DeepCwind Consortium Research Articles

Structural Evaluation of the DeepCWind Offshore Wind Foundation

Wind power technologies are being used as a mean of harvesting the energy from the wind with low carbon impact and with great potential to contribute to a new energy paradigm. Great benefits may arise from the use of offshore turbines, such as having a reduced visual impact, if far enough from the coast, or being able to produce more energy due to the use of bigger turbines. Current challenges for these technologies include producing a stable and profitable platform that allow turbines to be installed in deep waters, meaning that these solutions need to be of floating type. An example of such structure is the foundation developed by the DeepCWind consortium, which is evaluated for its structural integrity on this document. The evaluation consists of modal and static/transient analyses, according to a selected Design Loading Case, which is defined for the certification of wind turbines’ offshore foundations. The objective is to observe how this structure would perform when subjected to a simulated real life loading. The DeepCWind foundation exhibited great difficulties enduring the applied loads. Therefore, a structurally improved platform was design with the aim of successfully passing the load case.

Hydrodynamic response of a semi-submersible platform to support a wind turbine

In this research, a numerical investigation on the hydrodynamic response of a semisubmersible platform, developed by DeepCwind consortium, equipped with the National Renewable Energy Laboratory 5-MW reference wind turbine, and a typical 6-MW wind turbine is conducted. The main objective is to develop a numerical model validated by experimental test results of a semisubmersible platform with mooring lines made of various materials. A commercial software package is employed to model the floating platforms, and a coupled dynamic analysis is carried out on floater motions, mooring lines, and their interactions. The 5-MW numerical model is verified using experimental results of the reduced 1/50th scale semisubmersible platform model conducted by DeepCwind consortium. The results from the numerical model are also compared with FAST programme developed by the National Renewable Energy Laboratory, where an acceptable agreement is achieved to benchmark the parametric study presented herein. A semisubmersible platform with 6-MW wind turbine is designed through hydrostatic and hydrodynamic analyses, using the general arrangement, main structure, and mooring system from the verified 5-MW numerical model. A parametric study is carried out to study the influence of different parameters on the hydrodynamic response of the platform, including total mass, mooring material properties, and wave characteristics. Various mooring lines are examined with particular attention to polymer and synthetic materials to determine which is the most suitable for this platform. Results reveal that using the synthetic mooring lines due to their low Young's modulus provide better performance for the platform compared with other materials investigated in this research.

The effects of second-order hydrodynamics on a semisubmersible floating offshore wind turbine

The objective of this paper is to assess the second-order hydrodynamic effects on a semisubmersible floating offshore wind turbine. Second-order hydrodynamics induce loads and motions at the sum- and difference-frequencies of the incident waves. These effects have often been ignored in offshore wind analysis, under the assumption that they are significantly smaller than first-order effects. The sum- and difference-frequency loads can, however, excite eigenfrequencies of a floating system, leading to large oscillations that strain the mooring system or vibrations that cause fatigue damage to the structure. Observations of supposed second-order responses in wave-tank tests performed by the DeepCwind consortium at the Maritime Research Institute Netherlands (MARIN) offshore basin suggest that these effects might be more important than originally expected. These observations inspired interest in investigating how second-order excitation affects floating offshore wind turbines and whether second-order hydrodynamics should be included in offshore wind simulation tools like FAST. In this work, the effects of second-order hydrodynamics on a floating semisubmersible offshore wind turbine are investigated. Because FAST is currently unable to account for second-order effects, a method to assess these effects was applied in which linearized properties of the floating wind system derived from FAST (including the 6x6 mass and stiffness matrices) are used by WAMIT to solve the first- and second-order hydrodynamics problems in the frequency domain. The method was applied to the Offshore Code Comparison Collaboration Continuation OC4-DeepCwind semisubmersible platform, supporting the National Renewable Energy Laboratory's 5-MW baseline wind turbine. In this paper, the loads and response of the system caused by the second-order hydrodynamics are analysed and compared to the first-order hydrodynamic loads and induced motions in the frequency domain. Further, the second-order loads and induced response data are compared to the loads and motions induced by aerodynamic loading as solved by FAST.

Study on Mooring System Design of Floating Offshore Wind Turbine in Jeju Offshore Area

This paper presents a mooring design procedure for a floating offshore wind turbine. Offshore environmental data for Jeju are taken from KHOA (Korea Hydrographic and Oceanographic Administration) and used for the environmental conditions in numerical analyses. A semi-submersible-type floating wind system with a 5-MW-class wind turbine studied by the DeepCwind Consortium is applied. Catenary mooring with a studless chain is chosen as the mooring system. Design deliverables such as the nominal sizes of chain and length of the mooring line are decided by considering the long-term prediction of the breaking strength of the mooring lines where a 100-year return period is used. The designed mooring system is verified using a fatigue calculation based on rain-flow cycle counting, an S-N curve, and a Miner's damage summation of rule. The mooring tension process is obtained from time-domain motion analyses using ANSYS/AQWA.

Study of a Floating Foundation for Wind Turbines

Offshore wind farms are currently located predominantly in shallow water as it is possible to cost effectively install bottom fixed offshore turbines. Where shallow water sites are not available, floating offshore turbines could be a better solution than bottom fixed turbines. Currently three main concepts are promoted for the design of a floating wind turbine: a ballast stabilized floater (i.e., spar), a buoyancy stabilized floater (i.e., barge or semisubmersible), or a mooring stabilized floater (tension leg platform). In April 2011 the DeepCWind consortium visited MARIN to carry out model tests in the offshore wave basin with these three types of floating wind turbine platform. This paper reports a numerical study of a wind turbine supported by a semisubmersible floater. The response of the floating system to wind and wave conditions is compared to physical measurements at 1:50 model scale. The outcome of these comparisons is discussed in the conclusions of this paper.

The Effect of Second-order Hydrodynamics on Floating Offshore Wind Turbines

Offshore winds are generally stronger and more consistent than winds on land, making the offshore environment attractive for wind energy development. A large part of the offshore wind resource is however located in deep water, where floating turbines are the only economical way of harvesting the energy. The design of offshore floating wind turbines relies on the use of modeling tools that can simulate the entire coupled system behaviour. At present, most of these tools include only first-order hydrodynamic theory. However, observations of supposed second-order hydrodynamic responses in wave-tank tests performed by the DeepCwind consortium suggest that second-order effects might be critical. In this paper, the methodology used by the oil and gas industry has been modified to apply to the analysis of floating wind turbines, and is used to assess the effect of second-order hydrodynamics on floating offshore wind turbines. The method relies on combined use of the frequency-domain tool WAMIT and the time- domain tool FAST. The proposed assessment method has been applied to two different floating wind concepts, a spar and a tension-leg-platform (TLP), both supporting the NREL 5-MW baseline wind turbine. Results showing the hydrodynamic forces and motion response for these systems are presented and analysed, and compared to aerodynamic effects.

DeepCwind: Application of active and passive acoustics to evaluate potential impacts of deepwater offshore wind technology on fish.

The DeepCwind Consortium is conducting research and development to bring deepwater offshore wind technology to the Gulf of Maine. As part of the process, we are monitoring the potential impacts of 1/3 scale floating wind turbine platform on pelagic fishes and species of interest (e.g., sturgeon, Atlantic salmon) using active (hydroacoustic surveys) and passive (tagging) acoustic systems. We discuss our application of before‐after‐control‐impact designs to evaluate potential impacts, report on our preliminary monitoring activities and results from 2010, and highlight challenges encountered thus far in the context of improving monitoring protocols for future deployment(s) of full‐scale floating platforms.