Simplified Fatigue Assessment Method Applied to a Floating Offshore Wind Turbine Substructure

Offshore structural engineering has new challenges in the renewable energy sector due to the global trend to reduce the use of fossil fuels. However, structural phenomena (stresses, strains, buckling, compressions, rotations) are always the same. Here we analyze the Fatigue Limit State associated with the accumulation of damage caused by cyclic loading and unloading acting on offshore structures mainly subjected to waves during their design life. Due to this phenomenon, cracks can occur and propagate in areas of high stress concentration until they cause the fracture of the element or the failure of the whole structure even at very low nominal stress levels. This work presents the simplified fatigue evaluation method by analyzing the most critical location where fatigue failure of the support substructure of a Floating Offshore Wind Turbines, hypothetically located in the Gulf of Tehuantepec, Mexico, could occur. This method proposed by the American Bureau of Shipping basically consists of calculating a maximum allowable value of a range of stresses for a welded connection and comparing it with the maximum acting stress range during the design life. The FLEXCOM computer program was used to generate the global model and obtain the maximum acting stress range. The allowable stress ranges obtained with this methodology are mainly affected by the depth of operation and the number of cycles along the design life, so the deeper the operation zone is the higher the allowable stress ranges will be obtained on the contrary as the cycles increase during the design life the allowable stress ranges decrease. This methodology is considered in an engineering way as conservative, so its use is recommended when the values of the allowable stress ranges are far from the acting ones.