Permanent Mooring Anchors for Floating Facilities - A Review and Selection Process for Carbonate Soils

Abstract

Abstract The use of conventional catenary moorings provide a viable and cost effective anchoring system for the large FLNG field developments proposed along the deeper water regions offshore the North Western coast of Australia. The design of the catenary mooring systems and the anchors are highly dependent on the water depth and the seabed soil conditions. However, the extreme environmental loads associated with these deep water developments dominate the mooring anchor design with anchors required to sustain significant vertical and horizontal loads. Selection of an appropriate anchor design therefore becomes an intensive exercise involving many disciplines including geotechnical, structural and installation engineering to ensure the appropriate and optimum anchor type is selected. Due to the size and weight of some conventional anchor designs e.g. driven piles, suction anchors etc., over the last 10 years, a number of new anchor concepts have been proposed and developed including DEPLAs, SEPLAs, VLAs, Torpedo piles etc. to improve anchor installability and reduce costs. However, when considering extreme FLNG mooring loads technical challenges and uncertainties still remain for several of these concepts. Further considerable complexity is added to the anchor design and selection process given the wide range and nature of the seabed soils encountered in the North Western shelf which typically comprise carbonate soils. A methodical anchor concept screening process has been developed in this paper to review and rate various mooring anchor concepts for FLNG developments, although the process can also be applied to alternative floating facilities and mooring systems such as FPSO developments. The process considers geotechnical, structural and installation drivers that impact on the design to determine the most practical and technically viable solution for the water depth, design loads and soil conditions. As part of a mooring anchor project the results of the screening process can be presented at an early stage in the mooring anchor design for discussion and can act as a starting point to identify the major risks, help optimize the anchor design and highlight areas that require further analyses and testing.