Intact Stability Criteria Research Articles

Design Analysis Using Evaluation of Surf-Riding and Broaching by the IMO Second Generation Intact Stability Criteria for a Small Fishing Boat

Design Analysis Using Evaluation of Surf-Riding and Broaching by the IMO Second Generation Intact Stability Criteria for a Small Fishing Boat

A Numerical Method on Large Roll Motion in Beam Seas Under Intact and Damaged Conditions

The second-generation intact stability criteria, including five stability failure modes, were approved by the International Maritime Organization (IMO) in 2020, and it is an urgent task to develop the numerical method for the significant roll motion under dead conditions. Both intact and damaged stability focus on the large roll motion in beam seas. A unified numerical method is studied to predict the large roll motion in regular and irregular beam seas under intact and damaged conditions. Firstly, a sway–heave–pitch–roll–yaw-coupled equation named 5-DOF and a sway-, roll-, and yaw-coupled motion with the roll-righting arm in still water named 3-DOF are used to predict the large roll motion in regular beam seas under the intact and damaged conditions. Secondly, the method is extended for the large roll motion in irregular beam seas, where the diffraction force in the roll direction and the sway and yaw motion under intact and damaged conditions are calculated by the subharmonic superposition method. Thirdly, the roll-righting arm in the calm water, roll-damping coefficients, and the roll natural roll period, under the intact and damaged conditions, are obtained by software and a free roll decay experiment, respectively. Finally, the numerical results of a patrol boat under intact and damaged conditions are compared to the experimental results. The results show that the sway-, roll-, and yaw-coupled motion with the roll-righting arm in still water named 3-DOF can predict the large roll motion in regular and irregular beam seas under intact and damaged conditions.

An insight about roll damping within the second generation intact stability criteria

AbstractDespite the recognized complexity to estimate the damping influence on ship roll motion, some efficient semi-empirical methodologies have been developed along the years to this purpose. CFD methods, in principle the most appropriate tools to predict roll damping, are not a favorite option in preliminary design at present due to their implications in terms of complexity and computational time. In this paper, an integration of the semi-empirical Ikeda’s simplified method has been formulated with modification of the bilge keels and lift components, following a contribution found in the literature and relevant for Ro–Ro ships. Moreover, with focus on damping evaluation, the second generation intact stability criteria (SGISc) have been investigated: the second-level vulnerability criteria for dead ship condition and parametric roll have been applied to a Ro–Ro passenger ship. Both the consolidated and the new proposed roll damping prediction methods have been implemented in order to appreciate their effects on the final outcome.

Assessment of Dead Ship Condition by the IMO Second Generation Intact Stability Criteria for 5000HP Tug Boat

_ The International Maritime Organization (IMO) has been discussing technical issues by dividing the second-generation intact stability criteria for ships into five types. In this paper, we paid attention to the dead ship condition and introduced the process for the assessment of Levels 1 and 2 in detail. Dead ship condition refers to a case where a large angle of rolling motion occurs due to waves incident on the side of the ship’s hull after the ship’s engine has failed. Basically, in the dead ship condition, if the Level 1 criterion for analysis related to the GZ curve is not satisfied, an evaluation is performed against the Level 2 criterion considering the hydrodynamics of waves. The method for the effective wave slope function required to obtain the spectrum of the effective relative roll angle, which is the most important factor in the calculation of Level 2, was implemented. In particular, unlike existing ship types in terms of various experiences, this study performed stability evaluation using special ship data of a 5000HP tug boat and obtained results that satisfied both Level 1 and 2 standards. Through this example of dead ship condition evaluation for a tug boat, we aim to ensure the expansion of IMO second-generation intact stability evaluation targets for various types of ships. Keywords IMO second generation intact stability criteria; dead ship condition; GZ curve; effective wave slope function; tug boat

Review of the analytical prediction method of surf-riding threshold in following sea, and its relation to IMO Second-Generation Intact Stability Criteria

Review of the analytical prediction method of surf-riding threshold in following sea, and its relation to IMO Second-Generation Intact Stability Criteria

An Investigation about Stability in Waves of Large Pleasure Yachts

The IMO’s second-generation intact stability criteria (SGISc) have been applied to a set of seven megayacht units ranging from 40 m to 80 m. The application aims to evaluate their stability performance in a seaway. The assessment is based on two vulnerability levels, and the related criteria rely on the physics of the phenomena under investigation (dead ship condition, excessive acceleration, parametric rolling, pure loss of stability, and surf-riding). At the same time, SGISc provide methodological approaches to consider possible operational limitations related to the geographical area the vessel is meant to sail. Results of the comprehensive analysis carried out for the selected megayacht units are presented in term of limiting KG curves. Outcomes pointed out that inconsistencies among vulnerability levels still exist. Further comments about the relation of the investigated vessel typology and relevant operational profile have been reported. In addition, systematic analysis of the effect of the main design parameter on ship rules compliance has been carried out.

A Unified Numerical Method for Broaching and Loss of Stability in Astern Seas

The interim guidelines of second-generation intact stability criteria and their explanatory note were issued by the IMO in 2022. However, due to their complexity, the direct stability assessments of broaching and loss of stability still need to be made easier for users. Therefore, the mathematical models for broaching and loss of stability in astern seas are studied in this paper. Firstly, a time-domain 6 DOF numerical model is adopted, combining seakeeping and maneuvering mathematical models. Secondly, the hydrodynamic forces, heave, and pitch motions are obtained by an enhanced strip method with the upright hull at different speeds in the frequency domain. Then, their time-domain values are transferred from their frequency-domain values with the speed variation considered. Thirdly, the time-domain varied wet hull in waves is captured by the 6 DOF ship motion. Then, the Froude–Krylov and the hydrostatic forces in the surging, swaying, rolling, and yawing directions are simulated considering the wave pressure around the wet hull. Fourthly, the exposure of the twin rudders and the wave-particle velocity are considered for predicting broaching. Finally, the calculated results are compared with the published results. The results show that the time-domain 6 DOF coupled numerical model can be unified for predicting broaching and loss of stability in the astern seas.

The influence of ship stability in waves on naval vessel operational profiles

Ensuring a widely safe operational profile of naval vessels is one of the leading aspects of the design process. With reference to weather conditions, this kind of vessels often cannot avoid heavy situations to guarantee the continuity and the effectiveness of service. For this reason, an applied research investigation aimed to define guidance in operations is deemed as a worthwhile activity. In particular, the performance in waves under the perspective of intact stability has been considered in this work. The IMO second-generation intact stability criteria have been identified as a suitable tool for these investigations: as a result, there is evidence that operational guidance can be developed for a set of representative naval vessels, following the relevant guidelines. Outcomes have been represented by means of comprehensive polar diagrams addressing the intact stability failures modes. Bearing in mind the actual operational profile, results have been analysed and discussed considering how ship and operating decisions in a seaway condition may affect the performance in terms of ship stability.

On-board monitoring and estimation of lateral accelerations through extreme value theory

Lateral accelerations experienced on-board of ships with high initial stability, especially in beam waves and at low speed, can be a source of risk for people and cargo. In the present paper, an on-board system that informs the crew on the actual situation of the vessel is described. The system monitors the lateral acceleration and estimates the extreme values that could occur in the short-term.A mid-sized stern trawler typical of the Galician fleet (Spain) has been selected to test the system. Fishing vessels are susceptible to dangerous dynamic phenomena in waves due to their hull form, loading conditions, size and operational profile. The vulnerability of the vessel has been confirmed by the application of the excessive acceleration failure mode, one of the Second Generation Intact Stability Criteria. Results have shown that the operability criterion for lateral acceleration is exceeded in many sea states.The on-board system has been tested against weakly non-linear time domain simulations and time series from experimental tests conducted at the University of A Coruña (Spain) towing tank. The estimated extreme values are close to the ones experienced, confirming the ability of the system to identify potentially dangerous situations.

Extrapolation of ship capsize probability over significant wave height: Foundation on wave groups theory

The paper lays formal foundation for the “extrapolation over significant wave height” technique which has recently been incorporated into the Second Generation Intact Stability Criteria of the International Maritime Organization (IMO) for assessing the dynamic stability of ships in waves. The technique aims at estimating efficiently a ship's tendency to capsize in relatively mild sea conditions through parsimonious simulations of ship motion. To address the rarity of capsizes in such conditions, extrapolation is employed for the probability value obtained from severer sea states differing only in their significant wave heights. In its current version, the approach extrapolates as if capsize was caused solely by exceptionally large wave encounters, thus ignoring phenomena such as resonance or wave grouping. Herein, the concept is revisited from the perspective of the “critical wave groups” method which is demonstrated as a general framework enabling rigorous “extrapolation over significant wave height” formulations. In this setting, two new extrapolation models are developed from basic principles as improvements to the one suggested by the IMO. The first model accounts for the joint effect of wave heights and periods, while the second additionally integrates the temporal dependence between consecutive wave heights into the extrapolation scheme. The accuracy of the theoretical predictions as well as their sensitivity to the amount of available data are evaluated for the case of beam-sea rolling assuming two different spectrum types as possible characterizations of the prevailing wave conditions. Justification about the extrapolation capabilities of the derived models is provided in view of the advances in extreme wave stochastics accommodated by the theory of Quasi-Determinism.

Intact stability operational measures: Criteria, standards and examples

Harmonised treatment of design and operation is very efficient to provide a required safety level. However, operation is usually not addressed by intact stability rules of IMO or classification societies. A substantial change was introduced in the second-generation intact stability criteria of IMO, which address both design requirements and ship-specific operational measures. Moreover, this is done using the same criteria, standards, and safety level, so that operational measures can be used as a loading condition-specific alternative to design assessment. This paper proposes several operational measures, discusses relevant criteria, and defines corresponding acceptance thresholds. Further, it is discussed whether operational measures can always be acceptable as an alternative to the design assessment, when operational measures should be prepared, and how cross-sea and ship propulsion and steering abilities can be addressed. Examples of the proposed operational measures are shown for five ships.

Evaluation by a Quantitative Index about Intact Stability Performance in Waves of a Set of Megayacht Units

Intact stability represents one of the most important topics when addressing ship safety, and it is ruled by the IMO Intact Stability code, evaluating ship stability in a calm water scenario. However, the interest in ship stability in waves has increased in recent years and this has led to the formulation of the second generation intact stability criteria (SGISc), finalized at IMO in 2020. In this research, an approach to quantitatively and comprehensively evaluate the ship stability performance in waves has been pursued. A methodology is developed with reference to the SGISc. The intact stability in waves index (ISWI) has been proposed, with the aim to become a complementary tool for designers and shipbuilders in the assessment of stability performance in waves. The ISWI represents a comprehensive stability index, able to capture the stability in waves performance of a vessel. The stability index has been verified on a set of megayacht units and its sensitivity to the wave characteristics has been tested, changing the environmental conditions. The outcomes point to a good agreement between the ISWI and the influence of environmental condition changes on the stability performance.

Further Study on One of the Numerical Methods for Pure Loss of Stability in Stern Quartering Waves

The International Maritime Organization (IMO) finalized the second-generation intact stability criteria in 2022. However, an accurate and practical numerical method for stability loss has yet to be established. Therefore, a 6 DOF numerical model is further improved based on the previous study. Firstly, the rolling motion is simulated using a seakeeping model instead of the previous maneuvering mathematical model. Secondly, the roll-restoring variation is calculated directly considering the instantaneous wet hull instead of the previous pre-calculated method. Thirdly, transferring frequency to time is used to obtain heave and pitch motions, further considering yaw angle and sway velocity. Fourthly, the dynamic forces for sway, roll, and yaw motions are calculated, further considering the effect of the speed variation. Fifthly, the 6 DOF motions are used to determine the instantaneous wet hull, and the FK force and the hydrostatic force are calculated by the body’s exact method. Finally, a new conclusion is obtained that the sway and yaw motions’ effect on the ship speed loss, the relative longitudinal wave profile by the speed loss, the rudder angles, and the accompanying rudder forces in the rolling direction are significant, and much more than their centrifugal force or coupled force in the rolling direction.

Level 1 vulnerability criterion for the dead ship condition: A practical methodology for embedding operational limitations

The framework of second generation intact stability criteria (MSC.1/Circ.1627) allows introducing so-called “operational limitations”, by specifying alternative environmental conditions based on the expected operation of the ship. Relevant calculation parameters of the criteria are correspondingly modified, according to standardized procedures specified by the Explanatory Notes. This is a significant novelty, and information is available for implementing operational limitations for most of the failure modes and levels of assessment. However, this is not the case for the level 1 vulnerability criterion for the dead ship condition. Therefore, the paper investigates the development of a rational simple procedure to contribute filling this gap. The devised approach provides a tool for determining a modified reference wind speed based on the environmental conditions representative of the considered operational limitations. Specifically, the modified wind speed is defined as the wind speed with a specified probability of exceedance. The reference probability of exceedance is determined considering the standard conditions and assumptions in the framework of MSC.1/Circ.1627. The modified reference wind speed can then be used to re-define all the calculation parameters of the criterion, according to the relevant theoretical background. Two example applications of the devised approach are also provided.

Complements to Level 2 surf-riding/broaching vulnerability criterion regarding waterjet propelled ships and wave surge force correction

The Second Generation Intact Stability Criteria (SGISC) has officially entered the trial period for further validations and to collect feedback based on applications. This study focuses on the Level 2 surf-riding/broaching vulnerability criterion and proposes complements to the assessment procedures regarding two important issues. One is that the current procedure is only applicable to ships using propellers. However, waterjet propelled ships are common nowadays and they may be vulnerable to this stability failure mode as well. The other issue is that there lacks a both reliable and practical method for the wave surge force correction. The current procedure uses the Froude-Krylov (FK) component of the first order wave surge force as the input for the assessment, which significantly over-estimates the probability of surf-riding occurrence.Through the introduction of the thrust coefficient KT and the advance coefficient J for waterjet systems, the first issue is addressed by model the waterjet thrust based on the KT ∼ J relationship just as that of propellers. The validity of such treatment is first demonstrated through the actual data of a real pump, while the rationale behind the universality of such modelling is analyzed based on dimensional analysis. The second issue is addressed through the proposal of a practical method to estimate the wave surge force correction factor based on the ratio between the first order wave surge force and its FK component. Sample ship calculations are performed to demonstrate the reliability of the proposed method. Finally, the complete Level 2 surf-riding/broaching vulnerability assessment for a waterjet propelled ship is performed for demonstration. This study contributes to the improvement of SGISC, making it applicable to a wider range of ship types with higher accuracy.

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.

Standard Wave Scatter Table Limitation for Evaluating SGISC Based on Hindcast Data Analysis

The operational measure (OM) of the Second-Generation Intact Stability Criteria (SGISC) is the initial step toward the design of a performance-based dynamic stability assessment of the ship by considering the vessel’s operation, loading condition and weather parameters. The SGISC recommends the standard wave scatter table (WST) for the environmental data, an indefinite requirement for a simplified assessment pathway, which provides the probability of wave occurrence. The existing standard WST was developed based on the North Atlantic Ocean. The study aims to identify the discrepancy in the probability of wave occurrence in the IMO-recommended WST when compared with developed hindcast WST for smaller regions of the North Atlantic Ocean for the application of the SGISC. This study is one of a kind and very significant, given that the assessment using the standard WST provided unrestricted operational ability to ship under the SGISC. The findings of this research provide insights into significant differences in the existing standard WST when compared graphically with hindcast data, especially across different seasons of the North Atlantic Ocean. A case study of OMs on the C11 class post-Panamax container ship for excessive acceleration is provided to better represent the study. In the case study, the operational limitation defined by the hindcast WST for individual sea areas and seasons is compared with the standard WST. This investigation shows that regional and seasonal operational limitations are required for season 4 operation beyond the standard WST safety estimation and that the operational limitation achieved with standard WST is not adequate. The findings suggest that the identified limitations significantly limit the use of a standard WST for unrestricted operational validation in the SGISC. The provided recommendation is very relevant in improving the safety assessment using vulnerability criteria, given that the hindcast data are reliable and available for the season and are also region-specific, and hence the accuracy of the ship stability can be improved while using the SGISC.

A Vulnerability Criterion of Dynamic Instability for Ship Course-Keeping in Following Waves

_ A practical criterion which can be used for assessing the course-keeping capability of a ship in following waves, is proposed and evaluated. Presently, it accounts for regular waves and it accrued from an analytical estimation of the course instability region’s boundary by applying the method of harmonic balance. The calculation is performed with regard to the third-order yaw equation, derived from a classic sway-yaw-rudder model of ship maneuvering motions, with time-dependent coefficients at two places, which is like a Mathieu-type equation extended to third order. The proposed analytical criterion was evaluated thoroughly against simulations with regard to this sway-yaw model and it was found to be adequately accurate. A supplementary quasi-static yaw stability criterion (fitting to ship operation with frequency of encounter, with respect to the waves, close to zero) was also considered in order to determine which one yields more stringent requirements, for various operating conditions. The proposed criterion could be an extra vulnerability check for broaching-to, in the context of the Second Generation Intact Stability Criteria. Introduction The difficulties of steering of ships in following seas have received the attention of the research community since more than 70 years ago (Davidson 1948). A relevant direction of research refers to the avoidance of the broaching-to instability and, in particular, to the development of practical criteria that could ensure sufficient course-keeping capability for a ship encountering steep following waves. Some classic works on this topic, such as those of DuCane and Goodrich (1962), Wahab and Swaan (1964), and Motora et al. (1981), were focused on the quasi-static condition of a ship on the wave which could be practically realized if the ship was advancing with speed equal to the wave celerity (zero frequency of encounter). As implied, these works were essentially focused on the avoidance of a type of broaching-to instability that is preceded by the realization of surf-riding, a phenomenon where the ship is forced to move with the wave, usually riding a downslope.

Numerical study of the interaction between the pure loss of stability, surf-riding, and broaching on ship capsizing

Ship capsizing in stern quartering wave is very complicate with a strong correlation to the stability failure modes proposed in second-generation intact stability criteria. It is of great importance to shed some light on this complex phenomenon. In this study, the capsizing behavior of a free-running ONR Tumblehome model in stern quartering seas is investigated by an unsteady Reynolds-averaged Navier–Stokes (URANS) solver coupled with dynamic overset grid approach. Validation study of the URANS solver is performed for the prediction of extreme roll motion of the ship. The comparisons show satisfactory agreement, with an average difference less than 3% between the simulation and experimental results. The correlation between the occurrence of stability failure modes (surf-riding, pure loss of stability, and broaching) and ship capsizing is then assessed under different ship speed and heading angle conditions. It is concluded that the capsizing occurs as the free-running ship sailing with a high speed, which is caused by the interaction between the broaching and severe stability loss. In addition, influenced by the wave-induced moment, both the pure loss of stability and surf-riding modes may cause the broaching, indicating the similar occurring condition and strong relation between these stability failure modes.

Influence of sea areas and season in navigation on the ship vulnerability to the parametric rolling failure mode

The influence of areas and season in navigation on the ship vulnerability to the parametric rolling failure mode is investigated. The second check of level 2 vulnerability criterion, defined in the interim guidelines on the second generation intact stability criteria (SGISC), is used for the assessment of ship vulnerability to the parametric rolling failure mode. In the framework of SGISC, even ships failing to pass the vulnerability criteria are allowed to operate by implementing the operational limitations. In order to prepare operational limitations related to areas or routes, and season, area-specific and season-specific wave scatter tables are made using Global Wave Statistics. Service speed and major navigation routes/areas of containerships and pure car and truck carriers are determined by analysing global satellite AIS data. Then, the vulnerability is assessed for a C11 class containership and a pure car and truck carrier for the determined limited navigation areas to identify the failure risk depending on the sea areas. Further, the effect of seasonality on the risk of stability failure due to parametric rolling is discussed.