Direct Stability Assessment Research Articles

Qualitative validation on a numerical model for the direct stability assessment of surf-riding and broaching

A previously developed numerical model for surf-riding and broaching is qualitatively validated based on the criteria of direct stability assessment (DSA) proposed in SDC 7. Roll decay tests are conducted to obtain roll backbone curves, which are compared with calculated GZ curve to demonstrate their consistency. Then, roll response curves under different wave frequencies in regular beam wave are obtained. It is verified that roll response curve “fold around” the newly defined “peak backbone curve”. Cases with fixed yaw under different Fn are chosen to demonstrate the numerical model's capability of reproducing surf-riding equilibrium. Turning circle test is conduced to demonstrate correct modelling of maneuvering forces and the capability to reproduce heel due to ship turning. Wave surging forces are calculated under different wave numbers and amplitudes. The correct tendency of phase difference between wave surging force and the incident wave is observed, which reveals the correct modelling of wave forces in the model. Moreover, a preliminary quantitative validation is conducted under various cases. Through comparison with experimental results, it is recommended that the wave diffraction correction of the wave surging forces should be included in the simulation model in order to avoid overestimation of surf-riding and broaching.

Review and prospects on research methods of strength failure for shipped bulk granular cargoes

Bulk granular cargoes would encounter strength failure during sea transportation, and serious accidents may happen. Limiting their moisture content is the most widely utilized preventative measure. However, using this method is not enough for their sea transportation safety. It is essential to have an in-depth comprehension of their mechanism to encourage the development of crucial technologies and devices. The application of theoretical, experimental and numerical methods to study strength failure mechanism in bulk granular cargoes is reviewed. The mechanism from macroscopic, mesoscopic, and microscopic perspectives are then summarized. Based on the present studies, however the macroscopic and microscopic interactions between particles and water during strength failure remain unclear. Microscopic alterations in cargo structure and water-air development remain unknown. As a result, the techniques that could be used to investigate cargo particle movement and moisture migration are advised. So the macroscopic and microscopic relationship could be established. Further, the direct ship stability assessment model of the sea conditions-cargo-ship coupled motion could also be established. The effects between wave loads, cargo wet granular structure and water migration could be understood more deeply. A more accurate transport scheme for bulk granular cargoes can be developed to ensure the safety of their sea transport.

An Improved Method for Predicting Roll Damping and Excessive Acceleration for a Ship With Moonpool Based on Computational Fluid Dynamics Method

AbstractExcessive acceleration is one of the stability failure modes involving large roll motion of ships. The overset method is applied to solve the six degrees-of-freedom motion of the ship with moonpool in beam waves. Based on a computational fluid dynamics (CFD) method, the improved method of considering the roll damping of square moonpool is proposed. The improved method of considering moonpool damping is used in vulnerability assessment for excessive acceleration. The comparative analysis of the level 1 and level 2 vulnerability assessment of the excessive acceleration of a ship with moonpool is completed. The influence of moonpool on the vulnerability assessment of excessive acceleration is studied by comparing the model test results. The results show that the main factor affecting the estimation accuracy of lateral acceleration of the ship is the accuracy of roll amplitude calculation. The existence of moonpool will reduce the roll damping coefficient of the ship. The improved methods proposed in this paper can effectively improve the estimation of lateral acceleration of ships with moonpool in the level 1 vulnerability criteria and increase the safety margin in the level 2 vulnerability assessment. In the direct stability assessment, the CFD method can simulate the large-amplitude roll motion of the ship with moonpool and bilge keels, and can capture the strong nonlinear phenomena.

Simplified operational guidance for second generation intact stability criteria

The SGISC have been developed to account for dangerous dynamic phenomena in waves. Five failure modes are defined, and a multi-layered structure, consisting of vulnerability criteria and Direct Stability Assessment, is adopted to check the ship vulnerability against each phenomenon. If the ship does not comply with one or more criteria, Operational Measures can be provided to reduce the likelihood of stability failures.The present paper provides an overview of Operational Measures and focuses on the application of the Simplified Operational Guidance (OG) for four different ships, each vulnerable to at least one of the failure modes. OG are derived according to IMO Interim Guidelines, by limiting ships’ operation in terms of sailing conditions. The original contribution of this work is the development and application of simple procedures to account for the effect of heading variations, leading to less conservative results in the evaluation of sailing conditions to be avoided.Results are presented in tabular form, reporting the critical speed for which the ship is vulnerable in defined headings, and in polar diagrams, identifying safe sailing conditions for given sea states.

Probabilistic estimation of the large heel due to broaching associated with surf-riding for a ship in short-crested irregular waves and its experimental validation

The International Maritime Organization started using a numerical simulation to quantify the probability of a ship's stability failure as a direct stability assessment for broaching associated with surf-riding. Some of the authors proposed a critical wave method to estimate such probability, and its numerical verification and experimental validation in long-crested irregular stern-quartering waves were already published. The remaining issue is numerically verifying and experimentally validating the methodology applied to short-crested irregular stern-quartering waves. The work described in this paper is an attempt to resolve this remaining issue by executing numerical simulation and a free-running model experiment in both short-crested and long-crested irregular waves. The subject ship used in this work is an ocean research ship similar to the one lost in actual stern-quartering waves with higher speed, which was used in our previous study in regular waves. The experimental results indicate that the direct counting in the numerical simulation shows stochastically sufficient agreement with the model experiment, and the critical wave method provides reasonably conservative estimates even in short-crested irregular waves.

A fast time domain method for predicting of motion and excessive acceleration of a shallow draft ship in beam waves

The excessive acceleration has been incorporated into the framework of the second generation intact stability criteria (SGISC) at IMO 53rd meeting. The vulnerability criteria for excessive acceleration, including Level 1 and Level 2 and the direct stability assessment (DSA) are used to evaluate the sample ship. In DSA, the comparative study of five theoretical approaches is performed. One modified approach is to adopt a 5-DOF nonlinear mathematical model. Based on ordinary strip theory, the modified diffraction force is proposed to obtain more accurate motion of shallow draft ships in beam waves. The nonlinear restoring force is calculated through pressure integration on instantaneous wetted surfaces. Consider the three-dimensional effects of hydrodynamics force around the bow and stern of a shallow-draft ship, the added mass, damping coefficient and diffraction force in the direction of heave is modified respectively. Considering the nonlinear roll diffraction force caused by the non wall-sided hull, the instantaneous value of the roll diffraction force is obtained by adopting the method of invoking the steady-state solution in frequency domain corresponding to the corresponding draft of each section at each instant. In this paper, the sway, heave, roll response and lateral acceleration as well as vertical acceleration at measuring point are compared and analyzed by five approaches. The influence of modified diffraction force on time domain prediction of ship motion and excessive acceleration is studied. The model test has been carried out to verify the prediction method of ship nonlinear motion and acceleration response. The results show that the roll RAO and lateral acceleration obtained by the modified calculation method are the closest to the test values compared with other four methods. Compared with linear theory, the method proposed in this paper can obtain more accurate results of ship motion in a relatively short time, which is effective for direct prediction and evaluation of the ship’s lateral acceleration response in waves.

13K Chemical Tanker의 기관 제어 불능상태 IMO 2세대 안정성 평가에 관한 실험적 연구

The stability of the existing ships has been evaluated through numerical calculations in the steady-state, but recently the IMO proposed a new stability assessment criteria that the stability is evaluated in the state in which environmental loads from such as waves and wind act like the loads under actual ship operating conditions. In this study, IMO 2nd generation stability assessment method and procedure were summarized for the dead ship condition, and Direct Stability Assessment (DSA) was performed on 13K chemical tanker through basin model test. The model test is performed in the ocean engineering basin to implement wave and wind loads, and environmental conditions for waves were set height and period of the incident wave, considering the regular wave and wind generation range reproducible in the ocean engineering basin. In addition, to consider the effect of wind speed, the Beaufort Scale for wind speed was applied in the model test.

THE DIRECT EVALUATION METHOD OF DEAD SHIP STABILITY FOR INTACT SHIP

The International Maritime Organization is currently establishing second generation intact stability criteria, the dead ship stability is considered one important criterion, so the development of its direct stability assessment regulation has become a topic undergoing close review. In this paper a peak-over-threshold (POT) method is proposed to evaluate the dead ship stability, which focuses on the statistical extrapolation that exceed the threshold, also the traditional Monte Carlo simulation is carried out to approve the method. On the basis of verification calculation of the sample ship CEHIPAR2792, the capsizing probability of a certain warship is also conducted. Moreover, the influence of initial stability height GM and effective wave slope coefficient Y on the capsizing probability is analysed. The results and the possible reason for the difference are examined. This study is expected to provide technical support for the second-generation stability criteria and establish the capsizing probability of damaged dead ship stability.

Model experiments and direct stability assessments on pure loss of stability in stern quartering waves

The criteria for direct stability assessment of pure loss of stability are currently under development at IMO as part of second-generation intact stability criteria. For providing an accurate and practical mathematical model, a four-stage approach has been adopted herein. Firstly, a surge-sway-heave-roll-pitch-yaw coupled mathematical model (6-DOF) is newly established, with unified expressions based on an MMG standard method and existing mathematical models of broaching and parametric rolling. The heave and pitch motions are obtained by a strip method with the speed variation considered. They determine the instantaneous relative position of the ship to waves. Then, the corresponding nonlinear Froude-Krylov roll restoring variation is calculated by integrating the wave pressure up to the wave surface. Secondly, model tests are carried out in stern quartering waves to validate the predictions. Thereafter, the effect of some crucial terms on the prediction is studied. Terms such as the higher-order manoeuvring coefficients, the rudder force, the diffraction forces are investigated. Finally, the types of roll motions observed during pure loss of stability are identified. The results show that pure loss of stability is not “pure” but the proposed coupled 6-DOF model can be utilized for the direct assessment of the vulnerability at this failure mode.

IMO Second Generation Intact Stability Criteria: General Overview and Focus on Operational Measures

At the beginning of 2020, after a long and demanding process, the Second Generation Intact Stability criteria (SGISc) have been finalized at the 7th session of the International Maritime Organization (IMO) sub-committee on Ship Design and Construction (SDC). At present, SGISc are not mandatory, nevertheless IMO endorses their application in order to assess their consistency and validity. It is envisaged that SGISc can support the design of safer ships, nevertheless such a rules framework might have an impact also on the ship operational aspects in a seaway. In fact, within the SGISc framework, Operational Measures have also been implemented providing guidance and limitations during navigation. After a comprehensive overview about SGISc vulnerability levels and direct stability assessment, this paper provides a specific insight into the methodological approach for the Operational Measures extensively addressed as a complementary action to ship design.

A Comparison of Numerical Simulations and Model Experiments on Parametric Roll in Irregular Seas

The recently finalised Second Generation Intact Stability Criteria (SGISC), produced by the International Maritime Organisation (IMO), contain a level 3 assessment, the so-called Direct Stability Assessment (DSA). This assessment can be carried out using either model experiments or simulations. The fact that such a choice is given implies that the methods are equivalent in accuracy. This assumption has been verified, for one case, by the Cooperative Research Ships (CRS) community. The verification was based on new model experiments and calculated results, using four different programs owned by different CRS members. Results of the verification of the parametric roll failure mode in regular waves were published before, but this study concerns results in irregular seas. The experimental and numerical results are compared in both probabilistic and deterministic manners. The probabilistic comparison showed that the simulation programs considered are sometimes conservative and sometimes non-conservative in the prediction of the probability of an extreme value. The deterministic comparison in head seas showed that parametric roll events were predicted in the simulations in a wave train that showed no sign of important roll events in the measurement. The deterministic comparison in the following seas, on the other hand, showed an accurate fit of experimental and numerical results. It is suggested that predictions could possibly be improved by adding non-linear diffraction forces to the numerical model.

Seakeeping time domain simulations for surf-riding/broaching: investigations toward a direct stability assessment

Seakeeping time domain simulations are carried out to capture the likelihood of a surf-riding and/or broaching event, with the aim to gain a further insight into the ship performance in stern quartering seas. The applied numerical tool, named PANSHIP, is a 6-DoFs time domain seakeeping software based on panel method, combined with semi empirical viscous models. The investigated ships are two yacht vessels and a patrol boat. Calculations are also carried out with a tool implementing the IMO second level vulnerability criterion for surf-riding/broaching. Similarities, differences and critical issues of both approaches are discussed, as a first step in the perspective of a comprehensive application of the IMO Second Generation Intact Stability criteria, inclusive of the so-called “Direct Stability Assessment”.

Development of a Computation Code for the Verification of the Vulnerability Criteria for Surf-riding and Broaching Mode of IMO Second-Generation Intact Stability Criteria

Recently, the Sub-Committee on SDC (Ship Design and Construction) of IMO have discussed actively the technical issues associated with the second-generation intact stability criteria of ships. Generally, second generation intact stability criteria refer to vulnerability five modes ship stability which occurs when the ship navigating in rough seas. As waves passes the ship, dynamic roll motion phenomenon will affect ship stability that may lead to capsizing. Multi-tiered approach for second generation of intact stability criteria of IMO instruments covers apply for all ships. Each ship is checked for vulnerability to pure loss of stability, parametric roll, and broaching/surf-riding phenomena using L1(level 1) vulnerability criteria. If a possible vulnerability is detected, then the L2(level 2) criteria is used, followed by direct stability assessment, if necessary. In this study, we propose a new method to verify the criteria of the surf-riding/broaching mode of small ships. In case, L1 vulnerability criteria is not satisfied based on the relatively simple calculation using the Froude number, we presented the calculation code for the L2 criteria considering the hydrodynamics in waves to perform the more complicated calculation. Then the vulnerability criteria were reviewed based on the data for a given ship. The value of C, which is the probability of the vulnerability criteria for surf-riding/broaching, was calculated. The criteria value C is considered in new approach method using the Froude-Krylov force and the diffraction force. The result shows lower values when considering both the Froude-rylov force and the diffraction force than with only the Froude-Krylov force was considered. This difference means that when dynamic roll motion of ship, more exact wave force needs considered for second generation intact stability criteria This result will contribute to basic ship design process according to the IMO Second-Generation Intact Stability Criteria.

Model experiments and direct stability assessments on pure loss of stability of the ONR tumblehome in following seas

The guidelines for direct stability assessment of pure loss of stability are currently under development at IMO for the second generation intact stability criteria. A surge-heave-pitch-roll coupled equation named as 4 DOF is newly established for predicting pure loss of stability in following seas. Firstly, the thrust and the resistance of calm water are varied with the ship forward speed and the excited surge force by waves is varied with the relative position between the ship and waves. Secondly, the heave and pitch motions obtained by a strip method with the speed variation newly considered, are used to determine the simultaneous relative position of the ship to waves in the time domain, and then the nonlinear Froude-Krylov roll restoring variation is calculated by integrating the wave pressure up to the wave surface. Thirdly, the effect of the constant ship speed, the surge motion, the initial heeling angle, the heel-induced hydrodynamic forces and the heave and pitch motions on pure loss of stability are studied for direct stability assessment with the 4 DOF mathematical model. Finally, the new numerical approach on pure loss of stability in following seas is verified by experimental results using the standard ONR tumblehome.

The Direct Evaluation Method of Dead Ship Stability for Intact Ship

The International Maritime Organization is currently establishing second generation intact stability criteria, the dead ship stability is considered one important criterion, so the development of its direct stability assessment regulation has become a topic undergoing close review. In this paper a peak-over-threshold (POT) method is proposed to evaluate the dead ship stability, which focuses on the statistical extrapolation that exceed the threshold, also the traditional Monte Carlo simulation is carried out to approve the method. On the basis of verification calculation of the sample ship CEHIPAR2792, the capsizing probability of a certain warship is also conducted. Moreover, the influence of initial stability height GM and effective wave slope coefficient on the capsizing probability is analysed. The results and the possible reason for the difference are examined. This study is expected to provide technical support for the second-generation stability criteria and establish the capsizing probability of damaged dead ship stability.

The Direct Evaluation Method of Dead Ship Stability for Intact Ship

The International Maritime Organization is currently establishing second generation intact stability criteria, the dead ship stability is considered one important criterion, so the development of its direct stability assessment regulation has become a topic undergoing close review. In this paper a peak-over-threshold (POT) method is proposed to evaluate the dead ship stability, which focuses on the statistical extrapolation that exceed the threshold, also the traditional Monte Carlo simulation is carried out to approve the method. On the basis of verification calculation of the sample ship CEHIPAR2792, the capsizing probability of a certain warship is also conducted. Moreover, the influence of initial stability height GM and effective wave slope coefficient on the capsizing probability is analysed. The results and the possible reason for the difference are examined. This study is expected to provide technical support for the second-generation stability criteria and establish the capsizing probability of damaged dead ship stability.

An Overview of Assessment Methods for Synchronization Stability of Grid-Connected Converters Under Severe Symmetrical Grid Faults

Grid-connected converters exposed to weak grid conditions and severe fault events are at risk of losing synchronism with the external grid and neighboring converters. This predicament has led to a growing interest in analyzing the synchronization mechanism and developing models and tools for predicting the transient stability of grid-connected converters. This paper presents a thorough review of the developed methods that describe the phenomena of synchronization instability of grid-connected converters under severe symmetrical grid faults. These methods are compared where the advantages and disadvantages of each method are carefully mapped. The analytical derivations and a detailed simulation model are verified through experimental tests of three case studies. Steady-state and quasi-static analysis can determine whether a given fault condition results in a stable or unstable operating point. However, without considering the dynamics of the synchronization unit, transient stability cannot be guaranteed. By comparing the synchronization unit to a synchronous machine, the damping of the phase-locked loop is identified. For accurate stability assessment, either nonlinear phase portraits or time-domain simulations must be performed. Until this point, no direct stability assessment method is available which consider the damping effect of the synchronization unit. Therefore, additional work is needed on this field in future research.

Direct Stability Assessment for excessive acceleration failure mode and validation by model test

Abstract The excessive acceleration is one of five stability failure modes for intact stability being discussed at IMO. The trial calculation for Direct Stability Assessment (DSA) and the model test have been carried out to verify two prediction methods of ship motions in the short-crested irregular waves for DSA. The ship motions in the short-crested irregular waves calculated by the linear superposition of frequency responses and the time domain simulation agree well with the results of the model test. Using experimentally validated prediction method and wave scatter table of North Atlantic, the long term probabilities have been calculated and the results obtained by two prediction methods become closer to each other. DSA evaluated by using the long-term failure probabilities obtained by two prediction methods are same.

Further validation study of hybrid prediction method of parametric roll

The International Maritime Organization (IMO) is developing Direct Stability Assessment (DSA) criteria for Parametric Roll (PR) and other four failure modes under the framework of second generation intact stability criteria, which will be very helpful for further enhancement of the safety level of ship design. According to previous studies, roll damping plays an important role in the simulation of PR, and a hybrid prediction method for ship parametric roll has been developed, which uses direct CFD approach to estimate roll damping coefficients, as well as weakly-nonlinear model to predict 3 D.O.F motion responses. The well-known container ship C11 was adopted for validation in the previous study, and comparison study has been conducted among hybrid method, a 1 D.O.F non-linear dynamics method and direct CFD prediction method for PR. The results show that the proposed hybrid method has reached sufficient accuracy, and can be used at early design stage of a ship. From the engineering point of view, more validations are necessary to further explore the robustness of the hybrid method, especially the specific applicable condition. In this research, further validation study of the hybrid method is done with three more ships, including two container ships and one Ro-Ro ship. Based on the comparison of results from numerical simulations and experiments, the prediction accuracy of the hybrid method is investigated and limitations are discussed. From this study one can obtain experience of time-domain numerical simulation, and make contributions to the development of IMO DSA criteria guideline.

Study on Roll Restoring Arm Variation Using a Three-Dimensional Hybrid Panel Method

Providing a reliable numerical tool for the prediction of parametric roll and pure loss of stability is a difficult task. It is acknowledged that the occurrence of these two failure modes is mainly attributed to the variation of roll restoring arm, which should be calculated accurately. This work studies exclusively the roll restoring arm variation numerically and experimentally. First, partially restrained experimental tests at different constant heeling angles (up to 30°) are conducted to measure the variation of roll restoring arm in regular waves. Second, a numerical method for the calculation of roll restoring arm in waves taking radiation and diffraction moments into account is adopted. The calculation results are benchmarked against experimental model test data. Third, the characteristics of roll restoring arm variation and the effects of radiation and diffraction moments on roll restoring arm variation (GZRD) in head and following seas are investigated. Finally, the feasibility of GZ calculated by the integral of instantaneously average wetted surface is studied by comparing the results with that calculated by the integral of the fully instantaneously wetted surface. The research show that the GZRD can be ignored in following seas. The GZ in head seas consists of two important harmonic components, and GZRD is the main contribution of the second harmonic component. Therefore, GZRD should be calculated with consideration of the instantaneous heeling angles and Froude number in head seas, especially for the prediction of large amplitude roll motion at high speed. 1. Introduction Currently, the second-generation intact stability criteria are under development by the International Maritime Organization (IMO) (IMO SLF 53/WP.1-Add.1 2011; IMO SDC 3/WP.5 2016). Five different kinds of potentially dangerous dynamic stability failure modes in waves were proposed. Parametric roll and pure loss of stability are two of the stability failure modes that may lead to capsizing of a ship. According to the proposed drafts, a multilevel structure was adopted to make sure that numerical assessment procedures are only applied when the vulnerability to dynamical stability failures is beyond reasonable doubt. Assessment of the vulnerability criteria is normally based on simplified geometry characteristics or empirical formulas. However, direct stability assessment requires highly accurate prediction (Kobylinski 2012).