Section 3 Watertight shell boundaries

3.1.1 The requirements of Chapter 7 regarding watertight integrity are to be complied with.

3.1.2 The minimum requirements for watertight shell plating and framing of column-stabilised units, self-elevating units, tension-leg units, buoys and deep draught caissons are given in this Section.

3.1.4 The Rules are, in general, applicable to shell plating with stiffeners fitted parallel to the hull bending compressive stress. When other stiffening arrangements are proposed, the scantlings are to be specially considered and the minimum shell thickness is to satisfy the buckling strength requirements given in Pt 4, Ch 5 Primary Hull Strength, but the minimum requirements of this Section are to be complied with.

3.1.6 The scantlings of moonpool bulkheads will be specially considered with regard to the maximum forces imposed on the structure and the permissible stress levels are to comply with Pt 4, Ch 5 Primary Hull Strength.

3.1.7 The minimum scantlings of moonpool bulkheads on buoys and deep draught caissons are to comply with Pt 4, Ch 6, 3.4 Buoys and deep draught caissons and the load head in Pt 4, Ch 6, 3.4 Buoys and deep draught caissons 3.4.7 is to be measured to the top of the moonpool bulkhead.

3.1.8 The minimum scantlings of moonpools and drilling well bulkheads on column-stabilised and tension-leg units are to comply with Pt 4, Ch 6, 3.2 Column-stabilised and tension-leg units 3.2.5, but plating thickness is to be not less than 9,0 mm, see also Pt 3, Ch 13, 2 Floating structures and subsea buoyant vessels.

3.1.9 The scantlings of moonpools and drilling well bulkheads on surface type units and self-elevating units are to comply with Pt 3, Ch 13, 2 Floating structures and subsea buoyant vessels.

3.1.10 The scantlings of circumturret well bulkheads on ship units are to comply with Pt 10 SHIP UNITS.

3.1.11 Where column structures or superstructures extend over the side shell of the unit, the side shell/sheerstrake is to be suitably increased locally at the ends of the structure.

3.1.12 On units fitted with two chines each side the bilge plating should not be less than required for bottom plating. When units are fitted with hard chines the shell plating is not to be flanged, but where the chine is formed by knuckling the shell plating, the radius of curvature, measured on the inside of the plate, is not to be less than 10 times the plate thickness. Where a solid round chine bar is fitted, the bar diameter is to be not less than three times the thickness of the thickest abutting plate. Where welded chines are used, the welding is to be built up as necessary to ensure that the shell plating thickness is maintained across the weld, see also Pt 4, Ch 6, 3.4 Buoys and deep draught caissons 3.4.7.

3.1.13 The plating of swim ends is to have a thickness not less than that required for bottom shell plating.

3.1.14 Where a rounded sheerstrake is adopted, the radius should, in general, be not less than 15 times the plate thickness.

3.1.15 Sea inlets, or other openings, are to have well rounded corners and, so far as possible, are to be kept clear of the bilge radius. Openings on, or near to, the bilge radius are to be elliptical. The thickness of sea inlet box plating is to be the same as the adjacent shell, but not less than 12,5 mm. The ends of stiffeners should in general be bracketed and alternative proposals may be considered.

3.1.16 In general, secondary hull framing is to be continuous and the end connections of stiffeners to watertight bulkheads are to provide adequate fixity and, so far as practicable, direct continuity of strength.

3.1.17 The end connections of secondary hull framing and primary members are to comply with:

• Pt 10, Ch 3, 1 Scantling requirements for ship units; and
• Chapter 8 for other unit types.

3.1.18 The lateral and torsional stability of stiffeners together with web and flange buckling criteria are to be verified in accordance with Pt 4, Ch 5, 3 Buckling strength of plates and stiffeners.

3.1.19 Web frames supporting secondary hull framing are, in general, to be spaced not more than 3,8 m apart when the length, L, is less than 100 m and (0,006L + 3,2) m apart where L is greater than 100 m. For units which are also required to operate aground, see Pt 4, Ch 4, 2 Sea bed-stabilised units.

3.2.1 When the external watertight boundaries of columns, lower hulls and footings are designed with stiffened plating, the minimum scantlings for shell plating, hull framing and web frames, etc., are to comply with Pt 4, Ch 6, 3.3 Self-elevating units 3.3.4, see also Pt 4, Ch 6, 3.2 Column-stabilised and tension-leg units 3.2.3.

3.2.2 The scantlings determined from Pt 4, Ch 6, 3.3 Self-elevating units 3.3.4 are the minimum requirements for hydrostatic pressure loads only and the overall strength is to comply with Pt 4, Ch 4 Structural Unit Types.

3.2.3 Where cross ties are fitted in columns or lower hulls, the scantlings are to comply with Pt 4, Ch 6, 3.3 Self-elevating units 3.3.5 and Pt 4, Ch 6, 3.3 Self-elevating units 3.3.6 taking the head as the pressure head in accordance with Pt 4, Ch 6, 3.3 Self-elevating units 3.3.4 as appropriate. Where cross ties are fitted inside tanks, the requirements of Pt 4, Ch 6, 3.3 Self-elevating units 3.3.4 are also to be complied with.

3.2.4 When the scantlings of primary web frames or girders are determined by a frame analysis or where the boundaries of columns, lower hulls and footings are designed as shells either unstiffened or ring stiffened, the scantlings may be determined on the basis of an agreed analysis, see Pt 4, Ch 1, 2 Direct calculations. The minimum design loads are to be in accordance with Pt 4, Ch 3 Structural Design and the permissible stresses are to comply with Pt 4, Ch 5 Primary Hull Strength. The scantlings are not to be less than required by Pt 4, Ch 6, 3.2 Column-stabilised and tension-leg units 3.2.1.

3.2.5 The minimum scantlings of the external watertight boundaries of the upper hull structure are to comply with Pt 4, Ch 6, 3.3 Self-elevating units 3.3.5.

3.2.6 The shell plating and structure are to be reinforced in way of mooring fairleads, supply boat moorings, towing brackets and other attachments, see also Pt 4, Ch 6, 1 General requirements.

3.2.7 Columns, lower hulls, footings and other areas likely to be damaged by anchors, chain cables and wire ropes, etc., are to be protected or suitably strengthened.

3.2.8 Openings are not permitted in the shell boundaries of columns, lower hulls and footings except when they are closed with watertight covers fitted with closely spaced bolts, see Pt 4, Ch 7 Watertight and Weathertight Integrity and Load Lines.

3.3.1 The minimum scantlings of shell plating are to comply with Pt 4, Ch 6, 3.4 Buoys and deep draught caissons 3.4.7 and the secondary hull framing and primary members are to comply with Pt 4, Ch 6, 3.4 Buoys and deep draught caissons 3.4.7, see also Pt 4, Ch 6, 3.3 Self-elevating units 3.3.4.

3.3.2 The shell plating thickness is to be suitably increased in way of high shear forces in way of drilling cantilevers and other concentrated loads.

3.3.3 The scantlings and arrangements of the boundary bulkheads of leg wells will be specially considered with regard to the maximum forces imposed on the structure, and the permissible stress levels are to comply with Pt 4, Ch 5 Primary Hull Strength. The minimum scantlings are to comply with Pt 4, Ch 6, 7.3 Watertight and deep tank bulkheads 7.3.4 as a tank bulkhead with the load head measured to the upper deck at side. In no case is the minimum plating thickness to be less than 9 mm.

3.3.5 When cross ties are fitted to support shell web frames the scantlings of the web frames are to be determined from Pt 4, Ch 6, 3.4 Buoys and deep draught caissons 3.4.7 and Pt 4, Ch 6, 7.3 Watertight and deep tank bulkheads 7.3.4 and the area and least moment of inertia of the cross tie are to satisfy the following, see also Pt 4, Ch 6, 3.3 Self-elevating units 3.3.6 and Pt 4, Ch 6, 3.3 Self-elevating units 3.3.7:

where

=

one half the vertical distance in metres between the centres of the bottom or deck webs adjacent to the cross tie, see Pt 4, Ch 6, 3.3 Self-elevating units 3.3.5

=

vertical distance from the centre of the cross tie to deck, in metres, see Pt 4, Ch 6, 3.3 Self-elevating units 3.3.5

=

length of cross tie between the toes of the horizontal brackets on the web frames at the cross tie, in metres

=

span of web frames, see Pt 4, Ch 6, 3.3 Self-elevating units 3.3.5

=

least inertia of cross tie cross-section, in cm4

=

area of cross tie, in cm2

r	=	least radius of gyration of cross tie cross-section, in cm
	=

as defined in Pt 4, Ch 3, 3.3 Determination of span point.

Figure 6.3.1 Cross tie construction

Table 6.3.2 Watertight shell boundaries of the upper hull of column-stabilised units and tension-leg units

Items and requirement	Boundaries of lower hull or columns
(1) Shell plating thickness general See also Pt 4, Ch 6, 3.1 General 3.1.5	The greater of the following:
	(a) t = 0,004sf mm
	(b) t = 0,012
	but not less than 7,5 mm
(2) Bottom plating thickness between columns within outside of column shell but not less than two web frame spaces See also Pt 4, Ch 6, 3.1 General 3.1.5	The greater of the following:
	(a) t = 0,004s f + 2,5 mm
	(b) t = 0,012
	but not less than 7,5 mm
(3) Shell stiffeners and primary webs, general	To comply withPt 4, Ch 6, 7.3 Watertight and deep tank bulkheads 7.3.4 using the load head
(4) Shell stiffeners adjacent to columns as defined in (2):
(a) Modulus	Z = 8,5s k x 10-3 cm3
(b) Inertia	I = Z cm4
Symbols
Symbols as defined in Pt 4, Ch 6, 7.3 Watertight and deep tank bulkheads 7.3.4, except as follows:
= load head, in metres, as defined in Pt 4, Ch 6, 7.3 Watertight and deep tank bulkheads 7.3.4 for watertight bulkheads but not less than 6,0 m
= 470 + mm or 700, whichever is the smaller
= s but is not to be taken less than
W = greatest width or diameter of stability column, in metres
NOTES
In no case are the scantlings in way of tanks to be less than the requirements given in Pt 4, Ch 6, 7.3 Watertight and deep tank bulkheads 7.3.4 for tank bulkheads using the load head .

3.3.6 The scantlings of the webs and flanges of cross ties are to be checked for buckling by direct calculation.

3.3.7 Design of end connections of cross ties is to be such that the area of the welding, including vertical brackets, where fitted, is to be not less than the minimum cross sectional area of the cross tie derived from Pt 4, Ch 6, 3.3 Self-elevating units 3.3.5. To achieve this, full penetration welds may be required and thickness of brackets may require further consideration. Attention is to be given to the full continuity of area of the backing structure on the transverses. Particular attention is also to be paid to the welding at the toes of all end brackets on the cross tie.

3.4.1 Where the external watertight hull boundaries are designed with stiffened plating, the minimum scantlings for shell plating, hull framing and web frames supporting framing, etc., are to comply with Table 6.3.5 Watertight shell boundaries of buoys and deep draught caissons.

3.4.2 The scantlings determined from Pt 4, Ch 6, 3.4 Buoys and deep draught caissons 3.4.7 are the minimum requirements for hydrostatic pressure loads only and the overall strength is to comply with Pt 4, Ch 4 Structural Unit Types.

3.4.3 Where the scantlings of primary web frames are determined by a frame analysis or where the boundaries are designed as shells, either unstiffened or ring stiffened, the scantlings are to be determined on the basis of an established analysis using the appropriate design pressure heads as defined in Pt 4, Ch 3 Structural Design. The permissible stresses are to comply with Pt 4, Ch 5 Primary Hull Strength, but the scantlings are not to be less than required by Pt 4, Ch 6, 3.4 Buoys and deep draught caissons 3.4.1.

3.4.4 The shell plating and hull framing are to be reinforced in way of mooring line attachments, mooring fairleads, supply boat moorings, towing brackets and other attachments, see also Pt 4, Ch 6, 1 General requirements.

3.4.5 Areas of the hull which may be damaged by chain cables or wire ropes are to be protected or suitably strengthened.

3.4.6 Where cross ties are fitted to support shell web frames, the scantlings are to comply with Pt 4, Ch 6, 3.3 Self-elevating units 3.3.5 and Pt 4, Ch 6, 3.3 Self-elevating units 3.3.6 taking the head as the pressure head in accordance with Pt 4, Ch 6, 3.4 Buoys and deep draught caissons 3.4.7.

3.4.7 Where cross ties are fitted inside tanks, the requirements of Pt 4, Ch 6, 3.3 Self-elevating units 3.3.4 are to be complied with.

Table 6.3.3 Shell plating self-elevating units

Location	Thickness, in mm, see also Pt 4, Ch 6, 3.1 General 3.1.5
(1) Bottom shell plating	The greater of the following:
See Notes 1, 2 and 4	(a) t = 0,001(0,043L + 10)
	(b) t = 0,0052
(2) Bilge plating (framed) See Note 2	t as for (1)
(3) Side shell plating	(a) Above from base:
See Notes 1, 2, 3 and 4	The greater of the following:
	(i) t = 0,001 (0,059L + 7)
	(ii) t = 0,0042
	(b) At upper turn of bilge (see Note 2): The greater of the following:
	(i) t = 0,001 (0,059L + 7)
	(ii) t = 0,0054
	(c) Between upper turn of bilge and from base:
	The greater of the following:
	(i) t from (b)(i)
	(ii) t from interpolation between (a)(ii) and (b)(ii)
(4) Minimum plating	tm = (6,5 + 0,033L)
Symbols
L, D, , as defined in Pt 4, Ch 1, 5 Definitions
k = steel factor as defined in Pt 4, Ch 2, 1 Materials of construction
s = spacing of secondary stiffeners, in mm
= 470 + mm or 700 mm, whichever is the smaller
= s, but is not to be taken less than
NOTES
1. In no case is the shell plating to be less than tm .
2. When no bilge radius is fitted and the unit is fitted with hard chines, the bottom shell thickness required by (1) is, in general, to be extended up to from base, see Pt 4, Ch 6, 3.1 General 3.1.10.
3. The thickness of side shell need not exceed that determined from (1) for bottom shell when using the spacing of side shell stiffeners.
4. In no case are the scantlings of tanks to be less than the requirements given in Pt 4, Ch 6, 7.3 Watertight and deep tank bulkheads 7.3.4 for tank bulkheads using load head .

Table 6.3.4 Shell framing self-elevating units

Items and location	Modulus
(1) Hull framing, see Note 1
(a) Bottom frames	Z = 11,0s k x 10-3 cm3
(b) Side frames	Z = 8,0s k x 10-3 cm3
(2) Primary members, see Note 1
(a) Bottom web frames supporting framing	Z = 11,0k S x 10-3 cm3
(b) Side web frames supporting framing	Z = 8,0k S x 10-3 cm3
Symbols
D and as defined in Pt 4, Ch 1, 5 Definitions
= load head, in metres, and is to be taken as the distance from the middle of the effective length to a point 1,6 above the keel or to the upper deck at side whichever is the lesser but not less than 0,01L + 0,7
k = steel factor as defined in Pt 4, Ch 2, 1 Materials of construction
= effective length of member, in metres, as defined in Pt 4, Ch 3, 3.3 Determination of span point
s = spacing of frames, in mm
S = spacing or mean spacing of primary members, in metres
NOTES
1. In no case are the scantlings in way of tanks to be less than the requirements given in Pt 4, Ch 6, 7.3 Watertight and deep tank bulkheads 7.3.4 for tank bulkheads using the load head .
2. In no case are the scantlings to be less than the requirements given in Pt 4, Ch 6, 7.3 Watertight and deep tank bulkheads 7.3.4 for watertight bulkheads using the load head .
3. Where frames are not continuous they are to be fitted with end brackets in accordance with Pt 4, Ch 6, 7 Bulkheads or equivalent arrangements provided.

Table 6.3.5 Watertight shell boundaries of buoys and deep draught caissons

Items and requirement	Shell boundaries, see Note 5
(1) Shell plating thickness	t = 0,004sf + 2,5 mm
See also Pt 4, Ch 6, 3.1 General 3.1.5	but not less than 9,0 mm
(2) Hull framing:
(a) Modulus	Z = 8,5s k x 10-3 cm3
(b) Inertia	I = cm4
(3) Primary members: Web frames supporting framing
(a) Modulus	Z = 8,5k x 10-3 cm3
(b) Inertia	I = cm4
Symbols
f = 1,1 – but not to be taken greater than 1,0
= load head in metres measured vertically as follows:
(a) For shell plating the distance from a point one third of the height of the plate above its lower edge to the top of the highest predicted wave in the most unfavourable design situation or to a height 1,0 m above the uppermost deck, whichever is the greater with a minimum of 6,0 m, see Note 3
(b) For hull framing and primary members, the distance from the middle of the effective length to the top of the highest predicted wave in the most unfavourable design situation or to a height 1,0 m above the uppermost deck, whichever is the greater, with a minimum of 6,0 m, see Note 3
k = steel factor as defined in Pt 4, Ch 2, 1 Materials of construction
= effective length of member in metres as defined in Pt 4, Ch 3, 3.3 Determination of span point
s = spacing of frame in mm
S = spacing or mean spacing of primary members, in metres
NOTES
1. In no case are the scantlings in way of tanks to be less than the requirements given in Pt 4, Ch 6, 7.3 Watertight and deep tank bulkheads 7.3.4 for tank bulkheads using the load head .
2. In no case are the scantlings to be less than the requirements given in Pt 4, Ch 6, 7.3 Watertight and deep tank bulkheads 7.3.4 for watertight bulkheads using the load head .
3a. For shell plating of units defined in Pt 3, Ch 13 Buoys, Deep Draught Caissons, Turrets and Special Structures which are designed to follow the wave profile, need not exceed the distance measured from a point one third of the height of the plate above its lower edge to the top of the highest predicted wave in the most unfavourable design situation or to a height 1,0 m above the uppermost deck, whichever is the greater. (But note that t shall not be less than 9,0 mm.)
3b. For hull framing of units defined in Pt 3, Ch 13 Buoys, Deep Draught Caissons, Turrets and Special Structureswhich are designed to follow the wave profile, need not exceed the distance measured from the middle of the effective length to the top of the highest predicted wave in the most unfavourable design situation or to a height 1,0 m above the uppermost deck, whichever is the greater, but shall not be less than the calculated from the shell plating thickness formulation (Pt 4, Ch 6, 3.4 Buoys and deep draught caissons 3.4.7 (1)) that corresponds to the minimum thickness requirement of 9,0 mm.
4. Where frames are not continuous they are to be fitted with end brackets in accordance with Pt 4, Ch 6, 7 Bulkheads or equivalent arrangements provided.
5. The scantlings of shell boundaries derived from this Table are to be suitably increased in way of tanks which cannot be inspected at normal periodic surveys, see Pt 4, Ch 4, 7.10 Corrosion protection.