Bonjun Koo

Model Tests for a Floating Wind Turbine on Three Different Floaters

Wind energy is a promising alternate energy resource. However, the on-land wind farms are limited by space, noise, and visual pollution and, therefore, many countries build wind farms near the shore. Until now, most offshore wind farms have been built in relatively shallow water (less than 30 m) with fixed tower type wind turbines. Recently, several countries have planned to move wind farms to deep water offshore locations to find stronger and steadier wind fields as compared to near shore locations. For the wind farms in deeper water, floating platforms have been proposed to support the wind turbine. The model tests described in this paper were performed at MARIN (maritime research institute netherlands) with a model setup corresponding to a 1:50 Froude scaling. The wind turbine was a scaled model of the national renewable energy lab (NREL) 5 MW horizontal axis reference wind turbine supported by three different generic floating platforms: a spar, a semisubmersible, and a tension-leg platform (TLP). The wave environment used in the tests is representative of the offshore in the state of Maine. In order to capture coupling between the floating platform and the wind turbine, the 1st bending mode of the turbine tower was also modeled. The main purpose of the model tests was to generate data on coupled motions and loads between the three floating platforms and the same wind turbine for the operational, design, and survival seas states. The data are to be used for the calibration and improvement of the existing design analysis and performance numerical codes. An additional objective of the model tests was to establish the advantages and disadvantages among the three floating platform concepts on the basis of the test data. The paper gives details of the scaled model wind turbine and floating platforms, the setup configurations, and the instrumentation to measure motions, accelerations, and loads along with the wind turbine rpm, torque, and thrust for the three floating wind turbines. The data and data analysis results are discussed in the work of Goupee et al. (2012, “Experimental Comparison of Three Floating Wind Turbine Concepts,” OMAE 2012-83645).

Experimental Comparison of Three Floating Wind Turbine Concepts

Beyond many of the Earth’s coasts exist a vast deepwater wind resource that can be tapped to provide substantial amounts of clean, renewable energy. However, much of this resource resides in waters deeper than 60 m where current fixed bottom wind turbine technology is no longer economically viable. As a result, many are looking to floating wind turbines as a means of harnessing this deepwater offshore wind resource. The preferred floating platform technology for this application, however, is currently up for debate. To begin the process of assessing the relative advantages of various platform concepts for floating wind turbines, 1/50 th scale model tests in a wind/wave basin were performed at MARIN (Maritime Research Institute Netherlands) of three floating wind turbine concepts. The Froude scaled tests simulated the behavior of the 126 m rotor diameter NREL (National Renewable Energy Lab) 5 MW, horizontal axis Reference Wind Turbine attached via a flexible tower in turn to three distinct platforms, these being a tension leg-platform, a spar-buoy and a semi-submersible. A large number of tests were performed ranging from simple free-decay tests to complex operating conditions with irregular sea states and dynamic winds. The high-quality wind environments, unique to these tests, were realized in the offshore basin via a novel wind machine which exhibited low swirl and turbulence intensity in the flow field. Recorded data from the floating wind turbine models include rotor torque and position, tower top and base forces and moments, mooring line tensions, six-axis platform motions and accelerations at key locations on the nacelle, tower, and platform. A comprehensive overview of the test program, including basic system identification results, is covered in an associated paper in this conference. In this paper, the results of a comprehensive data analysis are presented which illuminate the unique coupled system behavior of the three floating wind turbines subjected to combined wind and wave environments. The relative performance of each of the three systems is discussed with an emphasis placed on global motions, flexible tower dynamics and mooring system response. The results demonstrate the unique advantages and disadvantages of each floating wind turbine platform.

Model Test Data Correlations With Fully Coupled Hull/Mooring Analysis for a Floating Wind Turbine on a Semi-Submersible Platform

The DeepCwind floating wind turbine model tests were performed at MARIN (Maritime Research Institute Netherlands) with a model set-up corresponding to a 1:50 Froude scaling. In the model tests, the wind turbine was a scaled model of the National Renewable Energy Lab (NREL) 5MW, horizontal axis reference wind turbine supported by three different generic floating platforms: a spar, a semi-submersible and a tension-leg platform (TLP) (Ref. [1] and [2]). This paper presents validation of the MLTSIM-FAST [3] code with DeepCwind semi-submersible wind turbine model test results. In this integrated program, the turbine tower and rotor dynamics are simulated by the subroutines of FAST [4], and the hydrodynamic loads and mooring system dynamics are simulated by the subroutines of MLTSIM. In this study, fully coupled hull/mooring dynamics and second-order difference-frequency response are included in MLTSIM-FAST. The analysis results are systematically compared with model test results and show good agreement.Copyright

Prediction of Motions and Loads for Floatover Installation of Spar Topsides

There are several substantial advantages to installing integrated topside onto a Spar using floatover method, particularly for large topsides which exceed the single lift capacity of the available heavy lift derrick barge fleet. These advantages include schedule and cost savings for the integration and commissioning of modules on land rather than at sea. Uncoupling the deck fabrication schedules from the availability of heavy lift vessels is another advantage. The performance of a successful floatover installation requires adequate design and analysis of each phase of the floatover installation, and a sufficient weather window in which to perform each phase. Design of floatover installation includes: a) Global motions / mooring analysis to determine motions and loads on mooring lines, fenders, and structural members, b) Structural design including structural integration of the topsides with the barges and shock cell design on the Spar and barges, and c) Operational procedures for mating and barge separation. Validated analysis tools are essential to ensure adequacy in the design of all stages in the floatover operation. This paper presents data from floatover installation model tests, performed at OTRC (Texas A&M University, College Station, Texas, USA), and results from numerical analysis tools for motion and load predictions. The scale of the model tests was 1:60, and the simulated topside was approximately 18,000Te. The simulated environmental conditions included expected upper limit operational sea states for the Gulf of Mexico. The details of the model tests are described in Ref [1]. The analytical challenges related to floatover installation simulations are several and include multi-body hydrodynamics, and prediction of relative motions and interface loads during the mating operations. Available numerical analysis tools include the time domain multi-body proprietary code MLTSIM, and WAMIT, a frequency domain potential code that is widely available in the industry. The validation of MLTSIM involves viscous damping, multi-body hydrodynamic interaction, and simulation of impact forces. This paper presents the results from the validation on the basis of full scale, and quantifies the accuracy of predictions by comparing the measured and predicted motions and loads.Copyright

Model Test Correlation Study for a Floating Wind Turbine on a Tension Leg Platform

The challenges related to floating wind turbine analysis simulations relate to the modeling of the flexible turbine tower dynamics, the rotor dynamics, and the floating platform dynamics. In order to simulate the interactions between the wind turbine and the floating platform, two existing numerical codes, FAST [1], developed by the National Renewable Energy Laboratory (NREL), and MLTSIM, a Technip proprietary software, were integrated into one code, MLTSIM-FAST. In this integrated program, the turbine tower and rotor dynamics are simulated by the subroutines of FAST, and the hydrodynamic loads and mooring system dynamics are simulated by the subroutines of MLTSIM.This paper presents validation of the MLTSIM-FAST code on the basis of data from the DeepCwind floating wind turbine model tests [2] and [3]. For the present MLTSIM-FAST validation study, the TLP floating wind turbine, which showed the strongest interactions between the wind turbine and the floating platform among the three platforms TLP, SEMI, and SPAR tested, is selected. The validation results are given on the basis of full scale measured and simulated motions and loads.Copyright

Model Tests for a Floating Windturbine on Three Different Floaters

Wind energy is a promising alternate energy resource. However, the on-land wind farms are limited by space, noise, and visual pollution, and therefore many countries build wind farms near shore. Up to now, most of offshore wind farms have been built in relatively shallow water (less than 30m) with fixed tower type wind turbines. Recently, several countries plan to move wind farms to deep water offshore locations to find stronger and steadier wind fields as compared to near shore locations. For the wind farms in deeper water, floating platforms have been proposed to support the wind turbine.The model tests described in this paper were performed at MARIN (Maritime Research Institute Netherlands) with a model set-up corresponding to a 1:50 Froude scaling. The wind turbine was a scaled model of the National Renewable Energy Lab (NREL) 5MW, horizontal axis reference wind turbine supported by three different generic floating platforms: a spar, a semi-submersible and a tension-leg platform (TLP). The wave environment used in the tests is representative of the offshore in the state of Maine. In order to capture coupling between the floating platform and the wind turbine, the 1st bending mode of the turbine tower was also modeled.The main purpose of the model tests was to generate data on coupled motions and loads between the three floating platforms and the same wind turbine for the operational, design, and survival seas states. The data are to be used for calibration and improvement of existing design analysis and performance numerical codes. An additional objective of the model tests was to establish advantages and disadvantages among the three floating platform concepts on the basis of test data.The paper gives details of the scaled model wind turbine and floating platforms, the set-up configurations, and the instrumentation to measure motions, accelerations and loads as well as wind turbine rpm, torque and thrust for the three floating wind turbines. The data and data analysis results are the subject of another paper in this conference [1].Copyright

Model Tests for Floatover Installation of Spar Topsides

There are several substantial advantages to installing an integrated deck on a Spar using floatover installation, particularly for large topsides which exceed the single lift capacity of the available heavy lift derrick barge fleet. These advantages include schedule and cost savings for the integration and commissioning of modules on land rather than at sea. Uncoupling the deck fabrication schedules from the availability of heavy lift vessels is another advantage. The purpose of the model tests described in this paper was to generate data on motions and loads for the operational sea states in the Gulf of Mexico, and to define and validate different approaches of transferring the topsides to the Spar, using a catamaran configuration. The data are intended to (1) demonstrate the feasibility of the installation method for the GOM, and (2) validate Technip’s analysis tools. The model tests were performed at OTRC (Texas A&M) with a model set-up corresponding to a 1:60 model scale. The simulated topsides was about 18,000Te, and Jones Act compliant barges were modeled for the catamaran configuration. The paper will describe the catamaran and spar models, and the instrumentation to measure motions and loads for transportation and installation. It will also describe the shock cell configuration used for the mating operation, and several alternative methods for performing the mating. The environmental conditions tested included several random sea states, harmonic waves, and three headings (beam, head, and quartering seas). Selected data will be shown to demonstrate the range of motions and loads associated with the floatover installation in the GOM. Estimates of limiting sea states for the GOM will be discussed. The validation of the analysis tools is the subject of another paper in Ref [1].© 2010 ASME

Wave Slamming Model Test Data and Analysis for a Spar Hull Design

This paper presents wave slamming loads on the Aasta Hansteen Spar from model test data, and it discusses analysis methodologies for extreme loads in order to derive slamming design pressures for the local and global design of the Spar hull.A 3×3 array of slamming load transducer panels was employed to measure the horizontal wave impact loads for the 100-year and 10,000-year storms at the Aasta Hansteen field in the Norwegian Sea. The wave height and period for these storms were varied to investigate wave steepness effects on slamming loads. Three-hour simulations and as many as 20 realizations per sea state were used to capture the statistics for the slamming loads.Gumbel distribution was used to derive extreme pressures for the local and global design at the measured direction by using various panel combinations. The short term target percentiles used in the Gumbel distribution were determined by long term analysis. The detailed long term analysis results are the subject of another paper [1].The main objective of this study is to establish the extreme pressure distribution along the length of the Spar above the mean water level (MWL). The linear correlation was found between the slamming pressure coefficient and incident wave steepness and it was used to obtain the extreme pressure profile for other wave directions around the Spar hull.The methodology presented in this paper can also be applied to slamming pressure of other platforms/floaters.Copyright

Long Term Analysis for Estimation of Wave Slamming Pressures for Spar Design

A long term analysis was performed to determine extreme wave slamming loads on the Aasta Hansteen Spar, the first production and storage Spar to be installed in the Norwegian Sea. The Spar will experience high slamming pressures on the hull due to harsh environments in the field. Extensive model tests were performed to measure the wave slamming pressure which is one of challenging design parameters.The slamming loads were measured with a 3×3 array of force transducer panels attached to the Spar hull. The extreme slamming loads were estimated from 3-hour simulations of the 100-yr and 10000-yr wave environments at the Aasta Hansteen field in the Norwegian Sea. The wave simulations included fourteen sea states, and each sea state was represented by as many as 20 realizations.Based on model test data, short term analysis of 3-hour extreme pressure at each tested sea state was performed using the Gumbel distribution. Due to high variability of 3-hour maximum pressures, a long term analysis was required to investigate the proper percentile level to be used in the design.The paper presents a long term statistical methodology for extreme wave slamming loads that is used to calculate long term slamming pressures corresponding to a specified annual exceedance probability of q (e.g., q = 10−2 and q = 10−4). The paper also derives the appropriate non-exceedance probability for a short term wave environment that reproduces the long term pressures of a specified annual exceedance probability, q.Various sensitivity analyses (e.g., on the two Gumbel parameters, number of realizations, etc.) were performed to validate the short term target percentiles and associated extreme pressures derived from this approach.Details of the model tests and methodology to define the design pressure profile above mean water level (MWL) are presented in a companion paper of this Conference.Copyright

Hydrodynamic Interactions and Relative Motions Analysis for Installation of an Extendable Draft Platform

The Extendable Draft Platform (EDP) is a deep draft, column stabilized platform with a deck box support for topsides and a single, deep draft heave plate that provides suitable motion characteristics to enable the use of dry tree top tensioned risers. The EDP can be fabricated with topsides installed on the deck box and commissioned quayside in a typical construction yard. With the columns in the retracted position, the EDP floats on its deck box and can be towed, in this configuration, to the location of interest. Once the EDP is transported to its final site, the columns and heave plate are lowered to their final operating draft. During the lowering sequence, the deck box and the lower hull become two relatively independent bodies, mechanically connected by chains that control the lowering of the columns and heave plate, and the guides between the deck box and the columns. This multi-body system is hydrodynamically coupled because of radiated and diffracted waves. The global performance analyses of the installation process (lowering of the lower hull) are carried out by three different methods. The first method is frequency-domain analysis by WAMIT and a frequency domain motion solver. In the frequency domain analysis, all the mechanical connections are modeled as linear springs. The second method is time-domain, partially coupled analysis using HARP/WINPOST. In this analysis, the off diagonal 6×6 hydrodynamic interactions are ignored. The last method is a time domain, fully coupled analysis using HARP/WINPOST. In this analysis, full 12×12 hydrodynamic interactions are considered. In the time domain analyses, the mechanical couplings between each column and deck box are modeled with linear springs and the chain connections are modeled with slender rods by using the nonlinear finite element method. This paper presents and compares analysis results based on the three methods for relative motions and loads between the deck box and the lower hull during the lowering of the columns and heave plate.© 2009 ASME