Composite Riser Lifetime Prediction

Abstract

Abstract A background on phenomenological behavior of CRP laminates is given and the power of the superior laminate analysis theory by PUCK is outlined as a basis to build the confidence in the long-term performance of CRP materials for offshore tubular applications. Special attention is drawn to the procedure for lifetime prediction of a riser with CRP load bearing structure. The importance of progressive laminate failure analysis is highlighted as the tool to tailor the design so that the component failure is fiber controlled. Based on this requirement the procedure for fatigue failure prediction developed by DNV and others is recommended. As improvements to this procedure a unique fatigue test specimen is introduced and the basic idea of the remaining strength approach is implemented. Both contributes to the target to increase the level of consistency of the prediction theory compared to the real physical behavior of the product CRP laminate and therefore raise the reliability of lifetime predictions for composite offshore tubulars. Introduction ABB Vetco Gray and others have developed the technology to design and manufacture offshore composite tubulars. The present overall status of these technologies is probably best described in [1]. It is expected, that 2002 will become a major milestone with more field experience and initial commercial projects. Regarding lifetime requirements the most challenging type of the composite tubulars are production risers for the reason, that a lifetime of 20-40 years needs to be guaranteed. Remaining reservations of potential oil industry customers seem to be focussed on concerns related to the long-term performance. This paper highlights ABB's efforts to address this key task. Composite risers will essentially be made from carbon fiber reinforced plastics (CRP) for the load bearing tubular structure. This material in general has the reputation of very superior long-term, especially fatigue properties. Whereas the ratio of remaining strength after 107 load cycles to initial static strength for the first load cycle is about 0.3 for steel it is about 0.8 for unidirectional (UD) CRP. This superior property is a major driver for the usage of CRP for the application of production risers. However, the wall thickness needs to be tailored to given load cases and other requirements. A lifetime prediction resp. a long-term static and fatigue loading design procedure is needed. It may have some impact on necessary wall thickness and therefore on product weight and cost. Since CRP composites have been used for commercial applications in other industries, especially aircraft for about 30 years one would expect to find long-term and fatigue design procedures in the public domain. This is, however, unfortunately not the case. Major sources like [2] are limited to very general information. A cause to this situation may have been that the use of CRP in most cases was motivated by stiffness requirements, leading to wall thicknesses that are very safe in terms of strength, which was excessively proven by testing.