Erin Elizabeth Bachynski

Point Absorber Design for a Combined Wind and Wave Energy Converter on a Tension-Leg Support Structure

A combined wind and wave energy extraction device is studied, consisting of a single column tension leg platform (TLP) which supports a 5MW wind turbine (WT) and 3 point absorber wave energy converters (WECs). Two variations of the WECs are considered: one that is constrained to purely heave motion relative to the TLP hull, and a hinged device which moves in coupled surge and pitch as well as heave. The effects of both types of WECs on the WT power takeoff; on structural loads in the turbine tower and blades, WEC supporting structure, and tendons; and on the platform motions are examined for operational and 50-year extreme environmental conditions.Copyright

Real-time hybrid model testing of a braceless semi-submersible wind turbine. Part I: The hybrid approach

This article presents a method for performing Real-Time Hybrid Model testing (ReaTHM testing) of a floating wind turbine (FWT). The advantage of this method compared to the physical modelling of the wind in an ocean basin, is that it solves the Froude-Reynolds scaling conflict, which is a key issue in FWT testing. ReaTHM testing allows for more accurate testing also in transient conditions, or degraded conditions, which are not feasible yet with physical wind. The originality of the presented method lies in the fact that all aerodynamic load components of importance for the structure were identified and applied on the physical model, while in previous similar projects, only the aerodynamic thrust force was applied on the physical model. The way of applying the loads is also new. The article starts with a short review (mostly references) of ReaTHM testing when applied to other fields than marine technology. It then describes the design of the hybrid setup, its qualification, and discusses possible error sources and their quantification. The second part of the article [1] focuses on the performance of a braceless semisubmersible FWT, tested with the developed method. The third part [2] describes how the experimental data was used to calibrate a numerical model of the FWT. ∗Corresponding author (thomas.sauder@marintek.sintef.no)

Real-time hybrid model testing of a braceless semi-submersible wind turbine. Part II: Experimental results

Real-Time Hybrid Model (ReaTHM) tests of a braceless semi-submersible wind turbine were carried out at MARINTEK’s Ocean Basin in 2015. The tests sought to evaluate the performance of the floating wind turbine (FWT) structure in environmental conditions representative of the Northern North Sea. In order to do so, the tests employed a new hybrid testing method, wherein simulated aerodynamic loads were applied to the physical structure in the laboratory. The test method was found to work well, and is documented in [1]. The present work describes some of the experimental results. The test results showed a high level of repeatability, and permitted accurate investigation of the coupled responses of a FWT, including unique conditions such as blade pitch faults. For example, the influence of the wind turbine controller can be seen in decay tests in pitch and surge. In regular waves, aerodynamic loads due to constant wind had little influence on the structure motions (except for the mean offsets). Tests in irregular waves with and without turbulent wind are compared directly, and the influence of the wave-frequency motions on the aerodynamic damping of wind-induced low-frequency motions can be observed. ∗Corresponding author: maxime.thys@marintek.sintef.no INTRODUCTION Floating wind turbines (FWTs) are an emerging technology which can be used to generate electricity from the significant wind resource in relatively deep water (>50 m). Scaled model tests are an important part of the qualification process for such novel concepts, and such tests may have many different objectives. Model tests can, for example, be used to confirm system behavior, evaluate nonlinear phenomena, assess extreme and detailed loads, validate computer codes, or convince decision makers of the feasibility of a concept [2]. There are, however, significant challenges related to carrying out scaled model tests of FWTs in an ocean basin. Hydrodynamic tests generally follow Froude scaling, but a consistent scaling of the wind turbine will then result in a reduced Reynolds number compared to the prototype, which leads to generally poor aerodynamic performance [3]. Furthermore, there are practical challenges related to generating (and measuring) constant and turbulent wind fields in a wave basin [4, 5]. In order to improve the aerodynamic load modeling in wave basin experiments, several researchers have attempted various forms of “non-geometrical” scaling of the wind turbine rotor. One form of non-geometrical scaling is to replace the wind turbine rotor with a drag disk (e.g. [6, 7]), which gives the correct mean thrust and provides some aerodynamic damping, and 1 Copyright c

Hydrodynamic Modeling of Tension Leg Platform Wind Turbines

In order to compute the system response of tension leg platform wind turbines (TLPWTs), it is important to accurately capture the hydrodynamic loading not only at the wave frequency, but also in the low (difference) and high (sum) frequency ranges. The current work compares the dynamic response of several single column TLPWT designs in different wind and wave conditions using three hydrodynamic models: first order potential flow with viscous drag, first and second order potential flow with viscous drag, and a Morison’s equation model. Second order wave forces were found to have a relatively small effect on the structural load predictions: increased tendon tension variation of approximately 2–10% was observed in storm conditions, while negligible effects were observed in operational conditions. The Morison model, however, gave significantly larger pitch forcing near the natural period, leading to larger structural load predictions in all sea states.Copyright

Hydrodynamic Modeling of Large-Diameter Bottom-Fixed Offshore Wind Turbines

For shallow and intermediate water depths, large monopile foundations are considered to be promising with respect to the levelized cost of energy (LCOE) of offshore wind turbines. In order to reduce the LCOE by structural optimization and de-risk the resulting designs, the hydrodynamic loads must be computed efficiently and accurately. Three efficient methods for computing hydrodynamic loads are considered here: Morison’s equation with 1) undisturbed linear wave kinematics or 2) undisturbed second order Stokes wave kinematics, or 3) the MacCamy-Fuchs model, which is able to account for diffraction in short waves. Two reference turbines are considered in a simplified range of environmental conditions.For fatigue limit state calculations, accounting for diffraction effects was found to generally increase the estimated lifetime of the structure, particularly the tower. The importance of diffraction depends on the environmental conditions and the structure. For the case study of the NREL 5 MW design, the effect could be up to 10 % for the tower base and 2 % for the monopile under the mudline.The inclusion of second order wave kinematics did not have a large effect on the fatigue calculations, but had a significant impact on the structural loads in ultimate limit state conditions. For the NREL 5 MW design, a 30 % increase in the maximum bending moment under the mudline could be attributed to the second order wave kinematics; a 7 % increase was seen for the DTU 10 MW design.Copyright

Second Order Wave Force Effects on Tension Leg Platform Wind Turbines in Misaligned Wind and Waves

Although the majority of studies of tension leg platform wind turbines (TLPWTs) have focused on aligned wind and wave conditions, it is not uncommon for the wind and waves to be significantly misaligned. Wind-wave misalignment is expected to influence both ultimate and fatigue loads. The present work compares the dynamic response of a representative TLPWT in both aligned and misaligned wind and wave conditions, with and without second order sum-frequency potential forces. The contribution of the second order loads to the maximum stress and to the short-term fatigue damage at the tower base, tower top, and tendon fairleads is examined for several operational conditions. The same TLPWT with softened tendons is also studied in order to examine the sensitivity of the results to the system natural frequencies. The fatigue damage decreased in misaligned wind and wave conditions, but the effect of second order forces increased. For the soft TLPWT design, second order forces had an important effect on fatigue in both aligned and misaligned conditions. Despite the increase in side-side loading in misaligned conditions, aligned conditions were associated with larger maximum stresses (in operational conditions).Copyright

Key Contributors to Lifetime Accumulated Fatigue Damage in an Offshore Wind Turbine Support Structure

A simulation study is performed to identify the key contributors to lifetime accumulated fatigue damage in the support-structure of a 10 MW offshore wind turbine placed on a monopile foundation in 30 m water depth. The relative contributions to fatigue damage from wind loads, wave loads, and wind/wave misalignment are investigated through time-domain analysis combined with long-term variations in environmental conditions. Results show that wave loads are the dominating cause of fatigue damage in the support structure, and that environmental condtions associated with misalignment angle > 45° are insignificant with regard to the lifetime accumulated fatigue damage. Further, the results are used to investigate the potential of event-based use of control strategies developed to reduce fatigue loads through active load mitigation. Investigations show that a large reduction in lifetime accumulated fatigue damage is possible, enabling load mitigation only in certain situations, thus limiting collateral effects such as increased power fluctuations, and wear and tear of pitch actuators and drive-train components.Copyright

Dynamic Responses of a Jacket-Type Offshore Wind Turbine Using Decoupled and Coupled Models

This paper presents numerical studies of the dynamic responses of a jacket-type offshore wind turbine using both decoupled and coupled models. In the decoupled (hydroelastic) model, the wind load is included through time-dependent forces and moments at a single node on the top of the tower. The coupled model is a hydro-servo-aero-elastic representation of the system. The investigated structure is the OC4 (Offshore Code Comparison Collaboration Continuation) jacket foundation supporting the NREL 5-MW wind turbine in a water depth of 50m. Different operational wind and wave loadings at an offshore site with relatively high soil stiffness are investigated. The objective of this study is to evaluate the applicability of the computationally efficient linear decoupled model by comparing with the results obtained from the nonlinear coupled model. Good agreement was obtained in the eigen-frequency analysis, decay tests, and wave-only simulations. In order to obtain good results in the combined wind and wave simulations, two different strategies were applied in the decoupled model, which are 1) Wind loads obtained from the coupled model were applied directly as time-dependent point loads in the decoupled model; and 2) The thrust and torque from an isolated rotor model were used as wind loads on the decoupled model together with a linear aerodynamic damper. It was found that, by applying the thrust force from an isolated rotor model in combination with linear damping, reasonable agreement could be obtained between the decoupled and coupled models in combined wind and wave simulations.Copyright

Analysis and Dynamic Scaling of Tethered Wave Energy Converters in Irregular Waves

Wave energy converters (WECs) are a technically and economically promising option for renewable electricity generation. This paper investigates the hydrodynamic characteristics of a tethered cylindrical wave energy absorber using analytical methods and derives the scaling relations for laboratory testing. The effects of the cylinder geometry, mooring system, and mass distribution on the idealized power takeoff and the pitch motions of a tethered point wave energy absorber in irregular seas are summarized. Analytical solutions for the hydrodynamic coefficients and wave forcing are based on potential flow formulations and eigenfunction expansions. The results show that a relatively light mooring system has little effect on the power takeoff, but introduces a low-frequency coupled pitch-surge resonance that can cause system failure in long period swells. While analytical solutions provide first-order estimates of the system response, laboratory experiments are required to evaluate the nonlinear, coupled system response. In order to design and interpret such experiments, appropriate scaling relationships are determined and validated using numerical simulations. The added mass, radiation damping, wave radiation and diffraction excitation forces, and mooring system mass and stiffness are found to be self-consistent using geometric and Froude number similarity. The effects of incomplete geometric similarity with a shallow wave tank and viscous forces are also discussed.Copyright

Modelling of Floating Offshore Wind Technologies

The modelling of FOWT forms a critical stage of the design process, as it allows a fully coupled dynamic assessment of the response of the concept while accounting for blade-rotor dynamics, support structure motions and mooring dynamics. For both new and for existing concepts, modelling offers the potential to test, in controlled environments, a series of assumptions and scenarios at a relatively minor cost. Two fundamental modelling approaches can be followed: numerical and experimental.