Oil exploration and production has been moving into steadily deeper water in recent years as shallow reserves are worked out. And with the recent rise in oil prices, ever more diffi ult areas are becoming economic, particularly off the coast of West Africa.
This has meant the use of new techniques because platforms simply cannot be built in depths of 1km or more of water. Instead, anchored rising pipelines are used attached to floating production offloading and storage vessels, or FPOS.
These provide a surface reservoir to which tankers can connect for on-loading.
The FPOSs are moored to the seabed, sometimes using versions of the traditional drag anchor but more usually these days, the anchorage is provided by piles, almost certainly installed by suction.
The suction pile concept is deceptively simple. 'Imagine a bean can with one end cut off and a valve in the top, ' says engineer Jamie Irving at specialist consultant SEtech which designs foundations for the offshore industry. 'You simply attach a pump and create a vacuum, or more accurately an under pressure, in the can and it is pushed into the seabed [using water pressure].' The advantage is that no piling rig is required, which would be diffi cult and expensive in very deep water. Instead the pile can be sunk from a barge.
It is then installed remotely, monitored by ROVs operated from the surface.
Suction piles are large, typically around 6m in diameter and up to 30m long, chunkier than a conventional pile serving the same purpose. 'About five times diameter is the longest you can go, ' says SEtech-UK's managing director Peter Allen. They have a steel wall between 20mm and 30mm thick and usually a cross piece for stiffening at the end and across the 'lid'.
Total weight is about 60t-100t.
The structure is quite simple, ' says Irving, 'which is an advantage when operating somewhere like West Africa because they can be made up on shore using relatively simple facilities with just rolled steel and welders.' Once in place the suction pile will serve various functions, usually anchoring cable to the surface but also for traditional load bearing in compression to support undersea structures like valves and manifolds.
Loads on anchoring piles are more complex. They may be acting in tension but more typically loads are lateral because the catenary curves of the long anchor lines and chains required mean that at the point of attachment a cable could be almost horizontal. Consequently attachment connections on the piles are a major factor in their design and can vary between top of the pile to somewhere halfway down or more, even underneath the seabed. Various modes of failure have been analysed usually involving rotation.
The shape of the pile also varies considerably with a variety of types from straightforward cylinders to assorted wedges - easier for stacking on board the installation vessels in heavy seas - and stubby 'winged' cylinders with side fi ns added to create additional lateral resistance in the ground.
Designs are still evolving, because the technique is relatively new.
'The Norwegians first began using them in the North Sea in the 1980s, ' says Allen, 'but there were a few unhappy experiences and only in the mid1990s did people begin using them seriously, working out the problems.' Difficulties included installation. If too great a pressure is applied the ground can fracture and a plug of soil is drawn into the pile rather than the pile being drawn into the ground.
The piles also require fairly uniform seabed conditions.
'In the areas we are typically working in the sea beds are mainly very soft clay, ' says Allen, 'with around only 2Kpa strength. The currents below 1,000m are usually very limited and that means fine particles are deposited out'. The site surveys needed for the pile designs usually produce a fairly homogenous profi e he says with 'a narrow band of readings for penetration tests for example'.
Soft clays are highly suitable for installation. Typically this is done from a specialist contractor's vessel, maintained on location with reference to a GPS signal. The pile is simply dropped over the side of the vessel. Once it reaches the seabed simple self-weight will carry it 5m or so into the ground providing a good seal for the 'vacuum'.
A detachable pump and monitoring skid is mounted on the top of the pile with an umbilical to the surface for control and to carry readings from a clutch of instruments including pitch and roll sensors.
A sonar altimeter measures the soil level within the pile to ensure that the soil plug is not heaving. The position of the pile is checked, usually with a ultra-short baseline acoustic detectors, which track a transponder on the pile pump skid.
'There are also ROVs in the water which can monitor by video - the piles have level marks on their sides, ' says Allen.
The actual installation under suction takes about two hours if all goes well and then the valve is closed and the pump and skid are released and hauled to the surface on the umbilical.
Getting installation right is important. The installation vessels cost around £250,000 a day so re-doing work is an expensive business. Even if all goes well the work can take some time; a typical installation will require perhaps six to a dozen piles.
In addition to the FPOS there may be a 'bridge' mid-water supporting a loop in the pipeline which gives it the slack to accommodate ship movement.
The use of the piles is most signifi cant in the West African offshore fields although some are found in the Gulf of Mexico, Australia and to some extent the North Sea. But around the North Sea coasts they may well appear soon in another form, where they could prove highly useful for offshore windfarm installation offshore.
Allen is cautious about the potential of these very shallow locations which mean installation in sandy seabeds which are much more difficult to work in. But they have a champion at Oxford University where Guy Houlsby, Professor of civil engineering has been researching the use of suction piles for near shore work.
'These are very different in shape, ' he explains, 'being wider than they are deep, typically 12m in diameter and 6m deep, though the limit is about one diameter in depth.' The different shape, which could be referred to as a suction caisson, is needed because of the typically sandy conditions which limit penetration.
But wind turbines mostly need to cope with lateral wind loads rather than vertical loads. The alternative is usually a long monopole.
Installation in the stiffer sand would be more complex than in clay with careful control of the suction to create enough groundwater flow over the edge of the caisson. 'The flow has an effect on the forces required for the edge to penetrate, ' says Houlsby. But go too fast and piping and other effects can occur with inrush of sand.
There is also a possibility of buckling in the cans.
Allen's doubts include reservations about the variability of ground in shallow water.
He believes that water flow in softer sections will drag out fines and increase differential permeability leading to ground failure. But Houlsby says there are a significant number of offshore sites which have good homogenous ground in the seabed and these would be suitable.
'There could be 3,000 turbines planned off the UK, and they are getting larger, up to 5MW or so, and even if just a proportion proved suitable for the technique it could be useful' he says.
Suction piles provide cost savings, not because the foundations are smaller - typically they use about the same 200t or so of steel that a monopole would involve - but because they do not need an expensive jack-up barge and pile rig for installation.
The next stage is to see what happens with various tests underway says Houlsby, including work off Denmark.
He has done initial research and hopes to win further funding with some input from industry and the UK government, possibly looking at using suction caissons to support anemometer masts being erected at various possible wind generation locations.