Analysis of Ship Structural Loading in a Seaway

1972 ◽  
Vol 9 (02) ◽  
pp. 173-194
Author(s):  
Dan Hoffman
Keyword(s):  
Pressure Distribution ◽  
Phase Relationship ◽  
Bending Moment ◽  
Green Water ◽  
Detailed Structural Analysis ◽  
Transverse Pressure ◽  
Structural Loading ◽  
Wave Induced

The recent advent of the large tanker and bulk carrier has promoted the requirements for more detailed structural analysis of a ship and the reevaluation of theories for calculating the static, quasistatic and dynamic loads. The paper begins with discussion of the methods available to determine the various types of loads expected, their phase relationship, and ways of superimposing them. It then proceeds to the treatment of sea loads based on theoretical and experimental data, and techniques of determining the ship response in a seaway are discussed. The response to regular waves is reviewed with special reference to the determination of pressure distribution on the hull. Statistical ship response, immediate and cumulative over the life of the ship, is demonstrated in relation to the prediction of long-term bending moment trends, and the distribution of the extremes is discussed. Special loading conditions are described with special emphasis on the transverse pressure distribution, dynamic effects due to motion of liquid cargo in tanks, shipping of green water, wave-induced vibrations, slamming pressures and whipping stresses due to various causes. The paper treats the above subjects in a broad manner and no attempt to illustrate the theory in detail is made.

Load Combination for Fatigue Analysis of Ship Structures

2004 ◽  
Author(s):  
Yung S. Shin ◽  
Booki Kim ◽  
Alexander J. Fyfe
Keyword(s):  
Bending Moment ◽  
Linear Summation ◽  
Load Combination ◽  
Longitudinal Stiffeners ◽  
Stress Components ◽  
Tank Pressure ◽  
Wave Induced ◽  
Vertical Bending Moment ◽  
Oil Tanker

A methodology for calculating the correlation factors to combine the long-term dynamic stress components of ship structure from various loads in seas is presented. The methodology is based on a theory of a stationary ergodic narrow-banded Gaussian process. The total combined stress in short-tem sea states is expressed by linear summation of the component stresses with the corresponding combination factors. This expression is proven to be mathematically exact when applied to a single random sea. The long-term total stress is similarly expressed by linear summation of component stresses with appropriate combination factors. The stress components considered here are due to wave-induced vertical bending moment, wave-induced horizontal bending moment, external wave pressure and internal tank pressure. For application, the stress combination factors are calculated for longitudinal stiffeners in cargo and ballast tanks of a crude oil tanker at midship section. It is found that the combination factors strongly depend on wave heading and period in the short-term sea states. It is also found that the combination factors are not sensitive to the selected probability of exceedance level of the stress in the long-term sense.

Prediction of Green Water Events on FPSO Vessels

2003 ◽  
Author(s):  
Alexander Fyfe ◽  
Edward Ballard
Keyword(s):  
Probability Distributions ◽  
Green Water ◽  
Design Stage ◽  
Wave Crest ◽  
Peak Period ◽  
Wave Conditions ◽  
Motion Characteristics ◽  
Central North Sea

Most floating vessels experience some sea states, not necessarily extreme storms, which cause large volumes of green water to flow across the deck. Due to the location of safety critical equipment on the deck of FPSOs, the determination of the likely occurrences and the magnitudes of such events are critical to safe design and operation. A method for the determination of green water heights on the deck of an FPSO has been presented in references 1–5. This paper examines the long-term distributions of heights implied by these references and the identification of sea states in which extreme events are likely to occur. The method is based upon the long term distribution of sea states at the intended location, combined with the motion characteristics of the vessel. Freeboard exceedance at the bow and at a point along the side is considered for two typical FPSO configurations. The methodology presented is widely applicable to many locations but wave conditions typical of the Central North Sea are used by way of illustration. The results presented include long term probability distributions of green water height on deck at locations of interest. Relative contributions of each combination of significant wave height and peak period to the probability of the largest single event in a defined return period are determined and discussed. It is shown that the wave conditions most likely to give rise to the most severe green water events are seldom those characterized by the largest wave crest heights. Instead, there exists a complex dependence on characteristic periods associated with vessel motions and on the long-term occurrences of particular sea states. The ability to predict conditions in which the largest green water events are most likely to occur offers the possibility of providing improved operational guidelines for FPSOs, allowing action to be taken to avoid unfavourable loading conditions and/or vessel headings in certain sea conditions. However, it is also shown that it may be difficult to identify some severe green water sea states from normally available forecast data and hence it is important that appropriate provision is made at the design stage.

Effects of Avoidance of Heavy Weather on the Wave Induced Load on Ships

2006 ◽  
Author(s):  
Zhi Shu ◽  
Torgeir Moan
Keyword(s):  
Green Water ◽  
Vertical Acceleration ◽  
Sea State ◽  
Hull Girder ◽  
Term Prediction ◽  
Strip Theory ◽  
Operational Criteria ◽  
Wave Induced ◽  
Long Term Prediction

This paper is concerned with evaluating the effect of avoidance of heavy weather on the long term wave induced loads on ships. Two hydrodynamic codes VERES based on a 2D strip theory and WASIM based on a 3D Rankine panel method are employed to calculate the wave induced loads and motions on various vessels. Two models for heavy weather avoidance are proposed. The first is based upon the assumption that operational criteria relevant to vertical acceleration, green water and bottom slamming are fulfilled. The second one is based upon the assumption that the sea state forecasts are available to the ship master, and that rerouting is made. And based on the first model considering avoidance of heavy weather and the hydrodynamics results calculated from two codes, the wave induced hull girder loads are obtained. The results are discussed. In particular, the effect of different hydrodynamic codes and various scatter diagrams are assessed. After all, the long term prediction of wave induced hull girder loads considering the effect of avoidance of heavy weather will give a relatively more realistic evaluation of the extreme hull girder loads. Finally the results from ship rules will also be re-evaluated compared with the long term prediction with and without heavy weather avoidance.

Structural Reliability of a Suezmax Oil Tanker Designed According to New Joint Tanker Project Rules

2006 ◽  
Author(s):  
C. Guedes Soares ◽  
Josˇko Parunov
Keyword(s):  
Structural Reliability ◽  
Progressive Collapse ◽  
Bending Moment ◽  
Single Step ◽  
Moment Capacity ◽  
Hull Girder ◽  
Term Operation ◽  
Reliability Method ◽  
Wave Induced

The paper aims at quantifying the changes in notional reliability levels that result from redesigning an existing suezmax tanker to comply with new Joint Tanker Project (JTP) rule requirement for ultimate vertical bending moment capacity. The probability of structural failure is calculated using a first-order reliability method. The evaluation of the wave-induced load effects that occur during long-term operation of the ship in the seaway is carried out in accordance to IACS recommended procedure. Comparative analysis of long-term distributions of vertical wave bending moment calculated by two independent computer seakeeping codes is performed. The still water loads are defined on the basis of a statistical analysis of loading conditions from the loading manual. The ultimate collapse bending moment of the midship cross section, which is used as the basis for the reliability formulation, is evaluated by JTP single-step procedure and by program HULLCOLL for progressive collapse analysis of ship hull-girders. The reliability assessment is performed for “as-built” and “corroded” states of the existing ship and a reinforced design configuration complying with new JTP rules. It is shown that hull-girder failure probability of suezmax tanker reinforced according to new JTP rules is reduced several times. Sensitivity analysis and a parametric study are performed to investigate the variability of results to the change of parameters of pertinent random variables within their plausible ranges.

Influence of Whipping on Long-term Vertical Bending Moment

2004 ◽  
Vol 48 (04) ◽  
pp. 261-272
Author(s):  
Gro Sagli Baarholm ◽  
Jørgen Juncher Jensen
Keyword(s):  
Nonlinear Response ◽  
Extreme Values ◽  
Bending Moment ◽  
Sea Waves ◽  
Short Term ◽  
Strip Theory ◽  
Long Time ◽  
Wave Induced ◽  
Vertical Bending Moment

This paper is concerned with estimating the response value corresponding to a long return period, say 20 years. Time domain simulation is required to obtain the nonlinear response, and long time series are required to limit the statistical uncertainty in the simulations. It is crucial to introduce ways to improve the efficiency in the calculation. A method to determine the long-term extremes by considering only a few short-term sea states is applied. Long-term extreme values are estimated using a set of sea states that have a certain probability of occurrence, known as the contour line approach. Effect of whipping is included by assuming that the whipping and wave-induced responses are independent, but the effect of correlation of the long-term extreme value is also studied. Numerical calculations are performed using a nonlinear, hydroelastic strip theory as suggested by Xia et al (1998). Results are presented for the S-175 containership (ITTC 1983) in head sea waves. The analysis shows that whipping increases the vertical bending moment and that the correlation is significant.

scholarly journals On movement control of deformable hull for displacement ship in waves

2020 ◽  
Vol S-I (2) ◽  
pp. 237-241
Author(s):  
R. Mudrik ◽  
◽  
P. Mudrik ◽  
Keyword(s):  
Target Function ◽  
Bending Moment ◽  
Movement Control ◽  
Irregular Waves ◽  
Regular Waves ◽  
Propulsion Performance ◽  
Design Variables ◽  
Internal Forces ◽  
Wave Induced

This paper discusses the optimization of ship heading in given regular waves taking into account the limitations for wave-induced midship bending moment and roll amplitude. External and internal forces in irregular waves are determined as per finite-element method used for motion calculations. This study presents the obtained surfaces of responses for the variables of state depending on wave parameters and design variables. The optimization problem discussed in this study was solved taking the non-linearity of target function and limitation functions into account. the study also formulated and solved an inverse problem, i.e. determination of controlling parameters (heading and speed) so as to obtain optimal propulsion performance in given wave conditions without prejudice to reliability and seakeeping.

Stress Combination for Fatigue Analysis of Ship Structures

2004 ◽  
Vol 127 (2) ◽  
pp. 175-181 ◽  
Author(s):  
Yung S. Shin ◽  
Booki Kim ◽  
Alexander J. Fyfe
Keyword(s):  
Bending Moment ◽  
Short Term ◽  
Linear Summation ◽  
Longitudinal Stiffeners ◽  
Stress Components ◽  
Tank Pressure ◽  
Wave Induced ◽  
Vertical Bending Moment ◽  
Oil Tanker

A methodology for calculating the correlation factors to combine the long-term dynamic stress components of ship structure from various loads in seas is presented. The proposed methodology is valid for a stationary ergodic narrow-banded Gaussian process. The total combined stress in short-term sea states is expressed by linear summation of the component stresses with the corresponding combination factors. This expression is proven to be mathematically exact when applied to a single random sea. The long-term total stress is similarly expressed by linear summation of component stresses with appropriate combination factors. The stress components considered here are due to wave-induced vertical bending moment, wave-induced horizontal bending moment, external wave pressure, and internal tank pressure. For application, the stress combination factors are calculated for longitudinal stiffeners in midship cargo and ballast tanks of a crude oil tanker. It is found that the combination factors strongly depend on wave heading and period in the short-term sea states. It is also found that the combination factors are not sensitive to the selected probability of exceedance level of the stress in the long-term sense.

Effects of Avoidance of Heavy Weather on the Wave-Induced Load on Ships

2008 ◽  
Vol 130 (2) ◽  
Author(s):  
Zhi Shu ◽  
Torgeir Moan
Keyword(s):  
Green Water ◽  
Vertical Acceleration ◽  
Sea State ◽  
Hydrodynamic Analysis ◽  
Hull Girder ◽  
Rankine Panel Method ◽  
Strip Theory ◽  
Operational Criteria ◽  
Wave Induced

This paper is concerned with evaluating the effect of heavy weather avoidance on the long-term wave-induced loads on ships. Two hydrodynamic codes, VERES based on a 2D strip theory and WASIM based on a 3D Rankine panel method, are employed to calculate the wave-induced loads and motions on various vessels. Two models for heavy weather avoidance are envisaged. The first one is based on the assumption that operational criteria relevant to vertical acceleration, green water, and bottom slamming are fulfilled. The second one is based on the assumption that the sea state forecasts are available to the shipmaster and that rerouting is made. Based on the first model, the effect of the heavy weather avoidance on the long-term wave-induced hull girder loads is evaluated. In particular, the effect of hydrodynamic analysis methods and wave scatter diagram are also assessed. The calculated values are compared with those given in ship rules. The results show that several factors should be critically assessed in evaluating the accuracy of the ship rule values.

The Prediction of Long-Term Distributions of Wave-Induced Bending Moment from Model Tests

1968 ◽  
Vol 5 (02) ◽  
pp. 137-149
Author(s):  
Roger H. Compton
Keyword(s):  
Random Sampling ◽  
Large Population ◽  
Bending Moment ◽  
Model Tests ◽  
Ocean Wave ◽  
Design Stage ◽  
Bending Moments ◽  
Actual Distribution ◽  
Wave Induced

A procedure is described for predicting the long-term distribution of wave-induced bending moments for a ship operating in a statistically defined ocean-wave environment, using results from model tests and realistic sea spectra. To illustrate the procedure a hypothetical tanker is subjected to a known wave environment, completely defined by a large population of sea spectra, and the long-term distribution of wave bending moment is predicted from a relatively small random sampling of sea spectra, using the proposed procedure. The predicted distribution is shown to compare favorably with the actual distribution of bending moment. Suggestions are made regarding the applicability of this procedure to the prediction of service wave bending moment distributions for actual ships in the design stage.

Estimation of Short-and Long-Term Probability Distributions of Wave-Induced Loads Acting on a Cruise Vessel in Extreme Seas

2017 ◽  
Vol 140 (2) ◽  
Author(s):  
Suresh Rajendran ◽  
Nuno Fonseca ◽  
C. Guedes Soares
Keyword(s):  
Time Domain ◽  
International Association ◽  
Probability Distributions ◽  
Bending Moment ◽  
Green Water ◽  
Cruise Ship ◽  
Short Term ◽  
Strip Theory ◽  
Radiation Forces

A time domain code based on strip theory is applied to calculate the probability distributions of relative motions and bending moments of a cruise ship in a set of extreme seas. The code includes two levels of complexity. The simpler one combines linear radiation and diffraction forces with nonlinear Froude–Krylov forces, hydrostatic forces, and shipping of green water on the bow. Cummins formulation is used to represent the radiation forces. The second approach is a generalization of the first one and, although the formulation is based on the linear assumption (of the radiation forces), the effects of body nonlinearity are considered by a simplified method: the memory functions, infinite frequency added masses, and the radiation restoring coefficients are assessed at each time instant as function of the instantaneous wetted surface. A similar procedure is used to calculate the diffraction forces. The code is used to analyze the responses of a cruise ship in a set of extreme sea conditions. The short-term nonlinear responses are represented by empirical probability distributions, obtained from the nonlinear time domain simulations, and the quality of the predictions is assessed by comparing with model tests experimental data. Finally, the long-term value of the bending moment is calculated from the short-term distribution of the nonlinear loads in a few extreme sea states selected based on coefficient of contribution method, and the results are compared with the International Association of Classification Societies (IACS) rule bending moment.

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