Geotechnical Considerations in Foundation Design of Offshore Gravity Structures
Authors: Young A.G. , Kraft L.M. , Focht J.A. 
Published: December 2007

Interest in offshore gravity structures as production platforms has increased because of their relatively short on-site installation time that provides an economic savings in rough-water areas. This paper presents provides an economic savings in rough-water areas. This paper presents the geotechnical considerations pertinent to foundation design qf gravity structures and provides a critique of procedures currently used for foundation design.

Introduction

The continuing search for petroleum offshore has led to exploration and production for oil and gas in deeper waters and in areas subject to severe storm conditions, such as the North Sea, northwestern Atlantic Ocean, Gulf of Alaska, and China Sea. Since the "windows" in the weather that permit heavy construction at sea in these hostile offshore environments are relatively short, there is considerable interest in production structures that can be constructed onshore or in sheltered waters, towed semisubmerged to location upon completion, and then installed on location in a short time by flooding. Gravity structures meet this requirement. These structures are supported by relatively large foundation elements bearing on the near-seafloor soils and depend principally on their weight to resist vertical and principally on their weight to resist vertical and horizontal loads. During installation, gravity structures require less time and reduced demands for costly marine equipment than do conventional pile-supported steel jacket structures; additionally, oil-storage facilities can be easily incorporated into the cellular configuration of many proposed gravity structures.

Many gravity-structure designs have been developed during the past few years; 16 different proposed designs were recently summarized. The proposed structures are either of concrete design consisting of cellular concrete construction with a large concrete-mat foundation, a hybrid design that features a typical steel-template structure supported by a concrete mat, or are principally all steel design. Examples of concrete and hybrid structures are illustrated in Fig. 1 along with an almost all steel design featuring multimat foundations.

Unlike conventional pile-supported structures, a gravity structure depends on single or multiple concrete mats bearing on the unprepared ocean floor to provide the foundation stability against the maximum environmental loads imposed on the structure. A pile-supported tower structure may sit on the sea bottom for a considerable period of time before it is safely pinned by piling and period of time before it is safely pinned by piling and before its superstructure is put in position. However, the gravity-structure foundation is designed and constructed to withstand design environmental loads when first placed on the sea floor. The significance of this ability to support design loads soon after installation was apparent when a storm wave that was 90 percent of the 78-ft design wave hit the first gravity percent of the 78-ft design wave hit the first gravity structure in the North Sea within 6 months after installation.

The gravity-structure concept is being used for locations with severe design conditions without significant prior experience in less severe conditions. In contrast, prior experience in less severe conditions. In contrast, the design concepts used for pile-foundation design have evolved over a 25-year period since the first pilesupported jacket was built in shallow waters of the Gulf pilesupported jacket was built in shallow waters of the Gulf of Mexico.

The purposes of this paper are to (1) present the geotechnical considerations pertinent to foundation design of gravity structures, and (2) summarize and provide a critique of procedures currently used for provide a critique of procedures currently used for foundation design.

JPT

P. 925