MSC.1/Circ.1281

EXPLANATORY NOTES TO THE INTERNATIONAL CODE ON INTACT STABILITY, 2008

(9 December 2008)

 

 

1. The Maritime Safety Committee, at its eighty-fifth session (26 November to 5 December 2008), adopted, by resolution MSC.267(85), the International Code on Intact Stability, 2008 (2008 IS Code). In adopting the 2008 IS Code, the Committee recognized the necessity of appropriate explanatory notes to ensure uniform interpretation and application.

 

2. To this end, the Committee approved the Explanatory Notes to the Intact Stability Code, 2008, set out in the annex, as prepared by the Sub-Committee on Stability and Load Lines and on Fishing Vessels Safety, at its fiftieth session (30 April to 4 May 2007).

 

3. The Explanatory Notes are intended to provide Administrations and the shipping industry with specific guidance to assist in the uniform interpretation and application of the intact stability requirements of the 2008 IS Code.

 

4. Member Governments are invited to use the Explanatory Notes when applying the intact stability requirements of the 2008 IS Code adopted by resolution MSC.267(85) and to bring them to the attention of all parties concerned.

 

Annex.

EXPLANATORY NOTES TO THE INTERNATIONAL CODE ON INTACT STABILITY, 2008

 

CHAPTER 1.
GENERAL

 

1.1 Introduction

 

The intact stability criteria given in part A (mandatory) and part B (recommendatory) of the 2008 IS Code are prescriptive rules developed from ship operation statistics and weather criterion collected in the middle of the twentieth century. To enable a proper understanding and application of these criteria, their origin and development are presented in chapter 3.

 

1.2 Purpose

 

The purpose of these explanatory notes is to deliver to the user of the Code information on the history, background and method of elaboration of the present stability criteria, as set out in part A of the 2008 IS Code.

 

CHAPTER 2.
TERMINOLOGY

 

It should be noted that, while the terms listed below are in common usage, they are not those given in MSC/Circ.920, MODEL LOADING AND STABILITY MANUAL, section 2.2, table 1, which are based on ISO standards (ISO 7462 and ISO 7463).

 

Particular care should be taken with regard to asymmetric weight and buoyancy distribution.

 

 

In all cases it is of utmost importance to define clearly the reference points/planes and the signs of the positive and negative directions along the vessel's coordinate system.

 

CHAPTER 3.
ORIGIN OF PRESENT STABILITY CRITERIA

 

3.1 General

 

3.1.1 The Maritime Safety Committee requested the Sub-Committee on Stability and Load Lines and on Fishing Vessels Safety (SLF), to develop a range of intact stability requirements to cover all ship types for eventual incorporation into the 1974 SOLAS Convention. At the thirty-third session of the Sub-Committee (SLF 33), the Working Group on Intact Stability (IS) considered this matter and foresaw the procedural problems that would arise by incorporating a wide range of stability criteria covering different ship types into the Convention, and also recognized that these criteria could not be developed in a short time. The group recommended that, alternatively, consideration should be given to developing a comprehensive code to incorporate the then existing stability requirements contained in all IMO recommendations and codes for various types of ships. Criteria for additional ship types could be added later as each ship type was considered and a criterion developed. The group also suggested that the 1974 SOLAS Convention should either: include a basic stability standard and refer to the Code for varying ship types or, alternatively, it should only refer to the Code. The proposed Code could be divided into two parts: part A, containing mandatory requirements; and part B, containing recommendatory requirements. Development of the proposed Code was given priority [IMO 1988].

 

3.1.2 In considering the proposal by the above group, SLF 33 agreed that the development of a stability code for all ships covered by IMO instruments (IS Code) would be of value, so that the generally accepted and special stability requirements for all types of ships' forms would be contained in a single publication for ease of reference. This was thought to be important because stability requirements were dissipated amongst various documents which made their use by designers and authorities difficult [IMO 1988a]. The SLF Sub-Committee emphasized that the Code should contain instructions on operational procedures as well as technical design characteristics. This course of action was approved by the Maritime Safety Committee at its fifty-seventh session.

 

3.1.3 The collation of the stability requirements contained in various IMO instruments and the preparation of the first draft of the Code was undertaken by Poland and submitted to IMO [IMO 1990]. This formed the basis for the development of the Code which was to include the following groups of requirements as proposed by Poland [Kobylinski 1989]:

 

.1 ship construction;

 

.2 physical characteristics of ships;

 

.3 information available onboard and navigational aids; and

 

.4 operations.

 

3.1.4 This framework was eventually adopted by SLF 35, which also agreed that the Code should have recommendatory status. The final draft of the Code was agreed by SLF 37 and subsequently adopted by resolution A.749(18) [IMO 1993]. It was subsequently amended in 1998 by resolution MSC.75(69). The Code was considered to be a "living" document under constant review, into which all new requirements developed by IMO would be incorporated.

 

3.2 Background of criteria regarding righting lever curve properties (part A of the 2008 IS Code)

 

3.2.1 Introduction

 

3.2.1.1 The statistical stability criteria were originally included in resolutions A.167(ES.IV) and A.168(ES.IV). They were developed as a result of discussions conducted at several sessions of the Sub-Committee on Subdivision and Stability Problems (STAB), a forerunner of the SLF Sub-Committee and the Working Group on Intact Stability (IS). There was general agreement that the criteria would have to be developed on the basis of the statistical analysis of stability parameters of ships that had suffered casualties and of ships that were operating safely.*

____________

* The detailed discussion of the work of these IMO bodies and of the method used in the development of stability standards was reported in the following papers: Nadeinski and Jens [1968] and Thompson and Tope [1970].

 

3.2.1.2 The IS Working Group agreed to a programme of work that eventually included the following item:

 

.1 collation, analysis and evaluation of existing national rules or recommendations on stability;

 

.2 evaluation of stability parameters which could be used as stability criteria;

 

.3 collection of stability characteristics of those ships that become casualties or experienced dangerous heeling under circumstances suggesting insufficient stability;

 

.4 collection of stability characteristics of those ships which were operating with safe experience;

 

.5 comparative analysis of stability parameters of ships becoming casualties and of ships operated safely;

 

.6 estimation of critical values of chosen stability parameters; and

 

.7 checking formulated criteria against a certain number of existing ships.

 

3.2.1.3 The analysis of existing national stability requirements (paragraph 3.2.1.2.1) [IMO 1964] revealed considerable consistency in the applicability of certain parameters as stability criteria. It was noted also that in many countries there was a tendency to adopt weather criterion. However, weather criterion was not considered by the IS Working Group at that time.

 

3.2.1.4 With regard to paragraph 3.2.1.2.2 of the programme, the IS Working Group singled out a group of parameters characterizing the curve of righting levers for the ship at rest (V = 0) in still water. This was done notwithstanding the fact that if a ship sails in a seaway, the curve of static stability levers changes. However, it was decided that the only practical solution would be to use the "stipulated" curve of righting levers and this curve could be characterized using the following set of parameters:

 

.1 initial stability - GM₀,

 

.2 righting levers at angles - GZ₁₀, GZ₂₀, GZ₃₀, GZ₄₀, GZ_φ, GZ_m,

 

.3 angles - φ_m, φ_v, φ_f, φ_fd,

 

.4 levers of dynamic stability - e₂₀, e₃₀, e₄₀, e_φ.

 

3.2.1.5 The number of stability parameters which could be used as stability criteria should be, however, limited. Therefore, by analysing the parameters used in various national stability requirements, the Working Group on Intact Stability concluded the following eight parameters have to be left for further consideration: GM₀, GZ₂₀, GZ₃₀, GZ_m, φ_m, φ_v, φ_fd, e.

 

3.2.1.6 During the realization of paragraph 3.2.1.2.3 of the programme, a special form of casualty record was prepared and circulated amongst IMO Member States [IMO 1963]. It was requested that the form be filled in carefully with as many details of the casualty as possible. Altogether there were casualty records collected for 68 passenger and cargo ships and for 38 fishing vessels [IMO 1966, 1966a]. In a later period, some countries submitted further casualty records so that, in the second analysis that was performed in 1985, data for 93 passenger and cargo ships and for 73 fishing vessels were available [IMO 1985]. On the basis of the submitted data, tables of details of casualties were prepared.

 

 

Figure 1.
Explanation of righting levers and heeling angles

 

3.2.1.7   Within paragraph 3.2.1.2.4 of the programme, data on stability characteristics for 62 passenger and cargo ships and for 48 fishing vessels, which were operated safely, were collected and for this purpose a special instruction containing detailed specifications for the manner how the stability information was to be submitted was developed. Also, for these ships, tables were prepared of stability parameters.

 

3.2.1.8  Paragraph 3.2.1.2.5 of the programme included analysis of the collected data, the results of which were submitted to IMO in several documents separately prepared for passenger and cargo ships and for fishing vessels [IMO 1965; 1966; 1966a; 1966b].

 

3.2.1.9 After IMO resolutions A.167(ES.IV) and A.168(ES.IV) had been adopted and further intact stability casualty data were collected, it was decided to repeat the analysis in order to find out if additional data might change conclusions drawn in the first analysis. This second analysis confirmed, in general, the results achieved in the first analysis [IMO 1985]. In the following text, the results of the second analysis that was based on the larger database are referred to.

 

3.2.1.10 The analysis performed consisted of two parts. In the first part, details relevant to casualties were evaluated, which allowed qualitative conclusions with regard to the circumstances of casualties to be developed and therefore the specification of general safety precautions. In the second part, stability parameters of ships reported as casualties were compared with those for ships which were operated safely. Two methods were adopted in this analysis. The first was identical with the method adopted by Rahola [Rahola 1939] and the second was the discrimination analysis. The results of the analysis of intact stability casualty data and of the first part of the analysis of stability parameters are included in paragraph 3.2.2.2. The results of the discrimination analysis are referred to in paragraph 3.2.2.3.

 

3.2.2 Results of the Analysis of Intact Stability Casualty Records and Stability Parameters

 

3.2.2.1 Analysis of details relevant to the casualties

 

3.2.2.1.1 The evaluation of details relevant to the casualties is shown in Figures 2 to 7.

 

 

Figure 2.
Distribution of length of capsized ships collated by IMO [1985]

 

3.2.2.1.2 In all 166 casualties reported, the ships concerned were: 80 cargo ships, 1 cargo and passenger ship, 1 bulk carrier, 4 off-shore supply ships, 7 special service vessels, and 73 fishing vessels. Distribution of ship's length is shown in figure 2. It is seen that the majority of casualties occurred in ships of less than 60 m in length.

 

3.2.2.1.3 A great variety of cargoes were carried so that no definite conclusions could be drawn. It may be noted, however, that in 35 cases of the 80 cargo ships reported, deck cargo was present.

 

3.2.2.1.4 The result of the analysis of the location of the casualty is shown in Figure 3. It may be seen that the majority of casualties (72% of all casualties) occurred in restricted water areas, in estuaries and along the coastline. This is understandable because the majority of ships lost were small ships of under 60 m in length. From the analysis of the season when the casualty occurred (Figure 4) it may be seen that the most dangerous season is autumn (41% of all casualties).

 

 

3.2.2.1.5 The result of the analysis of the weather conditions is shown in Figure 5. About 75% of all casualties occurred in rough seas at a wind force of between Beaufort 4 to 10. Ships were sailing most often in beam seas, less often in quartering and following seas.

 

3.2.2.1.6 The manner of the casualty was also analysed (Figure 6). It showed that the most common casualty was through gradual or sudden capsizing. In about 30% of casualties, ships survived the casualty and were heeled only.

 

3.2.2.1.7 In Figure 7 the results of the analysis of the age of ships are shown. No definite conclusions could be drawn from this analysis.

 

 

3.2.2.1.8 The distributions of stability parameters for ships' condition at time of loss are shown in Figures 8 to 14.