Nowadays, there is an increasing demand for use of jack-up crane vessels to install offshore wind turbines. These vessels usually have shallow soil penetration during offshore crane operations because of the requirement of frequent repositioning. The soil-structure interaction should thus be properly modeled for evaluating the motion responses, especially at crane tip at large lifting height. Excessive crane tip motion affects the dynamic responses of the lifted components and subsequently affects the safety and efficiency of operations. The present study addresses the effects of soil behaviour modeling of a typical jack-up crane vessel on the dynamic motion responses of a wind turbine blade during installation using a fully coupled method. The coupled method account for wind loads on the blade and the vessel hull, wave loads on the vessel legs, soil-structure interaction, structural flexibility of the vessel legs and crane, and the mechanical wire couplings. Three models for the soil-leg interactions and two soil types are considered. The foundation modeling is found to have vital effects on the system dynamic motion responses. The characteristics of system motion differ under different types of soil. Compared to the combined linear spring and damper model, the simplified pinned and fixed foundations respectively lead to significant overestimation and underestimation of the motion responses of the blade during installation by jack-up crane vessels. To ensure safe and efficient offshore operations, detailed site specific soil properties should be used in numerical studies of offshore crane operations using jack-up crane vessels.

The first issue of the DNV Offshore Standard, DNV-OS-J103 Design of Floating Wind Turbine Structures , was published in June 2013. The standard was based on a joint industry effort with representatives from manufacturers, developers, utility companies and certifying bodies from Europe, Asia and the US. The standard represented a condensation of all relevant requirements for floaters in existing DNV standards for the offshore oil and gas industry which were considered relevant also for offshore floating structures for support of wind turbines, supplemented by necessary adaptation to the wind turbine application. The development of the standard capitalized much on experience from development projects going on at the time, in particular the Hywind spar off the coast of western Norway, the WindFloat off the coast of Portugal and the Pelastar TLP concept. In July 2018, DNV GL published a revision of DNV-OS-J103 as a part of the harmonization of the DNV GL codes for the wind turbine industry after the merger between Det Norske Veritas (DNV) and Germanischer Lloyd (GL) in the fall of 2013. The standard was re-issued as DNVGL-ST-0119 Floating wind turbine structures . This new revision reflects the experience gained after the first issue in 2013 as well as the current trends within the industry. Since 2013, numerous guidelines addressing the design of floating structures for offshore wind turbines have been published by various certifying bodies, and an IEC technical specification on the subject is under way. In addition, several prototypes have been installed and the first small array of floating wind turbines, Hywind Scotland pilot park, are currently in operation. The most important updates in the revision of the standard include formulation of floater-specific load cases, requirements to be fulfilled to support the exemption for design of unmanned floaters with damage stability, and replacement of current consequence-class based requirements for design fatigue factors with low-consequence based factors dependent on the accessibility for inspection and repair, the aim being a safety level against fatigue similar to that which is currently targeted for bottom-fixed structures. Other topics which have been considered in the revision are the floater motion control system and its possible integration with the control and protection system for the wind turbine, the issue of how to deal with slack in tendons in the station keeping system, corrosion, anchor design and power cable design. In parallel to the revision of the standard, a new service specification for certification of floating wind turbines has been developed by DNV GL, identified as DNVGL-SE-0422 Certification of floating wind turbines . For technical requirements, the service specification refers to the revised standard, DNVGL-ST-0119. The technical paper summarizes the updates and changes in the revised standard, in addition to the content of the new service specification.

It is now widely accepted that, due to significant economies of scale, the levelized cost of energy in offshore wind industry decreases as the turbine size and rated power increases. For offshore wind turbines, fixed and floating foundations can be quite complimentary when sites span a large water depth range. This paper presents the new WindFloat semisubmersible design supporting a 10MW generic wind turbine made by DTU [1]. This study evaluates the initial global performance of the WindFloat 10MW hull. In addition, RAO and frequency domain accelerations at the nacelle are presented. A comparison of the OpenFAST model, that we plan to use in all the future analysis, will be conducted against the benchmarked OrcaFAST model used by PPI and validated against the WindFloat prototype [2].

The coupled wind-wave solver (Yang & Shen, 2011) is extended to offshore wind energy research, with the embedment of turbine model (Yang & Sotiropoulos, 2015). The composite solver consists of a high-order spectral method for surface waves, large-eddy simulation for offshore wind on wave-surface-fitted dynamic grid, and an actuator based model for the representation of blades and nacelles. The blades are discretized by line elements, and its bound motion with floating platform is considered. Some smoothed spectral operations are applied to alleviate Gibbs phenomenon brought by local jump of turbine force. By performing simulations for a turbine array on land and over ocean waves, we have analyzed the impacts of waves on the wind farm in terms of mean wind speed and mean kinetic energy budget. It is found that a fast-propagating swell at the sea surface leads to a higher mean velocity at turbine hub height, which affects the wind power extraction. The influence of floating platform motion is also investigated. It is found that pitch motion has stronger influence than other in-plane motions, i.e., surge and heave. The interaction between tip vortex sheet and nacelle wake meandering is also studied. The intersection of these two flow structures coincides with a change of growth rate of mean velocity deficit width. The results will be useful for the understanding and control of turbine wakes in offshore wind farm.

Fixed-bottom offshore wind turbines (OWTs) are typically located in shallow to intermediate water depth, where waves are likely to break. Support structure designs for such turbines must account for loads due to breaking waves, but predictions from breaking wave models often disagree with each other and with observed behavior. This variability indicates the need for a better understanding of each model’s strengths and limitations, especially for different ocean conditions. This work evaluates and improves the accuracy of common breaking wave criteria through comparison to Computational Fluid Dynamics (CFD) simulations of breaking waves. The simulated ocean conditions are representative of potential U.S. East Coast offshore wind energy development sites, but the discussion of model accuracy and limitations can be applied to any location with similar ocean conditions. The waves are simulated using CONVERGE, a commercial CFD software that uses a Volume of Fluid (VOF) approach and includes adaptive mesh refinement at the evolving air-water interface. First, the CFD model is validated against experimental data for shoaling and breaking wave surface elevations. Second, 2D simulations of breaking waves are compared to widely-used breaking wave limits (McCowan, Miche, and Goda) for different combinations of wave height, wavelength, water depth, and seafloor slope. Based on these comparisons, the accuracy and limitations of each breaking limit model are discussed. General usage guidelines are then recommended.

This paper presents wind speed measurements collected at 40m to 200m above sea-level to support the New England Aqua Ventus I 12 MW Floating Offshore Wind Farm to be located 17km offshore the Northeast United States. The high-altitude wind speed data are unique and represent some of the first measurements made offshore in this part of the country which is actively being developed for offshore wind. Multiple LiDAR measurements were made using a DeepCLiDAR floating buoy and LiDARs located on land on a nearby island. The LiDARs compared favorably thereby confirming the LiDAR buoy measurements. Wind speed shear profiles are presented. The measurements are compared against industry standard mesoscale model outputs and offshore design codes including the American Bureau of Shipping, American Petroleum Institute, and DNV-GL guides. Significant variation in the vertical wind speed profile occurs throughout the year. This variation is not currently addressed in offshore wind design standards which typically recommend the use of only a few values for wind shear in operational and extreme conditions. The mean wind shears recorded were also higher than industry recommended values. Additionally, turbulence measurements made from the LiDAR, although not widely accepted in the scientific community, are presented and compared against industry guidelines.

There is an increasing awareness that the exploitation of offshore wind may become the main technology option for decarbonization of the global energy production systems. Various floating wind turbines are therefore being studied at the present time. In this paper we propose to proceed from floating to moving platforms by equipping autonomous sailing ships with hydrokinetic turbines and thereby to open up vast wind rich ocean areas for renewable power generation. The turbine output power is stored either in electric batteries or is fed into electrolysers to produce hydrogen which is compressed and stored in tanks. We provide a summary of our latest technoeconomic optimization studies of this energy ship concept using Multi Pole Systems Analysis and we describe the design, development and testing of a small-scale autonomous hydrofoil boat to serve as a technology demonstrator.

Using classic approaches of analytical mechanics, this paper addresses the general problem and provides an analytic and explicit formulation for the stiffness matrix of a generic mooring system layout. This is done around a generic offset position and heading of the floating unit, given the curves of tension vs displacement for each mooring line, for a frictionless seabed. The international benchmark of the Offshore Code Comparison Collaboration Continuation – OC4 is taken as a case study. The use of the analytical formulation is exemplified by systematically varying the mean offset position and heading of the platform, as well as the pretensioning of the mooring system.

In offshore jacket design, it has long been recognized that an accurate global structural model requires implementation of the effects of local joint flexibility (LJF). However, there is still no general method for implementing these effects accurately and efficiently without complicating the application of loads. The literature describes several techniques for determining LJFs using parametric formulas and implementing these in global models of a jacket structure. These techniques are simple but associated with uncertainties and a risk of compromising the accuracy of the global model. Alternative methods, such as the use of superelements, provide very accurate results but complicate the consistent application of external loads as well as postprocessing. This paper introduces a new methodology which is called the Correction Matrix Methodology. This allows the effects of LJF from detailed three-dimensional (3D) finite-element (FE) shell or solid models to be incorporated in a global beam FE model via a simple correction matrix. The effectiveness of the methodology is improved by using interpolation between a limited number of correction matrices. The new methodology provides exact results when correction matrices associated with the actual geometry are applied. When using the interpolation procedure, the methodology provides accurate results and computational efficiency when the database has been established. The Correction Matrix Methodology is a significant improvement of the conventional methods for modelling LJF and is currently being implemented in a general form for arbitrary joints in Rambolls Offshore Structural Analysis Program (ROSAP).

This paper explores geometry optimization of an offshore wind turbine’s mooring system considering the minimization of the material cost and the cumulative fatigue damage. A comparison of time domain simulations against frequency domain simulations is made to explore the suitability of these methods to the design process. The efficient design options, the Pareto front, from the frequency domain study are also re-evaluated using time domain simulations and compared against the time domain Pareto front. Both the time and frequency domain results show optimal results utilizing similar design philosophies, however, the frequency domain methods severely under predict the fatigue loads in the mooring system and incorrectly class infeasible solutions as feasible. The frequency domain is therefore not suitable for optimization use without some external means of applying engineering constraints. Furthermore, re-evaluation of the frequency domain solutions provides guidance to the uncertainty and the necessary design fatigue factors required if implementing frequency domain methods in design.

A new concept has emerged for far offshore wind energy conversion. It is the wind energy ship (1). It consists of a ship propelled by wind sails towing a water turbine. The water turbine produces electricity. The electricity is converted into a fuel (hydrogen for example). When the tanks are full, the ship sails to a terminal where the fuel is unloaded. Then, it can start a new charging cycle. An energy ship consists in several sub-systems: wind propulsion subsystem, hull, water turbine, energy storage. The focus of this paper is on the wind propulsion subsystem because of the many options available. Indeed, it has been proposed to implement rigid sails (2, 3), kite wings (4, 5), airfoils (1, 7) or Flettner rotors (6). Applying systems engineering, key requirements for the wind propulsion have been identified for the energy ship application. They are presented in the paper. Next, the advantages and drawbacks of each technology are discussed and most promising options are highlighted.

Floating Offshore Wind Turbines (FOWT) are promising Marine Renewable Energy (MRE) technologies. Considering the emerging stage of development of MRE technologies, no dedicated certification scheme has been developed so far by international organizations. Technical specifications are under development in the framework of the International Electrotechnical Commission (IEC) Technical Committee (TC) 88 and IEC Renewable Energy (IECRE). Within IECRE, the Marine Energy Operational Management Committee (ME OMC) is in charge of the development of a conformity assessment system dedicated to Floating Wind Turbines. In this context, Bureau Veritas (BV) has issued a Guidance Note NI631 Certification Scheme for Marine Renewable Energy Technologies and also the NI572 for the Classification and Certification of FOWT to support technology developers and speed up commercial phases. The note NI631 describes the different schemes of MRE certification, whereas the NI572 details the technical requirements for FOWT. This paper will provide an overview of the certification schemes applicable to FOWT technologies, addressing prototype, component, type and project certification. Main objective, scope, intermediary steps to be completed and resulting certificates will be detailed for each certification scheme, as well as their interactions. A methodology relying on the qualification of new technology process will be detailed when no guidelines or standards are available for the most innovative parts of a FOWT, or when existing standards from related sectors, such as wind energy, shipping or offshore Oil&Gas, require adaptations to fit their requirements to the specific MRE conditions.

Floating wind solutions have developed significantly in the recent years, moving from single demonstrators to having several floating wind pilot wind farms currently under development and even in operation. This is an important step for the industry allowing the market to gain confidence in these solutions for offshore wind. Ideol is a leading floating platform designer and they have been working on a demonstration project for their innovative platform in France. The Floatgen demonstration project consists of a 2MW wind turbine mounted on the Damping Pool platform. During the design phase of the project, the coupled analysis of the full system — turbine, tower, floating platform and moorings needs to be carried out to verify the loading on the turbine and platform, adapt the turbine controller for the floating application and re-design the tower and transition piece. For this project, DNV GL performed the aforementioned analysis in Bladed whilst Ideol performed parallel analysis in OrcaFlex, focusing on the platform and mooring design. It is crucial that both numerical models used in the different software tools and parallel analysis workflows are equivalent and lead to the same overall system behavior. This paper describes the numerical model used for coupled analysis in Bladed and its verification against Ideol’s OrcaFlex model, with emphasis on the aspects related to the platform modelling. For the hydrodynamic loading of the platform, boundary element method was considered together with global and local viscous drag terms. To compare and verify the coupled model results in Bladed to Ideol’s own numerical results, a set of static and dynamic tests were run and the resultant kinematics were compared. Ideol’s model was previously validated against tank test experiments giving confidence in its behavior. The viscous drag coefficients in the Bladed model were adjusted to ensure a good agreement between the kinematics of Ideol’s model of the system and the Bladed model. This paper summarizes the results of this verification exercise, along with some recommendations on areas of further research in the floating wind modelling domain.

As offshore wind turbines (OWTs) increase in size and are placed farther offshore, hydrodynamic loads have increased contribution to total load, resulting in fatigue limit states becoming more important to consider in structural design. Previous literature shows that placing wave energy converters (WECs) peripherally to an OWT array results in a milder wave climate within the array. This reduction could affect the wave loads and fatigue damage of OWT support structures. In this exploratory study, the effect of peripherally-distributing WECs in front of a fixed-bottom OWT on the fatigue of the OWT monopile is investigated. Relative sea state reductions from WECs are quantified, and fully-coupled time-domain fatigue analyses are performed for a 10 MW reference OWT. Results indicate that both a single WEC and a WEC array can lead to up to 8% and 10% fatigue load reduction in unidirectional wave cases, respectively.

A Tension Leg Platform supporting a wind turbine was tested in combined waves, wind and current in the Offshore Basin of MARIN. The wind turbine loads were simulated in two ways for these tests. Firstly, the wind turbine was modelled by a physical wind turbine following the approach of performance scaling. This approach intends to scale down the wind, the wind turbine inertia, the rotation speed of the rotor according to Froude’s scaling laws while it keeps the thrust that is representative of the considered wind turbine. The Marin Stock Wind Turbine (MSWT) was used in these tests. Secondly, the wind turbine was replaced by a system actuating the wind turbine loads in which the aerodynamic loads are calculated by a numerical code and applied through a system of winches to a frame mounted on the floater. Lift and drag coefficients of the blades of the MSWT were used by the computer program during these tests. The TLP was tested with both techniques for the same environmental conditions. Tests were done in waves, wind and current with the two techniques in the same basin, enabling an objective comparison of these two methods. The experimental results of both methods are analyzed and compared to each other in this study.

The U.S. East Coast has the potential for offshore wind deployment and contribute to the energy demands of the area. A safe and cost-effective development of offshore wind farms requires detailed and accurate meteorological and oceanographic (MetOcean) analysis and, thus, a wave and hydrodynamic database has been produced along the U.S. East Coast. The high-resolution wave and hydrodynamic models are calibrated and validated against various NOAA measurements along the U.S. East Coast. The wave model open lateral boundaries are forced by spectral data from a global wave model. Wind forcing for both models were taken from CFSR wind fields, and the hydrodynamic model was also forced by atmospheric pressure field. Spatially and temporally-varying water level and depth-averaged currents were used to force the wave model. The hydrodynamic model was forced by tidal elevations at the boundaries. The model’s resolution varies from 15 km along the offshore boundaries to approximately 3 km near the coastline. Wave spectra resolution was of 25 frequencies and 16 directions. New wave model features are explored, including the effect of atmospheric stability on wave growth, the air-sea density ratio and the current impact on the wave growth rate. These processes are now included during the calibration process of the wave model. Hydrodynamic model results show good agreement with surface elevation measurements, with no bias and small errors. Results of the wave model show overall good results, although some overestimation of the extreme events is noticeable for the studied cases. The high-quality wave and hydrodynamic models developed within this work will serve as a MetOcean database for developers or alternatively as forcing for establishing local high-resolution (100s m resolution) models, i.e. for complex bathymetry at specific wind farm sites. Further work includes longer-term validations, data assimilation and assessment of other processes such as bottom friction and their impact on the estimation of extreme waves.

The mooring system for a floating offshore wind turbine ensures that the platform stays within pre-defined station keeping limits during operation, while it provides sufficient restraining forces in storm events to guarantee survival. This presents a challenge during the design process, since the cost of the mooring system is proportional to the peak loads, i.e. those that occur infrequently in extreme conditions. Mooring designs are governed by extreme and fatigue loads which determine the required Minimum Breaking Load (MBL) of the system. If uncertainties in the environmental loading or hydrodynamic coupled response exist, additional safety factors are required. This paper explores the application of a hydraulic based mooring system that enables a variable, non-linear line stiffness characteristic that cannot be achieved with conventional designs. This non-linear load-response behavior could function like a ‘shock absorber’ in the mooring system, and thus reduce the line tensions, enabling a more efficient mooring system that necessitates a lower MBL and thus lower cost. These claims are evaluated through numerical modelling of the NREL OC3 spar buoy and OC4 semi-submersible offshore wind platforms using the FAST-OrcaFlex interface. The simulations compare the dynamics with and without the inclusion of the hydraulic mooring component. The results suggest that mean mooring line loads can be reduced in the region of 9–17% through a combination of lower static and dynamic loads, while the peak loads observed in extreme conditions were reduced by 17–18%. These load reductions, however, come at the expense of some additional platform motion. The paper also provides an outlook to an upcoming physical test campaign that will aim to better understand the performance and reliability of the mooring component, which will provide the necessary evidence to support these load reduction claims.

As the offshore wind industry moves toward deeper water, with larger turbines and correspondingly larger monopile foundations, nonlinear loads from steep waves may become more important for the ULS design. Nonlinear numerical wave tanks (NWTs) for generating wave kinematics, to be used as input to i.e. Morison’s equation, have been applied by several research groups, but further validation of the obtained wave kinematics is needed. Furthermore, the load models for larger diameters also need to be evaluated. The present work first compares the wave elevation results from existing two-dimensional (2D) NWT tools. The nonlinear wave elevation is compared to experimental results from model tests of two regular waves. Several methods of wave generation are considered. The conclusion from the study is that the selected codes represent physics well, even though we see individual differences, which are discussed. Differences between codes are not necessarily only due to differences in the applied theory, but also modeling of wave flap, numerical beach etc. Load (and response) estimates from using the NWT kinematics combined with the Morison load model are compared to experimental results, two sets of CFD simulations, and to simpler load models. Alternative methods of selecting the load coefficients for the simplified load models are also discussed.

Offshore wind assets have reached multi-GW scale and additional capacity is being installed and developed. To achieve demanding cost of energy targets, awarded by competitive auctions, the operation and maintenance (O&M) of these assets has to become increasingly efficient, whilst ensuring compliance and effectiveness. Existing offshore wind farm assets generate a significant amount of inhomogeneous data related to O&M processes. These data contain rich information about the condition of the assets, which is rarely fully utilized by the operators and service providers. Academic and industrial research and development efforts have led to a suite of tools trying to apply sensor data and build machine learning models to diagnose, trend and predict component failures. This study presents a decision support framework incorporating a range of different supervised and un-supervised learning algorithms. The aim is to provide guidance for asset owners on how to select the most relevant datasets, apply and choose the different machine learning algorithms and how to integrate the data stream with daily maintenance procedures. The presented methodology is tested on a real case example of an offshore wind turbine gearbox replacement at Teesside offshore wind farm. The study uses k-nearest neighbour (kNN) and support vector machine (SVM) algorithms to detect the fault using supervisory control and data acquisition (SCADA) data and an autoregressive model for the vibration data of the condition monitoring system (CMS). The implementation of all the algorithms has resulted in an accuracy higher than 94%. The results of this paper will be of interest to offshore wind farm developers and operators to streamline and optimize their O&M planning activities for their assets and reduce the associated costs.

In this paper, the operability of an Offshore Service Vessel (OSV) is looked into for different heading orientations of the vessel. Traditionally, the OSV heading orientation has been into the predominant current direction to prevent large beam current forces and thruster utility. However, such an orientation can lead to unfavourable wave headings which can cause large first order motions making it difficult to operate the gangway on board the vessel. Hydrodynamic time domain simulations were run for 1-year of hindcast weather data from an operational wind farm site in the Netherlands for different OSV heading orientations. The results of the simulations can be used to optimize the OSV heading strategy and increase uptime, reduce waiting time and energy loss associated to this waiting time.