Evaluation of Fatigue during Pipeline Installationin Ultra Deep Waters
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The pipeline installation process as a part of developing oil and gas production fields in ultra deep water is a challenging task as it involves harsh environmental conditions, limited lay vessel capacity and a set of complicated and high standard procedures that must be fulfilled during the installation process. In order to learn about fatigue damage characteristics that occur with a pipeline during the installation process in ultra deep water, a set of analysis including static, dynamic and fatigue analysis is performed using the Orcaflex software version 9.2a for a case study located in North Sea area.Static, dynamic and fatigue analysis are performed for three pipeline installation system models with topangles of 4°, 7.4° and 40°, respectively. A pipeline installation system model with a top angle of 7.4° is selected as the main model for fatigue analysis since it gives the minimum forces and moments and the top angle variation values in the dynamic analysis are not more than 30° (we have also noted the limitation on the tensioner capacity of the vessel).The dynamic analysis is performed for a water depth of 2000 m and significant wave heights of 2, 3 and 4m with various combinations of wave and current directions. The dynamic analysis results includingvariations of tensile stresses and bending stresses are combined with the JONSWAP wave spectrum forsignificant wave heights of 2, 3 and 4 m and wave periods of 5-13 s to calculate the fatigue damage of the pipeline during the installation process. Fatigue analysis is performed based on the S-N curve approachand Miner’s rule.The dynamic analysis results of the case study show that there is a compression force in the pipeline especially at the touchdown area. Compression in a pipe can be avoided by increasing the tension through top angle or layback length adjustments or by providing additional tension from the tensioner.The fatigue analysis results show that high fatigue damages that occur on the pipeline members located at the sea surface area are the effects of the lay vessel movements and high fatigue damages that occur onthe pipeline members located at the touchdown area are due to a high bending stress variation that depends on the bending radius and the top angle. The conservative fatigue analysis results for the case study, furthermore, indicate that significant waveheights of 3 and 4 m with wave periods of 6-13 s give large contributions to the resulting high fatigue damages. Therefore, a significant wave height of 2 m with wave periods of 5-13 s will be suitable wave conditions for conducting pipeline installation process at the case study location (North Sea area).