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Introduction

OC4 Phase I

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 I of OC4, participants used an assortment of simulation codes to perform a coupled simulation of a 5-MW wind turbine installed on a jacket support structure in 50 m of water. Code predictions were compared from load case simulations selected to test different model features. 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.

Popko et al. (2012) summarise the modelling capabilities of the various software tools under the headings of aero-hydro-servo-elastic simulation capabilities. Using the OC4 terminology, Flexcom would be categorised as follows:

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

•Hydrodynamics (hydro): Airy theory with stretching method (Airystr) or User-defined subroutine (UD) or Dean’s stream function (Stream) + Morison’s Formula (ME)

•Control (servo): External dynamic link library (DLL)

•Structural dynamics: Finite Element Method (FEM)

Jacket

In OC4 Phase 1, the NREL 5MW reference turbine is supported by the UpWind reference jacket model described by Vorpahl et al. (2011). The supporting platform consists of a jacket substructure, a transition piece and a tower. The jacket legs are supported by piles, which are modelled as being clamped at the seabed. The legs are inclined from the vertical position and stiffened by four levels of cross braces. Additionally, mudbraces are placed just above the mudline to minimise the bending moment at the foundation piles. The jacket and tower are connected through a transition piece. The elevation of the entire support structure is 88.15 m with a hub height of 90.55 m.

For simplification reasons, it was decided not to include appurtenances on the jacket structure such as boat landings, J-tubes, anodes, cables, ladders etc. and joint cans are not taken into account. At joints, the connecting nodes of elements are defined at the intersection points of the members’ centerlines, which leads to overlap of elements in the analysed jacket. Hence the mass of the jacket is overestimated by about 9.7%, though Popko et al. (2012) state there is only a marginal influence coming from overlapping parts on eigenfrequencies and simulated loading. Additional masses such as hydrodynamic added mass, water in flooded legs and marine growth, have a strong influence on the dynamic response of the structure and so are included in the model. Marine growth mass and hydrodynamic added mass are overestimated by about 9.2% and 4.6%, respectively, due to the overlapping members.

As shown below, jacket legs are numbered L1 to L4, jacket sides are numbered S1 to S4, four levels of K-joints are numbered K1 to K4, and four levels of X-joints are numbered X1 to X4. The Flexcom model is built according to this numbering scheme. For example, the line named L1_K3toK2 represents jacket leg no. 1 from K-joint no. 3 to K-joint no. 2. The 64 braces are numbered top-down, from Leg 1 to Leg 4 with increasing leg number (counter clockwise in plan view) as shown below.

Load Cases

17 different load cases were performed in OC4 Phase I, 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 divided in five distinct sections.

0.Mass comparisons - not an official load case section, but verification of correct structural and additional masses is important, given the direct impact on structural dynamics

1.Eigenanalysis - examination of modal properties of flexible structure

2.Rigid offshore wind turbine - rigid structure excited by non-combined wind and wave loads

3.Land based turbine - jacket support structure replaced with tubular tower, designed to verify aerodynamics and control

4.Flexible offshore structure - flexible support structure with no aerodynamics examined under different wave loadings

5.Fully-flexible offshore wind turbine - fully flexible structure examined under the combined action of wind and wave loads

 

Software Tools and Project Participants

A set of state-of-the-art simulation codes for offshore wind turbine modeling are represented in OC4 Phase I, including: 3DFloat, ADAMS + AeroDyn, ADCoS-Offshore, ASHES, Bladed V3.8X, Bladed V4 Multibody, FAST-ANSYS, FEDEM WindPower, Flex-ASAS, Flex5-Poseidon, GAST, HAWC2, OneWind, Phatas-WMCfem and USFOS-vpOne.

A number of academic and industrial project partners from 10 countries participated in OC4 Phase I, including: Fraunhofer Institute for Wind Energy and Energy System Technology IWES (Germany), the National Renewable Energy Laboratory (NREL) (USA), Technical University of Denmark, Department of Wind Energy, campus Risø, Roskilde, Denmark (Risø DTU) (Denmark), Fedem Technology AS (Norway), Garrad Hassan & Partners Ltd. (UK), Institute for Energy Technology (IFE) (Norway), Pohang University of Science and Technology (POSTECH) (Korea), Centre for Ships and Ocean Structures (CeSOS) at the Norwegian University of Science and Technology (NTNU) (Norway), National Technical University of Athens (NTUA) (Greece), Institute of Steel Construction at the Leibniz Universität Hannover (LUH) (Germany), the Endowed Chair of Wind Energy at the Institute of Aircraft Design at Universität Stuttgart (SWE) (Germany), Norwegian University of Science and Technology (NTNU) (Norway), Knowledge Centre WMC (The Netherlands), Energy Research Centre of the Netherlands (ECN) (The Netherlands), American Bureau of Shipping (ABS) (USA), REpower Systems SE (Germany) and China General Certification (CGC) (China).