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Introduction

OC4 Phase II

OC4 (Offshore Code Comparison Collaboration Continuation) is a code-to-code verification project operated under the IEA (International Energy Agency) Wind Task 30 and coordinated by NREL (National Renewable Energy Laboratory). In Phase II of OC4, participants used an assortment of simulation codes to model the coupled dynamic response of a 5-MW wind turbine installed on a floating semi-submersible in 200 m of water. Code predictions were compared from load case simulations selected to test different model features. According to (Robertson et al., 2014), "the comparisons have resulted in a greater understanding of offshore floating wind turbine dynamics and modeling techniques, and better knowledge of the validity of various approximations. The lessons learned from this exercise have improved the participants’ codes, thus improving the standard of offshore wind turbine modeling". 21 different organizations from 11 different countries submitted results using 19 different simulation codes. Although Flexcom was not officially represented in OC4, the software has been retrospectively benchmarked against OC4 results, as NREL and the IEA have kindly made all data from the project publicly available.

Robertson et al. (2014) summarised the modelling capabilities of the various software tools under the headings of structural dynamics, aerodynamics, hydrodynamics and mooring model. Using the OC4 terminology, Flexcom would be categorised as follows:

•Structural dynamics: Tower – Finite Element (FE), Platform – Rigid

•Aerodynamics: Blade-Element Momentum (BEM) or Generalized Dynamic Wake (GDW), plus Dynamic Stall (DS)

•Hydrodynamics: Potential Flow (PF) plus Morison Equation (ME)

•Mooring model: Finite Element (FE) / Dynamic

Semi-submersible Platform

Phase II of the OC4 project involved the modeling of a semi-submersible floating offshore wind system developed for the DeepCwind project (Goupee et al., 2012) as shown below. This concept was chosen for its increased hydrodynamic complexity compared to the only other floating system analyzed in the OC3 and OC4 projects, the OC3-Hywind spar buoy.

A summary of the semi-sub’s main characteristics is presented here, and further details are available in Robertson et al., 2014.

 

Load Cases

21 different load cases were performed in OC4 Phase II, encompassing varying levels of model complexity and a variety of ambient loading conditions. A subset of these cases were reproduced using Flexcom, and this forms the basis of this example. The load cases are ordered in increasing complexity, with three distinct groupings. The first group (1.X) are focused on fundamentals, including a static equilibrium simulation, a modal analysis, and a series of free-decay simulations. These simulations are run with still water, and have the generator locked (blade rotation is prevented via a brake). The second group (2.X) are focused on wave loading without wind - again the generator is locked. The third and final group (3.X) examines combined wind and wave excitation, including regular and irregular waves, and steady and turbulent wind.  

Although not part of OC4 Phase II, one additional load case (OC4 P2 LC3.1 modified) was added here in order to verify the correct operation of the control system in Flexcom.

*Load case OC4 P2 LC3.1 modified) was not part of OC4 Phase II. It was added here in order to verify the correct operation of the control system in Flexcom

 

Software Tools and Project Participants

Results from Flexcom are presented alongside results from a subset of the software tools used in OC4 Phase II. This helps to reduce clutter on graph comparisons, whilst still including some of the most well-known software tools in industry. In some cases the same software tool was used by multiple OC4 participants, one of whom is included here.

Notes

1.Data files for FAST are available from NREL for the majority of load cases. For a small number of load cases, results from NREL were not found in the IEA Wind Task 30 results library, so we sourced data for FAST from another participant instead. Where this is the case, the relevant data series in the comparisons presented here are labelled appropriately.

2.Results from the standard version of Bladed are used in the comparisons presented here. For a small number of load cases, results from Garrad Hassan for Bladed were not found in the IEA Wind Task 30 results library, so we used results for 'Bladed Advanced Hydro Beta' instead. If you would like further information about this version, we suggest that you contact Garrad Hassan directly.

3.Principia provided two sets of results for data for DeepLines Wind, with the models differing in terms of how the hydrodynamic response of the platform was simulated. The first approach corresponds to diffraction-radiation theory, which Principia describe as “hydrodynamic data calculated with Diodore software (potential flow + quadratic damping), rigid body (no strains)”. The second approach corresponds to Morison’s equation, which Principia describe as “FE model (Morison elements)”. Most project participants provided one set of results only. In reality, many simulation tools adopt a combined approach, whereby the radiation and diffraction components are modelled using potential flow theory and the viscous drag terms are modelled using Morison’s equation. Given that the Morison approach ignores wave field disturbance, it is not generally used to model floating structures of significant size. So for the purposes of the comparisons presented here, results for DeepLines Wind are based on the potential flow modelling approach. However as this neglects viscous drag effects, the results presented here do not represent the full modelling capabilities of DeepLines Wind.

4.Flexcom was not part of OC4. It has been retrospectively benchmarked against OC4 results by Wood personnel.