Search

When tidal turbines are utilized in the most energetic waters where there are significant waves, the assessment of the surface wave effects are of great concerns. The objective of this paper is to contribute to a fundamental understanding of surface wave effects on tidal turbines. A numerical model was developed based on the modified Blade Element Momentum theory with an inclusion of added mass effects, wave excitation forces and a one degree-of-freedom (DOF) simulation for turbine rotational motion. The experiments on a 1:25 scaled tidal turbine were performed in a towing tank. It is shown that the surface waves did not affect the average loads and power output, but caused severe periodical oscillations. The amplitudes of the cyclic thrust and torque could reach up to 50% of the mean value induced by the incident waves with period of 1.6 s and height of 14 cm. Non-dimensional response amplitude operators (RAOs) of thrust and torque were proved to be sensitive to submergence of the turbine. The wave induced torque and thrust tend to a fixed value when the incident wave length is much longer than the water depth, which provides an approximate assessment of the surface wave effects on tidal turbines.

Installation of blades for wind turbines is challenging due to large lifting height and high precision. Assessment of blade dynamic responses during installation needs advanced simulation tools which are limited at present. This paper aims at developing an integrated simulation tool SIMO-Aero for single blade installation for both onshore and offshore wind turbines. Based on the cross-flow principle, the aerodynamic model is established by accounting for the effect of wind turbulence and dynamic stall. Then it is coupled with SIMO to achieve the integrated simulation tool SIMO-Aero which can account for blade aerodynamics, vessel hydrodynamics and system mechanical couplings. The aerodynamic code is verified by code-to-code comparisons with HAWC2. Furthermore, SIMO-Aero is applied in case studies on the wind-induced dynamic responses of a DTU 10 MW blade during installation using a jack-up crane vessel which is assumed to be rigid, including the crane, and rigidly fixed to the seabed. The characteristics of system dynamic responses prior to mating the blade onto the hub are studied. It is shown that the blade motions are dominated by the pendulum motion. Critical parameters of the installation process are identified. The extreme responses of critical parameters are further studied under turbulent winds and wind gusts.

This paper addresses the safety of marine operations related to a seafastening system for large-object transport. The aim is to present a structural reliability approach to accommodate the uncertainties affecting design checks. The probability of structural failure by use of design standards for assessing marine operations is studied using structural reliability analyses to shed light on the implicit reliability levels of such standards. A structural reliability model that includes the effect of uncertainty in weather forecasts is established. The reliability analyses show that the method to account for forecast uncertainty as defined in the standards compensates well for that uncertainty, and the failure probabilities in the case studies are between 1 0 − 4 and 1 0 − 3 per operation. A reliability model that includes the long-term statistical distribution of the environmental conditions is also established. This model is applicable for operations with a duration longer than three days and is used to study seasonal variations and the failure probability as a function of the duration of operations. The failure probability is calculated for execution in several months, showing the dependency on the time of the year and the duration of the operations. The failure probabilities are on the order of 1 0 − 4 per operation.

The risk of vessel capsizing is inherent to anchor handling operations (AHOs). Lessons learned from the Bourbon Dolphin accident reveal that the large static heeling angle could not be prevented due to the lack of awareness of the vessel's stability status, which can be improved with the help of a suitable on-board monitoring system. Therefore, an on-board monitoring system is proposed for assessing stability in terms of the static heeling angle. However, a complete mathematical model is not available for estimating a static heeling angle as a function of operational parameters. Therefore, an artificial neural network (ANN)-based functional relationship has been established between the operational parameters and the static heeling angle. Furthermore, a parametric study has been performed to investigate the effect of neural network topology on network performance. The results show that an ANN topology that contains one hidden-layer is efficient enough to predict a static heeling angle. The correlation coefficient between the ANN model predictions and the target values is 0.999. This result shows that the ANN provides an accurate estimate of the static heeling angle as a function of the operational parameters. Therefore, the proposed mathematical model can be used for assessing a vessel's stability during AHOs.

Floating wind turbines represent a cost-efficient energy solution in deep water where bottom-fixed wind turbine becomes excessively expensive. However, mooring design is quite challenging for all shallow water depths including the transition water depth between bottom-fixed and floating wind turbines, in the order of 50–80 m, for which floating concepts might become more cost-effective than bottom-fixed ones. In this paper, mooring system design for floating wind turbine in shallow water are studied considering both catenary and taut mooring systems. Seven mooring concepts designed for a 5 MW semi-submersible floating wind turbine at 50 m water depth are compared with the purpose to identify solutions that are structurally reliable and economically attractive. The concepts are made of different mooring line materials (chain and synthetic fibre rope), mooring components (clump weight and buoy) and anchors (drag embedment anchor and suction anchor). Based on the latest experimental data, the nonlinear tension-dependent stiffness of synthetic fibre rope are described with an improved numerical model. Performance of the seven mooring concepts are compared with respect to mooring line characteristics, motion response amplitude operator, utilization factor considering the ultimate limit state design and cost etc. Six mooring design concepts are finally recommended for future assessment regarding the application of floating wind turbines in shallow water say 50 m and deeper.

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

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

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

This paper describes a methodology for obtaining passive state space representations of wave radiation forces for floating bodies with zero forward speed oscillating in multiple degrees of freedom. The method is based on fitting rational transfer functions written on pole-residue form to radiation frequency responses calculated using standard boundary element codes. Determination of poles and residues is facilitated by the vector fitting algorithm, originally developed for fitting of frequency responses of electrical networks. Using the poles and residues as parameters in the fitting has advantages over the more common ratio-of-polynomials transfer function formulation when the model order is high, which is typically required when the frequency dependence is strong and wide-banded such as for multibody floating systems. A methodology for perturbing the residues of slightly non-passive systems such that they become passive is also presented. The method is demonstrated successfully for a five-body wave energy converter, an array of circular cylinders and a single cylinder. A discussion and investigation of the truncation of high frequencies is provided for the wave energy converter, and parameter constraints governing the extrapolation of the rational model above the frequencies present in the dataset are suggested.

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

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

The present paper deals with a mathematical model of a heaving-buoy Wave Energy Converter (WEC) equipped with high-pressure hydraulic power take-off machinery for energy conversion. This model is based on linear hydrodynamic theory, and a hybrid frequency–time domain model is used to study the dynamics of the heaving-body exposed to an irregular incident wave. For the power take-off system, end-stop devices are provided to protect the hydraulic machinery when the buoy is exposed to severe sea states. The model also takes into account the lubricated friction force and pressure drops of orifice flow through the valves in the hydraulic system. All the forces mentioned in the hydraulic power take-off machinery have non-linear features. A complete non-linear state space model for the WEC system is presented in this study. The WEC system was numerically simulated for different cylinder lengths under a fixed volume. The effect of fluid compressibility in the cylinder has been investigated in the mathematical model. High frequency oscillations (HFOs) caused by the compressibility of the fluid are displayed in the time series and in corresponding power spectra, and variation is shown for different cylinder sizes. Piston ring and cylinder bore wear damage is estimated by using Archard’s equation on the basis of the simulation results. A comparison of these results with a performance of an identical WEC system which neglects fluid compressibility has been done in this work. It shows that although the spectral power is small, HFO can make a large contribution to both the ring and cylinder bore wear. For the purpose of wear prediction, oscillations at or below the wave frequency and HFO may be equally important.

This paper deals with drag forces due to irregular waves on a vertical slender structure in the splash zone, i.e. in the vicinity of still-water free surface, by considering the inundation effect due to instantaneous wave elevation. The force turns out to be a third-order quantity with respect to wave elevation. The focus of this paper is however limited to extreme value prediction of this force in stochastic waves. Based upon a transformation of random variables and use of the Rice formula, the mean up-crossing rate of inundation drag force is obtained in the frequency domain both by direct numerical integration and asymptotic evaluation for high levels using the Laplace method. The extreme value distribution of this force is then established by the Poisson probability law assuming independent up-crossing events. The proposed method agrees very well with time-domain simulations both for the mean up-crossing rate and the extreme value prediction. The effect of correlation between wave elevation and horizontal water particle velocity and the presence of current have been studied.

Previous studies of response conditioned wave methods have been focused on their applicability to the prediction of extreme nonlinear wave-induced load effects. The results showed that theses methods can be used to accurately and efficiently predict the nonlinear short-term probability distributions for rigid hull responses. This has led us to investigate how response conditioned wave methods can be used for long-term nonlinear fatigue analyses, and with which accuracy this can be done. In this paper we present the results from our investigation. The studies were performed using a container vessel with a length between perpendiculars of 281 m. Calculations were done with a nonlinear strip theory method in which the hull of the vessel was assumed to be rigid. The most likely response wave (MLRW) method was used to condition the waves. Only head seas were considered. We found that the MLRW method under-predicted the long-term fatigue damage by 3%. The method, however, required a simulation time that was approximately three orders of magnitude less than that required for a conventional long-term nonlinear analysis based on random irregular waves. A preliminary investigation showed that due to lacking springing and whipping contributions the MLRW method under-predicted the fatigue damage for a flexible hull by approximately 50%. Several comments about a more accurate extension of the proposed method to flexible hulls are included.

Phase control may substantially increase the power absorption in point-absorber wave energy converters. This study deals with validation of dynamic models and latching control algorithms for an oscillating water column (OWC) inside a fixed vertical tube of small circular cross-section by small-scale testing. The paper describes experimental and numerical results for the system's dynamics, using simple and practical latching control techniques that do not require the prediction of waves or wave forces, and which will be relevant to any type of point-absorbing devices. In the experimental set-up, the upper end of the tube was equipped with an outlet duct and a shut-off valve, which could be controlled to give a latching of the inner free surface movement. The pressure drop through the open valve is used as a simplified measure of the energy extraction. The control was realized by using the real-time measurement signals for the inner and outer surface displacement. A mathematical model of the system was established and applied in numerical simulation. In the case the OWC's diameter is much smaller than the wavelength and the wave amplitude much smaller than the draft, the free surface movement inside the tube can be described as an oscillating weightless piston. For this hydrodynamic problem an analytical solution is known. In addition, the mathematical model includes the effects of viscous flow losses, the air compressibility inside the chamber and the pressure drop across the valve. Experimental results were used to calibrate some of the model parameters, and the total model was formulated as a coupled system of six non-linear, first-order differential equations. Time-domain integration was used to simulate the system in order to test the control strategies and compare with experimental results.

The violent motion (sloshing) of liquefied natural gas (LNG) in cargo tanks has attracted significant attention. Transformations of the LNG market have led to the increased transport of LNG in partially filled tanks, but established technology is mainly based on engineering experience with completely filled containers. This paper investigates a large sample of sloshing pressure measurements. It focuses on the magnitude of individual sloshing impact events, and their associated temporal and spatial patterns. The durations of these impacts are comparable to the natural frequency of an LNG container wall, so the details of their time histories are important in determining the structural response. Experiments are performed on tanks with high (92.5%) and low (30%) filling levels, for various wave headings. The common post-processing approach of representing impact pressure histories by a triangular profile is studied, and an alternative approach is presented. Two statistical models are used to describe the distribution of maximal pressures in sloshing impacts: a three-parameter Weibull model and a generalized Pareto model. The latter is found to be of questionable utility due to small sample sizes. It is observed that for low filling levels the sloshing impacts are of greater magnitude, having longer durations, smaller ratios of rise time to duration, and larger spatial extents. All these factors should in principle increase the structural response.

Time-domain models of marine structures based on frequency domain data are usually built upon the Cummins equation. This type of model is a vector integro-differential equation which involves convolution terms. These convolution terms are not convenient for analysis and design of motion control systems. In addition, these models are not efficient with respect to simulation time, and ease of implementation in standard simulation packages. For these reasons, different methods have been proposed in the literature as approximate alternative representations of the convolutions. Because the convolution is a linear operation, different approaches can be followed to obtain an approximately equivalent linear system in the form of either transfer function or state-space models. This process involves the use of system identification, and several options are available depending on how the identification problem is posed. This raises the question whether one method is better than the others. This paper therefore has three objectives. The first objective is to revisit some of the methods for replacing the convolutions, which have been reported in different areas of analysis of marine systems: hydrodynamics, wave energy conversion, and motion control systems. The second objective is to compare the different methods in terms of complexity and performance. For this purpose, a model for the response in the vertical plane of a modern containership is considered. The third objective is to describe the implementation of the resulting model in the standard simulation environment Matlab/Simulink.

A new hybrid method for the time-domain nonlinear simulation of the hydroelastic load effects and the peak over threshold (POT) method for the calculation of the short-term extreme responses are briefly described and applied to a flexible SL-7 class containership and a flexible liquefied natural gas (LNG) ship. Three stiffness levels, three stiffness distributions and three modal damping ratios are used to study the influence of the hull flexibility and structural damping on the short-term prediction of extreme vertical hydroelastic load effects. The results give justification for some simplified treatment of the first vertical flexible mode in early design stage when structural details are not available.

Three-dimensional hydroelasticity theory is used to predict the hydroelastic response of flexible floating interconnected structures. The theory is extended to take into account hinge rigid modes, which are calculated from a numerical analysis of the structure based on the finite element method. The modules and connectors are all considered to be flexible, with variable translational and rotational connector stiffness. As a special case, the response of a two-module interconnected structure with very high connector stiffness is found to compare well to experimental results for an otherwise equivalent continuous structure. This model is used to study the general characteristics of hydroelastic response in flexible floating interconnected structures, including their displacement and bending moments under various conditions. The effects of connector and module stiffness on the hydroelastic response are also studied, to provide information regarding the optimal design of such structures.

Marine operations, e.g., the sea transport of heavy objects and the installation of offshore units and equipment, need to be planned and executed with proper consideration for environmental conditions and operational limits with respect to vessel motions and structural loads. Marine operations with a limited duration, usually less than 72h, are typically designed as weather-restricted operations. The environmental design criteria are thus predefined, and the actual weather conditions are confirmed by weather forecasts issued immediately prior to the start of such an operation. Marine operations of longer duration are typically designed as weather-unrestricted operations, and the environmental conditions are calculated based on long-term statistics, possibly depending on the season. More detailed information about uncertainties in weather forecasts could increase the feasible duration of weather-restricted operations. The uncertainty inherent in weather forecasts, notably that in the significant wave height, is studied. Further, a method to assess the reliability of weather forecasts is described. Data from the Norwegian Sea are used to quantify the uncertainty in forecasted data. The probability of exceeding the design criteria used in the planning of a weather-unrestricted marine operation can be estimated based on forecast statistics. The corresponding uncertainty can be incorporated into structural reliability analyses.