In regions of high current velocity such as East Africa, South America and the GoM, VIV (Vortex Induced Vibrations) of drilling riser systems is a very real risk. Much work has been carried out by the offshore industry in order to predict the VIV and resultant fatigue in top tensioned production and drilling risers and to develop standardized methodologies for performing these calculations. In addition a number of commercially available VIV analysis tools have been developed over the years. These VIV analysis tools and industry methodologies however, are focused on the global response of the riser string alone and have not been developed to account for the intricacies of wellhead loading & stress distributions. VIV response of the top tensioned riser (TTR) system (whether production TTR or drilling riser) results in load transfer into the wellhead via VIV induced motions. As commercially available VIV analysis tools generally only consider a riser model consisting of a slender beam, the more complex geometry and load path associated with the wellhead and conductor are not accurately modeled. Thus VIV induced damage of the wellhead cannot be predicted with confidence using these VIV analysis tools alone.

Based on a review and critique of both existing and proposed methods it has become apparent that existing methods for the calculation of VIV induced fatigue of the wellhead and conductor system are overly conservative and do not capture the level of detail required to predict the level of fatigue damage with confidence. Thus a revised methodology for calculation of wellhead and conductor fatigue due to VIV is required to build up a more representative picture of the associated fatigue damage accumulation. Proposed VIV fatigue calculation methodologies, including the use of VIV analysis tools alone and application of the results of the VIV analysis tools in combination with non-linear global finite element models, are described and the challenges associated with each are discussed herein.

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