6 Openings

6.1 Openings and scallops in stiffeners

6.1.1 Figure 11 shows examples of air holes, drain holes and scallops. In general, the ratio of a/b, as defined in Figure 11, is to be between 0.5 and 1.0. In fatigue sensitive areas further consideration may be required with respect to the details and arrangements of openings and scallops.

6.1.2 Openings and scallops are to be kept at least 200 mm clear of the toes of end brackets, end connections and other areas of high stress concentration, measured along the length of the stiffener toward the mid-span and 50 mm measured along the length in the opposite direction, see Figure 12. In areas where the shear stress is less than 60 percent of the permissible stress, alternative arrangements may be accepted.

Figure 11 : Examples of air holes, drain holes and scallops

The details shown in this figure are for guidance and illustration only.

Figure 12 : Location of air and drain holes

6.1.3 Closely spaced scallops or drain holes, i.e. where the distance between scallops/drain holes is less than twice the width b as shown in Figure 11, are not permitted in stiffeners contributing to the longitudinal strength. For other stiffeners, closely spaced scallops/drain holes are not permitted within 20% of the stiffener span measured from the end of the stiffener. Widely spaced air or drain holes may be permitted provided that they are of elliptical shape or equivalent to minimise stress concentration and are cut clear of the welds.

6.2 Openings in primary supporting members

6.2.1 General

Manholes, lightening holes and other similar openings are to be avoided in way of concentrated loads and areas of high shear. In particular, manholes and similar openings are to be avoided in high stress areas unless the stresses in the plating and the panel buckling characteristics have been calculated and found satisfactory.

Examples of high stress areas include:

• Vertical or horizontal diaphragm plates in narrow cofferdams/double plate bulkheads within one-sixth of their length from either end.
• Floors or double bottom girders close to their span ends.
• Primary supporting member webs in way of end bracket toes.
• Above the heads and below the heels of pillars.

Where openings are arranged, the shape of openings is to be such that the stress concentration remains within acceptable limits.

Openings are to be well rounded with smooth edges.

6.2.2 Manholes and lightening holes

Web openings as indicated below do not require reinforcement

• In single skin sections, having depth not exceeding 25% of the web depth and located so that the edges are not less than 40% of the web depth from the faceplate.
• In double skin sections, having depth not exceeding 50% of the web depth and located so that the edges are well clear of cut outs for the passage of stiffeners.

The length of openings is not to be greater than:

• At the mid-span of primary supporting members: the distance between adjacent openings.
• At the ends of the span: 25% of the distance between adjacent openings.

For openings cut in single skin sections, the length of opening is not to be greater than the web depth or 60% of the stiffener spacing, whichever is greater.

The ends of the openings are to be equidistant from the cut outs for stiffeners.

Where lightening holes are cut in the brackets, the distance from the circumference of the hole to the free flange of brackets is not to be less than the diameter of the lightening hole.

Openings not complying with this requirement are to be reinforced according to [6.2.3].

6.2.3 Reinforcements around openings

Manholes and lightening holes are to be stiffened according to this requirement, except where alternative arrangements are demonstrated as satisfactory, in accordance with the analysis methods described in Ch 7.

On members contributing to longitudinal strength, stiffeners are to be fitted along the free edges of the openings parallel to the vertical and horizontal axis of the opening. Stiffeners may be omitted in one direction if the shortest axis is less than 400 mm and in both directions if length of both axes is less than 300 mm. Edge reinforcement may be used as an alternative to stiffeners, see Figure 13.

Figure 13 : Web plate with openings

In the case of large openings in the web of PSMs (e.g. where a pipe tunnel is fitted in the double bottom), the secondary stresses in PSMs are to be considered for the reinforcement of these openings.

Where no FE analysis is performed, this may be carried out by assigning an equivalent net shear sectional area to the PSM obtained, in cm², according from the following formula:

where:

l_1-n50, l_2-n50 : Net moments of inertia, in cm^4, of deep webs (1) and (2), respectively, with attached plating around their neutral axes parallel to the plating.

A_1-n50, A_2-n50 : Net shear sectional areas, in cm², of deep webs (1) and (2), respectively, taking account of the web height reduction by the depth of the cut out for the passage of the ordinary stiffeners, if any.

: Shear span, in m, of deep webs (1) and (2) as defined in Ch 3, Sec 7, [1.1.2].

Deep web (1) and (2) are defined in Figure 14.

Figure 14 : Large openings in the web of primary supporting members

6.3 Openings in the strength deck

6.3.1 General

Openings in the strength deck are to be kept to a minimum and spaced as far as practicable from one another and from the ends of superstructures. Openings are to be located as far as practicable from high stress regions such as side shell platings, hatchway corners, or hatch side coamings.

6.3.2 Small opening location

Openings are generally to be located outside the limits as shown in Figure 15 in dashed area, defined by:

• The bent area of a rounded sheer strake, if any, or the side shell.
• e = 0.25 (B - b) from the edge of opening.
• c = 0.07 + 0.1b or 0.25b, whichever is greater.

where:

b : Width, in m, of the hatchway considered, measured in the transverse direction, see Figure 15.

: Width, in m, in way of the corner considered, of the cross deck strip between two consecutive hatchways, measured in the longitudinal direction, see Figure 15.

Transverse distance between the above limits and openings or between hatchways and openings as shown in Figure 15 is not to be less than:

• g₂ = 2 a₂ for circular openings.
• g₁ = a₁ for elliptical openings.

Transverse distance between openings as shown in Figure 16 is not to be less than:

• 2 (a₁ + a₂) for circular openings.
• 1.5 (a₁ + a₂) for elliptical openings.

where:

a₁ : Transverse dimension of elliptical openings, or diameter of circular openings.

a₂ : Transverse dimension of elliptical openings, or diameter of circular openings.

a₃ : Longitudinal dimension of elliptical openings, or diameter of circular openings.

Longitudinal distance between openings is not to be less than:

• (a₁+ a₃) for circular openings.
• 0.75 (a₁ + a₃) for elliptical openings and for an elliptical opening in line with a circular one.

If the opening arrangements do not comply with these requirements, the hull girder longitudinal strength assessment is to be carried out by subtracting such opening areas, see Ch 5, Sec 1, [1.2.11].

Figure 15 : Position of openings in strength deck

Figure 16 : Elliptical and circular openings in strength deck