ASME Conference Presenter Attendance Policy and Archival Proceedings

2018;():V001T00A001. doi:10.1115/IOWTC2018-NS.

This online compilation of papers from the ASME 2018 1st International Offshore Wind Technical Conference (IOWTC2018) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Scientific Track: Floating Concepts

2018;():V001T01A001. doi:10.1115/IOWTC2018-1022.

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.

Commentary by Dr. Valentin Fuster
2018;():V001T01A002. doi:10.1115/IOWTC2018-1056.

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.

Topics: Propulsion , Ships , Wind
Commentary by Dr. Valentin Fuster
2018;():V001T01A003. doi:10.1115/IOWTC2018-1079.

New floating wind turbine designs are needed to reduce production costs and to increase mass production feasibility. The TetraSpar floating wind turbine achieves these goals by being constructed using components highly suitable for standardization and industrialization. The design makes use of a suspended submerged counter weight to obtain a low center of gravity of the floating system, while also allowing a low draft during transport and installation. This novel concept requires a multibody modeling approach to perform a dynamic load and response analysis, as the stiffness between the floating platform and the counter weight is provided by chains. Additional design criteria are required for the counter weight system dependent on a combination of chain capacity and maintaining positive tension in all of the lines. To satisfy these design criteria a global hydrodynamic load and response analysis of the floater and counter weight is performed. In this concept, the counter weight depth contributes significantly to the dynamic properties of the system and therefore a parametric study is conducted. The global response parameters of the rigid-body motion natural frequencies, nacelle accelerations, counter weight chain tensions, and maximum platform-pitch angles are compared. Design recommendations are made for the configuration of counter weight depth and suspension system layout.

Commentary by Dr. Valentin Fuster
2018;():V001T01A004. doi:10.1115/IOWTC2018-1096.

General Electric, the National Renewable Energy Laboratory (NREL), the University of Massachusetts Amherst (UMass), and Glosten have recently completed a US Department of Energy (DOE)-funded research program to study technologies for mitigating loads on floating offshore wind turbines through the use of advanced turbine controls and tuned mass dampers (TMDs).

The analysis was based upon the Glosten PelaStar tension leg platform (TLP) with GE Haliade 150 turbine, a system developed in a previous front end engineering design (FEED) study funded by the Energy Technology Institute (ETI) in the UK. The platform was designed for the WaveHub wave energy research site, with a mean water depth of 59-m.

Loads were analyzed by running time-domain simulations in four 50-year return period (50-YRP) ultimate load state (ULS) conditions and 77 fatigue load state (FLS) environmental conditions. In 50-YRP conditions advanced controls are not active. The influence of TMDs on ULS loads have been reported previously (Park et al. [2]). In FLS conditions advanced controls and TMDs afford dramatic reductions in fatigue damage, offering the potential of significant savings in tower structural requirements.

Simulations in turbine idling conditions were run in OrcaFlex, and simulations in operating conditions were run in FAST. Simulations were run with a baseline turbine controller, representative of the current state of the art, and an advanced controller developed by NREL to use collective and individual blade pitch control to maintain rotor speed and reduce tower loads. UMass developed a number of TMD types, with varying system configurations, including passive nonlinear dampers and semi-actively controlled dampers with an inverse velocity groundhook control algorithm.

Loads and accelerations in FLS conditions were evaluated on the basis of damage equivalent loads (DELs), and fatigue damage was computed by Miner’s summations of stress cycles at the tower base.

To study sensitivity to water depth, loads were analyzed at both the 59-m WaveHub depth and a more commercially realistic depth of 100 m.

TMDs reduce fatigue damage at the tower-column interface flange by up to 52% in 59-m water depth and up to 28% in 100 m water depth. Advanced controls reduce fatigue damage at the tower-column flange by up to 22% in 59-m water depth and up to 40% in 100 m water depth.

The most effective load-mitigation strategy is combining advanced controls with TMDs. This strategy affords a 71% reduction in fatigue damage in both 59-m and 100-m water depths.

Commentary by Dr. Valentin Fuster

Scientific Track: Metocean

2018;():V001T01A005. doi:10.1115/IOWTC2018-1003.

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.

Commentary by Dr. Valentin Fuster
2018;():V001T01A006. doi:10.1115/IOWTC2018-1009.

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.

Topics: Wind , Satellites
Commentary by Dr. Valentin Fuster
2018;():V001T01A007. doi:10.1115/IOWTC2018-1038.

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.

Commentary by Dr. Valentin Fuster
2018;():V001T01A008. doi:10.1115/IOWTC2018-1052.

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.

Commentary by Dr. Valentin Fuster
2018;():V001T01A009. doi:10.1115/IOWTC2018-1086.

This work validates a numerical model of the Fraunhofer IWES LiDAR-Buoy by using open sea measurements. Such floating LiDAR systems (FLS) have been deployed for almost twenty years, aiming at exploring the offshore wind resource with lower cost. However, the uncertainty of wind measurements from a moving LiDAR are not clear, particularly due to the wave- and current-induced motion of the buoy. Therefore a numerical model with state-of-the-art approaches in conventional oil and gas industry was developed to quantify uncertainty and understand the effect of environmental conditions on the buoy. The model was validated against data from a measurement campaign at the offshore research platform FINO 3. The results show the challenges and limitations when transferring the experience from the oil and gas industry directly because of the different geometries and the much smaller buoys used for FLS. It has been found that the position of the LiDAR is dominated by the current, which is however commonly simplified in the state-of-the-art approach; the rotational motions are significantly influenced by the wave and can be reproduced up to a certain limit.

Topics: Seas , Buoys
Commentary by Dr. Valentin Fuster

Scientific Track: Model Testing

2018;():V001T01A010. doi:10.1115/IOWTC2018-1043.

Offshore structures operating at sea are severely affected by slamming pressure. This slamming pressure significantly shortens the design life of offshore structures including offshore wind turbines and results in personal and material damage. This slamming load has to be fully considered in the design phase of the structure, and therefore good quality of experimental results should be supported.

In Korea, offshore wind turbines have excellent conditions geographically. In the West sea, the construction of the jacket-type wind turbine is advantageous because the water depth is low, and in the East sea, which is relatively deep, the floating wind turbine is possible; thus, it is easy to generate a large-scale wind farm. For offshore wind turbines, several technical aspects should be considered. Among them, the damage of the structure caused by the slamming load should be studied.

In the case of FOWTs operating in large waves, slamming can cause structural damage, and repeated slamming loads reduce the structural strength and shorten the design life. The slamming load should be calculated by applying the maximum peak, its width, duration and the dynamic load according to the natural period of the structure, and the importance of structural design.

The results of the experiments can be used to determine the structural design, and the slamming load can be estimated to provide the design techniques offshore wind turbines as the design variables.

In this study, we investigated experimentally the elastic effects of a cylindrical model on the slamming load characteristics by free drop test at height 1.0m and 1.7m.

Commentary by Dr. Valentin Fuster
2018;():V001T01A011. doi:10.1115/IOWTC2018-1057.

Floating offshore wind turbines (FOWT) have many pros and cons. Among the pros, we can mention the availability of more constant winds and a velocity more suitable to the use of turbines in their optimum efficiency. Among the cons, there are the high costs for installation, mooring lines and the large length of cables required for the energy transmission. In this context, saving structural weights of the floater is, certainly, very welcome.

This paper describes the results of an experimental campaign of a 1/80th scale model FOWT performed in a wave basin. The model consists of a central column connected by pontoons to three equally spaced columns by an angle of 120 degrees. The offset columns are connected to the central tower by guywires. Being structurally light, both pontoons and guywires are subject to the effects of hydroelasticity. In these preliminary tests of the concept, only waves were considered, hence wind effects are not yet addressed. The analysis is based on the first order motions of the FOWT, the tension at the guywires, and the strain at the pontoons.

The results are compared with numerical simulations obtained with the software NK-UTWind, developed at the University of Tokyo, and METiS - USP, currently being developed at the University of São Paulo. Furthermore, the floater motions are also analyzed using the commercial software WAMIT, to provide a better insight on the physics involved by using a different approach to the calculation of hydrodynamic forces.

Commentary by Dr. Valentin Fuster
2018;():V001T01A012. doi:10.1115/IOWTC2018-1068.

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.

Commentary by Dr. Valentin Fuster
2018;():V001T01A013. doi:10.1115/IOWTC2018-1081.

This article presents the Real-Time Hybrid Model (ReaTHM®) tests that were performed on a 10-MW semi-submersible floating wind turbine in the Ocean Basin at SINTEF Ocean in March 2018. The ReaTHM test method was used for the model tests to circumvent the limitations encountered when performing model tests with wind and waves. The physical model was subject to physical waves, while the rotor and tower loads were simulated in real-time and applied on the model by use of a cable-driven parallel robot. Recent advances in the ReaTHM test method allowed for extended testing possibilities and load application up to the 3p frequency and the first tower bending frequency.

Commentary by Dr. Valentin Fuster
2018;():V001T01A014. doi:10.1115/IOWTC2018-1084.

A variable-scale model wind turbine has been developed by the Advanced Structures and Composites Center for testing scale-model floating offshore wind turbines. This model has been designed to be lightweight with a robust individual blade pitch mechanism. Froude number similitude is used to develop scaling relationships, while specialized blades have been designed to produce representative aerodynamic forces despite mismatched Reynolds numbers. Numerical simulations show that the model turbine is able to match the scaled aerodynamic thrust of commercial wind turbines by altering blade pitch and maintaining Froude number and tip-speed-ratio similitude. This turbine has the capability to accurately simulate commercial turbines of varying sizes in complex loading conditions with the additional capability to implement and test new control algorithms.

Commentary by Dr. Valentin Fuster
2018;():V001T01A015. doi:10.1115/IOWTC2018-1095.

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.

Commentary by Dr. Valentin Fuster

Scientific Track: Mooring and Foundation Design

2018;():V001T01A016. doi:10.1115/IOWTC2018-1006.

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.

Commentary by Dr. Valentin Fuster
2018;():V001T01A017. doi:10.1115/IOWTC2018-1011.

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.

Commentary by Dr. Valentin Fuster
2018;():V001T01A018. doi:10.1115/IOWTC2018-1012.

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.

Commentary by Dr. Valentin Fuster
2018;():V001T01A019. doi:10.1115/IOWTC2018-1017.

Large-scale deployment of multiple structures within a Floating Offshore Wind Farm (FOWF) will place many challenges on both the approach to effective integrity management and the demand to reduce through-life operating costs. Additionally, wind farm designers and operators will need to consider the issues related to design robustness in the design of their systems, whereby rational robustness criteria are applied to address possible accidental conditions that are not explicitly addressed in code-specified design basis conditions.

This paper will review the significant advances in the approach to integrity management that have been recently made in the mature offshore oil industry and relate them to the nascent offshore wind industry. In particular, the requirement for consideration across the full lifecycle of the potential for threat introduction and the application of controls to prevent or mitigate the evolution of those threats will be described. This includes threats that may be inadvertently introduced during the design, manufacturing and installation phases, in addition to the more traditional rate-based degradation mechanisms such as fatigue, corrosion and wear that occur once the facility is in operation.

The challenges that commercial-scale wind farm developments will face relate particularly to integrity issues arising in the design, manufacturing and installation phases, where the focus needs to ensure that degradation threats are not being introduced into the deployment of multiple repeat-copy units. The issues of common cause/common mode degradation threats will have much higher significance for a commercial-scale wind farm, where rectification of a common issue across a large number of floating units could have significant impacts in terms of reliability, operating costs, insurance premiums and power purchase agreements.

FOWF mooring system designs are also likely to be more optimized than that for a typical one-off offshore oil facility. This will require the wind farm designer to have a deep understanding of the fundamental dynamic behavior of the overall system and the local dynamic behaviors of the components within the mooring system in order to be able to fully identify the types of threats that may be present. This may also involve robustness considerations where there may be step changes in the dynamic behavior of the system, often termed as cliff-edge effects.

The paper will outline issues that wind farm designers will need to consider in building integrity considerations into the design and execution phases of a development, as well as the opportunities that risk-based integrity management processes offer in terms of through-life condition verification and inspection optimization.

Topics: Wind farms
Commentary by Dr. Valentin Fuster
2018;():V001T01A020. doi:10.1115/IOWTC2018-1035.

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.

Commentary by Dr. Valentin Fuster
2018;():V001T01A021. doi:10.1115/IOWTC2018-1040.

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.

Commentary by Dr. Valentin Fuster
2018;():V001T01A022. doi:10.1115/IOWTC2018-1047.

There are several challenges facing the design of mooring system of floating offshore wind turbines (FOWTs), including installation costs, stability of lightweight minimalistic platforms, and shallow water depths (50–300m). For station keeping of FOWTs, a proper mooring system is required in order to maintain the translational motions in surge and sway and the rotational motions in yaw of the platform within an adequate range. A combination of light pre-tension, shallow water depth and large platform motions in response to a survival storm condition can result in snap-type impact events on mooring lines, thus increasing the line tension dramatically.

In this paper, we present a new snap load criterion applicable to a catenary mooring system and compare it with Det Norske Veritas’ criterion for marine operations. As a case study, we examine the extreme tension on a catenary mooring system of a semi-submersible FOWT exposed to a 100-year storm condition. The software OrcaFlex was used for numerical simulations of the mooring system. NREL’s FAST software was coupled to OrcaFlex to obtain aerodynamic loads along with hydrodynamic loads for FOWT analyses. Snap-type impact events were observed in the numerical simulations and were characterized by two criteria. Tension maxima were fitted using composite Weibull distributions (CWDs) and comparisons of probability exceedance were made for the two different snap load criteria.

Commentary by Dr. Valentin Fuster
2018;():V001T01A023. doi:10.1115/IOWTC2018-1076.

Suction caissons and anchors are widely used for the foundation of both bottom-fixed and floating offshore structures. They are installed by means of self-weight and underpressure applied to the inside of the skirt compartment. The penetration resistance during installation is the sum of the tip resistance and wall friction of the skirt, both of which are functions of the foundation geometry, soil properties, soil-state and boundary conditions.

Several authors have proposed methods to predict the penetration resistance during installation. In general, high estimates of soil strength parameters are used to predict the installation resistance for design purposes. In addition, safety factors may be applied when predicting the achievable penetration depth and the corresponding loads acting on the structural components. However, current standards and guidelines lack a consistent approach for evaluating these loads and safety factors.

In order to apply a consistent safety concept in the structural caisson design, two methods for assessing the penetration resistance and hence required suction pressure are investigated; a simplified CPT-based method and a more advanced bearing-capacity-based method. For that purpose, a probability-based analysis has been performed, assuming statistical distributions of the corresponding input parameters, and a representative target failure probability. The performance of the two methods methods is investigated using the example of a generic sand and a generic clay profile. Based on these analysis, partial safety factors for the use in a deterministic design are proposed.

Topics: Caissons , Safety , Suction , Soil
Commentary by Dr. Valentin Fuster
2018;():V001T01A024. doi:10.1115/IOWTC2018-1077.

Industrial realization of both floating offshore wind and wave energy technologies requires reductions in the current high levelized cost of energy. Reducing mooring fatigue loads could decrease levelized cost of energy, as mooring is expected to be a major cost of these systems. Previous work on improving mooring reliability and costs has focused on material and design. In this exploratory study, we quantify how placing WECs in front of a FOWT could reduce fatigue damage incurred by FOWT mooring cables in long-crested wave conditions. We use SWAN to quantify the WEC-induced sea state modifications and obtain wave spectra at the FOWT location. The spectra are then input into WEC-Sim and MooDy to model the mooring cable behavior. Fatigue analysis with Rainflow Counting is used to quantify the fatigue loading on the mooring cables. Results from this study show a 8% reduction in fatigue damage to mooring cables over the lifetime of the structure. These results indicate that co-location could have a beneficial effect on FOWT mooring cable fatigue. In future work, these results will be leveraged to perform optimal O&M planning and reliability-based design optimization of floating offshore wind turbines.

Commentary by Dr. Valentin Fuster
2018;():V001T01A025. doi:10.1115/IOWTC2018-1099.

This paper explores the impact of friction models on mooring line simulations. Seabed friction can play an important role in the determination of mooring loads of slack-moored floating offshore wind turbines. Most mooring models include a relatively simple seabed friction formulation, if any, and little examination of their accuracy is available in literature. Current implementations typically represent seabed contact as coulombic friction with ramping near zero velocity to mitigate instability in the numerical time integration. To assess the impact of this friction model’s use, we compare it against a more sophisticated friction model. This model differentiates between static and kinetic friction, where the former is dependent upon the forces acting on the line and the latter is a function of seabed’s normal response. Both friction models have been implemented into the MoorDyn mooring dynamics simulator and tested under a set of prescribed scenarios including snap loads and oscillatory motion, where the fairlead of a mooring line was driven along both linear and circular paths. Additionally, coupled floating wind turbine simulations using the OC4-DeepCwind semisubmersible show how the friction models affect the platform global response and the extreme and fatigue mooring loads. The results highlight practical differences between the models in terms of both loads prediction and simulation stability/consistency.

Topics: Friction , Mooring , Seabed
Commentary by Dr. Valentin Fuster

Scientific Track: Numerical Modeling

2018;():V001T01A026. doi:10.1115/IOWTC2018-1014.

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.

Topics: White noise
Commentary by Dr. Valentin Fuster
2018;():V001T01A027. doi:10.1115/IOWTC2018-1016.

This paper describes a fully coupled numerical simulation methodology which is tailored towards floating offshore wind turbines. The technique assembles three key components; an aerodynamic model of the applied wind loads based on blade element momentum theory, a structural model of the floating platform and its associated mooring lines based on the nonlinear finite element method, and a hydrodynamic model of the wave-induced forces based on potential flow theory. The simulation methodology has been implemented in a commercial software product called ‘Flexcom Wind’, and the technical validation involves comparisons with experimental data derived from model-scale tank test facilities.

The validation process centres on an innovative floating wind turbine concept developed by Eolink. Unlike most wind turbines in industry which are supported by a single mast, this patented design uses four separate pillars to connect the turbine structure to the corners of the floating platform. This unique configuration offers several advantages over conventional designs, including a more even stress distribution in structural members, reduced dynamic vibration, smaller floater size and lower overall capital expenditure. Data obtained from the numerical simulations combined with the empirical tests is helping to optimise the device, with a view to further improving its structural design and performance.

Commentary by Dr. Valentin Fuster
2018;():V001T01A028. doi:10.1115/IOWTC2018-1025.

The wind engineering community relies on multiphysics engineering software to run nonlinear time-domain simulations (e.g., for design-standards-based loads analysis). Although most physics involved in wind energy are nonlinear, linearization of the underlying nonlinear system equations is often advantageous to understand the system properties and exploit well-established methods and tools for analyzing linear systems. This paper presents the development of the new linearization functionality of the open-source engineering tool OpenFAST for floating offshore wind turbines, as well as the concepts and mathematical background needed to understand and apply it.

Commentary by Dr. Valentin Fuster
2018;():V001T01A029. doi:10.1115/IOWTC2018-1045.

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.

Commentary by Dr. Valentin Fuster
2018;():V001T01A030. doi:10.1115/IOWTC2018-1046.

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.

Commentary by Dr. Valentin Fuster
2018;():V001T01A031. doi:10.1115/IOWTC2018-1059.

The unsteady aerodynamics of floating offshore wind turbine rotors is more complex than that of fixed-bottom turbine rotors, due to additional rigid-body motion components enabled by the lack of rigid foundations; it is still unclear if low-fidelity aerodynamic models, such as the blade element momentum theory, provide sufficiently reliable input for floating turbine design requiring load data for a wide range of operating conditions. High-fidelity Navies-Stokes CFD has the potential to improve the understanding of FOWT rotor aerodynamics, and support the improvement of lower-fidelity aerodynamic analysis models. To accomplish these aims, this study uses an in-house compressible Navier-Stokes code and the NREL FAST engineering code to analyze the unsteady flow regime of the NREL 5 MW rotor pitching with amplitude of 4° and frequency of 0.2 Hz, and compares all results to those obtained with a commercial incompressible code and FAST in a previous independent study. The level of agreement of CFD and engineering analyses in each of these two studies is found to be quantitatively similar, but the peak rotor power of the compressible flow analysis is about 20 % higher than that of the incompressible analysis. This is possibly due to compressibility effects, as the instantaneous local Mach number is found to be higher than 0.4. Validation of the compressible flow analysis set-up, using an absolute frame formulation and low-speed preconditioning, is based on the analysis of the steady and yawed flow past the NREL Phase VI rotor.

Commentary by Dr. Valentin Fuster
2018;():V001T01A032. doi:10.1115/IOWTC2018-1071.

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.

Commentary by Dr. Valentin Fuster
2018;():V001T01A033. doi:10.1115/IOWTC2018-1083.

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.

Commentary by Dr. Valentin Fuster
2018;():V001T01A034. doi:10.1115/IOWTC2018-1087.

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.

Commentary by Dr. Valentin Fuster
2018;():V001T01A035. doi:10.1115/IOWTC2018-1104.

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].

Topics: Wind turbines
Commentary by Dr. Valentin Fuster
2018;():V001T01A036. doi:10.1115/IOWTC2018-1112.

The Floating Offshore Wind Turbine (FOWT) is a fairly new concept. There are limited number of full-scale prototypes to provide real data. Therefore, most of the research today relies on numerical models. This is required, so that an adequate amount of confidence can be gained before venturing into large scale production. The major challenge ahead is proving their reliability and robustness. There needs to be supporting studies that consider most factors that can go wrong. The computer program FAST was a groundbreaking contribution from NREL in this regard. FAST is capable of doing combined loading analysis of FOWTs. The numerical model used for the hydrodynamics can, however, be improved further.

Non-linear hydrostatic and wave forces on floating structures become very important during large amplitude waves. The computer program SIMDYN is a blended time domain program developed by Marine Dynamics Laboratory at TAMU and is capable of capturing the role of non-linear fluid forces. SIMDYN has previously been used to demonstrate that nonlinear hydrostatics become very important in the problem of parametric excitation. In the current work, SIMDYN is coupled with FAST. The FAST-SIMDYN is now a tool that is capable of studying large amplitude motions of FOWTs in extreme seas.

FAST-SIMDYN was then used to study the classic instability of negative damping that occurs in FOWTs that use conventional land based control. The development of platform pitch and platform surge instability are studied in relation to different wave and wind scenarios. The intent was to do an analysis to see if the non-linear forces do play a significant role in large amplitude motions induced by negative damping. This study gives an indication of whether the development of an even more sophisticated hydrodynamic modules is justified.

Commentary by Dr. Valentin Fuster

Scientific Track: Offshore Farms

2018;():V001T01A037. doi:10.1115/IOWTC2018-1024.

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.

Commentary by Dr. Valentin Fuster
2018;():V001T01A038. doi:10.1115/IOWTC2018-1058.

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.

Commentary by Dr. Valentin Fuster
2018;():V001T01A039. doi:10.1115/IOWTC2018-1061.

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.

Commentary by Dr. Valentin Fuster

Scientific Track: Structural Analysis

2018;():V001T01A040. doi:10.1115/IOWTC2018-1010.

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.

Commentary by Dr. Valentin Fuster
2018;():V001T01A041. doi:10.1115/IOWTC2018-1015.

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).

Commentary by Dr. Valentin Fuster
2018;():V001T01A042. doi:10.1115/IOWTC2018-1062.

The site-specific load verification for floating offshore wind turbines requires the consideration of the complex interaction of the different system components and their environment. Sensitivity analyses help reducing the simulation amount for both fatigue and ultimate load analysis significantly by highlighting relevant load parameters and increase the understanding for the system behavior in its real environment. Aligned with work in the H2020 project LIFES50+, this study investigates different approaches for global sensitivity analysis using quasi-random sampling for the independent variables.

Two different load case groups are analyzed: (1) fatigue loads during power production, (2) ultimate loads during power production and severe sea state. The considered system is the public DTU 10MW turbine’s rotor-nacelle assembly, installed on the public NAUTILUS-10 floating structure. Load simulations are performed by using FAST v8. Simulations are set up based on the LIFES50+ Site B (medium severity). A comparison is made to a similar study with a different platform (Olav Olsen semi-submersible) in order to observe if similar conclusions can be reached for the different floater types.

Commentary by Dr. Valentin Fuster
2018;():V001T01A043. doi:10.1115/IOWTC2018-1064.

The initial design of a 12-MW floating offshore wind turbine (FOWT) was made by the University of Ulsan (UOU) based on the 5-MW offshore wind turbine of the National Renewable Energy Laboratory (NREL) using the law of similarity. The tower design was checked through the eigenfrequency and fatigue strength analysis according to the GL guideline of tower design conditions. The direct expansion of the 5 MW wind turbine support structure caused a resonance problem of the tower of the 12-MW UOU FOWT and the tower length was adjusted to avoid the 3P resonance. Wind turbines are required to have a design life of more than 20 years and shall be designed to endure both ultimate and fatigue loads experienced during the design life. The platform pitch motion of FOWTs due to combined wave and wind loading may result severely in both fore-aft forces and moments at the base of the tower. In this study, we used the simplified fatigue analysis, which is generally applied when considering safety margins by stress to predict the fatigue life of tower. In order to calculate the fatigue load, the Markov matrix was constructed by using the cycle counting method to determine range, average value, and cycle number of loads from peak and valley values of actual load histories simulated by FAST v8 of the tower base. The predicted fatigue life at the tower base was follow by S/N curves for welded steel structures and it was calculated by the Palmgren-Miner’s rule.

Commentary by Dr. Valentin Fuster
2018;():V001T01A044. doi:10.1115/IOWTC2018-1074.

Intermittent brittle crushing often occurs in the movement of an ice sheet against an offshore structure. Matlock’s ice-structure interaction model is used to simulate the behavior of the ice crushing by modeling ice teeth indentation contacting a spring-mass-dashpot structure. The dynamic behavior of this analytical ice-structure interaction system is studied using Fourier analysis to efficiently predict the response amplitude of specific dynamic periodicity at a given indentation speed. The system’s equations of motion are established based on the assumption of continuous ice indentation. This assumption well captures the ice crushing failure by allowing immediate contact of the offshore structure with the next ice tooth at the time of fracturing of a previous tooth. The fourth mode shape of a numerical wind tower system is studied to convert the physical parameters to the parameters of the spring-mass-dashpot system.

The time histories of ice tooth deflections are expressed through the non-linear dynamic equations. The kinematic initial conditions and the response amplitude of the wind turbine structure can be predicted at targeted periodicity via the Fourier analysis. Given a representative offshore wind tower system, the mode shapes of the physical system are calculated as inputs for the ice-structure interaction model. As an extended validation, the amplitudes of the structural dynamic vibrations predicted by the analytical models at specific periodicity and using the mathematical closed-form simulation are compared with the numerical simulation results.

Commentary by Dr. Valentin Fuster
2018;():V001T01A045. doi:10.1115/IOWTC2018-1075.

Offshore wind turbines are subjected to combined static and cyclic loads due to its self weight, wind, current and waves. For the design of support structures, a point of concern is whether the highly varying loads may cause cyclic degradation of the soil leading to a permanent undesired pile settlement and tilting for the wind turbine. In particular during a severe storm, the large cyclic loads are being more critical as the wind and waves are typically from a single direction. The DTU 10MW wind turbine supported by a jacket at 33 m water depth is considered in this study, where the piles are axially loaded in order to bear the moment under wind and wave actions. This paper investigates the cyclic loads using traditional linear irregular waves and fully nonlinear irregular waves realized from the wave solver Ocean-Wave3D previously validated until near-breaking wave conditions. This study shows that the nonlinear irregular waves introduce more extreme cyclic loads, which result in significantly larger pile settlement than using linear wave realizations. For the case in this study, linear wave theory underestimates pile settlement at least 30% compared to nonlinear wave realizations.

Commentary by Dr. Valentin Fuster
2018;():V001T01A046. doi:10.1115/IOWTC2018-1078.

Fatigue limit state design check based on S-N curve and Palmgren-Miner rule approach is required by design standards for floating wind turbines since floating wind turbines are subjected cyclically varying environmental loads. Fatigue damage over a given period could be considered as a summation of short-term fatigue damage of each stationary environmental condition multiplying by the number of occurrence of the environmental condition during the given period. In reality, environmental condition varies continuously. While fatigue damage due to transient load processes that are induced by startup and shutdown operations cannot be captured by this conventional approach. Large data bases of real-time measurements show shutdown and startup operations may appear in any operational conditions and the number of the operations could be considerable. Analysis for startup and shutdown operations induced fatigue damage is required by standards for offshore wind turbines. However, relevant publications addressing this issue are very limited in particular for floating wind turbines. In contrast to bottom fixed wind turbines, floating wind turbines have rigid-body motions in 6 d.o.f.s, while the floating wind turbine hulls are moored by mooring lines rather than directly mounted on sea bed or land.

This paper focuses on shedding light on the importance of startup and shutdown induced transient load processes on fatigue damage in the tower of two MW-level horizontal axis semi-submersible wind turbines by comparing short-term fatigue damage in several environmental conditions with and without transient load processes induced by startup and shutdown of the wind turbines. In some situations, 4,600 seconds short-term fatigue damage may be dominated by the transient load process induced fatigue damage which may make the fatigue damage be increased by up to 300%, while in many cases, the fatigue damage may be increased by 10% to 100%. The importance of the transient load processes on long-term fatigue damage is related to occurrence frequency of startup and shutdown events and needs more analysis in future.

Commentary by Dr. Valentin Fuster

Project Development Track: Design and Operational Challenges

2018;():V001T02A001. doi:10.1115/IOWTC2018-1039.

In today’s world, the advent of bigger and higher building structures has pushed the limits of engineered lifts. A single heavy payload is preferable to many small loads. This practice cuts down on lead time, infrastructure and manpower to finish the job. This is where high-performance synthetic rope solutions are needed. Light weight yet high-strength fiber provides faster and safer rigging. These lifting operations require rigorous design and analysis to determine the ideal application parameters for the lift planning process... Simply, it can be a headache. Samson engineers, backed by historical big data and testing capabilities, have managed to simplify the process of finding the right sling in the most efficient way. Using proprietary technology, the time of choosing any rope size from available inventory to build that hi-capacity sling has come. The system is built to offer multiple options through its smart multi-loop sling configurator. Current capability includes building slings with rope ranging in size from 16 to 168mm, to a maximum number of 8 loops — which translates to the possibility of reaching up to 4,000mT maximum break strength on a short length of 3 meters. Smaller bend radius ratio is also possible because the system interacts with individual ropes, while working as a system. The use of a mechanical splice also allows for tighter length tolerances. Having this breakthrough at the tip of your fingers will set you apart.

Topics: Design , Ropes
Commentary by Dr. Valentin Fuster
2018;():V001T02A002. doi:10.1115/IOWTC2018-1069.

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.

Commentary by Dr. Valentin Fuster

Project Development Track: World Wide Projects

2018;():V001T02A003. doi:10.1115/IOWTC2018-1021.

The typical offshore wind turbine generator (WTG) currently being installed worldwide produces 6 to 8 megawatts of electrical power and stands more than 100 meters above the ocean surface. The next generation turbines will produce 12 megawatts or more. In the summer of 2016 five of these turbines were installed in the coastal waters of Rhode Island. They are representative of the latest in a constantly evolving series of WTGs. As manufacturers continue to develop more powerful turbines, larger and larger specialized vessels will be needed to lift the components at assembly sites offshore. The process these vessels use to build America’s offshore windfarms will need to be different from the approach normally used at sites outside the U.S. This is due in part to regulatory restrictions imposed by the Merchant Marine Act of 1920, which is more commonly known as the Jones Act. As it pertains to offshore windfarm construction, the Jones Act prohibits foreign-flagged vessels, (vessels built outside the United States) from moving cargo between two U.S. ports. Under the law, an offshore windfarm within territorial waters is considered a U.S. port. Therefore, employing a foreign built windfarm construction vessel to transport components from a nearby port to the construction site, as it typically does, violates the Jones Act. Although the Jones Act might appear to be a deterrent to the development of offshore wind in the U.S., it is not. The developers of the Rhode Island’s Block Island Wind Farm (BIWF) used a combination of U.S. and foreign-flagged vessels to construct BIWF, while remaining in compliance with the law. This paper reviews the approach used at Block Island and discusses how it can be applied to future projects. An analysis of alternatives for various vessel fleet combinations that can be used for this purpose is provided. The analysis is based on results produced by a throughput and scheduling model developed by the authors. The model leverages the experience gained during the construction of BIWF, and was validated with data collected during the project. The alternatives include scenarios utilizing existing vessels modified for windfarm construction service, foreign-flagged windfarm construction vessels, and new vessels specifically built for the U.S. market. The paper concludes with a review of the relative efficiencies of various fleet configurations while undertaking the construction of a notional windfarm located off the East Coast of the U.S.

Commentary by Dr. Valentin Fuster
2018;():V001T02A004. doi:10.1115/IOWTC2018-1026.

The state of California stands at a crossroads where many different enablers are now coming together to spur its leadership in a new offshore wind energy industry off the west coast of the US. This paper presents the rationale for this new industry to be built from the ground up and elaborates on the development efforts recently undertaken by Principle Power Inc. (PPI) to jumpstart this important opportunity. The paper will first focus on the unique value proposition offshore wind offers to the Golden State and discuss the path the company has taken to accelerate the development of the offshore wind industry along the coast, with the proposition of a flagship project in Humboldt County.

Commentary by Dr. Valentin Fuster

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