Definition of the Floating System for Phase IV of OC3

TLDR: In this article, the authors present the specifications of an offshore floating wind turbine, which are needed by the participants for building aero-hydro-servo-elastic models during the IEA Annex XXIII Offshore Code Comparison Collaboration (OC3).

Abstract: Phase IV of the IEA Annex XXIII Offshore Code Comparison Collaboration (OC3) involves the modeling of an offshore floating wind turbine. This report documents the specifications of the floating system, which are needed by the OC3 participants for building aero-hydro-servo-elastic models.

Figures

Figure 4-3. Hydrodynamic wave excitation per unit amplitude for the OC3-Hywind spar

Table 4-1. Periodic Sea State Definitions

Figure 5-2. Load-displacement relationships for the OC3-Hywind mooring system in 2D

Figure 5-3. Load-displacement relationships for one OC3-Hywind mooring line

Figure 4-2. Panel mesh of the OC3-Hywind spar used within WAMIT

Table 5-1. Mooring System Properties

Full text

Technical Report
NREL/TP-500-47535
May 2010
Definition of the Floating System
for Phase IV of OC3
J. Jonkman

National Renewable Energy Laboratory
1617 Cole Boulevard, Golden, Colorado 80401-3393
303-275-3000
•
www.nrel.gov
NREL is a national laboratory of the U.S. Department of Energy
Office of Energy Efficiency and Renewable Energy
Operated by the Alliance for Sustainable Energy, LLC
Contract No. DE-AC36-08-GO28308
Technical Report
NREL/TP-500-47535
May 2010
Definition of the Floating System
for Phase IV of OC3
J. Jonkman
Prepared under Task No. WE101211

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i
Table of Contents
1 Introduction ................................................................................................................................ 1
2 Tower Properties ........................................................................................................................ 2
3 Floating Platform Structural Properties ................................................................................. 4
4 Floating Platform Hydrodynamic Properties ......................................................................... 6
5 Mooring System Properties ..................................................................................................... 16
6 Control System Properties ...................................................................................................... 22
References .................................................................................................................................... 24

ii
List of Tables
Table 2-1. Distributed Tower Properties ....................................................................................... 2
Table 2-2. Undistributed Tower Properties ................................................................................... 3
Table 3-1. Floating Platform Structural Properties ........................................................................ 4
Table 4-1. Periodic Sea State Definitions ...................................................................................... 8
Table 4-2. Floating Platform Hydrodynamic Properties .............................................................. 15
Table 5-1. Mooring System Properties ........................................................................................ 16
Table 6-1. Baseline Control System Property Modifications ...................................................... 23
List of Figures
Figure 3-1. Illustrations of the NREL 5-MW wind turbine on the OC3-Hywind spar .................. 5
Figure 4-1. Dimensionless Parameters for the OC3-Hywind spar ................................................ 9
Figure 4-2. Panel mesh of the OC3-Hywind spar used within WAMIT ..................................... 10
Figure 4-3. Hydrodynamic wave excitation per unit amplitude for the OC3-Hywind spar ........ 10
Figure 4-4. Hydrodynamic added mass and damping for the OC3-Hywind spar ....................... 11
Figure 4-5. Radiation impulse-response functions for the OC3-Hywind spar ............................ 12
Figure 5-1. Load-displacement relationships for the OC3-Hywind mooring system in 1D ........ 19
Figure 5-2. Load-displacement relationships for the OC3-Hywind mooring system in 2D ........ 20
Figure 5-3. Load-displacement relationships for one OC3-Hywind mooring line ...................... 21

Frequently asked questions

Q1. How many linear dampings were needed for the OC3-Hywind system?

Additional linear damping of 100,000 N/(m/s) was needed for surge and sway motions, 130,000 N/(m/s) was needed for heave motions, and 13,000,000 Nm/(rad/s) was needed for yaw motions to match the free-decay responses supplied by Statoil. 

Q2. What is the effect of pitch-to-feather control of wind turbines?

A consequence of conventional pitch-to-feather control of wind turbines, though, is that steady-state rotor thrust is reduced with increasing wind speed above rated. 

Q3. What is the effect of the rotor azimuth on the mooring line?

In an idealized PI-based blade-pitch controller, the rotor azimuth responds as a second-order system with a natural frequency and damping ratio [4]. 

Q4. What is the common formulation used in the analysis of fixed-bottom support structures for offshore wind?

The popular hydrodynamic formulation used in the analysis of fixed-bottom support structures for offshore wind turbines—Morison’s formulation—is applicable for calculating the hydrodynamic loads on cylindrical structures when (1) the effects of diffraction are negligible, (2), radiation damping is negligible, and (3) flow separation may occur. 

Q5. What is the hydrodynamic load associated with the incident waves and radiation of outgoing waves?

The remaining hydrodynamic loads—those associated with excitation from incident waves and radiation of outgoing waves from platform motion—depend on whether flow separation occurs. 

Q6. How many components of LinesF depend on the load of q?

In general, all six components of LinesF depend nonlinearly on all six displacements of q. (Without the subscript, LinesF represents the set of mooring system loads, including three forces and three moments.) 

Q7. What is the effect of the change in the generator-torque controller?

With this change, the generator-torque controller does not introduce negative damping in the rotor-speed response (which must be compensated by the blade-pitch controller), and so, reduces the rotor-speed excursions that are exaggerated by the reduction in gains in the blade-pitch controller. 

Q8. What is the hydrodynamics model for the OC3-Hywind system?

This hydrodynamics model consists of linear potential-flow theory in the time domain augmented with the nonlinear viscous-drag term from the relative form of Morison’s equation [5]. 

Q9. What is the cylinder diameter of the NREL 5-MW spar-buoy?

The cylinder diameter of 6.5 m above the taper is more slender than the cylinder diameter of 9.4 m below the taper to reduce hydrodynamic loads near the free surface. 

Q10. What is the heave force for a deep-drafted spar?

For a deep-drafted spar, the heave force can be approximated as the change in buoyancy brought about by direct integration of the hydrostatic pressure dependent on the time-varying wave elevation (as was done by many modelers in OC3 Phase III). 

Q11. How much is the yaw inertia of the floating platform?

The roll and pitch inertias of the floating platform about its CM are 4,229,230,000 kg•m2 and the yaw inertia of the floating platform about its centerline is 164,230,000 kg•m2. 

Q12. What is the optimum frequency of the matrices?

The zero- and infinite-frequency limits of all elements of the damping matrix are zero (not all shown), as required by theory, and peak out at some intermediate frequency. 

Q13. How many components of LinesF were calculated for each combination of the displacements?

All six components of LinesF were calculated for every combination of these displacements, for a total of (13 × 13 × 7 × 11 × 11 × 11 =) 1,574,573 discrete combinations. 

Q14. What is the importance of the damping of the platform-pitch mode?

As the analyses of Refs. [5,9,11,12] have demonstrated, it is important that the damping of the platform-pitch mode be positive and kept as large as possible. 

Q15. What is the inverse of the linear memory effect?

The linear memory effect is captured within time-domain hydrodynamics models through the time-convolution of the radiation impulse-response functions (i.e. the wave-radiation-retardation kernel), Kij, with the platform velocities.