**THE ASSEMBLY,**

RECALLING Article 15(j) of the Convention on the International Maritime Organization concerning the functions of the Assembly in relation to regulations and guidelines concerning maritime safety,

RECALLING ALSO that, by resolution A.649(16), it adopted the Code for the Construction and Equipment of Mobile Offshore Drilling Units, 1989 (1989 MODU Code) to promote the international operation of mobile offshore drilling units,

NOTING that mobile offshore drilling units continue to be moved and operated internationally,

RECOGNIZING the need to establish a more rational method for reviewing the intact stability criteria based on advanced technological computerized means and more realistic environmental conditions,

FURTHER RECOGNIZING that chapter 3 of the 1989 MODU Code allows for alternative stability criteria to be considered by Administrations,

HAVING CONSIDERED the recommendation made by the Maritime Safety Committee at its fifty-seventh session,

**1.**ADOPTS an example of alternative intact stability criteria for twin-pontoon column-stabilized semisubmersible mobile offshore drilling units, 1989, as set out in the Annex to the present resolution;

**2.**RECOMMENDS that Governments concerned accept the application of this example as being an equivalent to the provisions of paragraph 3.3.1.2 of the 1989 MODU Code;

**3.**INVITES all Governments concerned to consider putting this alternative into effect and to evaluate its use;

**4.**FURTHER INVITES all Governments concerned, through the Organization, to exchange experience gained as a result of the application of this alternative;

**5.**AUTHORIZES the Maritime Safety Committee to amend the alternative as necessary after consultations with relevant organizations.

## Annex An example of alternative intact stability criteria for twin-pontoon column-stabilized semisubmersible units

### 1.1

The criteria hereunder constitute alternative intact stability criteria for column-stabilized units under the provisions of section 3.3.3 of the Code for the Construction and Equipment of Mobile Offshore Drilling Units, 1989 (1989 MODU Code). These criteria apply only to twin-pontoon column-stabilized semisubmersible units in severe storm conditions which fall within the following range of parameters

V

_{p}/ V

_{t}is between 0.48 and 0.58

A

_{wp}/ ( V

_{c})

^{2/3}is between 0.72 and 1.00

l

_{wp}/ [ V

_{c}× ( L

_{ptn}/ 2) ]  is between 0.40 and 0.70

The parameters used in the above equations are defined in paragraph 1.3.

### 1.2 Intact stability criteria

**1.2.1**The stability of a unit in the survival mode of operation should meet the following criteria :

**.1 Capsize criteria**

These criteria are based on the wind heeling moment and righting moment curves calculated as shown in section 3.2 of the 1989 MODU Code at the survival draught. The reserve energy area 'B' must be greater than 10% of the dynamic response area 'A' as shown in figure 1.1.

Area 'B'/Area 'A' > 0.10 Where:

Area 'A' is the area under the righting arm curve measured from

*θ*to (

*θ*

_{1}+ 1. 15

*θ*

_{dyn})

Area 'B' is the area under the righting arm curve measured from (

*θ*

_{1}+ 1. 15

*θ*

_{dyn}) to

*θ*

_{2}

*θ*

_{1}is the first intercept with the 100 knot wind moment curve

*θ*

_{2}is the second intercept with the 100 knot wind moment curve

*θ*

_{dyn}is the dynamic response angle due to waves and fluctuating wind

*θ*

_{dyn}= ( 10. 3 + 17. 8

*C*) / ( 1 +

*GM*/ ( 1. 46 + + 0. 28

*BM*) )

C= ( L

_{ptn}

^{5/3}× VCP

_{w1}× A

_{w}× V

_{p}× V

_{c}

^{1/3})/ ( l

_{wp}

^{5/3}× V

_{t})

Parameters used in the above equations are defined in paragraph 1.3.

**.2 Downflooding criteria**

These criteria are bases on the physical dimensions of the unit and the relative motion of the unit about a static inclination due to a 75 knot wind measured at the survival draught. The initial downfloodlng distance (DFDo) must be greater than the reduction in downflooding distance at the survival draught as shown in figure 1.2.

DF D

_{0}- RDF D > 0. 0 Where:

DF D_{0} | = | initial downflooding distance to D_{m} in metres |

RDF D | = | reduction in downflooding distance in metres |

= | SF ( k × QSD_{1}+ W ) | |

SF | = | 1.10, which is a safety factor to account for uncertainties in the analysis, such as non-linear effects. |

k(correlation factor) | = | 0. 55+ 0. 08( a - 4. 0 ) + 0. 056 ( 1. 52 - GM ) |

a | = | ( F BD_{0} / D_{m} ) ( S _{ptn} L _{ccc} ) / A _{w p} |

(a cannot be taken to be less than 4.0) | ||

(GM cannot be taken to be greater than 2.44 m) | ||

QSD_{1} | = | DF D_{0} - Quasi-Static downflooding distance at θ _{1}, in metres, but not to be taken less than 3.0 m. |

RMW | = | Relative motion due to waves about O_{1}, in metres |

= | 9. 3 + 0. 11 ( X - 12. 19 ) | |

X | = | D_{m} ( V _{t} / V _{p} ) ( A _{wp}^{2} / l _{wp} ) ( L _{ccc} / L _{ptn} ) |

( X cannot be taken to be less than 12.19 m ) |

The parameters used in the above equations are defined in paragraph 1.3.

### 1.3 Geometric parameters

A _{wp} | is the waterplane area at the survival draught including the effects of bracing members as applicable (in square metres). |

A _{w} | is the effective wind area with the unit in the upright position (i.e. the product of projected area, shape coefficient and height coefficient) (in square metres) . |

BM | is the vertical distance from the metacentre to the centre of buoyancy with the unit in the upright position (in metres). |

D_{m} | is the initial survival draught (in metres). |

F BD_{0} | is the vertical distance from D_{m} to the top of the upper exposed weathertight deck at the side (in metres). |

GM | for paragraph 1.2.1.1, GM is the metacentric height measured about the roll of diagonal axis, whichever gives the minimum restoring energy ratio, 'B'/'A'. This axis is usually the diagonal axis as it possesses a characteristically larger projected wind area which influences the three characteristic angles mentioned above. |

GM | for paragraph 1.2.1.2, GM is the metacentric height measured about the axis which gives the minimum downflooding distance margin (i.e. generally the direction that gives the largest QSD_{1} ) (in metres). |

l _{w p} | is the waterplane second moment of inertia at the survival draught including the effects of bracing members as applicable (in metres to the power of 4). |

L _{ccc} | is the longitudinal distance between centres of the corner columns (in metres). |

L _{ptn} | is the length of each pontoon (in metres). |

S _{ptn} | is the transverse distance between the centreline of the pontoons (in metres) . |

V _{c} | is the total volume of all columns from the top of the pontoons to the top of the column structure, except for any volume included in the upper deck (in cubic metres). |

V _{p} | is the total combined volume of both pontoons (in cubic metres). |

V _{t} | is the total volume of the structures (pontoons, columns and bracings) contributing to the buoyancy of the unit, from its baseline to the top of the column structure, except for any volume included in the upper deck (in cubic metres). |

VCP _{w1} | is the vertical centre of wind pressure above D_{m} (in metres). |

### 1.4 Capsize criteria assessment form

**Input data**

**Determine**

**Results**

*Note: The minimum GM is that which produces a 'B'/'A' ratio = 0.10*

### 1.5 Downflooding criteria assessment form

**Input data**

**Determine**

**Results**

*Note: The minimum GM is that which produces a downflooding margin = 0.0 m*