Note: Descriptions are shown in the official language in which they were submitted.
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_ACKGROUND OF T~E INVENTION
In rece~t years, the petroleum industry has employed
a number of semi-submersible drilling rigs in latitudes subject
to driftiny icebergs. These icebergs are tracked on radar,
and industry procedures require that a rig cast free of its
mooring system and move off location when an iceberg approaches
within a specified radius distan-e. To maximize drilling time,
it was 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 linear
shaped charge explosive device to sever rigs anchor chains.
5uch a device and method of operation are disclosed in U.S.
Patent Mo. 4,148,257, issued on April 10, 1979 to
Larry M. Orrill et al and assigned to Halliburton Company~
Briefly, the 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 ensure
absence of hull plate damage, this necessitates diver place-
ment or, alternatively, long term submergence of the chargeswith a subsequent reduction in charge detonation reliability.
It is therefore desirable to position the charges above
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the rig fairleads, which positioning permits rapid de-
ployment 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 in-tegrity of the rig legs is necessary
to maintain rig buoyancy, a means to attenuate the
shock wave and gas bubble effects of submerged charge
explosion was necessary.
5UMMARY OF THE INVENTION
The blast attenuator of the present invention
comprises a double-walled structure having an inner
cylindrical shell surrounded by a substantially con-
centric cylindrical outer shell, an air space beillgprovided therebetween. The blast at-tenuator is placed
about the shaped charge explosive device on the anchor
chain, above the fairlead and close to a rig column or
leg. Upon detonation of the charge, the resulting
explosion 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 expan-
sion of the inner shell. The oscillating gas bubble
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formed by the high pressure gaseous detonation products
of the explosive charge is also contained within the
outer shell and redirected suhstantially parallel to the
rig column. While the blast attenuator of the present
invention could be constructed of two continuous cylinders,
capped at each end by annular p'~ates, for ease of installation
the blast attenuators disclosed hereinafter comprise two half
right-circular cylindricaL assemblies hinged together, whi.ch
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 one aspect of the present 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.
In a further aspect of the present invention
there is provided an underwater blast attenuator comprising
a first haLf right-circular cylindrical assembly having a half
cylindrical inner shell, a substantially concentric half
cylindrical ou*er shell, side plates joining the lateral
extremities thereof, end plates joining the ax:iaL extremities
thereof, a second haLf right-circular cyli.ndrical 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
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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.
BRIEF DESCRIPTION OF THE DRA~INGS
The blast attenuator of the present invention will
be more fully understood by reference to the following
detailed description of the preferred embodiments thereof,
taken in conjunction with the appended drawings, in which:
FIG. 1 shows a top elevation of a first preferred
embodiment of the blast attenuator of the present invention,
FIG. 2 is a section taken along line A-A in FIG. 1,
FIG. 3 is an eleva-ion of the hinge assembly of
the embodiment shown in FIG. 1,
FIG. 4 is an elevation of the gastener assembly of
the embodiment shown in FIG. 1,
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FIG. 5 is a schematic sectional elevation of the
blast attenuator of FIG. 1 mounted on an offshore rig;
FIG. 6 is a top elevation of a second preferred
embodiment of the present invention;
FIG. 7 is a section taken along line B-B of FIG. 6;
and
FIG. 8 is a schematic sectional elevation of the
blast attenuator of FIG. 6 suspended from the anchor
chain of an offshore rig.
DETAILED DESCRIPTION OF A
FIRST PREFERRE~D EMBODIMENT
Referring to FIGS. 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 therewithu 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
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plates 22' ancl 24'. Side gusset plates 19' and 21' are
employed to reinforce the iunction of the side plates
18' and 20' with inner shell 14'. A11 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 a-ttenuator 10, assemblies 12
and 12' are joined together by hinge assembly 30, the de-
tail of which is depicted in FIG. 3. Hinge asse~bly
30 is made up of a plurali-ty 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 ad-
jacent flanges 42 and 42' welded to outer shells 16 and16', 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'. se-
tween each set of 1ange gusset plates, hex head cap
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screws 46 are inserted through holes in flanges 42 and
42' and nuts 48 threaded thereon and tightened after
blast attenuator lQ 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 ern-
ployed on a rig, a blast attenuator being used on
each anchor chain fairlead. Fairlead 62 is attached
as shown to rig column 68~in extremel~ 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 un-
shielded, 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
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and rig column 68. However, blast attenuator lO will
contain the explosive force of device 70 as outlined
below. Briefly, when shaped charge cutting device is
detonated, the shoc~ wave ~Jill plastically defor~ inner
shell 14 and 14' radially outwardly aqainst outer shell
16 and 16', which contains the remain~rg undissipated
energy of the shock wave and redirects the oscillating
gas bubble from the charge explosion parallel to the
rig eolumn 68 to avoid damage thereto.
The protection of an elastic structure (in this case,
the hull plates of the ric~ column 68), frc~ underwater
loading requires consideration of two load~n~) mechanis~s.
The initial loading of the hull plates occurs with the
arrival of the explosively generated shock wave at the front
surfaee of the plates, with subsequent diffraction and
refleetion of the wave at the water/steel interface.
The plates will respond with an initial velocity, and
rapidly unload due to the inertial effeets 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.
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Two mechanisms are therefore necessary to reduce
dynamic structural loading of the rig column plates: a
mechanism 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, and solves in a
straightforward manner, both of the above problems. The
shock wave from the charge explosion contacts the inner
shell 14 and 14', of blast attenuator lO. 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 sup-
port 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 re-
maining undissipated kinetic energy without signifi-
cant plastic deformation. Water motion resulting fromthe 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.
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The size and material strength of the blast at-
tenuator 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, He~ogen, T4, the
formula thereof being C3H6N6O6), an inner shell of 30
inches O.D. x 1/2 inch wall thickness x 70 1/2 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 com-
ponents 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 minimum. 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 de-
sign considerations are twofold: that the inner shell
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have adequate plastic strain absorption capability and
that the outer shell is strong enough as a whole (in-
cluding hinge and fastener assemblies) to preclu~e
rupture or complete failure.
DETAILED DESC~IPTION OF A
SECOND PREFERRED EM~ODIMENT
Referring now to FIGS. 6, 7 and 8, a second pre-
ferred 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 ex-
tremities by side plates 118 and 120, and at their ad-
jacent 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.
Unli]ce the top and bottom plates in blast attenuator
10, plates 122 and 124 perform the functions of sus-
pending blast attenuator 110 from the anchor chain,
and guiding the severed chain end from the bottom of the
attenuator 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
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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,
perpendicular to slot 127. At the bottom of blast at-
tenuator 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 togetherl as with assemblies 12 and 12' of blast
a-ttenuator 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 struc-
ture to fastener assembly 40, there being no need therefore
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to describe assemblies 130 and 1~0 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
10 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 11~ and 11~'. After chain 66 is
severed by device 70, blast attenuator 110 remains sus-
pended 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', afterwhich 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.
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It should be ~oted 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 inven-
tion has been described in use with respect to at-tenuat-
ing 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 inven-
tion have been described in detail above, but it is ap-
parent that additions, deletlons and rnodifications to
these preferred embodiments may be made without depart-
ing 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 arrangement, or a double-
shell spherical attenuator could be built. Such ad-
ditions, deletions and modifications would be readily
apparent to one of ordinary skill in the art.
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