Composite Riser Tubes: Defect Tolerance Assessment and Nondestructive Testing

Abstract

ABSTRACT The work presented in this paper was carried out as part of a Joint Industry Program to qualify composite tubes for use as TLP production/injection risers. The program included the study of the influence of different kinds of damage on tube performance and fatigue life, the assessment of NDT methods for detecting damage and its evolution, as well as ways of predicting the ultimate performance of severely fatigue tested tubes. The following oil companies supported the program: Agip, Chevron, Conoco, Elf Aquitaine, Shell IPM, Statoil. BACKGROUND Initial Development The Institut Francais du Petrole (IFP) and Aerospatiale have been developing high performance composite tubes for oil industry applications for a number of years. Initial work concentrated on exploring the possibilities of different fibers and on finding technical solutions for details such as the tube end piece connection and internal/external liners for assuring pressure tightness. A large number of small diameter tubes were then tested in tension, pressure, bending, fatigue, ageing and abrasion. This preliminary work led to a design consisting of a filament wound tube with a hybrid structure, comprising circumferential layers and longitudinal layers (see Fig.1), reinforced with glass fibers and carbon fibers respectively, set in an epoxy resin matrix. The end piece design (see Fig.2) consisted of a conical steel insert, fitted with steel shell pieces, to which the composite was bonded. There are many possible applications of composite materials to the offshore oil industry (see ref.1). The initial one considered by IFP/ Aerospatiale was 4" J.D. Kill & Choke lines of drilling risers. Several such tubes were built and tested and in 1983 six 15 m long tubes were successfully tested on a drilling riser in the North Sea. Composite TLP Risers Since 1985 IFP/ Aerospatiale have concentrated on the development of larger diameter (9r) composite tubes for applications to production/injection risers of deepwater TLPs. Work on the composite risers of the PLTB 1000 project (see ref.2) showed the potential benefits that could be gained from the principal characteristics of composites, which included high strength and low weight, as well as their elasticity and their excellent behavior in fatigue and corrosion. These promised to allow large reductions in riser top tensions and also the elimination of tensioners, leading to economic benefits. In 1985 IFP mounted a joint industry program (JIP No.1), supported by seven major oil companies, with the object of building samples of composite risers for TLPs and subjecting them to thorough mechanical testing. The tubes were 9" ID, with a hybrid wall structure made up of glass fiber reinforced circumferential layers (total thickness 9.57mm) and carbon fiber reinforced longitudinal layers (7.28mm thick). A cross-section of the tube is shown on Fig.3, where the layered structure of the wall can be clearly seen.