Installation of Subsea Equipment with focus on Slender Objects
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- Institutt for marin teknikk 
This thesis presents the investigation of an installation of a spool structure using a simulation software called AquaSim. AquaSim gives the possibility to calculate and visualize the internal stress distributions in the components, as well as the interaction between fluid and structure. In the simulations, a provided spool model from Technip is used. By applying this model to the simulation of relevant Ultimate Limit State, a strategy to follow to increase the design limits for subsea lifting operations is found. The strategy is to look at different time intervals for different sea states to find the maximum dynamic forces acting on the spool. By using these maximum forces, a statistical strategy based on avoiding the most extreme values and, therefore, increase the operational window is proposed. Critical internal forces were identified to study maximum axial and moment forces in the spool. Two elements with maximum axial forces (element 6 and 17), one element with maximum bending moments (element 24) and the axial forces in the wire was studied when lifted through the wave zone. The highest value for 20 different time intervals for four different sea states is shown as a Gumbel distribution to find rejected values based on a 90% confidence interval. The results showed that the dynamic forces were decreasing when the zero-up- crossing period increased from 6.20s to 8.04s for wave height 2.5m in most of the studied elements and the wire. The dynamic forces were doubling and tripling respectively to moment and axial forces when the wave height doubled from 2.5m to 5m. The forces increased for the wire and moment forces in element 24, and decreased in axial forces in element 6 and 17 when increasing zero-up-crossing period from 8.12s to 10.05s for wave height 5m. The distributions were, with some exceptions, right-skewed, meaning that there are a couple of (one or two) extreme values for each distribution. A 90% confidence interval seems, therefore, reasonable to use to avoid the most extreme forces and to find the likelihood of a successful operation in a subsea lift. By having distributions like these for conventional structures and different sea states, the probability of a successful operation can quickly be estimated. Based on probability distributions for maximum values in the most critical sections of the structure, the work has shown that it is possible to find a strategy to follow to widen the operational window for such lifting operations. The results do however not take into consideration the entire spool structure but only the spool its self. There are more critical internal forces in the bracing than in the spool. The probability that the structure is hitting the vessel while lowering or that snap loads occur, also depends on the weather, and will, therefore, limit the operational window.