Abstract

Vortex Induced Vibrations (VIV) may cause significant fatigue damage in Steel Lazy Wave Risers. The severity of the VIV fatigue damage is dependent on the natural frequency of the riser and the vortex shedding frequency which varies according to long-term current profiles and distribution. VIV suppression devices such as strakes, fairings and integrally straked buoyancy modules are used on riser pipe to reduce the fatigue damage caused by vortex induced vibrations. However, the higher the coverage of these suppression devices, the higher the Engineering, Procurement, Construction and Installation (EPCI) cost of the riser.

Strakes are the most commonly used suppression devices for deep water risers. The buoyancy section of the Steel Lazy Wave Riser needs to be straked if it becomes a driving power in region for vortex induced vibrations. As VIV strake coverage increases, the installation time for the riser increases which can significantly affect the EPCI cost of a riser due to the high day rate of installation vessel. Hence optimizing the coverage of strakes can result in significant savings. Typical strake coverages on risers can range from 30–90% of riser suspended length for Gulf of Mexico projects while in South America it can range up to 100% including the buoyancy region.

Genetic algorithm is applied within an artificial intelligence tool to optimize the coverage and location of strakes along the riser automatically using an objective function with the aim of minimizing the strake coverage length. The algorithm for the proposed artificial intelligence tool along with the objective function are presented in the paper followed by a few examples showing the efficacy of the tool. While optimizing the coverage and location of strakes on risers is an iterative and cumbersome process, the use of the artificial intelligence tool presented in this paper will help an engineer speed up the strake coverage design and improve the quality of VIV analysis performed together with savings on EPCI cost of steel lazy wave risers.

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