Kostas F. Lambrakos

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

Investigation on the VIM Mitigation of the HVS Semisubmersible

The mitigation of Vortex Induced Motion (VIM) of the HVS (Heave and VIM Suppressed) semisubmersible is investigated through extensive comparisons between CFD analysis and VIM model test results. It is shown that the lower VIM response of the HVS semisubmersible results from the break in coherence of vortex shedding along the length of column due to the column step. The present CFD application was carried out on the basis of in-house best practices for VIM analysis of multi-column floaters. The analysis results show excellent comparison with the model test results. The present findings and methodology can be applied to optimize semisubmersible hull designs for suppressed VIM response.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

A Methodology for In-Line VIV Analysis of Risers in Sheared Currents

As exploration and production move to even deeper water and more severe environment, the need to have a methodology for analyzing risers for in-line VIV fatigue damage without undue conservatism increases. The methodology presented in this paper reduces the conservatism in available methods by accounting for (1) the power-in region, (2) the power-out region (hydrodynamic damping), (3) competing modal excitation in the case of multiple mode excitation, and (4) the multiple constraints, if available, in the riser that result in irregular modal shapes. This methodology requires the use of a cross-flow VIV code with sheared flow capability such as SHEAR7, VIVA, or VIVANA. In this methodology the riser over the current profile is split into sections of cross-flow excitation and sections that have potential for in-line VIV excitation only. The cross-flow VIV code defines the sections for cross-flow excitation. All sections are analyzed for in-line VIV with the cross-flow VIV code using the appropriate in-line VIV force coefficients and Strouhal numbers. The assumptions implicit in the cross-flow VIV code regarding power-in, power-out, etc., are assumed valid for the in-line VIV analysis. The in-line VIV coefficients used in the analysis reported in this paper have been obtained from laboratory data, and are functions of both the VIV response amplitude and reduced velocity. The coefficients have been modified to give in-line VIV response amplitudes with the methodology presented that are consistent with DNV-RP-F105. The fatigue damage along the riser represents the sum of the damages produced by in-line VIV excitation for each of the riser sections.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

Coupled Dynamic Modeling of a Moored Floating Platform With Risers

A time domain coupled analysis capability has been developed to model the dynamic responses of an integrated floating system incorporating the interactions between vessel, moorings and risers in a marine environment. Hydrodynamic responses of the vessel allowing diffraction, radiation damping and wave drift forces on panelized bodies in addition to loads on Morison members, are modeled using the well-established program, MLTSIM. RodDyn, a finite element rod dynamics program, based on Garrett’s rod theory, is an efficient program to model the nonlinear dynamics of risers and moorings. The coupled MLTSIM-RodDyn suite integrates the nonlinear motions and structural analysis capabilities of RodDyn with the extensive hydrodynamic simulation capabilities of MLTSIM. With the fully-coupled dynamic analysis, the integrated system can be analyzed consistently. That is, the forces and moments applied by the rods to the platform are concurrent with the motions imposed on the rods by the platform at their multiple contact points. An asynchronous coupling of these two programs has been developed which allows for a fast simulation of this very complex problem. A worked example showing the nonlinear coupled analysis, is elaborated with systematical comparison with uncoupled analysis.Copyright

Time Domain VIV Prediction for Top Tensioned Risers

Top Tensioned Risers (TTRs) have been widely used with floating production systems such as Spars and TLPs in deepwater field developments. A TTR system provides direct access to subsea wells from a floating platform for drilling, workover, and completion operations. It is often subjected to Vortex-Induced Vibration (VIV) caused by ambient ocean currents or vessel motions. This paper investigates time domain VIV prediction for TTRs used in a typical Spar floating production system. A typical TTR has strong nonlinear and time-varying dynamic characteristics. The existing gaps between the riser and keel guide and between riser top centralizers and the supporting conductor result in intermittent VIV behaviors of the riser. In addition, hydraulic tensioners are widely used to provide tension to a TTR. The tension from tensioners varies with the riser’s dynamic response especially in the vertical direction. The time domain approach, which has been benchmarked and published in about ten technical papers, is thus more appropriate to predict TTRs’ VIV performance than a frequency domain method. This paper first introduces a typical TTR structure and then presents the analysis methodology and features of the time domain VIV prediction program ABAVIV. An example TTR is used to illustrate intermittent VIV behaviors such as top tension, interaction load at the keel guide, and VIV response at the location of top centralizers. This paper further studies the sensitivity of the VIV response to different current profiles. It finally uses the time domain approach to analyze the VIV response of the riser with its boundary conditions fixed and compares the results with those from a frequency domain program. A conclusion is finally drawn about the use of time domain VIV prediction for Spar TTRs.Copyright