A Modern Day Mulberry (Joint Port Opening)
Can we exploit modern technology and systems used in the shore engineering and offshore energy industries to create a modern day Mulberry Harbour?
Improving the UK’s expeditionary port survey, repair and operating capability are both possible and necessary. Building a modern-day Mulberry harbour is less so, but it is an interesting thought exercise nonetheless.
In the opening article in this series, I described the changing nature of coastlines and the proliferation of ports and then defined a requirement for the port survey and repair/augmentation.
This article builds on that, also looking at the changing nature of coastlines, but describes a requirement to rapidly built a direct ship offload capability that can exploit more varied coastline terrain than just beaches. It is certainly more aspirational than the previous two, more a collection of thoughts and ideas on potential solutions rather than a firm proposal.
Defining a Requirement for a Modern-Day Mulberry Harbour
How do you build up supplies onshore for an embarked force?
Liquids (fuel and water) will either be driven ashore in containers, tankers, bowsers or jerrycans. When risks have been reduced, there are bulk fuel systems that make use of transfer pumps and storage tanks that allow fuel to transfer, in large quantities, directly from tankers or floating dracones.
Vehicles are driven from amphibious shipping onto landing craft and lighters (e.g. Mexeflote) and then onto the beach.
Dry cargo (pallets or containers) are either placed onto vehicles (as above), or placed onto the landing craft and lighters using mechanical handling equipment (MHE), transferred to shore, and finally, unloaded at the beach area using more MHE, before being loaded onto other vehicles.
What characterises all these techniques is double handling.
Stores, vehicles and liquids all have to move off big ships and onto small boats at sea, the small boats have to move slowly to the shore, when they offload, and then return to the big ships and do it again. This is what makes amphibious logistics so slow, cycle time. Move further offshore and cycle times start to dramatically increase.
Increase the speed of landing craft to compensate and the penalty for speed is increased fuel consumption, cost, and generally, reduction in carrying capacity. The D-Day planners understood this fundamental point, and in the absence of ports, built their own, the Mulberry Harbour system. The absolutely crucial point about Mulberry was it allowed the logistics operation to cut out the landing craft middle man.
Large ships docked and offloaded directly.
Although we might initially look at increasing aviation assets to improve the throughput, or prepositioning equipment, investing in intra-theatre transport vessels and even buying lots and lots of high-speed connectors, these are not without obvious issues and as described in the context section, the ability of a wheeled combat team to transit large distances makes an approach from further afield more feasible.
The D Day Mulberry Harbour was a masterpiece of military engineering, but whilst the design was in many ways visionary, it was not without faults. The US Navy and Army picked up the baton and created the Elevated Causeway – Modular (ELCAS – M) as a component of the wider Joint Logistics Over The Shore (JLOTS) capability, but again, this is limited, not least because it relies on a beach environment and still requires stores double handling. There have been a handful of studies aimed at improving the concept of an expeditionary harbour but they have not resulted in any tangible improvements.
Stores ships still have to hang around waiting for the slow cycle to complete. If some type of docking landing platform were connected to the shore, cargo ships would be able to transfer stores, fluids and vehicles without landing craft and lighterage. Ships would still need to come into the shore but instead of hanging around for a long time whilst slowly offloading to slow lighters, and thus vulnerable to discovery and targeting, they would unload in very short order. This rapid offload capability will allow combat power and logistics to be built up quickly.
And, by exploiting a much greater range of shore terrain, the element of predictability is reduced that is inherent with beaches. Instead of offloading at suitable beaches, which as described previously, are likely to dramatically reduce in number as urbanisation and population-driven changes in land use put coastlines under development pressure, almost any coastal terrain can be exploited.
Because the bare beach solutions generally exclude any means of wave attenuation they are restricted by sea state and getting supplies onto lighters or landing craft is also a challenge in the rough stuff. The surf zone is a very difficult environment for cargo and vehicle transfer; if the ground is too soft, handling equipment will bog in, too shallow a beach gradient means lighters and landing craft will run aground in the surf zone and steeper gradients are often accompanied by stronger currents.
This requirement describes an ability to carry out a ship to shore logistics operation in varied terrain and at higher sea states than currently, with a much higher throughput derived from cutting out the lighterage middle man and avoiding operations in the surf zone of a beach.
Breaking the requirement down further…
At the most basic level, this must be able to accommodate a single ship at a time and that ship type should include ROPAX ferries, container feeder vessels, large CONRO’s, the UK’s Strategic RORO vessels, RN and RFA vessels, product tankers; container barges and allied shipping such as the US LMSR vessels. Cargo types will be vehicular, bulk liquids and containers/pallets and the system must include appropriate cranes and offload platforms, lighting and mooring systems.
The ship interface must be stable, not unduly affected by waves and tides. All these factors will be traded with others as any conceptual designs emerge, we may have to compromise on throughput in order to satisfy any notional installation time requirement, or we might be able to meet them all.
Link to Shore Causeway
A link roadway will connect the ship interface to the shore interface, this will allow vehicles, cargo handling equipment and bulk liquids to move to the shore without lighterage. It may be floating or fixed, but must be able to accommodate MLC120 class loads in single lane configuration with a twin lane configuration as a stretch option.
The shore interface is a significant departure from existing systems because it sets out to utilise a much broader range of terrain than beaches. It will provide a link from the Link to Shore Causeway to the shore, specifically, the shore that can be driven off directly.
The images below are representative examples of the type of industrial, port and natural terrain the shore interface must be able to connect to.
Build Time and Operations Duration
The basic system must be built and trafficking cargo within 4 working days of construction start, although for extra-long causeways this may be extended. Throughput will depend on many factors; the number of ship moves, sea state, type of cargo, and crucially, the ability of the shore location to handle it. However, the target would be to offload any ship within the target types in as little time as practicable. This does sound a little vague, granted, but there is some degree of ‘chicken and egg’ here with the throughput achieved with different design combinations that may be practical, or not.
The systems should be operable for a minimum of 180 days.
The nearshore environment is both complex and challenging.
It is into this environment that Requirement 3 will be placed and therefore has to operate.
Temperature Variation; although subsurface temperature variation is not as significant as offshore, any equipment must be able to work across typical temperature variations from the Arctic to the tropics. In the Arctic, air temperatures can be lower than those underwater.
Wind; wind velocities can often be the determining factor in nearshore operations, especially those involving cargo transfer and berthing activities.
Marine Growth; water chemistry will have a long term effect on marine construction but for the shorter term, marine organism growth will need to be considered.
Current; Tidal currents are not only horizontal but also have a vertical component and may be channelled by subsurface features. Tide induced currents may also lag the prevailing tide so, in some circumstances, the surface current will flow in a different direction to the subsurface current, exacerbated by changes in salinity in estuaries. Currents around structures can create eddies resulting in scour and erosion.
Waves and Swell; waves cause floating structures to move in six degrees.
Consideration of wave loading is likely to be the major factor for Requirement 3 design.
For most potential operation locations wave and swell prediction is well established, based on many years of observation. Wave energy is proportional to the square of its height and long wave periodicity can cause many problems where vessels are less than half a wavelength. Waves are thus characterised by significant height and significant period.
Sea State is a simplification of a very complex subject. Sea State is a combined measure of wave conditions whose components are local wind-generated waves and swell or waves that have travelled from outside of the local area. Sea State is manifested in three properties; wave height, period and direction. Wave height is the difference between the crest and trough, period, the time between successive crests and the direction from which the wave arrives. These are aggregated over a period of time to produce a simple guide to aid understanding. The build phase should be carried out in Sea State 3 and the completed system operable in Sea State 4, with comparable wind and tidal variations.
Scour; Scour can undermine foundations and spud legs leading to collapse.
When scour has occurred the structure can be subject to wave-induced ‘rocking’, total collapse in these conditions is a real threat. Scour from tugs, bow thrusters and waterjet propulsion devices should also be considered for this requirement.
Soil and Seabed Condition; Because Requirement 3 may utilise structures connected to the seabed the condition of the seabed is of critical importance. It may consist of combinations of sands of varying types and sizes, cobbles, clays, coral, gravel and large boulders.
It is recognised that most solutions to these requirements will require some form of dedicated transport and that taking up space on existing amphibious and general cargo vessels is undesirable. A high level of automation is a general requirement in order to reduce manpower requirements and thus keep through-life costs and manning manageable.
To summarise, this requirement calls for a semi-mobile system that will allow a wide variety of ships to offload their cargo directly to a variety of shore terrain and in a range of environmental conditions, in a timely manner. This will allow a logistics build up and ongoing sustainment to be conducted much quicker than with traditional methods that utilise landing craft and lighterage.
Meeting the Requirement for a Modern-Day Mulberry Harbour with Existing Systems
It would represent a significant improvement over anything in service anywhere today and be a unique capability, one that is hugely ambitious. Meeting this requirement would certainly produce a capability in excess of UK requirements but instead, a ‘signature system’ for multinational operations in conjunctions with allies.
There are three parts to the discussion on meeting this requirement;
- Review existing systems
- Pierhead, pier and shore interface
- Wave attenuation
The most obvious starting point for ideas to meet this requirement would be to examine existing solutions and previous research projects.
JLOTS ELCAS Pier
The Expeditionary Elevated Causeway (ELCAS) is an amazing portable modular structure that can be built out from the beach to a distance of 3,000 feet (914m) and up to a depth of 20 feet to allow small vessels to come alongside and discharge their cargo.
It is part of the US Department of Defense capability set called Logistics Over The Shore (LOTS), broadly defined as;
The process of loading and unloading of ships without the benefit of deep draft-capable, fixed port facilities; or as a means of moving forces closer to tactical assembly areas.
When Army and Navy LOTS operate together under a Joint Force Commander LOTS becomes JLOTS.
The threat environment is described as;
Joint logistics over-the shore (JLOTS) operations are normally conducted in a low threat environment. Primary threats to consider are air and rocket attacks, attack by adversary ground forces, guerrillas or insurgents operating behind the lines, and sabotage. Chemical, biological, radiological, nuclear, and high-yield explosives warfare is considered possible.
Sea Basing was supposed to largely replace JLOTS; instead of anchoring ships close inshore and using D Day era landing craft and pontoons, Sea Basing was high speed, largely airborne and of course very expensive. JLOTS on the other hand is in comparison, cheap as chips.
JLOTS is not for use when there is opposition in the area but then again, neither was the MPF(F). It is not very mobile and takes time to establish but when it does get going, throughput can be significant, unmatched by anything else. One of the initial proving exercises, despite many disruptions due to higher than expected sea states, demonstrated an offload of over 800 containers and 1,300 vehicles in 14 days. To do this required 67 different units, a total of 5,000 personnel, 70 small craft and a large onshore build.
Impressive, hell yes, especially when compared to the LCU/Mexeflote capability the UK enjoys but look again at those numbers. 5,000 personnel for a start, that is significant ration strength to support and if replicated in the UK would represent nearly 7% of the total British Army regular strength. 800 containers in two weeks, as a comparison, London Gateway can handle about 125,000 containers in the same period
JLOTS has a myriad of components and it would be impossible to list them all here but the main ones are shown in the diagrams below, JLOTS over a bare beach and JLOTS using an existing port.
Planning preference is to augment an existing port because of its proximity to main supply routes and other facilities. Just parking the whole JLOTS capability in the middle of nowhere means roads and offloading hardstanding have to be built from nothing. The vast majority of JLOTS is designed for installation and operation between Sea State 2 and 3. ELCAS for example ELCAS can survive up to sea state 5 conditions but only operate at a maximum sea state of 2 because of potential damage to the pier and the inability of the cranes to safely offload stores in the associated high wind speeds. Generally speaking, JLOTS only operates at a maximum of Sea State 2 because of the impacts of crane pendulation, dissimilar motions between large vessels and lighters, surf impinging on causeways and issues caused by surf zone waves on the stability and safety of all components.
ELCAS was part of a wider programme, Container Off-loading and Transfer System or COTS, ELCAS being a subsystem. It came into service with the US armed forces in the late seventies and was specifically designed to transfer containers and equipment, but mostly, containers, in the follow-on phases of an amphibious assault.
The objective for ELCAS was (and is) to avoid having landing craft and barges enter the surf zone.
Initial testing and concept studies recognised that its significant volume would displace equipment intended for the USMC and therefore, unlikely to find a home on USN amphibious shipping, hence the drive to deploy it on civilian and MSC shipping, especially SeaBee and LASH barge carriers, the LASH carrier being proven to have many advantages over the SeaBee. These tests also included using something called the Lightweight Modular Multi-Purpose Spanning Assembly or LMMSA which allowed a RORO ship to interface with ELCAS.
ELCAS modules were originally modified Navy Lighterage pontoon causeway units connected by a hinged ‘flexor pin’. Once the causeway modules had been manoeuvred into place and connected together using the flexor pin they were ready for elevating. Each module had a series of internal or external spud wells and the piles were inserted into these spud wells and then jacked up to the required working height.
The ELCAS pier head is simply another row of causeway modules.
A fender string was installed to resist berthing forces and provide some measure of a standoff that allowed the 140-tonne crawler crane to lift containers from the lighter to an awaiting truck and trailer. The piles used for the fender string are pointed instead of hollow to allow them to be driven deeper than the normal piles. Because the pier head was not wide enough to allow a truck to turn the 180 degrees needed a motorised turntable at the seaward end of the pier head was used, ingenious I think.
At the beach end, a pair of ramps were used to transition from the pier causeway to the beach.
The main problem with the original ELCAS was that because the causeway sections and piles had the be assembled on the water before jacking up, the whole thing was very susceptible to wind and waves limiting the construction sea state. It was also very labour intensive and therefore, not very quick to install. Because the deck was frequently overtopped by waves it was wet and hazardous when operating cranes.
Recognising these shortcomings and because of the physical condition of the ELCAS stock a decision was made to replace it, the new system coming into service, after a number of contract problems, in the mid-nineties. Instead of jacking floating modules out of the water, ELCAS-M uses a cantilever method to build out from the beach, without the deck modules touching the water, and thus, practically immune from wave conditions.
A number of alternatives were proposed and some initial testing conducted, these included a high wire transfer and aerostat system, neither was adopted. First used on operations during the 2003 operation to invade Iraq the ELCAS-M was used to augment an existing Kuwaiti naval base, providing additional loading and unloading capacity.
Apart from the method of construction, the ELCAS-M design was not that much of a change from ELCAS.
The beach ramps are grounded into an excavated area above the high water mark called a ‘duck pond’ that is 9 meters wide by 8 meters long. The 40 foot by 8 foot ISO container-sized modules is supported on 2 feet diameter hollow steel piles that are driven in during construction as the causeway is assembled from the beach to the sea.
The piles are driven in sequence and modules attached until the required length is achieved. Individual piles are 30 feet long and welded together as they are driven, usually between 10m and 12m depth. Softer soil conditions will require longer piles to be used, in Kuwait for example, many had to be 90 feet long to achieve sufficient load-bearing strength. Two pile drivers are used, one driving and one positioning. The full ELCAS-M system requires 193 piles.
In order to accommodate two-way vehicle traffic, the pier is 24 feet wide. When the piles are driven to the correct depth they are pinned to the module and cut off flush to avoid interfering with traffic and crane operations.
The pier head is wider to accommodate two 200 tonne cranes and two turntables that allow trucks to be turned within their diameter which negates the need for vehicles to reverse along the full length of the pier.
Lighting is also installed to allow 24×7 operations, together with safety restraints and power generation (total 260kW split between 4 generators). It can be built out to a maximum length of 915m and takes on average between 7 and 14 days to build, depending on pier length. In optimal conditions, the maximum stated throughput is 370 TEU per 24 hour period or about 15 per hour.
It is worth re-emphasising, the build sequence is to land the components on the shore using a combination of landing craft and barges, and the pier built out from the beach to the sea.
The fender strings shown above alleviate some of the lateral forces between ELCAS and the lighter, they are the same size as normal ELCAS modules but only one wide. Elevating the pier head.
It should also be noted that the ELCAS-M system is not designed to accommodate large ships but lighters, small logistic support vessels and pontoons/barges although the Lightweight Modular Multi-purpose Spanning Assembly (LMMSA) was tested in 1990 to interface ELCAS with a floating RORO discharge platform.
This video provides an excellent overview
Nothing about ELCAS-M is not impressive but it is not without a few downsides, and completely out of range for the MoD.
The first is simply the resources used to build it, well beyond the RE and RLC given other competing requirements. At 6m depth, it can only accommodate smaller vessels and lighters. RORO cargo is not offloaded except by sling. Because it can only use piles to support the pier deck it has to be sand and clay. ELCAS-M requires the landing of the components before construction can start, this is time-consuming and is limited to a beach.
It also requires a lot of MHE and construction equipment. ELCAS-M needs 7 marshalling areas for staging, storage, generators and others requirements. Because the modules are no larger than an ISO container they are strategically deployable but require a great deal of handling in use. Throughput is a difficult factor to quantify but a number of exercises have proven the stated throughput to be very optimistic.
Although both sides of the pier can be used and the pier is wide enough for two-way traffic the means of offloading containers is relatively slow, manually connecting slings and spreader bars to container twist locks. No automatic spreaders are used. High winds slow offload severely. As a whole system, the need for containers to be double handled (ship to lighter) decreases actual throughput significantly.
Finally, when installed, ELCAS-M is very resistant to wind, wave and tide but building and operating are limited by sea state, current velocity and the wind. Manoeuvring lighters into position and crane pendulation make offload operations hazardous.
Lightweight Modular Causeway System (LMCS)
The LMCS was developed by the U.S. Army Engineer Research and Development Center (ERDC), Coastal and Hydraulics Laboratory (CHL) as a lightweight alternative to rigid modular pontoons. The flotation tubes are neoprene-coated heavyweight nylon and are inflated to 3 psi to support rigid deck panels. LMCS sections are connected using pre-tensioned cables and connectors in order to form a floating causeway.
The total weight is approximately 25 tonnes per 25m section and can be packed down into a number of ISO containers, two per 25m. It was intended to be carried aboard the Joint High-Speed Vessel but has also been adapted for wet gap crossing where its lightweight, air portability and high load capacity (70 tonnes) is particularly valuable.
Even the USMC has taken an interest, using LMCS as a towed pontoon for moving light vehicles and stores to shore from a T-AKE ship, somewhat like a Mexeflote, and the air deployability of it is certainly attractive.
LMCS can support relatively heavy loads but is susceptible to deflection under wave loading which would severely limit offload rates with containers and tractor-trailers.
Advanced Cargo Transfer Facility
Following the 1982 Falklands conflict, the US Department of Defense initiated a number of studies to investigate means of improving cargo transfer over a beach and after a number of years published a paper on a system called the Advanced Cargo Transfer Facility (ACTF)
ACTF proposed a system that comprised a 2,500-foot long pier using 16 jack-up foundation modules, eight mooring modules and two berthing modules. These could be assembled and used to transfer containers to the shore in sea state 4 from in-service container ships without the need for lighters.
Jack-up platforms were used in combination with a purpose-designed universal foundation system that could accommodate sand, clay, rock or coral seabed conditions.
Ships would be manoeuvred using winch barges secured to the seabed with propellant driven anchors.
Once in position, they would be offloaded from both sides onto a rail-mounted tray that propelled them to the shore using linear induction motors. As can be seen from the diagram above, the facility did not have its own cranes but instead made use of the T-ACS crane ships, like the SS Cornhusker State
Two truss designs were described.
ACTF has a great deal to offer this requirement because it proposed dispensing with lighters and providing a facility for large 35,000-tonne cargo ships to offload directly whilst anchored in 15m of water at up to Sea State 4. The reduction in piling was also significant for installation time, as was the universal footing design.
The concept was developed further with improvements to the crane handling system but it was not taken into production.
Based on technology and systems developed for the North Sea oil industry, the Falkland Islands Intermediate Port and Storage System (FIPASS) was designed to resolve a number of issues; port access, refrigerated warehouse space and personnel accommodation. Six North Sea oil rig support barges (300×90 ft) were connected together and linked to the shore via a 600-foot causeway. Four of the barges carried warehouses, with provision for refrigerated storage. In addition, there were accommodation offices, which include a galley and messing facility for 200 persons.
The first cargo ship to use Flexiport unloaded 500 tonnes of general cargo and 60 ISO containers in 30 hours, by way of comparison, the same load, offloaded using Mexeflotes, took 21 days.
All this cost £23 million.
The company that designed FIPASS was ITM Offshore, realising the potential they developed the ‘Flexiport’ concept.
The design passed to ASP Ship Management and by the look of their website, Flexiport is now a company in its own right.
Much like ACTF, Flexiport has many relevant attributes for this requirement
The MOSES Pier
The MOSES pier is one of the more recent proposals for the provision of a ship to shore causeway system. The goal of the MOSES project is to design a causeway that can reach large ships in deep water, keep vehicles dry, and be safely operational in at least Sea State 4.
MOSES is a result of work carried out by students on the National Research Enterprise Intern Program sponsored by the Office of Naval Research, as such, it is relatively limited in maturity but it does show a great deal of promise.
The MOSES pier consists of a large fabric filled structure that is pumped full of seawater. When pressurised it forms a roadway 1m above sea level that can carry heavy vehicles. The weight of the water in the fabric creates sufficient downward force to ensure stability in the surf zone.
Like ELCAS and LMCS, it makes the assumption that only landing craft and other lighters will dock at the pier head. It only caters for RORO vehicular traffic and has no cranes or other means of offloading vessels. It also assumes that the pier will be landed at a beach, no other terrain can be accommodated.
Visualise it as a long RORO linkspan.
One of the more ingenious aspects of the proposal is that positive water pressure is maintained in the structure by large open-topped water reservoirs. This provides positive pressure for little energy expenditure and allows a ‘puncture reserve’ to be maintained. If the structure experiences a significant puncture the reservoir can provide enough time to maintain rigidity and thus allow safe evacuation of the pier. The cellular fabric construction is simply unspooled from a drum and filled using the reservoirs.
After a number of evolutions, the final proposal described a 150m continuous structure with roadway planking to protect the fabric.
Further investigations examined the ship interface and concluded that the proposed deployment method would be sub-optimal so one of the original concepts of using a jack-up barge were re-examined.
The jackup platform was called a Mobile Support Platform (MSP) and would be used both for the deployment of the pier, and interfacing to landing craft and lighters.
Floating Container Port
Scapa Flow is synonymous with the Royal Navy but a research study conducted a few years ago looked at using it for the ‘greenest’ container port in the world. Instead of developing shore facilities, the study looked at a number of floating concepts.
Scapa Flow, in the Orkney Islands could become the world’s greenest post if a clever plan that has been drawn up by researchers from Edinburgh Napier University’s Transport Research Institute (TRI) is adopted. The blueprint for the transhipment port could also could bring ‘substantial’ benefits to Scotland’s economy. The TRI estimate that the floating hub, which consists of a large storage vessel fitted with cranes, could nearly double the current £16bn value of Scotland’s exports of manufactured goods and that spin-off jobs would also be created for Scotland as the hub’s host nation.
Ultimately, the study was not taken forward for economic and other reasons but the research was sound. At around £40m, the proposed floating port is called the Floating Container Storage and Transhipment Terminal or FCSTT. It would cost approximately £80m less to build than a conventional land-based port offering similar capacity. What the proposed Scapa Flow Floating Container Storage and Transhipment Terminal system did was provide a floating transhipment capability, putting a storage and handling buffer in between vessels.
A couple of design concepts were considered, taking into account container vessel size, crane reach and other factors. The first design concept was considered a barge and travelling portal crane design.
The barge concept provided the most flexibility using existing crane designs. It could easily transfer containers from a 13 container wide Panamax ship to either an 8 or 10-row feeder or lighter. If a wider barge were used it would provide greater storage capacity but given the limitations of existing cranes would not allow direct transfer from a 13-row container ship to one with 10 rows. In this context, this compromise might be acceptable.
The throughput of a twin barge, twin travelling gantry crane system would be based on a 20 hour operation time, be in the order of 740 crane moves. These moves either being directly from the main vessel to the feeder or from the main vessel to barge storage/barge storage to the feeder, or any combination in between.
The second design concept considered converting a surplus Panamax container ship and fitting it with 4 pedestal cranes instead of the travelling portal cranes on the barge.
The conversion would be self-deploying and offer greater throughput than the barge option but would need larger cranes even to reach an 8-row feeder vessel and would be more costly in capital and operational terms. Throughput was estimated at 1488 crane moves per 20 hour day.
Crane pendulation is caused by a combination of operator input, relative ship motion and system dynamics. Crane operations become impossible in Sea State 2 to 3 and above and this is compounded if both platforms are in motion, in ship to ship transfer for example.
The US Navy Seabasing programme has been looking at various solutions to the problem of crane pendulation including heave compensating cranes, the Auto log cable transfer systems and others.
The latest solution is the Large Vessel Interface Lift On Lift Off (LVI LO/LO) from Oceaneering.
LVI LO/LO tackles the challenge of two ships moving in 6 degrees of freedom in relation to each other, akin to threading a needle being carried by a horse, whilst riding another horse!
The objective of the study was to prove vessel to vessel container transfer in Sea State 4.
Trials were conducted using the AC5 SS Flickertail State. It is an impressive feat of engineering, but excruciatingly slow
Self-powered barges are not the fastest of craft though, so it would be a trade-off between deployment speed and deployment ease. A good example of this type is the Damen Crane Barge 6324.
It is equipped with self-contained accommodation for up to 12 personnel, basic navigation and propulsion options and a large Liebherr CBG 350 Transhipment Crane.
The CBG 350 can lift up to 45 tonnes at 36m outreach and when equipped with a container spreader, easily handle fully laden 40 foot ISO containers.
Discussion on Existing Systems
Every single one of the systems described above is both impressive and extremely relevant to this requirement.
There is no doubt ELCAS-M is a marvellous piece of engineering that has no peer (no pun intended) in any armed force in NATO, or anywhere else for that matter. But, it has a number of issues that take the shine off and make it only partially suitable for Requirement 3.
Ship Interface; at 6m depth, it can only accommodate smaller vessels and lighters. Given one of the underlying requirements for Requirement 3 is to cut out the use of slow lighters and double handling, it needs to be approaching double that. The ELCAS crane cannot offload the LCU-2000 without the craft repositioning and the LSV at all. RORO cargo is not offloaded except by sling.
Bottom Conditions; because it can only use piles to support the pier deck bottom conditions have to be sand and clay. Requirement 3 has to be able to work in rocky conditions, concrete and other complex sea bed environments.
Build Sequence and Time; ELCAS-M requires the landing of the components before construction can start, this is time-consuming and is limited to a beach with plenty of space. It also requires a lot of MHE, personnel and construction equipment. ELCAS-M needs 7 marshalling areas for staging, storage, generators and others requirements. Because the modules are no larger than an ISO container they are strategically deployable but require a great deal of handling in use. It is easy to handle small Lego blocks, but it means a lot of handling.
Throughput; throughput is a difficult factor to quantify because there are so many variables, but a number of exercises have proven the stated design throughput to be very optimistic. Although both sides of the pier can be used and the pier is wide enough for two-way traffic the means of offloading containers is relatively slow, manually connecting slings and spreader bars to container twist locks. No automatic spreaders are used. High winds slow offload severely. As a whole system, the need for containers to be double handled (ship to lighter) decreases actual throughput significantly.
Sea State; when installed, ELCAS-M is very resistant to wind, wave and tide but building and operating are limited by sea state, current velocity and the wind. Manoeuvring lighters into position and crane pendulation make offload operations hazardous.
Skin to skin transfer, Mobile Landing platforms, automated cargo handling and heave compensated cranes are all there, but no pier. Of course, this is intentional, in the Seabasing concept, there is no requirement for ELCAS.
Lightweight Modular Causeway System
The use of inflatable pontoons for bridging and causeways has been a feature of US military bridging for many years, they have a great deal of experience. Whilst the floating nature of LCMS in use as a pier to shore would be valuable insomuch as it would be somewhat independent on bottom conditions, a floating pier creates many problems with waves and sea state. There are also questions on durability in an offshore environment.
Advanced Cargo Transfer Facility
The ACTF was actually a very interesting concept, perhaps ahead of its time. The combination of propellant driven anchors, expanding truss piers, winch barges and container shuttles made for a very effective system. The expanding supported truss system was particularly ingenious because by dispensing with a bridge deck, the volume and weight of material used for the pier to shore were dramatically reduced. But there lies a problem, it was only able to move containers and pallets, not vehicles or bulk liquids.
FIPASS revolutionised cargo handling after the Falklands conflict and the fact that it is still is a testament to the concept. Flexiport has built on this to create a concept that is well suited to this requirement but is at a scale that makes transport difficult and its semi-permanent nature is somewhat at odds with the general requirement.
The Moses Pier
The most interesting part of the Moses Pier concept is not actually the pier, but the use of jack-up barges to provide the ship interface.
Floating Container Port
The floating container port concept idea of using surplus Panamax vessels is worth exploring, it could be a relatively cheap means of providing the pierhead. The use of multiple large cranes also increased throughput significantly.
Floating Cranes and Crane Barges
Crane pendulation might be a significant issue of the pier head floats, the technology might be useful. It is expensive and slow, however. Crane barges are in widespread use and relatively inexpensive but they require calm water.
Meeting the Requirement for a Modern-Day Mulberry Harbour with a New Design
Whilst every single one of the designs described above has something to offer, the state of the art of offshore engineering has advanced considerably by virtue of the renewable energy sector. The three main components are pierhead, pier and a shore connector, the latter allowing the system to access a broad range of shore terrain, not just a beach.
The basic requirement for the Pier Head is a large flat platform that can support cranes and offload ramps, space for vehicles to turn and store cargo, and finally, some means of connecting to the shore via a pier.
It is as simple as that.
Size, Shape and Orientation to Shore
Looking at the potential solutions there is no single best approach to pierhead orientation. FIPASS and Mulberry were parallel to the beach, ELCAS and ACTF, perpendicular. The orientation of the pierhead may provide some protection from waves and thus improve stability and ease of berthing depending on prevailing wind direction, tidal currents and wave direction.
It would be desirable for it to operate in either orientation.
Vehicle Turning Geometry
It must allow articulated trucks or shuttle carrier vehicles to enter the pier head, be loaded directly from the moored ship, turn around and exit the pier head without having to use time-consuming turntables like the ELCAS-M pier. Throughput will greatly depend on the ability of vehicles to enter and leave the pierhead quickly, it cannot become a bottleneck.
For industrial buildings, roads and loading areas, there are numerous standards and norms for sizing turning circles. Vehicles using European roads, for example, must be able to turn within a circle of 5.3m inner radius, and 12.5m outer radii. A common design for industrial buildings and loading areas is called a ‘banjo’, with alternative ‘stub’ and ‘hammerhead’ designs less suitable alternatives for non-articulated vehicles.
Orientation and position relative to the pier will influence turning radius requirements. If the pier enters the pierhead whilst the pierhead is moored perpendicular to the shore, vehicles will require space to turn into the banjo. When oriented perpendicular to the shore a vehicle can enter the banjo without turning.
The turning banjo minimum width is 16.5m
To allow vehicles to turn into the pierhead at ninety degrees, the vehicle will have to perform a double eight which results in a minimum length of 35m, 50-60m preferable. Incidentally, this is not far off the dimensions of the Albion class well dock. A clear deck space of 20m wide and 60m long, just for vehicle loading, unloading and turning, is the basic requirement, but the longer the better.
A trade-off might be to accept a smaller pier head but use a motorised turntable.
This model assumes a single pier entry point. Two piers, each entering the pierhead (in parallel orientation) at opposite ends would reduce the need for a vehicle to turn back on itself and allow an easier double consecutive ninety-degree turn manoeuvre instead. If the pierhead were installed perpendicular to the shore both piers would need to be installed in close proximity to the narrow end and thus, a complete 180-degree turn would still need to be executed.
The Flexiport concept envisages an L shaped configuration to provide RORO offload for ships without a slewing or quarter ramp. Inverting the L may also provide some measure of shelter against wind and wave and the advantages of simultaneous cargo and vehicle offloading increases flexibility a great deal.
The connection could take the form of a simple coupling ramp.
If the pier head is of a simple rectangular shape any number of units could be connected but if transport and configuration requirements dictate a more conventional shape with the streamlined bow then only two units would be able to be connected in an L shape, or possibly three in a T shape.
The key to the pier is size, enough clear space to allow articulated vehicles to turn and cranes operate. Connecting two or more units into an L shape provides many advantages for RORO shipping and provision of some wave and wind shelter.
35-40m wide and 60m plus long is a minimum, longer would be better. The longer the pier the less the number of ships and crane moves and an opportunity for multiple cranes to operate simultaneously. However, the larger it goes the heavier it becomes, with foundations needing to be more robust, there are trade-offs.
Stability and Strength Considerations
Stringing multiple platforms might seem easy in the videos and images but stability is both a serious challenge and a critical success factor. The pierhead must have sufficient lateral strength to withstand berthing impacts, vertical load-bearing sufficient to accommodate multiple cranes and vehicles (plus itself) and general resistance to overturning and sliding from the wind, tide and wave.
In order to provide an environment where ships can safely and efficiently offload the relative motion caused by the wave, wind and tide have to be minimised. Without fitting expensive (and slow) motion compensating cranes on both the pier head and vessels it is logical that one of them has to exhibit little or no movement.
It is unlikely to be practicable to conduct any dredging operations at the pier head so it must be grounded in waters of such a depth to enable the target ships hazard-free access. Small to medium-sized vessels need between 4m and 8m with larger vessels needing 12m-14m, plus under keel clearance.
This leads to consideration about whether the pier head should be completely free of the surface or free-floating. Lifting the pier head completely clear of the surface of the water with vertical and lateral loads borne by piles or legs results in relatively low loading from tides and other currents and zero movements under the wind, wave and tide load. Partially submerging the pier whilst still being stabilised by pier legs (spuds), distributes loads, this technique was used by the Mulberry harbour system.
Where this requirement complicates things considerably is the requirement for high-speed installation on many different seabed geologies and construction in Sea State 3 or below.
If we look back at the Mulberry harbour system used on D-Day, the pier head platforms had to be able to offload 130m 10,000 ton Liberty ships with an 8.5m draught. Each pierhead platform had 90-foot spud legs that allowed it to be raised and lowered with the tide. The mode of the operation depended on the weather. In calm seas the pier head was anchored by the spud legs but not supported by them, the whole thing floating up and down guided by the legs. In heavier weather, although it was not completely supported by the spud legs, it was held a small distance above the free flotation level. At their full extent, the D-Day Mulberry Pier Head had 15.2m of water underneath it, well within the required range for the requirement.
The significant expansion in nearshore (and offshore) wind turbine construction has moved the state of the art considerably and it is worth examining for potential solutions. The most common form of foundation design for wind turbines is the monopile, typically a 4m diameter 35m long steel tube weighing 600-700 tonnes driven into the seabed. Pre-drilling and grouting may sometimes be carried out depending on seabed conditions. A transition piece is fixed to the pile and the turbine tower is fixed to the transition piece. At the base of the monopile, scour protection mattresses are installed. Installation time varies but a typically driven monopile takes approximately 24 hours, up to three times that if pre-drilling is required.
The forces monopoles need to resist, particularly over-turning moments, are extremely high, much higher than needed for Requirement 3, but they do provide a useful indicator and applicable installation techniques.
A typical deployment sequence for an offshore wind jack-up construction vessel involves mooring (or dynamic positioning) over the target location, deployment of the spud legs and pre-loading for a number of hours to confirm stability. When stability at pre-load levels have been confirmed the vessel will be jacked clear of the sea and work commenced.
This is a critical phase as the uncertainty of penetration depth and footing stability can give rise to the leg dangerously pushing through pockets of poor soils. Wave slap on the underside of the vessel as it is raised can produce dangerous levels of stress and wave height restrictions are placed on operations although the larger types can be jacked clear in conditions in excess of 2m significant wave height.
Construction vessels have to withstand very high vertical loads because of the weight of monopiles, transition pieces and turbine components. Although some resistance against collision loads is considered, they are simply not designed to withstand lateral loading i.e. from berthing ships.
Their legs are either of solid construction (circular or square section) or lattice. Spud cans fitted to the bottom allow deployment in a number of different conditions and jetting systems can be used to fluidise the topsoil layers allowing the spud to be placed on harder soils below.
If conventional monopiles take too long to install (and are too massive), and jack up vessels using spud legs do not have the lateral load resistance necessary some other solution will be required.
Conventional pile installation can be significantly quicker if the driving equipment is duplicated, installed of driving one at a time, all 4 (or 6) per pier head platform can be driven simultaneously. The Fistuca BLUE piling technology is an interesting innovation that might be applicable. Using a combusting mixture and water column avoids the need for hydraulic or impact hammers.
Suction piles are traditionally used in deep water jacket construction and for anchors but the Dutch company, SPT, have developed the concept for platform and foundations in shallower waters. They are open-ended steel tubes with an enclosed top. The top is fitted with a valve and when landed on the seabed, negative pressure is applied to remove water. This suction effect pulls the pile into the seabed to create a strong foundation. In contrast to conventional piles, they are much wider but penetrate to a much-reduced depth, in the offshore wind industry the typical monopile is driven to 30m, a suction pile usually less than 8m. Because seabed penetration is much smaller, installation times are much reduced, a few hours in some cases. Noise is eliminated and they can be easily released by injecting water under pressure.
There are a number of variations on the design, tripod clusters, monopiles and platform designs for example. The monopile shown below can be towed to the site and installed without heavy lifting equipment. SPT has also proven installation in 2m significant wave height, Sea State 4.
Suction anchors offer a potentially very fast and silent installation solution for the pierhead with no seabed preparation required. They could be permanently installed, as with the SIP-II platform, or installed separately as monopiles onto which the pierhead is fixed. With the piles fixed to the seabed, the pierhead can be floated adjacent to them and secured using a gripper assembly. Once attached, the pierhead would be raised using strand jacks.
The jack-up platform (legs) could form the pierhead whilst a number of driven or suction monopiles provide protection against collision and berthing forces, the distance between the pierhead and vessel would increase but this could easily be accommodated by crane selection.
There are many variations and combinations on the theme of foundation piling, constructed off the pierhead or permanently attached and conventional or suction. Selection would have implications for the construction process and equipment required but would ultimately, be the result of modelling and testing.
But for the purposes of this exercise, a conventional jack-up system combined with suction piles to provide protection against berthing loads is proposed.
Cranes and Other Deck Equipment
Considerations on cranes are very similar to those for port repair requirements, and it is not inconceivable that the same mobile harbour crane could be used for both. Whilst those cranes had to be portable, the key difference with this is that they can be fixed to the pierhead. This simple choice opens up a wider group of potential designs.
Ship to shore gantry cranes, available from manufacturers such as Liebherr, Kone and Terex, are generally sized according to the container ship size; feeder, Panamax and Post-Panamax for example. They are very fast, able to operate in high wind conditions and lift multiple containers at a time. Their design also allows them to easily reach the far side of a high stacked container vessel whilst maintaining lift capacity. Back each span can be up to 30m.
For maximum container throughput, the Ship to Shore Gantry Crane is the gold standard.
The fundamental problem with using one of these for this requirement, however, is one of size and weight. They are extremely large and because of their height would present a stability problem. For this reason, they are discounted from consideration.
A mobile harbour crane may be the best solution.
They can be wheeled, rail or pedestal mounted. Although there may be some stability benefits in rail or pedestal mounting the crane on the pierhead centreline greater flexibility will be achieved by using the wheeled base. The larger 550 and 600 series can easily lift fully loaded containers at maximum outreach distances of 50m.
This increases throughput significantly.
30 crane moves per hour is a reasonable average for this type of equipment and with a telescopic double spreader; that is 60 TEU per hour from a single crane.
For most large ships the simplest way of discharging RORO cargo onto the pierhead will be to simply lower its ramp on the deck but if the pierhead is raised or simply too high for smaller vessels they will not be able to lower their RORO ramps.
A simple solution might be to create a ‘steel beach’, notched into the side of the pierhead, protected by a gate during transit. The modular buffer pontoon described previously may also form part of the overall solution.
Whilst the primary objective for this requirement is solid cargo, vehicles and container offload, fuel is a vital commodity. Because of the nature of the fuel, it lends itself to deployable solutions, dracones and ship to shore pipelines for example. However, in order to increase flexibility, it might be desirable to incorporate some form of liquid handling system on the pierhead would be desirable
Fendering and Mooring
Fendering systems provide protection for vessels and pierheads. Pneumatic fenders, high-density foam and other systems can be fitted to the sides of the pierhead.
Although conventional mooring bollards should be fitted to the pierhead there is a relatively new suction mooring system available from Cavotec called the Moormaster. Moormaster is an automated system that eliminates the need for conventional mooring lines. Remote-controlled suction pads deploy from the quayside and secure the target vessel. The system is now well-proven and the larger units can accommodate up to 1m of heave and 7m tidal range. Automated mooring systems may well be worth considering for the pierhead.
From the above, a basic design has emerged that is a large flat deck, some means of elevating on spud legs, a RORO steel beach/ramp, the ability to clip together multiple units, liquids handling, a large harbour crane, fenders and some means of protecting the platform from berthing forces.
This all points to a relatively simple ‘big metal box’ construction.
What complicates matters significantly is the means by which it arrives on site.
There are three broad options;
If the towed or carried option is chosen the pierhead can be of extremely simple and cheap construction, offshore barges cost in the single or tens of millions of pounds. The Damen Stan Pontoon 9127 ‘North Sea Barge’ is a common example of this type.
There is a lot to commend this approach; cheap, simple and robust.
Fitting them with spud legs or whatever form of support is chosen is again, an established and commercially available option, as are semi-submersible configurations.
Whilst they are cheap and effective, unpowered barges are difficult to handle. Weather and Sea State restrictions are the norm, although the larger offshore barges are designed for severe weather, and tugs are required for both transport and installation activities. Instances of towlines parting and other losses due to severe weather are well documented.
Taking this approach would also mean an ocean-going tug or two would be required as part of the solution. This would not necessarily be a bad thing because tugs would be part of the overall package in any case.
Using modular or integral propulsion units can provide some measure of self-deployment capability they are not the sleekest of vessels and so transit times are high.
An alternative to towing is to use a semi-submersible heavy lift vessel, often called a float on flow off (FLO/FLO) vessel. These are commonly used for outsize and extreme weight cargoes and offshore construction modules. At the target site, the ship is ballasted down and the barge floated off. The image below shows FIPASS during transportation on a semi-submersible heavy lift vessel, the Divy Teal
Rolldock and OHT operate a diverse fleet of heavy lift vessels, some, semi-submersible. The semi-submersible FLOFLO type of vessel could be used to carry a pierhead barge, or two, arranged in an oblique pattern across the bearing deck, as shown above. Speeds between 15 knots and 18 knots are common.
Once deployed, the vessel can withdraw and the pierhead modules positioned using tugs or dynamic positioning before securing with their spud legs. Significant wave height varies between the different ships but the maximum allowable seems to be 1.5m to 2m. This option would require this approach to have its own heavy-lift vessel or some form of short notice chartering arrangements with a civilian operator.
Because heavy lift operations tend to be conducted in response to installation projects and schedules planned well in advance, the cost of such disruptive charting arrangements, whether actually used or not, would likely be very high. Incorporating a single semi-submersible heavy lift vessel into this capability set increases the cost significantly, although no doubt other uses would likely be found for it.
It may be possible to have the heavy lift vessel carry one leg of the pierhead and form the other leg itself, this would reduce redundancy and cost.
An interesting alternative is the Articulated Tug Barge (ATB) concept. Designs have evolved from simple notched barges to the extremely capable ATB designs from Intercon and Ocean Tug and Barge Engineering. ATB’s are quite common in the Gulf of Mexico and USA, but less so elsewhere. Instead of pulling, the barge is pushed, speeds of 13-15 knots are common.
The tug connects to the barge through an articulating connector unit. One of the spin-off benefits of this is that at the deployed location, the tug(s) can detach and be used to assist ships berthing at the pierhead.
Finally, one could dispense with the notion of unpowered barges and take the ‘self-propelled’ option.
Commercially available wind turbine installation vessels offer an obvious potential solution. Gusto MSC of the Netherlands is one of the leading designers of self-elevating platforms. A recent example of their Next Generation Self Elevating Platform was sold to Seajacks for $121m, the cost including design, construction and delivery. It entered service as the Seajacks Hydra, a similar design is shown below.
It has a length of 75m, a width of 36m and a top speed of 8 knots. Typical leg penetration at full load is between 3m and 5m and jacking can take place in a significant wave height of 2m, 21 knots wind speed and 2 knots surface current, analogous to somewhere between Sea State 3 and 4. The problem with using this design for this requirement is the 900 m2 deck area is too small, the 8-knot transit speed possibly too low and the 100 person accommodation and main crane too much. But with appropriate design changes, these issues could be resolved.
The Seafox 5 from Workfox is a larger vessel. As with the Seajacks Hydra, the crew accommodation for 150 persons, 1200 tonne crane capacity, 20m air gap, helicopter landing pad, 7,000-tonne deck load and 105m jack leg system far exceed Requirement 3 requirements.
These windfarm construction vessels are designed for loads far in excess of Requirement 3 loads, evident from the size of the spud legs and cranes, but they do show a design approach that could meet the requirements.
There are a number of different configurations and options for transport, either self-deploying or towed/carried. As with foundation/pile configuration, the optimal solution would be derived from modelling and testing.
If the pierhead is a relatively simple concept, the pier to shore throws up some very difficult design, installation and operation challenges.
The length of the pier is pretty much dictated by the seabed gradient. For a given water depth, the steeper the seabed gradient the quicker it will be achieved, and therefore, the shorter the pier. Other local conditions may influence the length as well, seabed obstructions, reefs and tidal range, for example, but the main factor is seabed gradient.
For comparison, the ELCAS pier can extend to approximately 1,000m, but given this requirement requires much greater depth at the pierhead, logically, the pier must be longer. Distances to the 30ft sounding line and mean low water vary between just a few tens of metres to over two thousand, all depending on location.
We accept compromise on the terrain that this requirement will be used in.
Pier width is driven by two factors, the number of lanes and maximum vehicle width.
Heavy plant such as bulldozers, container handlers and trackway dispensers are considerably wider than their track or wheelbase. Modern armoured vehicles also have considerable overhangs caused by applique armour. Container handlers such as the Rough Terrain Container Handler would be particularly problematic as they can only travel at a reasonable speed with the container at right angles to the vehicle. Pedestrian safety barriers also need to be considered. Ordinarily, this would not be a concern because these overhanging elements could simply overhang the pier kerb and offset pedestrian barriers used.
However, where it does become a problem is if a panel-type construction is used, a Bailey bridge deck for example, as the structural panels are considerably higher than the kerb and would thus impede the flow of wide vehicles. Panel bridge construction would tend to result in greater widths than other methods and the additional structural materials required might increase total deck loads and load-bearing capacity at each pier. The pier should provide a minimum clear space width of 4m, regardless of construction technique.
If the pier is a single lane it could form a bottleneck in a number of scenarios. For traffic across the pier that is primarily RORO and moving from ship to shore, a single lane is not likely to be a significant issue. Where it could be a problem is if the traffic on the pier is two way, articulated vehicles shuttling containers from the pierhead to a shore marshalling area for example.
The longer the pier the greater the time spent on the pier and the potential for bottleneck creation at either the pierhead or shore. Traffic control lights would optimise pier usage but the problem would remain. A solution that starts with a single lane pier that can be expanded to a double lane would potentially provide a higher speed of installation followed up with increasing throughput.
After dimensions, the next factor to consider is load-carrying capacity and it makes sense to define the pier in terms of Military Load Classification.
To provide assistance to bridge designers, tables of weights of commonly used equipment combinations have been compiled since the early days of military bridging. Civilian bridges were also classified using the new scheme and it ultimately developed into a NATO standard, the Military Load Class (MLC). The NATO system uses 16 hypothetical classes; 4, 8, 12, 16, 20, 24, 30, 40, 50, 60, 70, 80, 90, 100, 120 and 150 as defined in STANAG 2021.
The number is merely an indicator although weight plays a significant part in its definition and vehicles and bridge/ferry/raft MLC are calculated differently. Vehicle MLC’s use weight, wheelbase, axle loading, spacing and contact area. Because wheeled vehicles tend to be longer than tracked vehicles and therefore exert differing concentrations of load and bending moment, bridges often have both a wheeled and tracked MLC. Bridging equipment MLC calculations include weight, the spacing of vehicles, safety factors and dynamic effects or impacts.
The largest, MLC 120, will be able to accommodate a main battle tank, tractor and trailer. MLC 120 is demanding from a bridging perspective, it adds weight and reduces installation time. An appropriate compromise might be to lower the load classification of the pier to MLC 60, which would allow a 40-tonne container and articulated trailer at maximum UK road weight (and therefore a common configuration). This would mean the heavy armour would need to use traditional over the beach methods unless it can be driven off the ship directly i.e. not on a trailer. MLC 70 – 80 would allow for future growth and cover the vast majority of vehicle and cargo combinations.
Other considerations include lighting, kerbing, lane marking, pedestrian barriers and anti-skid surface coatings.
Conventional piers usually use densely packed steel or reinforced concrete piles driven into the seabed or grouted in place because they have to withstand frequent storms and continuous use over periods measured in decades. Requirement 2 design drivers have different priorities and longevity is not at the top of the list.
This is not to say it will be flimsy because the actions of a wave, wind and tide combined with loading from vehicles and personnel trafficking the pier will all require a fairly substantial structure, just not as substantial as a permanently installed pier.
In general, the speed of construction has a higher priority than absolute longevity. The pier is required to interface with the Pierhead but they may be at different heights, temperature-induced expansion and movement from wind, wave and tide will also need to be considered. This points to a general requirement to decouple the pier from the pierhead and use some form of the flexible connector.
One of the first questions to ask is should the pier float up and down with the tide and waves, or be decoupled from the sea by anchoring to the seabed? The pier can also be constructed like a bridge, with a clear span and supports, or a pontoon.
ELCAS uses modular pontoons for the complete structure, each container-sized building block is the same. They are secured by driving piles into the seabed at regular intervals. This is what drives up the installation time, the pier itself can only be built in ISO container-sized pieces and it needs a great deal of pile driving using conventional impact hammers suspended from a crawler crane. Hence, 2 weeks installation time.
Mulberry took the floating approach, this made installation times very low but resulted in a structure that was susceptible to waves and had a weight limit of approximately 25 tonnes.
For this requirement, there must be faster alternatives and ones that can support much higher payloads, as above, minimum 60 tonnes with 80 being desirable. The answer to the float or fix question may indeed be both; fixed in the surf zone and floating beyond. If floating, should the pier be water supported across its entire length, or held clear of the water by floating supports or pontoons?
Pier Deck Supports
Many of the issues are the same as with the pierhead, but in general, loads will be much lower.
The longer the deck, the greater the load transmitted to the seabed and they must also be resistant to wave, tide and wind loading. Because the pier will traverse the surf zone, wave loading may be significant, scour likely and soil bearing strength poor in some areas. A conventional approach would be to use driven tubular piles, supporting a spreader, onto which the deck sections can be landed. As described in the Pierhead section, pile driving is slow. A 1,000m pier comprised of 20x 50m individual deck spans would require in the order of 40-50 driven piles. Double the pier and half the span and the problem with using hammer driven piles becomes obvious, one of speed.
Many of the floating steel modular pontoons described in Requirement 2 are also suitable for use as a floating pier deck support pontoon and the same range of manufacturers could be used. Simply put, the larger the weight to be supported, the larger the pontoon.
MSP provide a range of pontoons and useful data sheets that show weight supported for each size at a given water displacement. Floating pontoon support would need to cope with at least 2m waves when fully loaded and so the sizing calculation must take this into account.
A 40ft x 8ft x 8ft ISO container-sized pontoon can support a maximum of 50 tonnes with a freeboard of only 30cm, in other words, not enough.
Doubling stacking, connecting multiple pontoons side by side or arranging in a triangular format would provide the necessary load-carrying capacity, stability and resistance to overtopping by the most likely wave conditions. Floating deck supports are more suited to the deeper water towards the pierhead. A floating pontoon pier deck support would need anchoring to the seabed, and clipping them together in any kind of significant waves would be impossible.
Spud Leg Pontoon
To provide greater stability than an anchor, the same floating pontoons can be stabilised using spud legs. All the pontoon manufacturers offer freefall spud legs in a variety of lengths and end cap designs.
The support pontoon would still float up and down, but the spud legs would limit this movement to the vertical plane only, reducing the need for anchors and twisting or rolling movement.
If the seabed provides sufficient load-bearing strength the spud legs can be used to support deck loads in addition to providing stabilisation. There would still be no driving as the freefall may provide sufficient seabed penetration load-bearing penetration. Jacking units can be attached and the pontoon jacked up, either completely clear of the water surface or partially clear, or semi-submerged.
The ACTF concept described in the previous section used something similar to this, defined as a ‘Jack-Up Foundation Unit’
If a greater surface area were needed to support each pier deck, the very common jack-up platform could be used.
Perhaps the simplest option is to use a mass with a large bearing surface to spread the load of the deck. This can consist of sand, gravel, boulders or other granular fill materials contained within the support. The support could be perforated containers, geotextile bags or reinforced geotextiles like gabions. Instead of granular fill materials; precast concrete sections could also be used.
Simply dumping boulders or precast concrete block into the sea at deck intervals would be the least complex but this would require a great deal of material and extended build times, neither desirable nor practical.
Steel mesh gabions filled with rocks or cobbles can be lifted into place from a working platform with a suitable crane. Built-up to the required height they would form a sturdy series of deck supports. Volume calculations are complex and would depend on the pier length and seabed profile but making a series of assumptions, a 1,000m pier would require 150 construction units, where each construction unit is 1m high and 4m wide.
This would create a 1mx4m wide rectangular unit to support the pier, stacked to create the correct height. The problem with this assumption is they would simply topple over under a kind of wave load so the support would have to be of prism shape, like a block Toblerone.
The prism could still be constructed of rectangular shapes, built up into the required shape. Again, applying a number of assumptions and making a series of approximations, the gabion volume required for a 1,000m pier would be approximately 4,000m3. A 20ft intermodal container has an internal volume of 29m3, 140 containers in equivalent volume. The number of crane moves would also be significant.
Perforated and filled ISO containers could be used, weights would be higher but crane moves, and therefore time to install, significantly reduced. The volume required is so high because of the size of the prism shape at its base in deeper water. Simple geometry results in high volumes. De-installation would also be very time consuming with solid concrete blocks or gabions.
This leads to a general conclusion that in deeper water, solid construction is not optimal. In deeper water, towards the pierhead, the only practical alternative to piling is to use some form of trestle, frame, floating system or other structure in place of mass.
A properly designed and constructed trestle or deck support would be strong and very quick to install. The Mabey MAT-75 and System 160 provide examples of what is possible, especially if pre-fabricated.
They would simply be craned into position, their weight and construction geometry providing stability and load-bearing strength.
The MAT 75 is a modular bridge support system made from basic column units that are braced at either 5 or 10 feet centres with individual columns available in 2, 3, 4, 6, 8 and 12 feet lengths.
Adjustable screw ends and hydraulic jacks can be fitted on top of the columns to support the grillage beams that would support the pier deck. Pre-assembled units would simply be lowered onto the seabed and in order to provide additional bracing against wave loads, outriggers made from the System 160 shoring system would be fitted.
This kind of support will need a wide base foundation or rock/concrete mattress.
Geotextiles have advanced enormously in recent years and are now in widespread use for coastal construction projects. The Tencate Geotube could be very useful in providing a pier support. A Geotube is permeable fabric tube of varying length. Secured in place with steel tie rods or frames it is pumped full of sand/water sludge using commonly available dredging pumps. Chemicals can be added to accelerate dewatering but over a period of time, the dewatering process takes place without external intervention leaving what is in effect a giant sandbag.
They are used for slurry and waste dewatering and the construction of jetties, artificial islands, groynes, temporary dams, breakwaters, scour protection and beach erosion barriers.
Although full dewatering takes months, stability of the tube and load-bearing capacity can be suitable in a period of only ‘hours’. Multiple Geotubes can be stacked and filling carried out using multiple entry points with locally available non-cohesive sand and soils.
The pier deck would be simply supported on a Geotube, or stack of Geotubes, placed parallel to the shore. Geobags are smaller tubes closed at both ends in ready to deploy configuration.
The main advantage of using geotextiles like the Geotube is their low weight and volume in transport, they are very efficient. Fill times will depend on many factors but a typical 2.5m high, 4m wide, 50m long Geotube with a medium-duty pump at 15% sand/water mixture will take 9 hours to fill.
Given that deck support only needs to be approximately 5m wide the potential installation times are quite modest and this would be a serious contender for deck supports in the shallower end of the depth spectrum.
Helical Screw Piling and Prefabricated Grillage
If pontoons or prefabricated trestles are used, they may need to be secured with piles driven into the seabed soils. As described above, if conventional hammer driven piles are too time-consuming alternatives may be used.
Composite or steel sheet piles can be quickly driven into support column shapes and backfilled but one option that may be promising is helical screw piling. Increasingly used for low disruption and displacement piling on land there have been a number of examples of their use in road/rail infrastructure construction and proposals for offshore wind foundations, they were actually used commonly for Victorian piers, Brighton Pier for example and most motorway overhead signs are screw pile-supported.
A helical screw pile is as it sounds, a large screw driven into the ground using a hydraulic torque motor. Installation times are very low. Once driven, a tubular or sectional grillage can be placed on top and this is used to support the pier deck.
A possible means of slashing installation times is to fabricate a grillage and have a series of helical anchors permanently fitted, ready for driving.
A grillage frame is shown below.
Instead of driving sequentially using a hydraulic excavator torque motor, motors could be permanently attached to each one with a slip ring. Hydraulic pipework and a manifold would allow a single hydraulic pump connection to be made that would drive all motors at once. This pump would be installed on a workboat or platform. If the simultaneous installation of multiple grounding grillage frames is needed, multiple workboats would be required.
This arrangement has the potential to produce an extremely short installation time, and if used with off-board hydraulic power, one that can be used at deeper depths, nearer the pierhead, or, where ground conditions are suitable.
The other advantage of using an underwater grillage frame is it can be driven in the undulating or sloping ground to level, making subsequent installation of support trestles much easier.
There are a number of trade-off points with the pier deck, I have covered load-bearing and span above but in general, the longer the clear span, the more difficult to install and heavier it will be. Heavier spans generally, although not always, mean stronger foundations that are more difficult and time-consuming to install.
Shorter spans, on the other hand, are lighter and easier to install with a lesser requirement for support strength; but require more supports and more lift operations to install for a given length.
Another factor that must be considered is volumetric efficiency. Given that the entire pier must be transported to site solutions that involve lots of free space and open trusses will require more space and present transportability challenges.
The optimum deck span to deck span support ratio would be determined from detailed modelling and depend on whether fixed or floating pier deck supports are used.
The pier deck is very simply, a bridge, and bridges come in many designs.
A simple beam bridge comprises steel I-beams simply supported at each end with a deck comprising steel plates, reinforced concrete or some other surface. The most common form of bridging in the UK is a ladder deck beam bridge. They are simple, easy to install and economic, although there are more economic methods for longer spans and practical limits to clear deck span are about 30-50m.
An over truss bridge (sometimes called deck or underslung) replaces I-beam with a truss design, this reduces the amount of steel used but creates a much deeper structure, approximately 10-15% of the span. Over truss bridges are light, easy to install (although more complex to fabricate) and relatively cheap.
The through truss bridges uses the same construction technique but places the deck inside the truss, reducing depth underneath the bridge whilst still retaining the lightweight construction, long span potential and ease of installation of the underslung or over truss approach.
A bowstring truss design uses an arch and truss combination with less sturdy foundations, the deck can be placed on top, through or underneath.
Truss bridges use variations on the Warren, Pratt, Howe and Parker truss designs.
For rapid installation and transportability, the Bailey type panel bridge is the next obvious solution to examine. These panels of a standard size that incorporate warren trusses and can be connected together to form longer and stronger spans. Various types of the transom and bracing can be used to provide additional stiffness or strength.
The modern version is produced by (among others) Mabey.
What makes the Bailey type panel bridge so successful is its versatility. It is a truly modular design, able to accommodate varying load requirements, floating, multi-span supported or clear span. They are also able to be constructed by hand or with mechanical assistance, and importantly, are easily transportable because the basic building blocks are small and light.
Launching can be by craning a complete span into the gap or launching across the gap using a launching nose and cantilever technique.
Despite their many fine qualities, they are not fast enough for combat operations and so are generally relegated to ‘logistic support’ type installation, often replacing bridges deployed in combat operations. This is intentional, each type of bridge is used in the most appropriate situation. The rapidly installed bridges tend to fall into two, those that are installed under direct fire, usually by tanks, and those that may be installed under indirect fire.
The familiar tank launched Combat Support Bridge and the less commonly known support bridges, characterised by the WFEL Dry Support Bridge or BAE General Support Bridge.
Although there are differences between the Dry and General Support bridges in service with the US and the UK respectively, the operational concept is the same. A launching beam is cantilevered over the gap and this is used to support bridge segments as they are lifted into position, connected and launched forward. Once the bridge landed, the launch beam is withdrawn.
Not only are they very quick to install, but they can also support great loads over long spans. The WFEL Dry Support Bridge can, for example, support an MLC 80(T) or MLC 96 (W) load over a span of 46m and be built from scratch to trafficking vehicles in less than 90 minutes. Individual 6m 4.4-tonne folding modules can be stored and transported stacked, two per 20ft ISO container. A 40m span bridge set comprises a launch vehicle and 5 ISO container-sized loads, transported on vehicle and trailer combinations.
One of the other benefits they have over panel bridges is during construction, they require little space on the home bank, the home bank in this application being a pier support. A complete 1,000m Pier set would still only require 1 launch vehicle but would need 166 bridging modules weighing 730 tonnes and requiring nearly 100 ISO containers. Installation could be in the order of 48 hours in good conditions.
If we accept a lower load-bearing capacity, the Medium Girder Bridge or Air Portable Ferry Bridge could be used. This well-established bridge design is flexible and very lightweight, a single story 16m span, for example, may only be able to carry MLC 30 vehicles but weighs less than 8 tonnes.
A bridge deck of 8 tonnes allows it to be emplaced using a Chinook helicopter. ‘Heli-Bridging’ has been used operationally in a number of theatres and remains a viable option for lightweight bridging. 20m spans installed by Chinook would produce a very low overall build time and should be seen as a realistic option for Requirement 3 should a load-bearing compromise be viable.
A number of manufacturers produce portable bridges in varying spans, most of them of simple beam type construction. The 1.75m wide Mabey Quickbridge is available in multiple fixed lengths, the largest, a 20m span, weighs 14 tonnes. Multiple bridge units could be crane installed quickly. Although economic factors are important, given the relatively small number of spans required, it may be possible to investigate the use of high strength low weight alloys.
Carbon fibre structural beams and shapes are being increasingly used in historic building restoration and repair. A number of products are now commercially available that could be used for either steel reinforcement or steel replacement in bridge sections. Carbon fibre beams have been shown to have the same strength as steel but with a 60% weight reduction. Foam-filled carbon fibre box section increases torsional rigidity and perforated arch beams have been shown to have the incredible load-bearing capacity for minimal weight, a 4kg 8ft arched beam can hold over 1 tonne for example.
In bridging applications, carbon fibre reinforced plastics are starting to gain traction. They are widely used for the repair and reinforcement of existing cast iron bridges with Plymouth University pioneering much of the research. The first CFRP bridge was built in the USA, the Bridge Street Bridge (no, that is its real name!) in Michigan. In Europe, they are increasingly used for lightweight footbridges, one of the longest being the 56m Ooypoort bridge in Nijmegen. In the UK, composite reinforced bridges have generally been used for short spans given the civil engineering world is by necessity, conservative, this is understandable. The UK Composites industry association has some very good case studies of composites in construction, click here to read.
The West Mill Bridge in Oxford is almost entirely constructed from Glass Fibre Reinforced Polyester (GFRP) and in 2011, a bridge across the River Tweed in Wales was installed that used steel beams reinforced with a composite of silica and recycled plastic water bottles. The Friedberg bridge in Germany is a good example of a steel and composite mix bridge. Fiberline, the Danish composite bridge specialist, has a number of very interesting case studies on their website for pedestrian and road bridges, including the 10m West Mill Bridge and 52m bridge across the M6 in Lancashire.
Composites, in all their variations, have great potential for this requirement for structural or decking components, they are light, strong and corrosion-resistant. Using lightweight bridge beams and decking reduces the need for foundation strength, in this context, that is a very good thing.
For those pier decks that are supported by a floating pontoon, likely closer to the pierhead, they will need to withstand twisting forces induced by the rise and fall of the tide and effects of wind and current. The means by which the pier deck is connected to the support would also need careful consideration.
One of the key objectives is that it must be able to access a variety of shore terrain, not just gently sloping beaches. This means seawalls, rocks, flood defences and other manmade structures, for example, both in and around port environments, or completely non-developed areas.
A jack-up platform would appear to offer the simplest and most effective solution for landing the final pier deck before transitioning to shore. From the platform, another ramp structure would be used to land onto the shore. A Pier segment with a self-levelling bankseat beam would then rest on the ground and provide a road surface to the shore.
A conventional jack-up platform using modular pontoons can get very close to and over, all manner of shore terrain. The images below show examples of jack-up platforms in close proximity to shore, different terrain types and water depths.
Where the shore landing spot is a beach, the geotextile solutions described in the Pier section would provide a simple, quick and cheap solution for landing the final Pier segment.
Construction and Sequencing
Construction methods and sequencing depend wholly on the selection of pierhead, pier, pier supports and shore interfaces described above. Detailed analysis of different options would demonstrate what the optimal solution for a range of pier lengths, speed requirements, environmental conditions and shore types is, but part of this analysis would examine construction methods.
There will be trade-offs to be made, but one of the most important questions will be whether the systems works as does ELCAS-M (equipment landed on the shore and built out towards the sea) or Mulberry (built entirely from the sea towards the shore)
It would be desirable to complete this requirement as the latter, from ship to shore, simply because of time and the need to double handle every single component. We may also examine whether construction of the pier and specifically, the supports, could be completed in parallel with each other.
One option would be to land the pier head, and then in sequence, install a pier deck support, built out the pier deck to it, and start again. This would edge the pier deck toward the shore, one pier deck section at a time, using the cantilevered technique.
With sufficient construction equipment, it may be possible to install multiple pier deck supports at a time, with the pier deck, as above, being built from one to the other in sequence. By using floating or jack-up construction platforms, multiple pier decks may be installed at once, simply lifting complete pier deck sections onto their supports.
If the pier deck support is something akin to the ACTF, it would be telescoped out in both directions at the same time, one half providing a counterbalance for the other. There are even realistic options for installing pier deck supports using a helicopter like the Chinook, even if this is only to be used for a launching beam.
Walking Leg Platform
If build out to the shore using multiple jack-up platforms is selected, slowness and susceptibility to wave conditions is likely penalty. What is needed is either a floating or walking platform that can expand the frontal edge of the pier by installing the pier deck supports in front of the pier.
The Wavewalker is a walking leg jack-up platform used for construction and geotechnical investigation in rough seas, surf zones and beaches.
To quote from the description;
WaveWalker 1 is an innovative jackup which can be operated in conventional 4-legged mode or as an 8-legged, self-contained walking jackup platform, capable of safely operating and bi directional movement whilst elevated allowing the jackup to move and relocate without floating.
The key is to ‘relocate without floating’
This allows it to operate in the surf zone, basically striding forward over the waves.
Wavewalker 1 has eight independently jacking legs. Each of these is carried in a leg bearing unit, which slides on bull rails built into the hull structure. Once on location, 4 legs on two opposite hull sides are lowered to seabed and the rig is jacked up in the conventional manner. The other 4 legs in raised position are then slid to the end of the walking stroke by the walking cylinders. These legs are then lowered to the seabed and weight transferred to them. The initial 4 jacklegs are then retracted clear of the seabed and the walking cylinders driven to the extent of their stroke, causing the hull to slide 4 m in the required direction. The raised legs are then lowered to the seabed and weight transferred to them. Finally, the unloaded legs are jacked up clear of the seabed and reset to the start position. In this way a 4 m “walk” is completed and the cycle can be repeated if required – overall walking speeds of up to 40 m per hour are achievable
As can be seen from the image below, it is quite large, at 32m by 32m, and has a coverable moon pool, a number of cranes and accommodation for multiple crew members.
The images below show how it can simply jack itself out of the waves, very high waves.
This ability to walk forward at 40m per hour opens up an interesting potential construction sequence. As it walks forward it maintains a steady height, this will allow it to drag an already built pier deck forward from the Pier Head.
Using the moon pool and cranes it could install any of the pier deck support systems described above and it has enough space and deck loading capacity (400 tonnes) to carry enough for the complete Pier.
The build sequence could be;
One; upon arrival at the build site, offloaded either from a semi-submersible vessel or towed, it secures the first Pier Deck section to the Pier Head.
Two; as it walks from the Pierhead to the first pier support location it drags behind it the pier beams/deck components or launching beams. The deck would be attached using a hydraulic mechanism and deployed from the Pierhead platform, positioned using the mobile cranes.
Three; when it arrives at the first Pier deck support position it stops, having deployed the pier deck. It then deploys the next pier deck support through the moon pool. Once complete the platform jacks up clear of the support, walks forward and once in position, sets down the deck onto the support. Whilst this is being carried out, the second Pier deck module is positioned at the pier head ready for movement forward.
Four; with the first Pier segment in place and secured to the support, the second Pier segment is moved forward, along the length of the first and secured by the Wavewalker platform using the hydraulic mechanism at one end, the other supported by rollers.
Five; the walking platform walks forward to the second Pier support location, again, dragging the Pier deck behind it. In the second pier support position, the sequences are repeated. Install the support through the moon pool, elevate, walk over a short distance, and secure the front edge of the Pier deck on the newly installed Pier support. Small deck segments would need to cover gaps between the multiple Pier deck segments but these would be quickly installed using mobile lifting equipment, likely sizes would be able to be lifted by a JCB type loader.
Six; the sequence is repeated for all Pier segments. Supplies of Pier decks are progressively depleted from stocks on the Pierhead and stocks of Pier supports from the walking platform.
This would be a sequential operation, with no ability for parallel or simultaneous installation but dwell time at each point would be relatively low and it is a simple process without the requirement for heavy lift cranes or large amounts of space for cantilever launch. As the walking platform would be raising and lowering over the Pier supports the Pier deck coupling mechanism would need a bearing or hinge mechanism to accommodate the movement.
Eliminating time-consuming crane lifts, launching beam or cantilever launch techniques by the simple expedient of walking Pier deck segments forward and placing them onto a series of supports would result in a much-reduced installation time.
By adjusting the design of the Wavewalker platform it may be possible to increase walking speed by sacrificing carrying loads and leg height. Given the likely operating water depths and loads, this is hopefully a feasible design change. If we could increase to 50m per hour, a 1,000m Pier could be ‘walked’ in 20 hours. With a 50m span and 1 hour spent at each location deploying the support and placing the deck, the target Pier length of 2,000m becomes achievable within two or three days.
This method also minimises personnel required. There are many assumptions here, this is acknowledged, and it would need to be confirmed with modelling. Is one hour enough for a pier support installation, would the cycle time of moving the Pier deck from Pierhead to installation location introduce delays, can the deck accommodate height changes and twisting forces whilst being installed, and much more.
A handy by-product of this method is that the walking leg platform becomes redundant at the end of the build sequence
Submersible Pier Deck Spans
This one might sound a bit far-fetched, but hear me out.
This idea is based on the understanding that none of the Pier components is naturally buoyant, and these three images…
The images above show early assault bridges (Inglis MkII, Mobile Bailey Bridge and M4 Plymouth Bailey Bridge). A 40m-50m ladder beam span could be fitted with a tracked drive unit at each corner and lowered to the seabed using a passive heave compensating crane attachment. At each corner would also be a spud leg, on a rotating fixture to allow them to be easily stacked whilst in transport.
When landed on the seabed, the hydraulic motors would be attached to a powerpack housed on a workboat and simply driven toward the shore. Once in position, the same workboat mounted hydraulic powerpack could power the jacking units to raise it clear of the surface. Finally, it would be locked in position and decked, perhaps even using a rolling trackway solution like that from Faun.
Underwater excavators are available from a number of manufacturers, existing commercial and mature technology. One of the benefits of this approach is the reduction of wind dependent crane moves for each span. For Pier spans supported by floating pontoons, the pontoon can be flooded and when in position, pumped full of air to raise the deck.
Surf Zone Construction Work
For general construction and excavating in the surf zone, moving Geotextiles, for example, a range of specialist plant is available.
Pre Build Survey
There is a common misconception that large offshore jack-up platforms simply turn up for work and jack down, this is not the case. Leg push through is a dangerous condition where one of the legs pushes through an unexpectedly soft layer of the seabed and in some cases the rig can be put at risk of collapse. Because jack-up platforms are likely to feature in solutions, this is a concern.
This potential need for seabed soil survey also poses a particularly difficult challenge for two reasons. It is likely that some potential operational locations for this do not fall within the ‘friendly nation’ category and are so unlikely to allow an overt survey using jack-up platforms or survey vessels of their nearshore areas. This would also provide an alert that a pre-surveyed is an area of interest and counter-productive to mission success.
And so the first difficult challenge for this requirement is that the seabed survey may need to be conducted covertly, underwater. Although given time, a number of pre-surveyed locations could be a sizeable database the time available for conducting the pre-build survey may be compressed by contingent operations. The second challenge is that the survey must be conducted and data analysed quickly. In speeds, favour is that the actual physical area to be surveyed is likely to be quite small.
Put the two together and the problems are amplified.
Conducting a rapid and covert subsea soil survey is not a trivial task.
The survey task would actually start in the UK by analysing existing data and data that may be obtained from other sources. This initial desktop survey would be used to support additional capture and analysis planning. Follow up hydrographic surveys, metals/debris detection and sub-bottom profiling could be carried out using vessels of opportunity or under the cover of routine hydrographic surveys carried out by NATO survey vessels. Standoff AUV’s and even handheld devices could also be used.
But this only gets so far, there is no substitute for physical sampling.
Existing special forces have quite capable covert beach survey capabilities (including manual core sampling) and when combined with stand-off systems deployed from conventional survey vessels the majority of the survey task can be covered. However, the aspect of the survey requirement that is not covered by existing capabilities is sea bed load-bearing capacity.
This requires penetrometers and possibly, core sampling.
At its simplest form, cone penetrometer testing involves inserting a rod into the seabed and measuring tip resistance and sleeve friction. The data collected is then analysed using specialist software. The traditional method of conducting a near-shore soil load-bearing survey is to use a jack-up platform and a cone penetrometer test using an integrated drill derrick or a standalone system deployed over the side or through a moon pool. The penetrometer is inserted through a hollow casing that provides lateral stability and buckling resistance. In very soft soils an additional device called a piezecone penetrometer may also be used.
Core samples may be taken to verify predicted data taken from penetrometers.
These are established techniques, but hardly covert.
For deeper water surveys, typically used for offshore energy installations and requiring significant load-bearing, underwater penetrometers are available from a number of manufacturers including AP Van Den Berg in the Netherlands and Datem in the UK. Both have developed compact coiled rod units that can be lowered to the seabed and remotely operated from a ship.
Although mostly used for deeper water they have also seen some use when mounted on a tracked underwater ROV for intertidal applications.
If they can be mounted on a tracked ROV or lowered from a surface vessel, they can be deployed from submarines or simply ‘driven in’ from a safe offshore distance.
A number of parameters will need to be analysed; Soil density, soil strength, friction angle, consolidation and permeability for example. This should also include analysis of the strata to the desired penetration. Finally, a scour potential survey should be carried out, analysing wave and current in addition to soil conditions.
Whilst sampling would in most cases be conducted covertly, analysis can be carried out away from the target site aboard one of the survey vessels supporting the operation, or in the UK via reach back. The output from a survey would be a decision on site suitability.
With the system in place, the operation becomes the next issue to consider. For these operations, there are two main (and interconnected) issues, mooring/berthing, and wave attenuation.
Some ships are fitted with thrusters, azipods or other means of fine adjustment and station keeping, but most do not. This means they will need some assistance to moor at the Pier Head.
Cargo vessels need to be manoeuvred into place and secured to the pierhead so they can unload or load. Most likely vessels do not have bow thrusters and dynamic positioning equipment so tugs or some other method is needed. The ACTF proposed a series of winch barges and propellant embedded anchors to position the cargo ship next to the crane ship.
This is time-consuming and so an alternative would be to simply take three tugs as deck loads, lifting, pushing or floating them into the water as required.
In order to allow safe and efficient offloading at the Pier Head, the relative motion between the free-floating ship and the fixed Pier Head must be minimised.
Conventional breakwaters are usually massive reinforced earthworks, sometimes with protective sheet piling. Protection against erosion caused by waves is provided by combinations of geotextiles, aggregates and in many cases, large boulders or concrete blocks lifted into position, such as those made by Xbloc Large concrete caissons are also sometimes used.
Concrete caissons are also used.
None of these solutions is practical for this requirement.
The first recorded use of a floating breakwater was at Plymouth in 1811 and since then the floating breakwater has evolved in a number of specialist areas such as marina and aquaculture. Going back to D Day, the problem of wave attenuation was a big one, the location and time the invasion was planned for meant waves would be significant and much thought was given to the issue. After many different ideas were tried the actual means of wave attenuation coalesced on three methods.
The image above clearly shows the effectiveness of the wave attenuation capabilities of the combinations of these methods.
First were a row of 60 blockships, old and obsolete ships that were stripped, towed into position and sunk. Preparations including destroying their watertight bulkheads to ensure they sank quickly when Amatol charges punched holes into their hulls. Once in place, they were overlapped and sunk. The overlapping was needed to prevent scouring at the bow and stern. The lack of such overlapping on UTAH meant the US blockships suffered from this effect and several of the ships had their backs broken by voids opening up underneath them. Their main advantage was they could get to the beaches under their own steam, not requiring precious tugs but due to their comparatively low height there were unable to be used across the entire area and thus, deeper concrete caissons were needed.
Second, large concrete caissons called PHOENIX, 150 of these were built in six depth variations to accommodate different water depths, the largest (Type A1) displacing 6,044 tons and the smallest (Type D), 1,672 tons. Each was a standard 60m in length with a boat hull to facilitate towing operations. Some of the PHOENIX caissons had anti-aircraft guns and barrage balloons, with the necessary crew quarters built into the structure. The caissons were prefabricated in the UK and when towed into position their scuttling valves were opened, flooded and sunk.
Finally, an anchored wave attenuation device called BOMBARDON was used. These were 200-foot long cruciform cross-section floating steel constructions that were anchored to the sea bed and each other in long ‘strings’. 24 Bombardons, each with a 50-foot gap between them, created a breakwater 1 mile long. Water was then let into the three lower fins as ballast. Each was designed for a Force 6 storm, unfortunately.
Of these three methods, only one could be considered as a floating breakwater, the BOMBARDON, and the only one that could be considered for this requirement
Floating breakwaters are often selected for deeper water, where soil conditions preclude the construction of soil and rock structures and where water quality needs to be maintained, aquaculture being particularly sensitive to this. Short choppy waves can be effectively attenuated with floating breakwaters but longer period waves encountered offshore can be difficult to deal with because the transmitted wave (that behind the breakwater) depends on the ratio between the incoming wave and width of the breakwater.
In the years since the BOMBARDON, there has been a great deal of research into floating breakwaters and many designs and configurations tested. The increasing size of container vessels and the need for smaller feeder ports has also resulted in a modest increase in research in floating breakwaters.
Floating tire or log mats can be used although they are not easy to deploy and less effective than other types, they are cheap, though.
A more common type are the floating box, concrete or steel in construction, often foam-filled, linked together with flexible pins and moored to the seabed.
The first image below shows a floating pontoon breakwater installed in Holy Loch, Scotland. The 240m long structure comprises twelve 20m long pontoons, each weighing 42 tonnes and the second image below, a similar concrete box construction floating breakwater in Italy from Ingemar
The unit is designed for a maximum wave height of 1.5m before being overtopped, with a maximum wavelength of 18m and a period of 5 seconds. This would correspond to a sea state of between 3 and 4. To improve efficiency they can be post-tensioned in order to increase stiffness although this creates significant forces.
Floating or bottom-tethered pontoons or tubes have also seen widespread use with individual designs being influenced by specific conditions encountered at the intended location. A significant advantage of inflatable breakwaters is their ability to absorb wave energy by structural deformation, unlike fixed pontoon or box structures. This can also result in lower stress on the mooring system. They can also be inflated and deflated in situ, an obvious space advantage for a deployable solution but can obviously be deflated by puncturing. Newer designs, generally aimed at the marina and leisure market used moulded plastics to form long flexible strings of breakwaters, examples include Waveeater
and Whisprwave Fixed tubular semi-submersible types are also available.
The Canadian company Narvaltech produce a floating breakwater that utilises 5 tubes arranged in a cruciform shape. Each module is 7.3m long and weighs 7.1 tonnes. They can be arranged in single or double parallel patterns to maximise wave attenuation.
Other proposals include the use of artificial kelp barriers used in conjunction with inflatable beam breakwaters.
The most likely failure point in any floating breakwater is the mooring because it has to maintain the floating structure across a vertical range, alternating between slack and taut. Many studies have shown that the influence of the mooring system on the overall effectiveness of the floating breakwater is significant. Mooring systems that can accommodate depth variations are most effective, those from Seaflex being good examples
In the mid-nineties, the USACE Military Engineering RDT&E Program of the Coastal and Hydraulics Laboratory (CHL) carried out what was probably the most comprehensive study into deployable breakwaters since the Bombardon.
As described above, floating breakwaters are most effective when their width is a quarter wavelength, in open water where long-wavelength waves are more common than ‘choppy’ short-wavelength waves closer to shore the problem becomes extremely difficult due to the simple dimensions required.
RIBS was required to reduce wave height by 50% in Sea State 3 to 2, be survivable at Sea State 5 and be able to be deployed and redeployed quickly using in-service shipping. It was not specifically aimed at creating a stable platform for ship offloading to a fixed pier but for offloading large ships to lighters. The same principle still applies to this though. After testing many designs the CHL came up with the ‘Double Delta’ shape. The cells were filled with expanding foam and a rigid curtain extended between them at a sufficient depth to stop wave energy from penetrating beneath the breakwater. It would be deployed in a V-shape with the tip of the V facing into the waves, the floating structure deflecting waves rather than reflecting them as traditional breakwaters might, this was a big departure from those described above.
At the point of the V was a nose buoy that was moored to the surface and allowed the angle to be varied between 0 and 60 degrees. Scale testing indicated up to 85% wave attenuation.
The first tests used a rigid structure but these were rapidly changed to inflatable beam structures, called the Hydro RIB.
The final test was at full scale using a design called XM-2001 that un-spooled from a large bobbin before pressurisation. The larger-scale testing demonstrated a reduction from upper sea state 3 to 1 in the lee of the breakwater.
Pretty damned impressive and more so because the deflection mechanism allowed moorings to be relatively low strength and simple.
RIBS was as cheap as chips and self-evidently, extremely effective. The deployment was simple and the whole assembly was very compact pre-installation. Although RIBS has not yet been deployed it is being developed further for the roll on roll off platforms used for ship to ship transfer. The RIBS system, perhaps combined with some of the commercially available systems would be a relatively simple drop-in for this requirement and provide localised sheltered water at the Pierhead
Pollution control barrier technology might also be utilised.
Summary and Trade-Offs
If you have read all this, I salute you!
Let me start the summary by saying I am not an offshore or geotechnical expert, just someone interested in the subject. This should be viewed in that context, an interested party with no great expertise. I make a number of assumptions that would need to be tested, even assuming the basic premise of the proposal is correct, which is equally open for debate.
We should absolutely recognise that the expertise to realise a modern-day Mulberry Harbour does exist, and exist in the UK and Europe. Whether that is in military bridging, port construction, offshore engineering or geotechnical surveys, the UK has a rich vein of research, technical and engineering capability as a result of decades of experience in the oil and gas and renewables industries which to mine for solutions.
It is in these organisations that we must look for solutions, not BAE or Lockheed Martin.
We benefit from an embarrassment of riches in offshore and port engineering.
This concept is also based on a couple of opinions; first, the changing nature of shorelines and ports, and the increasing proliferation of anti-access denial weapon systems means the barriers to entry for future amphibious operations are raised. If we are honest with ourselves, the UK cannot afford to play with the USMC, and this is why the Future Commando Force is moving away from establishing and maintaining a force ashore to raiding. If expeditionary forces are becoming increasingly wheeled, ports become more important, and if we can bring our own, all the better.
For a demonstration of where trade off’s might be found, the image below (of Flying Fish Cove, Christmas Island) shows a typical example.
Don’t worry about the specific location issues, consider it a representative example for a case study.
As can be seen, there is a small beach and loading jetty, and a larger phosphate loading facility with high sea walls.
How could we bring RORO ships in to rapidly offload vehicles onto the access road or bulk cargo facility? It is here that this proposal would fit, being able to create a pop-up landing jetty and strike a causeway pier directly onto the road, bypassing both existing facilities.
And it is here where a trad-off could be found.
All the items described above are large, heavy, and expensive, and they are thus because of the stated requirement to offload cargo ships quickly. By lowering this requirement ceiling to just include a RORO offload facility, things become smaller, simpler, cheaper.
Without having to handle large berthing loads or huge cranes, the pierhead would become a simple and relatively lightweight structure like any of the jack-up pontoons described above. This could be self-deploying, towed or carried as deck cargo, either whole or in modular form.
The causeway pier is still a challenge but in essence, a bridging issue. The number of pier supports versus the span, and the design of the pier support remain a challenge but with a combination of geotextile, walking leg jack-up barges, screw piling, rock bags and surf zone excavators, all challenges that can be overcome. Causeway pier spans could be any combination of lightweight bridging equipment.
And finally, for the shore access point, again, walking leg or conventional jack-up barges look eminently suited.
Anyway, a long article, apologies for that, see you in the comments
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