Optimization of Synthetic Mooring Systems for Floating Offshore Wind Turbines

As the floating offshore wind industry matures it has become increasingly important for researchers to determine the next generation materials and processes that will allow platforms to be deployed in intermediate (50-85 m) water depths which challenge the feasibility of traditional catenary chain mooring systems and fixed-bottom jacket structures. One such technology, synthetic ropes, has in recent years come to the forefront of this effort. A significant challenge of designing synthetic rope moorings is capturing the complex physics of the materials which exhibit viscoelastic and nonlinear elastic properties. Currently numerical tools for modeling the dynamic behavior of floating offshore wind turbines (FOWTs) are limited to mooring materials that lack these strain-rate dependent properties and have a linear tension-strain response. To address this limitation, a mooring modelling module, MoorDyn, which operates within the popular FOWT design and analysis program, OpenFAST, was modified to allow for nonlinear elastic mooring materials to add additional capabilities in the numerical tools. Simulations from the modified OpenFAST tool were then compared with 1:52-scale test data for a 6-MW FOWT Semi-submersible platform in 55m of water subjected to representative design load cases. A strong correlation between the simulations and test data was observed. In addition to reducing the cost of the mooring systems, synthetic systems can also reduce the footprint compared to a chain catenary system which frees areas around the turbine for other maritime uses such as commercial fishing. Both the mooring systems component cost and footprint are pertinent design criteria that lend themselves naturally to a multi-objective optimization routine. A new approach for efficiently screening the design space for plausible mooring systems that balance component cost and footprint using a multi-objective genetic algorithm is presented. This method uses a tiered-constraint method to avoid performing computationally expensive time-domain simulations of mooring system designs that are infeasible. Performance metrics for assessing the constraints of candidate designs are performed using open-source software such as Mooring Analysis Program (MAP++), OpenFAST and MoorDyn. A case study is presented providing a Pareto-optimal design front for a taut synthetic mooring system of a 6-MW floating offshore wind turbine. As the wind industry develops larger turbines for offshore deployment the problems with stationkeeping systems are exacerbated. While turbines increase in size so do the loads on the turbine. Meanwhile the offshore sites available for leasing in the intermediate water depth are still available to developers regardless of turbine and platform size. This complicates the process of designing mooring systems for these larger systems and emphasizes the importance of having a good methodology for automating this process. The final portion of this dissertation presents a method for mapping objectives for a multi-objective genetic algorithm to obtain the relationship between mooring system minimum cost and mooring radius. This work implements and expands on the aforementioned tiered-constraint evaluation scheme. These techniques are used to find the most cost-effective mooring designs for a 15-MW FOWT with a semi-taut mooring system over a range of mooring radii. New components and constraints are added to the system to allow the optimizer to find realistically deployable designs with reasonably accurate cost estimates