Section 4 Cargo tanks for Type G tankers

4.1.1 Pressure vessels intended for Type G tankers are independent tanks of the domed type. Fittings for the cargo piping system are to be mounted on the domes above the open deck (see Pt 5, Ch 13, 3.6 Connections to cargo tanks 3.6.6) All instrumentation and other connections are to be also accessible from the open deck.

4.1.2 The tanks are to be provided with at least one manhole with access from the open deck

4.1.3 The pressure vessels are to comply with Pt 5, Ch 13, 4.2 Cargo tank design to Pt 5, Ch 13, 4.10 Stress relieving independent tanks and, as far as applicable with the requirements of Ch 4 Cargo Containment for independent Type C tanks of the Rules for Ships for Liquefied Gases.

4.2.1 Scantlings based on internal pressure should be calculated as follows:

1. The thickness and form of pressure-containing parts of pressure vessels under internal pressure, including flanges should be determined according to an acceptable standard. These calculations in all cases should be based on generally accepted pressure vessel design theory. Openings in pressure-containing parts of pressure vessels should be reinforced in accordance with an acceptable standard.

2. The design liquid pressure defined in Pt 5, Ch 13, 4.3 Design loads 4.3.2 should be taken into account in the above calculations.

3. The welded joint efficiency factor to be used in the calculation according to Pt 5, Ch 13, 4.2 Cargo tank design 4.2.1(a) is depending on inspection and non-destructive testing requirements. See for guidance the Rules for Liquefied Gases Part E - Tank Types.

4. The maximum volume of a cargo tank is to comply with Pt 4, Ch 4, 3.2 Hold spaces, cargo tanks and service spaces

5. The length to diameter ratio of a cargo tank is not to exceed 7, see Pt 4, Ch 4, 3.5 Special requirements for Type G tankers 3.5.1

4.3.1  General. The independent tanks together with the supports and other fixtures should be designed taking into account proper combinations of the following loads:

• Internal pressure
• External pressure
• Loads on supports

4.3.2  Internal pressure. The internal pressure head P _eq in bars gauge resulting from the design vapour pressure P _o and the liquid pressure P _gd should be calculated as follows:

4.3.3  External pressure. External design pressure loads should be based on the difference between the maximum internal pressure (maximum vacuum) and the maximum external pressure to which any portion of the tank may be subjected simultaneously.

4.3.4 The loads on supports are covered by the Pt 5, Ch 13, 4.6 Supports.

4.4.1 The design temperatures for the calculations and selection of materials is to be as follows:

• The design temperature, T, for calculation purposes is to be not less than 50°C.
• A minimum temperature is to established for the selection of materials, see Pt 5, Ch 13, 5.4 Design temperature for Type G tankers 5.4.1.

4.5.1 For independent tanks the maximum allowable membrane stress to be used in the calculation according to Pt 5, Ch 13, 4.2 Cargo tank design 4.2.1(a) will be specially considered.

4.5.2 For guidance reference is made to the Rules for Ships for Liquefied Gases Part E - Tank Types.

4.6.1 Cargo tanks should be supported by the hull in a manner which will prevent bodily movement of the tanks while allowing contraction and expansion of the tank under temperature variations and hull deflections without undue stressing of the tank and of the hull.

4.6.2 Tank supports are generally to be located in way of the primary structure of the tank and the ship’s hull. Steel seatings are to be arranged, where possible on both the floors and underside of the cargo tank so as to ensure an effective distribution of the transmitted load and reactions into the cargo tanks and ship’s structure.

4.6.3 Suitable supports should be provided to withstand a collision force acting on the tank corresponding to half the weight of the tank and cargo in the forward direction and one quarter of the weight of the tank and cargo in the aft direction without deformation likely to endanger the tank structure.

4.6.4 Anti flotation arrangements should be provided for independent tanks. The anti flotation arrangements should be suitable to withstand an upward force caused by an empty tank in a hold space flooded to T _max + 0.4 m in which T _max is the maximum draught of the ship, without plastic deformation likely to endanger the hull structure.

4.6.5 An adequate clearance is to be provided between the anti flotation chocks and the ship’s hull in all operational conditions.

4.6.6 The support arrangements should be designed for the full weight of the tanks together with holding down arrangements.

4.6.7 The tank seatings shall be capable to keep the tank in place for a total heeling range up to and including the total capsised condition.

4.6.8 The saddles are to be extended to a point of not less than 10º below the horizontal centre line of the pressure vessel.

4.7.1 All welded joints of the shells of independent tank should be of the butt weld, full penetration type.

4.7.2 Manufacture and workmanship are to satisfy the requirements of Ch 13, 1 General welding requirements of the Rules for Materials for Class 1 pressure vessels.

4.7.3 Independent tanks should be subjected to a hydrostatic test or alternatively to a hydro pneumatic test as per Chapter 4, Section Part E - Tank Types for Type C independent tanks of the Rules for Ships for Liquefied Gases.

4.8.1 For independent tanks, inspection and nondestructive testing should be as far as applicable in compliance with Chapter 4, Section Part E - Tank Types of the Rules for Ships for Liquefied Gases and the requirements of Ch 13, 4 Specific requirements for fusion welded pressure vessels of the Rules for Materials for Class 1 pressure vessels.

4.9.1 For pressure vessels no corrosion allowance is generally required if the contents of the pressure vessel are non-corrosive and the external surface is protected by inert atmosphere or by an appropriate insulation with an approved vapour barrier. Paint or other thin coatings should not be credited as protection. Where special alloys are used with acceptable corrosion resistance, no corrosion allowance should be required. If the above conditions are not satisfied, the scantlings calculated according to Pt 5, Ch 13, 4.2 Cargo tank design should be increased as appropriate.

4.10.1 For independent tanks of carbon and carbon-manganese steel, post-weld heat treatment should be performed after welding. The post-weld heat treatment is to conform to the requirements of Ch 13, 4.10 Post-weld heat treatment and Ch 13, 4.11 Basic requirements for post-weld heat treatment of fusion welded pressure vessels in the Rules for Materials. For all carbon and carbon-manganese steel tanks requiring heat treatment, the requirements of Table 13.4.2 Post-weld heat treatment requirements in the Rules for Materials are to be complied with.