Comparative Analysis of 12,500 ft. Water Depth Steel and Advanced Composite Drilling Risers

Abstract

Abstract With deepwater drilling becoming an area of increased interest, the National Institute of Standards and Technology (NIST) sponsored composite drilling riser program objectives were extended in 1997 to include a comparative analysis of steel and advanced composite drilling risers for drilling in water depths down to 12,500 ft. A water depth of 12,500 ft was established as the maximum water depth thought to be of interest worldwide. Using requirements established by DeepStar and Reading & Bates, Northrop Grumman Marine Systems and ABB Vetco Gray synthesized a steel drilling riser system and an advanced composite drilling riser system such that the two drilling risers could be compared with regard to key system characteristics such as top tension required, buoyancy required, total system weight and stacked volume. This paper presents preliminary results of the comparative study which identified a weight savings of more than 50% and a volume reduction of 33% as benefits gained by the use of advanced composite materials for fabrication of the riser tubulars. Reduced buoyancy requirements are shown to be a major contributor to weight and volume savings, and also to reduced hydrodynamic drag forces. The comparative analysis performed included dynamic analysis in the hang-off modes using the Northrop Grumman developed Modal Dynamic Riser Analysis (MODRAN) program. MODRAN allowed comprehensive 3D analyses of coupled transverse and axial dynamics during hang-off for broad ranges of conditions. The differences (and similarities) in hang-off response are described. Introduction In April 1995, an industry/university team started work on a three year, $4.8 Million, NIST Advanced Technology Program (ATP) cooperatively funded project to apply advanced composite technology to a class of tubulars known as marine drilling risers. Previous studies have shown that composite tubulars offer strong weight advantages over steel, especially in very deep water.1,2 The NIST project team members are: ABB Vetco Gray, Northrop Grumman Marine Systems, DeepStar, Reading & Bates, Hexcel and Offshore Technology Research Center (a National Science Foundation Engineering Research Center at Texas A & M University and University of Texas). Significant progress has been made on this project3,4; however, when the project proposal was submitted in mid-1994, the focus for the development effort was on what was then considered to be "deepwater" systems of up to 6000 ft. water depth. Recent Gulf of Mexico Minerals Management Service lease auctions have attracted significant interest, and many of the tracts of interest are in water depths greater than 8000 ft. The rush to deeper water depths was not foreseen by our team when the proposal was submitted. Early in 1997, our team decided that we would have to change the focus to be consistent with the emerging marketplace. The team established a target of developing an advanced composite drilling riser system suitable for service in water depths of up to 12,500 ft. A modified scope of work was developed, appropriate budgets were defined and a request was s.ent to NIST for approval of the program revisions.