Design Considerations Particular to SCRs Supported by Spar Buoy Platform Structures
Authors: Jesudasen A.S. , McShane B.M. , McDonald W.J. , Vandenbossche M. , Souza L.F. 
Published: October 2008

This paper broadly discusses the design considerations particular to the large diameter export steel catenary riser (SCR) systems for the Holstein and Mad Dog Spar Buoy facilities. It also focuses on fatigue phenomena that are a challenge, but essential, to quantify to ensure feasibility and delivery of an optimized design solution.

Introduction

BP and its partners are developing several deepwater prospects in the Gulf of Mexico. These include the Holstein, Mad Dog, Thunder Horse and Atlantis floating production facilities in water depths ranging from 4500 feet to 7300 feet. Oil and gas will be exported via the Mardi Gras Transportation System, the deepwater segment of which consists of 16-inch to 28-inch oil and gas pipelines, steel catenary risers, and subsea wyes for current and future tie-ins.

The focus of this paper is the export riser systems supported by the Holstein and Mad Dog Spar Buoy platform structures. Each facility has two SCRs; the diameters of which are 24-inch (Oil, Caesar export system) and 16-inch (Gas, Cleopatra export system).

This paper discusses the effect of combining the predicted fatigue damage for these large diameter risers, as well as understanding how riser routing impacts and is impacted by vessel motions, to establish the optimum location for attaching the SCRs near the keel line of the Spar. This is not the standard hang off location of an SCR, as it incurs a significant increase in complexity of the system installation. Nevertheless, it was essential for Holstein and Mad Dog to achieve a feasible large diameter export riser system.

At this hang off location, the vessel motions that dominate fatigue are different from those that traditionally induce the highest damage to the riser. To resolve these fatigue issues required an iterative interactive process between the riser and vessel design teams to optimize the hull motions to minimize damage. This process was a requisite, not only to optimize the design solution, but also to deliver a feasible system configuration.

Holstein and Mad Dog System Description

The Holstein and Mad Dog facilities are located in Green Canyon blocks GC 645 and GC 782 respectively in a water depth of approximately 4400 feet. The Holstein and Mad Dog facilities are some of the largest Spars that have been built to date, with the Holstein spar being the largest, with a total length of 746 feet and diameter of 150 feet. Both Spars are a truss type, characterized by a large cylindrical buoyant hard tank penetrating the sea surface, below this is an open truss type structure terminating at the vessel keel with a smaller buoyant soft tank. The export systems depart the facilities in a northeasterly direction. Figure 1 shows a plan view of the two facilities, the export risers and associated Caesar and Cleopatra export pipeline routes in the immediate vicinity. A more detailed summary of the Spars and riser configuration is captured in Table 1, which lists the key spar configurations and Figure 2, which shows a typical general arrangement of an export SCR.