Note: Descriptions are shown in the official language in which they were submitted.
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~NCHORED OTi`FSllORr~' STRUCTUR~
WIT~I SW~Y CONTROI
BACICGROUND OF Tl-IE :[NVENTION
The present invention relates to an offshore structure
for disposition in a body of deep water; and more partic-
ularly, this invention relates to an offshore structure
carrying a platform thereon and held by guy wires secured to
anchors on the ocean floor.
offshore structures in current use in connection with
the drilling and production of hydrocarbon deposits in
offshore locations have generally included a platform held
above the surface of the water by support members which rest
on the ocean floor. Such structures, referred to as fixed
platforms, have been quite successfully utilized in opera
tions conducted in shallow water. However, as offshore
drilling operations move into areas having deeper waters,
such as about 300 feet, the fixed platform design becomes
less desirable, primarily because of the great expense to
fabricate and install such a structure. As a general rule
the cost of fabrication and installation of a fixed platform
designed in accordance with proven shallow water methods and
technology will increase exponentially with water depth.
~ lthough an alternative to the fixed platform design is
the floating platform concept, that approach is found to
present additional, and perhaps even more serious disad-
vantages. Floating platforms have the disadvantage of being
more suseeptible to rough sea conditions and exhibit the
undesirab~ie feature of signi~icant heave, pitch and roll
motion
Recently another type of structure for deep water
offshore~locations has been proposed which has a platform
supported on buoyant members that are held in position at
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the well-site by anchor lines extending to fixed anchors on
the ocean floor. Offshore platform structures of -this type
are referred to generally as "buoyant restrained platorm"0
The basic principle is to provide a platform with buoyant
chambers below the wave area to give a positive buoyancy and
to tie the structure down to the ocean floor, allowing the
buoyancy of the structure to hold the anchoring cables in
tension to prevent heaving. Offshore structures of this
type offer significant cost-savings for operation in deep-
water environments.
Illustrative of the concept of buoyant restrained plat~form design is that structure disclosed in the article
"Tension Leg OfEers Steady Base At Sea" in the November 1973
issue of OFFSHORE magazine, beginning at page 100. The
structure disclosed there comprises three buoyant vertical
columns having horizontal bracing structure. A working deck
is positioned on top of the vertical columns above the
surface of the water. Several anchoring cables attach to
each vertical column and are secured by dead weight anchors
on the ocean floor. Anchor loads to hold the structure are
significant, however, due to the resistance in the wave zone
o the structure.
` Additional designs based on the buoyallt restrained
platform concept are described in the OFFSHO~E ENG-~NEER of
May 1975, at page 55. Also, relevant teachings of the
buoyant restrained platform concept are found in U.S. Patent
3,256,537 to Clark and U.S. Patent 2,777,669 to Willis.
A yet another approach which has been proposed is th~t
of a structure comprising a s]ender column held in a vertical
position by guy wires extending from near the top of the
column to fixed anchors on the ocean floor. The column
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rests on the ocean floor and extends above the surface of
the water with a platform supported thereon. Although the
vertical column design has less resis-tance in i-ts wave zone
than the typical buoyant restrained platform design, some
resistance is still pre~ent and forces are developed from
periodic wave motion which act on the structure. A problem
associatèd with guyed structures of this type is that tilting
of the column off-vertical makes it very difficult to con~
duct drilling or production operations. However, increased
tension in the guy lines to restrict tilting is not desir-
able either.
One approach to solving the problem of tilting is that
described in U.S. Patent 3,903,705, assigned to Exxon Pro-
duction Research Company. There is disclosed in that patent
a guyed tower platform using a clump weighted guy line
system to control swaying and still relieve the structure of
excessive moment forces produced by waves. The wei~hts are
designed to lift off the bottom during large s-torm waves,
but remain in position during normal sea conditions. The
guy line system provides adequate sway restriction, yet,
relieves the guy lines of excessive stress and removes the
need for heavier anchoring equipment.
Another concept proposed for an offshore structure to
provide restricted movement with waves is that of the buoyant
tower. The buoyant tower is an elongate tower s-tructure
that is hèld near the ocean Eloor by a universal joint that
permits t~he tower to tilt. The force required to prevent
the tower from tilting excessively is provided by the es~
tablishment of buoyancy for the tower near the surface of
the water. One design based on the buoyant tower concept is
the structure illustrated and described in T~IE OIL AND G~S
OURNAL of October 28, 1~7~, beginning at page ~0.
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S[JMM~RY OF TEI:I~ INVENTION
srieEly, in accordance with the present invention,there is provided a guyed offshore structure carrying a plat-
form thereon from which drilling o:r pxoduction operations
may be conducted. Specifically, the instant invention provides
an offshore structure suitable for use in bodies of deep
water, which structure permits restricted movement with waves,
yet prevents excessiVe tilt;ng O.e the structure.
Thus, broadly, the invention contemplates an offshore
structure ~or placement in a body of water which comprises
an elongate member for disposition ~n a body of water *o
extend above and below the surface of the water, with the
member being anchored to the ocean floor to permit ti,lting, a
plurality of ~uy lines radially arranged around the elongate
member and connecting between the memher and anchors on the
ocean floor, and sway control apparatus operably connected
to each of the guy lines fo~ altering the length of the guy
lines between t~e elongate member and the anchors in response
~ to tilt~ng of ~he elongate member. The sway control apparatus
includes first and second winch means, with the first winch
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~eans ~ncluding a first group of reels mounted on a common
~drive sha~t which connects to a first group of guy lines and
; tilts the elongate structure along one direction by pulling
in and lettin~ out the first ~roup of ~uy lines, and with the
second winch means including a second group of reels mounted
on a common drive shaft which connects to a second group of
guy lines and tilts the elongate structure a}on~ another
~directLon at a right angle to the one direction by pulling in
and lettin~ out the second group of ~uy li~es~
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The present invention makes use of the concep-ts of -the
buoyant restrained platform and the guyed structure, by
providing an offshore structure comprising an elonga-te
member for vertical disposition in a body of water with a
buoyancy tank giving a po.si`ti~ve buoyancy to the member.
Anchoring lines connected between the lower end of the
elongate member and anchors on the ocean floor tie~down the
- member, and in addition permit tilting of the structure. To
restrict movement of the structure with waves~ a plurality
of guy lines connect between the elongate member and sepa-
rate clump weight anchors that are disposed on the ocean
floor. The clump weight anchors are adapted to raise off
the ocean floor upon o~f-vertical movement of the elongate
membex. Finally, sway control apparatus operably connected
to the guy lines alters line length between the connection
point on the elongate member and the respective clump weight
anchor. The sway control apparatus operates in response to
off-vertical movement of the elongate member and prevents
excessive tilting of the structure by reeling in or letting
out guy line to keep the structure substanti.ally vertical.
The sway control apparatus also regulates tension in the guy
lines.
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In a more specific embodiment of the presen-t inven-tion,
the anchoring lines connecting to the lower end of the
elongate member define a universal pivot point. ~lso, the
guy lines attach to the elongate member below the surface of
the water at a point between the longitudinal mid-point of
the elongate member and the top of the elongate member. The
sway control apparatus comprises a separate reel for each
guy line with means for driving the reels in response to an
inclinometer device that detects off-vertical movement of a
predetermined degree. The means for driving the reels may
comprise an electric motor or similar type of prime mover
device.
An offshore structure in accordance with the present
invention has the advantage of offering significant cost-
savings for operations in deep-water environments, with
smaller anchor loads being xequired to hold the struckure
than usually found in buoyant restrained platform designs.
Also, tilting of the elongate member, as usually found in
most guyed structures, is much less because of the dynamic
sway control apparatus provided. This invention, although
especially suitable for use in drilling and production
operations in deep water, is also useful in shallow water
operations.
Other aspects of this invention not outlined in the
above will be covered in the detailed description presented
below.
BRIEF DESCRIPTION OF THE DR~WXNGS
A more complete appreciation of the invention may be
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had by re~erence to the accompanying drawings illustrating a
preferred embodiment of the invention to be described in
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detail, in which like reference numberals designate iden-
tical or corresponding parts throughout the several use and
wherein:
Fig. 1 is an elevation view of an offshore structure in
accordance with the present invention having a drilling or
production plat~orm thereon;
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Fig. 2 is a planned view of the embodiment of the off-
shore structure of Fig. l, which view illustrates the ar-
rangement of the guy lines extending from the offshore
structure;
Fig. 3 illustrates a guy line tackle arrangement for
the preferred embodiment illustrated; and
Fig. 4 is a plan view of the sway control apparatus
dlsposed on the lower deck of the platform.
DESCRIPTION OF THE PRE~FERRED EMEiODIMENT
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Referring now to Fig. 1, there is shown an oEfshore
: structure I0 in accordance with the concepts of the present
: invention. The offshore structure lO is disposed in a body
of water 12 o~ any depth sufficient to accommodate the
~ particular dlmensions of the structure. However, the struc-
ture may be most advantageously used in a body of deep
: water, wherein the depth will be at least 300 feet, with a
prefe.rable depth range of 700-1500 feet.
An elongate tubular member 14, having an internal
buoyancy tank 16 extending over a substantial portion of its
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length, is the primary s-tructural component of the struc-
ture. The elongate tubular member 14 which houses the
buoyancy tank 16 is designed to withstand the total hydro-
static and hydrodynamic pressure of the surrounding sea wikh
only atmospheric pressure on the inside. The upper portion
of tubular member 14 consists of skeleton structure ]8 which
reduces resistance in the wave zone of the structure. The
skeleton structure comprises several upwardly extending
struts along with several diagonal braces which form the
lo X-braciny shown.
A two-level platform 20 is carried atop elongate member
14, and has mounted thereon a derrick 22 for carrying out
drilling or production operations. In addition, various
pieces of equipment such as, for example, a crane 2~ are
provided on the upper deck, as is a control building 26.
The upper deck 28 is built atop the lower deck 30, which
lower deck has a smaller floor area. Upper deck 28 is
supported above lower deck 30 by means of a support and
bracing structure generally denoted by the referenced nu-
meral 32. Located on the lower deck 30 is the sway control
apparatus 34 to be discussed in more detail in regard to
Fig. 4. The elevations of the upper deck 28 and the lower
deck 30 will be approximately 100 feet and 80 feet, respec-
tively, above khe surface of the water.
The buoyancy chamber 16 comprises a selectively bal-
lastable and deballastable chamber. Selective ballasting
of the buoyancy chamber 16 permits the exact amount of
buoyancy effect necessary to offset the weight of the struc-
ture and the equipment mounted on the pla-tform 20. Offshore
s~ructure 10 is floated in the water utilizing the positive
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buoyancy effec-ts provided by the buoyancy -tank 16, with
anchoring lines 36, 38, 40 and 42 tying the structure to the
ocean floor. The anchor lines connect between the lower end
of elongate member 14 and individual dead-weight anchors 44,
46, 48 and 50 (not shown). The portion 17 oE elongate
member 14 proximate the connection point of the anchor lines
is preferably ballasted with sea water or drilling mud to
provide a negative buoyancy in that region.
In order to prevent an excessive -tension from being
placed on thè anchoring lines, it is necessary to properly
ballast the buoyancy tank 16. To properly se-t and main-
tain the correct load on the anchoring lines, the weight of
the structure and the equipment which it supports must be
determined during construction to provide an accurate account
of total weight, as well as the resulting center of gravity
for the structure. In addition~ the buoyancy effect through
water displacement by elongdte member 14 must be considered.
As well as the buoyant effect of the structure and the
weight of the equipment load on the platform, in order to
? determine the amount of ballast necessary to be placed in
the buoyancy tank 16, the water depth as well as changes in
depth due to tidal variation should be considered.
By virtue of the tie-down arrangement provided by the
anchoring cables connected to the lower end of elongate
tubular member 14, the offshore structure 10 i5 in effect
pi~oted at its lower end, and therefore suscep-tible to
~ilting. In order to counteract swaying of the structure,
guy lines, of which only guy lines 50 and 52 are shown in
Fig. 1, extend radially outwardly (see ~ig. 2) rom the
`~ 30 eIongate tubular member 14. The guy lines attach to the
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elongate member 1~ at a poin-t between -the longitudinal mld-
point of elongate member 14 and the top of the elongate
member, and extend down at approximately a 60-degree angle
from vertical to a clump weight on the ocean floor. ~ach
clump weight serves as an anchor and consists of a number of
parallel segments oE large-diameter chain connected to the
guy line. The clump weights are designe~ to lift off the
bottom during the existence of large waves. A more detailed
discussion of the use of clump weights in connection wi-th a
guyed structure may be had by reference to U.S. Patent No.
3,903,705 and to the July 14, 1975 issue of THE OIL ~ND GAS
JOURNAL, at page 86 thereof. Beyond the clump weights 54
and 56, guy lines 50 and 52 extend to an anchor piles 5~ and
60, respectively.
Referriny next to Fig. 2, the symetrical pattern o~ the
guy lines 50, 85, ~7, 89, 52, 93, 95 and 97 may be appre-
ciated. Also shown in dotted outline is the platform 20 and
derrick 22. Although a fewer or greater number of guylines
than the eight illustrated may be used, the arrangement
illustrated is preferred. In any event, in determining the
number of guy lines to be used it must be appreciated that a
sufficient number must be used to distribute the load im-
posed. An excessive number of guy lines may make the sway
control apparatus 34 (see Fig. 4) too unwieldy, and thus
impractical.
The upper ends of the guy lines connect to guy line
tackle devices 62 and 64 which are secured to the outer
surface of the elongate member 14. The guy lines may often
comprise rather large diameter cables which would not be
adapted for winding on a winch drum. Hence, it is desirable
to permit adjustment oE the heavy guy lines using a smaller
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diameter cable windable on a winch. Turnlng to Fly. 3, one
guy line tackle device is shown in de-tail in position on a
section of elongate men~er 14. Specifically, yuy line
tackle device 62 comprises a fixed sheave 64 that is secured
to the outside o~ elongate member 14, with its axis of
rotation 66 being substantially perpendicular to the radi-
ally extending guy line S0 which passes over it. The end of
guy line 50 terminates in an eyelet 68 which further con-
nects to a traveling block 70. A pair of traveling block
pulleys 74 and 76 receive a cable 78. Cable 78 encircles
pulley 76 on traveling block 70 and passes over a fixed
sheave 80 that is fixed relative to elongate member 14. The
cable 78 then passes over the second pulley on traveling
block 70 before its fixed end 7.2 terminates at dead end 82
which is also secured to the outside of elongate member 14.
Thus, when the free end 79 of cable 78 is reeled in or let
out via a winch or the like ~not shown) the guy line 50 is
either pulled in or let out. Movement in this fashion
~ results in an increase or decrease in tension on guy line 50.
Furthermore, use of the tackle mechanism illus-trated main-
tains cable 78 under one-fourth the tension of guy line 50,
and permits four times the footage movement o~ the cable 7
as compared to guy line 50 to accomplish more accurate
adjustments of tension in the guy lins 50. The tackle
arrangement shown is illustrative only, and it will be
understood that various other mechanisms might be used to
adjust tension of the guy lines. Movement of guy line 50 in
this manner also adjusts the length of line between clump
weight S~ and the attachment point on elongate member 14
which is defined by sheave 66.
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Referring next to Fig. ~, there is illustrated one
configuration for sway control apparatus 34 which is dis-
posed on lower deck 30, and which regulates guy line te~sion
and alters the len~th of the guy lines between thelr point
of attachment to elonga-te member 14 and their respective
clump weight. Specifically, there is provided a separate
winch line for each guy line, which winch lines are desig-
nated with referenced numerals 78, 84, 86, 88, 90, 92, 94
and 96, with winch line 78 corresponding to the cable 78
illustrated in the Fig. 3. Each winch line connects to a
separate reel which either reels in or lets out its partic-
ular winch line to alter the length of and tension in the
guy line to which that particular winch line is connected.
The reels are arranged in groups, in this case of four,
and are skid mounted along with the prima mover device that
drives them to form winch means 99 and 101. For example,
winch means 99 comprises skid 98 having reel 100 for winch
line 96, reel 102 for line 94, reel 104 for line 86, and
reel 106 for line 88, The reels are all mounted on a single
20~ drive shaft 108 which is coupled to an electric motor 110
through a gear box 112 and a brake 114. Reels 102 and 100
are wound clockwise, while reels 104 and 106 ara wound
counterclockwise. This type of winding arrangement permits
a single direction of rotation for -the reels to be used to
provide the appropriate reeling in and letting out of the
various~winch lines.
A similar arrangement exists for winch means 101 com-
prising skid 115 having reels 116, 118, 120 and 122 mounted
thereon. These enumerated reels connect respectively to
winch lines 90, 92, 84 and 78. An electric motor 124
simultaneously drives the reels through a gear bo~ 126 with
a brake 128 being provided.
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In order to supply the required electric power for the
electric motors 110 and 124, a diesel generator set is
provided with a skid mounted diesel en~ine 130 driving an
electric generator 132 to genera.te the required ~lectric
power for the sway control apparatus.
The sway control apparatus is responsive through motor
controls to detection means 134 which senses off-vertical
movement of the elonga-te member 14. Detection means 134 may
be an inertial detector or an i.nclinometer, either of which
may be utilized to generate a signal that activates the winch
motors to drive the winches in the aforementioned manner to
alter the length and tension of the guy lines. The sway
control apparatus activates when the structure is displaced
from vertical due to wind, waves, or current.
To illustrate the operation of the apparatus of this
invention, if the tilting forces on the tower provide a
resultant force from a direction between the extensions of
lines 94 and 96 in Figure 4, then only reels operating off
: winch means 99 likely need be activated to accomplish any
adjustment. If, however, the resultant force is from a
direction between the extensions of lines 78 and 96, both
winch means 99 and 101 will require adjustment~ Similarly,
a resultant force from the direction between the extensions
of cables 90 and 92 requires only adjustment of wlnch means
101, whereas a resultant force between the extensions of
cables 92 and 94 would again require adjustments from both
winch means. : :
: The foregoing description of the invention has been : :
directed to a particular preferred embodiment of the present
invention for purposes of explanation and illustration. It
will be apparent, however, to those skilled in this art that
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many modifica-tions and changes in the appaxa-tus may be made
without depar-ting from the scope and spiri-t of the invention.
It is therefore intended that the following claims cover all
equivalent modifications and variations as fall within the
scope of the invention as defined by the claims.
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