A New Assessment of Offshore Wind Profile Relationships

Engineering design codes specify a variety of different relationships to quantify vertical variations in wind speed, gust factor and turbulence intensity. These are required to support applications including assessment of wind resource, operability and engineering design. Differences between the available relationships lead to undesirable uncertainty in all stages of an offshore wind project. Reducing these uncertainties will become increasingly important as wind energy is harnessed in deeper waters and at lower costs. Installation of a traditional met mast is not an option in deep water. Reliable measurement of the local wind, gust and turbulence profiles from floating LiDAR can be challenging. Fortunately, alternative data sources can provide improved characterisation of winds at offshore locations. Numerical modelling of wind in the lower few hundred metres of the atmosphere is generally much simpler at remote deepwater locations than over complex onshore terrain. The sophistication, resolution and reliability of such models is advancing rapidly. Mesoscale models can now allow nesting of large scale conditions to horizontal scales less than one kilometre. Models can also provide many decades of wind data, a major advantage over the site specific measurements gathered to support a wind energy development. Model data are also immediately available at the start of a project at relatively low cost. At offshore locations these models can be validated and calibrated, just above the sea surface, using well established satellite wind products. Reliable long term statistics of near surface wind can be used to quantify winds at the higher elevations applicable to wind turbines using the wide range of existing standard profile relationships. Reduced uncertainty in these profile relationships will be of considerable benefit to the wider use of satellite and model data sources in the wind energy industry. This paper describes a new assessment of various industry standard wind profile relationships, using a range of available met mast datasets and numerical models.

Analysis of Wind Speed Shear and Turbulence LiDAR Measurements to Support Offshore Wind in the Northeast United States

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.

Evaluating the Coupledness of the Aerodynamics and Hydrodynamics on the Estimation of Fatigue Damage Equivalent Load for a Floating Offshore Wind Platform

In this research, the estimation of the fatigue life of a semi-submersible floating offshore wind platform is considered. In order to accurately estimate the fatigue life of a platform, coupled aerodynamic-hydrodynamic simulations are performed to obtain dynamic stress values. The simulations are performed at a multitude of representative environmental states, or “bins,” which can mimic the conditions the structure may endure at a given site, per ABS Floating Offshore Wind Turbine Installation guidelines. To accurately represent the variety of wind and wave conditions, the number of environmental states can be of the order of 103. Unlike other offshore structures, both the wind and wave conditions must be accounted for, which are generally considered independent parameters, drastically increasing the number of states. The stress timeseries from these simulations can be used to estimate the damage at a particular location on the structure by using commonly accepted methods, such as the rainflow counting algorithm. The damage due to either the winds or the waves can be estimated by using a frequency decomposition of the stress timeseries. In this paper, a similar decoupled approach is used to attempt to recover the damages induced from these coupled simulations. Although it is well-known that a coupled, aero-hydro analysis is necessary in order to accurately simulate the nonlinear rigid-body motions of the platform, it is less clear if the same statement could be made about the fatigue properties of the platform. In one approach, the fatigue damage equivalent load is calculated independently from both scatter diagrams of the waves and a rose diagram of the wind. De-coupled simulations are performed to estimate the response at an all-encompassing range of environmental conditions. A database of responses based on these environmental conditions is constructed. The likelihood of occurrence at a case-study site is used to compare the damage equivalent from the coupled simulations. The OC5 platform in the Borssele wind farm zone is used as a case-study and the damage equivalent load from the de-coupled methods are compared to those from the coupled analysis in order to assess these methodologies.

Effect of Foundation Modeling of a Jack-Up Crane Vessel on the Dynamic Motion Response of an Offshore Wind Turbine Blade During Installation

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.

Fatigue Life Analysis of Offshore Wind Turbine Support Structures in an Offshore Wind Farm

This study evaluated, by time-domain simulations, the fatigue lives of several jacket support structures for 4 MW wind turbines distributed throughout an offshore wind farm off Taiwan’s west coast. An in-house RANS-based wind farm analysis tool, WiFa3D, has been developed to determine the effects of the wind turbine wake behaviour on the flow fields through wind farm clusters. To reduce computational cost, WiFa3D employs actuator disk models to simulate the body forces imposed on the flow field by the target wind turbines, where the actuator disk is defined by the swept region of the rotor in space, and a body force distribution representing the aerodynamic characteristics of the rotor is assigned within this virtual disk. Simulations were performed for a range of environmental conditions, which were then combined with preliminary site survey metocean data to produce a long-term statistical environment. The short-term environmental loads on the wind turbine rotors were calculated by an unsteady blade element momentum (BEM) model of the target 4 MW wind turbines. The fatigue assessment of the jacket support structure was then conducted by applying the Rainflow Counting scheme on the hot spot stresses variations, as read-out from Finite Element results, and by employing appropriate SN curves. The fatigue lives of several wind turbine support structures taken at various locations in the wind farm showed significant variations with the preliminary design condition that assumed a single wind turbine without wake disturbance from other units.

DNV GL Standard for Floating Wind Turbines

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.

Further Discussion on the White-Noise Approach for Predicting Slow-Drifts in FAST

Making use of theoretical approximations for the computation of the wave-induced slow-drift forces is a common procedure in the early stages of design of a new floating unit. They can help reducing the computational burden in two different fronts: for generating the QTFs in a frequency domain analysis, and during the subsequent execution of time-domain simulations. In a previous paper, we have discussed a simple procedure for making use of the white-noise approximation in FAST, without the need for any modification of the software. The proposal only requires restricting the computation of the QTFs to pairs of frequencies that are indeed essential to the slow-drift dynamics. For this, however, an additional assumption is made, considering that each motion is decoupled from those in the other dofs. In the present paper, a more detailed analysis of the subject is made, in order to clarify the theoretical aspects of the procedure and supplement the previous analysis. Once again, the results are based on the data available for the OC4 FOWT. The accuracy obtained with the procedure is discussed not only in terms of the resulting motions, but also comparing its effects on the second-order force spectra. A more detailed evaluation of the dynamic couplings is presented, and comparisons with the results obtained with Newman’s approximation are made in simulations involving waves only.

Simulation of a Floating Offshore Wind Turbine With an Integrated Response Mitigation Technology

This work investigates the implementation of a novel, NASA-developed Fluid Harmonic Absorber (FHA) technology to mitigate platform motions and structural loads that can lead to lighter platforms, increased turbine performance, and ultimately, a lower LCOE. The novel damping strategy takes advantage of existing water ballast in the VolturnUS semi-submersible platform to achieve significant performance gains with minimal additional equipment and complexity. NREL’s FOWT software FAST is modified to include the primary features of the FHA technology. A study of the University of Maine-developed VolturnUS semi-submersible FOWT augmented with FHA technology is undertaken to quantify global performance of the system. When compared to the baseline technology, numerical simulations of a redesigned platform utilizing the FHA dampers indicate a reduction of 15.8% in hull structural material. Finally, the improvements in LCOE resulting from this mass reduction are assessed to demonstrate the advantages of NASA’s FHA technology for FOWT applications.

Long Term Satellite Observations of the Global Wind Resource

A database of global satellite measurements of wind speed is calibrated and validated to provide a consistent set of global measurements over a period of 30 years. This database is used to describe the global wind resource including: mean monthly climatology, extreme value estimates of global wind speed and global estimates of trend (changes) in wind speed.

Dynamic Load Reduction and Station Keeping Mooring System for Floating Offshore Wind

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.