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

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

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

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

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

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

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/).

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.

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.

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.

This study addresses numerical simulations of the lifting operation of a monopile for an offshore wind turbine with a focus on the lowering process. A numerical model of the coupled system of the monopile and vessel is established. The disturbed wave field near the vessel is investigated and observed to be affected by the diffraction and radiation of the vessel. The shielding effects of the vessel during the continuous lowering operation are accounted for in this study by developing an external Dynamic Link Library (DLL) that interacts with SIMO program in the time-domain simulations. The DLL is implemented by interpolating fluid kinematics between pre-defined wave points near the vessel. Based on the time-domain simulations, the critical responses, such as the motions of the monopile, the tensions in the lift wire and the contact forces in the gripper device in the disturbed wave fields, are compared with those in incident wave conditions. The results indicate that a great reduction in these extreme responses can be achieved when the shielding effects are considered. The sensitivity study of the responses in different wave directions is performed. The results indicate different behaviours with different wave directions and with short or long waves. A comparison of the responses when using a floating vessel and a jack-up vessel is also studied and can be used to support the choice of installation vessel type. © 2014. 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/

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.

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.

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 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.

A numerical model is introduced in this paper to investigate both global and local ice loads on ship hulls. This model is partly based on empirical data, by which the observed phenomena of continuous icebreaking can be well reproduced. In the simulation of a full-scale icebreaking trial, the interdependence between the ice load and the ship’s motion is considered, and the three degree-of-freedom rigid body equations of surge, sway and yaw are solved by numerical integration. The variations in the level ice thickness and in the strength properties of ice can also be taken into account. The simulated ice loads on ship hulls are discussed through two case studies, in which the ship’s performance, the statistics of ice-induced frame loads, and the spatial distribution of ice loads around the hull are analyzed and compared with field measurements. As far as we know the present paper is the first to integrate all the features above. It is hoped that further studies on this numerical model can supplement the field and laboratory measurements in establishing a design basis for the ice-going ships especially for ships navigating in the first-year ice.

The alternate hold still-water loading in hogging combined with wave loading is critical for the safe design of bulk carriers. The ultimate longitudinal strength of the hull girder of bulk carriers in this condition has been found to be considerably reduced by the action of local lateral pressure loads. In the present paper, an interaction equation based on the ultimate hull girder strength assessment obtained by nonlinear finite element analyses is adopted to consider the relationship between ultimate longitudinal bending capacity and average external sea pressure over the bottom. This interaction equation is used as the basis for the failure function. The annual probability of failure is obtained by FORM analysis considering two typical load cases, namely, pure longitudinal hogging bending moment and combined global hogging bending moment and local lateral pressure loads. The effect of heavy weather avoidance on the failure probability is evaluated. The results show that the local lateral pressure has a significant influence on the annual probability of failure of bulk carriers in the hogging and alternate hold loading condition.

This is the second of two companion papers dealing with nonlinear finite element modelling and ultimate strength analysis of the hull girder of a bulk carrier under Alternate Hold Loading (AHL) condition. The methodology for nonlinear finite element modelling as well as the ultimate strength results from the nonlinear FE analyses was discussed in the companion paper (Part 1). The purpose of the present paper is to use the FE results to contribute towards developing simplified methods applicable to practical design of ship hulls under combined global and local loads. An important issue is the significant double bottom bending in the empty hold in AHL due to combined global hull girder bending moment and local loads. Therefore, the stress distributions in the double bottom area at different load levels i.e. rule load level and ultimate failure load level are presented in detail. The implication of different design pressures obtained by different rules (CSR-BC rules and DNV rules) on the stress distribution is investigated. Both (partially) heavy cargo AHL and fully loaded cargo AHL are considered. Factors of influence of double bottom bending such as initial imperfections, local loads, stress distribution and failure modes on the hull girder strength are discussed. Simplified procedures for determination of the hull girder strength for bulk carriers under AHL conditions are also discussed in light of the FE analyses.

The coefficient of contribution method, in which the extreme response is determined by considering only the few most important sea states, is an efficient way to do nonlinear long-term load analyses. To furthermore efficiently find the nonlinear short-term probability distributions of the vessel responses in these sea states, response conditioned wave methods can be used. Several researchers have studied the accuracy of response conditioned wave methods for this purpose. However, further investigations are necessary before these can become established tools. In this paper we investigate the accuracy by comparing the short-term probability distributions obtained from random irregular waves with those from response conditioned waves. We furthermore show how response conditioned wave methods can be fitted into a long-term response analysis. The numerical and experimental investigations were performed using a container vessel with a length between perpendiculars of 281 m. Numerical simulations were done with a nonlinear hydroelastic time domain code. Experiments were carried out with a flexible model of the vessel in the towing tank at the Marine Technology Centre in Trondheim. The focus was on the probability distributions of the midship vertical hogging bending moments in the sea states contributing most to the hogging moments with a mean return period of 20 years and 10 000 years. We found that the response conditioned wave methods can very efficiently be used to accurately determine the nonlinear short-term probability distributions for rigid hulls, but either accuracy or efficiency is to a large effect lost for flexible hulls, when slamming induced whipping responses are accounted for.

An accurate determination of the global load effects in a ship is vital for the design of the vessel. This paper addresses an experimental and numerical study of containership responses in severe head seas. Experimental results were obtained using a flexible model of a containership of newer design. The experiments showed that, taking hull flexibility into account, the fourth and sixth harmonic of the vertical bending moments had a maximum value of between 25% and 50% of the first harmonic. We also demonstrated that hull flexibility can increase the vertical bending moment by up to 35% in sea states relevant for design. Comparisons of moments found experimentally with results from a nonlinear hydroelastic strip theory method showed that the effect of nonlinearities on the rigid body moments was slightly over-predicted in the aft body. The method also tends to over-predict the increase of the bending moments due to hull flexibility. In general however, the numerical results compared reasonably well with the experimental ones.

This is the first of two companion papers dealing with nonlinear finite element modelling and analysis of the ultimate strength of a bulk carrier hull girder under alternate hold loading (AHL) condition. The purpose is to contribute to establishing rational ultimate longitudinal strength criteria for the hull girder under combined loading. The focus is on the hogging condition. An important issue is the significant double bottom bending in empty holds in AHL due to combined global hull girder bending moment and local loads. The local loads may substantially reduce the strength of the hull girder. Different AHL conditions, i.e. fully loaded cargo and (partially) heavy cargo are considered. A critical review of external and internal design pressures for different AHL conditions is accomplished using both CSR-BC rules and DNV rules. A methodology for nonlinear finite element modelling of hold tanks of a bulk carrier under AHL is presented by use of ABAQUS. A mesh convergence study is carried out in order to find the appropriate mesh for the model. The implication of using different design pressures on the hull girder strength is assessed. The FE results can be used as a basis for establishing simplified methods applicable to practical design of ship hulls under combined loadings. This issue is discussed in the companion paper.

In this paper, a consistent structural analysis procedure to estimate the global and local load effects considering symmetric and anti-symmetric hydroelastic vibrations in high waves is proposed. The procedure consists of motion analysis followed by a structural analysis. First, a system equation of motion by modal approach is set-up and solved in the time domain. In the load evaluations, not only linear but also nonlinear forces such as quadratic component in Bernoulli's theory and body nonlinearity due to the fluctuation of wetted surface, which might have high priority from the viewpoint of structural analysis, are considered by using three-dimensional potential theory. The accuracy of the structural analysis results depends upon the consistency between the models employed in each analysis as well as the accuracy of the analysis at each stage itself. In order to maintain the consistency, the mass terms and the modal forces are evaluated based on the FE model of the whole ship in which all the mass properties are included, and also on modal analysis results on the same model. Then, the inertia forces and pressure distributions at selected time steps are statically applied on to the FE model considering dynamic amplification effects arising from the flexible modes. The procedure is applied to a large container ship. Local and global effects are evaluated. Fluctuating component in torsional mode, or springing in torsion is found.

Uncertainty of wave-induced response of marine structures due to long-term variation of extratropical wave conditions

This paper deals with a estimation of long-term extreme value for a given return period, say D=100 yr . In principle, this response is obtained by combining the response in all the sea states. The long-term response for a linear system can be effectively obtained by determining the response for each sea state, specified by the significant wave height, Hs, and the peak period, Tp, in the frequency domain. However, if the response is nonlinear, time domain simulation and a long time series would be required, to limit statistical uncertainty. Therefore, the long-term analysis becomes rather complicated and time consuming. For the long-term analysis, it is crucial to introduce ways to improve the efficiency in the calculation. In this work, it is shown that, the long-term extremes can be estimated by considering only a few short-term sea states. A long-term analysis based on identifying the most important sea state, defined by the coefficient of contribution, using linear analysis is applied. An iteration procedure is thereafter used to find the nonlinear long-term extreme values. It is concluded that only a limited number of sea states is necessary to get an acceptable estimate of the nonlinear D-year response as long as the most important sea states are included, i.e., the sea state with the maximum coefficient of contribution.

Stresses and deformations relevant for transverse strength analysis are calculated for a 60 m catamaran by a compartment model and are compared with corresponding results from a global model. It is found that the flexibility of the partial bulkheads typical for a twin-hull Ro/Ro-vessel is so large that the interaction with the surrounding structure has to be accounted for. If springs are applied, the results are sensitive to the spring stiffness. A web frame model that does not include any bulkheads, gives large errors and is not recommended.

The main objective of this paper has been to review and to verify already published hot-spot stress extrapolation procedures for plate structures, and to develop and verify a new and general method for the structural stress extrapolation to be used together with a hot-spot design S-N curve for aluminum ship structures. The proposed extrapolation method has been based on the asymptotic behavior of the stresses adjacent to an idealized notch (‘singularity’). On basis of the fatigue test S-N data obtained in this study, relevant S-N curves to be used together with a proposed extrapolation procedure and with already published extrapolation methods have been suggested. A hot-spot design S-N curve with a characteristic strength of 32 was suggested as a suitable choice for the fatigue assessment of profile ground fillet welded stiffener/bracket connections while a design hot-spot design S-N curve with a characteristic strength of 25 was suggested for as-welded stiffener/bracket connections.

Probabilistic models are presented for the still water bending moment (SWBM) and vertical wave bending moment (VWBM). Different stochastic and deterministic methods are applied for the combination of the long-term SWBM and VWBM. It is found that commonly applicable deterministic load combination methods, namely, the peak coincidence method, the Turkstra's rule and the SRSS rule, are either very conservative or nonconservative. Using stochastic methods, including the exact point-crossing method, approximate load coincidence method and Ferry Borges method, leads to identical predictions for the current load combination problem. For code application, load combination factors are introduced for both sagging and hogging conditions to reduce the conservatism inherent in the existing ship rules. Case studies are performed for an offshore production ship. A considerable reduction of the total design bending moment is found when the load coincidence is properly considered instead of combining the maximum of each load.

This paper is concerned with ultimate strength formulations for long circular tubes under combined pressure, tension and longitudinal bending. The study is motivated primarily by the design needs of tendon and riser bodies used in the offshore industry, but it also relevant for offshore pipelines. The tubulars considered have a diameter-to-thickness ratio between 15 and 40 and a yield stress primarily in the range 300–600 MPa. The stress-strain characteristics are typically like those of X-52 and X-77. The goal has been to develop a simple, explicit strength formulation that can be used in reliability analysis as well as load and resistance factor design (LRFD) checks of riser and tendon bodies. This goal is achieved by a systematic comparison of existing formulations for tubes subjected to pressure, tension and longitudinal bending, with relevant experimental data as well as numerical analyses. The study resulted in a new formulation for the interaction between the three loads.

The uncertainty of the long-term distribution of the wave-induced wave bending moments in ships has been assessed, relating it to ocean areas, ship routes, ship speeds and ship types. Weibull distributions have been fitted to the calculated values of long-term distributions and their shape factor has been related to the ship length. The number of load cycles in a ship lifetime has been calculated, and characteristic values were derived for different levels of probability. A two-parameter Weibull distribution defined by a characteristic value and a shape factor has been proposed as a load model to be adopted in design rules for the assessment of fatigue damage in ship structures. The model uncertainty of the two parameters defining the distribution is assessed as a function of the method of fitting the long-term distribution, of ship speed and of the climatological data.