Compared with central manifold, PLET is much smaller and the load combination is also simpler. The foundation analysis result is shown that shallow foundation can meet all the requirements of LS17-2 project. This paper presents the design methodology of shallow foundation for PLETs. A design example is also be demonstrated in this paper.
The development of LS17-2 gas field consists of subsea production system, one SEMI and subsea pipelines. The subsea production system comprises one west manifold and three east manifolds. LS17-2 SEMI is a SEMI platform and located near Manifold EAST1. Well fluid from west production loop will be transported to LS17-2 SEMI for process through two 10" pipelines and two 10" SCR risers. Well fluid from all east manifolds will converge at Manifold EAST1 and then be transported to LS17-2 SEMI for process through two 12" SCR risers. After processing, qualified natural gas shall be pressurized and transported to subsea pipelines from YC13-1 platform to HongKong through a 18" subsea export pipeline. Condensate oil shall be sent to the storage tank on LS17-2 SEMI and be exported by DP Shuttle oil tanker. Except four manifolds, the subsea production system also comprises twenty-five PLETs to connect manifolds or SEMI. Fig.1 shows the development layout of LS17-2 Gas Field.
Most facilities in subsea production system are fixed structures, that is, they need to be fixed onto the seabed or sliding in a small area by the foundation structures which can ensure they will not sink into the seabed by the vertical load and prevent the great displacement by the horizontal load. Driven piles, suction piles and shallow foundations are three common foundation types for subsea production system. Driven piles are suitable for a wide range of soil conditions but restricted by the water depth for the pilling technology and economic. Suction piles can be used in very soft geological conditions and deep water but the cost of design, construction and installation is very high. Shallow foundation is that foundation for which the depth of embedment is less than the minimum lateral dimension of the foundation element. Mudmat with skirts or without skirt is the typical shallow foundation. It is not sensitive to the water depth and easy to be installed. So, it is widely used for the foundation of subsea products in deep water if the geophysical condition is appropriate. For LS17-2 project, a suction pile is selected as the central manifold's foundation, because the soft seabed cannot bear the large external load and self-weight from central manifold. In comparison, PLET is much smaller and the load combination is also simpler. The foundation analysis result is shown that shallow foundation can meet all the requirements of LS17-2 project.
