1 General

1.1 Introduction

1.1.1 Purpose

1.1.1.1 The purpose of these Guidelines is to enable the use of the second generation intact stability criteria for the assessment of dynamic stability failure modes in waves, as requested in section 1.2 of part A of the 2008 Intact Stability (IS) Code. These dynamic stability failure modes are as follows: dead ship condition, excessive acceleration, pure loss of stability, parametric rolling and surf-riding/broaching. In this sense, the overarching aim is to use the latest technology and knowledge on ship dynamics to provide guidance for ship designers on dynamic stability failure modes and to provide operational guidance for ship masters. This is undertaken to further improve the safety level of a ship beyond the mandatory intact stability criteria.

1.1.1.2 The main purpose of these criteria is to enable the use of the latest numerical simulation techniques for evaluating the safety level of a ship from an intact stability viewpoint. By using such tools for simulating the dynamic ship behaviours in a random seaway, the safety level of a ship can be estimated with a probabilistic measure. This approach is hereby called direct stability assessment. However, applying such tools to all new ships that are subject to the 2008 IS Code is not practical due to the limitation of human resources and facilities that are required for experimentally validating the numerical tools. Thus, the vulnerability of a ship can be assessed using simpler vulnerability criteria or more comprehensive direct stability assessment. The guidance for vulnerability criteria and the guidance for direct stability assessment are provided in chapters 2 and 3 of the Interim Guidelines, respectively.

1.1.1.3 It is noted that compliance with the criteria contained within part A of the 2008 IS Code, good seamanship, appropriate ship-handling and appropriate operation may avoid the potential danger of excessive roll, excessive lateral accelerations or capsizing due to a dynamic stability failure mode. Mindful of this fact, operational measures for a ship may be provided as an alternative to the vulnerability criteria or direct stability assessment. For this purpose, the guidelines for operational measures are provided in chapter 4 of the Interim Guidelines. Whereas the natural order of application is from the vulnerability criteria to direct stability assessment and operational measures, all these alternatives are equivalent in the regulatory sense and any of them can be used independently of others, in the way that is most suitable for the particular design.

1.1.2 Framework

1.1.2.1 For the purpose of this framework, the following definitions apply:

.1 criterion is a procedure, an algorithm or a formula used for the assessment on the likelihood of a stability failure;
.2 standard is a boundary separating acceptable and unacceptable likelihood of a stability failure; and
.3 rule (or regulation) is a specification of a relationship between a standard and a value produced by a criterion.

1.1.2.2 The second generation intact stability criteria are tools to judge the likelihood of intact stability failures. Intact stability failure is an event that includes the occurrence of very large roll (heel, list) angles or excessive rigid body accelerations, which may result in capsizing or impairs normal operation of the ship and could be dangerous to crew, passengers, cargo or ship equipment. Three subtypes of intact stability failure are included:

.1 heel/list exceeding a prescribed limit;
.2 roll angles exceeding a prescribed limit; and
.3 lateral accelerations exceeding prescribed limit.

1.1.3 Application logic

1.1.3.1 The application logic is summarized in figure 1.1.3. Although the user may be guided by a sequential logic of the Interim Guidelines (see 1.1.3.2), it is also acceptable that the users apply any alternative design assessment or operational measure option (see 1.1.1.3). For example, a user may wish to immediately commence with the application of direct stability assessment procedures without passing through Levels 1 and 2 of the vulnerability criteria or develop operational measures without performing design assessment.

1.1.3.2 A sequential application logic can be summarized, as follows:

As the simplest options, the vulnerability criteria are presented in two levels: Level 1 and Level 2. The assessment of the five stability failure modes should begin with the use of these levels. Level 1 is an initial check and then, if the ship in a particular loading condition is assessed as not vulnerable for the tested failure mode, the assessment for that failure mode may conclude; otherwise, the design would progress to Level 2. If the ship in a particular loading condition is assessed as not vulnerable for the tested failure mode in Level 2, then the assessment would conclude; otherwise, the design would progress to the application of direct stability assessment, application of operational limitations, revising the design of the ship or discarding the loading condition. If the ship in a particular loading condition is not found acceptable with respect to direct stability assessment procedures, then the logic is that the design would then progress to the application of operational measures or operational guidance, revising the design or discarding the loading condition.
Figure 1.1.3 – Simplified scheme of the application structure of the second generation intact stability criteria. For actual application details, reference is to be made to the text of these Interim Guidelines.

1.1.4 Testing

1.1.4.1 The second generation intact stability criteria have been developed envisioning a future incorporation into the 2008 IS Code. However, they require testing before using them as mandatory criteria. This is because the robustness of the new criteria is not the same for the different stability failure modes.

Specifically, results obtained in the development process, indicate that:

.1 Level 1 and Level 2 vulnerability criteria for dead ship stability failure mode sometimes provide non-consistent results, i.e. Level 2 may be more conservative than Level 1 for some ships;
.2 vulnerability criteria for excessive acceleration may require further refinements;
.3 Level 2 vulnerability criterion for the pure loss of stability failure mode provides very conservative results for ships with low freeboard; therefore, results of testing for such ships should be treated with care; and
.4 parametric rolling and surf-riding/broaching Level 1 and Level 2 vulnerability criteria have sufficient scientific background and feasible methods for regulatory use.

1.1.4.2 Therefore, these criteria should be used on a trial basis at this stage. Such criteria usage and subsequent reporting are necessary to gain experience and consequently enable the introduction of this approach to the analysis of intact stability. It is also highly recommended to apply the criteria to ships already in service and to compare the results with operational experience.

1.1.5 Feedback

1.1.5.1 The second generation intact stability criteria methodology has been developed using the latest technology and scientific knowledge for assessing ship dynamics in waves. The methodology has been tested on a number of sample ships and, to this end, these draft Interim Guidelines are intended to generate data and feedback for a large number of ships.

1.1.5.2 These guidelines have been issued as "Interim Guidelines" in order to gain experience in their use. They should be reviewed in order to facilitate future amendments based on the experience gained.

1.1.5.3 Member States and international organizations are invited to submit information, observations, suggestions, comments and recommendations based on the practical experience gained through the application of these Interim Guidelines. To support the objective of obtaining robust criteria for regulatory use, suggestions for alternatives to and/or refinements of the criteria elements contained in the Interim Guidelines are encouraged. The suggestions should compare the outcomes with the criteria elements included in the Interim Guidelines.

1.1.5.4 With such feedback not only on the technical results but also their usability and clarity, the Organization will be able to subsequently refine the second generation intact stability criteria, if necessary.

1.1.6 Relationship with mandatory criteria

1.1.6.1 These Interim Guidelines are not intended to be used in lieu of the mandatory intact stability criteria contained in the 2008 IS Code. They are intended for use as a guide for ship designers to assess the aspects of ship stability not adequately covered by the mandatory criteria and to provide operational guidance for ship masters. Therefore, they should be used as a supplementary set of stability assessment methods.

1.1.7 Notes for application

1.1.7.1 These Interim Guidelines are intended to be applied to ships that are also subject to the 2008 IS Code.

1.1.7.2 These Interim Guidelines have not been specifically developed for multihulls. Moreover, for ships with an extended low weather deck, additional application provisions are provided in the relevant chapters.

1.2 Definitions

1.2.1 Loading condition, in the context of these Interim Guidelines, is defined by the mean draught d, trim angle θ, metacentric height GM and mass moments of inertia I_xx(or natural roll period T_r), I_yyand I_zz.

1.2.2 Fully loaded departure condition means the loading condition, as defined in section 3.4.1 of part B of the 2008 IS Code.

1.2.3 Sea state is the stationary condition of the free water surface and wind at a certain location and time, described in these Interim Guidelines by the significant wave height H_S, mean zero-crossing wave period T_Z, mean wave direction μ, wave elevation energy spectrum S_zz, and mean wind speed, gustiness characteristics and direction. For combined wind sea and swell, significant wave height, mean zero-crossing wave period and mean wave direction may be defined separately for each of the two wave systems.

1.2.4 Sailing condition is a short notation for the combination of the ship forward speed V_s and heading relative to mean wave direction μ.

1.2.5 Assumed situation is a condition of the ship that refers to the sailing condition combined with sea state. Thus, a situation is defined by the ship forward speed v₀, mean wave direction μ, significant wave height H_Sand mean zero-crossing wave period T_Z, direction and gustiness characteristics of wind.

1.2.6 Design situation is an assumed situation representative for a particular stability failure mode.

1.2.7 Wave scatter table is a table containing the probabilities of each range of sea states encountered in the considered operational area or operational route. In these Interim Guidelines, the probabilities contained in a wave scatter table are defined to sum to unity.

1.2.8 Limited wave scatter table is a table obtained from the full wave scatter table by removing all sea state ranges with the significant wave height above a certain limit.

1.2.9 Operational area and operational route are the geographical areas specified for the ship operation. In the context of these Interim Guidelines, operational area or operational route are specified by the long-term wave statistics (wave scatter table) and wind statistics.

1.2.10 Nominal ship forward speed means the ship speed in calm water under action of the ship's propulsion at a given setting.

1.2.11 Maximum service speed means maximum ahead service speed, as defined in SOLAS regulation II-1/3.14.

1.2.12 Design assessment corresponds to the application of vulnerability criteria according to chapter 2 or direct stability assessment according to chapter 3 of these Interim Guidelines or a combination of the two.

1.2.13 Operational measures mean operational limitations or operational guidance.

1.2.14 Guidelines for vulnerability assessment means the content of chapter 2 of these Interim Guidelines.

1.2.15 Guidelines for direct stability assessment means the content of chapter 3 of these Interim Guidelines.

1.2.16 Guidelines for operational measures means the content of chapter 4 of these Interim Guidelines.

1.2.17 2008 IS Code means the International Code on Intact Stability, 2008, as amended.

1.2.18 Mean 3-hour maximum amplitude means the average value of several maximum amplitudes, each of which is determined for an exposure time of 3 hours.

1.3 Nomenclature

1.3.1 The general nomenclature used in these Interim Guidelines is set forth in 1.3.2, 1.3.3, 1.3.4 and 1.3.5. Nomenclature that is specific to a particular section is defined in that location and prevails over the general nomenclature reported here. If not otherwise stated, reference should be made to the nomenclature used in the 2008 IS Code.

1.3.2 General ship characteristics:

L = length of the ship, as defined in paragraph 2.12 of the introduction part of the 2008 IS Code (m)

B = moulded breadth of the ship (m)

B_wl = moulded breadth at waterline (m)

D = moulded depth, as defined in the 2008 IS Code (m)

V_s = service speed (m/s)

v₀ = forward speed (m/s)

Fn = Froude number =

A_k = total overall area of the bilge keels (no other appendages) (m²)

∇_D = volume of displacement at waterline equal to D at zero trim (m³)

D_p = propeller diameter (m);

x_i = longitudinal distance from the aft perpendicular to a station i (m), positive forward

1.3.3 Constants:

g = acceleration due to gravity, taken as 9.81 (m/s²)

ρ = density of salt water, taken as 1025 (kg/m³)

ρ_air = density of air, taken as 1.222 (kg/m³)

1.3.4 Loading condition characteristics:

d_full = draft corresponding to the fully loaded departure condition in calm water (m)

C_B,full = block coefficient of the fully loaded departure condition in calm water

C_m,full = midship section coefficient of the fully loaded departure condition in calm water

d = mean draught, i.e. draft amidships corresponding to the loading condition under consideration in calm water (m)

L_WL = length of the ship on the waterline corresponding to the loading condition under consideration (m)

KB = height of the centre of buoyancy above baseline corresponding to the loading condition under consideration (m)

KG = height of the centre of gravity above baseline corresponding to the loading condition under consideration (m)

∇ = volume of displacement corresponding to the loading condition under consideration (m³)

C_B = block coefficient corresponding to the loading condition under consideration (-)

Δ = displacement (t)

A_W = waterplane area at the draft equal to d (m²)

I_T = transverse moment of inertia of water-plane area (m⁴)

I_xx = dry roll moment of inertia (t m²)

I_yy = dry pitch moment of inertia (t m²)

I_zz = dry yaw moment of inertia (t m²)

m = mass of the ship (t)

k_xx = dry roll radius of gyration around axis (m)

k_yy = dry pitch radius of gyration around axis (m)

k_zz = dry yaw radius of gyration around axis (m)

GM = metacentric height of the loading condition in calm water (m), with or without correction for free surface effect, as required

A_L = projected lateral area of the portion of the ship and deck cargo above the waterline (m²)

Z = vertical distance from the centre of A_L to the centre of the underwater lateral area or approximately to a point at one-half the mean draft, d (m)

T_r = linear natural roll period in calm water (s)

ω_r = natural roll frequency = 2 π / T_r(rad/s)

φ = angle of roll, heel, or list (rad or deg)

θ = angle of pitch or trim (rad or deg)

ψ = angle of yaw, heading or course (rad or deg)

φ_S = stable heel angle under the action of steady heeling moment calculated as the first intersection between the righting lever curve (GZ curve) and the heeling lever curve, (rad or deg)

φ_V = angle of vanishing stability. In presence of a heeling moment, it should be calculated as the second intersection between the righting lever curve (GZ curve) and the applied heeling lever curve (rad or deg)

1.3.5 Environmental condition characteristics:

λ = wavelength (m)

H = wave height (m)

H_S = significant wave height for the short-term environmental condition under consideration (m)

s = wave steepness = H/λ

T_Z = mean zero-crossing period for the short-term environmental condition under consideration (s)

T_p = wave period corresponding to peak of spectrum for the short-term environmental condition under consideration (s)

μ = mean wave direction with respect to ship centre plane (deg)

S_zz = wave elevation energy spectrum (m2/(rad/s))

ω = circular frequency (rad/s)

k = wave number = 2π/ λ (rad/m)

1.3.6 Other parameters

N_s = number of simulations

f_s = joint probability density of sea state (probability of sea states per unit range of significant wave heights and mean zero-crossing periods) (1/m·s)