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The Second-Generation Intact Stability Criteria: New Yardstick of the Future

For all ships at sea, Intact Stability (IS) is one of the important foundations of ship safety and a mandatory statutory requirement for ship design and construction. Ship design, especially the development of new ship types, is closely related to it. As a mandatory statutory requirement, the ship stability criterion is very important to the safe navigation of ships. With the enforcement of the second-generation IS criterion in the next few years, it will have a great impact on China's shipbuilding industry.

 The huge impact of the enforcement

For many years, people have been persistently exploring the IS criteria that can ensure the safety of navigation. Although people do not fully understand the motion of ships in complex sea conditions, they still gain a lot of experience and draw lessons from a large number of ship operation and navigation accidents. It is based on these experiences and lessons and the deepening understanding of ship hydrodynamics that the International Maritime Organization has been working on the formulation and revision of IS criteria applicable to all types of ships since the middle and late last century. In recent years, with the continuous development of modern ship design technology and research on ship fluid performance, after years of work by the IMO Sub-Committee on Stability, Load Lines and Fishing Vessels Safety, "2008 IS Code" (2008 IS Code), the first generation of IS Code, has been drafted. IMO adopted and promulgated this code on December 4, 2008, of which PART A was officially enforced on July 1, 2010.

The currently effective "2008 IS Code " is mainly described by the ship statics theory based on the parameters of the righting arm curve under the action of cross wind and beam waves when the ship is stationary in the still water, and is used as the assessment of ship stability method. This criterion is a prescriptive code formulated based on ship operation statistics and meteorological criteria collected in the middle of the 20th century. It is mainly based on experience and cannot reflect the diversification of ship types and their operating and environmental conditions, nor can it reflect the new achievements in modern ship hydrodynamics research. The most important thing is that the current IS criterion is only one of the ship stability failure modes, and it does not involve other stability failure modes that may cause ship capsizing, personal injury, and cargo damage. Therefore, the current prescriptiverules based on experience are not enough to prevent ships from capsizing caused by dynamic stability.

Since the beginning of the 21st century, because other failure modes of IS have become increasingly  dangerous to ships, IMO has put the development of the second-generation IS criteria on the agenda. This stability criterion, which is composed of the weakness criterion and the direct calculation criterion, will include the stability of the dead ships in the event of loss of power, the maneuverability-related broaching and the parameter rolling caused by the change of resilience. Among them, the weakness criteria are divided into two levels. First, ships are judged based on the first level criteria in the stability evaluation, among which those that do not meet their requirements shall be determined based on the second-level criteria, and those that do not meet the weakness criteria shall be determined by direct calculation criteria.

The second-generation IS criteria based on the new stability failure modes (5 failure modes such as pure stability loss, parametric rolling, wave riding/broaching, dead ships, and excessive acceleration)will change the previous method of setting the stability based on the empirical formula, introducing the concept of direct stability assessment, and providing personalized stability assessment for the stability of new ships and super-scale ships, which is a brand-new revolution to the existing IS criteria. The second-generation IS criteria will be part of the mandatory requirements of the 2008 IS Code as a complementary and alternative approach to the 2008 IS Code as well as a guidance for the operation of newly designed and built ships and existing operating ships, providing the basis and guidance for the design and operation of the ships, and also putting forward higher requirements for ship design.

The second-generation IS criteria takes into account the overturning phenomenon and risks of ships in actual navigation in waves, which is completely different from the 2008 IS Code currently being implemented based on hydrostatic and empirical background. Its enforcement will have a huge impact on the shipbuilding industry, which is one of the new maritime regulations that urgently need the active participation and response of the Chinese industry. At the 6th meeting of the SDC in February 2019, the IMO has initially completed the formulation of all technical contents of the weakness criteria, direct stability assessment, and operational restrictions/guidelines. The formal drafting of all criteria texts will be completed in early 2021 and will be submitted to the Maritime Safety Committee of IMO for approval and release. According to the plan of IMO, the second-generation IS criteria will first be recommended as a non-mandatory requirement for trial use, collecting the use experience during a trial period of 4 to 5 years. After its improvement, it will be promoted as a mandatory requirement. It takes into account some special overturning phenomena and risks of ships, complementing accident modes that are not covered by existing international regulations, and can effectively improve the safety level of ship dynamic stability in bad sea conditions. It is of great significance to improve the safety of ships and reduce the risk of human life. Once the second-generation IS criteria is enforced, the ship design must improve the new ship types that do not meet the requirements or adopt the method of operating restrictions/operating guidelines, so that the product can enter the international market only after meeting the criteria.

 Difficulties in key technology breakthroughThe second-generation IS criteria has become a key issue that needs to be solved urgently in the global maritime sector, shipbuilding and shipping industries, and is an important part of ship safety. IMO has decided to propose a second-generation IS code for new ships in the next few years, which will bring new challenges to China's ship design and shipbuilding industry.

The ability to conduct theoretical and experimental research on various stability failure modes reflects a country's ability to research and design modern ships, especially novel ships.  Therefore, the shipping and shipbuilding industries should take measures to establish the relevant technical reserves according to the nature of the globalization of the ship market. Otherwise, it may restrict Chinese ship design, construction and shipping industry, and lose the great opportunity for the development of shipbuilding industry brought by the development of ship types which meets the second-generation IS code.

In the process of forming  second-generation IS criteria of IMO, Europe and the United States and other major western countries play a leading role and are in a leading position in terms of technology. Experts from China Classification Society (CCS) said that in order to provide more support in the formulation of IS criteria, safeguard the interests of our country and the rights of China's shipbuilding industry. We must conduct extensive and in-depth research on the connotation, prediction and verification technology of the second-generation IS criteria, breaking through its core technology and the monopoly of the United States, Japan and other developed countries. It is advisable to put forward proposals and suggestions that reflect China’s national interests, to provide important guarantee and support for effectively expanding China's share in the world shipbuilding market.

The second-generation IS criteria will include five failure modes, and each failure mode includes three levels of evaluation methods, which is a complex evaluation system. Among them, there are not only simple evaluation formulas, but also advanced ship hydrodynamic calculation technology, of which some complex calculation methods have not been fully mastered in China, especially not forming mature and reliable assessment technology and software. Once the criteria are implemented, if China lacks the means of prediction and evaluation, it will bring serious adverse effects on the design and manufacture of ships in China. Therefore, it is very important to break through the key technology and establish the systematic evaluation means and software of the second-generation IS criterion. Only in this way, the implementation of the criteria of IMO can ensure the smooth implementation of the new generation of IS criteria by Chinese ship design and manufacturing enterprises, making the stability criteria of ships designed and manufactured in China in line with the international advanced level.


The key contents of the second-generation IS criteria is to use advanced ship hydrodynamic technology to calculate and evaluate the stability of ships in wind and waves more accurately according to different stability failure modes. Therefore, for the coming into effect of the  second-generation IS criteria of IMO, we will track, analyze and study five failure modes, including loss of pure stability, parametric rolling, wave riding / broaching, dead ships and excessive acceleration, focusing on solving the weakness criteria of these five failure modes and the key technology in the direct assessment technology of the pure stability loss; we will establish and improve the technology of IS model test, numerical calculation and evaluation and verification, develop evaluation methods for the main failure modes of IS and direct evaluation software for pure stability loss, improve the research level of IS criteria technology in China, form the second-generation IS criteria of IMO evaluation software with independent intellectual property rights, improve the research level of IS criterion technology in China. It is necessary to develop the second-generation IS evaluation software of IMO and relevant guidance with independent intellectual property rights. The proposal of the second-generation IS assessment of IMO is an urgent task and a problem to effectively improve the design level of Chinese ships and the ability of ship stability assessment and verification, as well as to improve the R&D capability of Chinese new ships.

The main contribution of CCS

As the only professional organization engaged in ship classification inspection in China, CCS has been actively participating in the work of the IMO and IACS, submitting a large number of technical proposals concerning ship breaking, new shipbuilding design index, nitrogen oxide emissions, lifesaving, noise, fire protection, and stability, protecting the interests of the country and the industry to the maximum extent.

In recent years, IMO has proposed to study the second-generation IS criteria. CCS took the lead in cooperating with relevant domestic scientific research institutions and universities, to get involved in the research of the second-generation IS criterion at an early stage, and to keep track of the research progress of IMO member states. Since 2010, China has participated in the IMO Communications Working Group to develop the second-generation IS criteria. In the past eight years, CCS has submitted  a total of 24 proposals and assisted other units in submitting 23 proposals, all of which have been adopted or considered by IMO. Following CCS's presentation of China's work on second-generation IS at SDC2 Meeting in 2015, CCS has also conducted extensive research on the five failure modes of second-generation IS, including conducting large-scale real-ship validation calculations for a total of 52 real ships, using the six calibration modules of the weakness criterion, and cumulatively completing about 700 calibration analysis of real-ship loading conditions. Among them, the over-acceleration criterion scheme jointly led by China and Germany was incorporated into IS CODE as the final scheme after the coordination with Germany

It is worth mentioning that CCS has made groundbreaking research on the formulation of over- acceleration and weakness criterion. Through carrying out model tests of 4 sample ships and numerical simulation studies of 52 real ships, CCS has thoroughly studied the mechanical mechanism and main influencing factors of the over-acceleration mode. On behalf of China, CCS has proposed a intact weakness criteria of Level 1 and Level 2, involving meteorological conditions, criteria calculation methods, rotational inertia estimation formulas, damping coefficient estimation methods for rolling, and correction coefficient processing methods and standard values for other degrees of freedom affecting by Level 2 acceleration calculation. At the same time, quantitative and qualitative requirements for numerical methods for direct stability assessment have been studied and formulated.

When IMO formulated this criterion, the main focus of contention mainly included: Level 1 calculation formula for rolling criterion angle; Level 2 motion equation selection rolling criterion; whether the irregular wave direction spectrum is considered in the level 2 criterion; the selection of criterion environmental conditions; and level 2 calculation method of criterion sensitivity index; estimation method of rolling moment of inertia and whether additional inertia is considered; estimation method of rolling damping coefficient; treatment method of correction coefficient for influence of level 2 criterion acceleration on other degrees of freedom; selection of acceleration standard value and freedom degree of numerical simulation method for direct stability evaluation. After a large number of verification and analysis for sample ships, CCS representing China and Germany jointly developed the level 1 and level 2 weakness criteria, which was officially adopted by IMO as one of the five failure modes. IMO also adopted CCS's suggestion on the selection of motion freedom in the direct evaluation method of over-acceleration stability. In addition, CCS has also carried out criteria verification analysis on a large number of domestic ships , which fully verified the feasibility and rationality of the above criteria in engineering applications.

Through the above research and work, China has greatly improved its understanding of the physical mechanisms for several failure modes of the second-generation IS criteria, enabling China to fully participate in the decision-making process of the criteria. At the same time, through the adoption of relevant proposals, it has further enhanced our country’s right of speech and influence, demonstrating our country’s strength in hydrodynamic theory of ships, numerical calculations, and model tests. This has made the second-generation IS criteria fully meet our country’s interests, providing important guarantee and support for effectively expanding China's share in the world shipbuilding market.