Geotechnical Design of the Barracuda and Caratinga Suction Anchors

Abstract The Paper presents the methodologies adopted for the design of mooring and riser anchors of the P43 and P48 FPSOs due to be installed in 2003 in deep water Campos Basin, offshore Brazil. It is shown how an efficient semi-analytical 3D finite element model can be used as a routine design tool for calculating the pull-out capacity of cylindrical suction anchors under combined horizontal and vertical loading. The speed of the FE model has allowed pull-out capacity contours to be developed which help in the selection of limiting installation tolerances. Fully coupled axisymmetric finite element consolidation analyses of the set-up phenomenon have been described in which the initial excess pore pressure field is established by simulating the final penetration stages of the skirts. Introduction The Barracuda and Caratinga fields are located in deepwater Campos Basin, some 150 km offshore Brazil. The average water depth is 825m at Barracuda and 1030m at Caratinga. A taut-leg spread mooring system has been adopted for both the Barracuda (P43) and Caratinga (P48) FPSOs. Each FPSO is moored by 18 lines, ten at the stern and eight at the bow. The mooring lines are made of polyester rope, combined with lengths of chain at the fairlead and seabed ends, Wibner1. Various types of anchor were considered in the initial phases of the project. Suction anchors were selected as the preferred option for the following reasons, Sparrevik2:Relatively accurate positioning on the seabed, which isimportant in fields congested with flow lines and subsea facilities,Economical and simple installation method, avoiding the need for pile driving in deepwater or dragging and proof loading.Reasonably well-established design methods compared to other types of anchor, even though no Standard Code of Practice is yet available. In addition to the 36 mooring suction anchors, 52 suction anchors in four different sizes were designed for restraint of the risers. The subject of this paper is the geotechnical in-place capacity of mooring anchors only and excludes installation aspects. The suction anchors were designed to comply with ABS Rules3 and current industry practice for 20 years service life. Site Investigation An integrated approach to SI was taken4,5. A comprehensive geophysical and geotechnical site investigation was performed in spring 2001 from the vessel Rockwater-1. Subsea7 (formerly Halliburton Subsea), performed the geophysical surveys consisting of swathe bathymetry, ROV high resolution sub-bottom profiling and side scan sonar imaging, while Fugro BV performed the following geotechnical SI from the same vessel:66 WHEELDRIVE Piezo-cone penetration testing (CPTU) to 30m penetration,12 WHEELDRIVE insitu vane tests up to 20m. One test every 0.4m and remoulded shear strength determination every second test,39 Push-in Piston cores up to 20m. The SI was followed by a programme of monotonic and cyclic laboratory testing both in Holland and the UK.