GB2110179A - Blast attenuator - Google Patents

Abstract

An underwater blast attenuator (10) comprises an inner, plastically deformable shell (14, 14') surrounded by an outer shell (16, 16') adapted to contain the remaining explosive force of the blast to be attenuated after the inner shell has been plastically deformed, there being an air space (26, 26') between the inner and outer shells. Such attenuators are placed around explosive devices to shield adjacent structures from the explosive forces. One use is around explosive devices on anchor chains of offshore rigs, the explosive devices being utilised to sever the anchor chains. <IMAGE>

Description

SPECIFICATION Blast attenuator This invention relates to a blast attenuator for underwater explosive charges.

In recent years, the petroleum industry has employed a number of semi-submersible drilling rigs in latitudes subject to drifting icebergs. These icebergs are tracked on radar, and industry procedures require that a rig be cast free of its mooring system and moved off location when an iceberg approaches within a specified distance. To maximize drilling time, it is desirable that a quick, preferably instantaneous method be developed to free rigs from their mooring systems. One such instantaneous release method involves the use of a linear shaped charge explosive device to sever rigs' anchor chains. Such a device and method of operation are disclosed in U.S.

Patent No. 4,148,257, to which reference should be made for further details. Briefly, a shaped charge cutting device, which is electrically detonated from the rig floor, is clamped to each of the rig anchor chains. While it is possible to place these devices on the anchor chains at a distance from the rig to avoid hull plate damage, this necessitates diver placement or, alternatively, long term submergence of the charges with a subsequent reduction in charge detonation reliability. it is therefore desirable to position the charges above the rig fairleads, which positioning permits rapid deployment of the charges, maximum reliability of charge detonation and minimum probability of chain fouling in the fairlead after severance.Such positioning, however, places the charges underwater very close to the legs of the rig, exposing the plating of the legs to the destructive forces caused by charge explosion. As structural integrity of the rig legs is necessary to maintain rig bouyancy, some means of attenuating the shock wave and gas bubble effects of submerged charge explosion is necessary.

We have now devised a blast attenuator which can be used for this purpose.

According to the invention, there is provided an apparatus for attenuating the destructive effects of an underwater explosion, comprising: an inner shell substantially surrounding the source of said explosion; an outer shell substantially surrounding said inner shell; and a water-free space between said inner and outer shells.

The invention includes an underwater blast attenuator comprising: a first half right-circular cylindrical assembly having a half cylindrical inner shell, a substantially concentric half cylindrical outer shell, side plates joining the laterial extremities thereof, end plates joining the axial extremities thereof; a second half right-circular cylindrical assembly having a half cylindrical inner shell, a substantially concentric half cylindrical outer shell, side plates joining the lateral extremities thereof, end plates joining the axial extremities thereof; said first and second assemblies defining first and second substantially air - tight chambers; hinge means joining said two assemblies in facing relationships at one side; and fastener means adapted to join said two assemblies at the side opposite said hinge means.

The blast attenuator of the present invention comprises a double - walled structure having an inner, preferably cylindrical, shell surrounded by a preferably substantially concentric cylindrical outer shell, an air space being provided therebetween. The attenuator is placed about the shaped charge explosive device on an anchor chain (or the like), above the fairlead and close to a rig column or leg. Upon detonation of the charge, the resulting explosion preferably plastically deforms the inner shell and is contained by the outer shell, the air space inhibiting transmission of the explosive shock wave to the outer shell and permitting substantially free plastic expension of the inner shell.The oscillating gas bubble formed by the high pressure gaseous detonation products of the explosive charge is also preferably wholly contained within the outer shell and redirected substantially parallel to the rig column. While the blast attenuator of the present invention could, for example, be constructed of two continuous cylinders, capped at each end by annular plates, for ease of installation the preferred blast attenuators hereinafter described comprise two half right - circular cylindrical assemblies hinged together, which assemblies are then closed like a clam shell about the chain and explosive cutting device and fastened together opposite the hinge location. The hinge and fastener assemblies are designed equal in strength to the hoop strength of the outer shell of the blast attenuator.

In order that the invention may be more fully understood, reference is made to the accompanying drawings, in which: Figure 1 shows a top elevation of a first preferred embodiment of the blast attenuator of the present invention; Figure 2 is a section taken along line A-A of Figure 1; Figure 3 is an elevation of the hinge assembly of the embodiment shown in Figure 1; Figure 4 is an elevation of the fastener assembly of the embodiment shown in Figure 1; Figure 5 is a schematic sectional elevation of the blast attenuator of Figure 1 mounted on an offshore rig anchor chain; Figure 6 is a top elevation of a second preferred embodiment of the present invention; Figure 7 is a section taken along line B-B of Figure 6; and Figure 8 i a schematic sectional elevation of the blast attenuator of Figure 6 suspended from the anchor chain of an offshore rig.

Referring to Figures 1 and 2, blast attenuator 10 comprises two half right - circular cylindrical assemblies 12 and 12'. Assembly 12 comprises inner shell 14 and concentric outer shell 16, shells 14 and 16 being joined at their adjacent lateral extremities by side plates 18 and 20, and at their adjacent axial extremities by top and bottom plates 22 and 24. Side gusset plates 19 are employed to reinforce the junction of side plate 18 and inner shell 14, while side gusset plates 21 are used at the junction of side plate 20 therewith. In a similar fashion, assembly 12' comprises inner shell 14' and outer shell 16', joined together at their lateral adjacent extremities by side plates 18' and 20', and at their adjacent axial extremities by top and bottom plates 22' and 24'.

Side gusset plates 19' and 21' are employed to reinforce the junction of the side plates 18' and 20' with inner shell 14'. All components of assemblies 12 and 12' are preferably formed of steel, joined together by welds to form airtight chambers 26 and 26'. Assemblies 12 and 12' define attenuator bore 28.

On one side of blast attenuator 10, assemblies 12 and 12' are joined together by hinge assembly 30, the detail of which is depicted in FIG. 3. Hinge assembly 30 is made up of a plurality of hinge blocks 32 and 32', backed with hinge gusset plates 34 and 34', alternately welded to outer shell 16 and outer shell 16'. Steel rod 36, threaded at both ends, is run through the aligned bores of hinge blocks 32 and 32', after which nuts 38 and 38' are threaded thereon.

Due to the presence of hinge assembly 30 in blast attenuator 10, assemblies 12 and 12' may be opened and closed like a clam shell.

On the opposite side of blast attenuator 10 from hinge assembly 30, fastener assembly 40 comprises adjacent flanges 42 and 42' welded to outer shells 16 and 16', respectively, and backed by flange gusset plates 44 and 44', which are welded to their respective flanges 42 and 42' and outer shells 16 and 16'.

Between each set of flange gusset plates, hex head cap screws 46 are inserted through holes in flanges 42 and 42' and nuts 48 threaded thereon and tightened after blast attenuator 10 has been installed around an anchor chain.

Referring now to FIG. 5, blast attenuator 10 is schematically shown in position on an offshore rig.

Blast attenuator 10 is secured to the bellmouth 60 of fairlead 62. The securing means, which have not been shown, may comprise bolting together flanges on blast attenuator 10 and bellmouth 60, clamp mechanisms or other securing means well known to one of ordinary skill in the art. The spokes 64 of fairlead 62 guide anchor chain 66 as it is played out in setting the anchor for the rig. Of course, a plurality of anchors are employed on a rig, a blast attenuator being used on each anchor chain fairlead. Fairlead 62 is attached as shown to rig column 68 in extremely close proximity, in many instances a distance of less than three feet. Shaped charge cutting device 70 is secured to anchor chain 66, detonating wires (not shown) being run to the rig deck.The detonation of such a device 70, if unshielded, can cause severe damage to rig column 68, particularly due to the fact that fairlead 62 is below the water line and the explosive force of the device 70 is transmitted through the water between the device 70 and rig column 68. However, blast attenuator 10 will contain the explosive force of device 70 as outlined below. Briefly, when shaped charge cutting device is detonated, the shock wave will plastically deform inner shell 14 and 14' radially outwardly against outer shell 16 and 16', which contains the remaining undissipated energy of the shock wave and redirects the oscillating gas bubble from the charge explosion parallel to the rig column 68 to avoid damage thereto.

The protection of an elastic structure (in this case, the hull plates of the rig column 68), from underwater loading requires consideration of two loading mechanisms. The initial loading of the hull plates occurs with the arrival of the explosively generated shock wave at the front surface of the plates, with subsequent diffraction and reflection of the wave at the water/steel interface. The plates will respond with an initial velocity, and rapidly unload due to the inertial effects of the adjacent water. Reloading subsequent to the shock wave occurs until the plates' kinetic energy is converted to strain potential energy. Following the initial shock loading of the hull plates, the motion of the adjacent water reloads the plates again due to the expansion and oscillation of the high pressure gas bubble comprising gaseous products of charge detonation.This latter effect eventually ceases as inherent buoyancy carries the bubble to the water surface.

Two mechanisms are therefore necessary to reduce dynamic structural loading of the rig column plates: a r; nggSchanism for reducing initial shock strength; and a second mechanism for redirecting the motion of the water due to gas bubble expansion.

The present invention addresses both of the above problems. The shock wave from the charge explosion contacts the inner shell 14 and 14', of blast attenuator 10. While a single - layer shell would be "transparent" to the shock wave, e.g., the shock wave force would be transmitted from the water on one side of the shell to that on the other, the air chambers 26 and 26' do not provide support for shell 14 and 14'; hence the relatively ductile steel of inner shell 14 and 14' plastically deforms radially outward against the outer shell 16 and 16', dissipating the shock wave energy in the deformation process by converting it to plastic strain energy. The outer shell 16 and 16' is designed to absorb any remaining undissipated kinetic energy without significant plastic deformation.Water motion resulting from the oscillating gas bubble is redirected by the still-intact outer shell 16 and 16' to a path substantially parallel with the rig column, therefore eliminating the dynamic pressure of the water motion.

The size and material strength of the blasfatte- nuator 10 are determined by the size of the blast to be attenuated. For example, to attenuate a 3200 gram/foot linear shaped charge in the configuration shown in the aforesaid U.S. Patent No. 4,148,257, the explosive being 1.162 pounds of RDX explosive (Cyclotrimethylenetrinitramine, Hexahydro -1, 3, 5- Trinitro - 5 - Triazine, Cyclonite, Hexogen, T4, the formula thereof being C3H6N606), an inner shell of 30 inches O.D. X 1/2 inch wall thickness x 701/2 inches height and an outer shell of 42 inches O.D. and the same wall thickness and height as the inner shell may be employed. The inner shell is preferably formed of ASTM grade A-53 structural steel due to its high ductility and allowable elongation, the outer shell being of the same grade steel. Other steel plate components may be preferably ASTM A-36, SAE 1020 steel. As noted before in discussing the design, the attenuator is made in two hinged and fastened together sections, the hinge pin being at least 160 KSI steel and bolts employed being grade 8 mini mum. Welding of all components is preferably effected using multiple passes and a low hydrogen electrode. Of course, both sections of the attenuator are tested for air tightness. The key design considerations are twofold: that the inner shell have adequate plastic strain absorbtion capability and that the outer shell is strong enough as a wholy (including hinge and fastener assemblies) to preclude rupture or complete failure.

Referring now to FIGS. 6,7 and 8, a second preferred embodiment of the blast attenuator of the present invention will be hereafter described.

Blast attenuator 110, like blast attenuator 10, comprises two half right - circular cylindrical assemblies, in this instance designated as 112 and 112'. Assembly 112 comprises inner shell 114 and outer shell 116, shells 114 and 116 being joined at their adjacent lateral extremities by side plates 118 and 120, and at their adjacent axial extremities by top and bottom plates 122 and 124. Side gusset plates 119 and 121 are employed to reinforce the side plate/inner shell junctions. Unlike the top and bottom plates in blast attenuator 10, plates 122 and 124 perform the functions of suspending blast attenuator 110 from the anchor chains, and guiding the severed chain end from the bottom of the attenuator after severance after severance.Accordingly, top plate 122 is of half circular, rather than half - annular shape, with a slot 127 cut therein of slightly larger cross - section than a half - section of the clain from which blast attenuator 110 will be suspended. Bottom plate 124, while half-annular as in the case of bottom plate 24, has a smaller inner diameter which acts as a part of a chain guide described hereafter. On the underside of top plate 122, a plurality of suspension gusset plates 123 are circumferentially spaced about the inner periphery of inner shell 114, bracing top plate 124 to assist in the suspension of blast attenuator 110 from the anchor chain. Chain alignment tabs 125 are also secured to the underside of top plate 122, perpendicularto slot 127.At the bottom of blast attenuator 110, half-frustoconical shaped chain guide 129 is welded to the inner periphery of inner shell 114 and the inner edge of bottom plate 124.

Assembly 112' is a mirror image of assembly 112, all components corresponding to those of assembly 112, all parts of both assemblies being formed of steel and welded together, as with assemblies 12 and 12' of blast attenuator 10.

Assemblies 112 and 112' define attenuator bore 128, and air chambers 126,126',131 and 131'.

On one side of blast attenuator 110, assemblies 112 and 112' are joined by hinge assembly 130, identical in design and structure to hinge assembly 30. Similarly, fastener assembly 140 is identical in design and structure to fastener assembly 40, there being no need therefore to describe assemblies 130 and 140 in detail. After blast attenuator 110 has been placed around an anchor chain, fastener assembly 140 is secured.

Referring now to FIG. 8, blast attenuator 110 is schematically shown suspended from an anchor chain 66 in position over the bellmouth 60 of fairlead 62 attached to rig column 68 of an offshore rig. This position, while higher than that of blast attenuator 10, is still below the waterline. Link 66a of chain 66 is partially within the slot formed when slots 127 and 127' are placed together. Link 66b, perpendicular to 66a, is held in place by alignment tabs 125 and 125', so that chain 66 and shaped charge cutting device 70 suspended thereon, are centered within blast attenuator 110. Such centering is desirable so that the force of exploding shaped charge cutting device 70 is equally distributed around inner shell 114 and 114'.After chain 66 is severed by device 70, blast attenuator 110 remai s suspended from the top portion of the chain, while the portion immediately below the cut is guided out of blast attenuator 110 by chain guide 129 and 129', after which the cut portion passes through fairlead 62, and the rig is freed for movement away from any approaching danger.

Blast attenuator 110, like blast attenuator 10, has eliminated charge to rig column 68.

It should be noted that the air chambers of both blast attenuators described herein provide buoyancy to the attenuators, making them easier to handle underwater.

While the blast attenuator of the present invention has been described in use with respect to attenuating the explosive force of anchor chain cutters, it is to be understood that such use is by way of example and not limitation. The present invention may be used to attenuate explosive force in an underwater environment whenever it is desirable to minimize damage to structures adjacent an exploding charge.

For example, it may be employed in salvage operations, or in removing unwanted temporary structural members of an offshore rig after it has been placed on site. These and other uses will be readily apparent to one of ordinary skill in the art.

Several preferred embodiments of the present invention have been described in detail above, but it is apparent that additions, deletions and modifications to these preferred embodiments may be made without departing in spirit and scope from the invention as claimed. For example, as noted previously, two circular cylinders may be used as shells, rather than employing hinges and fasteners to effect a clam shell arrangements, or a double-shell spherical attenuator could be built. Such additions, deletion and modifications would be readily apparent to one of ordinary skill in the art.

Claims (22)

CLAIMS:

1. An apparatus for attenuating the destructive effects of an underwater explosion, comprising: an inner shell substantially surrounding the source of said explosion; an outer shell substantially surrounding said inner shell; and a water-free space between said inner and outer shells.

2. Apparatus according to claim 1, wherein said inner shell is plastically deformable in response to said explosion.

3. Apparatus according to claim 1 or 2, wherein said inner shell and said outer shell are both substantially cylindrical and are substantially concentric.

4. Apparatus according to claim 1 or 2, wherein said inner shell comprises two half cylindrical sections and said outer shell comprises two half cylindrical sections, each inner sections being affixed to, but radially separated from, an outer section so as to form a half right-circular cylindrical assembly, the two of said assemblies when joined together forming a right-circular cylindrical assembly.

5. Apparatus according to claim 4, wherein said half right-circular cylindrical assemblies are joined at one side by hinge means, and at the other side by fastener means.

6. Apparatus according to claim 4 or 5, wherein said half right-circular cylindrical sections of said outer shell comprise a material possessing sufficient hoop strength to contain the remaining force of said explosion without substantial deformation after said inner shell has been plastically deformed.

7. Apparatus according to claim 5, wherein said hinge means and said fastener means are of substantially the same tensile strength as the hoop strength of said outer shell.

8. Apparatus according to claim 7, wherein all components thereof are formed of steel.

9. An underwater blast attenuator comprising: a first half right-circular cylindrical assembly having a half cylindrical inner shell, a substantially concentric half cylindrical outer shell, side plates joining the lateral extremities thereof, end plates joining the axial extremities thereof; a second half right-circular cylindrical assembly having a half cylindrical inner shell, a substantially concentric half cylindrical outer shell, side plates joining the lateral extremities thereof, end plates joining the axial extremities thereof; said first and second assemblies defining first and second substantially air-tight chambers; hinge means joining said two assemblies in facing relationships at one side; and fastener means adapted to join said two assemblies at the side opposite said hinge means.

10. Apparatus according to claim 9, wherein said inner shells are of a plastically deformable material.

11. Apparatus according to claim 9 or 10, wherein said inner shells are braced at their said junctures with said side plates by a plurality of side gusset means.

12. Apparatus according to claim 9, 10 or 11, wherein each of said end plates is of half-annular shape, the inner diameter of which is substantially the same as said inner shell diameter, and the outer diameter of which is substantially the same as said outer shell diameter.

13. Apparatus according to claim 9 or 10, wherein said end plates at the top axial extent of said shells are of half-circular cross section, and having a diametrically oriented substantially equal length slot cut in each of said top end plates, said slots being contiguous when said fastener means are fastened.

14. Apparatus according to claim 13, further comprising a plurality of suspension gusset means secured to the underside of said top end plates and the innerwall of said inner shells.

15. Apparatus according to claim 9 or 10, wherein said bottom end plates are of half-annular shape, the inner diameter of which is substantially less than said inner shell diameter, and the outer diameter of which is substantially the same as said outer shell diameter.

16. Apparatus according to claim 15 further comprising half-frustoconical chain guide means affixed to the inside of said shell and the inner diametrical extent of each of said bottom end plates, thereby defining third and fourth substantially airtight chambers.

17. Apparatus according to claim 9, wherein said inner shells are plastically deformable in response to said underwater blast, and said outer shells possess sufficient hoop strength to contain the remaining force of said blast without substantial deformation after said inner shells have been piastically deformed.

18. Apparatus according to claim 17, wherein said hinge means and said fastener means are of substantially the same tensile strength as the hoop strength o- said outer shells.

19. Apparatus according to claim 18, wherein all components thereof are formed of steel.

20. Apparatus for attenuating the destructive effects of an underwater explosion substantially as herein described with reference to Figures 1 to 4, or Figures 6 and 7 of the accompanying drawings.

21. An anchor chain or the like to which is attached an explosive device around which is disposed attenuating apparatus as claimed in any preceding claim.

22. An anchor chain or the like as claimed in claim 21 substantially as herein described with reference to Figure 5 or 8 of the accompanying drawings.