Suction anchors for FPSO at Shelley Field

NGI was geotechnical consultant to Sevan Marine and the client Oilexco for the design and installation of suction anchors for the novel cylindrical FPSO at the Shelley Field in the UK Sector of the North Sea.

The cylindrical FPSO, SEVAN VOYAGEUR, prior to installation at the Shelley Field in the UK Sector of the North Sea. In front above the waterline the four anchor lines of one of the three clusters can be sees.

The soil conditions at the three clusters for the suction anchors were quite variable, and thereby a challenge. The area consisted in general of clay with varying thickness of dense to very dense sand on top. Due to the dense sand and internal stiffener of the anchors, the anchors to be installed were equipped with a water-flow system to reduce the resistance during penetration.

Predictions for high and low estimate penetration resistance in terms of required suction with depth were made for the three clusters. The installation sequence and operational aspects could be adjusted during the installation, based on experience from previous installations. The predictions compared favourably with the measured values during penetration. The water-flow system enabled penetration in areas of dense sand and detailed back-calculation investigated the benefit of the water-flow system.

Predicted versus measured required underpressure, and soil/structural capacity vs. depth, for Cluster 3.

The installation of suction anchors at the Shelley Field again proved the success of this anchoring solution and the good correspondance between predicted and measured behaviour. The work will serve as a reference for future installation of suction anchors in a variety of soil conditions.

The work was presented at the OTC 2009, see paper OTC 20294.