Whilst every single one of the designs described in the previous section have something to offer, the start of the art of offshore engineering has advanced considerably by virtue of the renewable energy sector, it is obvious, but these need to be examined and exploited.
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 than 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 pier head 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 Requirement 3 to operate in either orientation.
Vehicle Turning Geometry
Requirement 3 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 radius. 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 in 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 to, the vehicle will have to perform a double eight which results in a minimum length of 35m, 50-60m preferable.
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.
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 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 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 a 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 on to deliberation 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 movement under wind, wave and tide load.
Partially submersing the pier whilst still being stabilised by pier legs (spuds), distributes loads, this technique was used by the Mulberry harbour system.
Where Requirement 3 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. 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 Requirement 3.
The significant expansion in nearshore (and offshore) wind turbine construction has moved the state of the art on 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 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 jackup 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 top soil 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 it 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 have 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 jackup 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 Requirement 2, and it is not inconceivable that the same mobile harbour crane could be used for both. Whilst Requirement 2 cranes had to be portable, the key difference with Increment 2 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 Requirement 3, however, is one of size. 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.
For Requirement 2, portability and deployability were weighted higher in preference than pure performance but for Requirement 3, reach and lift speeds can be optimised. The larger 550 and 600 series can easily lift fully loaded containers at maximum outreach distances of 50m. Multiple containers can be lifted at the same time using twin lift or telescopic spreaders. Available from Bromma or Stinnis for example.
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 is 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 in Requirement 2 may also form part of the overall solution.
Whilst the primary objective for Requirement 3 is solid cargo, vehicles and container offload, fuel is a vital commodity. Because of the nature of 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 vessel and pierhead. 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 consideration 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 the 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
Dockwise, Rolldock, OHT and Hansa 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 maximum allowable seems to be 1.5m to 2m. This option would require Requirement 3 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 Requirement 3 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, width 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 Requirement 3 is the 900 m2 deck area is too small, the 8-knot transit speed too low and 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 Requirement 3 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 Requirement 3 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 overhang caused by applique armour. Container handlers such the Rough Terrain Container Handler would be particularly problematical 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 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 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 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. As far back as 1887, the ‘Instructions in Military Engineering’ manual listed likely weights of things as diverse as heavily laden elephants and cavalry in marching order. These allowed assumptions to be made about the safe operation of a bridge and a follow on simplification started from this point was the concept of a bridge classification. During WWI, this was refined and loads were divided into 4 classes, light, medium, heavy and tank, with each one having a defining weight, spacing and where relevant, axle loads. Despite this advance, an improvement was sought and in 1928 the Royal Engineer Board started collating details on military loads and the increasing number and type of vehicles in service. The details were used to further categorise loads into Light (Brigade), Medium (Division), Heavy (Corps) and Super Heavy but the fundamental problem remained, matching bridge capacity to vehicle weight as they had to be looked up in tables before allowing to cross. This was cumbersome and time-consuming and relied on every type of vehicle appearing in the tables so in 1938 the system was looked at again by the Royal Engineers Board.
The rather elegant and simple solution they came up with was to invent a scale, or classification, related to weight but crucially, not only weight.
Each bridge type was allocated load class number and each vehicle was also given a load class number. Instead of looking up and cross referencing a vehicle against a bridge classification a simple comparison of load class was performed, if the numbers matched or the vehicle was less than the bridge classification then it could pass. A spacing of 80ft was assumed at the bridge classification took into account bending moment and other factors, it was not simply a weight (this is a key distinction) Instead of weights of vehicles, each vehicle had a class, these starting at 3 and moving up to 24 in regular intervals.
If a vehicle’s load class was smaller than the bridges load class then it could cross and to assist with the rapid cross checking a standardised series of markings was designed, both bridge and vehicle had the marking in the same colours so a driver could simply compare the bridge sign with that painted on his vehicle and make the decision whether to cross without reference to bridge commanders or complex tables.
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 concentration 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 NATO MLC system also defines normal crossing conditions but allows for a reduction of safety factors in an emergency or tactical conditions. It also describes a method for defining a temporary vehicle classification if none exists. The 16 classes are defined by 16 hypothetical wheeled and 16 hypothetical tracked vehicles of various weights, sizes and axles. It must be understood that MLC is not a vehicle’s weight but simply a reference number. Each hypothetical vehicle has the bending moment and shear forces calculated and plotted on a graph at 1m intervals up to 100m, assuming a single span simply supported bridge. For other conditions, the curves are re-plotted. Vehicle and trailer combinations form part of the 32 definitions for tracked and wheeled vehicles. The standard defines treatments for exceptions such as towing and vehicles outside of the basic 16×2 classes.
At the end of these complex calculations, the MLC number is derived and rounded up to provide a safety factor. A temporary MLC can be calculated by multiplying the vehicles mass in metric tonnes by 1.20 for a tracked vehicle and 1.25 for a wheeled vehicle. Different methods are used for single and multiple span military bridges, civilian bridges, rafts, ferries and floating bridges that take into account a diverse range of factors. Normal, caution and risk classes are defined for each bridge that recognises safety factors may be eroded in wartime but these may also need more restrictions placed on crossing speeds, for example. STANAG 2021 provides a couple of interesting examples; the Leyland DAF 4 tonner has an MLC of 11, the Leopard 2A5 an MLC of 66 and the M3 Rig vehicle an MLC of 26. From the early work and subsequent refinement in the years prior to WWII by the Royal Engineers Board a simple and robust classification system for bridges, ferries and raft, is now a NATO standard.
One of the decisions that therefore needs to be made is the required MLC for the pier.
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 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 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 Requirement 3, 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. An Increment 2 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.
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 a 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 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 the 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. Two of the most promising for Requirement 3 are the Elcorock Container and Tencate Geotube
A Geotube is permeable fabric tube of varying length. Secured in place with steel tie rods or frames it is pumped full of a 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 a 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 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 simultaneous installation of multiple grounding grillage frames is needed, multiple workboats would be required.
This arrangement has 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 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. This panels of a standard size that incorporate warren trusses and can be connected together to form longer and stronger spans. Various types of 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 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, they can 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.
A complete 1,000m Requirement 3 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. These well-established bridge designs 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.
Another option is the Unibridge iBridge, this is a modular bridge that uses sealed 11m box beams that are interlinked with a simple pin, and launched using the cantilever or lift in method. Deck panels are added once the beams are in place. The larger Unibridge can span 45m.
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 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 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.
Pipex installed the 18m Dawlish Footbridge in a single lift. 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. Triangulated composite decking is also increasingly being used, much of the research being carried out by Bristol University.
Composites, in all their variations, have great potential for the Requirement 3 Pier 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 for Requirement 3 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 transition to shore.
From the platform, another ramp structure would be used to land onto shore. A Pier segment with self-levelling bankseat beam would then rest on the ground and provide a road surface to the shore.
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 depends 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 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 Requirement 3 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 we do chose to build out to the shore using multiple jack-up platforms we have to accept two large compromises, slowness and susceptibility to wave conditions. 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 shows 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. The deck would be attached using a hydraulic mechanism and ‘payed out’ from the Pierhead platform, positioned using the mobile cranes.
Three; when it arrives at the first Pier deck support position it stops, having ‘payed out’ the pier deck. It then deploys the 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, no ability to 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 methods is that the walking leg platform becomes redundant at the end of the build sequence.
A redundant walking leg platform would be an ideal shore connector, neat eh!
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 are 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.
TABLE OF CONTENTS