The suction pile has proved to be an attractive alternative to a driven-in or drilled-in anchor pile or a gravity block and is intended to serve as a fixed seabed mooring point for floating platforms and surface buoys. The suction pile consists of a hollow steel cylinder with closed top which is installed by creating an underpressure within the pile, thus utilising the available hydrostatic pressures. The suction operation is stopped after installation of the pile, which means that the underpressure is then eliminated.
After laboratory tests and a small-scale field test, the suction pile concept was tested full-scale on inshore locations of which two with sandy soils and one with hard clay (potclay). Piles of 3. 8 m diameter and between 5 and 10 m length were tested in order to determine their installation characteristics and their lateral and axial load capacities. In these tests holding capacities of the order of 200 t were measured. The suction-pile method was licensed to SBM Inc, who will exploit it commercially for fixed mooring points.
Floating production facilities ranging from SBM's to tethered buoyant platforms require to be moored effectively for many years. The selection of a suitable fixed seabed mooring point will strongly depend on the application concerned, on the load condition, the soil and environmental conditions and on the availability of equipment.
To date modern ships' anchors, dead-weight blocks or anchor piles have been used as permanent mooring points in the North Sea. A ship's anchor digs itself into the soil as a result of the lateral forces exerted upon it. It can hardly resist inclined loads and requires very long chains when used in deep waters. With concrete blocks the underwater weight must be at least twice the maximum anticipated mooring force, which almost invariably makes the blocks heavy and difficult to install, especially in rough and deep-sea areas. Anchor piles are either installed by driving or drilling, which becomes increasingly difficult as the water depth and load become greater.
Furthermore, the stringent requirements that are set with respect to water depth and holding capacity represent a complicating factor in the design, fabrication and installation of the above-mentioned anchors. This induced Shell to look for alternatives to fixed seabed mooring points that could easily be installed and could accept horizontal and vertical loads. Of all concepts studied the features of a pile installed by suction were considered most promising. The principle development and theoretical approach for this type of pile are discussed in the following sections of this paper.
Basically, the suction pile consists of a hollow tubular member with closed top (Fig. 1). This top has to be provided with a suction connection to pump out water from the pile. In order to make the air escape rapidly from the inside, air valves can be mounted .at the top of the pile.
