Example No. B14GB1310142

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Nov 8, 2013 (3 years and 9 months ago)

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Example No. B14

GB1310142


BUOYANT OFFSHORE STRUCTURES



Patent number: GB1310142

Publication date: 1973
-
03
-
14

Inventor:

Applicant: TRANSORLD DRILLING CO

Classification:

-

international: B63B35/44

-

european: B63B35/44B

Applicat
ion number: GB19700038672 19700811

Priority number(s): US19690881315 19691201



View INPADOC patent family


Also published as:




US3605669 (A1)


NL7012519 (A)


FR2061001 (A5)


DE2041925 (A1)



Abstract not available for GB1310
142







Description of
GB1310142



PATENT SPECIFICATION


( 11) 1310 142 ( 19 ( 21) ( 31) ( 33) ( 44) ( 51) DRAWINGS ATTACHED Application No
38672/70 ( 22) Filed 11 Aug 1970 Convenilon Application No 881 315 ( 32) Filed 1 Dec
1969 in United States of Am
erica (US)


Compleite Specification published 14 March 1973


Interintational Classification B 63 B 35/44 ( 52) Index at acceptance Ei 1 H 11 ( 54)
BUOYANT OFFSHORE STRUCTURES ( 71) We, TRANSWORLD DRILLING
COMPANY, a Corporation organized and existing under

the laws of the State of Delaware,
United States of America, of KerrMcGee Building, Oklahoma, City, Oklahoma, United States
of America, do hereby declare the invention, for which we pray that a patent may be granted
to us, and the method by which it is to

be performed, to be particularly described in and by the
following statement:


The present invention relates to a buoyant offshore structure and particularly a floating vessel
for marine operations such as well drilling in deep offshore waters.



Offsho
re structures, both fixed and mobile, for performing marine operations such as well
drilling in offshore waters are well known.


Offshore structures of the mobile type, which are floated to a location and then either sunk to
the underlying land bottom or p
artially submerged to a level above the land bottom and
anchored into' position have been used for many years In some cases, drilling is done from a
platform on a conventional ship which has been modified to allow drilling through an opening
in the ship, o
r a platform is mounted between two vessels floating side by side.


The use of a conventional ship as a drilling vessel has certain advantages, particularly with
regard to stability while relocating, and also with regard to mobility, as the ship type struc
ture
is much easier and faster to relocate than are other types of mobile drilling structures.


Also, because of the nature of their design, a ship type vessel can get through narrow channels
and canals which will not handle other types of mobile drilling
structures However, the ship
type vessel has a serious drawback when used to support a drilling platform.


This disadvantage is its large exposure to wave forces, combined with an inherently low
radius of gyration This disadvantage results in excessive rol
l during operation, rand in some
cases results in damage to drilling equipment.


One of the most successful types of mobile structure used to date comprises a unitary
conlPrice 25 pl struction of a working platform fixed to the tops of a group of elongated
,
buoyant stabilizing columns spaced about the structure in a stabilizing pattern The spacing
and size of the stabilizing columns is selected so that the structure has a high degree of
stability during submergence and refloating operations Another related
type of floating
structure includes a platform movable with respect to open
-
ended stabilizing columns.


While these prior art devices have generally been quite satisfactory, a need still exists for a
design which offers advantages over existing equipment.


According to the present invention we provide a buoyant offshire structure that floats when in
transit and when in situ comprising:


a buoyant hull having a working platform thereon, at least three elongated buoyant hollow
stabilizing columns, each column

being provided with means for independent vertical
movement relative to the hull; means on the structure, including an opening therethrough for
each column, for guiding the columns through said vertical movement, means for securing the
columns relative to

the structure at at least one predetermined position thereon, the lower ends
of the columns, when in their lowered positions, extending into the body of water to a
distance above its mud line; and means for raising the hull above the surface of the body o
f
water, the structure being capable of floating while in such position; said hull being elongated
in the direction of its longitudinal axis to reduce water resistance during said transit; at least
one of said columns and its opening being located in an ap
pendage disposed outwardly from
and rigidly connected to the hull, and said openings for the columns being positioned in such
manner that the columns guided thereby provide the sole stabilization for the structure by
maintaining the centre of gravity of th
e structure below its metacenter when the hull is
positioned above the surface of the body of water.



By mud line we mean that line of substance above the bedrock under any body of water, No
Mr cn 1,310,142 The preferred form of the present invention pro
vides a stable structure
without the necessity of additional stabilizing means such as ballast chambers or the cable
-
supported ballast mass required by prior art structures which utilize open
-
ended columns It is
only necessary to make the stabilizing colum
ns of sufficient size, and locate them in a
stabilizing pattern a sufficient distance from the centre of the structure, to provide moments of
inertia of the water plane areas of the structure of sufficient magnitudes to establish the
metacentre of the stru
cture above the centre of gravity for every anticipated operating
condition of the structure The above requirement is essential to the stability of the structure
because the centre of gravity of the structure may be above the centre of buoyancy during
many

of its operating conditions, and without adequate water plane areas located in a
stabilizing pattern, the structure would be dangerously unstable When the centre of gravity of
the structure is below the centre of buoyancy, the size and location of the wat
er plane areas is
not critical, as the structure is then stable due to a pendulum effect However, when the centre
of gravity is above the centre of buoyancy as will be the case when the barge is elevated
above the water and a heavy drilling rig is mounted
on the platform, the requirement of water
plane areas Iocated in a stabilizing pattern about the structure is critical, and unless the
moments of inertia of these water plane areas about any axis of heel are sufficient to establish
the metacentre of the st
ructure above the centre of gravity, the structure will be inherently
unstable.


According to one embodiment of the invention, the platform is elevated solely by means of
deballasting the columns after it has been locked to the previously ballasted and low
ered
columns It is not necessary that prior art jacking equipment or rack and pinion means capable
of supporting and elevating the full weight of the barge be provided However, as a matter of
safety and convenience, it may be desirable to include means for

restraining or controlling the
movement of the columns with respect to the platform Such means might, for example, be
light weight jacking means similar in operation to the heavy duty types used to elevate
platforms in conventional jackup type rigs These
light weight jacking means need only be
capable of providing enough restraint to movement of the columns relative to the platform to
prevent undesired movement caused by wave action during the times that the columns are
being raised or lowered relative to
the platform The jacks may be used in conjunction with a
notched rail or rows of openings in the columns.


According to another embodiment of the invention, the columns may be raised and lowered
by conventional elevating devices such as heavy duty hydrauli
c jacking means or rack and
pinion means The inclusion of these elevating devices provides additional flexibility of
operation, and when used in conjunction with ballasting and deballasting, permits the columns
to be raised or lowered while maintain 70 ing

the platform a fixed distance above the water
level This is useful as a means of controlling the roll period of the structure, which should
always be different from the wave period to prevent undesirable amplification of 75 roll The
period of roll may als
o be varied by adding ballast to or removing ballast from the barge itself
This changes the centre of gravity of the structure and thereby varies the roll period of the
vessel It is obvious from 80 the above discussion that the stabilizing columns must pro
vide
enough buoyancy to float the structure with the barge portion out of the water.



If the structure is to be used for drilling 85 of wells, appropriate drilling and auxiliary
equipment is provided on the barge and platform portions of the structure Al
so, living
quarters, a heliport, storage facilities and other supporting equipment may be provided 90 The
structure is such that it has a high order of stability during movement or relocation of the
structure, which may be accomplished by towing, or by sui
table self
-
propulsion means.


The barge and the stabilizing columns are pre 95 ferably deballasted during relocation, with
the result that the structure is partially supported by the buoyancy of the barge portion, and
partially supported by the buoyancy of

the columns, except that in one particular embodi 100
ment, to be described below, the structure may be floated by the barge portion alone during
relocation.


Following is a description of the method of raising and lowering the barge solely by means
105 o
f adding and removing ballast After reaching a selected working location, the columns
are ballasted, for example, by pumping sea water into the interior of the fluid tight columns.


This causes the columns to sink through the 110 openings in the barge or o
ther appended
structure affixed to the barge portion, and when the columns have settled to the desired level,
they are locked into place Subsequent deballasting of the columns causes the entire struc 115
ture to rise as a unit The columns provide enough bu
oyancy that the barge portion can be
elevated sufficiently above the water level to be clear of normal wave action.


At this point, or in some cases before the 120 ballasting and deballasting is performed,
suitable anchoring means may be utilized to mainta
in the structure above a fixed point of the
underlying seabed Alternatively, the structure may be maintained in position above a 125
fixed point by suitable dynamic positioning systems.


After the barge is elevated and the anchoring means are in place, or
the dynamic positioning
system is functioning, a marine opera 130 1,310,142 tion, such as drilling or servicing a well,
is performed After completion of the operation, ballast is added to the columns, causing the
structure to begin to submerge When the bar
ge portion has reached its normal floating level,
the means locking the columns to the platform are released, and deballasting of the columns is
begun The columns should have a neutral buoyancy at this point to prevent sudden upward or
downward movement wh
en the locking means are released Complete deballasting of the
columns results in their being raised to the towing position, and the entire cycle can then be
repeated The use of restraining means, such as hydraulic jacks, is very useful during the times
th
at the columns are being ballasted or deballasted, since this lets the columns move against a
predetermined load, and prevents hazardous unrestrained back and forth relative movement
due to wave action or other causes.


In the embodiment wherein elevating
means sufficient to lift the entire barge out of the water
are provided, it is simply a matter of driving the columns down into the water until the
submerged portions of the columns provide enough buoyancy to float the entire structure with
the barge out o
f the water Continued operation of the elevating means after this point will
raise the barge above the water level.


Ballasting and deballasting of the columns is not necessary in this embodiment, but in some
cases will be desired to provide additional con
trol of the operation.



The objects and advantages of the present invention will be best understood from the
following description of the accompanying drawings, in which: Figure 1 is a perspective view
of the preferred structure in the operating conditio
n; Figure 2 is a front elevation of the
structure shown in Figure 1 in the towing or relocating condition; Figure 3 is a front elevation
of the structure with the hull floating and the columns lowered; Figure 4 is a front elevation of
the structure with th
e hull raised to just above the water level; Figure 5 is a front elevation of
the structure with the hull elevated to the operating level; Figure 6 is a front elevation of a
modification of the structure in the towing or relocating condition; Figure 7 is a

side elevation
showing mechanism for locking a column in place; and Figure 8 is a front elevation showing
mechanism for moving a column relative to the hull.


With reference to Figure 1 the buoyant offshore structure designated generally as 10 has a hull
or so
-
called barge 12, hollow buoyant stabilizing columns 14 and 16 located at the fore and
aft ends, respectively, of the barge and hollow buoyant stabilizing columns 18 and 20 located
on either side of the barge All of the columns can be flooded for ball
asting purposes The
barge has a deck 22 and a working platform 24 mounted thereon A derrick 26 is supported by
the working platform 24, and drilling rig equipment such as drawworks, pumps, motors, and
related equipment is indicated generally at 28 A stop m
ember 30 is located near the top of
each of the columns 14, 16, 18 and 20 Columns 14, 16, 18 and pass through respective
apertures in column machinery housings 32, 34, 36 and 38 respectively The two column
machinery housings 36 and 38 are rigidly attached
to the barge 12 by means of frameworks
40 and 42 respectively The column machinery housings serve as guides for the columns
Anchor lines 44 are shown extending from the barge A buoyancy tank is attached to the lower
end of each column as shown at 46.


With

reference to Figures 2, 3, 4 and 5, the structure 10 is shown in various stages of
operation Figure 2 shows the barge 12 in a floating condition with all of the columns raised to
the towing position (column 16 is not visible) As shown, the buoyancy tanks
46 attached to
the lower ends of the columns 14 and 16 rest against the bottom of the barge, while the
buoyancy tanks 46 attached to the lower ends of the columns 18 and 20 rest against the
column machinery housings 36 and 38 The buoyancy tanks are prefera
bly streamlined or
elongated in the direction of the longitudinal axis of the barge for minimum resistance or drag
during relocation of the structure.


Figure 3 shows the structure after it has reached a working location and after all of the
columns have b
een lowered, during which they are progressively flooded, until the stop
members 30 near the tops of the columns have contacted the column machinery housings At
this point in the operation the columns are locked in place.


Figure 4 shows the structure afte
r the columns have been locked in place and partially
deballasted by pumping The barge is shown just as it emerges from the water, at which point
the total stability of the structure is provided by the columns.


Figure 5 shows the structure in a completely

deballasted condition with the barge raised well
above the water level The structure in this condition is floated by the submerged portions of
the columns and attached buoyancy tanks.



Figure 6 shows a modified structure in which the frameworks 40 and 4
2 are constructed and
attached to the barge in such a manner that the columns 18 and 20 and their attached
buoyancy tanks can be elevated completely abcve the water level when the structure is
floating This further reduces resistance to movement of the str
ucture through the water, and in
addition enables the structure to move down narrow canals which could not accom1,310,142
modate the structure shown in Figure 2.


It will be obvious that a structure in accordance with the invention could have all of its
st
abilizing columns pass through housings secured outboard of the barge generally in the
manner of the housings 36 and 38 with at least three columns supplied in a stabilizing pattern
about the structure.


The process of operating a structure as shown in Fig
ures 1 to 6, in the embodiment described
previously where the barge is elevated by ballasting and deballasting, involves floating it to a
location as shown in Figure 2, and then adding water to the columns to cause them to sink to
the position shown in Fig
ure 3.


When fully lowered, the columns can rest against the top of the housings 32, 34, 36 and 38 by
means of the stop members 30 The columns are then locked to the machinery housings by any
suitable means, and the columns are deballasted As shown in Figu
re 4, as ballast is removed
from the columns, the entire structure, with the barge securely fixed to the top portions of the
columns, begins to rise in the water Since the columns alone have sufficient buoyancy when
d&ballasted to support the structure wit
h the barge completely out of the water, the barge and
platform will be elevated above the water level The barge is further elevated with continuing
deballasting, and the final stage of the operation is illustrated in Figure 5, which shows the
barge elevat
ed sufficiently above thie water level to clear a preselected level of wave action.


The structure may be maintained at the working location (such as directly above a drilling
site) by conventional means, which might be large sea anchors or a dynamic posit
ioning
system comprising a series of outboard motors arranged about the structure.


After drilling a well or otherwise completing an operation with the barge elevated, the raising
process is reversed, and the structure is returned to the towing or moving s
tate for relocation
Water is admitted to the columns and/or the barge until the barge reaches its normal floating
level Usually it is preferred to lower the structure by ballasting the columns, rather than the
barge as this gives the structure a lower cent
re of gravity and, therefore, is much safer Also, it
would not be desirable for the columns when submerged to have maximum buoyancy,
because when the locking means locking the barge to the top of the columns are released, the
columns would tend to spring u
pward, with resulting hazards.


After the barge has been lowered to its normal floating level, the locking means are released,
and the columns are deballasted The columns thus rise through the guides until they reach the
towing condition While it is possib
le to tow the structure with the barge out of the water, and
this might be desirable for short moves during calm weather, it is obviously much safer to
first lower the barge into the water.



Figure 7 shows a suitable locking means for locking each column

to the respective machinery
housing The locking means includes a pin 70 52 slidably mounted in a sleeve 54 which is
secured to the machinery housing Pin 52 is actuated by a hydraulically actuated piston rod 56
and engages with an opening 58 in the column
75 While the structure is illustrated in the
drawings with four stabilizing columnns, only three columns need be provided, the columns
be of sufficient size, and should preferably be located 80 in a stabilizing pattern a sufficient
distance from the centra
l portion of the structure, to provide adequate buoyancy and stability
during any planned use of the structure The stabilizing columns need not be all the same 85
size, and need not be spaced uniformly about the center of the structure, but in most cases t
his
would be the preferred design.


As mentioned above, in some cases it will be desirable to include in association with each 90
column elevating means for raising the barge relative thereto, instead of relying solely on
deballasting of the structure Such

elevating means may be a conventional heavy duty jack, or
may be a rack and pinion mechanism as 95 illustrated in Figure 8 Here a motor 60 drives a
spur gear 62 through a gear reduction system 64 Spur gear 62 meshes with a rack 66 mounted
on the column In

practice, two or more racks are mounted on each column to 100 distribute
the load The elevating means may also serve as the lockig means for securing the column
relative to the barge at a selected position.


The embodiments described above illustrate 105
the invention, but are not to be considered as
limiting the scope thereof It has been shown that the offshore structure in accordance with this
invention is mobile, and can be moved fromn location to location, either by towing or 110 by
self
-
propulsion, mu
ch more easily than the prior art column
-
stabilized offshore structures.


Also, the structure in accordance with the invention, whien in the operating or raised
condition, is much less susceptible to rolling due 115 to wave action than are the prior art
of
fshore drilling structures due to the smaller surfaces presented to the waves It is also
apparent that the structure in accordance with the invention is stable, due to the size and
spacing of the 120 stabilizing columns, even when the barge is raised above

the water level
and the centre of gravity of the structure is above the centre of buoyancy due to a heavy load
carried on the barge 125


Claims of
GB1310142



WHAT WE CLAIM IS:
-


1. A buoyant offshore structure that floats when in transit and when in si
tu comprising:


a buoyant hull having a working platform thereon, at least three elongated buoyant hol 130
1,310,142 low stabilizing columns, each column being provided with means for independent
vertical movement relative to the hull; means on the structu
re, including an opening
therethrough for each column, for guiding the columns through said vertical movement,
means for securing the columns relative to the structure at at least one predetermined position
thereon, the lower ends of the columns, when in t
heir lowered positions, extending into the
body of water to a distance above its mud line; and means for raising the hull above the
surface of the body of water, the structure being capable of floating while in such position;
said hull being elongated in t
he direction of its longitudinal axis to reduce water resistance
during said transit; at least one of said columns and its opening being located in an appendage
disposed outwardly from and rigidly connected to the hull, and said openings for the columns
be
ing positioned in such manner that the column's guided thereby provide the sole
stalilization for the structure by maintaining the center of gravity of the structure below its
metacentre when the hull is positioned above the surface of the body of water.


2. A buoyant offshore structure as defined in claim 1, wherein the hull has the general
configuration of a ship.


3. A buoyant offshore structure as defined in claim 1 or 2, including enlarged buoyancy tanks
attached to the lower ends of at least one of th
e columns.


4. A buoyant offshire structure as defined in claim 1, 2 or 3, including means for varying the
period of roll of the structure, said means comprising means for controllably varying the
centre of gravity of the structure while maintaining the st
abilizing columns fixed with respect
to the hull.


5. A buoyant offshore structure as defined in claim 1, 2, 3 or 4, wherein the centre of gravity
of the buoyant structure is above the center of buoyancy of the buoyant structure at least when
the hull is e
levated to a working level above the surface of the body of water.


6. A buoyant offshore structure as defined in claim 1, 2 or 3, including elevating means
adapted to vary the position of the columns with respect to the hull when the hull is above the
wat
er level.


7. A buoyant offshore structure as defined in claim 6, including means for varying the period
of roll of the structure, said means including means capable of maintaining the hull a fixed
height above the water level while varying the position of

the columns with respect to the
hull.


8. A buoyant offshore structure as defined in any of the preceding claims, wherein at least one
of the columns may be raised completely out of the water while the hull is floating.


9. A buoyant offshore structure co
nstructed and adapted to operate substantially as herein
described with reference to the accompanying drawings.