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Abstract Deep Draft Caisson Vessels (DDCV) or spars have been used in the Gulf of Mexico in water depths up to 4,800 ft and their use is being considered in ultra-deep waters up to 10,000 ft. A major study sponsored by DeepStar was used to provide a systematic and credible evaluation of DDCV mooring and riser technology alternatives for a challenging 10,000 ft water depth application in the Gulf of Mexico. This paper provides an overview of the DeepStar study objectives, scope and the significant conclusions. A base case DDCV was first developed for 10,000 ft water depth using extensions of current technologies for hull, mooring and risers. Several cost saving alternatives were then considered. These included polyester mooring, composite risers and a high level evaluation of other lightweight risers. A high pressure and high temperature reservoir and a high current environment (HP-HT-HC) were considered. The comprehensive study resulted in a number of significant conclusions. Introduction This paper provides an overview of a major study carried out in DeepStar Phase V (Ref. 1). The primary objectives of the study were to assess credible technology alternatives for mooring and top-tensioned production risers (TTRs) for a large ultra-deepwater DDCV, and to quantify the total system incentives for pursuing synthetic moorings and composite production risers. In addition to the primary scope described above, the study team was also asked to seek industry input and to provide a preliminary assessment of other potential alternate lightweight risers. The six month study was initiated in October 2001, with Halliburton KBR as the primary contractor. ExxonMobil was the oil company champion and the manager of this study within DeepStar. DeepStar is a research and development consortium of 15 oil and gas companies and about 50 service companies (see www.deepstar.org). The design basis chosen for the study included the following challenges:A high pressure high temperature fieldA very high current environmentLarge topsides of 40,000 short tons operating weightA significant number of steel catenary risers (13 SCRs and 5 umbilicals).20 top tensioned risers (TTRs). The scope of the study included consideration of several configuration cases listed in Table 1. The initially agreed scope included cases 1, 2 and 3. Cases 1A, 1B, 2A and 3A were added subsequently as the study developed. The scope of the study was comprehensive, including:DDCV hull sizingMooring designGlobal performance calculationsMooring fatigue analysisRiser configuration designProduction/drilling riser clearance assessmentProduction riser strength and fatigue calculationsHull scantling checksHull fabrication, transportation and installation methodsMooring installationCost and schedule estimatesConsideration of other lightweight riser materials Mooring design and mooring fatigue analysis were performed for both steel and polyester mooring systems. Riser clearance and strength, including fatigue, were analyzed for both steel and composite risers. |
