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This paper describes the development of a fault detection and diagnosis method to automatically identify different fault conditions of a hydraulic blade pitch system in a spar-type floating wind turbine. For fault detection, a Kalman filter is employed to estimate the blade pitch angle and valve spool position of the blade pitch system. The fault diagnosis scheme is based on an artificial neural network method with supervised learning that is capable of diagnosing a predetermined fault type. The neural network algorithm produces a predictive model with training, validation and test procedures after the final performance evaluation. The validation and test procedures of the artificial neural network model are conducted with the training model to prove the model performance. The proposed method is demonstrated in case studies of a spar floating wind turbine with stochastic wind and wave conditions and with consideration of six different types of faults, such as biases and fixed outputs in pitch sensors and excessive friction, slit-lock, wrong voltage, and circuit shortage in actuators. The fault diagnosis results from the final performance evaluation show that the proposed methods work effectively with good performance.

This paper proposes a stress analysis method for three-dimensional structures, with the consideration of the global hydroelasticity which is calculated through the beam connected discrete modules method. First, the continuous structure is discretized into rigid modules connected by elastically equivalent beams. The hydroelastic responses are solved by the coupling of the hydrodynamics of rigid modules and the structural stiffness of elastic beams. Then, according to six degrees of freedom motions of each module extracted from the hydroelasticity, the pressure distributions of rigid modules can be obtained based on the three-dimensional potential theory. Finally, the inertia force and the pressure distributions are statically loaded on three-dimensional finite element model. The process is applied to a barge under the regular wave. In rigid condition, the local stress using the proposed method is verified against the result from the Quasi-static method. Furthermore, the springing effect on the local stress is investigated. In the test model, the result shows that the springing effect induces an obvious difference.

Utformingen av et drivverk med middels hastighet for 10-MW referanse offshore vindturbin for Danmarks Tekniske Universitet (DTU). Det foreslås en fire-punkts drivlinjeoppsett som er utstyrt med en girkasse med to planetrinn og ett parallelt trinn. Deretter er drivkomponentene designet på grunnlag av designbelastninger og kriterier som er anbefalt i relevante internasjonale standarder. Til slutt oppnås en optimalisert drivstasjonsmodell via en iterativ designprosess som minimerer vekt og volum. En high-fidelity numerisk modell etableres via multikroppssystemtilnærmingen. Deretter sammenlignes den utviklede drivlinjemodellen med den forenklede modellen som ble foreslått av DTU, og de to modellene stemmer bra. I tillegg, en resonansevaluering av drivlinje er utført basert på Campbell-diagrammer og modal energidistribusjon. Detaljerte parametere for drivverkdesign og dynamisk modellering er gitt for å støtte reproduksjonen av drivstoffmodellen. En avkoblet tilnærming, som består av global aero-hydro-servo-elastisk analyse og lokal drivanalyseanalyse, brukes til å bestemme drivverkets dynamiske respons. De 20 år lange utmattelsesskadene på gir og lagre beregnes ut fra fordelingen av spenning eller belastningstid, Palmgren-Miner lineær akkumulerende skadeshypotese og langsiktige fordeling av miljøtilstand. Deretter etableres et inspeksjonsprioritetskart basert på feilrangeringen av drivlinjekomponentene, som støtter drivstoffinspeksjon og vedlikeholdsvurdering og videre modelloptimalisering. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2020 The Authors. Wind Energy Published by John Wiley & Sons, Ltd.

Conventional drivetrains with gearbox failures are associated with major downtime in offshore wind turbines. This impacts the maintenance cost and cost of energy. A hydraulic transmission eliminates the need for gearbox and potentially improves the turbine reliability. This paper explores the application of a novel high-pressure transmission machinery to a utility-scale spar floating wind turbine. We present a dynamic model of a hydraulic system consisting of a hydraulic pump, pipelines, a hydraulic motor, and an induction generator. The motor is placed inside the spar platform and operates at a fixed displacement. The hydraulic system is coupled with the aero-hydro-elastic code HAWC2 through an external dynamic link library. The dynamic responses of a 5 MW spar floating wind turbine and a land-based reference wind turbine with this hydraulic system were investigated under various wind and wave conditions. A comparison is made between the characteristics of the two wind turbines under operational conditions and both turbines show decent behaviors, and the statistics of power generation is at the same level compared to that of conventional wind turbines. Through simulation studies, we verify the feasibility and the engineering practice of the proposed hydraulic drive solution to floating wind turbines.

Compared with jack-up crane vessels that are now widely used in offshore wind turbine installation, floating crane vessels are more flexible with respect to working water depth and are much faster in relocation. They are thus a promising alternative to install offshore wind turbine components, especially in intermediate and deep water. However, the wave-induced motions of the floating vessels make the operations challenging. This study deals with a preliminary feasibility study on offshore single blade installation using floating crane vessels. Two typical floating crane vessels are considered, i.e., a mono-hull vessel and a semi-submersible vessel. They are assumed to be equipped with dynamic positioning systems that can well mitigate the slowly varying horizontal motions. Their overall performance during the blade installation is numerically evaluated by comparing their performance against a typical jack-up crane vessel. The crane dynamics plays a less important role for blade installation by floating vessels, compared to the jack-up crane vessel. The floating vessels’ wave-induced motion greatly affects the blade motion. The semi-submersible vessel causes a much smaller blade motion than the mono-hull vessel. The results indicate that it is feasible to install offshore wind turbine blades by using floating crane vessels provided that the vessel type is properly selected. From the operability point of view, semi-submersible vessels are more feasible than mono-hull vessels for offshore single blade installations.

This study investigates the effect of variability and uncertainty of wind and wave conditions on the short-term fatigue damage of a 10-MW floating wind turbine drivetrain. Global dynamic responses of a semi-submersible wind turbine are calculated by aero-hydro-servo-elastic simulations in various environmental conditions. Then, rotor and generator loads, as well as nacelle motions from the global analysis are provided to a drivetrain model to investigate its dynamics. One-hour fatigue damage of the drivetrain bearings is calculated based on the bearing loads and speeds, and the effect of uncertainties related to wind and waves is assessed. The results indicate that the variations of mean wind speed, turbulence intensity and wind shear have great effects on the studied drivetrain. The effect of uncertainties of irregular waves on the drivetrain fatigue damage is small. Five wind and wave random samples are sufficient for dynamic analysis of the drivetrain to achieve accurate results at a reasonable computational cost. Among the drivetrain components, the main bearings are generally most sensitive to the investigated environmental variables. Finally, this study discusses the variation of environmental variables in terms of their relative importance for drivetrain analysis. The results provide a basis for establishing improved design standards and engineering practice for design and analysis of floating wind turbine drivetrains.

Traditionally, drivetrain responses are obtained by a de-coupled analysis, which first involves a global analysis with a simplified representation of the drivetrain, followed by a detailed analysis of the drivetrain with the input of global response on the drivetrain interface. As the wind turbine size increases, it is questionable whether this de-coupled analysis method yields sufficiently accurate results. To address this question, a comparative study of the drivetrain dynamic behaviour obtained by a fully coupled method and a de-coupled one, is conducted and reported in this paper. A 10-MW fully coupled aero-hydro-servo-elastic floating wind turbine dynamic model is developed, including a high-fidelity drivetrain. The developed fully coupled model is assessed to be reasonable via the comparison of drivetrain first-order natural frequency and code-to-code comparisons in terms of global responses between two simulation tools Simpack and Fast. Resonance analysis of the 10-MW drivetrain in the fully coupled model is performed, with focus on rotor-drivetrain-bedplate-tower coupled modes in the low frequency range. Time domain simulations of the drivetrain in the fully coupled and the de-coupled models are carried out in different environmental conditions. One-hour fatigue damage of drivetrain gears and bearings in the fully coupled and de-coupled models are compared. Effect of nacelle motion on drivetrain fatigue damage in the de-coupled analysis is discussed. The results are presented to demonstrate whether the de-coupled method could be confidently used for drivetrain dynamic analysis. This study provides a basis for drivetrain design and dynamic analysis in floating wind turbines.

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.

The regulatory requirements for the Norwegian Continental Shelf specify that ship impact actions and other accidental actions should be determined by risk assessment. However, when the first requirements on collision energy from supply vessels were introduced by DNV around 1980, the frequency of impacts by attendant vessels were high – of the order of 10−3 per installation year. Therefore, it was assumed in the initial requirements that the impact action associated with attendant vessels should, as a minimum, be calculated for the maximum authorized vessel assumed to service the installation. At that time the resulting minimum impact energies were 11 and 14 MJ for head on and side impact, respectively; and have remained the same since then. However, the supply vessel size has since increased and design of supply ship bow and platform has changed. Further, the use of DP controlled supply vessels has increased, which may imply larger velocities at impact. Moreover, the consequence of ship impacts might change e.g. due to the change in design of supply vessels by providing ice-strengthened bows in supply vessels and platforms with cantilevered decks. In the revision of the NORSOK N-003 standard on “Actions and action effects” the requirements to ship impacts are being reassessed and updated based on statistics on supply vessel sizes and collision energies; as well on service experiences. Besides revisiting the requirements to attendant vessels, other ship impact scenarios are also considered. This especially includes the collision risk associated with shuttle tanker – FPSO. This paper presents the background for the revised standard; in terms of ship impact actions relating especially to supply vessels and shuttle tankers, recognising that the main risk control relating to ship impact is to limit the probability of impacts by operational control. Moreover, the consequences in terms of damage for different types of platforms are addressed, by e.g. demonstrating the feasibility of satisfying more restrictive requirements and especially the effect of providing ice-strengthened bows in supply vessels and designing platforms with cantilevered decks.

The Norwegian Public Road Administration is planning to upgrade Coastal Highway E39 by replacing ferry connections with floating bridges or submerged floating tunnels (SFTs). This study considers a potential pontoon-supported curved SFT designed for crossing Sognefjorden at a submergence of 12 m. It consists of two identical tubes with a diameter of 12.6 m each in a tandem configuration and with a length of approximately 4 km. The natural frequencies of the low-order modes are well within the energy content in the spectra of the second-order difference-frequency wave excitation forces and the vortex shedding-induced forces. In this paper, numerical simulation of wave-induced hydroelastic response and flow-induced vibrations of the twin-tube SFT is performed. Long- and short-crested waves, the first and second order wave loads, are considered. A time-domain approach to simulate crossflow vortex-induced vibration (VIV) and VIV-amplified inline drag forces, partly based on the coefficients obtained experimentally, is established and applied. The focus is on extreme conditions – relating to ultimate strength limit states. The second-order wave load substantially affects the lateral motion and lateral bending moment, as expected. The short-crested waves influence the response in both the lateral and vertical directions by exciting asymmetric eigenmodes. In strong flow conditions, once VIV is excited, the standard deviation of the vertical motion (of about 30% of the diameter) and the bending moment about the horizontal axis is more that an order of magnitude larger than that induced by the wave loads. The simulation of the wave- and flow-induced load effects provides a good reference for the design of SFTs.

Multi-body time-domain finite element models, which implement a recently developed numerical approach for determining forces and moments in floaters, are developed to simulate rigid-body motions and sectional forces and moments of a reference 5-MW braceless semi-submersible wind turbine in turbulent winds and irregular waves corresponding to below rated, at rated and above rated conditions. The simulated responses are compared with measurements of a 1:30 scaled model test using a real-time hybrid testing approach. In general, agreement between simulations and measurements are very good. Differences in spectral densities of the measurements and simulations have been quantified while the reasons for the differences have been thoroughly analyzed and discussed based on comparisons of measurements in different conditions and numerical parametrical study. Effects of non-linear wave excitation loads and drag forces on the rigid-body motions and sectional forces and moments are analyzed while dominant load components in fore-aft bending moments in five cross-sections in the hull of the reference model are identified. The interface between the pontoons and central column of the reference model is identified as the most critical part. Both low frequency and wave frequency load effect should be accounted for. Mean forces and moments from wind and waves result in a change in configuration of mean wetted body surface of the hull when compared to its configuration in calm water. This may result in a considerable change in resultant sectional forces and moments even though change in resultant of the hydro pressure forces on whole of the wetted body surface could be very limited. For the analyzed model, simulated fore-aft bending moments of the model in wind and waves could be obtained by superimposing the results for wind only condition, and wave only condition except that the corresponding averaged wind induced forces and moments should be applied on the numerical model. This simplification can significantly reduce number of cases of short-term analysis required in long-term analysis. However, applicability of the simplification should be analyzed case by case in particular for a blunt structure with relatively large volume of displaced water in waves with relatively small wave length. Analysis and discussions given in this paper are based on available measurements of the model test. Hydroelastisity and structural vibration of the columns and pontoons of the hull are not accounted for by the numerical and experimental models. Suggestions for design of future model tests are given in this paper. Copyright 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/)

The characteristic values of the extreme environmental load effects should correspond to a specified annual probability of exceedance. These load effects can be calculated using short-term or long-term methods. The full long-term method is considered the most accurate approach, but it requires tremendous computational effort for complicated structures, especially when nonlinearities must be considered. In a case study of the dynamic behavior of a three-span suspension bridge with two floating pylons, these nonlinearities are found to have a significant effect on the extreme values of some of the load effects. It is thus recommended to determine these responses in the time domain. However, time-domain simulations can be very time consuming even by using simplified approaches such as the environmental contour method (ECM) and the inverse first-order reliability method (IFORM). Therefore, this paper introduces a computationally efficient approach utilizing the ECM and the IFORM to determine long-term extreme values based on responses from combined frequency- and time-domain simulations.

When tidal turbines are utilized in the most energetic waters where there are significant waves, the assessment of the surface wave effects are of great concerns. The objective of this paper is to contribute to a fundamental understanding of surface wave effects on tidal turbines. A numerical model was developed based on the modified Blade Element Momentum theory with an inclusion of added mass effects, wave excitation forces and a one degree-of-freedom (DOF) simulation for turbine rotational motion. The experiments on a 1:25 scaled tidal turbine were performed in a towing tank. It is shown that the surface waves did not affect the average loads and power output, but caused severe periodical oscillations. The amplitudes of the cyclic thrust and torque could reach up to 50% of the mean value induced by the incident waves with period of 1.6 s and height of 14 cm. Non-dimensional response amplitude operators (RAOs) of thrust and torque were proved to be sensitive to submergence of the turbine. The wave induced torque and thrust tend to a fixed value when the incident wave length is much longer than the water depth, which provides an approximate assessment of the surface wave effects on tidal turbines.

The Norwegian Public Road Administration (NPRA) is currently developing the E39 ferry-free project, in which several floating bridges will be built across deep and wide fjords. In this study, we consider the floating bridge that was an early concept for crossing the Bjornafjorden with a width of about 4600 m and with a depth of more than 500 m. The floating bridge concept is a complex end-anchored curve bridge, consisting of a cable-stayed high bridge part and a low bridge part supported by 19 pontoons. It has a number of eigen-modes, which can be excited by wave loads. Wave loads and their effects should thus be properly modeled and assessed. Therefore, the effect of hydrodynamic load modeling are investigated in homogeneous wave conditions, including varying water depth at the ends of the bridge, viscous drag force on pontoons, short-crestedness and second order wave loads. It is found that the varying water depth has negligible effect, while the other features are important to consider. Second order difference-frequency wave loads contribute significantly to sway motion, axial force and strong axis bending moments along the bridge. However, these effects can be reduced by viscous drag forces, which implies that an appropriate model of viscous drag force effect on the pontoons is important. short-crested waves greatly affect the heave motion and weak axis bending moment. All these considerations on hydrodynamic load modeling are further applied to analyze the wave load effect of a floating bridge in a fjord considering inhomogeneous waves [1].

Installation of blades for wind turbines is challenging due to large lifting height and high precision. Assessment of blade dynamic responses during installation needs advanced simulation tools which are limited at present. This paper aims at developing an integrated simulation tool SIMO-Aero for single blade installation for both onshore and offshore wind turbines. Based on the cross-flow principle, the aerodynamic model is established by accounting for the effect of wind turbulence and dynamic stall. Then it is coupled with SIMO to achieve the integrated simulation tool SIMO-Aero which can account for blade aerodynamics, vessel hydrodynamics and system mechanical couplings. The aerodynamic code is verified by code-to-code comparisons with HAWC2. Furthermore, SIMO-Aero is applied in case studies on the wind-induced dynamic responses of a DTU 10 MW blade during installation using a jack-up crane vessel which is assumed to be rigid, including the crane, and rigidly fixed to the seabed. The characteristics of system dynamic responses prior to mating the blade onto the hub are studied. It is shown that the blade motions are dominated by the pendulum motion. Critical parameters of the installation process are identified. The extreme responses of critical parameters are further studied under turbulent winds and wind gusts.

Aiming to overcome the limitations of conventional offshore field development concepts (dry tree or subsea tree) for petroleum production in ultra-deep water, a new alternative offshore field development solution, termed as Deepwater Artificial Seabed (DAS) system, is proposed. The DAS system works in concert with dynamic positioning (DP) floaters, such as dynamically positioned Floating Production, Storage and Offloading (FPSO) vessels. Rather than relying on the passive mooring system, the DP maintains the reliable position of the FPSO with steering and propulsion units. Nonetheless, critical DP failures, which has potential to cause the drift-off scenario for the FPSO, poses a serious threat to the structural safety of the DAS system. Therefore, it is crucial to establish operational limits for the DP FPSO to prevent such accidents. In this study, a 3-phase probabilistic modelling methodology is proposed to predict safety limits for the operation of the DP FPSO. A surrogate model is established by the Support Vector Machine (SVM) algorithm so as to decrease the computational cost due to the generation of large statistical samples. The statistical distribution of the operational safety limits of FPSO is simulated by the successive approximations through the fully-coupled drift-off analysis. The accuracy of the proposed methodology is verified by a series of mathematical tests. In order to validate the effectiveness of the methodology, the safety limit prediction of the FPSO for the DAS system is taken as a case study. The critical positions of the FPSO are predicted in real time and provides ample time and information for operators’ decision-making by the visualization of the safe moving range of the FPSO. The study contributes to the safety control of DP operations on floating production units in an efficient manner.

In this paper, a time-domain hydroelastic method for very large floating structures (VLFS′) in inhomogeneous waves is developed based on Cummins' equation. By discretizing the continuous VLFS into rigid modules connected by elastic beam elements, the inhomogeneous wave effect can be considered by adopting different wave spectra over different regions of the VLFS. In this method, the frequency-domain hydrodynamic coefficients, considering the hydrodynamic interactions between each floating module, are transformed into the time-domain hydroelastic model using Cummins' equation. Moreover, the time-domain wave excitation forces on the modules in different regions are solved using different wave spectra. The hydroelastic responses of a freely floating structure in inhomogeneous regular and irregular waves are investigated. The results show that the inhomogeneity of waves has a significant effect on the bending moments, shear forces and torsional moments of the structure, especially for a wave direction of 90°, in which larger forces may be induced compared with the homogeneous waves.

The probability density of crest amplitude and duration that exceeds a given level is used in many theoretical and practical problems in engineering that are subjected to fluctuating loads such as wind and wave loads. The presently available joint distributions of amplitude and period are limited to excursion through a mean-level or to describe the asymptotic behavior of high level excursions. This paper extends the knowledge by presenting a theoretical derivation of probability of wave exceedance amplitude and duration for a stationary narrow-band Gaussian process. A density function is suggested that has the salient feature to depend only on the three lowest spectral moments m0, m1, and m2 and desired level of exceedance, H. It does not require any condition on the autocorrelation function. This paper shows how increase in H, increases the correlation between excursion periods and amplitude. This paper also shows that how the accuracy of the proposed joint distribution relates to spectral width parameter, ν, and that accuracy increases for higher levels of H, especially for a spectrum describing a physical phenomenon such as a sea state spectrum. It was demonstrated that the marginal distribution of amplitude is Rayleigh distributed, as expected, and that the marginal distribution of excursion duration works for asymptotic and non-asymptotic levels. Results demonstrate that the established distribution fits well with ideal narrow-band Gaussian processes as well as the sea states at three European sites —in the Atlantic Ocean and the North Sea. The suggested model is found to be a good replacement for the existing empirical distributions. (C) 2018 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission.

This paper addresses the safety of marine operations related to a seafastening system for large-object transport. The aim is to present a structural reliability approach to accommodate the uncertainties affecting design checks. The probability of structural failure by use of design standards for assessing marine operations is studied using structural reliability analyses to shed light on the implicit reliability levels of such standards. A structural reliability model that includes the effect of uncertainty in weather forecasts is established. The reliability analyses show that the method to account for forecast uncertainty as defined in the standards compensates well for that uncertainty, and the failure probabilities in the case studies are between 1 0 − 4 and 1 0 − 3 per operation. A reliability model that includes the long-term statistical distribution of the environmental conditions is also established. This model is applicable for operations with a duration longer than three days and is used to study seasonal variations and the failure probability as a function of the duration of operations. The failure probability is calculated for execution in several months, showing the dependency on the time of the year and the duration of the operations. The failure probabilities are on the order of 1 0 − 4 per operation.

Based on the three-dimensional (3D) potential theory and finite element method (FEM), this paper proposes a new numerical method for hydroelastic predictions of floating structures in inhomogeneous seabed and wave field conditions. The continuous floating structure is first discretized into rigid modules connected by elastic beams. The motion equations of the entire floating structure are established according to the six degrees of freedom (6DOF) motions of each module by coupling the hydrodynamics of the modules with the structural stiffness matrix of the elastic beams in the frequency domain. By applying different wave excitation forces onto different modules, this discrete-modules-based method then uniquely realizes application of various wave excitation forces onto different modules of the structures in inhomogeneous waves. The hydroelastic responses of a plate and a Wigley hull under an even and uneven seabed using the proposed method are verified against the results from the published model tests and the conventional 3D hydroelastic method. Finally, the effects of inhomogeneous waves on the distributions of the bending moment, shear force and vertical displacements of the freely floating plate are investigated. The results show that the inhomogeneity of waves may induce about 2∼3 times increase of the force responses in a specific wave frequency. This is the authors' accepted and refereed manuscript to the article. Locked until 13 July 2019 due to copyright restrictions.

Current installation costs of offshore wind turbines (OWTs) are high and profit margins in the offshore wind energy sector are low, it is thus necessary to develop installation methods that are more efficient and practical. This paper presents a numerical study (based on a global response analysis of marine operations) of a novel procedure for installing the tower and Rotor Nacelle Assemblies (RNAs) on bottom-fixed foundations of OWTs. The installation procedure is based on the inverted pendulum principle. A cargo barge is used to transport the OWT assembly in a horizontal position to the site, and a medium-size Heavy Lift Vessel (HLV) is then employed to lift and up-end the OWT assembly using a special upending frame. The main advantage of this novel procedure is that the need for a huge HLV (in terms of lifting height and capacity) is eliminated. This novel method requires that the cargo barge is in the leeward side of the HLV (which can be positioned with the best heading) during the entire installation. This is to benefit from shielding effects of the HLV on the motions of the cargo barge, so the foundations need to be installed with a specific heading based on wave direction statistics of the site and a typical installation season. Following a systematic approach based on numerical simulations of actual operations, potential critical installation activities, corresponding critical events, and limiting (response) parameters are identified. In addition, operational limits for some of the limiting parameters are established in terms of allowable limits of sea states. Following a preliminary assessment of these operational limits, the duration of the entire operation, the equipment used, and weather- and water depth-sensitivity, this novel procedure is demonstrated to be viable. © Springer Verlag. The final publication is available at https://link.springer.com/article/10.1007%2Fs11804-017-1418-6. This is the authors' accepted and refereed manuscript to the article.

The risk of vessel capsizing is inherent to anchor handling operations (AHOs). Lessons learned from the Bourbon Dolphin accident reveal that the large static heeling angle could not be prevented due to the lack of awareness of the vessel's stability status, which can be improved with the help of a suitable on-board monitoring system. Therefore, an on-board monitoring system is proposed for assessing stability in terms of the static heeling angle. However, a complete mathematical model is not available for estimating a static heeling angle as a function of operational parameters. Therefore, an artificial neural network (ANN)-based functional relationship has been established between the operational parameters and the static heeling angle. Furthermore, a parametric study has been performed to investigate the effect of neural network topology on network performance. The results show that an ANN topology that contains one hidden-layer is efficient enough to predict a static heeling angle. The correlation coefficient between the ANN model predictions and the target values is 0.999. This result shows that the ANN provides an accurate estimate of the static heeling angle as a function of the operational parameters. Therefore, the proposed mathematical model can be used for assessing a vessel's stability during AHOs.

Currently development of floating wind turbines for deep water is mainly based on horizontal axis wind turbines (HAWTs). However, floating vertical axis wind turbines (VAWTs) are possible alternative due to their potential in the cost of energy reduction. This study deals with a comparison of stochastic dynamic responses of floating HAWTs and VAWTs with emphasis on the extreme structural responses and fatigue damages. A 5 MW three-bladed HAWT and three 5 MW VAWTs with blade number ranging from two to four were mounted on a semi-submersible platform. Their stochastic dynamic responses, short-term extreme structural responses and fatigue damages were estimated in both operational and parked conditions. The results show that the three- and four-bladed floating VAWTs and the three-bladed floating HAWTs considered have similar performances in the variation of generator power production, in the maximum tower base bending moment and in the fatigue damages at tower base and mooring lines. However, the maximum tensions in mooring line for the three- and four-bladed floating VAWTs are approximately four times higher than that of floating HAWTs, which implies a significant challenge for their mooring systems. The maximum tower base bending moment and fatigue damage in the two-bladed floating VAWT are extremely significant. © 2017. This is the authors’ accepted and refereed manuscript to the article. LOCKED until 24.2.2019 due to copyright restrictions. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/

Floating vertical axis wind turbines (VAWTs) are promising solutions for exploiting the wind energy resource in deep waters due to their potential cost-of-energy reduction. The number of blades is one of the main concerns when designing a VAWT for offshore application. In this paper, the effect of blade number on the performance of VAWTs and dynamic behavior of floating VAWTs was comprehensively studied in a fully coupled aero-hydro-servo-elastic way. Three VAWTs with straight and parallel blades, with identical solidity and with a blade number varying from two to four, were designed using the actuator cylinder method and adapted to a semi-submersible platform. A generator torque controller was also designed based on a PI control algorithm. Time domain simulations demonstrated that the aerodynamic loads and structural responses are strongly dependent on the number of blades. In particular, by increasing the number of blades from two to three reduces the variation in the tower base bending moment more significantly than increasing it from three to four. However, the blade number does not significantly affect the generator power production due to the control strategy employed, and the platform motions and tension in mooring lines because of the compliant catenary mooring system. © 2016. This is the authors’ accepted and refereed manuscript to the article. LOCKED until 10.11.2018 due to copyright restrictions. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/

In this paper, we present a numerical study on the hydroelastic response of a 4.6 km long fjord crossing floating bridge subjected to wave loads. The bridge is straight in design and supported by 35 pontoons along its full length. To limit the response to horizontal loads, four clusters of deep water mooring lines are engaged to increase the transverse stiffness of the bridge. Owing to the very large span across the fjord, inhomogeneity in the wave field exists. This study examines the various effects of inhomogeneous wave loads on the dynamic responses of the floating bridge. These include the spatial variations of the wave direction, significant wave height and peak period as well as the coherence and correlation of waves along the entire length of the floating bridge. For the purpose of comparison, the dynamic bridge responses under homogeneous wave load cases are also studied. In addition, the effects of wave load components and short-crestedness are presented and discussed. /© 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)

This study compares the dynamic behaviour of a conventional and a compact gearbox for the DTU 10 MW wind turbine supported on a monopile offshore structure. The conventional gearbox configuration is composed of two planetary epicyclic stages and one parallel stage, while the compact gearbox configuration consists of a fixed planetary stage and a differential compound epicyclic stage. The design methodology for these two gearboxes is described, and the final gearbox specifications show a lighter weight and smaller volume for the compact gearbox design compared to the conventional one. Computational gearbox models are established using the multi‐body system dynamic analysis method. A decoupled approach is employed for the gearbox load effect analysis. Comparisons of the dynamic behaviour between these two gearboxes are conducted under pure torque load cases, tangential pin position error conditions and non‐torque load cases. The results demonstrate that the compact gearbox has better dynamic performance under different torque load cases and is more robust to withstand the effects of manufacturing errors and rotor non‐torque loads compared to the conventional gearbox. It is believed that the proposed compact gearbox concept is promising and would be a good alternative for multi‐megawatt floating wind turbines, although challenging with respect to the design and operation complexity. © 2020 The Authors. Wind Energy published by John Wiley & Sons Ltd. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Mooring systems are important structural components of very long floating bridges. They effectively limit the transverse motions of the bridge under environmental loads. They also add viscous hydrodynamic damping to the entire system. The safe and economical design of mooring systems is thus important but also challenging especially when the wave conditions are inhomogeneous. In this paper, a computational study is carried out to investigate the responses of the mooring lines for a 4.6 km long fjord crossing floating bridge accounting for inhomogeneous wave conditions. Based on the structural responses, this study also attempts to evaluate the fatigue damage in the mooring lines by using different fatigue analysis methods. The accuracy of the spectral methods is examined by comparison with the conventional rainflow cycle counting algorithm. Numerical studies are conducted to obtain an indication of the effect of various wave inhomogeneities on the fatigue damage in the mooring lines. DOI: 10.1016/j.oceaneng.2020.108304. 0029-8018/© 2020 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

This paper deals with the effect of bedplate flexibility on drivetrain dynamics of a 10 MW spar type floating wind turbine. The 10 MW drivetrain bedplate is designed based on extreme design loads and ultimate limit state (ULS) design criteria. A decoupled analysis approach is employed. Global dynamic analysis of the 10 MW floating turbine is firstly conducted using an aero-hydro-servo-elastic code, then the global response is used as input to the drivetrain dynamic analysis. Load effects and fatigue damage of gears and bearings in the rigid and flexible bedplate models in different environmental conditions are compared. In addition, sensitivity of the drivetrain fatigue damage to varying fidelity in the bedplate modelling is studied. The results indicate that the bedplate flexibility would increase the load effects on bearings inside the gearbox, while it would reduce the load effects on the main bearings. Reasonable bedplate modelling fidelity is of great importance, because it could save a great deal of computational costs without loss of the drivetrain dynamic response accuracy. The present work provides a reference for a proper drivetrain design and dynamic analysis in the future, by accounting for bedplate flexibility. © 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).