Master thesis : Fatigue Analysis of Temporary Offshore Structures using Monte Carlo Simulations

Abstract

The Orion, an offshore heavy lift DP3 installation vessel owned by DEME Group, is deployed worldwide to install monopiles of up to 2,500 tons with its 5,000-ton crane. A seafastening structure holding the monopiles on board of the vessel is exposed to cyclic loading due to waves encountered at sea. The fatigue life of this monopile seafastening, therefore, needs to be analyzed to ensure its integrity during installation and transport operations. For this, the applicability of different fatigue estimation methods is investigated throughout the thesis. Firstly, simplified fatigue analysis methods are tested to obtain an order of magnitude of the expected fatigue damage and to determine its criticality. The lambda factor method is adapted to offshore structures providing an approach to represent a variable load history by estimating equivalent stress cycles at a constant stress range. In another simplified method, the fatigue damage is approximated using a weighted sum approach. By calculating the fatigue damage for one hour spent in specified nautical areas, the total damage of different operational profiles can easily be assessed. This approach is further developed by applying the Monte Carlo technique to determine the damage expected for one hour of operation. Secondly, Monte Carlo simulations are implemented in Python to study the fatigue life of the seafastening in detail. In the simulation, the considered sea states are defined based on scatter diagrams and the vessel’s motion response is determined for every hour of operation. The stress ranges resulting from the accelerations of the monopile, as well as the number of stress cycles that the seafastening structure on the vessel experiences are calculated. Based on the stress occurrences, the partial fatigue damage of every hour and, ultimately, the total fatigue damage are estimated. By varying the input parameters of the analysis and evaluating the effect on the fatigue damage, recommendations on the optimal operational profile with respect to the fatigue life of the seafastening are made. With a predicted fatigue life of 660 years, the seafastening is not expected to fail due to fatigue damage during the specified operational period of 10 years. This result is obtained with the Monte Carlo approach which considers the most accurate representation of the sea state and the seafastening’s reactions to loads out of the tested methods. Nevertheless, the weighted sum method is found to be a valuable tool for fast fatigue estimations providing accurate results but missing the effects of quadratic roll damping.