Increment 2 – Pierhead
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 and Shape
Deck size and shape will depend upon a number of factors.
Orientation Relative to the Shore
Looking at the case studies and potential solutions there is no single 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 Increment 2 to operate in either orientation.
Vehicle Turning Geometry
Although 40ft containers will be the exception, Increment 2 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 pier.
Throughput will greatly depend on the ability of vehicles to enter and leave the pierhead quickly, they cannot become a bottleneck because the speed of offloading is such a key requirement for Increment 2.
Industrial buildings, roads and loading areas have numerous standards and norms for sizing turning circles. Vehicles using European roads must be able to turn within a circle of the 5.3m inner radius and 12.5m outer radius. A common design for industrial buildings and loading areas is called a ‘banjo’, with ‘stub’ and ‘hammerhead’ designs being 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 it will have to perform a double eight and this 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 two consecutive ninety degree turn manoeuvre instead. If the pierhead in 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.
Creating a L shape for Increment 2 would have a number of benefits.
Inverting the L may allow 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.
Size and Shape Requirement Summary
The key to Increment 2 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 and foundations 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 general resilience 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.
A large semi-submersible vessel anchored to the seabed can be quite stable but that same wave and tidal loads act upon a much larger area and so anchor systems would be needed.
In a normal offshore construction, these are known challenges and many solutions exist to meet them.
Where Increment 2 complicates things considerably is the requirement for high-speed installation (sub 48 hours) 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 Increment 2.
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 Increment 2, 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.
The diagram below shows spud can pockets on the underside of the construction vessel.
So, although legs may penetrate the seabed, they are not driven.
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. Ther 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.
Finally, the pierhead could have a combination of legs and driven/suction piles.
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.
Cranes and Other Deck Equipment
Considerations on cranes are very similar to those for Increment 1, and it is not inconceivable that the same mobile harbour crane could be used for both.
Whilst Increment 1 cranes had to be portable, the key difference with Increment 2 is that they can be fixed to the pierhead. This simple factor 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 Increment 2 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.
All of this brings us back to the same conclusions as Increment 1, a mobile harbour crane.
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 Increment 1, portability and deployability were weighted higher in preference than pure performance but for increment 2, 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 cargos 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.
An alternative would be to use a buffer pontoon, similar to Increment 1.
Whilst the primary objective for Increment 2 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.
Accommodation for crew and operators, smaller hydraulic jibs, remote operated vehicle (ROV) operating equipment and space for the access pier to deploy from the pierhead are also required.
Berthing and Mooring
Seabasing developments have concentrated on fender systems and distance measuring equipment, all to compensate for the simple fact that both platforms are in motion. By providing a stable platform for the crane, Increment 2 can dispense with these very complex and expensive systems, none of which are in commercial use or fully developed.
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 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.
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.
As described above, the Increment 2 pierhead is at its simplest, a two large flat platforms that can be arranged in an L shape and secured to the seabed. With the addition of cranes and tugs the system will be able to manoeuvre multiple ships into place, secure them, and allow them to be rapidly unloaded or loaded.
The final issue to resolve is transport, although this will also have a great deal of impact on final configuration and modes of operation.
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.
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.
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
They are also increasingly used for yacht transport.
Combilift, 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 Increment 2 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 Increment 2 increases the cost significantly, although 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 Increment 2 is the 900m2 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 Increment 2 requirements but it does show what is possible.
One of this type of vessel would most likely meet all the requirements, self-deployable at speed, stable, operable in Sea State 3 or above and able to accommodate a large mobile harbour crane and pile fixing cranes/hammers if needed.
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.
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