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This paper presents the design of a compact gearbox for the DTU 10 MW reference offshore wind turbine. An innovative gearbox concept consisting of a fixed planetary stage with a differential compound epicyclic stage is proposed. Power splitting and compound epicyclic transmission technologies are employed, which could effectively reduce the gearbox’ size. Power transmission principle of the gearbox is described, and power distribution on two transfer paths is derived by the geometrical and mechanical relationships among the components. The gearbox is designed based on the design loads and criteria with reference to the relevant international standards, and all of the critical components, gears and bearings, are designed by performing fatigue limit state (FLS) check. A high fidelity drivetrain dynamic model, consisting of the compact gearbox and one four-point support drivetrain configuration, is established by means of multi-body system (MBS) approach. Then, validation of the power distribution is conducted by the comparison of the simulation results and design values. Resonance analysis of the drivetrain model is conducted by employing Campbell diagram, energy distribtuion of components and time domain simulation approach, and the results show that no resonance phenomenon appears in this drivetrain model during the normal operating conditions. In addition, load sharing performance of the MBS model is assessed, indicating the a favorable dynamic operating behavior of the gearbox. It is believed that such compact design could be good alternative for floating offshore wind turbines.

To improve the efficiency of land transport, bridges, submerged tunnels and subsea tunnels are introduced to replace ferries to cross straits. For wide and especially straits with a large depth or very soft bottom, floating bridges or submerged tunnels are attractive. Modern floating bridges can be traced back to the pontoon bridge design implemented in the 1940s, and the notable Hood Canal bridge in 1961. More recent floating bridges include the two Norwegian floating bridges: the 845-m long Bergsoysund and the 1246-m long Nordhordland floating bridges built in 1992 and 1994, respectively. Submerged floating tunnels have been considered as an option for strait crossings, especially wide crossing such as the Gibraltar and Messina straits and the Hogsfjord in Norway. So far submerged floating tunnels have not been built, while immersed tunnels have been used in many places, essentially in relatively shallow water. Currently, the Norwegian Public Road Administration (NPRA) is assessing replacing ferries across 8 fjords by providing bridges or submerged tunnels on the Coastal Highway Route E39 Project. The width of the strait crossings is up to 5 km and the water depth is up to 1300 m. The NPRA is currently considering three alternative floating bridge concepts: curved, end anchored floating bridge or straight, side anchored floating bridge with mooring system and floating suspension bridge with pylons supported by TLP or spar floating bodies; as well as submerged tunnel type concepts. This paper presents an overview of relevant concepts, their characteristic behaviour and design criteria for serviceability and safety, especially dynamic response due to environmental and accidental loads, with a highlight on development trends.

During the design of a floating bridge, extreme structural responses are required to be properly evaluated for ultimate limit state (ULS) design check. This study addresses the estimation of extreme structural responses for an end-anchored curved floating bridge. The floating bridge, about 4600 m, consists of a cable-stayed high bridge part and a pontoon-supported low bridge part. The long-term extreme responses are approximated by using a engineering approach, i.e., the environmental contour method. The sea state with 100-year environmental conditions is considered, and a 90% fractile is used to calculate the short-term extreme responses by using the Gumbel method and the mean up-crossing rate (MUR) method based on 100 1-hour simulations with different seeds. The extreme responses are expressed as μ + κσ, where μ and σ are the ensemble mean and standard deviation, and κ is a multiplying factor. Numerical results show that structural responses are close to Gaussian distributed. κ of axial force and strong axis bending moment along the bridge girder estimated by both the Gumbel and MUR methods vary in the vicinity of 4. κ estimated by the two method deviates, especially for axial force. Moreover, for both methods the estimated κ deviates more significantly if fewer ensembles are used.

Structural design of the floater is an important aspect in developing cost efficient and reliable floating wind turbines. It is difficult to well account for the effect of strong non-linear dynamic characteristics and transient loading events, e.g. wind turbine faults, of floating wind turbines in a frequency-domain finite element analysis. The time-domain approach which implements the Morison's formula cannot accurately account for the hydrodynamic loads on the hull of floating wind turbines. While, the conventional hybrid frequency-time domain approach (based on the potential flow theory) fails to capture structural responses of the hulls since a rigid-body global model rather than a finite element model of the hull is employed. The present paper deals with the development and verification of a time-domain approach that can be easily implemented in various state-of-the-art computer codes for wind turbine analysis, e.g. Simo/Riflex/Aerodyn, OrcaFlex and FAST + CHARM3D, to extend their capabilities to analyze global forces and moments in structural components of a generic floater subject environmental loads from e.g. wind and waves. The global forces and moments in the structural components might be used as inputs of design formulas for structural strength design checks and/or used as boundary conditions in a sub-model finite element analysis to determine structural responses such as stresses. The proposed approach focuses on modeling of the inertia and external loads on the hull and mapping of the loads in the finite element model of the hull. In the proposed approach, floating wind turbines are considered as a system of several structural components, e.g. blades, rotational shaft, nacelle, tower, mooring lines, columns, pontoons and braces, rather than one rigid-body, while a finite element model for the hull is developed to represent the global stiffness of the structural components. The external and inertial loads on the hull are modeled as distributed loads rather than the integrated forces and moments. The conventional hybrid frequency-time domain approach, which is available in the state-of-the-art computer codes, is implemented to model the hydrodynamic loads on each structural component with essential modifications with respect to the corresponding hydrodynamic coefficients, e.g. added mass and potential damping coefficients and wave excitation forces. Approaches for modeling the hydrostatic pressure forces, gravity loads, drag forces and inertial loads on each structural component are also illustrated. Second order and higher order terms of the hydrostatic and hydrodynamic loads and the hydroelasticity effects are not accounted for in the present paper but can be further included. So far, the proposed approach has been implemented in the computer code Simo/Riflex/Aerodyn to analyze global forces and moments in the hull of a semi-submersible wind turbine. Good agreement between the reference values and the simulation results has been observed and indicates that the developed time-domain numerical models are reliable. The simulation results show that the low-frequency aerodynamic loads and fluctuations of hydrostatic pressure forces on and gravity of the floating wind turbine are important contributions to the structural responses, in particular, in the low-frequency range. ©2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Recently the interest in developing vertical axis wind turbines (VAWTs) for offshore application has been increasing. Among the aerodynamic models of VAWTs, double multi-streamtube (DMST) and actuator cylinder (AC) models are two favorable methods for fully coupled modeling and dynamic analysis of floating VAWTs in view of accuracy and computational cost. This paper deals with the development of an aerodynamic code to model floating VAWTs using the AC method developed by Madsen. It includes the tangential load term when calculating induced velocities, addresses two different approaches to calculate the normal and tangential loads acting on the rotor, and proposes a new modified linear solution to correct the linear solution. The effect of dynamic stall is also considered using the Beddoes-Leishman dynamic stall model. The developed code is verified to be accurate by a series of comparisons against other numerical models and experimental results. It is found that the effect of including the tangential load term when calculating induced velocities on the aerodynamic loads is very small. The proposed new modified linear solution can improve the power performance compared with the experiment data. Finally, a comparison of the developed AC method and the DMST method is performed using two rotors and shows that the AC method can predict more accurate aerodynamic loads and power than the DMST method, at least for the considered rotors.

In the present paper the functionality of the Semisubmersible wind energy and Flap-type wave energy Converter (SFC) is examined experimentally. In order to study the functionality of the SFC, the focus is on operational environmental conditions. SFC is a combined concept that utilizes offshore wind energy and ocean wave energy for power production. Details are presented as far as the physical modelling of the wind turbine with the use of a redesigned small-scale rotor and of the Power Take-Off mechanism of the Wave Energy Converters (WECs) with the use of a configuration that is based on a mechanical rotary damper. Tests with quasi-static excitation, motion decay, regular and irregular waves without and with wind that is uniform are conducted on an 1:50 scale physical model. The experimental data are compared with numerical predictions obtained by a fully coupled numerical model using Simo/Riflex tool. A good agreement is observed between experimental and numerical predictions. The combined operation of WECs doesn't affect the tension of mooring lines nor the acceleration of nacelle and the bending moment in tower's base. The produced power of the WECs of the SFC and consequently the functionality of the SFC is estimated. © 2016. This is the authors’ accepted and refereed manuscript to the article. LOCKED until 19.3.2018 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/

Interest in the exploitation of offshore wind resources using floating wind turbines has increased. Commercial development of floating horizontal axis wind turbines (FHAWTs) is emerging because of their commercial success in onshore and near-shore areas. Floating vertical axis wind turbines (FVAWTs) are also promising because of their low installation and maintenance costs. Therefore, a comparative study on the dynamic responses of FHAWTs and FVAWTs is of great interest. In the present study, a FHAWT employing the 5MW wind turbine developed by the National Renewable Energy Laboratory (NREL) and a FVAWT employing a Darrieus rotor, both mounted on the OC3 spar buoy, were considered. An improved control strategy was introduced for FVAWTs to achieve an approximately constant mean generator power for the above rated wind speeds. Fully coupled time domain simulations were carried out using identical, directional aligned and correlated wind and wave conditions. Because of different aerodynamic load characteristics and control strategies, the FVAWT results in larger mean tower base bending moments and mooring line tensions above the rated wind speed. Because significant two-per-revolution aerodynamic loads act on the FVAWT, the generator power, tower base bending moments and delta line tensions show prominent two-per-revolution variation. Consequently, the FVAWT suffers from severe fatigue damage at the tower bottom. However, the dynamic performance of the FVAWT could be improved by increasing the number of blades, using helical blades or employing a more advanced control strategy, which requires additional research. Copyright © 2016 John Wiley & Sons, Ltd.

Because of scale effects and inappropriate hydrodynamic models, the nonlinear hydroelastic response of net cages used for fish-farming cannot be analyzed precisely with traditional model testing or combinations of finite element methods (FEMs) and load models. In this study, an innovative hybrid method is proposed to determine the hydroelastic response of full-scale floater-and-net systems more accurately. In this method, the net for the fish cage was vertically and peripherally divided into similar interconnected sections with different hydrodynamic parameters, which were assumed to be uniformly distributed over each section. A model of a typical section was subjected to various towing velocities, oscillation periods, and amplitudes in a towing tank to simulate the potential motions of all sections in the net under various currents, waves, and floater movements. By analyzing the measured hydrodynamic force from this test section, a hydrodynamic force database for a typical net section under various currents, waves, and floater motions was built. Finally, based on an FEM, the modified Morison equation and the hydrodynamic force database, the hydroelastic behavior of the full-scale fish cage was calculated with an iterative scheme. It is demonstrated that this hybrid method is able to produce correct hydroelastic response for both steady and oscillatory flows. The hydroelastic response of a two-dimensional example of a full-length net panel with steady currents and floater oscillations was studied in detail.

Due to the nature of the fatigue phenomena it is well known that small changes in basic assumptions for fatigue analysis can have significant influence on the predicted crack growth lives. Calculated fatigue lives based on the S–N approach are sensitive to input parameters. Fracture mechanics analysis is required for prediction of crack sizes during service life in order to account for probability of detection after an inspection event. Analysis based on fracture mechanics needs to be calibrated to that of fatigue test data or S–N data. Calculated probabilities of fatigue failure using probabilistic methods are even more sensitive to the analysis methodology and to input parameters used in the analyses. Thus, use of these methods for planning inspection requires considerable knowledge and engineering skill. Therefore the industry has asked for guidelines that can be used to establish reliable inspection results using these methods. During the last years DNV GL has performed a joint industry project on establishing probabilistic methods for planning in-service inspection for fatigue cracks in offshore structures. The recommendations from this project are now included in a Recommended Practice. The essential features of the probabilistic methods developed for this kind of inspection planning are described in this paper.

Offshore wind is an attractive source of renewable energy. In regions with cold climates, such as Canada, the Baltic Sea, and Bohai Bay, a good understanding of ice loads is essential to design a reliable and cost effective support structure for an offshore wind turbine. This paper presents a study of the dynamic ice–structure interaction of a commonly used monopile-type offshore wind turbine in drifting level ice in both parked and operating conditions. A semi-empirical numerical model for ice–structure interaction was coupled to the aero-hydro-servo-elastic simulation tool HAWC2. A convergence study was performed to determine the proper time step and simulation length to obtain reliable response prediction. The simulated ice load was compared with the formulations in international standards. The coupling between the ice loads and the structural response of the monopile-type wind turbine was investigated and found to be important. The effects of the ice characteristics (e.g., ice thickness and drifting speed) were examined with the turbine being in parked and operating conditions. Compared with current numerical model, standards from International Electrotechnical Commission (IEC) and International Organization for Standardization (ISO) predict lower ice loads. 3-D effect of ice–monopile interaction and failure of local ice sheet are considered. More stochastic dynamic phenomena are captured for ice–structure interaction in our numerical model. The effect of the ice thickness on the response was found to be significant. A negligible drifting speed effect is found on the bending moment response in the fore-aft direction. There is a large increase in the fore-aft response as the inclination angle of the cone increases. Further sensitivity studies and validation against model tests will be performed for the conical waterline of the wind turbine. © 2015. This is the authors’ accepted and refereed manuscript to the article. LOCKED until 22.12.2017 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 wind turbines represent a cost-efficient energy solution in deep water where bottom-fixed wind turbine becomes excessively expensive. However, mooring design is quite challenging for all shallow water depths including the transition water depth between bottom-fixed and floating wind turbines, in the order of 50–80 m, for which floating concepts might become more cost-effective than bottom-fixed ones. In this paper, mooring system design for floating wind turbine in shallow water are studied considering both catenary and taut mooring systems. Seven mooring concepts designed for a 5 MW semi-submersible floating wind turbine at 50 m water depth are compared with the purpose to identify solutions that are structurally reliable and economically attractive. The concepts are made of different mooring line materials (chain and synthetic fibre rope), mooring components (clump weight and buoy) and anchors (drag embedment anchor and suction anchor). Based on the latest experimental data, the nonlinear tension-dependent stiffness of synthetic fibre rope are described with an improved numerical model. Performance of the seven mooring concepts are compared with respect to mooring line characteristics, motion response amplitude operator, utilization factor considering the ultimate limit state design and cost etc. Six mooring design concepts are finally recommended for future assessment regarding the application of floating wind turbines in shallow water say 50 m and deeper.

Environmental conditions along a very large floating bridge are inhomogeneous due to complex topography and hydrography. Evaluating the inhomogeneity in environment conditions is important for safety design of floating bridges but challenging because of their large size. This study addresses the assessment of long-term spatial inhomogeneity in wind and wave parameters for environmental conditions in a Norwegian fjord for design of floating bridges based on 16-year hindcast data. The wind and wave data are numerically simulated by using the Weather Research and Forecasting (WRF) model and Simulating Waves Nearshore (SWAN) model, respectively. The accuracy of hindcast wave data is validated by comparison against field measurements. Inhomogeneity in wind and wave parameters are revealed by contour maps by considering two dominant wave directions. Significant wave height and peak period are fairly close in the south part and decrease gradually towards the north end. Mean wave direction decreases from south end to north end. Mean wind speed has a relatively small variation in the south part and a great variation in the north part, so do the mean wind direction. Correlation of wind and wave parameters along a potential bridge route is analyzed. Wind parameters are less correlated than the wave parameters.

Long-term extreme load effects are one of the primary concerns in the design of civil and offshore structures. Such load effects can be evaluated using the accurate but computationally demanding full long-term method or the more efficient but approximate first-order and second-order reliability methods. Monte Carlo based methods enhanced with machine learning algorithms offer efficient alternatives to the traditional methods. Therefore, artificial neural networks and support vector machines are used as surrogate models for the limit state function to speed up the prediction of long-term extreme load effects. A three-span suspension bridge with two floating pylons under combined wind and wave actions is used as a case study. The cumulative density functions of the long-term extreme values corresponding to a bending moment value due to vertical deflections at the critical position of the girder are calculated. It is then shown that the artificial neural network and support vector machine-based approaches require less computational effort and yield more accurate results than the first- and second-order reliability methods.

Very-long floating bridges represent an innovative marine structure for crossing wide and deep fjords. During the design of a floating bridge, extreme structural responses at a specified probability of exceedance are required to be properly evaluated for ultimate limit state (ULS) design check. This study addresses the estimation of extreme structural responses due to wind and wave loads and associated uncertainties. An end-anchored floating bridge, about 4600 m, is considered in a case study. The long-term extreme responses are estimated by using a simplified engineering approach, in which the long-term extreme response is approximated by the one-hour short-term extreme responses at a high fractile (90% in this study) for selected short-term sea states. The extreme responses are expressed as μ + κ · σ , where μ and σ are the ensemble mean and standard deviation, and κ is a multiplying factor. Statistical analyses indicate that the structural responses, including axial force, strong and weak axis bending moments of the bridge girder, are close to follow a Gaussian distribution. A simplified analytical method, the Gumbel method and the mean upcrossing rate (MUR) method are employed to estimate the multiplying factor κ and extremes. The κ estimated by these three methods are generally close, varying in the vicinity of 4. The κ and extremes estimated by the simplified method have a much smaller variation than the Gumbel and MUR methods. Statistical uncertainties and model uncertainties in the extreme value prediction are also addressed. Based on the results of 10 sets of 10 1-h ensembles, the mean and coefficient of variation (CoV) of μ , κ , σ and extremes of structural responses of 10 1-h simulations under two selected sea states are evaluated. The CoV of σ is less than 0.045, but the CoV of κ is relatively large, mainly between 3.5 × 10 - 2 and 6.5 × 10 - 2 . The CoV of extremes estimated by the simplified analytical method is fairly small, less than 0.035. While the CoV of extremes estimated by the Gumbel and MUR methods are much larger and can reach 0.137 and 0.158, respectively. In practical design of floating bridge, only a limited number of simulations (e.g. 10 1-h) are conducted to predict the extreme structural responses. This will introduce statistical uncertainties and should be corrected by a factor for a conservative estimate. A simplified procedure to derive the correction factor is presented in this study. For the floating bridge considered with 10 1-h simulations, the correction factor is recommended to be 1.1 when the absolute value of mean μ is smaller than σ , and be 1.2 when the absolute value of mean μ is larger than σ , in order to achieve a 90% conservative estimation of extreme.

Advances in the design of large scale wind turbines have raised an interest for more optimized life cycle design of mechanical components (e.g. gears, bearings, shafts, etc.), with a better understanding of their performance over time. Development of the non-systematic, often deterministic design methods followed by the application of corresponding design standards that incorporate experience from current common practice have managed to produce mechanical components that comply with minimum safety requirements. However, such approaches cannot achieve optimized mechanical components, neither account for the performance over their service lives. Reliability-based design currently represents the most advanced method in order to achieve the targets mentioned above, by explicitly considering uncertainty of design variables. This paper describes a structural reliability method (SRM) for fatigue analysis of mechanical components of wind turbines. The method is based on the ‘so-called’ limit state functions of relevant failure modes. Two gear tooth fatigue failure modes (surface and subsurface pitting) were considered. The method is exemplified by a time-domain based gear contact fatigue analysis of the National Renewable Energy Laboratory's 750 kW land-based wind turbine. The sensitivity of the reliability index on some random parameters used for reliability-based gear contact fatigue analysis is estimated. The effect of inspection on reliability analysis is also briefly investigated.

A coupled numerical model has been developed and validated to study the fluid–structural interaction responses of a three-bladed tidal turbine in aligned waves and current. The unsteady blade element momentum (BEM) theory was combined with modal analysis for hydro-elastic calculation. Both the loading and deflection of the blade were studied. The dynamic loading on the blade due to structural deformation was much smaller than the wave-induced loading under linear wave conditions for the given condition. The linear response amplitude operators (RAOs) of the loads and the blade tip deflections were obtained and used to predict the linear responses. Although both sum- and difference-frequency responses can be identified from time domain simulations, the wave-induced load and the deflection of the blade are dominated by the first-order contributions. The maximum deflection of the blade tip could reach 1.3 m (203% of the means) in the flapwise direction and 0.35 m (210% of the mean) in the edgewise direction with a wave peak period of 11.3 s and a significant wave height of 5.5 m.

Common industrial practice for designing floating wind turbines is to set an operational limit for the tower-top axial acceleration, normally in the range of 0.2–0.3 g, which is typically understood to be related to the safety of turbine components. This paper investigates the rationality of the tower-top acceleration limit by evaluating the correlation between acceleration and drivetrain responses. A 5-MW reference drivetrain is selected and modeled on a spar-type floating wind turbine in 320 m water depth. A range of environmental conditions are selected based on the long-term distribution of wind speed, significant wave height, and peak period from hindcast data for the Northern North Sea. For each condition, global analysis using an aero-hydro-servo-elastic tool is carried out for six one-hour realizations. The global analysis results provide useful information on their own—regarding the correlation between environmental condition and tower top acceleration, and the correlation between tower top acceleration and other responses of interest—which are used as input in a decoupled analysis approach. The load effects and motions from the global analysis are applied on a detailed drivetrain model in a multibody system (MBS) analysis tool. The local responses on bearings are then obtained from MBS analysis and postprocessed for the correlation study. Although the maximum acceleration provides a good indication of the wave-induced loads, it is not seen to be a good predictor for significant fatigue damage on the main bearings in this case.

Floating wind turbine has been the highlight in offshore wind industry lately. There has been great effort on developing highly sophisticated numerical model to better understand its hydrodynamic behaviour. A engineering-practical method to study the nonlinear wave effects on floating wind turbine has been recently developed. Based on the method established, the focus of this paper is to quantify the wave nonlinearity effect due to nonlinear wave kinematics by comparing the structural responses of floating wind turbine when exposed to irregular linear Airy wave and fully nonlinear wave. Critical responses and fatigue damage are studied in operational conditions and short-term extreme values are predicted in extreme conditions respectively. In the operational condition, wind effects are dominating the mean value and standard deviation of most responses except floater heave motion. The fatigue damage at the tower base is dominated by wind effects. The fatigue damage for the mooring line is more influenced by wind effects for conditions with small wave and wave effects for conditions with large wave. The wave nonlinearity effect becomes significant for surge and mooring line tension for large waves while floater heave, pitch motion, tower base bending moment and pontoon axial force are less sensitive to the nonlinear wave effect. In the extreme condition, linear wave theory underestimates wave elevation, floater surge motion and mooring line tension compared with fully nonlinear wave theory while quite close results are predicted for other responses. © 2019 The Authors. Published by Elsevier Ltd. Open Access CC-BY-NC-ND

The influence of fully nonlinear wave effects on floating wind turbine has been studied in this paper by comparing both floater motions and structural responses of tower base and mooring lines exposed to linear and fully nonlinear long-crested irregular waves. Wave kinematics of the linear and nonlinear wave are calculated in a 2D Harmonic Polynomial Cell wave tank. The wave kinematics are further processed by a polynomial fitting method to scale down the data size so that it fulfills the memory requirement of HAWC2 where coupled dynamic analysis is carried out. The external DLL used to provide wave kinematics to HAWC2 is extended from one dimensional (Wkin.dll 2.4) to two dimensional (Wkin.dll 2D) wave field so that fully nonlinear wave can be implemented on floating wind turbine through reading pre-generated wave kinematics manner. The whole work procedure including wave generation, polynomial fitting and implementation in HAWC2 has been verified with a linear regular wave case. Two extreme irregular wave conditions are focused to study the nonlinear wave effects regarding critical responses, such as wave elevation, floater motions and mooring line tension. The results have not only proved the accuracy and applicability of the polynomial fitting method and the extended Wkin.dll 2D for HAWC2 but also revealed the importance to consider fully nonlinear wave model in hydrodynamic analysis compared with linear wave theory especially for high sea states in shallow and intermediate water. As a result of development of computer capacity and numerical wave tank, this paper has demonstrated that fully nonlinear wave effect can be considered in an engineering manner with acceptable efficiency. © 2019 The Authors. Published by Elsevier Ltd. Open Access CC-BY-NC-ND

The cross-flow vortex-induced vibration features of a submerged floating tunnel element, which is composed of two rigidly connected cylinders in a tandem configuration, were investigated via a self-oscillation model test in a steady flow. The Reynolds number ranged from 2 × 104 to 9 × 104, and the ratio of the center-to-center distance between the two cylinders and cylinder diameter varied within a range of 2-4. The vortex induced vibration responses and lift forces on the up- and downstream cylinders were studied under different spacing ratios and compared with those on a single cylinder. The results show that the spacing ratio plays an important role in VIV until the ratio reaches 4. For a small spacing ratio, a significant difference between the lift forces on the up- and downstream cylinders appears and induces a large torsional moment. For the convenience of engineering application, a torsional coefficient was proposed. The maximum torsional coefficient can reach 2.9, 1.2 and 0.98 for spacing ratios of 2, 3 and 4 at the reduced velocity of 5, respectively. Considering the vortex induced vibration responses as well as the torsional moment, a spacing ratio of 3 was recommended for tandem floating tunnel design.

This paper presents a time-domain hydroelastic analysis method for bridges supported by floating pontoons in inhomogeneous wave conditions. The inhomogeneous wave effect is accounted for by adopting different wave spectra over different regions along the structure, then the time history of inhomogeneous first-order wave excitation forces on the floating pontoons can be obtained. The frequency-domain hydrodynamic coefficients are transformed into the time-domain hydroelastic model using Cummins' equations. The linear hydroelastic responses of a curved floating bridge with end supports, subjected to irregular waves with spatially varying significant wave heights and peak periods, are investigated. Moreover, sensitive analyses are performed to study the effects of the inhomogeneity on the hydroelastic responses. The primary results indicate that the inhomogeneity of the waves has a significant effect on the dynamic responses of the floating bridge.

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.

Designing reliable and cost-effective floating bridges for wide and deep fjords is very challenging. The floating bridge is subjected to various environmental loads, such as wind, wave, and current loads. All these loads and associated load effects should be properly evaluated for ultimate limit state design check. In this study, the wind-, wave-, and current-induced load effects are comprehensively investigated for an end-anchored curved floating bridge, which was an early concept for crossing the Bjørnafjorden. The considered floating bridge is about 4600 m long and consists of a cable-stayed high bridge part and a pontoon-supported low bridge part. It also has a large number of eigen-modes, which might be excited by the environmental loads. Modeling of wind loads on the bridge girder is first studied, indicating that apart from aerodynamic drag force, aerodynamic lift and moment on the bridge girder should also be considered due to their significant contribution to axial force. Turbulent wind spectrum and spatial coherence play an important role and should also be properly determined. The sway motion, axial force, and strong axis bending moment of the bridge girder are mainly induced by wind loads, while the heave motion, weak axis bending moment, and torsional moment are mainly induced by wave loads. Turbulent wind can cause significant larger low-frequency eigen-mode resonant responses than the second-order difference frequency wave loads. Current loads mainly contribute damping and reduce the variations of sway motion, axial force, and strong axis bending moment.

A new numerical method, which is based on three-dimensional (3D) potential flow theory and finite element method (FEM), is used to predict the wave-induced hydroelastic responses of flexible floating bridges. The floating bridge is discretized into several modules based on the positions of the pontoons which are 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 rigid module coupled with the dynamics of the elastic beams. The hydrodynamics loads on each module are considered as external loads and simultaneously applied. The method is extended to take into account the shore side effect, which is obtained from the 3D potential flow theory and considered as a hydrodynamic boundary condition. The effects of inclination of shore side on the responses of the bending moment, horizontal and vertical displacements of the pontoon and their distribution along the bridge are investigated. The results show that the displacement response increase with an increasing steepness of the shore side. © 2018. This is the authors' accepted and refereed manuscript to the article. The final authenticated version is available online at: http://dx.doi.org/10.1115/OMAE2018-78738

Designing floating bridges for wide and deep fjords is very challenging. The floating bridge is subjected to wind, wave, and current loads. All these loads and corresponding load effects should be properly evaluated, e.g. for ultimate limit state design check. In this study, the wind-, wave- and current-induced load effects of an end-anchored floating bridge are numerically investigated. The considered floating bridge, about 4600 m long, was an early concept for crossing Bjørnafjorden, Norway. It consists of a cable-stayed high bridge part and a pontoon-supported low bridge part, and has a number of eigen-modes, which might be excited by the relevant environmental loads. Numerical simulations show that the sway motion and strong axis bending moment along the bridge girder are primarily induced by wind loads, while variations of heave motion and weak axis bending moment are mainly induced by wave loads. Current loads mainly provide damping force to reduce the variations of sway motion and strong axis bending moment. Turbulent wind can cause significantly larger low-frequency resonant responses than second-order difference-frequency wave loads. Copyright © 2018 by ASME

Based on relevant accident experiences with oil and gas platforms, a brief overview of structural integrity management of offshore structures is given; including an account of adequate design criteria, inspection, repair and maintenance as well as quality assurance and control of the engineering processes. The focus is on developing research based design standards for Accidental Collapse Limit States to ensure robustness or damage tolerance in view damage caused by accidental loads due to operational errors and to some extent abnormal structural damage due to fabrication errors. Moreover, it is suggested to provide robustness in cases where the structural performance is sensitive to uncertain parameters. The use of risk assessment to aid decisions in lieu of uncertainties affecting the performance of novel and existing offshore structures, is briefly addressed.

Floating vertical-axis wind turbines (FVAWTs) provide the potential for utilizing offshore wind resources in moderate and deep water because of their economical installation and maintenance. Therefore, it is important to assess the performance of the FVAWT concept. This paper presents a stochastic dynamic response analysis of a 5 MW FVAWT based on fully coupled nonlinear time domain simulations. The studied FVAWT, which is composed of a Darrieus rotor and a semi-submersible floater, is subjected to various wind and wave conditions. The global motion, structural response and mooring line tension of the FVAWT are calculated using time domain simulations and studied based on statistical analysis and frequency-domain analysis. The response of the FVAWT is compared under steady and turbulent wind conditions to investigate the effects of turbulent wind. The advantage of the FVAWT in reducing the 2P effect on the response is demonstrated by comparing the floating wind turbine with the equivalent land-based wind turbine. Additionally, by comparing the behaviour of FVAWTs with flexible and rigid rotors, the effect of rotor flexibility is evaluated. Furthermore, the FVAWT is also investigated in the parked condition. The global motions and structural responses as a function of the azimuthal angle are studied. Finally, the dynamic response of the FVAWT in selected misaligned wind and wave conditions is analysed to determine the effects of wind-wave misalignment on the dynamic response. Copyright © 2016 John Wiley & Sons, Ltd.

With a successful and rapid development of offshore wind industry and increased research activities on wave energy conversion in recent years, there is an interest in investigating the technological and economic feasibility of combining offshore wind turbines (WTs) with wave energy converters (WECs). In the EU FP7 MARINA Platform project, three floating combined concepts, namely the spar torus combination (STC), the semi-submersible flap combination (SFC) and the oscillating water column (OWC) array with a wind turbine, were selected and studied in detail by numerical and experimental methods. This paper summarizes the numerical modeling and analysis of the two concepts: STC and SFC, the model tests at a 1:50 scale under simultaneous wave and wind excitation, as well as the comparison between the numerical and experimental results. Both operational and survival wind and wave conditions were considered. The numerical analysis was based on a time-domain global model using potential flow theory for hydrodynamics and blade element momentum theory (for SFC) or simplified thrust force model (for STC) for aerodynamics. Different techniques for model testing of combined wind and wave concepts were discussed with focus on modeling of wind turbines by disk or redesigned small-scale rotor and modeling of power take-off (PTO) system for wave energy conversion by pneumatic damper or hydraulic rotary damper. In order to reduce the uncertainty due to scaling, the numerical analysis was performed at model scale and both the numerical and experimental results were then up-scaled to full scale for comparison. The comparison shows that the current numerical model can well predict the responses (motions, PTO forces, power production) of the combined concepts for most of the cases. However, the linear hydrodynamic model is not adequate for the STC concept in extreme wave conditions with the torus fixed to the spar at the mean water level for which the wave slamming on the torus occurs and this requires further investigation. Moreover, based on a preliminary comparison of the displacement, the PTO system as well as the wind and wave power production, the STC concept will have a lower cost of energy as compared to the SFC concept. However, the cost of energy of either the STC or the SFC concept is higher than that of a pure floating wind turbine with the same floater. This is a submitted manuscript of an article published by Elsevier in Journal of Ocean Engineering and Science, 29 January 2016

This work addresses a combined wind and wave energy concept called the Spar–Torus-Combination (STC) system. Two additional survival modes for the original STC system (in a survival mode with the tours locked to the spar) have been proposed, namely the submerged mode and the low power take-off (PTO) damping mode. The performance of the STC system with different modes has been investigated based on a coupled analysis of wind–wave-induced stochastic response using the SIMO-TDHMILL code in the time domain. The energy production, structural fatigue damage and extreme responses of the STC system have been estimated based on the long-term wind–wave joint distribution at two selected sites in European waters. The hydrodynamic loads on the spar and torus were estimated using potential theory by accounting for the hydrodynamic interactions. The aerodynamic loads of the rotor were calculated by a validated simplified thrust model (TDHMILL). The annual wind and wave power production have been obtained using the hourly mean output power of each short-term condition and its occurrence probability. The annual fatigue damage of both mooring lines and the tower has been calculated based on the S – N curve approach and the Palmgren–Miner׳s linear damage hypothesis. The extreme responses of the STC system have been investigated by assuming that the largest values in each short-term case follow the Gumbel distribution. The results show that the new proposed survival strategy can significantly reduce the long-term fatigue damage and extreme responses of the original STC system without sacrificing power production by avoiding the possible heave resonance effect when the torus is locked to the spar at the mean water level (MWL). Furthermore, considering the fact that long-term analysis with all sea states is time consuming, the contour line method has also been applied to effectively estimate the extreme responses of the original STC system and has been validated by a comparison with the full long-term analysis method.

The Spar Torus Combination (STC) concept consists of a spar floating wind turbine and a torus-shaped heaving-body wave energy converter (WEC). Numerical simulations have shown a positive synergy between the WEC and the spar floating wind turbine under operational conditions. However, it is challenging to maintain structural integrity under extreme wind and wave conditions, especially for the WEC. To ensure the survivability of the STC under extreme conditions, three survival modes have been proposed. To investigate the performance of the STC under extreme conditions, model tests with a scaling factor of 1:50 were carried out in the towing tank of MARINTEK, Norway. Two survival modes were tested. In both modes, the torus WEC was fixed to the spar. In the first mode, the torus WEC is at the mean water surface, while in the second mode, it is fully submerged to a specified position. The model tests recorded the 6 degrees of freedom (D.O.F.s) rigid body motions, mooring line tensions, and the forces between the spar and torus in 3 directions (X, Y and Z). The wind speed was also measured by a sensor in front of the model and the wind force on the wind turbine disk was measured by a load cell installed on top of the tower. This paper describes the model test setup for the two survival modes, and presents and compares the results from the tests and the numerical simulations. Several nonlinear phenomena were observed during the tests, such as wave slamming, Mathieu instability and vortex induced motion. The current numerical model based on linear potential theory cannot capture these nonlinear phenomena and requires a further development. © 2015. This is the authors’ accepted and refereed manuscript to the article. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/

This paper presents detailed descriptions, modeling parameters and technical data of a 5MW high-speed gearbox developed for the National Renewable Energy Laboratory offshore 5MW baseline wind turbine. The main aim of this paper is to support the concept studies and research for large offshore wind turbines by providing a baseline gearbox model with detailed modeling parameters. This baseline gearbox follows the most conventional design types of those used in wind turbines. It is based on the four-point supports: two main bearings and two torque arms. The gearbox consists of three stages: two planetary and one parallel stage gears. The gear ratios among the stages are calculated in a way to obtain the minimum gearbox weight. The gearbox components are designed and selected based on the offshore wind turbine design codes and validated by comparison to the data available from large offshore wind turbine prototypes. All parameters required to establish the dynamic model of the gearbox are then provided. Moreover, a maintenance map indicating components with high to low probability of failure is shown. The 5 MW reference gearbox can be used as a baseline for research on wind turbine gearboxes and comparison studies. It can also be employed in globalmore » analysis tools to represent a more realistic model of a gearbox in a coupled analysis.« less

This paper deals with the feasibility of using a 5 MW drivetrain which is designed for a land-based turbine, on floating wind turbines. Four types of floating support structures are investigated: spar, TLP and two semi-submersibles. The fatigue damage of mechanical components inside the gearbox and main bearings is compared for different environmental conditions, ranging from cut-in to cut-out wind speeds. For floating wind turbines, representative wave conditions are also considered. All wind turbines are ensured to follow similar power curves, but differences in the control system (integral to different concepts) are allowed. A de-coupled analysis approach is employed for the drivetrain response analysis. First, an aero-hydro-servo-elastic code is employed for the global analysis. Next, motions, moments and forces from the global analysis are applied on the gearbox multi body model and the loads on gears and bearings are obtained. The results suggest that the main bearings sustain more damage in floating wind turbines than on land-based. The highest main bearing damage is observed for the spar floating wind turbine. The large wave induced axial load on the main shaft is found to be the primary reason of this high damage in the spar wind turbine. Apart from the main bearings - which are located on the main shaft outside the gearbox - other bearings and gears inside the gearbox hold damages in floating wind turbines equal or even less than in the land-based turbine. It is emphasized that the results presented in this study are based on a drivetrain with two main bearings, which considerably reduces the non-torque loads on the gearbox. Copyright © 2015 Elsevier Ltd. All rights reserved. This is the authors' accepted and refereed manuscript to the article.

A liquefied natural gas (LNG) containment system (LCS) No. 96 type is built up as a box system consisting of plywood plates and bulkheads. Plywood components are connected using staple connectors. The aim of this paper is to explain the crushing and buckling failure of plywood component of an LCS No. 96. A systematic study of crushing and buckling behavior of plywood components is conducted by means of experiments and finite element (FE) analysis. The FE study is based on numerical material modeling using Hashin’s theory and fracture energy approach as damage initiation and evolution, respectively. Observations on strength and progressive deformation during the crushing and buckling tests are discussed. It is observed that crushing failure of plywood bulkheads determines the ultimate crushing capacity. Crushing failure of plywood plate which is due to shear through the thickness failure of the veneer, causes reduced stiffness before the ultimate crushing capacity is reached. It is found from buckling test that staple connectors have no influence on structural strength nor stiffness, i.e. they keep the structure together during manufacturing. During buckling of bulkheads, withdrawal of staple legs from the plywood bulkhead is observed. Thus, plywood bulkheads are supported on their edges and creates hinge like support. The numerical simulations – including the use of the damage model of plywood components – agree well with the test results.

When designing a floating bridge crossing a deep and wide fjord, a homogeneous wave field is usually assumed for simplicity. However, waves in fjords are commonly inhomogeneous, and hence the mentioned design practice introduces uncertainty, which should be assessed for the design. In this study, we proposed an approach to account for the inhomogeneous wave load effect on a floating bridge and applied it on a floating bridge that was initially proposed for crossing the Bjornafjorden. The floating bridge considered is end-anchored, about 4600 m long and consists of a cable-stayed high bridge and a low bridge supported by 19 pontoons. Wave excitation loads on each pontoon were proposed to be modeled and applied separately to account for inhomogeneous waves. By considering 1-year and 100-year wave conditions, dynamic responses of the floating bridge subjected to homogeneous and inhomogeneous waves were analyzed and compared. It is found that inhomogeneous waves cause relatively larger sway motion, axial force, and strong axis bending moment, as well as significantly larger weak axis bending moment along the bridge girder than homogeneous waves. These responses depend on the inhomogeneity level of waves considered. Proper description of the wave field is therefore very important for evaluating the effect of inhomogeneous waves and associated uncertainties.

Due to the interaction between elastic structural deformations and fluid motions, the responses of floating bridges should be analyzed by hydroelasticity methods. We firstly employed a linear time-domain approach, based on the discrete-module-based hydroelastic method to investigate the responses of two surface bridge concepts under first order wave forces, a straight bridge with mooring system and an end-anchored curved bridge. The results show that the displacement of the curved bridge is smaller, compared with the straight bridge. However, the reaction force and the vertical bending moment of the curved bridge are larger. Furthermore, a nonlinear time-domain hydroelasticity approach for analysis of the floating bridge under nonlinear wave forces within inhomogeneous wave conditions is established. Finally, a comparison between the linear and nonlinear responses of the moored straight bridge is made. The results show that the nonlinearity of the wave excitation forces has a significant influence on the mooring forces and horizontal displacement. Copyright © 2018 by ASME

This paper presents model-based fault detection, fault isolation, and fault-tolerant control schemes focused on blade pitch systems in floating wind turbines. Fault detection, isolation, and accommodation techniques are required to achieve high power capture efficiency and structural reliability in floating wind turbines. Faults in blade pitch systems should be detected at an early stage to prevent catastrophic failures. To detect faults of the blade pitch systems, a Kalman filter is designed to estimate the blade pitch angle of the system. The fault isolation algorithm is based on inference methods and capable of determining the fault type, location, magnitude and time. The fault-tolerant controller based on a reconfiguration block with a virtual sensor and shutdown mode controls the floating wind turbine to avoid unexpected external loads. The proposed methods are demonstrated in case studies with stochastic wind and wave conditions that considering different types of faults, such as biases and fixed outputs in pitch sensors and stuck pitch actuators. The simulation results show that the proposed methods can detect and isolate multiple faults effectively at an early stage. Additionally, the effectiveness of the fault-tolerant control systems for different load cases for single and multiple fault conditions is verified by numerical simulations.

Jack-up crane vessels are commonly used to install offshore wind turbine blades and other components. A jack-up crane vessel is subjected to wind and wave loads, which cause motion at crane tip. Excessive motion at crane tip can lead to failure of lifting operations. Therefore, the crane tip motion should be properly assessed for jack-up crane vessels. In this study, a fully coupled model is developed for a typical elevated jack-up crane vessel, considering the hydrodynamic and aerodynamic loads on the vessel, the soil-structure interaction, and the structural flexibility of the jack-up legs and crane. The vessel model developed is further coupled with the SIMO-Aero code to achieve a fully coupled aero-hydro-soil-elastic-mechanical code SIMO-RIFLEX-Aero for numerical modeling and dynamic analysis of offshore single blade installation using jack-up crane vessels. The SIMO-RIFLEX-Aero code is then applied to study the dynamic response of the DTU 10 MW wind turbine blade installed by a typical jack-up crane vessel under various wind and wave conditions. The results show that significant motion is induced at crane tip, mainly due to wave loads. It is important to consider the structural flexibility of the jack-up legs and crane when modeling the installation of offshore wind turbine blades.

An overview of structural integrity management of offshore structures in the oil and gas energy sector is presented in this article. Based on relevant experiences with the hazards, accidents and means to control the associated risks are categorised from a technical and physical as well as human and organisational point of view. Structural risk relates to extreme environmental and accidental events, as well as structural degradation and can be controlled by use of adequate design criteria, inspection, repair and maintenance as well as quality assurance and control of the engineering processes. Such measures are briefly outlined, with an emphasis placed upon a quantitative design approach for dealing with a life cycle approach especially relating to crack degradation phenomena. The current status of risk and reliability methodologies to aid decisions in the safety management of novel and mature offshore structures is briefly reviewed. This is an Accepted Manuscript of an article published by Taylor & Francis in Structure and Infrastructure Engineering on 23.02.2018, available at https://doi.org/10.1080/15732479.2018.1438478

This paper presents a systematic approach for designing the hydraulic mechanism as a part of the gripper system employed in offshore wind turbine monopile installation. Traditionally, such equipments used in marine operation are designed based on deterministic approach, selecting actuators and power pack by applying a safety margin which is not explicitly derived from a systematic load/load effect analysis, or a reliability based method. The method in this article offers a systematic way of designing the hydraulic power system, actuators and supporting structure to overcome extreme and fatigue loadings during operation. The design starts with a global analysis and modelling of monopile and installation vessel. The forces and motions from global analysis are then employed for designing hydraulic actuators and power system. A dynamic model of hydraulic system is built to analysis dynamic response in hydraulic system. The results from this local dynamic model can be used in power management and system optimization. The proposed method is a step forward to apply reliability-based design on mechanical components in marine applications through a systematic long-term load and load effect analysis. Copyright © 2018 by ASME

The construction of a three-span suspension bridge with two floating pylons is currently being considered for crossing the 5-km-wide and 550-m-deep Bjørnafjorden in Norway. The bridge design represents a novel concept that requires a detailed dynamic analysis to improve the current understanding of its dynamic behavior. Geometric nonlinearities in the cables and mooring system and nonlinearities in the load models are of particular interest; in addition, the relative influence of the buffeting wind forces and the first- and second-order wave excitation forces were carefully studied. The response predictions were obtained using state-of-the-art time domain methods. This is a submitted manuscript of an article published by Elsevier Ltd in Marine Structures, 7 February 2018.

Due to the complex topography in a fjord, wave conditions differ from those of ocean waves. In this study, characteristics of wave conditions in the Bjørnafjorden in Hordaland County, Norway were thoroughly investigated based on field measurements. Bjørnafjorden is approximately 4,600 m wide and more than 500 m deep, with a complex hydrography and topography. Three Datawell Waveriders (DWRs, Datawell, Haarlem, Netherlands) were deployed to measure the wave data. Due to two ferry routes nearby, the measured raw data were found to be influenced by ship waves. Thus, a band-pass filter based on wavelet and inverse wavelet analyses was proposed and developed to detect and remove ship waves from raw data. The wave data analyzed were measured over approximately 19 months. The wave conditions measured by each DWR were characterized by several parameters, such as significant wave height, average zero upcrossing period, and dominant direction. The values of each wave parameter at each DWR usually differed, which indicates that the wave field in Bjørnafjorden is inhomogeneous. The statistical values of these parameters among the three DWRs were correlated to some extent. Their distribution could not be fitted by a suitable distribution function unless more data were available. The coherence among the three DWRs was fairly low. © 2018. This is the authors' accepted and refereed manuscript to the article. The final authenticated version is available online at: http://dx.doi.org/10.1061/(ASCE)WW.1943-5460.0000481

A large number of offshore wind turbines have been installed recently, mostly in water depth up to 30 m based on monopile foundations. It is expected that floating wind turbine becomes more competitive than bottom fixed wind turbine when the water depth exceeds 50 m. In this paper, the focus is on the environmental loads and responses of mooring systems for a semi-submersible in water depths from 50 m to 200 m. Mooring design for moderate water depths is relatively easy to achieve, but it is challenging for shallow water. The effect of environmental load modelling should be studied based on designing a reasonable mooring system. With a mooring system design for 200 m water depth as a reference, two mooring system design concepts in 100 m and 50 m water depth have been proposed for a 5-MW-CSC semi-submersible floating wind turbine. Preliminary design has been carried out to determine mooring line properties, mooring system configurations and document the static performances. A fully coupled time domain dynamic analysis for extreme environmental conditions was performed using Simo-Riflex-AeroDyn. Four different load models were applied in order to check the influence of different load components including the effect of wind, current and second order wave forces by means of Newman's approximation and a full QTF method. Content from this work may be used under the terms of theCreative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd

To ensure safety in anchor handling operations, maintaining anchor handling vessel stability is recognized as a critical and complex task. The vessel’s stability depends primarily on the vessel’s design and operational parameters, which are expressed by the vessel’s dynamic rolling angle, static heeling angle, and capsizing angle. The vessel’s stability during anchor handling operations (AHOs) is possible to attain by means of maintaining the vessel’s heading in line with the mooring line direction. However, lessons learned from past accidents show that normal and abnormal events and gross errors can lead to capsizing. The occurrence of a large deviation between the vessel heading and the mooring line direction due to abnormal events is considered to be an accidental limiting condition. In this study, two stability criteria are established: (1) the critical static heeling angle criterion and (2) the critical rolling angle criterion. The first criterion is useful in the design phase for assessing the vessel’s allowable static heeling angle for a well-defined operational sea state. The second criterion is useful for assessing the vessel’s stability in the analysis and planning phase of the operation. A case study is conducted on the Bourbon Dolphin (BD) accident for assessing the stability in a capsizing scenario. The predicted results show that the BD can maintain stability under normal conditions but not under an accidental condition during anchor deployment. While assessing a vessel’s intact stability, it is essential to account for the effect of normal uncertainty and variability on the operational parameters; this aspect is investigated in this paper through a sensitivity study. This is a post-peer-review, pre-copyedit version of an article published in [Journal of Marine Science and Technology]. The final authenticated version is available online at: https://doi.org/10.1007/s00773-017-0465-7

The combined wind and wave concept semisubmersible wind energy and flap-type wave energy converter was developed in the EU FP7 project MARINA Platform. It consists of a four-column semisubmersible with a 5-MW wind turbine placed on top of the central column and three flap-type wave energy converters on top of three pontoons that connect the four columns. Numerical and experimental studies have been performed to demonstrate the functionality and the survivability of the combined concept. In extreme conditions, both wind turbine and wave energy converters are set in a protection mode which reduces the dynamic loads and responses. In this article, different methods for predicting long-term (50-year) extreme responses considering the wind and wave conditions at two given European offshore sites are carried out, and structural response quantities are calculated, compared and presented. The full long-term analysis was performed and regarded as the reference method, and the corresponding results are compared with the modified environmental contour method and the environmental contour method. The response quantities studied here are the axial forces and bending moments of the semisubmersible wind energy and flap-type wave energy converter, including those of the wind turbine (blade, shaft and tower), arms of the flap-type wave energy converters and mooring lines, as well as the platform motion in 6 degrees of freedom. The extreme responses that are dominated by the aerodynamic loadings are effectively calculated either by the full long-term analysis or the modified environmental contour method. Compared to the full long-term analysis, the environmental contour method gives an under-prediction of the long-term extreme responses of quantities related to the wind turbine (e.g. internal loads of blades and tower). For the extreme responses that are dominated by the hydrodynamic loadings, all the three methods provide similar results. © 2018. This is the authors' accepted and refereed manuscript to the article. The final authenticated version is available online at: https://doi.org/10.1177%2F1475090217726886

The aim of this paper is to study anchor handling vessel (AHV) thrust capacity during anchor deployment, especially in a deep water situation when high external forces are expected. The focus is on obtaining realistic external forces and evaluating the positioning capability of an AHV. Wind, wave and current loads on the AHV are considered. Current load on the mooring line, which is usually excluded in practice, is included in the model as well. The thrust utilisation plot, a concept widely used in the Dynamic Positioning system, is proposed to illustrate the positioning capability of an AHV. The Bourbon Dolphin accident was investigated as a case study using the proposed model and methodology. First, load analysis was performed. The results indicated the importance of applying a reasonable current profile and taking the mooring line effect into account. Then, thrust utilisation plots for normal and accident conditions were compared. The comparison showed that the Bourbon Dolphin might have been in the most unfavourable weather direction in terms of position capability during the accident event. Finally, the effect of mooring line configuration was studied. The results signified that a very long mooring line might challenge the propeller thrust capacity and the propeller thrust loss due to lateral thrust usage needs to be considered. Such an analysis and documentation prior to the commencement of the operation can be used for defining vessel specific limitations and selecting the proper vessel for a specific task.

Long term time domain analysis of the nominal stress for fatigue as- sessment of the tower and platform members of a three-column semi- submersible was performed by fully coupled time domain analyses in Simo- Ri ex-AeroDyn. By combining the nominal stress ranges with stress con- centration factors, hot spot stresses for fatigue damage calculation can be obtained. The aim of the study was to investigate the necessary simulation duration, number of random realisations and bin sizes for the discretisa- tion of the joint wind and wave distribution. A total of 2316 3-hour time domain simulations, were performed. In mild sea states with wind speeds between 7 and 9 m/s, the tower and pontoon experienced high fatigue damage due to resonance in the rst bending frequency of the tower from the tower wake blade passing frequency (3P). Important fatigue e ects seemed to be captured by 1 hour simulations, and the sensitivity to number of random realisations was low when run- ning simulations of more than one hour. Fatigue damage for the tower base converged faster with simulation duration and number of random realisations than it did for the platform members. Bin sizes of 2 m/s for wind, 1 s for wave periods and 1 m for wave heights seemed to give acceptable estimates of total fatigue damage. It is, however, important that wind speeds that give coinciding 3P and tower resonance are included and that wave periods that give the largest pitch motion are included in the analysis. © 2014 Elsevier Ltd. All rights reserved. This is the authors' accepted and refereed manuscript to the article. Locked until 2017-01-01 due to the copyright restriction.

In this paper, a methodology suitable for assessing the allowable sea states for installation of a transition piece (TP) onto a monopile (MP) foundation with focus on the docking operation is proposed. The TP installation procedure together with numerical analyses is used to identify critical and restricting events and their corresponding limiting parameters. For critical installation phases, existing numerical solutions based on frequency and time domain (TD) analyses of stationary processes are combined to quickly assess characteristic values of dynamic responses of limiting parameters for any given sea state. These results are compared against (nonlinear and nonstationary) time domain simulations of the actual docking operations. It is found that a critical event is the structural damage of the TP's bracket supports due to the potential large impact forces or velocities, and a restricting installation event (not critical) is the unsuccessful mating operation due to large horizontal motions of the TP bottom. By comparing characteristic values of dynamic responses with their allowable limits, the allowable sea states are established. Contact–impact problems are addressed in terms of assumed allowable impact velocities of the colliding objects. A possible automatic motion compensation system and human actions are not modeled. This methodology can also be used in connection with other mating operations such as float-over and topside installation.