Offshore steel jackets have traditionally been fixed to the sea bed by piles, which as a foundation solution is expensive. A new method for supporting jackets has been developed by Statoil. This new method involves the use of circular flat stiffened foundation plates under each jacket leg, enabling the jacket to stand as a gravity structure. Each plate is surrounded by a skirt penetrating the sea bed. The skirts will ensure suction effects under the plates thereby stopping the jacket toppling over during storm conditions. Particularly on dense sand and hard clay the Skirt-plate foundation will give small and cost effective structures. The suction effect has recently been tested offshore through model tests on the Sleipner and 16/11 areas.
The skirt-plate foundation is in many ways similar to the gravity foundations of the Maureen platform and spud-can foundations for jack-ups. The main difference from the gravity solution is the use of skirts. The plate foundation with skirts will develop suction forces resisting tensile loading. This avoids the need for continuous compressive load in the foundations that otherwise had to be ensured by a combination of ballast and width between the legs. Experience from gravity structures with skirts have provided the detailed background for the installation and load. transfer mechanisms. Most notibly is the Gullfaks C structure with 22 metres deep skirts which has been monitored since installation in 1989.
The main objective of the skirt-plate foundation studies has been to develop a safe and economically attractive foundation system that avoids the use of piles. The background for this is that piles constitute a very expensive part of jacket structures. The foundation plate is in principle a flat plate welded to the lower part of the leg, as shown in figure 1.
