No abstract

Floating offshore wind turbines (FOWTs) contribute to an emerging green energy technology, by exploiting higher and consistent wind speeds above the ocean. There are several challenges facing the design of mooring system of FOWTs, including installation costs, stability of light-weight minimalistic platforms, and shallow depths (50–300m). The extreme tension in mooring lines of a light displacement platform in shallow-water is dominated by snap loads. This is because light pre-tension requirements in the line may be insufficient to prevent the mooring lines from being exposed to wave motion induced slack and shock events. In this paper, we present a comparative analysis of a semi-submersible based FOWT exposed to a 100-year storm condition, based on model test data and numerical simulations of well-known industry standard software. The data was obtained from a 1/50th-scale FOWT with the wind turbine modeled after the NREL 5MW wind turbine. The software, OrcaFlex, was used for numerical simulations of the mooring system. NREL’s FAST software was coupled to OrcaFlex to obtain aerodynamic loads along with hydrodynamic load for FOWT analyses. The numerical simulation of the moored FOWT in a 3-hour storm was executed in both the frequency-domain and the time-domain to determine the dynamic behavior of the platform and mooring system, respectively. Snap–type impact events were observed in both test data and numerical simulation. Tension maxima were fitted into extreme value distributions and comparisons are made between simulated and measured data. It is seen that snap events follow a different exceedance probability distribution compared to the cycle-to-cycle tension maxima.

As the offshore wind industry matures and projects begin to expand to deeper water regions, various floating systems are being considered to support wind turbines. This paper explores the feasibility of a Tension Leg Platform (TLP) support system anchored with synthetic rope tendons attached to a gravity base template to provide a platform for a wide range of water depths with acceptable operating nacelle accelerations. In this paper, the NREL 5MW wind turbine is selected in order to provide a comparison to previous studies of steel tendon TLPs. A fully-coupled numerical modeling tool is used to assess the effects of extreme irregular sea loads on the TLP. A series of numerical simulations are carried out to compare the response of a Single Column (SC) TLP for three different water depths and three different environments. The responses are compared with the steel tendon model. The use of synthetic rope tendons potentially offers more efficient installation options and enlarges the range of acceptable water depths. The use of a gravity base/suction pile foundation may improve the installation cost and schedule. The fully coupled nonlinear, time domain analysis tool used provides a unique look into the fully operating wind turbine under stable motion characteristics of the TLP.

No abstract

Model tests were conducted on three generic floating wind turbine systems in 2011, and reported in a series of papers at OMAE 2012. These tests were conducted at the MARIN facility in the Netherlands, by a consortium of universities, government research organizations and industry. As part of the testing program, decay tests in platform pitch were conducted with and without wind. It was found that for spar and semi-submersible type structures, resonant pitch motion was damped due to wind in storm sea conditions. The nonlinear decay motion of a floating wind turbine platform is modeled using a one degree-of-freedom nonlinear oscillation equation about a mean offset angle. Attention is paid to the turbine thrust coefficient and its variability with respect to oncoming flow speed, which in turn is affected by the structure pitch motion. The equation of motion reveals that the mean offset position has an important role in the stiffness, damping and consequently the natural period of pitch motion. Several important dimensionless parameters are introduced. The paper discusses a simple thrust model for an offshore wind turbine based on rudiments of blade element theory. Using the simplified thrust coefficient formulation, the increase in platform pitch damping due to wind is formulated. Experimental data reported from prior tests described above show good agreement with the theoretical model.