Note: Descriptions are shown in the official language in which they were submitted.
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1 ~OORING SYSTEM FOR
. SEMISUBMERSIBLE DRILLING PLATFORM
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~ Field o~ the Invention
;'~' 20 This invention relates to a mooring system for a sem~-
. 8ubmersible drilling plat~orm or drill ship, and moxe particularly, -
.. i8 concerned with a mooring system w~ich can be detached and
~ stored on the ooean floor when not in use. :~ :
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Since the discovery of oil and gas in the arctic regions
. ~ off the north ~lope of Alaska and the northern shores of Canada,.~ there has developed a need ~or off-shore drilling equipment
~. capable of operating in the icy waters of the arctic region. . ~;
;~...... 3 Th~ has given ri~e tD the developmen~ of semisubmersible ~ .
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drilling platforms and drillships capable of operating in ice-
covered waters. One such platform, described in Canadian Patent
No. 1,021,582, issued November 29, 1977, inventor Harry Smith
entitled Ice Cutter for Monopod Drilling Platform, and assigned
to the same assignee as the present invention, is a monopod type
structure in which a single supporting column extends between
a submerged hull and an elevated platform. Drilling takes place
through a moon pool extending from the deck of the platform
down through the center of the single supporting column and
opening in the bottom of the submerged hull. A comminuting ice
cutting mechanism rotates about the outside of the supporting
column to permit the platform to be moved through the ice or to
i permit the platform to remain stationary in the presence of
moving ice. .;.
In conventional practise, a semisubmersible drilling
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; platform is maintained in position over the drill site by an `~
, anchoring system in which a plurality of anchors are secured to
the ocean floor at spaced points around the drill site. Anchor ;
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chains and cables from the anchors are attached to and controlled ;
,~ 20 by mooring winches on the platform so that, with changes in `
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wind direction, currents and other forces acting on the platform
to shift its position, the tension on the various cables can be
adjusted to maintain the position of the platform substantially
fixed during the drilling operation. The anchors are initially
set in place by means of a workboat which drops each of the
anchors at the widely-spaced selected anchor points. This
must be done during the short summer season when the surface
;, waters are relatively free of ice.
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However, with the development of a platfonm capable of
. operating in ice-covered waters and capable of being moved from
: location to locs~ion through ice-covered waters, the conven-
tional mooring practice~ described above have not proved adequate.
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,SummarY of the Xnventisn
.
The present invention i~ directed to a~ improved mooring
system for a semisubmersible drilling platform which operates in ..
ice-covered wa~ers. The mooring system of the preæent invention
10 p~rmits the anchors to be laid a~ a drill site during a time : :
when the sur~ace water i~ relatively free from ice while .~ .
permitt~ng the cables to be easily attached or detached from the .. -
semisubmersible platform at any desired subsequent.time even
when the drill site later becomes covered wi~h ice.
In brief, this is accomplished by providing a mooring
8ystem which includes a sel~-powered mooring ring that acts as
both a guide and support for mooring cables attached to widely-
paced anchors on the ocean ~loor. The mooring ring includes
bsllast tanks and compressed air tanks~ wi~h remotely controlled
valves which control the buoyancy of the ring by admi~ting
controlled amounts of air into the ballast tanks from the
compres~ed air tanks. The ring i5 detachably secured ~o the : ;
~:` platform and operates as a guide for ~he cables which are in
turn detachably secured to drawworks on the platform. The
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.~ ~ ring can be raised and lowered between the ocean floor and the
bottom of ~he submerged plat~orm structure, permit~ing ~he ring
. and cable to be detached and loweret to the ocean floor when
.: not i~ use and later raised and attached to the bo~tom of the
.... platform below the ice and associated drawwork~ when the plat-
. 3 fonm i8 moved on location.
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:` In accordance with the present invention there is
provided a mooring system for anchoring a semisubmersible
. drilling platform to the ocean floor, comprising a mooring
. ring, a plurality of anchor cables extending from the mooring
ring, anchor means connected to the cables for securing the
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. cables at one end to the ocean floor, latch means ~or detachabl~ : :
' locking the mooring ring to the drilling platform, means for
i releasing the latch means to separate the mooring ring from :
the platform, ballast tank means in the mooring ring, and means
; 10 in the ballast tank means to change to buo~ancy of the mooring
ring for raising and lowering the mooring ring between the
.~ ocean floor and the platform. .
Also in accordance with the invention there is provided
a mooring system for an offshore drilling vessel, comprising
lS means defining an opening extending vertically through the :.
bottom of the vessel, an annular mooring ring~ latch means . :
detachably securing the ring to the bottom of the vessel in ~.
: coaxial relationship to said opening, means releasing the latch
means to separate the ring from the vessel, a plurality of :~
anchors; cable means attached to the anchors and extending into
the opening in the vessel, ballast tank means in the mooring
ring, and means attached to the mooring ring controlling the
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,~. amount of water in the ballast tank means to change to buoyancy
: of the mooring ring for raising and lowering the mooring ring
between the ocean floor and the vessel, the ring including guide :.
` means receiving the cable means to guide the cable means into . -
.. ~ said opening, and tensioning means mounted in the vessel for - :.
. applying controlled tension on the cable means for positioning
:~ the vessel relative to the anchors.
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Brief Description of the Drawings
. For a more complete understanding of the invention,
reference should be made to the accompanying drawings, wherein:
. FIG 1 is an elevational view of the drilling platform
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and associated anchoring system before they are attached;
FIG 2 shows the platform and mooring system after
. . they are attached;
FIG 3 i5 a fragmentary view partially in section of :
i the mooring ring attached to the bottom of the platform; :
;, 10 FIG 4 is a top view of the suppor~ing column of the
~ platform;
. FIG 5 is a side elevational view of the mooring ring;
FIG 6 is a top view of the mooring ring; and
FIG 7 is an enlarged partial view taken on the line ~ .
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Detailed_Descrlption
.
Referring to FIGS 1 and 2, the numeral 10 indicates
generally a monopod type semisubmersible drilling platform
having a lower qubmerged hull 12 to which is supported a deck
.l 20 14 on which is mounted a drilling derrick 16. A single
`~1. cylindrical column 18 supports the upper deck 14 above the ~ .
surface of the water and ice from the submerged lower hull 12.
. The column 18 is provided with suitable rotating ice cutters
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. 19, such as described in the above-identified copending applicat-
,. .! . : ion, for comminuting any ice encroaching on the supporting
~`' column and enabling the platform to move through surface ice,
,l such as indicated at 20.
~ Before the semisubmersible platform is moved through l:
., the ice to the drilling location, part of the site preparation
30 involves placing a plurality of anchors 22, preferably eight in
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.: 1 number, at a plurality of wide-spaeed posi~ions on the ocean ~
floor by means of a workboat. This is usually done in the ~ -
~ummer when the open wa~er permit~ ~he workboat to maneuver
. freely. Connected to each anchor is a length of anchor chain ~ ~ .
and/or cable 23 which is connected to a single mooring ring
24~ As hereinafter described in detail~ the moor~ng ring can
be remotely controlled from the platfonm or other surface
.. ve~sel ~o change the buoyancy of the ring by release of - ~:
compressed air into ballas~ tanks. When not in use the mooring
ring 24 has itB ballast tanks flooded so that the ring sinks
to the ocean floor. After the drilling platform i~ moved into ~;
location on the drilling site~ compressed air carried in the
; mooring rlng 24 i8 released into the ballast ~anks increasing
.~ the buoyancy of the ring sufficiently to cause it to rise
toward the 3urface, lifting the anchor chain and cables 23 with
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As shown in FIG. 2, the bottom of ~he lower hull 12
;'.' can be attached to the mooring ring 24. The cables are then - .
.. attached to control winches or drawworks mounted within the :
plat~orm 10 to contxol tension on each of ~he aDchor chains 80 ~...
as to hold the platform in the des~xed position over the drill
site.
. The manner in which the mooring ring 24 is attached to
;;;, the platform snd the anchor chains controlled ~8 shown in FIGS. .
~~5 3 and 4. The suppcrting column 18 includes an outer cylindrical .
.~wall 30 and an inner cylindrical wall 32. The inner ¢ylindrical
~wall 32 forms a moon pool open to the water through which the
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.~., drilling equipment passes from the derrick 1~ down through the
.;.platform to the ocean floor during the drilling operation.
:~3 When in operating position, the mooring ring 24 is ~cured to
~the bottom o the lower hull 12 by suitable grappling mean~, ;
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such automatic couplers that engage on contact between the m~or- :
. ing ring and hull bottom, with the central openlng of the ring
axially aligned wi~h the moon pool. The cables attached to the
anchor chains pass through hawsepipes, such as indicated at 34, -
extending from the lower outside surface of the mooring ring to
- ~he upper inside surface. Each cable then passes into a hawse-
pipe, such as indicated at 3~, opening in the bottom of the
lower hull 12 in the crescen~-shaped space between the outer
and inner walls 30 and 32 of the support column. The hawse-
pipes 36 extend from ~he bo~tom wall of the lower hull 12 up
; to a point well above the water level where the cables pass over ~ ~
pulleys, such as indicated at 38. The cables pass down again ~ ~ -
through the support column through ~awsepipes 40 to associated -~
winches 42 located in the l~wer hull 12. As shown in FIG. 4,
each of the anchor cables passes around its own pulley and back
down to the lower hull within the crescent-shaped space between
; the inner and outer walls 30 and 32 of the suppor~ column. Each
~ anchor cable~is controlled by its own winch, so that the tension".,
on each cable can be individually controlled to maintain the
proper position of the drilling platform. Alternatively, the
cables may be brought up from the ring through the moon pool to
.~ mooring winches on the upper deck.
` Details of the mooring ring are shown in FIGS. 5 and 6.
; The donut shaped ring is flattened at the top to provide a flat
~5 surface 50 which, as shown in FIG. 3, engages the bottom wall
of the platform around the lower open~ng Of~e moon pool. By
way of example, a typical mooring rin& might have an outer
diameter of 40 ft. or greater and a height of 12 ft., with an
inner diameter o~ 17 ~t. The inner and outer perimeters of the
annular flat ~urface 50 are provided with concentric pres~ure
~ealB 51 which pres~ agalnst the bo~tom o the platfonn when
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the mooring rln~ is locked in place. A plurality of stab
; connectox pins 52 project up from the surface 50 for engaging
mating lock~ng connectors 53 in the bottom of the platform.
I With the mooring ring locked in position against the bo~tom of
the platfonm, the annular space between the seals 51 ~s isolated
from ~he sea water. This permits access to the interior o the
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- mooring ring through the bottom of the platform for maintenance
service and repairs. To this end, the top surface 50 ~s ~-
prov~ded with a i~uitable manhole with a removable watertight
; ~0 hatch, as indicated at 54. A i~imilar hatch in the bottom of
the platform penmi~s access to the interior of the mooring ring
from the interior of the lower platorm hull 12.
Ths interiox of the moorLng ring is pr~ferably divided
into four compartments by four bulkheads 56, 58, 60, and 62.
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15 Each compartment in turn is t~vided into an upper and lower -
region by a divider wall 64. The lower portion of each of the
~ ~our sections forms a ballast tank, while the~iupper portion actsr! ~ as a buoyancy tank and as a space or mounting the neces~ary ~`
operating equipment.
~; 20 Each quadrant section of the mooring ring is provided
with a pair o hawsepipes, ~uch as indicated at 66 and 68. The ~ -
howsepipe 66 opens at one end ad~acent the upper interior
surace of the ring, as indicated at 70. The hawsepipe is -~
curved in an arc ~o that the other end opens in the lower outer
surface of the mooring ring. Slmilarly the hawsepipe 68
terminates at the upper inner surace in an opening 74 and at
the lower outex surface at an opening 76 which is subs~antially
tangential to the surface. Thus the anchor chains entex the
mooring ring substantially tangential to the surface of the
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~`~ 3 mooring rin~ rather than radially. This arr~ngement provides ;~
better anchoring of the mooring rinS and associated plstform ;~
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1 agains~ rotational forces, such as produced by the torque o~
the ice cutters which are rotata~ly mounted on the outslde of
the supporting column of the platform.
Mounted within the buoyancy tank section of each
~uadrant of ~he mooring ring i a high-pressure tank 77 for
storing compressed air. This ~ank is connected by an elertri-
cally controlled valve 78 to the associated ballas~ ~ank. When
opened, ~he valve 78 xeleases air under pressure into the
.. ~ ballast tank forcing water ou~ of the ballast tank through a flood valve 80 in the bottom of the a~sociated ballast tank.
.. Operation of valves 78 and 80 for controlling the buoyancy of
the associated quadrant of the mooring ring is provided by a
control unit 82 which responds ~o acoust~c signals trsnsmitted
to the mooring ring and received by a hydrophone 84. In
addition to receivlng command signals from the platform or
other surface vessel, the mooring ring includes a ~ransmitting
transducer 86 or transmitting position and attitude information `~
to the controlling vessel. Each quadrant has its own hydrophone
and transducex so that the buoyancy of the quadrant can be
~0 individually controlled from a remote point. The control unit
82 may also be oper~ted by wire control from a surface buoy or
manually by a diver or submarlne.
; The control circuit 82 responds to commands received on
the hydrophone 84 as well as at~itude sensors 88 mounted in the
mooring ring. The attitude sensors, through suitable servo
controls, operate the respective valves 78 and 80 in each o~
the quadrants so as to maintain the mooring ring substantially -
level in the water regardless o variations in the pull imposed
by the anchor chains on the mooring ring as the ring moves
` between the ocean bo~tom and the bottom of the drilling platfonm.
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1 In response to commands reeeived ~rom the surface vessel over
~he hydrophone, the control circùi~ also controls the valves 78
and 80 in the respective quadran~s to make the mooring ring
rise, lif ing the anchor chains off the bottom, or let the ring
sink to the ocean floor. The ring can also be stopped at any
intermediate level. The use o gravity type vertical sensors ~ -
and hydrostatic pressuxe sensors to actuate a sexvo contxol
: ~ystem ~or controlling the buoyancy o the individual ballas~ ~-
; tanks involves conventional techniques which are well k~own in -
the art o~ servo control.
A~ter passing through the hawsepipes 66 and 68, the
ends o~ the anchor chains are provided with locking pins which
;;` prevent the chains from pulling out oi the hawsepipes as they
are being lifted from the ocean floor by the rising mooring
ring. A~ter the mooring ring reaches the desired level, the
platorm is moved directly over the ring and the ring and ` ;;
pla~form are mated. At this time, the moon pool can be closed
o~f at the ~op and the water forced ou~ to the bottom of the
ciollector ring permitting an operator to gain direct access to
the lock pins on the anchor chains. In the alternative, rather
than attempting to remove the water from the moon pool, a diver
can be used. In either case, at the same time the locking pins
are removed ~rom the anchor chains, they are connected directly
to the ends o the control cables going to the winches, permit-
;~ ~5 ting the tension on the anchor chains to be controlled by the ;~
winches 42. At any time it becomes necessary to move she
platonm of~ location, the cables can be disconnected ~rom the -
i anchor chains and the locking pins inserted to secure the ends
; o the chains to the mooring ring. ; -
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i 3 While the ring i8 attached to the bottom of the platfonmg
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1 the ba~teries for providing power within the mooring ring can
be recharged and the sir tanks can be recharged from ~he
platform by sui~able electrical and pneu~natic connectors 90
and 92 in the top fla~ annular wall 50 of the mooring ring.
Ater drilling operations are complete, the mooring ring can be
co~manded to rise to the s~rface where a workboat aan ~etrieve
the anchors and chain~.
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