Section 5 Buoyancy and stability

5.1.1 The designer is to specify the type of buoyancy required, whether submerged beneath the effective wave height or floating. Drawings and calculations are to be submitted to show the make-up of the weight and centre of gravity and also the make-up of the buoyancy of the capsule/chamber in the operational condition. These calculations are to be verified by a weighing test on each completed capsule/chamber and by a flotation test in calm water on each new design or configuration of capsule/ chamber. Where the position of the vertical centre of gravity is of special importance an inclining experiment may be necessary in order to verify the calculated centre of gravity.

Whether or not these criteria operate independently, or in combination, should be indicated in the calculation. The stability recommendation a) should apply irrespective of whether or not the wheel traction and propulsion power (or their equivalent) will permit the operation of the vehicle on such gradients.

5.2.2 If a manned submersible vehicle is not fitted with an escape trunk extending above the water surface, then consideration should be given to incorporating into the design some means of releasing the pressure hull from the remaining structure, or of using jettisonable ballast so that the pressure hull may float to the surface in case of an emergency, and float in such a position so that the crew can disembark.

5.3.1 All submersible craft are to have sufficient reserve of buoyancy and stability to enable a properly trained crew and launching staff to launch, operate and recover them in all sea states and conditions for which they are intended, including emergency recovery conditions with jettisonable ballast dropped.

5.3.2 Hatches

5.3.3 Passenger submersibles should be fitted with a means of speedily altering trim and ballast dependent on the number of passengers embarked on each tour.

5.3.4 During submergence and surfacing, all submersible craft should have sufficient stability and sufficiently sensitive means of adjusting buoyancy to enable a properly trained crew to maintain effective control over the craft as a whole. Positive stability should be maintained in the event of a power failure. Attention should be paid to the possibility of air becoming trapped below suspended types of submersible craft during launching.

5.3.5 Self-propelled types of submersible craft should be capable of maintaining neutral buoyancy at any predetermined depth down to the maximum diving depth over the full range of water salinities in which they are intended to be able to operate. Neutral buoyancy should be maintained for all service conditions of loading; both without and with passengers, when passengers are carried, and at all speeds of operation.

5.3.6 Towed unmanned submersible craft should normally be arranged to have slight positive buoyancy at zero velocity, relying on dynamic forces to maintain their required depth of submergence. Towed manned craft should operate with either slight positive buoyancy or neutral buoyancy.

5.3.7 Suitable ballast arrangements should be fitted to all submersible craft and special attention is to be paid to the attachment of the system to the hull of the craft. Where ballast can be jettisoned, the arrangements should be such that the system can be operated with the craft at a trim and/or angle of heel at least 20° in excess of the maximum anticipated under normal operating service conditions, and the craft should retain positive stability during the ascent, and on the surface, with the ballast jettisoned. In the case of a diving bell the angle of heel considered in the design should be at least 10° in excess of the maximum anticipated under normal operating conditions.

5.3.8 The stability, while submerged, of the submersible craft in pitch and roll should be such that movements of the crew, passengers or equipment within the submersible, or the exit or entrance of crew or passengers and work operations carried out by the submersible, will not result in an uncontrollable change in attitude exceeding 5°, or such small angle as can be accepted by any hydroplanes or other control surfaces, without causing unexpected stall or negative incidence of attack within the normal operating range of the controls. In particular, such movements should not result in an uncontrolled dive.

5.3.9 Designers and builders should ensure, by means of calculation, model experiments and/or full scale trials, that submersible craft remain controllable in pitch, yaw, heave, roll, direction, etc., for all craft speeds; and also for all anticipated current speeds when the craft is anchored, moored or suspended. Particular care should be taken to avoid pitch instability on self-propelled craft, roll and directional instability on towed craft and oscillations and rotations of suspended craft. See also Pt 4, Ch 2, 5.3 Submersible craft 5.3.8.

5.3.10 Where submersible craft are designed to operate also as habitats or submersible vehicles, they are to comply with these Rules.

5.3.11 Passenger submersibles should have at least two means of emergency return to the surface and the capability of remaining on the surface in a stable condition.

5.4.1 Submersible habitats intended for saturation diving to depths in excess of 10 m should be so arranged that the submersible as a whole, the pressure hull, or the hyperbaric escape chamber can be released from the sea bed and float to the surface in an emergency. This facility need not be applicable where the habitat is manned only when a submersible craft, with pressurized access and sufficient accommodation for all crew and passengers, is in attendance.

5.4.2 Diving bells whose emergency ascent is initiated by the release of ballast at its maximum service weight and with its trunk flooded, must exhibit a positive buoyancy equal to at least 3 per cent of its displacement at maximum operating depth. In these circumstances, the bell should have sufficient stability to maintain a substantially upright position after release of ballast.

5.4.3 Where a habitat is arranged with more than three legs, then it should remain stable if the configuration of the sea bed is such that only three legs (irrespective of which three) are in contact with the ground in all currents and at all values of orbital wave motions for which the submersible is classed to operate. In the event of subsidence of the sea bed, settling of one or more legs or scouring of the sea bed in way of the legs, the habitat should remain stable for angles of heel up to 25°.

5.5.1 The habitat should have sufficient reserve buoyancy and stability to allow it to be towed, together with the means of altering the buoyancy so that it may be controlled during emplacement on the sea bed. The negative buoyancy after emplacement may be increased by the use of additional anchoring, having regard to tide flows, scouring, external loads, etc. The habitat should be stable when on the sea bed, even when subsidence may occur under one or more legs, and not impose unacceptable loads on any associated equipment. For this purpose model testing should be carried out and the results submitted for consideration.

5.6.1 Tests are to be carried out to prove the buoyancy and stability of the completed submersible to the satisfaction of the Surveyors. Results and hydrostatic curves are to be submitted for consideration.

5.6.2 When diving bells and submersibles are fitted with releasable ballast weights, a shallow water buoyancy test, with ballast released, should be carried out. For atmospheric submersible units, this test should be achieved with the access hatches open to ensure that stability is maintained on the surface. It is recommended-that the lifting wire remains attached to the submersible during this test.