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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.

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.

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/

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/)

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

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/).

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.

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.

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

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.

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.

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

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.

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 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.

Floating suspension bridges, one of several new designs to make it possible to cross deep and wide fjords, consist of three spans and supported by two tension leg platforms and two fixed traditional concrete pylons. Geometric nonlinearities, nonlinear aerodynamic and hydrodynamic forces and nonlinear mooring systems can become of high importance. Time domain methods are commonly applied when nonlinearities need to be considered. The main challenge in time domain simulation of the floating suspension bridge is the modelling of frequency-dependent aerodynamic self-excited forces and hydrodynamic radiation forces. This paper shows how rational functions fitted to aerodynamic derivatives and hydrodynamic added mass and potential damping can be converted to state space models to transform the frequency-dependent forces to time-domain. A user element is implemented in the software ABAQUS to be able to include the self-excited forces in the dynamic analysis. The element is developed as a one node element that is included in the nodes along the girder and the tension leg platforms. The responses of the floating suspension bridge under turbulent wind forces and first-order wave excitation forces are calculated in a comprehensive case study and compared with results obtained using a multi-mode frequency domain approach to illustrate the performance of the presented time-domain methodology. (c) 2017 by ASME

This paper focuses on the influences of the dynamic effects of ship–ice–water interaction on ship performance, ship motions, and ice resistance. The effects of the dynamic bending of ice wedges and ship speeds are especially investigated. The study is carried out using a numerical procedure simulating ship operations in level ice with ship motions in six degrees of freedom (DOFs). A case study is conducted with the Swedish icebreaker Tor Viking II. The 3-D hull geometry of the ship is modeled based on the lines drawing. The predicted performance of the ship is compared with data from full-scale ice trials.

This thesis is a summary of studies that were carried out as part of candidacy for aPhD degree. The purpose of these studies was to evaluate some factors in shipdesign that are intended for navigating in ice using numerical simulations. A semiempiricalnumerical procedure was developed by combining mathematical modelsthat describe the various elements of the continuous-mode icebreaking process inlevel ice. The numerical procedure was calibrated and validated using full- andmodel-scale measurements. The validated numerical model was in turn used toinvestigate and clarify issues that have not been previously considered.An icebreaker typically breaks ice by its power, its weight and a strengthened bowwith low stem angle. The continuous icebreaking process involves heave and pitchmotions that may not be negligible. The numerical procedure was formulated toaccount for all of the possible combinations of motions for six degrees of freedom(DOFs). The effects of the motion(s) for certain DOF(s) were investigated bycomparing simulations in which the relevant motion(s) were first constrained andthen relieved.In the continuous-mode icebreaking process, a ship interacts with an icebreakingpattern consisting of a sequence of individual icebreaking events. The interactionsamong the key characteristics of the icebreaking process, i.e., the icebreakingpattern, ship motions, and ice resistance, were studied using the numericalprocedure in which the ship motions and excitation forces were solved for in thetime domain and the ice edge geometry was simultaneously updated.Observations at various test scales have shown that the crushing pressure arisingfrom the ice–hull interaction depends on the contact area involved. A parametricstudy was carried out on the numerical procedure to investigate the effect of thecontact pressure on icebreaking.The loading rates associated with the ship’s forward speed have been anticipatedto play an important role in determining the bending failure loads, in view of thedynamic water flow underneath the ship and the inertia of the ice. The dynamicbending behavior of ice could also explain the speed dependence of the icebreakingresistance component. A dynamic bending failure criterion for ice was derived,incorporated into the numerical procedure and then validated using full-scale data.The results obtained using the dynamic and static bending failure criteria werecompared to each other.In addition, the effect of the propeller flow on the hull resistance for ships runningpropeller first in level ice was investigated by applying the information obtainedfrom model tests to the numerical procedure. The thrust deduction in ice wasdiscussed.

This paper presents an approach for performing a long-term fatigue analysis of rolling element bearings in wind turbine gearboxes. Multilevel integrated analyses were performed using the aeroservoelastic code HAWC2, the multibody dynamics code SIMPACK, the three-dimensional finite element code Calyx and a simplified lifetime prediction model for rolling contact fatigue. The National Renewable Energy Laboratory’s 750 kW wind turbine and its planetary bearing were studied. Design load cases, including normal production, parked and transient load cases, were considered. To obtain the internal bearing load distribution, an advanced approach combining a finite element/contact mechanics model and a response surface model were used. In addition, a traditional approach, the Harris model, was also applied for comparison. The long-term probability distribution of the bearing raceway contact pressure range was then obtained using Weibull and generalized Gamma distribution functions. Finally, we estimated the fatigue life of the bearing, discussed the differences of the methods used to obtain the bearing internal loads and analyzed the effects of the environmental conditions and load cases on the results. The Harris model may underestimate the inner raceway life by 55.7%, which can cause large load fluctuations along the raceways. The bearing fatigue life is very sensitive to the wind distribution and less affected by the transient and parked load cases. Copyright © 2014 John Wiley & Sons, Ltd.

The collapse behaviours of ship structures are usually validated by testing their small scale models. For thin-wall structures, the scale models obtained by complete geometrical similarity might not be economical or practical in test. The present study aims to figure out an approach to determine the dimensions of small scale models, which have similar collapse behaviours and load carrying capacity with full scale prototypes. For this purpose, the partial similarity methods are used to design the dimensions of scale stiffened panels considering the influence of collapse modes, which could be employed for the experiment in laboratory condition. The proposed approaches assume that three principle parameters dominate the buckling behaviours of stiffened panels, including plate, column and torsional slenderness, which are considered as determinate variables in the analyses of partial similarity methods. The stiffened panels under uniaxial longitudinal compression are simulated by finite element method. The dimensional and similitude analyses of stiffened panels with flanged profiles and flat bars are also performed. The ultimate strength and collapse mode are compared between the small and full scale models. From the comparison, the small scale stiffened panels designed by the partial similarity methods could reasonably represent the collapse behaviours of full scale models.

Emergency shutdown is always a challenge for an operating vertical axis wind turbine. A 5-MW vertical axis wind turbine with a Darrieus rotor mounted on a semi-submersible support structure was examined in this study. Coupled non-linear aero-hydro-servo-elastic simulations of the floating vertical axis wind turbine were carried out for emergency shutdown cases over a range of environmental conditions based on correlated wind and wave data. When generator failure happens, a brake should be applied to stop the acceleration of the rotor to prevent the rotor from overspeeding and subsequent disaster. In addition to the traditional mechanical brake, a novel hydrodynamic brake was presented to apply to the shutdown case. The effects of the hydrodynamic brake on the platform motions and structural loads under normal operating conditions and during the emergency shutdown events were evaluated. The use of both the hydrodynamic brake and mechanical brake was also investigated. The application of the hydrodynamic brake is expected to be efficient for rotor shutdown and for reducing the platform motions and structural loads. © 2014 Elsevier Ltd. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/3.0/).

Vertical bending moment (VBM) is of crucial importance in ensuring the survival of vessels in rough seas. With regard to conventional vessels, wave-induced maximum VBM is normally considered to be experienced in head seas. It is conservative to determine the extreme VBM based on either numerical simulations or model tests in long-crested head seas. Extensive model tests have been conducted in head seas with focus on the nonlinear vertical responses in severe seas, and the measured results were compared with numerical calculations for validation. Unexpected phenomena, however, were observed during the model tests of an ultra-large containership. The maximum sagging and hogging VBMs were encountered in oblique seas. Furthermore, the significant wave height used in oblique seas was even smaller than that used in head seas. The nonlinear vertical load effects in oblique seas require further investigations for this particular vessel. Limited experimental results in oblique seas have been reported, in which the lateral responses were always more concerned than the vertical responses. Up to now, rare systematic comparisons of the nonlinear vertical responses between head and oblique seas have been published, especially when the hydroelastic effects are also accounted for. A 13000-TEU ultra-large containership model, which was designed by Hyundai Heavy Industries (HHI), has been tested in the towing tank and the ocean basin at the Marintek center in Trondheim. The experimental results in regular waves are first compared between head and oblique seas. The statistical characteristics of the VBM amidships under nineteen irregular wave conditions are then investigated. Next, the extreme hogging and sagging VBMs are compared under different wave conditions with focus on the extreme hogging VBMs. At the end of the paper, the uncertainties in the experiments are discussed.

A new numerical model simulating ship–ice interaction is proposed in this paper. Since icebreaking is in its nature a three-dimensional nonlinear dynamic problem, the numerical model is developed to look into the intricate interaction process by considering ship motions in 6 degrees of freedom (DOFs). The effect on the icebreaking pattern and ship's performance of ship motions in heave, roll and pitch is investigated. Moreover, pressure–area relation is included in calculating the contact force. Semi-empirical method is used in developing the numerical model and the results are validated by comparing with full-scale data from an icebreaker.

The Gearbox Reliability Collaborative has conducted extensive field and dynamometer test campaigns on two heavily instrumented wind turbine gearboxes. In this study, the load sharing behaviour between six bearings in the planetary stage is described using a combined approach of measurement and simulation. First, planet-bearing data are analysed to characterise planetary stage behaviour in different environments. Second, a method is described for integrating the measured responses of the planetary stage into an advanced model of the bearing life that significantly changes the life prediction. Third, a sensitivity study of the planet bearings is conducted using multibody gearbox models. Various levels of gearbox flexibility and different planet assembly fits are investigated and compared with experimental observations. Measurements in the dynamometer and field show that bearing loading differs significantly between the six planet bearings. The relative loading behaviour of the planetary stage bearings is directly influenced by boundary conditions of the planet carrier pins. Assembly differences between two identically designed gearboxes cause different load sharing behaviour. Simulations are used to quantify the effect of different component flexibilities. Reduced order models are developed to accurately predict bearing loading in a cost-efficient manner.

This paper deals with fatigue analysis of the high-pressure hydraulic power take-off (PTO) machinery in a heaving-buoy wave energy converter (WEC) exposed to irregular incident waves, with a focus on the hydraulic cylinder. To estimate the fatigue damage of the relevant critical joints of the cylinder, time-domain simulations are required to obtain the pressure loads inside the hydraulic pump. In this paper, the relevant dynamic responses are calculated by using the previously established nonlinear WEC model. Long-term pressure loads are obtained by combining the short-term time-simulation results and the occurrence probability of each sea state, and two models, Weibull and generalized Gamma distributions, are applied to fit the long-term rainflow-counted pressure cycles. Based on the pressure cycle distribution, the long-term fatigue damage of the cylinder with two different kinds of material, carbon, low alloy steel and nickel–chromium–molybdenum–iron alloy, is obtained by using the SN-Miner Palmgren approach. Two main parameters, cylinder diameter and thickness-diameter ratio, are investigated. Moreover, the parameter combination with which the cylinder has an expected fatigue life and possesses minimum material volume is supplied. Finally, the relative contribution of fatigue damage from different sea states is obtained and the most important wave conditions are identified.

To achieve cost-effective and reliable structural design of floating wind turbines, efficient and accurate time domain numerical approaches are required to analyse structural responses in design conditions, e.g. wind and waves. This paper focuses on validation of a time-domain numerical approach for determining forces and moments in structural components of floaters. The approach considers floating wind turbines as a system of several structural components, e.g. blades, rotational shaft, nacelle, tower, mooring lines, columns, pontoons and braces. A finite element model is developed to represent global stiffness of the structural components. The external and inertia loads on the structural components are modelled as distributed loads. Hydrodynamic loads on each structural component are derived from the corresponding hydrodynamic coefficients obtained by solving the first order boundary value problem using WAMIT. A 1:30 scaled braceless semi-submersible model test which implements the ReaTHM® testing approach was done by SINTEF Ocean, formerly MARINTEK, in its ocean basin. Measurements of the global forces and moments at the base of a side column of the model and rigid-body motions of the model are compared to the corresponding simulations. This paper focuses on responses in moderate waves for which linear hydrodynamic loads are applicable. Differences in the corresponding simulations and measurements are found to be small, while possible reasons, e.g. synchronizations, non-linear effects, and uncertainties in the measurements and simulations, for the differences are analysed. Essential information about the model test, descriptions of the numerical models, calibrations and results and discussions of the validation are given in this paper. © 2017 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 objective of this paper is to investigate the frequency-based fault detection of a 5MW spar-type floating wind turbine (WT) gearbox using measurements of the global responses. It is extremely costly to seed managed defects in a real WT gearbox to investigate different fault detection and condition monitoring approaches; using analytical tools, therefore, is one of the promising approaches in this regard. In this study, forces and moments on the main shaft are obtained from the global response analysis using an aero-hydro-servo-elastic code, SIMO-RIFLEX-AeroDyn. Then, they are utilized as inputs to a high-fidelity gearbox model developed using a multi-body simulation software (SIMPACK). The main shaft bearing is one of the critical components since it protects gearbox from axial and radial loads. Six different fault cases with different severity in this bearing are investigated using power spectral density (PSD) of relative axial acceleration of the bearing and nacelle. It is shown that in severe degradation of this bearing the first stage dynamic of the gearbox is dominant in the main shaft vibration signal. Inside the gearbox, the bearings on the high speed side are those often with high probability of failure, thus, one fault case in IMS-B bearing was also considered. Based on the earlier studies, the angular velocity error function is considered as residual for this fault. The Hilbert transform is used to determine the envelope of this residual. Information on the amplitude of this residual properly indicates damage in this bearing. © 2017 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/).

Many long-span bridges that are slender and susceptible to wind-induced vibrations are currently under construction all over the world. As nonlinear structural behavior and nonlinear displacement-dependent wind loads become of higher importance, time-domain methods are commonly applied instead of frequency-domain approaches. This paper discusses how rational functions, fitted to aerodynamic derivatives, can be converted to a state-space model to transform the frequency-dependent aerodynamic forces into the time domain. A user element has been implemented in ABAQUS to include the self-excited forces in the dynamic analysis. The flutter stability limit and buffeting response of the Hardanger Bridge have been calculated in a comprehensive case study to illustrate the performance of the presented methodology. The Average Conditional Exceedance Rate method is used to estimate long-term extreme load effects. A simplified full long-term method that ignores wind velocities which contribute little to the long-term extreme values is introduced to reduce the required computational effort. The long-term predictions are also compared with results obtained using a short-term approach and it is concluded that the long-term extreme load effects are approximately 14% higher than the short-term extreme values. This is a submitted manuscript of an article published by Elsevier Ltd in Engineering Structures, 30 June 2017

Based on the three dimensional potential theory and finite element method (FEM), this paper presented a method for time-domain hydroelastic analysis of a floating bridge in inhomogeneous waves. A floating bridge in both regular and irregular waves, is taken as a numerical example. This method is firstly validated by the comparisons of the results between frequency domain method and presented time domain method under regular wave condition. Then the hydroeleastic responses of the floating bridge in waves with spatially varying significant wave height/peak period are presented, with the purpose to illustrate the feasibility of the proposed method. The primary results at this stage indicate that the inhomogeneity of the waves might affect the structure dynamic responses of the floating bridge in waves.

The one-dimensional (1-D) unsteady-pressure-flow equations of hydraulic pipeline can be approximated and solved numerically using a finite-mode approach. This paper deals with constructing bond graph representations for the hydraulic pipelines with laminar flow by using the previously established 1-D linear-friction bond graph structures and modal approximation techniques when 2-D frequency-dependent friction is taken into account. To model hydraulic pipelines as components in different systems using bond graphs, there are four possible input–output causalities: pressure inputs at both ends, flow rate input at both ends and the two cases of mixed inputs. In this paper, all four configurations are considered. The developed bond graph structures together with their corresponding modal parameters are presented. A comparison is made for three models, namely lossless, 1-D linear fluid friction and 2-D fluid friction. Simulation results show that the modified bond graph models studied in this work can give more satisfactory results.

Offshore wind is an important source of renewable energy and is steadier and stronger than onshore wind. Offshore areas not only have strong winds but also contain other potential renewable energy sources, such as ocean waves and tidal currents. Therefore, it is interesting to investigate the possibility to utilise these energy potentials simultaneously, particularly the combination of wind and ocean wave energy due to their natural correlation. For this reason, previous researchers have examined the use of a floating wind turbine (FWT) and a wave energy converter (WEC) on a single platform ( Aubault et al., 2011; Peiffer et al., 2011; Soulard and Babarit, 2012 ). In this paper, a combined concept involving a spar-type FWT and an axi-symmetric two-body WEC is considered and denoted as STC. With respect to operational conditions, a previous study ( Muliawan et al., 2013 ) indicates that the STC not only reduces the total capital cost but also increases the total power production compared to the use of segregated FWT and WEC concepts. As with other floating systems, the STC must be designed to ensure serviceability and survivability during its entire service life. One of the design criteria is the ultimate limit state (ULS), which ensures that the entire STC system will have adequate strength to withstand the load effects imposed by extreme environmental actions. Therefore, in the present study, coupled (wave- and wind-induced response mooring) analysis is performed using SIMO/TDHMILL in the time domain to investigate such responses of the STC system as mooring tension, spar-tower interface bending moment, end stop force, and contact force at the Spar-Torus interface under extreme conditions. Environmental conditions that pertain to the northern North Sea metocean data are selected and include operational, survival and 50-year conditions. Finally, the ULS level responses that are capital cost indicators for both FWT alone and for the STC system are estimated and compared.

With the increase in the size of containerships, the lowest wet natural frequencies can be as low as 0.5 rad/s for vessels whose lengths are beyond 300 m. These lower frequencies make the estimation of wave loads when considering the hull flexibility quite challenging. Because no numerical tools are able to account for all of the nonlinear effects properly and there is not enough experimental data available for validation, the extent that current numerical tools can correctly predict the hydroelastic effects is a concern. This paper adopts a nonlinear hydroelastic code, WINSIR, to calculate the nonlinear hydroelastic response of an 8600-TEU containership model advancing in regular waves. The calculated results are compared with test data in terms of the first, second, third, fourth and fifth harmonics of the vertical bending moment amidships. Finally, the uncertainties in the experiments are discussed, and conclusions are given at the end of this paper.

Tension leg platform wind turbines (TLPWTs) represent one potential method for accessing offshore wind resources in moderately deep water. Although numerous TLPWT designs have been studied and presented in the literature, there is little consensus regarding optimal design, and little information about the effect of various design variables on structural response. In this study, a wide range of parametric single-column TLPWT designs are analyzed in four different wind-wave conditions using the Simo, Riflex, and AeroDyn tools in a coupled analysis to evaluate platform motions and structural loads on the turbine components and tendons. The results indicate that there is a trade-off between performance in storm conditions, which improves with larger displacement, and cost, which increases approximately linearly with displacement. Motions perpendicular to the incoming wind and waves, especially in the parked configuration, may be critical for TLPWT designs with small displacement. Careful choice of natural period, diameter at the water line, ballast, pretension, and pontoon radius can be used to improve the TLPWT performance in different environmental conditions and water depths.

Due to the high level of fatigue loads as well as a large number of load cycles caused by wind and wave loads together, fatigue performance of welded connections is a design driving criterion for offshore wind turbine support structures. In this paper prediction of fatigue reliability of welded multi-planar tubular joints of the support structure of a fixed jacket offshore wind turbine designed for a northern North Sea site in a water depth of 70 m is performed. Dynamic response of the jacket support structure due to wind and wave loads is calculated by using a decoupled procedure with good accuracy. Hot-spot stresses at failure-critical location of the relative reference brace of the selected tubular joint are derived by summation of the single stress components from axial, in-plane and out-of-plane action, the effects of planar and non-planar braces are also considered. A two-parameter Weibull function is used to fit the long-term statistical distribution of hot-spot stress ranges by combination of time domain simulation for representative environmental conditions (wind/sea states) in operational condition. The main uncertainties associated with the whole procedure of reliability analysis are identified and quantified. The load histories are normalized to ensure that fatigue design criteria based on the SN-Miner-Palmgren approach is satisfied. Hence the reliability estimates obtained refer to fatigue design of tubular joints that satisfy design criteria. The reliability analysis is based on fracture mechanics (FM) analysis of crack growth. The corrosion-induced increased crack growth rate is taken into account by considering the increased hot-spot stress range due to changes of nominal stress and stress concentration factors (SCFs) produced by the thickness thinning (wastage) effects of all braces and chord of the selected tubular joint with a general uniform corrosion model. The geometry function effect and the material degradation effect due to corrosion on the reliability analysis are also investigated. The effects of inspection and repair with and without consideration of corrosion are quantified based on the quality of inspection in terms of probability of crack detection curves. The sensitivity of the reliability index on important random variables is estimated.

Accurate estimation of the wave-induced extreme hogging vertical bending moment (VBM) is of vital importance for the design of container ships because container ships are normally under hogging conditions in still water. According to the empirical formulas proposed by the classification society rules, the design hogging VBM can be approximately 20 % smaller than the design sagging VBM for vessels with small block coefficients. High-order harmonic components in the vertical load effects, which are induced by the nonlinearities in the hydrodynamic forces and ship hull geometry, contribute to the asymmetry. Previous studies have shown that the nonlinear hydrostatic and Froude–Krylov forces increase the sagging VBM significantly. Current numerical tools are able to reveal this asymmetry to a certain extent. There is, however, little focus on the nonlinear pressure under the bow bottom, which is a more likely contributor to the hogging VBM. Several unexpected phenomena have been observed for large container ships. The wave-frequency sagging and hogging VBMs followed each other closely, and hence did not reflect the significant nonlinear factors as expected. In this paper, the test data of two (8600-TEU and 13000-TEU) ultra-large containership models in both regular and irregular head waves are systematically studied. In regular waves, the influence of the second and third harmonics on the fundamental hogging peaks and sagging troughs is estimated by comparing both the amplitude and phase difference relative to the first harmonic peaks. In irregular waves, the focus is on the statistical characteristics of the wave-induced nonlinear vertical load effects. To achieve a balance between results in regular and irregular waves, the influence of the second harmonics is evaluated through bispectral analysis.

Floating wind turbines can be the most practical and economical way to extract the vast offshore wind energy resources at deep and intermediate water depths. The Norwegian Ministry of Petroleum and Energy is strongly committed to developing offshore wind technology that utilises available renewable energy sources. As the wind is steadier and stronger over the sea than over land, the wind industry recently moved to offshore areas. Analysis of the structural dynamic response of offshore wind turbines subjected to stochastic wave and wind loads is an important aspect of the assessment of their potential for power production and of their structural integrity. Of the concepts that have been proposed for floating wind turbines, spar-types such as the catenary moored spar (CMS) and tension leg spar (TLS) wind turbines seem to be well-suited to the harsh environmental conditions that exist in the North Sea. Hywind and Sway are two examples of such Norwegian concepts; they are based on the CMS and TLS, respectively. Floating wind turbines are sophisticated structures that are subjected to simultaneous wind and wave actions. The coupled nonlinear structural dynamics and motion response equations of these turbines introduce geometrical nonlinearities through the relative motions and velocities. Moreover, the hydrodynamic and aerodynamic loading of this type of structure is nonlinear. A floating wind turbine is a multibody aero-hydro-servo-elastic structural system; for such structures, the coupled nonlinear equations of motion considering nonlinear excitation and damping forces, including all wave- and wind-induced features, should be solved in the time domain. In this thesis, the motion and structural responses for operational and extreme environmental conditions were considered to investigate the performance and the structural integrity of spar-type floating wind turbines. The power production and the effects of aerodynamic and hydrodynamic damping, including wind-induced hydrodynamic and wave-induced aerodynamic damping, were investigated. Negative damping adversely affects the power performance and structural integrity. In this thesis, the controller gains were tuned to remove servo-induced instabilities. The rotor configuration effect on the responses and power production was investigated by comparing the upwind and downwind turbines. To develop robust design tools for offshore wind power, the competencies of the offshore technology and wind technology must be combined. Both the offshore and wind energy industries have begun to extend their existing numerical codes to account for the combined aerodynamic and hydrodynamic effects on the structure. As a result verifications of extended codes by doing experiments and code-to-code comparisons are needed. One of the aspects of the present research was to fill this gap by performing hydrodynamic and hydro-elastic comparison between commercial codes. For both CMS and TLS concepts, the comparisons were carried out prior to using the tools to study the behaviour of the CMS and TLS under wave- and wind-induced loads. Offshore structures encounter a variety of operational and harsh environmental conditions. Limit states such as ultimate, fatigue, accidental collapse and serviceability limit states (ULS, FLS, ALS and SLS) are defined as the design criteria for offshore structures. In performing realistic ultimate limit state analysis, the extreme responses of a floating wind turbine over its life should be estimated. This estimation requires detailed analysis of the extreme response. In the present thesis, extreme value analysis for spar-type wind turbines subjected to simultaneous wave and wind actions was preformed. The structural responses and the effect of modelled forces such as turbulence on these responses were investigated. The joint distribution of the environmental characteristics of the wave and wind was applied through the contour surface method. Stochastic wave and wind analysis showed that, while rigid body modelling was sufficient for obtaining accurate motions, consideration of the elastic behaviour of the tower/support structure was necessary to predict structural responses. The blades structural responses were found to be significantly affected by the turbulent wind. However, the mean and standard deviation of global motion and structural responses were not affected by the turbulence. Thus, to reduce the simulation time in fatigue analysis, a constant wind speed model can be applied. The CMS and TLS wind turbines are inertia-dominated structures, and the hydrodynamic viscous drag did not affect their wave-induced responses, while an increase in viscous drag could effectively reduce the resonant responses of such turbines. Under operational conditions, aerodynamic damping was found to be active in reducing both wave frequency and resonant responses. The results showed that, for a floating wind turbine, extreme response could occur in survival conditions, while for a fixed wind turbine, the extreme response occurs in operational cases related to the rated wind speed. To estimate the extreme value responses, extrapolation methods were used to reduce the sample size in Monte Carlo simulations. The accuracy of methods to estimate the extreme responses as a function of sample size and methods applied was investigated. The normalized responses for both CMS and TLS offshore wind turbines were presented to draw more generalized conclusions.

The coupled analysis of floating wind turbines is time consuming. This article presents a simplified approach for dynamic response analysis of floating wind turbines that are subjected to wave and wind loads. The goal of the approach presented here is to investigate a simplified method for the aerodynamic forces to minimize the computational time while maintaining acceptable accuracy. Hence, the simplified method is validated against a comprehensive aero-hydro-servo-elastic approach. A dynamic link library (DLL) called “TDHMILL” provides the aerodynamic loads as an external input to the Simo-Riflex. The coupled Simo-Riflex-TDHMILL, “SRT”, is compared with the HAWC2 code for spar-type wind turbines. The agreement between the SRT and HAWC2 codes is very good. The hydrodynamic code-to-code comparison between the Simo-Riflex and HAWC2 codes is carried out before the coupled wave-wind-induced analyses. The results show that the codes agree for the wave-only case, as well as for the wave- and wind-induced cases. The statistical characteristics of the dynamic motions and structural responses are compared for spar-type wind turbines. Two spar-type wind turbines are considered in the present article to investigate the sensitivity of the method to different designs. The code-to-code comparison between the SRT and HAWC2 codes is satisfactory for both the ShortSpar and DeepSpar configurations, at moderate and deep water depths, respectively. The simplified approach is intended and suitable for feasibility and pre-engineering studies of alternative substructures where effects from rotor dynamics are of minor importance. The results show that the simplified method is faster than the comprehensive method by a factor of 100, while its accuracy is acceptable in comparison to the sophisticated analyses.

The structural reliability analysis of damaged vessels has commonly been conducted by neglecting the effect of sloshing. In this paper, the coupled motion of the damaged vessel and sloshing is set up based on the assumption that the damaged tanks are of rectangular shape. The sloshing effects in tanks are modeled by a linear multimodal approach. Then the effect of sloshing in tanks on the hull girder bending moments and the failure probability are studied for an oil tanker in various damage conditions. It is concluded that in certain tank resonance conditions, sloshing effects cannot be neglected. In such cases sloshing damping plays an important role on the sloshing effects.

The main load-carrying component of the containment system of membrane gas carriers is made of plywood. One of the failure modes of the containment system due to violent liquid loading inside the tank is bending the plywood plate. A systematic study of the plywood bending failure is conducted by means of experiments and finite element (FE) methods. The objective is to propose an FE model that can replicate the nonlinear tests results of the plywood bending failure. The FE software ABAQUS has been utilized as a basis for this study. The material is modelled by linear elasticity combined with damage initiation using Hashin's theory. The brittle behaviour is indicated by linear softening. The softening is modelled by a fracture energy approach. The sensitivity study of the damage parameter on ultimate bending load is conducted. The numerical results from FE models are compared with the test results. A nonlinear material and FE model of plywood that could demonstrate the brittle failure is proposed.

An extended 3D hydroelasticity theory is used in the frequency domain to predict the dynamic response of 5 by 2 floating fish farm collars in regular waves. Hinge modes in two directions and flexible torsional modes are all considered in the analyses. The dry and wet resonant natural frequencies for each rigid relative motion and the flexible modes are also calculated. The results show that the modes, flexible bending and torsional modes, as well as their interactions, greatly contribute to the dynamic response of the floating collar system. From a design point of view, the effects of the connector rotational stiffness on the dynamic response of the structures are also analysed.

For bulk carriers in hogging, the most critical situation is the alternate hold loading (AHL) condition with odd numbered holds loaded with high density cargoes and even numbered holds empty. The effect of the local lateral pressure loads should be considered in the assessment of ultimate hull girder strength in the hogging and AHL conditions. In the present paper the ultimate strength of a Capesize bulk carrier hull girder under combined global and local loads in the hogging and AHL condition is extensively and systematically investigated using nonlinear finite element (FE) analysis with ABAQUS software. Since the bulk carrier used as a reference vessel in this study is an old design we also studied the effect of modified scantlings by multiplying the plate thickness in the bottom structure by a design modification factor (DMF). In particular, it should be noted that a DMF of 1.4 gives a design in accordance with the new CSR rules. Based on the results obtained by nonlinear FE analyses, a practical interaction equation is established between global hogging bending capacity and average external sea pressure over the bottom.

The development of robust design tools for offshore wind turbines requires knowledge of both wave and wind load models and response analysis. Verification of the numerical codes is required by the use of experiments and code-to-code comparisons. This paper presents a hydroelastic code-to-code comparison between the HAWC2 and USFOS/vpOne codes for a tension leg spar (TLS) wind turbine with a single tether. This concept is hence based on the TLP and Spar concepts. The comparison is performed using coupled hydroelastic time domain simulations. Several aspects of modelling, such as wave simulation, hydrodynamic and structural modelling, are addressed for the TLS. Wave-induced motions of the support structure affect the power performance of a wind turbine. Furthermore, overload of the tension leg should be avoided. In this paper, the motion and tension responses are compared. The tension leg introduces nonlinear effects on the spar motion. These nonlinear effects include combined-frequency effect such as double, difference and sum of wave, as well as natural pitch and surge frequencies. Hydrodynamic loads are based on a combination of the Morison formula and the pressure integration method. A comparison indicates that the motion and tension responses obtained in the two codes are in good agreement.

Although the coupled horizontal–torsional vibrations of open ships have been investigated numerically for decades, the available experimental data in oblique seas seem rare. Model tests, considering natural frequencies of bending and torsional modes, have been conducted by the Centre for Ships and Ocean Structures (CeSOS) in the towing tank and ocean basin. A flexible backbone model was designed with five cut-outs on the top side of the aluminum beam to realize approximately torsional stiffness as well as vertical and horizontal bending stiffness. This paper mainly deals with measured bending and torsional vibrations in regular and irregular waves. The damping ratios, mode shapes, and modal moments were derived from experimental data, and a numerical model based on modal superimposition is established according to the measured hydroelastic properties. Some time-domain simulations are carried out considering the structural characteristics, and compared with measured results. The test data in regular waves are presented to show the possible factors of influence on the vibrations. The effect of bending and torsional vibrations on the extreme response values in irregular waves is estimated. The uncertainties in the experiments are discussed and conclusions are presented at the end of this paper.