Design and Automated Optimization of Mooring Systems for Shallow Water and Harsh Environments
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- Institutt for marin teknikk 
The present market places significant cost pressure on the design of a mooring system. It is there- fore important to be able to produce a mooring system at the lowest possible cost, and at the same time, to ensure a safe design that meets the requirements set by standards. Various optimization procedures are available for this purpose, although limited work has been conducted regarding the automated cost optimization of mooring systems. This is the governing motivation for the work executed in this thesis.A model of the semi-submersible Safe Scandinavia is established in SIMO, together with suitable mooring systems. One system with 12 lines of a chain-steel wire-chain configuration, termed the steel system, and one system with 12 lines comprised of a chain-polyester rope-chain configuration, which is referred to as the polyester system, are considered for the optimization. The main objective of this thesis is to optimize both the polyester and steel system with constraints that correspond to four different cases, and evaluate the results and investigate the sensitivity of the optimization formulation. The optimization problem is formulated in the SIMA workbench developed by MARINTEK. The mooring systems are optimized with respect to hardware cost, whereby the cost of each material (chain, polyester rope and steel wire) depends on the respective length and diameter. Constraints in the various optimization cases involve maximum allowable tension in the mooring lines and maximum allowable offset in the horizontal plane. The time domain simulation within the optimization procedure is executed in SIMO, whereby the motions are separated into wave frequency and low frequency contributions, and a quasi-static method to calculate the mooring line tension is provided. The final or optimum mooring system configurations that result from each optimization case are verified through the run of several time domain realizations in order to obtain reliable statistics of the response, i.e. the most probable largest motions and line tension. A sensitivity study of the optimization formulation in SIMA is conducted in order to investigate the robustness of the formulation. The polyester system provides the most cost efficient configuration in comparison to the steel system. The optimization case with the softest constraints on maximum allowable mooring line tension and horizontal offset results in the cheapest system. In the comparison of the polyester system and the steel system, it is evident that a stricter constraint on safety factor results in the most expensive polyester system, whereas a reduced maximum allowable offset significantly increases the cost of the steel system. The sensitivity study concludes that the optimization formulation appears overly sensitive to a mi- nor change in the environmental conditions, whereby two optimization runs with a 10cm difference in significant wave height result in almost a 15% cost difference for the steel system. Additionally, the verification of the optimized systems highlights the importance of running several time domain simulations in order to obtain reliable statistics, for instance in terms of most probable largest tension and offset. The heaviest loaded line from both the steel and polyester system is analyzed in Riflex, to investigate the influence of mooring line dynamics on the tension. A quasi-static approach proves to be adequate to compute the tension in the polyester line, whereas this approach results in an underestimate of the tension in the steel line, whereby the dynamics from drag and inertia in the line enhances the tension.