Design and Installation of Suction Anchor Piles at a Soft Clay Site in the Gulf of Guinea

Abstract

Abstract Innovative shallow suction anchor piles have been used for permanent mooring of a process and accommodation barge at an oil and gas field developed by ELF Congo in the Gulf of Guinea. The 12 mooring lines, equipped with 5m diameter and 13m high steel suction anchor piles, were successfully installed at the site in July/August 1995. The process barge is scheduled to be connected early 1996 for start of production. With a water depth of about 170m, the Nkossa field is the deepest in offshore Western Africa to date. The soils at the site consist of soft normally consolidated clays, representative of deep water sites. The anchoring system design is presented, together with the description of the suction pile installation by under pressure. The monitored performance of the suction piles during installation is evaluated, including orientation and tilt at touch-down, and penetration rate during suction penetration. The installation behaviour appeared to be very smooth, and the measured penetration resistances were in perfect agreement with the predictions, thus demonstrating the reliability of the design installation method. Introduction The suction anchor piles, penetrating into the seabed by means of creating an under pressure by pumping out the water entrapped inside the caisson, are found as 'buckets', 'skirted foundations' or 'suction anchor piles' in the literature. The principle was first introduced by Senpere and Auvergne (Ref. 1), as alternative anchor for mooring a storage tanker at the Germ field offshore Denmark. More recently, and because of its and reliability, this foundation concept has renewed interest in the offshore industry, simplicity found a both as foundation for fixed platforms and as mooring anchor point. In addition to the Germ tanker mentioned above, mooring systems with suction anchor piles have been installed at three different sites in the course of the summer 1995, at Harding and Yme fields in the North Sea, and at Nkossa field offshore Congo. The Nkossa oil and gas field presently developed by ELF Congo is the deepest field in the Gulf of Guinea to date, with a water depth ranging between 150 and 300m. It includes two steel jackets (installed early 1995) with tender-assisted drilling, one process and accommodation barge (scheduled to be at the site early 1996 for start of production), and two floating storage and offloading tankers, one for oil and one for LPG (see Figs. 1 and 2). The design of the anchoring system for the process is given in the paper, together with the description of the suction anchor pile installation, with an emphasis on the monitored performance of the suction piles during installation. Barge and Site Characteristics Process Barge, The process and accommodation barge to be installed at the Nkossa field is presently under hookup and commissioning in South of France, near Marseilles. The hull, 220m long, 46m wide and 16m deep, was built by Bouygues Offshore in high performance pre-stressed concrete.