Design study of composite repair system for offshore riser applications

Abstract

Risers in offshore operations are subjected to corrosion during their service life cycle. The use of relatively inexpensive, high strength to weight ratio fibre reinforced polymer composite (FRPC) as a load bearing pipe repair sleeve is an emerging technology that is becoming common for offshore applications. Risers experience complex loading profiles and experimental investigations often incur substantial time, complicated instrumentation and setup costs. The main aim of this research is to develop a design tool for the repair of offshore riser that suffers from external corrosion damage on its surface using FRPC material. The simplest configuration of a fixed platform riser in the form of a vertical single-wall pipe is being considered. Characterization of the stress-strain behaviour of the FRPC laminate in the composite repair system subjected to various load profiles of a common riser is performed. The means of composite repair takes into account the ease of automated installation. The final repair method considers the use of unidirectional pre-impregnated (prepreg) FRPC that is assumed to be helically wounded around the riser. Finite element models of the composite repair system were developed via ABAQUS. Global analysis of the entire length of the riser was omitted as external corrosions usually occurs in a localised manner on the surface of the riser. Instead, local analyses were conducted where boundary conditions were applied to mimic an infinitely long cylindrical structure such as the riser. The local analyses FEA models were made to capture the stress-strain behaviour of the FRPC laminate subjected to different load profiles including static loadings such as internal pressure, tensile load and bending load. The design loads were calculated based on a limit analysis known as Double-Elastic Curve method developed by Alexander (2008). Proper element selection and mesh convergence were carried out to determine the FE model that can minimize the time and CPU memory needed for the simulation without compromising the accuracy of the results. The second part of this research integrated experimental tests to validate the FE model developed using the ABAQUS general purpose code. Due to constraints on cost and supply of materials and equipment, small-scale tests were conducted. Similitude relations were used to determine the scale properties between the model and the prototype. The final results showed that the FE model can represent the real-life tests of corroded riser repaired with off-axis FRPC laminate with great accuracy of more than 85%. Hence can be a useful tool for design and parametric study of the composite repair system. Using the validated FE model, an extensive parametric study of the composite repair system with respect to varying corrosion defects was conducted. The thickness and length of the repair laminate were compared to the ASME PCC-2 standard. Optimum thickness and length of the composite laminate were determined based on the maximum allowable strains computed using the Double-Elastic Curve method. In addition, varying fibre angle orientation of the unidirectional prepreg was considered as it is one of the main factors in helical winding. Based on the results from the parametric study, a simple relation was developed to predict the required thickness of the composite repair system subjected to combined loading. This relation combined with the developed FE model can be used to provide a quick design and performance validation of a composite repair system for offshore riser, which is the main novelty aspect of this research