Abstract

The Holstein truss spar is the largest spar hull ever built with a diameter of 45.5m. Holstein's strakes are unique for this and many other reasons. Vortex Induced Motion (VIM) model tests showed that in this area of high loop currents in the Gulf of Mexico, hull strake coverage and size as a percentage of diameter would need to be increased. This resulted in strakes that were closer to the free surface and therefore subject to much larger wave action loads.

Typical spar strakes consist of a main top plate and a supporting lower plate that form an enclosed triangular cross section against the hull. Previous spar strakes have large holes for passage of mooring lines and construction openings into this enclosed space. Holstein holes for mooring chain were designed smaller than other spars and construction holes were closed to increase the efficiency of the strake. Furthermore, external strake stiffeners and anodes were moved or removed to clean up the exterior strake surface.

New analysis procedures and techniques were found to be necessary to analyze and design the Holstein strakes. As these processes were being developed and implemented, the hull fabrication was well underway, adding to the overall challenge. This paper addresses how new procedures, through model testing and hydrodynamic analyses, were developed and implemented. The paper describes the process of load development for both strength and fatigue cases, and how the resulting loads were applied in a detailed finite element analysis. It outlines the techniques applied to post-processing the detailed stress results from the finite element software, and how fatigue life estimates were obtained using a filtering process to achieve greater efficiency. Finally it gives sample details from the structure that show design enhancements that were required as a result of the analysis.

Introduction

The Holstein Field is located in Green Canyon Blocks 644 and 645 in the Southern Gulf of Mexico, 190 miles south of New Orleans, Louisiana in 4,344 feet water depth. BP Exploration and Production Inc. and Shell Offshore Inc. are each 50 percent owners in the project, with BP the operator.

The Holstein field is developed with a truss spar with a semi-taut chain and steel wire mooring system with sixteen mooring lines. The Holstein spar is the largest spar ever built. The design topsides payload is approximately 26,000 short tons. It is the first spar to utilize hyhdraulic riser tensioners instead of buoyancy cans. The diameter of the spar is 45.5m (149.28 ft) and the hard tank depth is 88.74m (291 ft).

Strake Design Considerations

Classic spar hulls and the hard tank of the truss spar hulls are typically fitted with helical strakes to minimize hull VIM due to current. The Cell spar similarly has strakes along its length. Due to construction and transportation considerations, the strakes on truss spars built to date do not provide full coverage, but are truncated or have reduced height along one side, as shown in Figure 1.

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