Presentations

Abstract

The vessel which will be used for jacket transportation is usually not known during the early design phases of an offshore wind project. A simplified transportation fatigue analysis based on expected vessel characteristics may be undertaken, or fatigue allowances based on prior experience are assumed. During Detailed Design, the motions of the contracted vessels and availability of more accurate estimations of jacket-on-vessel durations can lead to fatigue damages above early estimates. For a recent design project in Saint Brieuc, France (Ailes Marines), the traditional static transportation fatigue analysis at Detailed Design showed total fatigue damages (whole life damages, including in-place damages) which would have led to significant limitations during transport. However, these type of transportation fatigue analyses have several limitations, including lack of consideration of vessel Response Amplitude Operators (RAOs) phasing, conservative assumptions around surge and sway gravity components, inability to consider wave spreading, and inability to consider non-linear Stress Concentration Factors (SCFs). To address these limitations, a dynamic time-domain transportation fatigue analysis was developed. This analysis captured vessel motion characteristics more accurately, leading to stress time history of hotspots for rainflow counting. Due to the computational intensity of dynamic time-domain analyses and project time constraints, it was not feasible to analyse every load condition. Therefore, the damages from time-domain assessments for specific environmental conditions were compared to those from static assessments to determine "Fatigue Benefit Factors" (FBFs). These were derived for various significant wave heights, peak periods, wave seeds, wave directions, and wave spreading coefficients. These factors were subsequently rationalised into a set of conservative FBFs for various fatigue details that were applied to the static transportation assessments. This approach significantly reduced the calculated transportation fatigue damages using static method and ultimately enabled Ailes Marines to demonstrate acceptability of total fatigue damage for design certification and validation.