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The design of catenary risers, riser towers, and export lines is key to oil and gas production in the deep ocean. Winds, waves, surface currents and current profiles are essential inputs to the design of these riser systems. With regards to current loading, the variety and abundance of current profiles can make it difficult to determine which profiles are most appropriate for determining extreme and fatigue loading on risers. The choices become increasingly difficult on the frontier of deepwater oil and gas developments as risers are increasingly being required to successfully operate in harsher environments and deeper water. However, longer and more detailed current measurements from in-situ deployments continue to become available. Two or three year current profile records are now common. In order to progress in the specification of current profiles and in the methodologies of using these profiles in the riser design, four oil & gas companies, BP Exploration Operating Company Limited, PETRÓLEO BRASILEIRO S.A. - PETROBRAS, Statoil ASA and TOTAL E&P Recherche Développement SAS, have funded the WACUP project (Worldwide Approximations of Current Profiles). The first objective of the WACUP project is to establish best practices for reducing large current profile databases into a smaller, representative set of profiles that can be used for designing risers. Three techniques have been compared in their ability in reducing measured in-situ databases: Empirical Orthogonal Function (EOF), Self Organizing Maps (SOM) and classical Current Profile Characterisation (CPC). For Vortex Induced Vibration (VIV) fatigue analysis we evaluate the skill of these three techniques in estimating VIV damage to a Steel Catenary Riser (SCR). Our measure of skill is the comparison of the results from the reduced data sets with results from the complete database. The standard techniques are modified, improving the accuracy of the database reduction. We recommend that any of these techniques can be made suitable for concept selection or preliminary engineering, but it may be desirable to run the full set of Gold Standard profiles for full detailed structural design. The second objective is to better understand the means to deal with extreme current profiles, in one part for fatigue damage and in another part for static loads analyses, to compare to traditional methods and to propose, if necessary, better practices. For fatigue, due to the complexity of the relation between current profile shape and damage, extrapolation on Gold Standard damage is the only way to produce quality n-year damage return values. Concerning extreme static response, the first analyses show that CCA profiles, used in exhaustive directions, produce realistic n-year return values. The ability of the available numerical current models to complement the in-situ measurements is also assessed. Models are superior to measurements for providing a large scale regional interpretation of key oceanographic processes. They can also be used to cost-effectively quantify spatial variability in the current regime. However they must be properly validated and calibrated before use in any engineering application. The primary source of data for characterisation of current profiles in riser design remains full water column, site specific, in-situ measurement. |
