Section 5 Submersible Hull

5.1.1 Plating thickness for the pressure hull structure should be sufficient in combination with stiffeners (of suitable geometric proportion) to control overall buckling and orient the first failure mode encountered with increasing design pressure to inter-stiffener buckling.

5.1.2 Ring stiffeners of suitable shape "I" or "T" and suitably sized are to have an effective means of load transfer between the frame and the pressure hull plate. Stability of the stiffeners is to be checked against web buckling/frame tripping.

5.1.3 Frame /web tilting can alter the circumferential stress on the pressure hull plate and therefore adequate safety margins are to be ensured in the frame design. The largest value of bending stress in the frame design in conjunction with the worst buckling mode is to be considered.

5.1.4 Stiffeners which are external to the pressure hull plating are to be attached to the plating by full penetration welding (as per internal frame stiffeners) to avoid local stress concentration points.

5.1.5 Common sites for discontinuities such as the junction of hemisphere-cylinder/cone-cylinder/cylinder – bulkhead are to be considered during design for suitable strengthening such as providing local higher thickness inserts to minimise the stress/mean stress levels.

5.1.6 Suitable precautions shall be taken such as not to attach structure and components to the knuckle portion of torispherical head so as to avoid the creation of a stress concentration that could adversely affect the structural performance under load.

5.1.7 Buckling response is sensitive to the geometry of the shell. The greater the deviation of the profile from the design requirement the greater the adverse effect on collapse pressure. Design of the pressure hull structure shall be checked for final approval based on the as-built circularity/out of roundness, and any deformed stiffeners including deformations as a result of any external pressure tests.

5.1.8 Internal structures such as transverse bulkheads, decks and tanks are to be adequately designed against instability and buckling due to collapse of the hull structure to which the components are directly connected/welded. Bulkheads and deck plates should be designed against applicable lateral loads. The effect of attachment across the hull diameter may affect the out of circularity due to the interaction with the welded component resulting in lowered collapse pressure. These sites are to be suitably assessed.

5.1.9 Consideration should be given to the indirect attachment of deck plates to the shell, where the deck would otherwise be subject to in plane loads due to shell deflection under hydrostatic pressure.

5.1.10 Consideration should be given to hull girder bending and the resultant stresses to determine if they are significant with respect to stresses from external pressure.

5.1.11 Viewports shall be designed as per Pt 3, Ch 2 Acrylic Windows and ASME PVHO-1. Design by analysis may have to be carried out for configurations not covered by code. Deflection of the frame at the sealing area is to be assessed against serviceability limits. The same considerations should be applied to other openings penetrations and hatches as required.

5.1.12 Suitable allowances on scantling thickness shall be provided to compensate for the possible corrosion, erosion and wear and tear during service.

5.1.13 Internal tanks that form part of the pressure hull boundary, and are therefore subject to external pressure should be adequately strengthened to prevent failure due to pressurisation.

5.2.1 Analysis shall be carried out to a recognised method from a relevant and recognised international code of pressure vessel design, to determine collapse pressures due to general elastic stability/overall buckling of pressure envelope, symmetric and asymmetric buckling in the interframe areas in conjunction with frame tripping. Additional areas of discontinuities/stress concentration and the effects of external loads should be checked for failure by appropriate analytical methods, where not adequately covered by the pressure vessel codes. The collapse curves used from the pressure vessel code should be an equivalent to the actual material used for hull construction in terms of ultimate and yield strength. A conservative approach should be taken in arriving at the collapse pressure if there are differences in the material properties.

5.2.2 Alternatively the above may be checked by elastic/elastic-plastic FEA, eigenvalue/bifurcation buckling analysis and by elastic-plastic buckling analysis as required, specifically considering the as-built deformed geometry. No strain hardening shall be considered in the elastic-plastic material model.

5.3.1 Noting the likelihood of tensile residual stresses in structure created during the fabrication. Suitable consideration should be given to cyclic stresses and therefore fatigue during the life of the pressure hull structure. Fatigue assessment methods as per recognised design codes may be used to arrive at the cumulative damage ratio at the site of peak stress ranges. The magnitude of residual stresses locked in should be suitably considered. A suitable reduction factor of not less than 2 should be applied to overall fatigue life if the components are not adequately protected against corrosion. Further factors should be considered where the locations are not readily inspectable. FEA may need to be carried out to determine the peak stresses at locations not covered by code rules.

5.4.1 The submersible pressure hulls of metallic construction are to be constructed in accordance with the requirements specified in Ch 13 Requirements for Welded Construction of the Rules for the Manufacture, Testing and Certification of Materials, July 2022, unless more stringent requirements are specified in the applicable design and construction code.

5.4.2 All weld designs for the pressure hull, primary and secondary stiffeners, bulkheads, decks, and other structural members, are to be submitted to LR for approval.

5.4.3 Weld joints of main seam welds and attachment welds shall be staggered and located in such a way to avoid intersection of welds.

5.4.4 All the weld joints within plating and shell cut-out attachments are to be full penetration type with suitable groove/bevel. Penetrations into the shell are to be attached by full penetration groove and fillet weld. The leg length of the fillet should not be the less than the smaller of the thickness of the two parts being joined or as recommended by the applicable code.

5.4.5 Where frames/stiffeners/rings are attached to pressure hull plating by welding, suitable consideration is to be given to the weld profile to avoid stress concentrations. Spacing of the frames should be such that there is access for effective welding and NDE.

5.4.6 Where internal bulkhead/deck plates are to be attached to pressure hull plating, weld types and profiles are to be suitable so as to avoid stress concentrations. Sizing of the weld is to be based on the applicable load with required safety margin.

5.4.7 Caution should be exercised on the use of welds having any form of taper transition and it is recommended that such joints are avoided and replaced by alternative integral design such as forgings etc.

5.5.1 All main seam welds are to be examined by RT (100 per cent), and examined by MT/PT (100 per cent) for surface flaws or indications.

5.5.2 Attachment welds for shell cut-out and frame/stiffeners are to be examined by UT (100 per cent) and examined by MT/PT (100 per cent) for surface flaws or indications.

5.5.3 NDE is to be performed after the completion of fabrication/welding/rework/repair/PWHT, if and where applicable.

5.6.1 Pressure testing is to be carried out as per Pt 3, Ch 1, 12 Fabrication and testing requirements of the Rules, after completion of fabrication/welding and heat treatment, if and where applicable.