Soil Parameters for Design of Suction Caissons for Gulf of Mexico Deepwater Clays

Abstract

Abstract The results of one of the earliest deepwater geotechnical site investigation which specifically addressed the design of suction caissons in the Gulf of Mexico (GoM) are presented herein. The extensive monotonic and cyclic laboratory testing program is discussed and key results are summarized in simple design diagrams. These failure interaction diagrams provide soil parameters that can be used in practical design to calculate the holding capacity of suction caissons for floating production systems at Gulf of Mexico deepwater locations. Introduction Suction caissons have become a proven means of anchoring floating production systems and fixed offshore structures in several areas of the world. Usually such caissons are used in clay soils but applications in sands and layered profiles also exist. Optimal design of caissons for use in clay profiles at deepwater sites requires a careful geotechnical site investigation. Special care must be taken to ensure that the soil shear strength is properly determined for the expected loading conditions on the anchor. Figure 1 shows a generic failure surface for a caisson under lateral loading. Load inclination, load attachment point, caisson geometry, and soil profile are all parameters which influence the holding capacity and the shape of the failure surface. The undrained shear strength of the clay must be determined for the stress conditions along the failure plane. This is usually accomplished by running triaxial compression, triaxial extension, and direct simple shear laboratory tests. The cyclic nature of the loading during the design event must also be properly accounted for: cyclic triaxial and direct simple shear tests are usually performed for several average and cyclic shear stress combinations to establish the clay strength and deformation characteristics under the cyclic storm loading. Although used worldwide, suction caissons had not been used in the Gulf of Mexico at the time of this site investigation (January 1996) and have so far (January 1998) been used only for supply vessels single point mooring. Several oil companies however have definite plans to use them in the near future for production platform spread mooring. Very limited information exists in the open literature on the monotonic and cyclic behavior of deepwater GoM highly plastic clays, as it pertains to suction caisson design. As a consequence, the detailed soil information needed for the design of caissons was not available when their use was considered at the Marlin site. Amoco therefore engaged in an in-depth geotechnical program to obtain the required data. In addition to more routine testing, the extensive laboratory testing program included constant rate of strain (CRS) consolidation tests, monotonic triaxial tests in compression consolidated under zero lateral strain condition (KoCAUC tests), anisotropically consolidated monotonic triaxial tests in compression (CAUC tests) and in extension (CAUE tests), monotonic direct simple shear (DSS) tests, as well as cyclic triaxial and DSS tests performed at various combinations of average and cyclic shear stresses. Partial results are presented and summarized herein and extend the knowledge of GoM deepwater clays behavior for suction caisson design purposes.