Section 10 Design features and loading factor

10.1.1 The design of external attachments and shell/hull penetrations subjected to external loads are to be given careful consideration in order to ensure that the chamber structure is not overstressed under the combined pressure and external loads.

10.1.2 Unless otherwise agreed, the unfactored accelerations of the submersible as a whole, due to collisions or other emergencies, should be assumed to be not less than 3 g_n in the horizontal direction. When a submersible/diving bell is designed to be slung or jacked up and down during launching, recovery or storage, then upward and downward accelerations should be assumed to be not less than 2 g_n. Generally the sea state for man-submersible handling systems should not exceed Sea State4. Special consideration will be given to cases where diving operations service in more severe sea state is envisaged for manned diving operations.

10.1.3 When the effect of impact loads, sudden control forces and similar loads are being calculated, the resulting accelerations of the submersible as a whole should be such as to give dynamic balance.

10.1.4 Viewports and windows of manned submersibles are to be arranged in positions suitable for the control of the submersible. Where control can be shared between the pilot and another crew member, then both pilot and crew members should be provided with viewports having similar views to one another in the forward and one other direction (generally downwards in the case of submersible craft).

10.1.5 All diving bells and compression chambers are to be fitted with viewports to allow observation of the occupants from the outside.

10.1.6 Tanks forming an integral part of the hull of a submersible are generally to be designed to be suitable for a hydraulic test pressure of 1.5 times the maximum differential pressure for which they are designed to be used in service.

10.1.7 Attention is drawn to the danger of leaving hatches, etc., open when a submersible is being launched, towed, or recovered, and consideration should be given to the introduction of design features to minimize such an occurrence. It is presumed that the operating instructions will prohibit the opening of surface access hatches during launching and recovery operations. Where appropriate, alarm systems should be provided to avert this kind of situations.

10.1.8 Submersibles and chambers designed for atmospheric internal pressure are to be provided with means to accommodate under/over pressure on return to the surface.

10.1.9 Transfer under pressure systems such as chamber/bell mating systems, diver evacuation system, food and equipment locks are to be provided with safety interlocks which prevent inadvertent uncontrolled loss of pressure. Also doors between such compartments should not be able to be opened until pressures have been equalized. Interlock should prevent the lock or mating trunk being pressurized unless the clamp or door is properly secured.

10.1.10 Hatches' manway can be designed in line with the guidelines of Appendix 1-6(g) of ASME Sect VIII Div 1, or the appropriate requirements of the adopted national or international design code/standard. Care is to be taken so that the centreline of the spherically dished head passes through the centroid of the flange. O-ring if applicable is to be located at the most appropriate radius of the flange. The connection of the dished head to flange shall include fillet(s) of radius not less than 10mm.

10.1.11 The retainer ring and the fasteners for the viewports shall be fabricated from materials that are compatible with the viewport flanges. The retainer ring and the associated fastening arrangement shall be designed with an adequate safety factor based on UTS, and a design pressure forcing the window against the retainer ring shall not be less than 5 psi(g) if there is no higher pressure acting during operation. ASME PVHO-1 may be used for guidance.

10.1.12  Diving bells, pressure vessels and hard structure of Diving Systems subjected to greater external pressure than internal pressure, the Designer shall provide evidence that the structure has sufficient collapse strength to withstand maximum operating pressure (including a factor of safety).