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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Numerical simulation based on finite element modelling is used to study the influence of welding sequences on the distribution of residual stress and distortion generated when welding a flat-bar stiffener to a steel plate. The simulation consists of sequentially coupled thermal and structural analyses using an element birth and death technique to model the addition of weld metal to the workpiece. The temperature field during welding and the welding-induced residual stress and distortion fields are predicted and results are compared with experimental measurements and analytical predictions. The effect of four welding sequences on the magnitude of residual stress and distortion in both the plate and the stiffener is investigated and their effects on the ultimate strength of the stiffened plate under uniaxial compression are discussed. Appropriate conclusions and recommendations regarding the welding sequence are presented.
Iconic lighthouses constructed on offshore reefs around the British Isles in the 19th century continue to play a crucial role in safe navigation, but the longevity of these historical structures is threatened by extreme weather. A program of experimental dynamic investigations has been carried out to support characterisation of extreme impulsive breaking wave loads on these structures, using monitored response data. This paper describes the procedures and outcomes of this program, which included modal tests of a collection of six of these lighthouses between June 2016 and October 2017. Five of the six lighthouses tested (Les Hanois, Wolf Rock, Longships, Bishop Rock and Eddystone) feature a 20th century metal helideck atop a 19th century masonry tower, with a Scottish lighthouse (Dubh Artach) being the exception that provides baseline behaviour of a relatively simple tower. All the masonry towers are imperfectly axisymmetric to some degree and all present logistical challenges for experimental work as they can only be accessed by helicopter flights subject to severe weather and time constraints. Against such challenges it was possible to identify key modal parameters, and to highlight some interesting effects due to symmetry and helideck retrofit. Notable findings were that most important modes have frequencies ranging between 4 Hz and 7 Hz and modal masses as low as ∼200 t. The rarely investigated effect of imperfect axisymmetry on forced vibration testing is studied, along with the introduction of additional modes due to retrofitted helideck. The implications of these effects on experimental modal analysis from forced vibration test data is illustrated. Finally, accelerations recorded on Wolf Rock Lighthouse during the 2017–2018 winter storm season show the modal test data can be used to infer breaking wave modal impulses up to 8 kNs.
Interaction of sea or lake ice with vertically sided offshore structures may result in severe structural vibrations commonly referred to as ice-induced vibrations. With the surge in offshore wind developments in sub-arctic regions this problem has received increased attention over the last decade, whereas traditionally the topic has been mainly associated with lighthouses and structures for hydrocarbon extraction. It is important for the safe design of these offshore structures to have the ability to predict the interaction between ice and structure in an expected scenario. A model for simulation of the interaction between a drifting ice floe and a vertically sided offshore structure is presented. The nonlinear speed dependent ductile and brittle deformation and local crushing of ice are considered phenomenologically. A one-dimensional sea ice dynamics model is applied to incorporate the effects of floe size, wind and current. The structure is modelled by incorporating its modal properties obtained from a general-purpose finite element software package. Alternatively, the model can be coupled to in-house design software for fully coupled simulations. Examples of application of the model to simulate dynamic ice-structure interaction are provided. Simulation results are validated with public data from forced vibration experiments, small-scale intermittent crushing and frequency lock-in, and full-scale interaction with the Norströmsgrund lighthouse. Effects of floe size and environmental driving forces on the development of ice-induced vibrations in full-scale are studied. It is shown that sustained frequency lock-in vibrations of the structure can only develop for very specific combinations of environmental driving forces and ice floe size. In all other cases, the ice floe slows down and comes to a stop, or accelerates to a drift speed which exceeds the range where frequency lock-in develops. This results in only a few cycles of vibration per interaction event, such as observed for the Norströmsgrund lighthouse in the Baltic Sea.
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.
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.
The paper presents finite element simulations of a small-scale stiffened plate specimen quasi-statically punched at the mid-span by a rigid indenter, in order to examine its energy absorbing mechanisms and fracture. The specimen, scaled from a tanker side panel, is limited by one span between the web frames and the stringers. The paper provides practical information to estimate the extent of structural damage within ship side panels during collision accidents. Moreover, the results of this investigation should have relevance to evaluate grounding scenarios in which the bottom sustains local penetration. This is possible since the structural arrangement of the double hull and the double bottom of tanker vessels is very similar. The experimentally obtained force–displacement response and shape of the deformation show good agreement with the simulations performed by the explicit LS-DYNA finite element solver. The numerical analysis includes aspects of particular relevance to the behaviour of ship structures subjected to accidental loads which could give rise to difficulties in interpreting finite element calculations. In particular, the paper comments on the material nonlinearities, the importance of specifying the precise boundary conditions and the joining details of the structure. The considerable practical importance of these aspects has been the focus of attention in previous publications of the authors which evaluate the experimental-numerical impact response of simple ship structural components, such as beams and plates. Therefore, this paper uses the definitions proposed in those references to evaluate its applicability in the scaled tanker side panel, as an example of a more complex ship structure.
One type of submarine composite pipeline structure, with carbon steel-concrete-stainless steel (CCS) double-skin tube (DST), was introduced in this paper. This composite pipeline was expected to make optimal use of the three types of the materials, and provide significant structural and internal corrosion resistance. This study investigated the compressive and flexural behavior of the composite pipeline under internal content pressure and external hydrostatic pressure through finite element analysis (FEA). Finite element models were developed, where non-linear material properties of stainless steel and composite actions between constituent parts were considered. The models were verified through the comparisons between the numerically and experimentally determined results, in terms of load-deformation histories, failure modes and ultimate strength. Structural behaviors of the composite pipeline under pressures were compared with those without content and hydrostatic pressure. Parametric studies were carried out to investigate the effects of the outer carbon steel strength, inner stainless steel strength, concrete strength and hollow ratio on the compressive and flexural behaviors of the composite pipelines subjected to pressures.