Increment 2 – Wave Attenuation
In order to allow safe and efficient offloading at the Pier Head the relative motion between the free floating ship and fixed Pier Head must be minimised.
Conventional breakwaters are usually massive reinforced earthworks, sometimes with protective sheet piling. Protection against erosion caused by waves is provided by combinations of geotextiles, aggregates and in many cases, large boulders or concrete blocks lifted into position, such as those made by Xbloc Large concrete caissons are also sometimes used.
None of these solutions are practical for Increment 2.
The first recorded use of a floating breakwater was at Plymouth in 1811 and since then the floating breakwater has evolved in a number of specialist areas such as marina and aquaculture.
Going back to D Day, the problem of wave attenuation was a big one, the location and time the invasion was planned for meant waves would be significant and much thought was given to the issue. After many different ideas were tried the actual means of wave attenuation coalesced on three methods.
The image above clearly shows the effectiveness of the wave attenuation capabilities of the combinations of these methods.
First were a row of 60 blockships, old and obsolete ships that were stripped, towed into position and sunk. Preparations including destroying their watertight bulkheads to ensure they sank quickly when Amatol charges punched holes into their hulls. Once in place, they were overlapped and sunk. The overlapping was needed to prevent scouring at the bow and stern. The lack of such overlapping on UTAH meant the US blockships suffered from this effect and several of the ships had their backs broken by voids opening up underneath them. Their main advantage was they could get to the beaches under their own steam, not requiring precious tugs but due to their comparatively low height there were unable to be used across the entire area and thus, deeper concrete caissons were needed.
Second, were large concrete caissons called PHOENIX, 150 of these were built in six depth variations to accommodate different water depths, the largest (Type A1) displacing 6,044 tons and the smallest (Type D), 1,672 tons. Each was a standard 60m in length with a boat hull to facilitate towing operations. Some of the PHOENIX caissons had anti-aircraft guns and barrage balloons, with the necessary crew quarters built into the structure. The caissons were prefabricated in the UK and when towed into position their scuttling valves were opened, flooded and sunk.
Finally, an anchored wave attenuation device called BOMBARDON was used. These were 200-foot long cruciform cross section floating steel constructions that were anchored to the sea bed and each other in long ‘strings’. 24 Bombardons, each with a 50-foot gap between them, created a breakwater 1 mile long. Water was then let into the three lower fins as ballast. Each was designed for a Force 6 storm, unfortunately.
Of these three methods, only one could be considered as a floating breakwater, the BOMBARDON, and only one that could be considered for increment 2.
Floating breakwaters are often selected for deeper water, where soil conditions preclude the construction of soil and rock structures and where water quality needs to be maintained, aquaculture being particularly sensitive to this. Short choppy waves can be effectively attenuated with floating breakwaters but longer period waves encountered offshore can be difficult to deal with because the transmitted wave (that behind the breakwater) depends on the ratio between the incoming wave and width of the breakwater.
In the years since the BOMBARDON, there has been a great deal of research into floating breakwaters and many designs and configurations tested. The increasing size of container vessels and the need for smaller feeder ports has also resulted in a modest increase in research in floating breakwaters.
Floating tire or log mats can be used although they are not easy to deploy and less effective than other types, they are cheap, though.
A more common type are the floating box, concrete or steel in construction, often foam filled, linked together with flexible pins and moored to the seabed.
The first image below shows a floating pontoon breakwater installed in Holy Loch, Scotland. The 240m long structure comprises twelve 20m long pontoons, each weighing 42 tonnes and the second image below, a similar concrete box construction floating breakwater in Italy from Ingemar
The second image clearly shows their utility in exposed water and the unit is designed for a maximum wave height of 1.5m before being over topped, with a maximum wave length of 18m and period of 5 seconds. This would correspond to a sea state of between 3 and 4. To improve efficiency they can be post-tensioned in order to increase stiffness although this creates significant forces.
Floating or bottom-tethered pontoons or tubes have also seen widespread use with individual designs being influenced by specific conditions encountered at the intended location. A significant advantage of inflatable breakwaters is their ability to absorb wave energy by structural deformation, unlike fixed pontoon or box structures. This can also result in lower stress on the mooring system. They can also be inflated and deflated in situ, an obvious space advantage for a deployable solution but can obviously be deflated by puncturing. Newer designs, generally aimed at the marina and leisure market used moulded plastics to form long flexible strings of breakwaters, examples include Waveeater and Whisprwave Fixed tubular semi-submersible types are also available. The Canadian company Narvaltech produce a floating breakwater that utilises 5 tubes arranged in a cruciform shape. Each module is 7.3m long and weighs 7.1 tonnes. They can be arranged in single or double parallel patterns to maximise wave attenuation. Other proposals include the use of artificial kelp barriers used in conjunction with inflatable beam breakwaters.
The most likely failure point in any floating breakwater is the mooring because it has to maintain the floating structure across a vertical range, alternating between slack and taut. Many studies have shown that the influence of the mooring system on the overall effectiveness of the floating breakwater is significant. Mooring systems that can accommodate depth variations are most effective, those from Seaflex being good examples
In the mid-nineties, the USACE Military Engineering RDT&E Program of the Coastal and Hydraulics Laboratory (CHL) carried out what was probably the most comprehensive study into deployable breakwaters since the Bombardon.
As described above, floating breakwaters are most effective when their width is a quarter wavelength, in open water where long wavelength waves are more common than ‘choppy’ short wavelength waves closer to shore the problem becomes extremely difficult due to simple dimensions required.
RIBS was required to reduce wave height by 50% in Sea State 3 to 2, be survivable at Sea State 5 and be able to be deployed and redeployed quickly using in service shipping. It was not specifically aimed at creating a stable platform for ship offloading to a fixed pier but for offloading large ships to lighters. The same principle still applies to Increment 2 though. After testing many designs the CHL came up with the ‘Double Delta’ shape. The cells were filled with expanding foam and a rigid curtain extended between them at a sufficient depth to stop wave energy penetrating beneath the breakwater. It would be deployed in a V-shape with the tip of the V facing into the waves, the floating structure deflecting waves rather than reflecting them as traditional breakwaters might, this was a big departure from those described above.
At the point of the V was a nose buoy that was moored to the surface and allowed the angle to be varied between 0 and 60 degrees. Scale testing indicated up to 85% wave attenuation. The first tests used a rigid structure but these were rapidly changed to inflatable beam structures, called the Hydro RIB. The final test was at full scale using a design called XM-2001 that un-spooled from a large bobbin before pressurisation. The larger scale testing demonstrated reduction from upper sea state 3 to 1 in the lee of the breakwater.
Pretty damned impressive and more so because the deflection mechanism allowed moorings to be relatively low strength and simple.
RIBS was as cheap as chips and self-evidently, extremely effective. The deployment was simple and the whole assembly was very compact pre-installation. Although RIBS has not yet been deployed it is being developed further for the roll on roll off platforms used for ship to ship transfer. The RIBS system, perhaps combined with some of the commercially available systems would be a relatively simple drop in for Increment 2 and provide localised sheltered water at the Pierhead
Pollution control barrier technology might also be utilised.
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