Caisson Foundations as Alternative Anchors for Permanent Mooring of a Process Barge Offshore Congo

ABSTRACT Shallow caisson foundations are considered as alternative anchors for permanent mooring of a process barge at an oil and gas field developed by ELF Congo in the Gulf of Guinea. The 12 mooring lines are scheduled to be installed on site in July 1995 and the process barge will be connected in 1996 for start of production. b14tha water depth of about 170m, this field will be the deepest one offshore Western Africa to date. The soils at the site consist of soft normally consolidated clays. The paper presents the results of a comparative study of potential anchor solutions for this production barge, including high capacity drag anchors, driven piles, and shallow caisson foundations installed by under pressure. This comparative study, including design, construction, and installation of the mooring system, has shown that the caisson foundations represent the best suited solution, technically as well as on an economical point of view, because of the following advantages:vertical load capability, thus allowing to reduce the anchoring pattern,oncepositioned on the seabed, the location of each anchor is fixed and known with accuracy, andmore simple and shorter installation procedure, since caisson anchors, as piles, do not require to be proof-loaded on site. INTRODUCTION With oil exploration wells and development of offshore fields coming into deeper water depths, innovative concepts of structures and foundations are considered, with the aim to develop alternative solutions that can yield cost savings, but with similar or improved reliability. The NKossa oil and gas field presently developed by ELF Congo in 170m of water depth will be the deepest field in the Gulf of Guinea to date. It includes two steel jackets (installed early 1995) with tender-assisted drilling, one process and accommodation barge (presently under construction and scheduled to be at the site in 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 present paper gives the results of a comparative study on potential anchor solutions forpermanent mooring of the process barge, including high capacity drag anchors, driven piles, and shallow cylindrical steel caissons. The shallow caisson foundations, penetrated into the seabed by means of under pressure in the water inside the caisson (also called "buckets", "skirted foundations" or "suction anchors" in the literature), were first introduced by Senpere and Auvergne (1) as alternative anchors for mooring of a storage tanker at the Germ field offshore Denmark. More recently, and due to its simplicity and reliability, this foundation concept has found a renewed interest in the offshore industry. The geotechnical analysis of skirted foundations is based on the experience gained from large North Sea gravity structures. Model tests have been performed in clays by the NGI for application to the Snorre TLP (Dyvik et al, 2; Andersen et al, 3), as well as for studying the performance of anchors under horizontal loading (Keaveny et al, 4). Since then, it has been applied for different types of structures.