Sensitivities in fatigue analysis of offshore wind turbine support structures

A fatigue analysis gives an answer to the question of whether a structure will resist fatigue throughout its lifetime or not. The hot spot stress approach is one of the methods used to find the solution, by looking at points around the circumference of a weld, and finding the stress values at these...

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1. Verfasser: Hammerstad, Benedicte Hexeberg
Format: Dissertation
Sprache:eng
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Zusammenfassung:A fatigue analysis gives an answer to the question of whether a structure will resist fatigue throughout its lifetime or not. The hot spot stress approach is one of the methods used to find the solution, by looking at points around the circumference of a weld, and finding the stress values at these points. This master thesis investigates fatigue analysis using the hot spot stress approach, through a sensitivity analysis By changing what parameters and methods the fatigue analysis uses, several fatigue damage values are calculated for each joint. The design chosen for the study is a single support structure for an offshore wind turbine (OWT) the UpWind reference jacket from Phase I of the Offshore Code Comparison Collaboration Continuation (OC4) project. [5] The jacket structure supports the NREL 5-MW reference wind turbine. [6] It consist of K-joints, X-joints and Y-joints. With fatigue damage being the design driving factor, the method for estimating fatigue damage may necessitate different design in the structure. [12] That means that different fatigue damage estimations may result in different costs and a more reliable structure. As anything can be safe and reliable if you just overspend and over-design it, an ideal fatigue analysis can give reliable results, without being too conservative. During the master work, an application was developed in the .NET Framework to work in conjuncture with FEDEM WindPower, a simulation program used for wind turbine systems. The developed application performs all extractions and calculations needed for estimating the fatigue damage using the hot spot stress method. The HSSs calculated are linearly extrapolated structural hot spot stress, with the nominal stress in the chord included for the HSSs on the chord side of the weld. The application performs the fatigue damage estimations using several procedures of the hot spot stress approach. This leads to several fatigue damage values for each joint, giving the basis for the sensitivity analysis concerning the hot spot stress approach. Common for all variations in the options, are the load simulations and the usage of hot spot stress approach, rainflow counting, S-N curve and Palmgren-Miner rule. This is all explained in chapter 2. They also have in common general fixity conditions, chord-end fixity parameter equal to 0.7, and for the joints with two braces, loads occur in both. The variations consist of the L parameter used in the SCF equations, the order in which maximum