Annex 13 – Criteria for Longitudinal Strength of Hull Girder for Double-Hull Oil Tankers

1 General

1.1 These criteria shall be used for the evaluation of the longitudinal strength of the ship's hull girder as required by 9.1.2.

1.2 In order that the ship's longitudinal strength to be evaluated can be recognized as valid, fillet welding between longitudinal internal members and hull envelopes shall be in sound condition so as to keep the integrity of longitudinal internal members with hull envelopes.

2 Evaluation of longitudinal strength

On oil tankers of 130 m in length and upwards and over 10 years of age, the longitudinal strength of the ship's hull girder shall be evaluated in compliance with the requirements of this annex on the basis of the thickness measured, renewed or reinforced, as appropriate, during the renewal survey of the Cargo Ship Safety Construction Certificate or Cargo Ship Safety Certificate (SC renewal survey). The condition of the hull girder for longitudinal strength evaluation shall be determined in accordance with the methods specified in appendix 3.

2.1 Calculation of transverse sectional areas of deck and bottom flanges of hull girder

2.1.1 The transverse sectional areas of deck flange (deck plating and deck longitudinals) and bottom flange (bottom shell plating and bottom longitudinals) of the ship's hull girder shall be calculated by using the thickness measured, renewed or reinforced, as appropriate, during the SC renewal survey.

2.1.2 If the diminution of sectional areas of either deck or bottom flange exceeds 10% of their respective as-built area (i.e. original sectional area when the ship was built), either one of the following measures shall be taken:

.1 to renew or reinforce the deck or bottom flanges so that the actual sectional area is not less than 90% of the as-built area; or
.2 to calculate the actual section of moduli (Z_act) of transverse section of the ship's hull girder by applying the calculation method specified in appendix 1, by using the thickness measured, renewed or reinforced, as appropriate, during the SC renewal survey.

2.2 Requirements for transverse section modulus of hull girder

2.2.1 The actual section moduli of the transverse section of the ship's hull girder, calculated in accordance with 2.1.2.2, shall satisfy either of the following provisions, as applicable:

.1 for ships constructed on or after 1 July 2002, the actual section moduli (Z_act) of the transverse section of the ship's hull girder calculated in accordance with the requirements of paragraph 2.1.2.2 shall be not less than the diminution limits determined by the Administration, taking into account the recommended diminution limit adopted by IMO resolution MSC.108(73): 90% of the required section modulus for new buildings specified in IACS' Unified Requirements S7 (C=1.0Cn shall be used for the purpose of this calculation) or S11, whichever is the greater; or
.2 for ships constructed before 1 July 2002, the actual section moduli (Z_act) of the transverse section of the ship's hull girder calculated in accordance with the requirements of 2.1.2.2 shall meet the criteria for minimum section modulus for ships in service required by the Administration, provided that in no case Z_act shall be less than the diminution limit of the minimum section modulus (Z_mc) as specified in appendix 2.

Appendix 1 – Calculation Criteria of Section Moduli of Midship Section of Hull Girder

1 When calculating the transverse section modulus of the ship's hull girder, the sectional area of all continuous longitudinal strength members shall be taken into account.

2 Large openings, i.e. openings exceeding 2.5 m in length or 1.2 m in breadth, and scallops, where scallop welding is applied, shall always be deducted from the sectional areas used in the section modulus calculation.

3 Smaller openings (manholes, lightening holes, single scallops in way of seams, etc.) need not be deducted, provided that the sum of their breadths or shadow area breadths in one transverse section does not reduce the section modulus at deck or bottom by more than 3% and provided that the height of lightening holes, draining holes and single scallops in longitudinals or longitudinal girders does not exceed 25% of the web depth, for scallops of maximum 75 mm.

4 A deduction-free sum of smaller opening breadths in one transverse section in the bottom or deck area of 0.06(B –Σb) (where B = breadth of ship, Σb = total breadth of large openings) may be considered equivalent to the above reduction in sectional modulus.

5 The shadow area shall be obtained by drawing two tangent lines with an opening angle of 30°.

6 The deck modulus is related to the moulded deck line at side.

7 The bottom modulus is related to the baseline.

8 Continuous trunks and longitudinal hatch coamings shall be included in the longitudinal sectional area provided they are effectively supported by longitudinal bulkheads or deep girders. The deck modulus shall then be calculated by dividing the moment of inertia by the following distance, provided this is greater than the distance to the deck line at side:

where

distance from neutral axis to top of continuous strength member;

distance from top of continuous strength member to centreline of the ship;

x and y shall be measured to the point giving the largest value of y _t.

9 Longitudinal girders between multi-hatchways shall be considered by special calculations.

Appendix 2 – Diminution Limit of Minimum Longitudinal Strength of Ships in Service

1 The diminution limit of the minimum section modulus (Z_mc) of oil tankers in service is given by the following formula:

where:

L	=	Length of ships. L is the distance, in metres, on the summer load waterline from the fore-side of stem to the after side of the rudder post, or the centre of the rudder stock if there is no rudder post. L shall not be less than 96%, and need not be greater than 97%, of the extreme length on the summer load waterline. In ships with unusual stern and bow arrangement, the length L may be specially considered.
B	=	Greatest moulded breadth in metres.
C _b	=	Moulded block coefficient at draught d corresponding to summer load waterline, based on L and B. C _b shall not to be taken less than 0.6. C _b = c = 0.9c_n c _n = c _n = c _n =
k	=	material factor, e.g. k = 1.0 for mild steel with yield stress of 235 N/mm² and over k = 0.78 for high-tensile steel with yield stress of 315 N/mm² and over k = 0.72 for high-tensile steel with yield stress of 355 N/mm² and over.

2 Scantlings of all continuous longitudinal members of the ship's hull girder based on the section modulus requirement in 1 above shall be maintained within 0.4L amidships. However, in special cases, based on consideration of type of ship, hull form and loading conditions, the scantlings may be gradually reduced towards the end of 0.4L part, bearing in mind the desire not to inhibit the ship's loading flexibility.

3 However, the above standard may not be applicable to ships of unusual type or design, e.g. for ships of unusual main proportions and/or weight distributions.

Appendix 3 – Sampling Method of Thickness Measurements for Longitudinal Strength Evaluation and Repair Methods

1 Extent of longitudinal strength evaluation

Longitudinal strength shall be evaluated within 0.4L amidships for the extent of the hull girder length that contains tanks therein and within 0.5L amidships for adjacent tanks which may extend beyond 0.4L amidships, where tanks means ballast tanks and cargo tanks.

2 Sampling method of thickness measurement

2.1 Pursuant to the requirements of section 2.5, transverse sections shall be chosen such that thickness measurements can be taken for as many different tanks in corrosive environments as possible, e.g. ballast tanks sharing a common plane boundary with cargo tanks fitted with heating coils, other ballast tanks, cargo tanks permitted to be filled with seawater and other cargo tanks. Ballast tanks sharing a common plane boundary with cargo tanks fitted with heating coils and cargo tanks permitted to be filled with seawater shall be selected where present.

2.2 The minimum number of transverse sections to be sampled shall be in accordance with annex 2. The transverse sections chosen shall be located where the largest thickness reductions:

.1 are suspected to occur; or
.2 are revealed from deck and bottom plating measurements prescribed in 2.3.

The transverse sections chosen shall be clear of areas which have been locally renewed or reinforced.

2.3 At least two points shall be measured on each deck plate and/or bottom shell plate required to be measured within the cargo area in accordance with the requirements of annex 2.

2.4 Within 0.1D (where D is the ship's moulded depth) of the deck and bottom at each transverse section which shall be measured in accordance with the requirements of annex 2, every longitudinal and girder shall be measured on the web and face plate, and every plate shall be measured at one point between longitudinals.

2.5 For longitudinal members other than those specified in 2.4 to be measured at each transverse section in accordance with the requirements of annex 2, every longitudinal and girder shall be measured on the web and face plate, and every plate shall be measured at one point between longitudinals.

2.6 The thickness of each component shall be determined by averaging all of the measurements taken in way of the transverse section on each component.

3 Additional measurements where the longitudinal strength is deficient

3.1 Where one or more of the transverse sections are found to be deficient in respect of the longitudinal strength requirements given in this annex, the number of transverse sections for thickness measurement shall be increased such that each tank within the 0.5L amidships region has been sampled. Tank spaces that are partially within, but extend beyond, the 0.5L region, shall be sampled.

3.2 Additional thickness measurements shall also be performed on one transverse section forward and one aft of each repaired area to the extent necessary to ensure that the areas bordering the repaired section also comply with the requirements of the Code.

4 Effective repair methods

4.1 The extent of renewal or reinforcement carried out to comply with this annex shall be in accordance with 4.2.

4.2 The minimum continuous length of a renewed or reinforced structural member shall be not be less than twice the spacing of the primary members in way. In addition, the thickness diminution in way of the butt joint of each joining member forward and aft of the replaced member (plates, stiffeners, girder webs and flanges, etc.) shall not be within the substantial corrosion range (75% of the allowable diminution associated with each particular member). Where differences in thickness at the butt joint exceed 15% of the lower thickness, a transition taper shall be provided.

4.3 Alternative repair methods involving the fitting of straps or structural member modification shall be subject to special consideration. In considering the fitting of straps, it shall be limited to the following conditions:

.1 to restore and/or increase longitudinal strength;
.2 the thickness diminution of the deck or bottom plating to be reinforced shall not be within the substantial corrosion range (75% of the allowable diminution associated with the deck plating);
.3 the alignment and arrangement, including the termination of the straps, shall be in accordance with a standard recognized by the Administration;
.4 the straps shall be continuous over the entire 0.5L amidships length; and
.5 continuous fillet welding and full penetration welds shall be used at butt welding and, depending on the width of the strap, slot welds. The welding procedures applied shall be acceptable to the Administration.

4.4 The existing structure adjacent to replacement areas and in conjunction with the fitted straps, etc., shall be capable of withstanding the applied loads, taking into account the buckling resistance and the condition of welds between the longitudinal members and hull envelope plating.