Abnormal wave-induced load effects in ship structures

Ocean Engineering, 2016

The aim of this paper is to critically assess the methods used for the evaluation of wave-induced loads on ships examining analytical, numerical and experimental approaches. The paper focuses on conventional ocean going vessels and loads originating from steady state and transient excitations, namely slamming, sloshing and green water, for the latter, and including extreme or rogue waves, as well as the more occasional loads following damage. The advantages and disadvantages of the relatively simpler potential flow approaches against the more time consuming CFD methods are discussed with reference to accuracy, modelling nonlinear effects, ease of modelling and of coupling with structural assessment procedures, suitability for long term response prediction and suitability for integration within design and operational decision making. The paper also assesses the uncertainties involved in predicting wave-induced loads and the probabilistic approaches used for the evaluation of long term response and fatigue analysis. The current design practice is reviewed and the role of numerical prediction methods within the classification framework and goal based design approach discussed. Finally the suitability of current developments in prediction methods to meet the needs of the industry and future challenges is assessed.

Journal of Offshore Mechanics and Arctic Engineering, 2008

The structural loads on a floating production, storage, and offloading platform induced by a large set of realistic (measured) abnormal waves are compared with the loads induced by "design storms" and also with the minimum values required by Classification Society rules. The design storms have a duration of 3 h and correspond to the 100 yr contour of the scatter diagram of the Northern North Sea. Time domain simulations are performed with a time domain seakeeping program that accounts for the most important nonlinear effects. The results are analyzed to obtain probability distributions of the sagging and hogging peaks of the vertical bending moment. Several theoretical distributions are fitted to the simulated realizations.

Ships and Offshore Structures

This paper presents a new methodology to determine the design values of wave-induced hull girder loads acting on ships. The method is based on probabilistic approaches associated with selected scenarios that represent possible events in terms of the ship's functionality, operation and environment. As illustrative examples, the method is used to determine the design values of wave-induced vertical bending moments for as-built ships (a VLCC class tanker, a 9,300 TEU containership and a 22,000 TEU containership) and a hypothetical 25,000 TEU containership. The probabilities of exceedance for wave loads acting on ships are discussed in association with the design load values determined from classification society rules. It is found that both the class rule method and the present method are in good agreement for the considered example ships. The present methodology can of course be applied to determine other components of design wave loads such as horizontal bending moments and torsional moments.

Ships and Offshore Structures, 2012

A computational tool was applied based on a two dimensional linear method to predict the hydrodynamic loads for damaged ships. Experimental tests on a ship model have also been carried out to predict the hydrodynamic loads in various design conditions. The results of the theoretical method and experimental tests are compared to validate the theoretical method. The extreme waveinduced loads have been calculated by short term prediction. For the loads in intact condition, the prediction with duration of 20 years at sea state 5 is used, while for loads in damaged conditions the prediction in 96 hours exposure time at sea 3 is used. The maximum values of the most probable extreme amplitudes of dynamic wave induced loads in damaged conditions are much less than those in intact condition because of the reduced time. An opening could change the distribution of not only stillwater bending moment but also wave-induced bending moment. It is observed that although some cross sections are not structurally damaged, the total loads acting on these cross sections after damage may be increased dramatically compared to the original design load in intact condition.

For the analysis of non-linear processes such as large rolling and capsizing of ships as well for the evaluation of forces and motion behavior of offshore structures in extreme sea states, experimental investigations are still indispensable, both for the validation of numerical simulation tools and for basic insights into the underlying mechanism. Especially in ship design numerical simulation tools have improved significantly and are already considered routinely within the design process but are still under development and require further experimental confirmation. One decisive point in such experimental investigations is the generation of deterministic wave sequences tailored for the individual test. This requires modelling of the non-linear wave propagation in order to know the wave evolution in space and time which allows the analysis of the non-linear process as a cause-reaction chain. In this paper methods of analyzing non-linear wave propagation are presented and compared to results of linear wave theory as well as to corresponding measurements from wave tank experiments. A discussion of various practical applications closes the paper.

Ocean Engineering , 2020

Results are presented of a benchmark and uncertainty assessment study organised by the MARSTRUCT Virtual Institute on global linear wave loads on damaged ship. The study has two aims: to acquire valuable information regarding damage modelling in seakeeping analysis of damaged ships and to contribute to a rational approach for definition of the model uncertainty of linear seakeeping tools. Eight institutes participated in the benchmark, with six codes, representative of important linear seakeeping theories in use nowadays. The benchmark ship is the DTMB 5415 hull, with well documented and accessible data to perform seakeeping analysis and experimental results for motion and global wave loads. The uncertainty analysis is performed using the Frequency Independent Model Error as the uncertainty measure. The analysis is performed for vertical motions, vertical and horizontal global wave load components, and for torsional moments. Uncertainty measures of individual motion and load predictions are presented and compared. In addition, a comparative analysis of linear seakeeping theories is performed and the accuracy of the simplified methods used for the prediction of seakeeping of a damaged ship is assessed. Finally, recommendations are provided for efforts to improve modelling uncertainties in transfer functions of wave loads on damaged ships.

Transactions-Society of …, 2003

The European cooperative research project WAVELOADS investigated use of a linear frequency-domain numerical technique and developed a nonlinear time-domain numerical technique to predict wave-induced global loads for a high-speed ferry and round-bilge fast monohull. For the frequency-domain technique, besides adding viscous effects, an advanced software module was developed to evaluate hydrodynamic influence coefficients, whereby forward speed effects were fully accounted for. The procedure relied on the Fourier-Kochin free-surface forward-speed Green function method that promised to provide a solid mathematical basis for obtaining robust and computationally efficient predictions. Incorporation of this module into two existing frequency-domain panel codes resulted in accurate predictions of wave-induced global response for a slender Wigley hull advancing in regular head waves at high forward speed. However, for the reference ships, predictions deviated excessively from experimental data because numerical inaccuracies could not be resolved. Therefore, subsequent frequency-domain computations were performed on the basis of the so-called encounter-frequency approach. To validate the numerical techniques and to specify limits of application, numerical predictions were compared to experimental data obtained from systematic model tests. For ship speeds corresponding to Froude numbers up to about 0.4, this encounter-frequency approach produced accurate predictions, albeit only in waves of small steepness. To obtain long-term design values in a realistic seaway consisting of finite amplitude waves, linearly computed loads were corrected for nonlinear effects. As a practical demonstration, a statistical analysis yielded long-term (design) values of wave-induced midship vertical bending moments for the high-speed ferry. In sagging, the resulting bending moment deviated by five percent from classification society rule values; in hogging, by thirteen percent.

… conference on non-linear analysis, non …, 2011

Recent researches have shown that, under certain conditions, the impact waves on maritime structures generate a high intensity pressures that vary quickly in time. Even if the forces of waves are often applied in verification calculations like static forces, in such cases it requires a dynamic analysis. The dynamic analysis requires the loading history's definition and it can be approximated like a triangular time-history described by three quantities: the maximum amount of impact force, rise time and decay time. This paper presents the results from a study of the structural response (the form of top displacement) in order to determine the dynamic amplification factor and it depend of the rise time (t r) and the decay time (t d) of shock.

2017

This paper presents a brief summary of the work conducted by the MSRC at Strathclyde University in which the effect of operational location on the estimation of a vessel’s survival probability has been investigated and new s- factor formulations proposed. Further work is presented in which updated accident wave statistics have been used in order to assess the impact of vessel specific data on the predicted survivability. A test case on a large container ship has been conducted in order to gauge the effect of the new s-factor formula on the Attained Subdivision Index and thus the vessel safety level with regards to collision damage.

Marine Structures, 2016

In this paper, the effects of the nonlinearity and irregularity of ocean waves on the nonlinear dynamic response of a jack-up structure are investigated. A finite element model of a sample jack-up platform is formulated in USFOS to include the effect of material and geometrical nonlinearity and spudcan-soil-structure nonlinear interactions. Second-order NewWave and Constrained NewWaves are developed to simulate the nonlinearity of ocean waves. The nonlinear water surface and water particle kinematics for a typical extreme wave condition are estimated and implemented in the developed model, and the results are compared in terms of deck displacements. The results obtained from the analyses indicate that the inclusion of wave nonlinearity and irregularity for the studied case produces a considerable increase in the deck displacements and in the probability of failure of the sample jack-up structure.

Proceedings of the Institution of Mechanical Engineers, Part M: Journal of Engineering for the Maritime Environment, 2016

This article describes a series of model tests conducted to examine extreme wave events associated with tropical cyclonic conditions and their impacts on an offshore deck structure. Extreme waves of a representative cyclonic sea state were examined in a towing tank within long-crested irregular wave trains. Experimental results presented include global forces and localised slamming pressures acting on a rigidly mounted box-shaped deck, which represents a simplified topside structure of a tension leg platform. The effect of static set-down on the still-water air gap was investigated by applying an equivalent reduction for the deck clearance. It was found that a small reduction of 20 mm (2.5 m full scale) in the original deck clearance can lead to a doubling of the magnitude of the horizontal force and the vertical upward-directed force components, as well as significantly increased slamming pressures in many locations on the deck underside.

Journal of Offshore Mechanics and Arctic Engineering, 2007

We present a numerical procedure to predict impact-related wave-induced (slamming) loads on ships. The procedure was applied to predict slamming loads on two ships that feature a flared bow with a pronounced bulb, hull shapes typical of modern offshore supply vessels. The procedure used a chain of seakeeping codes. First, a linear Green function panel code computed ship responses in unit amplitude regular waves. Ship speed, wave frequency, and wave heading were systematically varied to cover all possible combinations likely to cause slamming. Regular design waves were selected on the basis of maximum magnitudes of relative normal velocity between ship critical areas and wave, averaged over the critical areas. Second, a nonlinear strip theory seakeeping code determined ship motions under design wave conditions, thereby accounting for the nonlinear pressure distribution up to the wave contour and the frequency dependence of the radiation forces (memory effect). Third, these nonlinearly computed ship motions constituted part of the input for a Reynolds-averaged Navier-Stokes equations code that was used to obtain slamming loads. Favorable comparison with available model test data validated the procedure and demonstrated its capability to predict slamming loads suitable for design of ship structures.

Volume 4: Ocean Engineering; Ocean Renewable Energy; Ocean Space Utilization, Parts A and B, 2009

During their lifetime ships often operate in severe weather and rough sea states. To ensure survival, a precise knowledge of global and local loads is an inevitable integral part for a safe design. One of the key parameters for ship design is the vertical bending moment. Not only vertical forces but also longitudinal forces can contribute to this bending moment. As the overall effect of longitudinal forces is still not fully understood, all structural loads are investigated in detail, especially in extreme seas. Within the project "Handling Waves", funded by the European Union, three segmented ships, equipped with force transducers, are investigated systematically at several cruising speeds and in various deterministic wave sequences to identify structural loads, i.e. the vertical bending moment as well as the superimposing longitudinal forces. Within this paper a detailed overview of the results for the bulk carrier is given and both, frequency-and time-domain results are presented. With Response Amplitude Operators (RAOs), delivered by frequencydomain analysis, the profound data for the standard assessment of structures, concerning seakeeping behaviour, operational limitations and fatigue are obtained. In addition, time domain analyses in rogue waves such as the so called "New Year Wave" provide essential data for extreme motions and structural loads. Former investigations on a FPSO revealed that, due to the location of the neutral axis, the longitudinal forces are significant and generate a counteracting moment compared to the vertical bending moment [1]. The new results show to what extent the above mentioned conclusions are applicable for various hull designs.

Journal of Marine Science and Technology, 2014

Currently, little information exists on the validity of interface-capturing methods in predicting local ship wave loads in short and steep waves. This study compares computational and experimental results in such a case (kA = 0.24, L wave /L ship = 0.16). The results allow the variation of wave loading between ten locations in the bow area of the ship to be observed. The computations were performed with an unstructured RANS solver that models free-surface flows with a volume-of-fluid method. In the model tests, the wave loads were measured with pressure sensors. The analysis of the results focuses on the wave conditions and on the pressure histories of the local wave loads. The computational and experimental results are in good qualitative agreement and encourage the further use of the computational results.