Generation Intact Stability Criteria Research Articles

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

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.

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.

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.

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.

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.

An alternative methodology for the simplified operational guidance in the framework of second generation intact stability criteria

Within the framework of the second-generation intact stability criteria (SGISc), a procedure to provide simplified operational guidance (OG) has been proposed. In particular, the phenomenon of parametric rolling has been investigated. The proposed methodology is meant to be an intermediate level between the suggested simplified OG and the deterministic OG, as defined in the MSC.1/Circ.1627. The proposed guidance relies on a simplified numerical tool able to assess ship roll motion due to non-linear irregular wave effects in the time domain. Outcomes are elaborated in the post-process phase, taking the maximum roll angle as leading criterion. Once defined the structure of this tool, an application has been carried out to a representative model of post-panamax container vessel. As benchmark study, the results have been compared to those obtained by the application of MSC.1/Circ.1228 and MSC.1/Circ.1627 (simplified OG) by means of polar diagram representation. Outcomes have been discussed and a general agreement can be found between the simplified OG and the proposed one. Although, it appears that the latter can detect dangerous roll motion for a more comprehensive set of sailing condition.

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.

Experimental and numerical study on the parametric roll of an offshore research vessel with extended low weather deck

After over 10 years' development, the International Maritime Organization (IMO) published the 2nd Generation Intact Stability Criteria (SGISC), including Parametric Roll (PR) criteria to avoid its serious harm to the safety of ship navigation. With the trial of SGISC, ships with extended low weather deck, such as typical Offshore Research Vessel (ORV), are found may be vulnerable to PR too. Relative research on such kind of ship is insufficient, because most of previous researches focus on conventional high freeboard ships such as container ships or passenger ships. With an aim to study the PR characteristic of ships with extended low weather deck, an experimental investigation and numerical comparison investigation are carried out. The effect of highly nonlinear water-on-deck phenomena caused by ORV's extended low weather deck on PR response is specially investigated. Regarding numerical simulation of ORV's parametric roll, the effects of assessment accuracies of GZ curves in waves and roll damping coefficients are analyzed. As a result, it was found that parametric roll may occur for ORV's in adverse sea conditions when overtaking of green water is seen on the deck in following wave conditions. It is recommended to account for this specific issue when analyzing PR for ORV's.

An analysis of stability requirements for large inland passenger ships

The regulatory approach to stability assessment of seagoing ships has significantly evolved during the last two decades, which resulted in the introduction of the probabilistic damage stability regulations and development of the Second Generation Intact Stability Criteria by the International Maritime Organization. The ship stability regulations for inland vessels, however, have not followed a similar path. This paper investigates the present intact and damage stability requirements intended for large inland passenger ships (LIPS), offers a critical analysis of the present stability regulations, highlights the need for a global harmonization of regulations, and probes into the hierarchy of the stability requirements for LIPS. The investigation is performed on two ships which belong to the classes of the largest inland passenger ships presently used for river cruises in Europe. It was found that damage stability generally dominates the stability requirements of LIPS, and that the stability assessment may substantially depend on the choice of Flag, even if the ships are intended for the same inland waterway. Significant discrepancies in stability assessments performed according to different regulations bring into focus the question of reliability of the present regulatory framework for stability of LIPS.

Effect of environmental data uncertainty in the framework of second generation intact stability criteria

The framework of second generation intact stability criteria in MSC.1/Circ.1627 includes a number of novelties for intact stability assessment. One of such novelties is the inherent possibility of designing a vessel for a specific operational area, by embedding area-specific environmental conditions. This is practically achieved through modification of the relevant calculation parameters in the criteria, on the basis of available MetOcean data. However, MetOcean data for a given area are generally available from multiple sources, and different sources provide information on environmental conditions on the basis of different approaches. This leads to a source-related variability that eventually reflects in the results of stability assessment, to an extent that is yet to be fully explored. The present study aims at providing quantitative indications in this respect. The Mediterranean Sea is considered as operational area, five different sources of MetOcean data are used, and level 1 and level 2-check 1 vulnerability criteria for parametric rolling are applied to two sample ships. The variability in the obtained safe zones of loading conditions is presented and discussed, also with reference to the effect of bilge keels. Some difficulties in the practical implementation of the procedure for embedding operational limitations are also highlighted and discussed.

Towards evaluation of the second generation intact stability criteria - Examination of a fishing vessel vulnerability to surf-riding, based on historical capsizing

The Second Generation Intact Stability Criteria (SGISC) are the latest achievements in the field of intact ship stability assessment. The criteria are the result of massive international cooperative efforts and extensive research that lasted more than 15 years. Even though they are based on a solid scientific foundation, the trial period has commenced to collect user experience and further validation. One of possible approaches to evaluation of the credibility of the criteria is to demonstrate that ships which capsized in the past due to a stability failure are identified by the criteria as being vulnerable to that specific stability failure mode. The approach combines a critical analysis of the actual accident and calculations that follow procedures provided by the SGISC framework on Level 1 and Level 2. Such approach was applied to a historical catastrophe of a ship regarding the surf-riding stability failure mode. Polish flag fishing vessel m/t Cyranka, which capsized in the North Sea, has been examined for her vulnerability to the surf-riding/broaching-to phenomenon. The analysis shows that the vulnerability criteria related to surf-riding phenomenon can identify the observed vulnerability of the ship. This study contributes to the recognition of the new criteria as being reliable regarding the surf-riding. The proposed approach can be adopted for the evaluation of any stability failure mode covered by the SGISC.

Assessment of Excessive Acceleration of the IMO Second Generation Intact Stability Criteria for the Tanker

Recently, the IMO (International Maritime Organization) has reviewed technical issues considering the second generation intact stability criteria in the design stage of ships. In this paper, the evaluation procedure for Lv1 (Level 1) and Lv2 (Level 2) was introduced by focusing on the excessive acceleration mode. Based on real ship data, the calculation process has been explained in detail to make it easy to understand. When the Lv1 criteria considering simple hydrostatic calculations are not satisfied, the computational results of the Lv2 criteria based on mathematical modeling and the hydrodynamics are presented. The relatively low ship roll periods and large changes in the hull shape in the vertical direction make the ships potentially vulnerable to excessive acceleration phenomena. Therefore, the minimum value of height KG that satisfies the stability criteria evaluation in consideration of loading conditions for actually navigating of the ship in the sea should be estimated and operated. In particular, roll damping coefficients using the Ikeda’s method, which are essential for Lv2 vulnerability calculation, were obtained and verified by comparing them with other ship results.

Alternative Method for Stability Assessment of Indonesian Traditional Wooden Boats

Due to the small down-flooding angle, the available intact stability criteria were difficult to apply to an Indonesian traditional wooden. This paper discusses the application of the dead ship criteria of Second Generation Intact Stability Criteria as an alternative method to assess the stability of the Indonesian traditional wooden boats. The hydrodynamics factors correspond to the formula to calculate the heel angle due to wave were determined by model experiments. The calculations were conducted for four different down-flooding angles consisting of 20 degrees, 25 degrees, 30 degrees and 35 degrees with scattered wave data of Lombok Strait. The results showed that the critical metacentric height decreases if the down-flooding angle increases. The minimum area under the righting arm curve up to the down-flooding angle tends to be independent of the down-flooding angle. The capsizing index corresponds to the critical metacentric height decreased when the down-flooding angle increased. These results showed that the method to assess ship stability in the Second Generation Intact Stability Criteria could be used to evaluate the stability of the Indonesian traditional wooden boats.

Excessive acceleration simplified Operational Guidance

Excessive acceleration criterion, developed by the International Maritime Organization within the Second Generation Intact Stability Criteria, deals with lateral accelerations experienced by people onboard. Level 2 criterion considers the ship at zero speed in beam waves, neglects diffraction and gives the expressions for the Froude-Krylov roll moment. In this paper, the Level 2 procedure is generalized for any ship speed and heading by developing the expressions for the Froude-Krylov exciting roll moment as a function of the heading angle and introducing a variance preserving transformation from encounter to wave frequency. The proposed procedure can be easily implemented in a user's code to assess the simplified Operational Guidance without the use of commercial software. The proposed expression is validated comparing the Froude-Krylov roll moment with the one obtained by the 3-D potential code HydroStar®, referring to a barge and a bulk carrier. The bulk carrier is selected as test case to develop the simplified Operational Guidance according to the proposed procedure. A polar diagram representation is chosen to identify safe combinations of ship speeds and heading while a tabular representation is proposed for a given heading to identify the minimum ship speed to avoid large lateral accelerations, for the sea states reported in standard wave scatter table.

Implementation of the IMO Second Generation Intact Stability Guidelines

This paper provides a discussion of the technical and theoretical ambiguities, requirements, and limitations to develop a practical implementation of the IMO Second Generation Intact Stability criteria. This discussion is the result of industry collaboration, where two implementations of the guidelines were developed jointly, albeit independently. Both implementations were then used to assess four sample cases: C11 container ship, KRISO container ship (KCS), barge, and fishing vessel, for which the detailed particulars and results are given. Conclusions on the practicalities of use, a comparison of the results, and suggestions on how the criteria might be integrated into a workflow are also given.

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.