Meeting Requirement 3 – Part C
A number of issues remain to be covered;
There is a common misconception that large offshore jack-up platforms simply turn up for work and jack down, this is not the case. Leg push through is a dangerous condition where one of the legs pushes through an unexpectedly soft layer of the seabed and in some cases the rig can be put at risk of collapse. Because jack up platforms are likely to feature in Requirement 3 solutions this is a concern.
This potential need for seabed soil survey also poses a particularly difficult challenge for two reasons.
It is likely that potential operational locations for Requirement 3 do not fall within the ‘friendly nation’ category and so unlikely to allow an overt survey using jack-up platforms or survey vessels of their nearshore areas. This would also provide an alert that a pre-surveyed is an area of interest and counter-productive to mission success.
And so the first difficult challenge for Requirement 3 that the seabed survey may need to be conducted covertly, underwater.
Although given time the number of pre-surveyed locations could be a sizeable database the time available for conducting the pre-build survey may be compressed by contingent operations. The second challenge is that the survey must be conducted and data analysed quickly. In speeds favour is that the actual physical area to be surveyed is likely to be quite small.
Put the two together and the problems are amplified.
Conducting a rapid and covert subsea soil survey is not a trivial task.
The survey task would actually start in the UK by analysing existing data and data that may be obtained from other sources. This initial desktop survey would be used to support additional capture and analysis planning.
Follow up hydrographic surveys, metals/debris detection and sub bottom profiling could be carried out using vessels of opportunity or under the cover of routine hydrographic surveys carried out by NATO survey vessels. Standoff AUV’s and even handheld devices could also be used.
But this only gets us so far, there is no substitute for physical sampling.
Existing special-forces have quite capable covert beach survey capabilities (including manual core sampling) and when combined with stand-off systems deployed from conventional survey vessels the majority of the survey task can be covered. However, the aspect of the survey requirement that is not covered by existing capabilities is sea bed load bearing capacity.
This requires penetrometers and possibly, core sampling.
At its simplest form, cone penetrometer testing involves inserting a rod into the seabed and measuring tip resistance and sleeve friction. The data collected is then analysed using specialist software. The traditional method of conducting a near shore soil load bearing survey is to use a jackup platform and a cone penetrometer test using an integrated drill derrick or a standalone system deployed over the side or through a moon pool. The penetrometer is inserted through a hollow casing that provides lateral stability and buckling resistance. In very soft soils an additional device called a piezecone penetrometer may also be used.
Core samples may be taken to verify predicted data taken from penetrometers.
These are established techniques, but hardly covert.
For deeper water surveys, typically used for offshore energy installations and requiring significant load bearing, underwater penetrometers are available from a number of manufacturers including AP Van Den Berg in the Netherlands and Datem in the UK. Both have developed compact coiled rod units that can be lowered to the seabed and remotely operated from a ship.
Although mostly used for deeper water they have also seen some use when mounted on a tracked underwater ROV for intertidal applications.
If they can be mounted on a tracked ROV or lowered from a surface vessel, they can be deployed from submarines or simply ‘driven in’ from a safe offshore distance.
Any equipment will need to fit within the Project Chalfont dry shelters that can be fitted to the Astute class submarines, and other similar shelters or torpedo/missile launch tubes used by allies, this may require some design engineering but the systems are available and well understood, the task would be repackaging not starting from scratch.
A number of parameters will need to be analysed; Soil density, soil strength, friction angle, consolidation and permeability for example. This should also include analysis of the strata to the desired penetration.
Finally, a scour potential survey should be carried out, analysing wave and current in addition to soil conditions.
Whilst sampling would in most cases be conducted covertly, analysis can be carried out away from the target site aboard one of the survey vessels supporting the operation, or in the UK via reach back.
The output from a survey would be a decision on site suitability.
If Requirement 3 were taken forward it would also be desirable to pre-survey potential locations in ‘peacetime’. This could be a standing and regular task shared between allies, although of course, there is some element of risk if those locations on not in the ‘friendly nation category’!
With the system in place, operation becomes the next issue to consider.
For these operations, there are two main (and interconnected) issues, mooring/berthing, and wave attenuation.
Some ships are fitted with thrusters, azipods or other means of fine adjustment and station keeping, but most do not. This means they will need some assistance to moor at the Pier Head.
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.
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.
Concrete caissons are also used.
None of these solutions are practical for Requirement 3, in fact, none of them are.
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 Requirement 3.
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 Requirement 3and provide localised sheltered water at the Pierhead
Pollution control barrier technology might also be utilised.
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