The invention relates to an inner explosion resistant membrane door for a marine vessel according to the preamble of claim 1.

Such an inner membrane door for a marine vessel is known from EP-B- 1144783 and is depicted in Figures 1, 2 and 3. In Figure 1 the known inner membrane door 31 is shown comprising a first plate 32 serving as fixed frame with an opening 34 enclosed by a peripheral edge 33. A door 37 is connected pivotally to this first plate 32 by two hinges 35, 36 such that with its peripheral zone 38 the door 37 can co-act sealingly with said peripheral edge 33. The first plate 32 carries a number of peripherally arranged clamps 39, which clamps 39 are simultaneously rotatable by a collective operating mechanism 40 with an operating handle 41 and through rotation can co-act with the peripheral surfaces 42 of corresponding non round continuous holes 43 in door 37. For each clamp 39 the peripheral surfaces 42 can have a contact surface 44 inclining relative to the main plane of the inner membrane door 31 in the closed situation, which continuous holes 43 have a form such that a clamp 39 can pass

therethrough. The door 37 of the known inner membrane door for a marine vessel further comprises a second plate or door leaf 45 made of steel which is received in a framework 46, which framework co-acts with the peripheral edge 33 in the closed situation of the inner membrane door 31. The clamps 39 are arranged on rotation shafts 47. Continuous holes 43 in door 37 are correspondingly formed slotted holes. The peripheral edge 33 is deformable under the influence of an air pressure load acting on the inner membrane door in the closed position such that the whole inner membrane door consisting of both the first plate with the peripheral edge and the door can be pressed out of the main plane it occupies when at rest. By means of handle 41 the rotation shaft 47 of each clamp can be rotated via

transmission arms 50 between a closing position and a free position, this via a system of mutually coupled rods generally designated 49. As shown in Figures 2 and 3, the peripheral edge 33 comprises a relatively thin (6-9 mm), sheet-metal strip 51 having the general shape of a wave, with at least one wave. Strip 51 extends wholly around door opening 34 and is welded to the first plate 32 and the remaining part 52 of peripheral edge 33. Figures 2 and 3 shows the situation in which the door 45, 46 is placed in closed position and depicts hooking means present on respectively the peripheral edge 33 and the door 37, which hooking means only enter into hooking co action when a certain minimal deformation of the inner membrane door 31 in closed position is exceeded as a result of a pressure load acting thereon. The hooking means comprise prismatic beams 57, 58 respectively forming part of respectively the peripheral edge and framework 46. These beams have undercut surfaces 59, 60 respectively directed toward each other which, in the case of the described substantial deformation under the influence of an air pressure load, can enter into hooking co-action with each other. This known inner membrane door for a marine vessel meets high standards of resistance to explosion.

It is an object of the invention relative to this prior art to provide an inner membrane door for a marine vessel which meets the higher standards of explosion resistance.

This object is achieved with an inner membrane door for a marine vessel having the characterizing features of claim 1. The invention is for one part based on the insight that the explosion resistance of the inner membrane door for a marine vessel can be improved if fracture resistance of the inner membrane door for a marine vessel is increased in case of combined blast loading and fragment perforations. The invention is in particular based on the insight that in case an exploding warhead, shell or bomb explodes in a inner compartment of a marine vessel closed by an inner membrane door fragments of the exploding warhead reach the inner membrane door before significant deformation of the door occurs due to the pressure load and these fragments can penetrate the inner membrane door forming holes therein. Thereafter the pressure load of the explosion causes significant deformation of the inner membrane door and, fractures or cracks can be created or propagate in the inner membrane door starting from the holes formed by the fragments, which fractures can strongly decrease the explosion resistance of the inner membrane door as (part of the) door leaf 45 can be blown open. The invention is for the other part based on the insight that in case the second plate or door leaf is made of stainless steel this fracture creation or growth can be strongly reduced. Please note, that using stainless steel for an inner door of a marine vessel is not an obvious choice in particular since stainless is more expensive than normal steel grade (e.g. Fe235) and further since inner doors are not susceptible to corrosion as they are not subjected to the aggressive marine environment. However, after numerous studies and experiments the inventors unexpectedly found that by using a second plate or door leaf of stainless steel fracture resistance under air pressure load of the inner membrane door, in particular the second plate or door leaf, was increased substantially in case of fragment holes.

In an embodiment of an inner membrane door for a marine vessel according to the invention the second plate or door leaf is made of, austenitic stainless steel of the AISI 300 series which are weldable, possess the proper strength values and have the required large toughness. This aspect of the invention is based on the insight that in particular a second plate or door leaf made of austenitic stainless steel provides increased explosions resistance while meanwhile being easy to integrate in an inner membrane door. It is in particular advantageous when the second plate or door leaf is made of austenitic stainless steels of the 300 series from AISI or S3xxxx series from ASTM/UNS. Preferably, the second plate or door leaf is made of, austenitic stainless steel, especially 316L low carbon, austenitic stainless steel.

In a still further embodiment of an inner membrane door for a marine vessel according to the invention the second plate or door leaf has a thickness of 4-5 mm. This is in contrast to EP-B- 1144783 which teaches that the second plate or door leaf requires a thickness of a maximum of 3 mm, the present invention is further based on the insight that for most kinds of explosions the fracture and explosion resistance of an inner membrane door for a marine vessel is adequate when the second plate or door leaf has a thickness of 4-5 mm.

The invention has been described above with reference to an inner membrane door for a marine vessel known from EP-B- 1144783, which known inner membrane door for a marine vessel is shown in the Figures. In the drawing:

Fig. 1 shows a perspective view of an inner membrane door for a marine vessel known from EP-B- 1144783 in opened position; and

Figs. 2 and 3 show a cross-section through a part of an inner membrane door for a marine vessel known from EP-B- 1144783 in closed position, elucidating the hook structure.

When compared to the inner membrane door for a marine vessel known from known EP-B- 1144783 the construction of the inner membrane door for a marine vessel according to the invention is substantially identical, except for the fact that according to the invention the second plate or door leaf 45 is made of stainless steel. As discussed above using stainless steel for an inner door of a marine vessel is not an obvious choice in particular since stainless is more expensive than normal steel grade (e.g. Fe235) and further since inner doors are not susceptible to corrosion. However, after numerous studies and experiments the inventors unexpectedly found that by using a second plate or door leaf 45 of stainless steel, preferably weldable, austenitic stainless steel, in particular austenitic stainless steels of the 300 series from AISI or S3xxxx series from ASTM/UNS explosion resistance in particular fracture resistance of the inner door structure can be increased

substantially. Preferably , the second plate or door leaf is made low carbon, austenitic stainless steel, especially 316L low carbon, austenitic stainless steel. With such a second plate or door leaf explosion resistance in particular fracture resistance of the inner membrane door can be increased

substantially. Please note that the chemical compositions of 300 series from AISI or S3xxxx series from ASTM/UNS steels, and in particular 316L are generally known in the technical field. In particular the second plate or door leaf 45 has a thickness of 4-5 mm. For most kinds of explosions such a thickness provides adequate fracture and explosion resistance to an inner membrane door for a marine vessel.