As the offshore wind industry develops, more lease sites in the intermediate water depth (50–85 m) are being released to developers. In these water depths floating wind turbines with chain catenary systems and fixed-bottom turbines with jacketed structures become cost prohibitive. As such, industry and researchers have shifted focus to floating turbines with taut or semi-taut synthetic rope mooring systems. 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. In this paper 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.

In this work, the usability of the Burger body model (BBM) for determining the behavior of oriented strand boards (OSBs) under long-term loads was evaluated. The actual bending strain data and predicted strain data as a function of different stress levels and load durations under constant environmental conditions (25 ± 2°C and 50% relative humidity) were compared. Two test groups, short-term bending tests and long-term creep-rupture bending tests, were performed according to relevant ASTM standards. Specimens were randomly assigned to three groups and loaded at 47% (132.2 kg), 51% (137.4 kg), or 55% (154.9 kg) of the mean static short-term flexural strength. Specimen creep was monitored for 10,000 h using an automated measurement system. The four-parameter BBM parameters were obtained for all specimens at 2000-h time intervals, providing five different estimates. Measured strain values were compared with strain predictions from the BBM and with the goal of evaluating length of experiment on prediction accuracy. Each stress level provided statistical differences based on the error between the actual strain and predicted strain values. Group 3 provided minimum error compared to group 1 and group 2. The 10,000 and 8000 h loading provided the most accurate predictions compared to 6000, 4000, and 2000 h of data. Overall, the longer the actual data is collected the more accurate predictions were obtained. As a result, the BBM was found useful tool for predicting the creep behavior of OSBs under different loads and load durations. It was also shown that the increased duration of practical loading minimizes the error between the prediction. Therefore, the BBM is suggested for use predicting the creep behavior of OSBs over 8000 h load durations.

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 efficiency of traditional catenary chain mooring systems and fixed-bottom jacket structures. One such technology, synthetic ropes, have in recent years come to the forefront of this effort. The challenge of designing synthetic rope moorings is the complex nonlinear tension-strain response inherent of some rope material choices. Currently, many numerical tools for modeling the dynamic behavior of FOWTs are limited to mooring materials that have a linear tension- strain response. In this paper an open source FOWT design and analysis program, OpenFAST, was modified to capture the more complex tension-strain responses of synthetic ropes. Simulations from the modified OpenFAST tool were then compared with 1:52-scale test data for a 6MW 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.

Floating offshore wind turbines (FOWTs) over the past decade have been targeted as a solution to reducing dependence on fossil fuel. At this point the hulls of FOWTs have been a huge point of emphasis for the research community. FOWT mooring systems, however, have recently started to garner more attention. One solution that has gained traction to reduce cost is mooring the turbine with a synthetic mooring system as opposed to the traditional chain catenary system. Currently there is guidance provided for designing synthetic mooring systems, but it is more geared to the needs of the offshore oil and gas industry, and often leads to conservative designs. This work investigates the stiffness models recommended by the design guides and their influence on FOWT global response.

As the wind industry develops larger turbines for offshore deployment the problems with stationkeeping systems are exacerbated. As turbines increase in size, so do the loads on the turbine. Meanwhile, many offshore sites available for leasing occur in intermediate water depths (55-85m) which will appear ever smaller relative to the increasing platform size of floating offshore wind turbines (FOWTs). 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. In this paper a routine is developed that will map objectives for a multi-objective genetic algorithm (MOGA) to obtain mooring radius-lowest cost designs over a range of radii simultaneously. This work will implement and expand on a tiered-constraint evaluation scheme that was developed in previous work by West et al. [1]. New components and constraints are added to the optimization problem to allow the optimizer to find realistically deployable designs with reasonably accurate cost estimates. These techniques will then be used to find the most economic mooring designs for a 15-MW floating offshore wind turbine with a hybrid mooring system.