Floating Support Structures Research Articles

Full scale testing of fatigue resistant composite joints for offshore wind Jacket and Floating structures

Complex welds in the joint region reduce the fatigue resistance of structural joints of circular hollow sections with factors that results in increased wall thicknesses of the tubes and overspending of steel in the multi-membered part of the jacket and floating support structures for offshore wind turbines. The wrapped composite joint is a breakthrough technology utilising bonding and fatigue resistant composite material to connect steel tubular members instead of welding. The main technical advantage is that the superior fatigue life of wrapped composite joints. Full-scale wrapped composite joint is tested by cyclic out-of-plane bending loading in resonance-based Cronos testing rig at OCAS N.V. in Belgium. Results reveal exceptional fatigue life of wrapped joints to combined loading when compared to welded joints, showcasing a potential that can radically reduce steel use for offshore structures.

Large-eddy simulations of interaction between surface waves and a tidal turbine wake in a turbulent channel

Tidal stream turbines are now being developed for array deployments, largely at sites with relatively shallow water depths on either bed-supported, or floating support structures. Proximity to the free-surface presents design challenges with increased exposure to wave-induced kinematics leading to potential for increased peak- and fatigue-loads. Free surface proximity can also alter wake recovery rates which can influence the siting, and operation, of further turbines. To-date the impact of waves on turbine loading and on wake recovery has receivedlimited attention, generally for specific combinations of conditions reproducible in experimen tal facilities [1-2] or numerical models [3]. Improved understanding of how waves affect both turbine loading, and wake dynamics is necessary to inform the development of appropriate load prediction and mitigation methods and to further parameterise wake recovery to inform array siting.
 This work presents CFD analysis of the loading and wake of a three-bladed horizontal axis tidal turbine within a wide, shallow channel with a turbulent inflow developed using a synthetic eddy method and free surface waves modelled using a Level-Set Method (LSM). The turbine and wave conditions considered are based on prior experimental studies [1] within a channel depth h = 0.45 m and mean flow U = 0.47 m/s. The simulated channel is width 3.67h and length 88h and long-crested regular waves are imposed, with the focus of this study on waves with non-dimensional wavenumber kh = 2.8, comparable with experimental conditions. Simulations are accomplished using the in-house large-eddy simulation (LES) code DOFAS(Digital Offshore Farms Simulator) [4], which adopts an Actuator Line Method (ALM) to resolve the turbine blades. LSM is used to model the air-water interface in an accurate manner [5] with waves can be generated in DOFAS using linear or second-order Stokes theories with an absorption layer placed at the outlet of the domain to avoid wave reflections. At the inlet a mean logarithmic velocity profile is imposed over which artificial turbulence is added. Simulations were run for 400 s of physical time on 8,000 cores using ARCHER2.
 The waves studied are shown to alter the rate of wake recovery, increasing the rate in the near-wake of the turbine and slightly reducing the rate beyond eight diameters downstream. Analysis of turbulence characteristics indicates a significant variation across the water column due to wave action. Propagation over the turbine wake also introduces directionality to the wave field, which is associated with the change of wave-speed over the wake region. Modal decomposition analysis of the velocity fluctuations with Proper Orthogonal Decomposition(POD) reveals that the wake dynamics behind the turbine change due to the waves.

Floating Offshore Vertical Axis Wind Turbines: Opportunities, Challenges and Way Forward

The offshore wind sector is expanding to deep water locations through floating platforms. This poses challenges to horizontal axis wind turbines (HAWTs) due to the ever growing size of blades and floating support structures. As such, maintaining the structural integrity and reducing the levelised cost of energy (LCoE) of floating HAWTs seems increasingly difficult. An alternative to these challenges could be found in floating offshore vertical axis wind turbines (VAWTs). It is known that VAWTs have certain advantages over HAWTs, and in fact, some small-scale developers have successfully commercialised their onshore prototypes. In contrast, it remains unknown whether VAWTs can offer an advantage for deep water floating offshore wind farms. Therefore, here we present a multi-criteria review of different aspects of VAWTs to address this question. It is found that wind farm power density and reliability could be decisive factors to make VAWTs a feasible alternative for deep water floating arrays. Finally, we propose a way forward based on the findings of this review.

Drivetrains on floating offshore wind turbines: lessons learned over the last 10\xa0years

This paper presents lessons learned from own research studies and field experiments with drivetrains on floating wind turbines over the last ten years. Drivetrains on floating support structures are exposed to wave-induced motions in addition to wind loading and motions. This study investigates the drivetrain-floater interactions from two different viewpoints: how drivetrain impacts the sub-structure design; and how drivetrain responses and life are affected by the floater and support structure motion. The first one is linked to the drivetrain technology and layout, while the second question addresses the influence of the wave-induced motion. The results for both perspectives are presented and discussed. Notably, it is highlighted that the effect of wave induced motions may not be as significant as the wind loading on the drivetrain responses particularly in larger turbines. Given the limited experience with floating wind turbines, however, more research is needed. The main aim with this article is to synthesize and share own research findings on the subject in the period since 2009, the year that the first full-scale floating wind turbine, Hywind Demo, entered operation in Norway.

Large‐scale offshore wind energy installation in northwest India: Assessment of wind resource using Weather Research and Forecasting and levelized cost of energy

AbstractIn India, there are plans for 5 GW of installed capacity of wind power by 2022. This study establishes that the wind resource along the west coast of Gujarat is sufficient for offshore wind farms of between 500 MW to 2 GW rated power to be operated with capacity factors of 40% to 54%. For the majority of sites, a levelized cost of energy (LCOE) in the range 68 to 86 £/MWh can be achieved with installation on floating support structures. The LCOE is up to 3.2 £/MWh greater for installation on bed‐fixed structures closer to shore. The wind resource and energy yield are predicted using Weather Research and Forecasting (WRF). Wind speed occurrence is predicted to within 1.3% for multiple sites simultaneously, and this provides wind turbine energy yield to within 3.4%. Wind speeds occurring due to sea breeze are accurately predicted and contribute 6.2% of annual energy generation at the nearshore locations and 3.8% at the deeper water offshore sites. While these events improve the viability of nearshore locations, floating installations offer lower LCOE for most locations along the Gujarat coast.

Structural integrity assessment of floating offshore wind turbine support structures

Floating offshore wind turbines are becoming more attractive to the wind industry due to their capability to operate larger turbines in deeper waters. The floating support structures are anchored to the seabed via mooring chains to impede the structure's unwanted movements. The combination of cyclic stresses and the corrosive marine environment makes the floating support structures vulnerable to corrosion pitting and subsequently fatigue crack initiation and propagation. In this study a framework is proposed to simulate fatigue crack growth from multiple corrosion pits at critical spots of the Spar-type floating support structures to examine the status of the crack during several years of operation. The proposed advanced fracture mechanics based approach provides a methodology to assess the integrity of the structure and subsequently plan for preventive or curative maintenance. The crack growth rate is examined for both singular and multiple cracks at different R ratios and for different stress levels using ABAQUS XFEM. Following numerical simulations, a sensitivity analysis is carried out using Crackwise software for different values of plate thickness, R ratio and initial crack size. The numerical results are discussed in terms of the corrosion pitting effects on fatigue life assessment of floating offshore wind turbines.

Critical review of floating support structures for offshore wind farm deployment

Floating structures enable offshore wind power deployment at numerous deep water sites with promising wind potential where bottom-fixed systems are no longer feasible. However, the large diversity in existing floater concepts slows down the development and maturing processes of floating offshore wind turbines. Thus, in this work, different floating support structures are assessed with respect to their suitability for offshore wind farm deployment. A survey is conducted to examine the capacities of selected floater types, grouped into ten categories, with respect to ten specified criteria focusing on wind farm deployment. By this means, a multi-criteria decision analysis (MCDA) is carried out, using the technique for order preference by similarity to ideal solution (TOPSIS). With the individual scores of the different systems, considering the weighting of each criterion, suitable concepts are identified and potential hybrid designs, combining advantages of different solutions, are suggested.

Nonlinear Lifting Line Theory Applied to Vertical Axis Wind Turbines: Development of a Practical Design Tool

Recently, a new interest in vertical axis wind turbine (VAWT) technology is fueled by research on floating support structures for large-scale offshore wind energy application. For the application on floating structures at multimegawatt size, the VAWT concept may offer distinct advantages over the conventional horizontal axis wind turbine (HAWT) design. As an example, VAWT turbines are better suited for upscaling, and at multimegawatt size, the problem of periodic fatigue cycles reduces significantly due to a very low rotational speed. Additionally, the possibility to store the transmission and electricity generation system at the bottom, compared to the tower top as in a HAWT, can lead to a considerable reduction of material logistics costs. However, as most VAWT research stalled in the mid 1990s, no sophisticated and established tools to investigate this concept further exist today. Due to the complex interaction between unsteady aerodynamics and movement of the floating structure, fully coupled simulation tools modeling both aero and structural dynamics are needed. A nonlinear lifting line free vortex wake (LLFVW) code was recently integrated into the open source wind turbine simulation suite qblade. This paper describes some of the necessary adaptions of the algorithm, which differentiates it from the usual application in HAWT simulations. A focus is set on achieving a high robustness and computational efficiency. A short validation study compares LLFVW results with those of a two-dimensional (2D) unsteady Reynolds-averaged Navier–Stokes (URANS) simulation.

Statistical Estimation of Extreme Loads for the Design of Offshore Wind Turbines during Non-Operational Conditions

The International Electrotechnical Commission (IEC) design standard (IEC 61400-3) for offshore wind turbines includes a design load case which considers loads on the turbine during extreme conditions, when the wind turbine is not operating and the blades are feathered. The recommendation of this design standard is to simulate 6 one-hour periods, each with wind and wave fields modeled as random processes, and to calculate design loads as the mean of the maximum from the six simulations. Previous studies have investigated this recommendation for fixed bottom offshore wind turbines and raised concerns about the stability of the estimate of the design load calculated using these guidelines. The research presented in this paper calculates statistics of extreme design loads as a function of the number of one-hour simulations considered for three offshore wind turbine support structures: a fixed-bottom turbine supported by a monopile and two floating turbines, a spar buoy and a semi-submersible. The study considers one extreme design load, the moment at the tower base, one set of 50-year metocean conditions representive of the Northeast U.S. Atlantic coast, and five combinations of wind and wave models, including linear and nonlinear representations of irregular waves. The data generated from this study are used to assess the stability of the estimate of the extreme load for various numbers of one-hour simulations for each turbine type. The study shows that, for the considered metocean conditions and models, the monopile exhibits the least stability in the estimate of the extreme load and therefore requires more one-hour simulations to have comparable stability as the two floating support structures. Explanations for this result are provided along with a probabilistic formulation for estimating variability of design loads more efficiently, using fewer one-hour simulations.

Stochastic dynamic load effect and fatigue damage analysis of drivetrains in land-based and TLP, spar and semi-submersible floating wind turbines

This paper deals with the feasibility of using a 5 MW drivetrain which is designed for a land-based turbine, on floating wind turbines. Four types of floating support structures are investigated: spar, TLP and two semi-submersibles. The fatigue damage of mechanical components inside the gearbox and main bearings is compared for different environmental conditions, ranging from cut-in to cut-out wind speeds. For floating wind turbines, representative wave conditions are also considered. All wind turbines are ensured to follow similar power curves, but differences in the control system (integral to different concepts) are allowed. A de-coupled analysis approach is employed for the drivetrain response analysis. First, an aero-hydro-servo-elastic code is employed for the global analysis. Next, motions, moments and forces from the global analysis are applied on the gearbox multi body model and the loads on gears and bearings are obtained. The results suggest that the main bearings sustain more damage in floating wind turbines than on land-based. The highest main bearing damage is observed for the spar floating wind turbine. The large wave induced axial load on the main shaft is found to be the primary reason of this high damage in the spar wind turbine. Apart from the main bearings - which are located on the main shaft outside the gearbox - other bearings and gears inside the gearbox hold damages in floating wind turbines equal or even less than in the land-based turbine. It is emphasized that the results presented in this study are based on a drivetrain with two main bearings, which considerably reduces the non-torque loads on the gearbox.

A Comparison on the Dynamics of a Floating Vertical Axis Wind Turbine on Three Different Floating Support Structures

To increase the competitiveness of offshore wind energy in the global energy market, it is necessary to identify optimal offshore wind turbine configurations to deliver the lowest cost of energy. For deep waters where floating wind turbines are the feasible support structure option, the vertical axis wind turbine concept might prove to be one of these optimal configurations. This paper carries out a preliminary investigation into the dynamics of a vertical axis wind turbine coupled with three generic floating support structures originally intended for horizontal axis wind turbines. The modifications to the original characteristics of the support structures were kept to a minimum to illustrate the use of floating horizontal axis wind turbine platforms for floating vertical axis wind turbines Issues regarding the adequacy of the mooring systems are outlined and an overview of platform responses in a number of varying met-ocean conditions is presented and discussed.

Tendon Response of 10 MW Offshore Wind Turbine TLP Platform in Extreme Environment Condition

Deep-sea offshore wind resources are extremely abundant. Large offshore wind turbine is the future trend to utilize of deep-sea offshore wind resources. Because of excellent heaving, pitching and rolling performances, tension-leg platform ( TLP ) is one of best floating support structures for large wind turbine. However, under extreme environment condition, the large tendons which are required for large offshore wind turbine TLP platform will meet extreme response, even lead to damage. Extreme response of tendon of a 10 MW offshore wind turbine TLP platform ( an improved MOSES TLP ) in the extreme environment condition is studied here. It is showed that the global motions can meet the basic requirements for 10 MW floating wind turbine, where the maximum angle of TLP is less than 100. Meanwhile, the maximum tendon tension of the TLP in the simulation is less than the breaking force, which meets the requirements of API rules on tendon of TLP.

Design of a Spar Buoy for Offshore Wind Turbines

Offshore wind Energy demonstrated to be one of the most promising technologies for growing electric energy demand worldwide. The main objective of this research was to conceive a floating offshore structure for supporting wind turbines. The Mediterranean Sea is characterized by deep water, especially in the western area, from Italy towards Spain, and this makes floating support structures desirable, since in deep water they are cheaper than bottom fixed piles [1]. An aerodynamic, electric and mechanic model was developed and tested against experimental results of laboratory and wind tunnel tests on a small scale wind turbine, demonstrating its reliability, so it was used to determine actions on the floater and a first concept design was performed using commercial software for marine structures analysis. Further steps in this process will include integration of the turbine model with hydrodynamic calculations, as well flume, tank and open sea tests for the designed floater.

Application and extension of the TOPSIS method for the assessment of floating offshore wind turbine support structures

The offshore wind industry is shifting its focus towards deeper water sites, more suited to floating rather than bottom fixed support structures. Floating support structures currently used for oil and gas platforms provide a starting point for the floating wind industry; however, the selection of an optimum structure is not trivial with several unique factors that contribute to its complexity. In this work programme, a methodology to rapidly assess several concepts for wind turbine floating support structures is proposed. Using the multi-criteria decision-making method, TOPSIS, configurations are rated for a range of attributes. In addition, two complementary methodologies have been developed that extend the TOPSIS capability by including an estimated impact of the uncertainties associated with each of the selected criteria, on the final choice of an optimum support structure. This methodology represents a robust yet flexible design tool to complement the early conceptual design process that is able to rapidly investigate a broad design space and narrow down the number of potential options suitable for floating wind turbines.

Conceptual design of a floating support structure for an offshore vertical axis wind turbine: the lessons learnt

The design of floating support structures for wind turbines located offshore is a relatively new field. In contrast, the offshore oil and gas industry has been developing its technologies since the mid 1950s. However, the significantly and subtly different requirements of the offshore wind industry call for new methodologies. An Energy Technologies Institute (ETI) funded project called NOVA (for Novel Vertical Axis wind turbine) examined the feasibility of a large offshore vertical axis wind turbine in the 10–20 MW power range. The development of a case study for the NOVA project required a methodology to be developed to select the best configuration, based on the system dynamics. The design space has been investigated, ranking the possible options using a multi-criteria decision making (MCDM) method called TOPSIS. The best ‘class’ or design solution (based on water plane area stability) has been selected for a more detailed analysis. Two configurations are considered: a barge and a semi-submersible. The iterations to optimise and compare these two options are presented here, taking their dynamics and costs into account. The barge concept evolved to the ‘triple doughnut-Miyagawa’ concept, consisting of an annular cylindrical shape with an inner (to control the damping) and outer (to control added mass) bottom flat plates. The semi-submersible was optimised to obtain the best trade-off between dynamic behaviour and amount of material needed. The main conclusion is that the driving requirement is an acceptable response to wave action, not the ability to float or the ability to counteract the wind turbine overturning moment. A simple cost comparison is presented.

Preliminary design of a floating support structure for a 5 MW offshore wind turbine

The offshore wind industry is already exploiting near shore sites, using bottom fixed support structures, and is moving toward further and deeper sites: around 100 km from the coast and in 50–150 m water depth. As already happened for the oil and gas offshore industry in the 1960s, the floating support structure option for 5 MW (and future 7–10 MW) offshore wind turbines is becoming not only a technically feasible but also an economically viable solution with respect to the fixed solution. In the present article, taking as input the NREL 5 MW turbine and the Dogger Bank site, in the North Sea, seven preliminary floating support structure concepts have been investigated, and compared through a preliminary techno-economic analysis. Then the optimum concept among the seven, the tri-floater configuration, has been further developed and refined through hydrostatic, hydrodynamic, and structural analyses. Due to the novelty of this research field, there are no established guidelines, recommended practices, or standards to design floating support structures: this work presents a relatively simple and quick methodology to use in the conceptual and preliminary design phase, using and adapting the standards developed for oil and gas offshore floating structures.

Floating Offshore Wind Energy — A Review of the Current Status and an Assessment of the Prospects

In principle, appropriately designed floating support structures need not be more massive nor costly than the jackets or tripods being deployed in German North Sea waters and elsewhere. A number of significant challenges remain, not least the current limited operating experience but also the limited availability of comprehensive design and modelling capabilities, and demonstrated and widely applicable safe and cost-effectively installation methods. This paper assesses the prospects in terms of technology principles, challenges and the potential resource.

Concrete Structures for Development of Offshore Fields

Summary Concrete structures represent competitive solutions for developing many offshore fields. They are of particular interest when storage is required and as floating support structures. Furthermore, concrete reusable platforms may represent cost-saving solutions for many applications. To design optimum structures, designers must be realistic with respect to the structural capacity of concrete for offshore use. Introduction Experience in the North Sea has shown that concrete support structures are competitive with steel jacket structures in terms of initial investments and maintenance costs. This experience is considered relevant for other areas, particularly for countries that must import steel, for icy areas, for offshore areas without infrastructure, for places where oil storage facilities are required, and for areas where large crane vessels are not available to lift heavy steel structures or large topside modules. Furthermore, concrete floaters [tension-leg platforms (TLP's) and catenary anchored structures] are attractive as minimum-cost solutions for deepwater applications. This paper discusses the requirements and infrastructure needed to design and fabricate concrete structures. Particular attention is given to availability of cement, aggregate, reinforcement steel, and prestressing cables. This paper also discusses design competence and required computer facilities, as well as the availability of barges, a shipyard dock, or a special-purpose construction dock. We describe the fabrication, installation, and removal processes. Removal is becoming increasingly important in a world gradually becoming more aware of environmental stability. We give examples of efficient designs suitable for optimum development of small fields and fields requiring storage. These examples include traditional concrete structures constructed with form work or slip forming. Furthermore, novel concrete truss structures fabricated by joining precast tubes and nodes by prestressing cables and grout are discussed. We provide charts that give concrete volumes for a range of applications to enable operators to perform their own economic analyses.