Offshore risers

Figure 1. a) Testing Insulated Pipes b) Testing Offshore Risers. 

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. 

Finite element analysis (FEA) modelling of fiber-reinforced polymer (FRP) repair in offshore risers... 

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. 

The chapter concludes with the positive viability of using FEA to model the CRS. Fumre trends on the application of FRPC in repairing offshore risers such as optimization, automation, and possible studies are summarized. 

Conventional offshore riser repairs and their limitations... 

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

ASME B31.4 (2006) Pipeline Transportation for Hydrocarbon Liquid and Other Liquids and ASME B31.8 (2003) Gas Transmission and Distribution Piping System are standards related to the design of pipelines which provide information on the stress and strain limits of industrial oil and gas pipelines. Although not specihcally intended for offshore risers, these standards provide a good foundation for pipeline design that in turn defines the requirements on the repair using FRPC materials. 

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. 

When an offshore riser is subjected to combined loadings, it is bound to experience different levels of principal stresses, that is, axial, hoop and radial stresses. However, the out-of-plane radial stress is negligible in the current case due to the usage of the thin shell formulation. A variation in the design of wrapping angle/orientation of the FRPC is prudent in providing an effective reinforcement to resist potential failure in these principal directions. 

This section provides further information on research, industrial projects, pubhcations and current development of the use of FRPC in repairing offshore risers. 

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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