Seagoing cargo ships for carrying dry bulk goods, such as forest products <BR> <BR> (pulp, wood, cellulose, etc. ) require easy access to their cargo holds for loading and unloading, e. g. by crane mounted grabs, but must also have watertight covers to those holds to prevent the cargo becoming wetted and spoiled by precipitation or seawater in rough (green seas) conditions. Conventionally, the cargo holds are provided with hatches extending for most of the length and width of the ship which are covered by hatch covers whilst the ship is at sea and removed for loading and unloading. The hatch covers are typically flat and made of steel plate with relatively complex framing and stiffening structures. A crane is provided on the ship or dockside for removal and replacement of the hatch covers.

Existing designs of hatch covers are relatively heavy and complex structures. Because of their complexity their construction involves a large number of welds and results in many points that are prone to fatigue and corrosion as well as allowing water or ice build-up. Hatch cover failure has been identified as a cause of marine accidents in some cases.

Accordingly it is an aim of the present invention to provide an improved hatch cover, for example that is lighter, simpler in construction and/or more easily protected from corrosion.

According to the present invention, there is provided a hatch cover having an arcuate longitudinal profile and comprising a sandwich structure having upper and lower metal plates and an intermediate layer of a compact plastic material bonded to said metal plates so as to transfer shear forces therebetween.

The sandwich structure plates used in forming the hatch cover have increased stiffness as compared to steel plates of comparable thickness and avoid or reduce the need to provide stiffening elements. This results in a considerably

simpler structure with fewer welds leading to both simplified manufacture and a reduction in the area vulnerable to corrosion. Further details of sandwich plate structures suitable for use in the present invention can be found in US Patent 5,778, 813 and British Patent Application GB-A-2 337 022. The intermediate layer may also be a composite core as described in British Patent Application No. GB 2 355 957.

Preferably, the arcuate profile is a circular arc with a radius r such that 1. 15*1 < r 1. 30*1, where 1 is the length of the hatch cover in the longitudinal direction.

The profiled weather hatch cover of the invention is particularly useful for use on seagoing cargo ships and can maximize the internal volume of the cargo hold for dry goods (minimises ullage) such as forest products (pulp, wood, cellulose, etc. ). The hatch cover of the invention also provides space for non-fitting packages, which are normally stored on top of conventional flat hatch covers, and so eliminates cargo lashing costs. Lashing of non-fitting packages may place restrictions on the in-hold cargo carrying capacity since the additional mass resting on the hatch covers will negatively affect the hydrodynamic stability of the ship.

A typical cargo ship may require 10 hatch covers to enclose a 50m long by 10m wide cargo hold opening. The hatch cover of the present invention can meet the geometric requirements and provides the required structural performance for such use. All exposed surfaces are entirely smooth (no external framing) thus eliminating the build-up of water and ice or snow and any resulting corrosion associated with trapped water. The complexity of standard maintenance and inspection procedures are greatly reduced. The profiled hatch cover of the invention has excellent impact resistance, superior dampening characteristics (effectively dampens high frequency vibrations), and provides effective sound insulation (deadens sound like wood).

The hatch cover of the invention can readily meet specified design loads and relevant design criteria for a Type'B'ship with load line length (LL) of 65m

and Type (a) hatch covers, which are defined by the International Load Line Convention (1966), as steel plated hatch covers used for cargo ships not designed for carrying liquid cargoes and stiffened by webs or stiffeners, secured by clamping devices, and with weather tightness achieved by means of gaskets. Based on the relevant criteria the profiled weather hatch cover of the invention can provide a design with a weight approximately 15-20% less than typical all-steel flat hatch cover designs.

The present invention will be described further below with reference to the following description of exemplary embodiments and the accompanying schematic drawings, in which: Figure 1 shows a cargo vessel having hatch covers according to the present invention ; Figure 2 is a perspective view of a hatch cover according to the present invention ; Figure 3 is side elevation of the hatch cover of Figure 2; Figure 4 is a vertical cross-section of the hatch cover of Figure 2; Figure 5 is a vertical cross-section of part of the hatch cover of Figure 2 showing the arrangements for mounting the hatch cover to the coaming top plate; Figure 6 shows a stack of hatch covers according to the present invention; Figure 7 is an enlarged cross-sectional view showing an alternative arrangement for mounting the hatch covers to the coaming top plate; Figure 8 is an enlarged cross-sectional view of an arrangernent for clamping the hatch covers in place; Figures 9 and 10 show an arrangement of pads for receiving vertical forces; Figure 11 is a cross-sectional view showing a sealing arrangement between adjacent hatch covers; Figure 12 is a cross-sectional view of a part of a modified hatch cover according to the present invention; and

Figures 13 to 18 show successive steps in a method of manufacturing the hatch cover according to the present invention.

In the various drawings, like parts are denoted by like reference numerals.

Figure 1 shows a cargo vessel 1 having its cargo holds covered by hatch covers 2 according to the invention. A crane 3 is provided for removing the hatch covers and the removed hatch covers can be stacked conveniently, as shown.

The hatch cover 2 is shown in greater detail in Figures 2 to 4. The hatch cover is rectangular in plan, with a length 1 sufficient to span the width of the cargo hatch. The width w is chosen so that a convenient number of hatch covers covers the full length of the cargo hatch. The hatch cover may, for example have a length 1 of about 10m and a width w of about 5m. In profile, the hatch cover is arcuate, for example conforming to a circular arc. For example, the circular profile may have an inner plate radius of 12. 0m and an outer plate radius of 12.15m ; this then provides an additional 36m3 of enclosed volume above the coaming top plate per hatch cover. The clear height h at mid-span is about 1150mm from the coaming top plate to the inner cover plate.

The hatch cover arch is constructed, for example, of SPS 5-140-5, which consists of two 5mm steel plates 10,11 separated by a 140 mm thick voided core layer 12, such that the total section depth d is 150mm. The hatch cover has a centre of gravity at approximately 785mm above the coaming top plate. Figure 4 is a vertical cross-section taken across the circular arc and illustrates the composition of the core :- this consists of a form comprising alternating rigid foam regions 13 and solid elastomer core regions 14. Approximately 72% of the core by volume consists of rigid foam while 28% consists of elastomer. Bulbflat profiles 18 maintain the inner (bottom) and outer (top) plates 10,11 at a constant distance and transfer loads to the coamings.

Figure 5 shows the edge detail of the hatch cover 2 and its manner of mounting on the coaming top plate 20. The hatch cover 2 has, spaced along the edge of the underside, sockets 15, with a resilient lining forming a gasket (e. g. of

vulcanised rubber), for engagement with studs 21 provided on the coaming top plate 20. On the upper side, a horizontal landing plate 16 having the same width is provided for receiving the sockets 15 of a hatch cover stacked above.

A stacked configuration of 5 hatch covers 2-1 to 2-5 is shown in Figure 6; this has a maximum height of about 2200mm. Stacking cones, clamping devices, and pads to transfer the vertical forces to the coaming top plate are provided on either side of the hatch cover as shown in Figures 7,8, 9 and 10. Lifting connections (not shown) are provided on the external edges of the coaming sections to allow the covers to be lifted into place or stacked by the gantry crane 3 as needed.

As shown in Figure 7, stacking cones 17a, 17b are provided to assist in locating hatch covers when stacking and ensuring the stack is properly aligned.

The stacking cones 17a, 17b are fixed to the outer edge of the hatch covers; several spaced apart on each side. On one side, the fixed side, the stacking cones 17a have a conical recess in their lower surface that engages a conical projection 22 on the coaming top plate 20 or the stacking cone 17a of a hatch cover below it in a stack.

On the free side, the stacking cone is effectively halved, so that the recess is open on one side and butts against the projection 23 on the coaming top plate or the stacking cone 17b of the hatch cover below it in a stack. The hatch covers are therefore fixed on two directions on the fixed side but only one on the free side.

Figure 8 shows details of a clamping mechanism for retaining the hatch covers in place. A hook 25 is fixed to the edge of the hatch cover 2 and engaged by a rod projecting through the coaming top plate 20. Beneath the coaming top plate 20, a manually operated clamp 26 is provided to tension the rod 24 and pull the hatch cover 2 down onto the coaming top plate 2.

Figures 9 and 10 show an arrangement of pads for transferring vertical forces, i. e. bearing the weight of the hatch cover when in place.

The shallow arch geometry of the profiled hatch cover efficiently supports loads normal to the plated surface in compression across the width. The use of a structural sandwich plate system to form the hatch cover distributes the loads

evenly to the structural elements reducing stress concentrations that are normally associated with stiffened steel plate construction. The outward thrust of the arch is transferred to the vulcanized rubber gasket and the vertical pads and stacking cones along the longitudinal edges of the hatch cover. With the hatch cover clamped in place, the gaskets will provide sufficient lateral restraint under all normal working conditions. Under extreme (overload) situations the stacking cones and vertical pads will resist the increased lateral loads.

Figure 11 illustrates a cross joint for sealing adjacent hatch covers. A flexible, vulcanized-rubber backing-sling 30 with gaskets is simply laid along the join and tensioned with suitable clamps at one or both ends. Given the circular arch profile of the hatch cover, the sling 30 exerts a constant pressure along its full length to create a watertight seal across the joint.

A variation on the hatch cover of the invention, to maximize the advantages of the cross joint shown in Figure 11, has a modified coaming section 16'as illustrated in Figure 12. In this variant, the weight of each stacked hatch cover rests entirely on the stacking cones and the landing plate is omitted. The circular arch of the top plate continues to the edge of the profiled hatch cover to simplify the sealing details required between adjacent covers. The vertical plate that formed the back of the coaming section has been replaced with a folded plate.

Backing bars welded to this plate provide a landing surface for the top faceplate 10 and the appropriate weld preparation for a butt weld.

A fabrication process for the hatch cover according to the invention is illustrated in Figures 13 to 18. To maintain dimensional accuracy and to facilitate the handling of thin plates a mandrel (not shown) should be constructed to support the radius of the bottom plate 11 (inner hatch cover plate). Once the completed hatch cover 2 is lifted off the mandrel, the dead load (self weight) will be transferred from the mandrel to the hatch cover to form the final configuration illustrated in Figure 3. The profile of the mandrel should account for the vertical and transverse deflections of the hatch cover from a fully supported position on the mandrel to a position where it is suspended by the gantry crane.

Thus the first step (Figure 13) is to place the bottom plate 11 on the mandrel. The bottom plate may be welded together from two half plates lla, llb in situ. Next (Figure 14), the core is subdivided into six cavities along the circular arc by the longitudinal and transverse bulbflat profiles 18,19 which are welded onto the bottom plate 11 and will support the top plate 10. Each of the outer cavities has a total volume of 1. 132m3 and the two inner cavities have volumes of 1. 332m3. Pre-fabricated coaming sections 15 (Figure 15) are attached and pre- fabricated rigid foam sections 13 are inserted (Figure 16). Then (Figure 17), the top plate 10 can be welded to the bulb flat profiles 18,19 ; again this can be formed in a number of convenient sections 10a, b, c. Finally (Figure 18), the landing plates 16 and the final details (e. g. lifting points, hooks, etc. ) in the coaming sections are welded in place and elastomer is injected to form the solid sections 14. Since approximately 72% of each volume is occupied by rigid foam, the volumes of elastomer required for injection are 0. 298m3 and 0. 330m3, respectively. Elastomer injection for each cavity can be completed in approximately 132s. The hatch cover can be moved 10-15 minutes following the casting when the injection and venting ports are sealed and the temporary support is removed. The elastomer is fully cured after 12 hours.

The upper and lower metal plates 10,11, and other metal parts of the hatch covers, are preferably structural steel (Lloyd's Grade A (or equivalent) with a minimum yield stress (so) of 235 MPa and a minimum tensile strength (su) of 400 MPa), as mentioned above, though may also be aluminium, stainless steel or other structural alloys in applications where lightness, corrosion resistance or other specific properties are essential. The metal should preferably have a minimum yield strength of 240MPa and an elongation of at least 10%.

The intermediate layer should preferably have a modulus of elasticity, E, of at least 250MPa, most preferably 275MPa, at the maximum expected temperature in the environment in which the member is to be used. In ship building applications this may be 100 °C.

The ductility of the elastomer at the lowest operating temperature is preferably greater than that of the metal layers, which is about 20%. A preferred value for the ductility of the elastomer at lowest operating temperature is 50%.

The thermal coefficient of the elastomer should also be sufficiently close to that of the steel so that temperature variation across the expected operating range, and during welding, does not cause delamination. The extent by which the thermal coefficients of the two materials can differ will depend in part on the elasticity of the elastomer but it is believed that the thermal expansion coefficient of the elastomer may be about 10 times that of the metal layers. The coefficient of thermal expansion may be controlled by the addition of fillers to the elastomer.

The bond strength between the elastomer and metal layers is preferably at least 0.5, most preferably 6, MPa over the entire operating range. This is preferably achieved by the inherent adhesiveness of the elastomer to metal but additional bond agents may be provided.

Additional requirements if the ramp is to be used in a ship building application, include that the elastomer must be hydrolytically stable to both sea and fresh water. The elastomer may therefore essentially comprise a polyol (e. g. polyester or polyether) together with an isocyanate or a di-isocyanate, a chain extender and a filler. The filler is provided, as necessary, to reduce the thermal coefficient of the intermediate layer, reduce its cost and otherwise control the physical properties of the elastomer. Further additives, e. g. to alter mechanical properties or other characteristics (e. g. adhesion and water or oil resistance), and fire retardants may also be included.

Whilst an embodiment of the invention has been described above, it should be appreciated that this is illustrative and not intended to be limitative of the scope of the invention, as defined in the appended claims. In particular, the dimensions given are intended as guides and not to be prescriptive.