Composite riser repair

This chapter will first outline the conventional offshore riser repair techniques and their limitations which, in turn, provide the motivation for adoption of composite repair. A brief discussion about the application of FRPC in pipeline repair, its limitations, current practices, and material types will be given. A subsequent section provides information on the industrial design and safety standards used in designing and assessing the performance of offshore risers, corroded pipelines, and pipelines repaired with FRPC. The typical loading conditions of an offshore riser are also discussed so that the requirements of CRS can be understood. 

Additional information such as material and bonding properties between steel and composite can be obtained by reviewing literature related to composite patch repair on a steel structure. A comprehensive amount of studies have been conducted and can be used as a basis to model the bonding between the composite repair material and steel riser. 

Key words fiber-reinforced polymer composites, pipes, risers, bonded joints, composite repair systems. 

A very recent study to investigate composite repair of offshore riser tubes has been described by Alexander and Ochoa (2010). Prefabricated carbon/epoxy half shells were adhesively bonded to steel tubes, and proved satisfactory during testing under internal pressure, tensile, and flexural loads. 

Alexander, C., Ochoa, O.O., 2010. Extending onshore pipeUne repair to offshore steel risers with carbon-fiher reinforced composites. Composite Stmctures 92, 499—507. 

Limitations of the appiication of composite repair for offshore risers... 

Before conducting a repair on a corroded pipe, analysis must be done to determine whether a repair is sufficient to restore the strength of the riser. This can be evaluated using the codes provided in ASME B31G (1991). The standard used here is compliant for onshore pipelines where the equations are only functions of the corrosion dimensions and internal pressure. Nevertheless, it provides a basis for determining the required thickness of the composite laminate. 

In FEA, if the ratio of wall thickness to radius is equal to or less than 10%, the component can be considered a thin walled stmcture. For both the riser and the composite repair sleeve. 

In a CRS, the composite laminate is bonded to the steel riser surface such that effective load transfer from the weakened riser to the composite can take place. Within the FEA model, perfect bonding is assumed between the contact surface interfaces of the riser and the composite laminate. This is an idealization of the FRPC repair in offshore risers. However, variation in material types, installation techniques and parameters tend to result in localized microvoids between surface interfaces of the repair (i.e., between the riser-FRPC and between FRPC laminates) where less than perfect bonding is often observed. 

Table 9.2 Mechanical properties of the two types of laminates used in the study of the composite repair of corroded riser (Soden et al., 1998h)...

Stress Engineering, Inc. is one of the major companies that has run multiple full-scale tests to validate the performance of numerous onshore pipeline composite repair products that are available in the market. Full reports documenting the vahdation of composite repair products are available (Alexander, 2005 Worth, 2005 Francini and Kiefner, 2006). Various publications on the development of the CRS for offshore riser application are also available under Chris Alexander of Stress Engineering, Inc. A program cosponsored by the Pipeline Research Council International, Inc. (PRCI) involves the full-scale testing of corroded risers with a CRS placed in a seawater test facility for 10,000 h subjected to combined pressure, tension and bending loads (Alexander, 2012). 

Alexander, C., 2007. Development of Composite Repair System for Reinforcing Offshore Riser (Ph.D. thesis), Texas A M University. 

Alexander, C., April—May 2012. Design of an optimized composite repair system for offshore risers using integrated analysis and testing techniques. In Offshore Technology Conference. http //dx.doi.org/10.4043/23164-MS. 

Alexander, C., Cercone, L., Lockwood, J., 2008. Development of a carhon-fibre composite repair system for offshore risers. In The 27th International Conference on Offshore Mechanics and Arctic Engineering, OMAE2008. The American Society of Mechanical Engineering, Estord, Portugal. 

Leong, A.Yi., Leong, K.H., Tan, Y.C., Liew, P.F.M., Wood, C.D., Tian, W., Kozielski, K.A., 2011. Overwrap composite repairs for offshore risers at topside and splash zone. In 18th International Conference on Composite Materials, Korea. 

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