Combination of primary load effects in ship structures

Marine Structures, 2020

This paper deals with the estimate of uncertainties affecting still water hull girder loads of bulk and dry cargo ships. In strength assessment of ships, two main categories of acting loads are considered: still water loads and wave induced ones. While the latter are generally defined bearing in mind their stochastic nature, this is not the case for still water loads, which are basically deterministically considered. The underlying assumption is that there is an overall control of the operational profile during the service of a ship. However, this is not the case in actual fact, especially for general dry cargo ships and bulk carriers, since the loading/unloading process cannot be fully controlled by the crew, often resulting into loading conditions rather different from those planned by the designer. Based on an earlier work, where loading conditions of the above-mentioned ship types were statistically analyzed, in the present paper Monte Carlo simulations are used to estimate the uncertainties affecting the hull girder still water loads of ships in service, showing that their allowable values can be exceeded due to inaccuracies in ship and cargo management.

Journal of Marine Science and Technology, 2006

An analysis is presented of the vertical bending moments induced in a containership by a set of abnormal waves measured at different locations and on different occasions. A systematic investigation was carried out by using a large set of wave traces that included abnormal waves. In this way it was possible to assess the influence of the height, length, and shape of the abnormal waves on the wave-induced structural loads. The probability distributions of the ship responses to the sea states that included the abnormal waves were also calculated and were compared to the responses induced by the abnormal waves and to fitted distributions. Finally, the structural loads induced by the abnormal waves were compared with rule values and with long-term predictions.

Volume 2: Structures, Safety and Reliability, 2012

In order to investigate the local response of a ship structure, it is necessary to transfer the seakeeping loading to a 3DFEM model of the structure. A common approach is to transfer the seakeeping loads calculated by a BEM method to the FEM model. Following the need to take into account the dynamic response of the ship to the wave excitation, some methods based on a modal approach have been recently developed that include the dry structural modes in the hydro-structure coupling procedure and allow to compute the springing and whipping response of the ship structure to the seakeeping loads. This method is applied for the calculation of the elongation of a strain gauge which is installed in the passage way of an ultra large container ship. NOMENCLATURE ULCS Ultra Large Container Ship

2012

Long-Term Properties and Combinations of Stochastic Hull Girder Loads on Ships

Up to date rule based methods used for the prediction of wave-induced loads, have been proven to be reasonably adequate with problems arising dealt by relying on empirical rules supported by service experience. However, with the increasing market demands for more slender, higher speed open deck ocean going carriers and the continuously updated unconventional multi-hull designs, industry has noticed the advantage of assessing the usefulness and applicability of alternative load prediction tools in ship design.

In this work, values determined from existing design load criteria are compared with loads determined by a first principles based method that has the novelty of using a wave trace including an abnormal wave from a real field measurement. A time domain seakeeping code is used in the linear and non-linear variants to solve the equations of motion and assess the structural loads for oceangoing vessels. Linear and nonlinear calculations are compared in time domain for the S-175 containership with speed of advance. Nonlinear time domain computations are compared with experimental results from physical model tests with a moored FPSO. For both, the S-175 and the FPSO, probability domain comparisons are made between long-term probability distributions, experimental results and minimum rule requirements. The uncertainty associated even with methodologies strongly based on first principles is also discussed.

Naval Engineers Journal, 2002

2000

Nowadays there is a tendency to move from empirical procedures to methods based on the first principles to define the criteria for reliability based structural design of ships (e.g. Guedes Soares et. al, 1996). The approach relies on a probabilistic model of structural strength and on a long-term probability distribution of wave induced loads and it allows the identification of

Ocean Engineering, 2019

Ocean crossing ship structures are continuously suffering from wave loads when sailing at sea. The wave loads cause large variation of structural stresses, leading to fatigue accumulation in ship structures. For the fatigue life prediction of ship structures, it is important to obtain both the long-term distribution and the time history of wave-induced loads. An essential step is to get reliable wave statistics and accurate description of the stochastic nature of sea state along a ship's sailing routes during her service time. Generally, the wave statistics are provided by the classification societies as a joint probability of significant wave height and mean wave period, also known as wave scatter diagram. In addition, different statistical wave models have been developed to describe wave environments along arbitrary shipping routes based on different data sources, e.g., hindcast data, satellite measurements, buoys, etc. In this paper, two statistical wave models based on hindcast data and satellite wave measurements are briefly introduced and compared with the wave measurements carried out by onboard radar. Both of the wave models are then used to generate the wave environments along given shipping routes. The effectiveness of the wave models is demonstrated by comparing the stochastic nature and the statistical characteristics of simulated sea state histories with those of the source oceanographic data. Finally, an application of the wave model to the fatigue assessment is presented.

Applied Ocean Research, 2012

This paper presents a stochastic model in space and time for significant wave height, a Bayesian hierarchical space-time model. The model consists of different components in a hierarchical manner including a component to model the contribution from long-term trends in the wave climate. As far as the authors are aware, no such model of significant wave height to date exploits the flexible framework of Bayesian hierarchical space-time models, which allow modelling of complex dependence structures in space and time and incorporation of physical features and prior knowledge, yet at the same time remains intuitive and easily interpreted. Furthermore, including a trend component in the model is a novel feature.

Marine Structures, 2011

This paper proposes a new method for combining the lifetime wave-induced sectional forces and moments that are acting on the ship structure. The method is based on load simulation and can be used to determine the exceedance probabilities of any linear and nonlinear long-term load combination. It can also be used to determine the long-term correlation structure between these loads in the form of the long-term correlation coefficients. They are essential part of the load combination procedures in design and strength evaluations as well as in the fatigue and reliability analysis of ship structures. The simulation method treats the non-stationary wave elevations during the ship's entire life (long-term) as a sequence of different stationary Gaussian stochastic processes. It uses the rejection sampling technique for the sea state generation, depending on the ship's current position and the season. Ship's operational profile is then determined conditional on the current sea state and the ship's position along its route. The sampling technique significantly reduces the number of sea state-operational profile combinations required for achieving the convergence of the longterm statistical properties of the loads. This technique can even be used in combination with the existing long-term methods in order to reduce the number of required weightings of the short-term CDFs. The simulation method does, however, rely on the assumption that the ship is a linear system, but no assumptions are needed regarding the short-term CDF of the load peaks. The load time series are simulated from the load spectra in each sea state, taking into account the effects of loading condition, heading, speed, seasonality, voluntary as well as involuntary speed reduction * Corresponding author. 485 Ohlone Way,

1980

: Existing probabilistic structural design methods are reviewed, their applicability to ship hull structural design considered and the most promising probabilistic analysis techniques are identified. The current state of knowledge concerning structural modes of failure and load distribution is considered with respect to its impact on probabilistic structural analyses. The emphasis is on longitudinal strength considerations. Factors influencing strength, in terms of uncertainties in ship strength distribution, are reviewed. Different methods are proposed to obtain coefficients of variation for various types of data on the uncertainties. Sample calculations are performed for a number of ships using an approximate probabilistic method and yielding safety margins for each. This method requires that only the coefficients of variation of the strength and load be known. A computer program is developed to perform this calculation for any ship subjected to any load or mode of failure. (Author)

Journal of Marine Science and Engineering

Structural failures in the barge midship sections can cause operational delay, sinking, cargo loss and environmental damage. These failures can be generated by the barge and cargo weights, and wave load effects on the midships sections. These load types must be considered in the design of the barge midship sections. Here, we present the structural analysis of a barge midship section that has decreased up to 36.4% of its deck thickness caused by corrosion. This analysis is developed using finite element method (FEM) models that include the barge and cargo weights, and wave load effects. The FEM models regarded three cargo tanks in the midship section, containing the main longitudinal and transverse structural elements. In addition, the hull girder section modulus and the required deck thickness of the barge were calculated using Lloyd’s Register rules. These rules were applied to estimate the permissible bending stresses at deck and bottom plates under sagging and hogging conditions,...

HAL (Le Centre pour la Communication Scientifique Directe), 2020

Une approche stochastique est mise en oeuvre pour traiter la question d'une structure marine exposéè a des impacts de vagues. L'étude se concentre sur (i) la fréquence moyenne des impacts de vagues et (ii) la distribution de probabilité des variables cinématiques associéesà ces impacts. Le champ de vagues est modélisé comme la réalisation d'un processus Gaussien. Les mouvements de tenueà la mer du corps considéré sont pris en compte dans l'analyse. Le couplage de l'approche stochastique avec un modèle d'impact hydrodynamique est illustré sur le cas d'étude d'un aileron exposéà des impacts de vagues. Summary A stochastic approach is implemented to address the problem of a marine structure exposed to water wave impacts. The focus is on (i) the average frequency of wave impacts, and (ii) the related probability distribution of impact kinematic variables. The wave field is assumed to be Gaussian. The seakeeping motions of the considered body are taken into account in the analysis. The coupling of the stochastic model with a water entry model is demonstrated through the case study of a foil exposed to wave impacts.

Ocean Engineering, 2017

This work is focused on experimental investigation of the hull girder loads on an intact and damaged naval ship DTMB 5415 at zero speed. The experimental campaign was carried out in head and beam regular waves at the University of Strathclyde. The effect of the use of moorings in the model experimental setup was investigated in the context of loads assessment, and the moorings are shown to influence the measured hull girder loads at some wave frequency compared to the free drift case. Therefore the tests in beam seas are performed with free drifting model while the moored model setup was adopted for head seas. The results for ship motions are compared with those from a previous campaign giving an insight into repeatability and uncertainty of measurements. The roll decay of the ship in both intact and damaged conditions is analysed and the linear and quadratic extinction coefficients for the model and the ship scale are reported and detailed discussion on intact-versus-damaged ship roll damping behaviour is given. The results for the hull girder loads are presented for intact and damaged ship. An investigation of the nonlinear effects due to wave height variation in the range wave height to wave length from 1/50 to 1/22 on shear force and bending moment was carried out for a range of wave lengths to ship length ratios from 0.8 to 1.4. The results of the extensive campaign are compared against similar experimental studies forming a benchmark for validation of numerical methods. Keywords: 5415 DTMB model, wave loads on intact and damaged ship; nonlinear responses; experimental shear force and bending moments assessment, roll decay, equivalent linear roll extinction coefficient NOMENCLATURE A -wave amplitude, m B OA -beam over all, m B WL -beam at waterline, m C B -block coefficient C M -midship section coefficient C P -prismatic coefficient D -depth, m g -acceleration of gravity, 9.80665 m/s 2 GM -transversal metacentric height, m H -wave height, m HBM -horizontal bending moment, Nm HSF -horizontal shear force, N H/λ W -ratio between wave height and wave length k -wave number, 2π/λ W KG -vertical position of the centre of gravity, from BL, m KM -vertical position of the metacentre, from BL, m k XX -radius of gyration with respect to x axis, m,