Comparing ocean-wave energy with its origin, wind energy, the former is more persistent and spatially concentrated. In this paper wave spectrum parameters related to transport, distribution and variability of wave energy in the sea are educed. Many different types of wave-energy converters, of various categories, have been proposed. It is useful to think of primary conversion of wave energy by an oscillating system as a wave-interference phenomenon. Corresponding to optimum wave interference, there is an upper bound to the amount of energy that can be extracted from a wave by means of a particular oscillating system. Taking physical limitations into account, another upper bound, for the ratio of extracted energy to the volume of the immersed oscillating system, has been derived. Finally, the significance of the two different upper bounds is 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. ► Nonlinear coupled analysis of tension leg platform wind turbines (TLPWTs). ► 5 baseline single column designs and 40 variations were analyzed and compared. ► Changes in dimensions, ballast, and water depth affected structural loads. ► Out-of-plane motions were critical for TLPWT designs with small displacement. ► Tower and tendon loads were more design-dependent than blade loads.
In many marine and coastal engineering applications, the simultaneous distribution of several met-ocean variables is required for risk assessment and load and response calculations. For example, a joint probabilistic description is needed to construct environmental contours for probabilistic structural reliability analyses. Typically, the joint distribution of significant wave height and wave period is needed as a minimum, but other environmental parameters such as wind, current, surges and tides might also be relevant. This paper presents a study on various joint models for the simultaneous distribution of significant wave height and zero-crossing wave period. The alternative models that have been investigated are a conditional model, a bivariate parametric model and several models based on parametric families of copulas. Each of the models is fitted to data generated from a numerical wave model for the current climate and for two future climates consistent with alternative climate scenarios. Additionally, the potential effect of climate change on the simultaneous distribution will be investigated. Initial investigation reveals that straightforward application of some of the most commonly used copulas will not give reasonable joint models. The reason for this is that they are symmetric whereas the empirical copulas display asymmetric behaviour. However, asymmetric copulas can be constructed based on these families of copula, and this significantly improves the fit. Analyses of the extremal dependence in the data indicate that the variables are asymptotically independent. Furthermore, the results suggest that extreme significant wave height and zero-crossing wave period tend to be more correlated in a future climate compared to the current climate.
T-joints are one of the most common welded joints used in the construction of offshore structures, including ships and platforms. In the present study, a sequentially coupled thermo-mechanical finite element model that considers temperature-dependent material properties, high temperature effects and a moving volumetric heat source was used to investigate the effect of welding sequence on the residual stresses and distortions in T-joint welds. The parameters of Goldak's double ellipsoidal heat source model were predicted using a neural network. The numerical models were successfully validated by the experimental tests. The results show that the welding sequences have significant effects on the residual stresses and distortions, both in the magnitude and distribution mode. The optimization of the welding sequences should be investigated numerically or experimentally before the construction welded structure.
Several floating wind turbine designs whose hull designs reflect those used in offshore petroleum industry have emerged as leading candidates for the future development of offshore wind farms. This article presents the research findings from a model basin test program that investigated the dynamic response of a 1:50 scale model OC3 spar floating wind turbine concept designed for a water depth of 200 m. In this study the rotor was allowed to rotate freely with the wind speed and this approach eliminated some of the undesirable effects of controlling wind turbine rotational speed that were observed in earlier studies. The quality of the wind field developed by an array of fans was investigated as to its uniformity and turbulence intensity. Additional calibration tests were performed to characterize various components that included establishing the baseline wind turbine tower frequencies, stiffness of the delta type mooring system and free decay response behaviour. The assembled system was then studied under a sequence of wind and irregular wave scenarios to reveal the nature of the coupled response behaviour. The wind loads were found to have an obvious influence on the surge, heave and pitch behaviour of the spar wind turbine system. It was observed from the experimental measurements that bending moment at the top of the support tower is dominated by the 1P oscillation component and somewhat influenced by the incoming wave. Further it was determined that the axial rotor thrust and tower-top shear force have similar dynamic characteristics both dominated by tower’s first mode of vibration under wind-only condition while dominated by the incident wave field when experiencing wind-wave loading. The tensions measured in the mooring lines resulting from either wave or wind-wave excitations were influenced by the surge/pitch and heave couplings and the wind loads were found to have a clear influence on the dynamic responses of the mooring system.
For wave energy to become a fully-fledged renewable, efficient and reliable Wave Energy Converters (WECs) must be developed. The objectives of this article are to present WaveCat, a recently patented WEC, and its proof of concept by means of an experimental campaign in a large wave tank. WaveCat is a floating WEC whose principle of operation is oblique overtopping; designed for offshore deployment (in 50–100 m of water), it has two significant advantages: minimum (if at all) impact on the shoreline, and access to a greater resource than nearshore or shoreline WECs. It consists of two hulls, like a catamaran (hence its name); unlike a catamaran, however, these hulls are not parallel but converging. Using a single-point mooring to a CALM buoy, the bows of WaveCat are held to sea, so incident waves propagate into the space between the hulls. Eventually, wave crests overtop the inner hull sides, and overtopping water is collected in reservoirs at a level higher than the (outer) sea level. As the water is drained back to sea, it drives turbine-generator groups. The freeboard and draught, as well as the angle between the hulls, can be varied depending on the sea state. After preliminary tests with a fixed model of WaveCat in a wave flume, which constituted the first step in the development of the WaveCat patent, in this work a floating model was tested in a large wave tank. In addition to serving as a proof of concept of the WaveCat model, this experimental campaign allowed to gather data that will be used to calibrate and validate a numerical model with which to optimise the design. In addition, it was found in the tests that the overtopping rates (and, therefore, the power performance) greatly depended on the angle between hulls, so that the possibility of varying this angle (as contemplated in the patent) should indeed be incorporated into the prototype. ► The concept of WaveCat, a recently patented wave energy converter (WEC), is presented. ► WaveCat is a floating, offshore WEC based on oblique overtopping, with a low impact. ► As a proof of concept, a 1:30 model was tested in a large wave tank. ► The laboratory tests allowed to gain insight on the importance of the configuration. ► The power performance was found to vary substantially with the angle between hulls.
In this paper, we investigate the damage to offshore platforms subjected to ship collisions. The considered scenarios are bow and stern impacts against the column of a floating platform and against the jacket legs and braces. The effect of the ship–platform interaction on the distribution of damage is studied by modeling both structures using nonlinear shell finite elements. A supply vessel of 7500-ton displacement with bulbous bow is modeled. A comprehensive numerical analysis program is conducted, and the primary findings are described herein. The collision forces from the vessel are compared with the suggested force–deformation curves in the NORSOK code. For collisions with floating platforms we particularly focus on the crushing behavior and potential penetration of the bulbous bow and stern sections into the cargo tanks or void spaces of semi-submersible platforms. For fixed jacket platforms we investigate whether jacket braces can penetrate into the ship without being subjected to significant plastic bending or local denting. Adequate treatment of the relative strength between the interacting bodies is especially relevant for impacts with high levels of available kinetic energy, for which shared energy or strength design is aimed at. Simplifying one body as rigid quickly leads to overly conservative and/or costly solutions, and is in some cases . The numerical analysis is used to develop a novel pressure–area relation for the deformation of the bulbous bow and stern corners of the supply vessel. Procedures for strength design of the stiffened panels are discussed. Refined methods and criteria are proposed for strength design of platforms, including both floating and jacket structures. The adequacy of the NORSOK design guidance for collisions against jacket legs is evaluated. The characteristic strength of a cylindrical column is used to develop a novel criterion for the resistance to local denting from stern corners and bulbous bows.
It is the purpose of the paper to present a review of prediction and analysis tools for collision and grounding analyses and to outline a probabilistic procedure for which these tools can be used by the maritime industry to develop performance based rules to reduce the risk associated with human, environmental and economic costs of collision and grounding events. The main goal of collision and grounding research should be to identify the most economic risk control options associated with prevention and mitigation of collision and grounding events.
Environmental contours are often applied in probabilistic structural reliability analysis of marine structures in order to identify extreme environmental conditions that may give rise to extreme loads and responses. The perhaps most common way of establishing such environmental contours is based on the IFORM approximation (Inverse First Order Reliability Method), but recently an approach based on direct Monte Carlo simulations with importance sampling has been proposed as an alternative. Even though these contours should be used in the same way and address the same problem, there might be rather large differences between such contours in certain cases. In particular, the alternative contour method will always yield convex contours, whereas the traditional contours may be either convex or non-convex. In this paper, recent comparison studies are extended to include applications on simplified response examples. The contours are applied to simple response problems with known response surfaces in order to study how large the differences between the methods may be in terms of estimated maximum response and associated return periods. These case studies clearly illustrate the influence of the environmental contour method on the estimated extreme structural response. Whereas the different methods yield comparable results for some structural problems, they may give very different estimates of the extreme response for other. The estimated extreme responses and associated return periods from either method will also be compared to the correct extreme response, as estimated by simulation studies, for the desired return period. It is demonstrated that in certain cases, the estimates from some of the contour methods are highly conservative, whereas they in other cases might be very optimistic. The reason for these results are discussed and some requirements on the response functions for obtaining conservative estimates will be stated.
This paper presents a procedure to analyse ship collisions using a simplified analytical method by taking into account the interaction between the deformation on the striking and the struck ships. Numerical simulations using the finite element software LS-DYNA are conducted to produce virtual experimental data for several ship collision scenarios. The numerical results are used to validate the method. The contributions to the total resistance from all structural components of the collided ships are analysed in the numerical simulation and the simplified method. Three types of collisions were identified based on the relative resistance of one ship to the other. They are denoted Collision Types 1 and 2, in which a relatively rigid ship collides with a deformable ship, and Collision Type 3, in which two deformable ships are involved. For Collision Types 1 and 2, estimates of the energy absorbed by the damaged ships differ by less than 8% compared to the numerical results. For Collision Type 3, the results differ by approximately 13%. The simplified method is applicable for right angle ship collision scenario, and it can be used as an alternative tool because it quickly generates acceptable results. ► A new procedure to analyse ship collisions using a simplified method is proposed. ► Numerical simulations using LS-DYNA software are conducted. ► Three types of collisions were identified based on the relative resistance of ships.
The use of lightweight aluminium sandwiches in the shipbuilding industry represents an attractive and interesting solution to the increasing environmental demands. The aim of this paper was the comparison of static and low-velocity impact response of two aluminium sandwich typologies: foam and honeycomb sandwiches. The parameters which influence the static and dynamic response of the investigated aluminium sandwiches and their capacity of energy absorption were analysed. Quasi – static indentation tests were carried out and the effect of indenter shape has been investigated. The indentation resistance depends on the nose geometry and is strongly influenced by the cell diameter and by the skin – core adhesion for the honeycomb and aluminium foam sandwich panels, respectively. The static bending tests, performed at different support span distances on sandwich panels with the same nominal size, produced various collapse modes and simplified theoretical models were applied to explain the observed collapse modes. The capacity of energy dissipation under bending loading is affected by the collapse mechanism and also by the face-core bonding and the cell size for foam and honeycomb panels, respectively. A series of low-velocity impact tests were, also, carried out and a different collapse mechanism was observed for the two typologies of aluminium sandwiches: the collapse of honeycomb sandwiches occurred for the buckling of the cells and is strongly influenced by the cell size, whereas the aluminium foam sandwiches collapsed for the foam crushing and their energy absorbing capacity depends by the foam quality. It is assumed that a metal foam has good quality if it has many cells of similar size without relevant defects. A clear influence of cell size distribution and morphological parameters on foam properties has not yet been established because it has not yet been possible to control these parameters in foam making. The impact response of the honeycomb and foam sandwiches was investigated using a theoretical approach, based on the energy balance model and the model parameters were obtained by the tomographic analyses of the impacted panels. The present study is a step towards the application of aluminium sandwich structures in the shipbuilding. ► Application of lightweight aluminium sandwich structures in the shipbuilding. ► Comparison of static and low-velocity impact response of aluminium foam and honeycomb sandwiches. ► Quasi – static indentation tests were carried out using indenters with different shape. ► Simplified theoretical models were applied to explain the collapse modes observed during the static bending tests. ► The impact response was investigated using an energy balance model and the thomographic analyses of the impacted panels.
This article aims at proposing a new frequency-domain response estimation method for floating structures by dealing with fluid memory effects from the view point of signal decomposition. One development is that the convolution terms are decomposed and replaced by a series of poles and corresponding residues in the Laplace domain, based on the estimated added mass and damping matrices of the structure. The advantage of the proposed method is that the frequency-dependent motion equations in the time domain can then be transformed to the Laplace domain without requiring Laplace-domain expressions of the added mass and damping. The second is that exciting wave forces are not limited to harmonic components, and each component can be purely harmonic, damped harmonic or purely damped exponential. Therefore, the problem of frequency leakage caused by using the Fourier transform can be avoided, which also means estimations of frequency-domain responses can be improved accordingly. Four examples are used to investigate the performance of the proposed method. The first is a simple retardation function, which is based on a purely analytical relationship, and it satisfies all the properties of the convolution terms in the time and frequency domains simultaneously, demonstrating the correctness of using the complex exponentials to represent the retardation functions. The second is a single degree of freedom (SDOF) mathematical model subjected to a synthesized exciting wave force. The numerical results show that the frequency response function (FRF) estimated using this approach matches well with that of the traditional method. For exciting wave forces, the proposed method can yield a better frequency-domain estimation because the proposed method is not limited to a frequency resolution. To extend the proposed method to multiple DOF systems and address applications to marine structures, a semi-submersible (SEMI) and a FPSO model in SESAM is employed respectively. One can draw the following conclusions from the numerical results: (1) the FRFs of the floating structures can be obtained by replacing the convolution terms with a series of poles and corresponding residues in the Laplace domain, and they match well with those from the traditional method; and (2) the new method can provide almost identical response estimates as those of the traditional frequency-domain method when the exciting wave forces are truly composed of purely harmonic components.
Nowadays explosion weld is widely used in marine structures, thanks to its reliability. AL/FE explosive welded joints, used in shipbuilding applications, were investigated in this research activity. Static and fatigue bending tests were carried out on rectangular specimens made of ASTM A516 low carbon steel, clad by explosion welding with A5086 aluminum alloy and provided with an intermediate layer of pure aluminum. Two full-field techniques were applied during the bending tests: Digital Image Correlation and Infrared Thermography. The digital image correlation technique allowed determining the displacement and strain fields showing that the aluminium side has a higher strain with respect to the steel side, and Infrared Thermography was used to detect the superficial temperature of the specimen allowing the determination of the fatigue limit. The fatigue limit values, predicted using the Thermographic Method during static and fatigue tests, are in good agreement with the experimental values of the fatigue limit.
Predicting extreme responses is very important in designing a bottom-fixed offshore wind turbines. The commonly used method that account for the variability of the response and the environmental conditions is the full long-term analysis (FLTA), which is accurate but time consuming. It is a direct integration of all the probability distribution of short-term extremes and the environmental conditions. Since the long-term extreme responses are usually governed by very few important environmental conditions, the long-term analysis can be greatly simplified if such conditions are identified. For offshore structures, one simplified method is the environmental contour method (ECM), which uses the short-term extreme probability distribution of important environmental conditions selected on the contour surface with the relevant return periods. However, because of the inherent difference of offshore wind turbines and ordinary offshore structures, especially their non-monotonic behavior of the responses under wind loads, ECM cannot be directly applied because the environmental condition it selects is not close to the actual most important one. The paper presents a modified environmental contour method (MECM) for bottom-fixed offshore wind turbine applications. It can identify the most important environmental condition that governs the long-term extreme. The method is tested on the NREL 5 MW wind turbine supported by a simplified jacket-type support structure. Compared to the results of FLTA, MECM yields accurate results and is shown to be an efficient and reliable method for the prediction of the extreme responses of bottom-fixed offshore wind turbines.
A large-scale model test of a truncated steel catenary riser (SCR) was performed in an ocean basin to investigate the vortex-induced vibration (VIV) and its fatigue damage under pure top vessel motion. The top end of the test model was forced to oscillate at given vessel motion trajectories. Fiber Bragg grating (FBG) strain sensors were used to measure both in-plane and out-of-plane responses. Four different factors have been discussed to understand the VIV responses and fatigue damage results: instantaneous shedding frequency, touch down point (TDP) variation, tension variation and traveling waves. Out-of-plane VIV associated with strong time-varying features was confirmed to have occurred under pure vessel motion. Both number and maximum shedding frequency were investigated and indicated that the middle part of the truncated model riser was the ‘power-in’ region for out-of-plane VIV. Meanwhile, fatigue damage caused by out-of-plane VIV was found to be strongly dependent on both top motion amplitude and period. The probability distribution of the maximum damage exhibits 3 critical locations in the test model: TDP, upper sag-bend and top of the SCR. Strong traveling waves, TDP variation and end wave reflection have been proven to cause the maximum damage locations to shift from the ‘power-in’ region to these three positions. Finally, a maximum fatigue damage diagram with top motion amplitude, period and maximum shedding frequency was constructed.
In this investigation, ductile fracture in stiffened and unstiffened panels is simulated employing the fracture criterion, which depends on the mesh size, stress state and damage induced softening. The aim of the study is to show that employed fracture criterion removes mesh size effects more efficiently than traditional fracture criteria adjusted only on the basis of uniaxial tension. Fracture model is implemented into Finite Element software ABAQUS using user-defined material, VUMAT-subroutine, available for shell elements. Mesh size sensitivity analysis is carried out. Finite element simulation results are validated with experimental measurements available in literature. Comparison of numerical and experimental results shows that simulations effectively capture most of the experimentally observed features, especially when considering different mesh densities. In most cases, mesh size effects are considerably reduced compared with the fracture criteria adjusted on the basis of a uniaxial tension.
Flow-induced vibration (FIV) of multiple marine risers frequently occurs in deepwater applications and might result in serious structural failure due to fatigue damage accumulation. It is known that long marine risers may experience high modes of vibration and behave multi-mode vibration features. Moreover, the interactions of multiple risers subject to FIV are very complex and still unclear. In this paper, a series of experimental tests were carried out to investigate FIV of two side-by-side flexible cylinders with high aspect ratio (length to diameter, / = 350) in a towing tank. Four cases of different spacing ratios (center-to-center separation distances to cylinder diameter, = 3.0, 4.0, 6.0 and 8.0) were adopted to examine the effect of spacing on the multi-mode FIV of the two flexible cylinders. The maximum dominant modes are 4th and 6th in cross-flow (CF) and in-line (IL) directions for both side-by-side cylinders, as well as the single one. In the switching region of the adjacent modes of vibration, higher-order mode vibrations are less difficult to excite for side-by-side cylinders. The IL displacement amplitudes of the two cylinders could be enhanced by the remarkably strong interaction between cylinders, even with a center-to-center distance of up to 8.0 cylinder diameter. In addition, the IL FIV behaviors are much more complicated than those in CF direction, for instance the response spectra in IL direction exhibit several large peaks and lots of small spikes around. The IL and CF interactions of the two side-by-side flexible cylinders were also investigated by using the response trajectories collected from seven measurement points at different reduced velocities.
The paper presents an application of Structural Health Monitoring system based on Fibre Bragg Grating sensors dedicated to an offshore wind turbine support structure (tripod) model. The experimental investigation was performed in a water basin for the wind turbine model fixed to a turntable that allowed to change the angle between the tripod legs and a wave generator direction. During measurements the structure was excited by artificial waves and rotor blades rotating simulating effect of wind blowing with different strength. The excitation simulated environmental loading typical for offshore wind turbine. One of the tripod's upper braces contained a flange simulating artificial circumferential crack occurrence. For the structure under simulating environmental condition four damage indexes (relative and absolute) were developed that allowed to detect the damage and localise it with accuracy to the tripod's leg.
A series of finite element analyses are conducted to investigate the influence of boundary conditions and geometry of the model on the predicted collapse behaviour of stiffened panels. Periodic and symmetric boundary conditions in the longitudinal direction are used to calculate the ultimate strength of stiffened panels under combined biaxial thrust and lateral pressure. The calculated ultimate strength of stiffened panels are compared with those by different FEM (finite element method) code and are assessed. The periodic boundary condition in the longitudinal direction for two spans or bays model provides an appropriate modelling to a continuous stiffened panel and can consider both odd and even number of half waves and thus, is considered to introduce the smaller model uncertainty for the analysis of a continuous stiffened panel.
This paper deals with an experimental study of the survivability of the offshore combined concept Semisubmersible wind energy and Flap-type wave energy Converter (SFC) and with comparisons of the experimental data with numerical predictions. The SFC is a combined energy concept consisting of a braceless semisubmersible type floating wind turbine and three fully submerged rotating flap-type Wave Energy Converters (WECs). In order to study the survivability of the concept the focus is on extreme environmental conditions. In these conditions the SFC will not produce wind or wave power; the wind turbine is parked with the blades feathered into the wind and the WECs are released to freely rotate about their axis of rotation. Firstly the development and set-up of the physical model are presented. Static, quasi-static, decay, regular waves and irregular waves with wind loading tests are conducted on an 1:50 scale physical model. Aligned and oblique wave with wind loading conditions are considered. Measured variables that are presented include motions of the semisubmersible platform in six rigid body degrees of freedom, rotation of the flap-type WECs, tension of mooring lines, internal loads of the arms that connect the flap with the pontoon of the platform and tower base bending moment. The experimental data are compared with numerical predictions obtained by a fully coupled numerical model. The comparison is made at model scale. A good agreement between experimental data and numerical predictions is observed confirming the accuracy of the numerical models and tools that are used. The discrepancy between numerical and experimental results is smaller for regular than irregular waves. Compared to oblique conditions a better agreement between experimental and numerical results is obtained for the case of aligned wave and wind loadings. The results obtained demonstrate the good performance of the SFC concept in extreme environmental conditions. No strong nonlinear hydrodynamic phenomena are observed in the tests.