4.6 Mobile offshore drilling units (MODUs)

• .1 The provisions given hereunder apply to mobile offshore drilling units as defined in 1.3.7, the keels of which are laid or which are at a similar stage of construction on or after 1 May 1991. For MODUs constructed before that date, the corresponding provisions of chapter 3 of resolution A.414(XI) should apply.

• .2 The coastal State may permit any unit designed to a lesser standard than that of this chapter to engage in operations having taken account of the local environmental conditions. Any such unit should, however, comply with safety requirements which in the opinion of the coastal State are adequate for the intended operation and ensure the overall safety of the unit and the personnel on board.

 For the purposes of this section, the terms used herein have the meanings defined in the following paragraphs:

• .1 coastal State means the Government of the State exercising administrative control over the drilling operations of the unit;

• .2 mode of operation means a condition or manner in which a unit may operate or function while on location or in transit. The modes of operation of a unit include the following:

  • .2.1 operating conditions - conditions wherein a unit is on location for the purpose of conducting drilling operations, and combined environmental and operational loadings are within the appropriate design limits established for such operations. The unit may be either afloat or supported on the seabed, as applicable;

  • .2.2 severe storm conditions - conditions wherein a unit may be subjected to the most severe environmental loadings for which the unit is designed. Drilling operations are assumed to have been discontinued due to the severity of the environmental loadings, the unit may be either afloat or supported on the seabed, as applicable;

  • .2.3 transit conditions - conditions wherein a unit is moving from one geographical location to another.

  4.6.3.1 Curves of righting moments and of wind heeling moments similar to figure 4.6-1 with supporting calculations should be prepared covering the full range of operating draughts including those in transit conditions, taking into account the maximum deck cargo and equipment in the most unfavourable position applicable. The righting moment curves and wind heeling moment curves should be related to the most critical axes. Account should be taken of the free surface of liquids in tanks.

Figure 4.6.1 Righting moment and wind heeling moment curves

  4.6.3.2 Where equipment is of such a nature that it can be lowered and stowed, additional wind heeling moment curves may be required and such data should clearly indicate the position of such equipment.

  4.6.3.3 The curves of wind heeling moment should be drawn for wind forces calculated by the following formula:

• F = 0.5C_SC_Hρ V²A (Newtons)

  where:

  F	=	is the wind force (Newtons)
  CS	=	is the shape coefficient depending on the shape of the structural member exposed to the wind (see table 4.6-1)
  CH	=	is the height coefficient depending on the height above sea level of the structural member exposed to wind (see table 4.6-2)
  ρ	=	is the air mass density (1.222 kilogrammes per cubic metre)
  V	=	is the wind velocity (metres per second)
  A	=	is the projected area of all exposed surfaces in either the upright or the heeled condition (square metres)

  4.6.3.4 Wind forces should be considered from any direction relative to the unit and the value of the wind velocity should be as follows:

• .1 In general a minimum wind velocity of 36 m/s (70 knots) for offshore service should be used for normal operating conditions and a minimum wind velocity of 51.5 m/s (100 knots) should be used for the severe storm conditions.

• .2 Where a unit is to be limited in operation to sheltered locations (protected inland waters such as lakes, bays, swamps, rivers, etc.) consideration should be given to a reduced wind velocity of not less than 25.8 m/s (50 knots) for normal operating conditions.

  4.6.3.5 In calculating the projected areas to the vertical plane, the area of surfaces exposed to wind due to heel or trim, such as under decks, etc., should be included using the appropriate shape factor. Open truss work may be approximated by taking 30% of the projected block area of both the front and back section, i.e. 60% of the projected area of one side.

  4.6.3.6 In calculating the wind heeling moments, the lever of the wind overturning force should be taken vertically from the centre of pressure of all surfaces exposed to the wind to the centre of lateral resistance of the underwater body of the unit. The unit is to be assumed floating free of mooring restraint.

  4.6.3.7 The wind heeling moment curve should be calculated for a sufficient number of heel angles to define the curve. For ship-shaped hulls the curve may be assumed to vary as the cosine function of ship heel.

  4.6.3.8 Wind heeling moments derived from wind tunnel tests on a representative model of the unit may be considered as alternatives to the method given in 4.6.3.3 to 4.6.4.7. Such heeling moment determination should include lift and drag effects at various applicable heel angles.

  4.6.4.1 The stability of a unit in each mode of operation should meet the following criteria (see also figure 4.6-2):

• .1 For surface and self-elevating units the area under the righting moment curve to the second intercept or downflooding angle, whichever is less, should be not less than 40% in excess of the area under the wind heeling moment curve to the same limiting angle.

• .2 For column-stabilized units the area under the righting moment curve to the angle of downflooding should be not less than 30% in excess of the area under the wind heeling moment curve to the same limiting angle.

• .3 The righting moment curve should be positive over the entire range of angles from upright to the second intercept.

Figure 4.6.2 Angle of inclination

  4.6.4.2 Each unit should be capable of attaining a severe storm condition in a period of time consistent with the meteorological conditions. The procedures recommended and the approximate length of time required, considering both operating conditions and transit conditions, should be contained in the operating manual, as referred to in 2.1.2. It should be possible to achieve the severe storm condition without the removal or relocation of solid consumables or other variable load. However, the Administration may permit loading a unit past the point at which solid consumables would have to be removed or relocated to go to severe storm condition under the following conditions, provided the allowable KG requirement is not exceeded:

• .1 in a geographic location where weather conditions annually or seasonally do not become sufficiently severe to require a unit to go to severe storm condition; or

• .2 where a unit is required to support extra deckload for a short period of time that is well within the bounds of a favourable weather forecast.

The geographic locations and weather conditions and loading conditions when this is permitted should be identified in the operating manual.

  4.6.4.3 Alternative stability criteria may be considered by the Administration provided an equivalent level of safety is maintained and if they are demonstrated to afford adequate positive initial stability. In determining the acceptability of such criteria, the Administration should consider at least the following and take into account as appropriate:

• .1 environmental conditions representing realistic winds (including gusts) and waves appropriate for world-wide service in various modes of operation;

• .2 dynamic response of a unit. Analysis should include the results of wind tunnel tests, wave tank model tests, and non-linear simulation, where appropriate. Any wind and wave spectra used should cover sufficient frequency ranges to ensure that critical motion responses are obtained;

• .3 potential for flooding taking into account dynamic responses in a seaway;

• .4 susceptibility to capsizing considering the unit's restoration energy and the static inclination due to the mean wind speed and the maximum dynamic response;

• .5 an adequate safety margin to account for uncertainties.

An example of alternative criteria for twin-pontoon column-stabilized semi-submersible units is given in section 4.6.5.

  4.6.5.1 The criteria given below apply only to twin-pontoon column-stabilized semi-submersible units in severe storm conditions which fall within the following range of parameters:

The parameters used in the above equations are defined in paragraph 4.6.5.3.

  4.6.5.2 Intact stability criteria

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 4.6.3 of the Code at the survival draught. The reserve energy area 'B' must be equal or greater than 10% of the dynamic response area 'A' as shown in figure 4.6-3.

  Area 'B'/Area 'A' ≥ 0.10

where:

• Area 'A' is the area under the righting moment curve measured from θ₁ to (θ₁ + 1.15 θ_dyn)

  Area 'B' is the area under the righting moment curve measured from (θ₁ + 1.15 θ _dyn) to θ₂

  θ₁ 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 + 17.8C)/(1 + GM/(1.46 + 0.28BM))

  C = (L_ptn 5/3 * VCP_w1 * A_w * V_p * V_c ^1/3)/(I_wp ^5/3 * V_t)

Parameters used in the above equations are defined in paragraph 4.6.5.3.

• .2 Downflooding criteria

  These criteria are based 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 downflooding distance (DFD_o) should be greater than the reduction in downflooding distance at the survival draught as shown in figure 4.6-4.

  DFD_o - RDFD > 0.0

The parameters used in the above equations are defined in paragraph 4.6.5.3.

  4.6.5.3 Geometric parameters

  4.6.5.4 Capsize criteria assessment form

Input data

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

  4.6.5.5 Downflooding criteria assessment form

Input data

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

Figure 4.6.3 Righting moment and heeling moment curves

Figure 4.6.4 Definition of downflooding distance and relative motion