The final piece of the jigsaw in regards of CONCEPT 2 is wave attenuation.
No you could reasonably argue that the crew and shore personnel managed to conduct an offload and load operation in high sea and wind states onto a RORO platform but it was very quick, a handful of vehicles only. This is not acceptable and the ships crew would be very familiar with the particular port
The video below shows the effects of wave motion induce crane pendulation
The load in question is a heavy cargo drum but still a minnow compared to an armoured vehicle or loaded container.
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 concrete blocks lifted into position such as those made by Xbloc
Obviously, within the time and logistics constraints of CONCEPT 2, bringing ones own concrete is not the best of ideas. Neither is using large concrete caissons and old ships like on D Day, imagine the environmental impact statement!
Some form of floating breakwater is therefore required.
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 water tight 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 pre-fabricated 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 CONCEPT 2.
Types of Floating Breakwater
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 affectively 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 tyre 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 types is the floating box, concrete or steel in construction, often foam filled, linked together with flexible pins and moored to the sea bed.
The image above 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 image below, a similar concrete box construction floating breakwater in Italy from Ingemar
The image above shows clearly 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.
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.
Traditional inflatable semi submersible types are also available
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 of Superflex being good examples
Rapidly Installed Breakwater System (RIBS)
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 apply to CONCEPT 2 though.
After testing many designs the CHL came up with the ‘Double Delta’ shape, as below
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 unspooled 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.
Then funding was stopped as the OMFTS and STOM crowd started hoovering up funding for sea basing and high speed over the horizon amphibious operations.
RIBS was as cheap as chips and self evidently, extremely effective. Deployment was simple and the whole assembly was very compact pre installation.
Putting a single CONCEPT 2 pier head in the lee of the V shape would need a much longer leg length than envisaged but there is no reason to suppose that combining some of the newer plastic moulded systems with the pressurised beam technology of RIBS a similarly deployable yet effective system could not be created.
Sources and Further Reading
Other Posts in the Series