mudmat bearing capacity
TRANSCRIPT
OTCD-ROM DOCUMENT REPORT
OTC 3750
TEMPORARY SEAFLOOR SUPPORT OF JACKET STRUCTURES
Steven C. Helfrich, Alan G. Young, and Clarence J. Ehlers, McClelland Engineers, Inc.
Copyright 1980 Offshore Technology Conference
This paper was presented at the 12th Annual OTC in Houston, Tex., May 5-8, 1980. The material is subject to correction by the author. Permission to copy is restricted to an abstract of not more than 300 words.
ABSTRACT
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.
INTRODUCTION
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: (1) the soil resistance developed on the jacket leg extensions, and (2) 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