Ship Design and Stability
The IMO instruments governing safe ship designs
The work of the IMO on ship design is mainly carried by the
Sub-Committee on Ship Design and Construction (SDC) which is directed by the Maritime Safety Committee as the parent IMO organ. The safe design of a ship is primarily regulated in
SOLAS chapter II-1, parts A (General) , A-1 (structure of ships) and B (subdivision and stability),
the 1966 Load Line Convention and the 1988 Protocol relating thereto, the 1969 Tonnage Measurement Convention and
the International Code on Intact Stability, 2008.
SOLAS chapter II-1
SOLAS chapter II-1 requires ships to comply with safety regulations concerning the construction, structure, subdivision, stability, the machinery and electrical installations on board ships. IMO's Sub-Committee on Ship Design and Construction (SDC) is the responsible IMO body tasked to develop any necessary amendments to relevant conventions and other mandatory and non-mandatory instruments, as well as the preparation of new mandatory and non-mandatory instruments, guidelines and recommendations, for:
design, construction, subdivision and stability, buoyancy, sea-keeping and arrangements, including evacuation matters, of all types of ships, vessels, craft and mobile units covered by IMO instruments;
testing and approval of construction and materials;
load line matters;
tonnage measurement matters;
safety of fishing vessels and fishermen; and
survey and certification.
1966 Load Line Convention
Introduction and history
It has long been recognized that limitations on the draught to which a ship may be loaded make a significant contribution to her safety. These limits are given in the form of freeboards, which constitute, besides external weathertight and watertight integrity, the main objective of the Convention.
The first International Convention on Load Lines, adopted in 1930, was based on the principle of reserve buoyancy, although it was recognized then that the freeboard should also ensure adequate stability and avoid excessive stress on the ship's hull as a result of overloading. In the 1966 Load Lines convention, adopted by IMO, provisions are made determining the freeboard of ships by subdivision and damage stability calculations. The regulations take into account the potential hazards present in different zones and different seasons. The technical annex contains several additional safety measures concerning doors, freeing ports, hatchways and other items. The main purpose of these measures is to ensure the watertight integrity of ships' hulls below the freeboard deck. All assigned load lines must be marked amidships on each side of the ship, together with the deck line. Ships intended for the carriage of timber deck cargo are assigned a smaller freeboard as the deck cargo provides protection against the impact of waves
Load Lines 1966 - Annexes
The Convention includes Annex I, divided into four Chapters:
Chapter I - General;
Chapter II - Conditions of assignment of freeboard;
Chapter III - Freeboards;
Chapter IV - Special requirements for ships assigned timber freeboards.
Annex II covers Zones, areas and seasonal periods.
Annex III contains certificates, including the International Load Line Certificate.
Adoption of tacit amendment procedure 1988
The 1988 Protocol
Adoption: 11 November 1988
Entry into force: 3 February 2000
The Protocol was primarily adopted in order to harmonize the Convention's survey and certification requirement with those contained in SOLAS and MARPOL 73/78.
All three instruments require the issuing of certificates to show that requirements have been met and this has to be done by means of a survey which can involve the ship being out of service for several days.
The harmonized system alleviates the problems caused by survey dates and intervals between surveys which do not coincide, so that a ship should no longer have to go into port or repair yard for a survey required by one Convention shortly after doing the same thing in connection with another instrument.
The 1988 Load Lines Protocol revised certain regulations in the technical Annexes to the Load Lines Convention and introduced the tacit amendment procedure (which was already applicable to the 1974 SOLAS Convention).Amendments to the Convention may be considered either by the Maritime Safety Committee or by a Conference of Parties.
Amendments must be adopted by a two-thirds majority of Parties to the Convention present and voting. Amendments enter into force six months after the deemed date of acceptance - which must be at least a year after the date of communication of adoption of amendments unless they are rejected by one-third of Parties. Usually, the date from adoption to deemed acceptance is two years.
Intact Stability Code
IMO has long developed intact stability criteria for various types of ships, culminating in the completion of the Code on Intact Stability for All Types of Ships Covered by IMO Instruments (IS Code) in 1993 (resolution A.749(18)) and later amendments thereto (resolution MSC.75(69)). The IS Code included fundamental principles such as general precautions against capsizing (criteria regarding metacentric height (GM) and righting lever (GZ)); weather criterion (severe wind and rolling criterion); effect of free surfaces and icing; and watertight integrity. The IS Code also addressed related operational aspects like information for the master, including stability and operating booklets and operational procedures in heavy weather.
In 2008, the Maritime Safety Committee, at its eighty-fifth session, adopted the International Code on Intact Stability, 2008 (2008 IS Code), following extensive considerations by the SLF Sub-Committee and taking into account technical developments, to update the 1993 Intact Stability Code. MSC 85 also adopted amendments to the SOLAS Convention and to the 1988 Load Lines Protocol to make the 2008 IS Code mandatory, which entered into force on 1 July 2010. The 2008 IS Code provides, in a single document, both mandatory requirements and recommended provisions relating to intact stability that will significantly influence the design and the overall safety of ships.
Second Generation Intact Stability Criteria
Ships vary widely in type, size, operational profile and associated environmental conditions which has made it difficult to develop generic dynamic stability criteria which are applicable for all ships subject to the International Code on Intact Stability, 2008 as it has been acknowledged that some ships are more at risk of encountering critical stability in waves than others.
The IMO is currently in the process of developing performance-based criteria for assessing five dynamic stability failure modes in waves, namely, dead ship condition, excessive acceleration, pure loss of stability, parametric rolling and surf-riding/broaching. One of the obstacles encountered by the IMO has been that the physics and evaluation methods for these five stability failure modes had not been well understood or developed when the mandatory intact stability criteria were established.
The current draft Interim Guidelines on second generation intact stability criteria (Interim Guidelines) have been finalized by the IMO Sub-Committee on Ship Design and Construction (SDC) at its seventh session (3 to 7 February 2020) and awaiting approval at the next session of the Maritime Safety Committee (MSC). The Interim character of the draft Guidelines reflect a certain degree of uncertainty in the recommendations developed but it is the first standalone instrument developed by IMO* to address dynamic stability failures building on best practices and the most advanced scientific tools available. The methodologies contained in these Interim guidelines are based on general first-principle approaches derived from the analysis of ship dynamics and latest technology, as opposed to predominant use of casualty records which form the basis of the mandatory intact stability criteria. For this reason, the presented dynamic stability criteria may be considered as the second generation intact stability criteria. However, in the development process, it was also necessary to simplify some of the assessment methodologies and to perform some semi-empirical tuning.
Once MSC approves the Interim Guidelines on second generation intact stability criteria, they may be used by Administrations to assess and approve ship designs which deviate from conventional concepts. In order to facilitate the use of the Interim Guidelines the SDC Sub-Committee is also in the process of developing associated Explanatory notes on the second generation intact stability criteria. However, neither the Interim Guidelines nor their associated Explanatory Notes are intended to be mandatory.
* Currently ship masters are advised to follow the Revised guidance to the master for avoiding dangerous situations in adverse weather and sea conditions (MSC.1/Circ.1228)
Damage Stability
In 2006, MSC 82 adopted comprehensive amendments to SOLAS chapter II-1 in relation to subdivision and damage stability requirements in order to harmonize the provisions for passenger and cargo ships. The revision of SOLAS chapter II-1 was intensively debated over the past decade by the SLF Sub-Committee, based on the "probabilistic" method of determining damage stability, which is different from the previously used "deterministic" method. However, although the method is different, the objective of both methods is the same as, i.e. “Ships shall be as efficiently subdivided as is possible having regard to the nature of the service for which they are intended. The degree of subdivision shall vary with the subdivision length of the ship and with the service, in such manner that the highest degree of subdivision corresponds with the ships of greatest subdivision length, primarily engaged in the carriage of passengers.”
The deterministic regulations for passenger ships in SOLAS were such that, based on the assumed damage scenario (i.e. one-compartment or group of compartments flooding) according to the ship’s factor of subdivision (function of length, number of passengers and other elements), the maximum permissible length of a compartment (between two adjacent bulkheads: subdivision) is obtained, which should ensure the ship remains afloat and stable. The probabilistic requirements are such that the attained subdivision index A (A=Σpisi), calculated as the summation of pi (the product of the probability that the one compartment or group of compartments under consideration may be flooded) by the si(probability of survival after flooding of the compartment or group of compartments in question), is not less than the required subdivision index R (function of length).
The damage control plan and damage control booklet, which are required by SOLAS regulation II 1/19, are intended to provide ships’ officers with clear information on the ship’s watertight subdivision and equipment related to maintaining the boundaries and effectiveness of the subdivision so that, in the event of damage to the ship causing flooding, proper precautions can be taken to prevent progressive flooding through openings therein and effective action can be taken quickly to mitigate and, where possible, recover the ship’s loss of stability.
Goal-Based Ship Construction Standards (GBS) for bulk carriers and oil tankers of 150m in lenght and above
In the 1990s, the Maritime Safety Committee recognized that the prescriptive-based regulations were unable to cope with the new ship design challenges and already took action to incorporate the goal-based philosophy into the technical regulations of the SOLAS Convention. The IMO has taken a new perspective – one that is goal and performance-oriented, in lieu of the traditional prescriptive-based approach.
The basic principles of IMO goal-based standards/regulations are:
- Broad, over-arching safety, environmental and/or security standards that ships are required to meet during their lifecycle.
- The required level to be achieved by the requirements applied by class societies and other recognized organizations, Administrations and IMO.
- Clear, demonstrable, verifiable, long standing, implementable and achievable, irrespective of ship design and technology.
- Specific enough in order not to be open to differing interpretations.
Goal-based ship construction standards (GBS) are required for oil tankers of 150 m in length and above and to bulk carriers of 150 m in length and above, constructed with single deck, top-side tanks and hopper side tanks in cargo spaces, excluding ore carriers and combination carriers. It is required that that such ships are designed and constructed for a specified design life to be safe and environmentally friendly, when properly operated and maintained under the specified operating and environmental conditions, in intact and specified damage conditions, throughout their life.
For more information on GBS click here.
Frequently asked questions
Can the we use alternative materials within ship structures, what regulations apply?
SOLAS does not explicitly prohibit the use of alternative materials for ships' hulls, systems or equipment but any unconventional material used needs to meet the requirements of SOLAS and that of the flag State which include, among many others, watertight integrity, fire safety standards, …. For some systems and equipment the use of steel is required, such as the use of steel, bronze or other approved ductile material for shell fittings and valves or steel only for pipes with respect to openings in the shell plating below the bulkhead deck of passenger ships and the freeboard deck of cargo ships. "A" class divisions on ships shall be constructed of steel or other equivalent material.
Does the IMO type-approval for ship systems and equipment ?
The International Maritime Organization (IMO) does not test or approve products for marine use as this is the responsibility of the national maritime Administration (i.e. the Government of the State whose flag the ship is entitled to fly). For specific systems tests may be carried out in test laboratories which have been recognized by the national maritime Administrations. The approval itself is generally issued by the Administration, on the basis of such laboratory test. In this regard, the expression "IMO approved" is used to mean "approved in accordance with the IMO Convention or other instruments concerned". If you require further information on type approval or testing of equipment or authority to its onboard use you may contact the flag Administration concerned or your national maritime Administration and seek their advice as appropriate.
The corresponding contact details can be found on IMO's Global Integrated Shipping Information System (GISIS) (https://gisis.imo.org/Public/) under "Contact Points".
I would like to request a technical opinion from IMO or have a request of interpretation for a regulation.
The International Maritime Organization (IMO) is a specialized agency of the United Nations and, as an impartial body, does not provide opinions on specific queries or interpretations of current regulations and rules. While we try to pinpoint any person seeking assistance to the relevant regulations or applicable standards and guidelines, we do not endorse any individual interpretation of regulations as this is a matter for the flag State or the relevant Administration. Please note that any questions or queries sent to the IMO may be answered to the best knowledge of the officer in charge of the subject but any response you receive from us has no bearing on the sovereign interpretation or enforcement of national regulations of Administrations and in case the information you receive from us seem to be in conflict with those of an Administration, the latter prevails.
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How to order IMO publications including the international Code on intact Stability, 2008?
Information on how to order the IMO publications can be obtained from our website (http://www.imo.org/en/Publications/Distributors/Pages/default.aspx)
Alternatively, the Publishing Service can be reached at the following address and phone numbers:
Publication Service/Sales
International Maritime Organization
4 Albert Embankment
London SE1 7SR
United Kingdom
Tel: +44 207 735 7611 (ask for publication sales)
Fax: +44 207 587 3241
Any questions in addition to the above?
Please contact IMO by sending an email to info@imo.org stating, in the subject line, the key words or IMO instrument you have a question or query on.
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