Composite Production Riser Reliability Assessment: The Influence Of Probabilistic Fiber-Composite Strength
Authors: Wang S.S. , Lu X. , Gong Z.Q. 
Published: October 2008

ABSTRACT
Composite production riser (CPR) joints are being seriously considered in the development of deep water tension leg platforms (UPs), because of their inherent light weight, superior fatigue and corrosion resistance, and outstanding specific strength and stiffness properties. Current efforts on the development of CPR joints have been mainly focused on low cost manufacturing and failure strength evaluation of CPR tube body and CPR joint connection. The important issue of system dynamics of UPs containing multiple CPR strings, has not been addressed. In this paper, system analysis of a UP containing 16 CPR strings and 12 tendons subjected to Gulf of Mexico environment loading have been conducted. The riser system is configured for 3,000 ft water depth with CPR joints, standard steel riser joints, splash zone joints, stress joint, and top tensioners. The study embraces several disciplines, including naval architecture, riser dynamics analysis, and composite failure mechanics to develop an iterative algorithm for evaluation of the top tension and stress joint requirement. Specifically, optimum top tension requirements have been determined based on riser dynamics and the failure envelope of the CPR joints. For comparison, the optimum top tension requirements are further used to size the UPs with all-steel riser and with CPR. For the 3,000 ft water depth case study, reduction in riser weight is magnified by 3.31 times in the UP size. It is demonstrated that the weight reduction in the riser string is nonlinearly related to the tensioner requirement and UP size.

INTRODUCTION
Composite materials offer many advantages for deep water applications because of their excellent corrosion and fatigue performance, high strength-to-weight ratio, and design flexibility. As the offshore industry moves aggressively to pursue deeper water developments, composite materials are finding a wide range of new applications for both topside and subsea structures. While most of the current applications are secondary structures in the top side facilities, several major U.S. and international initiatives are underway to develop primary load-bearing system components [1–6] In deep water exploration and production, significant advantages may be realized when composite materials and structures are incorporated in the offshore system design strategy during conceptual and pre-engineering stages. For a UP, the effective use of light weight composites may result in a significant cost savings and, perhaps, also enabling benefits. Synergistic reductions in the deck loads, hull, tendon mooring system, and platform size account for the reduced topside facilities weight [2]. This study couples fundamental failure mechanics of composites, dynamic analysis of the composite riser strings, and naval architecture to size the UP structure. It is demonstrated that an integrated interdisciplinary effort is required to overcome technological barriers in the utilization of composite materials in the offshore industry and effective UP design.

COMPOSITE PRODUCTION RISER JOINTS
CPR joints are currently being developed in a major project jointly supported by the industry and DoC NIST/ATP. The tube body of the CPR joint is a hybrid material system design in which the axial load is carried by helical carbon-fiber plies and hoop pressure is carried by both carbon and glass fibers wound close to the hoop direction.