Prior to foundation pile installation, offshore jacket structures require temporary seafloor support provided by the jacket leg extensions, lowest level of horizontal bracing, and mud mats. Since the leg extensions are the first foundation element to contact the seafloor, the designer must first select their length to assure full penetration. A second design consideration requires an analysis to determine the amount of support provided by the jacket leg extensions and lowest horizontal bracing. If additional support is needed, mud mats must be provided and designed based on the bearing capacity of the seafloor soils.
This paper recommends rational design procedures for the various foundation elements in contact with the seafloor. Additional sections describe design loadings and suggested factors of safety. Finally, five case histories are given illustrating the validity of the recommended procedures and highlighting potential problems caused by inadequate designs.
Offshore jacket structures must be temporarily supported by the near-seafloor soils before driving of the foundation piles. The foundation elements that bear on the seafloor soils include the jacket leg extensions, lowest level of horizontal bracing, and mud mats if needed (see Fig. 1). All these foundation elements must be designed to support the weight of the jacket plus any additional loads imposed by environmental or construction conditions. The purpose of this paper is to recommend analytical procedures to compute:
the soil resistance developed on the jacket leg extensions, and
the ultimate bearing capacity provided by the soils supporting the mud mats and lowest level of horizontal bracing.
Several case histories illustrate the validity of the proposed analytical methods and highlight potential problems that may occur during installation when foundations are not designed adequately.
To achieve a safe yet economical design, the structural and geotechnical engineers must coordinate their activities in sizing the foundation elements. The first step in the design sequence requires that the geotechnical engineer explore soil conditions at the proposed structure location. By drilling and sampling a boring, geotechnical data (strength and deformation properties) are determined for the critical zone generally located to a depth of 40 to 50 ft below the seafloor. After the structural engineer provides sizes of the jacket leg extensions and lowest horizontal bracing, the geotechnical engineer can then compute the soil resistance developed on these two foundation elements. Depending on the soil resistance, the length of leg extensions may need to be shortened to assure full penetration. If the jacket leg extensions and lowest horizontal bracing do not provide adequate temporary support, mud mats must be provided and should be designed based on the bearing capacity of the seafloor soils as determined by the geotechnical engineer. The following sections describe design procedures for each of the foundation elements. Additional sections describe the effects of mud mat and bracing shape, computational procedures for immediate settlement, design loadings, and suggested factors of safety.