Girassol: Design Analysis and Installation of the Suction Anchors

Abstract

Abstract Suction anchor piles are the main foundation and anchoring system for the Girassol development in 1,400m water depth, including the FPSO, the offloading export buoy, the riser towers and the subsea manifolds. This paper summarizes the main assumptions in the geotechnical design analysis, together with key features of installation behavior. Based on installation results, a revised method for the penetration analysis is proposed. The Girassol experience will be relevant for future applications of suction anchors at other deepwater locations in West Africa. Introduction For the permanent mooring of production floaters in waters over 1,000m, taut leg moorings are used, i.e. at an angle ranging between 35° and 45°at mudline, with steel wire cables or fibre ropes and with anchors having a vertical loading capability. In the soft clays generally found at deepwater sites, suction anchors and vertically loaded plate anchors (VLA) have become the two preferred options. A significant number of applications for the suction anchor have been found due to the following advantages [1]:fixed location on seabed, which is important in congested subsea field developments;simple installation procedure with no need for proof-load testing; andreliable design methods for ultimate capacity calculation, providing a larger confidence in their long term pull-out capacity. The Girassol field, the deepest offshore development off West Africa to date, is located in about 1400m of water in Block 17 operated by TotalFinaElf EP offshore Angola. The Girassol FPSO is connected to 40 subsea wells by means of three-riser towers and pipeline bundles [2]. In relation to the suction anchors, the installation operations took place between March and August 2001 and were carried out by Alto Mar Girassol (AMG) and Mar Profundo Girassol (MPG)), two consortia of Stolt Offshore and Bouygues Offshore Serviices, and SBM, from the crane barge Seaway Polaris and the vessel Seaway Eagle. Girassol Seabed and Soil Conditions From high-resolution 3-D seismic and shallow geophysical data, the Girassol seabed appears as sloping gently to the southwest, with a gradient of about 1:50 (1.2°). The soil investigation campaign carried out at the Girassol field comprised:four borings taken down to 30m with alternative push sampling and cone penetrometer testing (CPTU), with one boring at the site of each group of four FPSO anchors;three 30m deep borings, with combined sampling and CPTU testing, covering the subsea well locations over an area of about 8km by 7km;one 100m deep central boring for designing the penetration depth of the well conductors;two 30m deep continuous CPTU profiles, performed at a riser tower and FPSO anchor location; anda number of 5m deep CPTU profiles, aiming at improving the knowledge of seabed conditions for the pipelines and manifold foundations. The soil conditions, consistent and uniform over the whole Girassol field, are composed of soft organic clay of high plasticity.