The Increment 2 Pier connects the Pierhead to the Shore Connector/Interface and is used by vehicles to move between ship and shore.
There are three general requirements for the pier;
And a number of secondary attributes to consider
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 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 gradient.
The pre-installation will determine the actual pier length but Increment 2 should be able to accommodate a range of potential pier lengths. The figure n, required water depth is a function of under keel clearance and ship draught. The target is 12m-14m although this may be extended or reduced depending on mission requirements.
The ELCAS pier can extend to approximately 1,000m but Increment 2 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.
As a minimum, the Increment 2 pier must be capable of spanning two thousand metres, a very tall order. This coupled with the requirement to be rapidly built and globally transportable makes the pier a significant challenge.
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. Increment 2 is predicated on the speed of offloading and resultant reduction of time spent by ships at the pierhead so any issue that slows this down needs to be addressed.
A solution that starts with a single lane pier that can be expanded to double lane would 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.
When looking at the Increment 2 pier it is important to appreciate that the 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 and 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.
Design and Construction
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 increment 2, 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 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?
Different combinations may suit shallow water in the breaking waves or deeper water at the pierhead. Increment 2 should, therefore, be able to employ a variety of techniques, fixed and floating, the most appropriate used depending upon the local conditions.
Pier Deck Supports
Assuming that the approach of using a pier deck supported at each end is preferable that a floating pier deck some of the issues to be resolved with foundation design are similar to those at the pierhead.
Loads transmitted to the seabed will, of course, be greatly reduced, but the fundamental issues ae the same.
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.
Alternatives might include;
Many of the floating steel modular pontoons described in Increment 1 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 jack up up, either completely clear of the water surface or partially clear.
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.
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 Increment 2 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. Additives 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.
Although full dewatering takes months the stability of the tube and load bearing capacity can be suitable in a period of several hours. 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.
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 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.
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. 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 arrangement would produce an extremely short installation time.
There are a number of trade-off points with the pier deck, I have covered load bearing and span above but the in general, the longer the clear span, the more difficult to install and heavier it will be. Heavier spans 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
Pier Deck Design
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 Increment 2 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.
Accept a lower load bearing capacity and 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 Increment 2 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 completed in the USA, the Bridge Street Bridge (no, that is its real name!) in Michigan. In Europe, they are increasingly used for lightweight footbridges. 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 god 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 Increment 2 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.
There is no doubt that the pier deck remains the most difficult challenge to address for Increment 2, not because there are no options, but simply because whichever option is chosen will take a long time to install purely as a product of the length of the pier.
The requirement is 2 days, at the upper end of the potential pier length, this may not be possible.
Pier Deck Installation
One of the factors that make conventional military bridge slow (compared to the distances in Increment 2) is the inability to install bridge segments in parallel, installation starts at one end and moves to the other. There are obvious practical reasons for this, ELCAS is the same, it is built out to the sea from the beach, one module at a time.
For the Pier, there may be opportunities for construction in parallel to reduce overall construction times, or even, build from the pier and build from the shore, meeting in the middle.
Otherwise, combinations of cantilever and crane lift in could be used.
The most likely practical general arrangement will be to carry the Pier components on the Pierhead ships/barges and once in place, build out towards the shore, using the Pierhead as a construction and stores base.
This means the pier supports have to be installed before spanning with the pier deck. Given they will be installed in ‘free space’ this will be difficult from a gradually moving forward pier. What is needed is either a floating or walking platform that can expand the frontal edge of the pier by instaling the pier deck supports in front of the pier.
The Wavewalker system is integral to the Increment 2 pier, 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 8 no. independently jackable 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
It is quite large, at 32m by 32m, and has a coverable moon pool, a number of cranes and accommodation for multiple crew members. As it walks forward it maintains a steady height, this will allow it to drag an already positioned pier deck forward.
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 is therefore;
A Wavewalker like walking jack-up platform is launched from the semi-submersible Pierhead upon arrival at the operational location or towed in from a nearby friendly port if one is available. Onboard are the correct type and correct quantities of pier supports for the complete Pier span as confirmed by the pre-deployment survey.
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.
At the first pier support position, it deploys the 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.
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.
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.
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.
Increment 2 does not necessarily need the exact same specification as Windwalker 1 but certainly, something like it.
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 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 an placing the deck, the target Pier length becomes achievable within the target 2 day installation period. This method also minimises personnel required.
There are many assumptions here, this is acknowledged.
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.
This is merely a suggestion for a possible combination of Pier deck, pier support and an installation method that maximises speed whilst minimising the need for personnel and heavy lift cranes.
To repeat the oft repeated phrase, it would all need modelling and confirmation by trials.
There are a number of alternatives.
The first would be to use a smaller walking leg jack up platform as a working platform fitted with a heavy lift crane. Operating in a conventional mode it could provide a stable work platform for crane operations.
The second alternative is based on the understanding that none of the Pier components are naturally buoyant, and these three images…
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.
Once on the seabed, the hydraulic motors would be attached to a floating powerpack and simply driven toward the shore. Once in position, the same 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 trackway solution like that from Faun.
Underwater excavators are available from a number of manufacturers.
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.
Table of Contents