TLP Foundation Design And Installation For 2,000 To 4,000-Ft Water Depths

Abstract

ABSTRACT This paper describes the considerations and procedures developed for the design and installation of tension leg platform (TLP) anchors for water depths from 2000 to 4000 feet. Both piled and gravity anchor foundations are discussed. A pile foundation design is described that consists of four eight pile templates held together with a space frame which allows the templates to be launched and lowered to the sea floor as an integrated unit. The space frame has been designed to withstand the hydrostatic pressure at a 4000 foot water depth. Also described is a gravity anchor foundation system which consists of steel tanks that are filled with high density ballast to achieve the necessary in-place weight. In-place design and installation considerations are identified and discussed. INTRODUCTION The purpose of this study was to investigate alternative foundation systems for tension leg platforms (TLp1s) for typical locations in the Gulf of Mexico and the North Sea. Water depths from approximately 2000 to 4000 feet were considered. Typical site information, environmental data, and loading conditions were assumed based on published information and on data derived from other Fluor in-house studies and designs. Two basic types of foundations were considered. These were (1) the piled foundation and (2) the gravity foundation. For each of these two basic foundation types, options exist either to make the foundation independent for each leg of the TLP or to have the foundation as one or more integral units as illustrated in Figures 1, 2, 3, and 4. DESIGN CRITERIA Following is a summary of the criteria that were selected to represent the typical site locations and TLP configurations that were common to the designs for both types of foundations. Description General The pile foundation systems considered feasible were the independent pile template system as shown in Figure 1 and the integrated template system as shown. in Figure 2. The design considerations for the two systems are similar since each corner template is required to resist separately the axial and lateral loads imposed by the TLP tendons. Corner Template Based on initial tendon and pile configurations derived in previous in house studies, each corner template for a Gulf of Mexico installation was designed to be approximately 40 feet square with eight piles spaced around the perimeter at the corners and at the center of each side (See Figure 5). Four tendons are anchored in a twenty-foot square, with the tendons intersecting the main diagonals of the pile template. The corner template considered for installation in the North Sea was similar in design concept to the Gulf of Mexico template but was of an octagonal shape (See Figure 6). This shape was used to accommodate a different layout of the TLP tendons. For the design of the template and piles it was assumed that three of the tendons are required to carry the entire design load. This assumption causes the foundation template to be eccentrically loaded, resulting in unequally loaded anchor piles.