Note: Descriptions are shown in the official language in which they were submitted.
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Ballasting of Semi-submersible Rig
This invention relates to a semi-submersible structure and to a
method of ballasting such a structure. The invention is of
particular application to semi-submersible drilling rigs, such
as are used in the field of oil extraction. but is also
applicable to other types of semi-submersible structure, for
example production rigs or tender rigs.
BACKGROUND OF THE INVENTION
Semi-submersible drilling rigs are generally regarded in the
marine industry as being the most versatile of all drilling
platforms. The rigs are capable of being used in shallow water
or water which is too deep for self-contained fixed platforms.
Generally, such a rig comprises a platform which is supported on
a pair of parallel elongate pontoons by a number of vertical
columns. although many other shapes and geometrics of supporting
structure are possible. The pontoons contain ballast tanks for
allowing the rig to be partially submerged in a controlled
manner.
When the ballast tanks are empty, the rig is in a transit
condition, in which it floats on its pontoons and may therefore
be readily transported, either by being towed or using an
onboard propulsion system, to a site of interest. Once the rig
has reached that site, ballast (ie sea water) is introduced into
the tanks in the pontoons, causing the rig to become partially
submerged. When ballasted down to the desired draft, the semi-
submersible rig is in a semi-submerged condition, in which it
provides a relatively stable platform from which the drilling
operation can be conducted. When in this state, the rig is
supported mainly by the immersed portions of the columns.
Numerous semi-submersible rigs have been designed, built and
operated over the last thirty years, all striving to meet the
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design goals of optimizing design size, weight, deck load and
cost while ensuring that the two rig conditions can be achieved:
a first, ballast free transit condition. and a ballasted, semi-
submerged operating condition (in which the rig is stable).
To that end, various design considerations have to be taken into
account, including
1. The pontoon size
2. The weight of the rig
3. The total volume of the ballast tanks
4. The volume of the rig legs
5. The positions of the centres of gravity and buoyancy of the
rig (in both transit and semi-submerged conditions).
a0 As a result, the rigs, when in the transit condition, are able
to sail much like ships. but operate in a stable, fully
ballasted and semi-submerged condition in which a relatively
small surface area (i.e. that of the legs) is exposed to wave
action.
a5
It is desirable for the semi-submersible rig to be of as compact
and cost effective a design as possible for a given deck load.
However, constraints on the size of the rig design are imposed
by the need to be able to accommodate various items of equipment
30 and consumable materials for use in the drilling process. The
equipment includes the drilling derrick, pipe handling
equipment. and the or each engine (usually an internal
combustion engine such as a diesel engine) for providing the
power to operate the rig. Such an engine is generally referred
35 to as a prime mover.
For the purposes of the present specification, consumable
materials for a drilling rig are defined as solid or liquid
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materials which are used up or taken off the rig during the
drilling process, thereby reducing the rig's gross mass. Thus
the consumable materials include drilling mud or the initially
separate solid and liquid constituents of the mud, and fuel oil
for operating the prime mover. During operation, the total mass
of the rig is also reduced by the removal or deployment of
drilling equipment, for example drilling tubulars (such as drill
pipes, casings and risers). Such items of equipment are
hereinafter referred to as removable equipment.
It will be appreciated that other types of semi-submersible
structure may carry different consumable materials and items of
removable equipment.
In order to be able to maintain the necessary draft in all
possible operating conditions, known types of semi-submersible
rigs need sufficient capacity for ballast to maintain stability
of the rig both with and without the consumable materials,
and/or drilling equipment (such as drilling tubulars) on the
rig.
The present invention seeks to provide a semi-submersible
structure which is of a more compact design while providing a
relatively large deck load and starting capacity, and
flexibility in deep sea conditions. A preferred form of
invention seeks to enhance the provision of storage for
equipment and consumables on the rig while limiting the size of
the pontoons and still providing a sufficient amount of ballast
to enable the structure to achieve the desired draft in its semi
submerged condition.
SUMMARY OF THE INVENTION
According to a first aspect of the invention, there is provided
a semi-submersible structure having a transit condition in which
the structure is movable to a site of use, and a semi-submerged
condition in which the structure is sufficiently stable to be
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used at said site, the structure being adapted to carry
consumable solid or liquid materials and removable equipment,
which are to be removed therefrom during use. the structure
comprising pontoon means containing first compartment means for
containing consumable liquids, ballast control means for
introducing ballast into the first compartment means to
compensate for the removal of consumable materials from the
structure. and second compartment means for receiving ballast,
Wherein the arrangement is such that, if the first compartment
means were empty and no consumable material or removable
equipment is being carried by the structure, the filling of the
second compartment means with ballast will not cause the
structure to be submerged to an extent sufficient to achieve
said semi-submerged condition (it is in this sense "over-
buoyant"), whereas said condition would be achieved if the first
compartment means were also filled.
Conventionally, the pontoons of semi-submersible structures can
provide little or no storage for consumable materials or
equipment. In order to provide such storage, a pontoon of
conventional design would have to be of a considerably larger
volume than would otherwise be necessary. This in turn would
increase the amount of ballast water which Would have to be held
by the structure if it is to achieve its semi-submerged
condition, thus negating the advantages of using the pontoons
for storage. The problem would not be solved by designing the
pontoon to have an increased capacity since this would further
increase the volume, and hence displacement. of the pontoons,
and would correspondingly increase the amount of ballast which
the structure would have to be able to hold.
By contrast. the pontoon means of a structure in accordance with
the present invention can provide a significant amount of
storage for consumable materials, since the same part of the
pontoon means, ie the first compartment means. can be used to
hold consumable liquid and ballast, thus decreasing the
proportion of the pontoon means (relative to conventional
design) which is required for dedicated ballast tanks.
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Indeed, the pontoon means can be designed to provide further
storage in addition to that provided by the first compartment
means, sad to that end preferably includes third compartment
means for housing other consumable materials or fixed equipment.
The consumable materials contained in the third compartment
means may, for example, comprise fuel oil, drilling equipment or
anchor lines. Preferably, the third compartment means comprises
a compartment of sufficient size to house at least one prime
mover for the structure.
The situating of one or more prime movers in the pontoon means
not only increases the amount of usable space (for a given size
of structure) above the pontoons, but also significantly lowers
the center of gravity of the structure, giving rise to a
significant increase in deck load capacity for a given steel
weight of the structure.
Preferably, the first compartment means comprises a plurality of
tanks for the consumable liquid and ballast. Furthermore, the
structure is preferably adapted to carry a sufficient weight of
consumable materials, outside the first compartment means, to
enable the semi-submerged condition to be achieved when at least
one of the tanks of the first compartment means is empty.
Consequently, the ballast control means can be arranged to add
water only to those tanks of the first compartment means which
contain substantially no consumable liquid, thus enabling
ballast to be added to those tanks even if the consumable
liquids must not be mixed with the ballast.
According to a second aspect of the invention, there is provided
a semi-submersible structure comprising pontoon means having
first compartment means for containing consumable liquid, second
compartment means, exclusively for containing ballast, and
ballast control means for adding ballast to the first
compartment means, wherein the capacity of the second
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compartment means lies in the range of 40~ to 45% of the total
volume of the pontoon means.
Preferably. the capacity of the second compartment means is
approximately 40% of the total volume of the pontoon means.
It has been found that, when the structure is in its semi-
submerged condition, the pontoon means should constitute 70% of
the total volume of the submerged portion of the structure, if
the structure is to have the desired stability. Accordingly, if
all the ballast compartments are to be housed in the pontoon
means, those compartments must be able to receive a volume of
water equivalent to at least 30% of the total submerged volume
of the structure in order to counteract the increase in
displacement as the structure moves into the semi-submerged
condition. This defines the lower limit of the above range: if
the capacity of the second compartment means is less than
approximately 40% of the pontoon volume, the semi-submerged
condition will not be achieved by flooding the second
compartment means if, before said flooding, the pontoon means
were at the water surface.
The same criterion applies to conventional semi-submersible
structures, but such designs require additional capacity in
their ballast tanks to take into account the removal of
consumable materials or removable equipment on deck. On the
other hand, a structure in accordance with the present invention
does sot need the extra capacity since the first compartment
means is also used to contain ballast water when necessary.
If the capacity of the second compartment means exceeds 45% of
the volume of the pontoon means. then the structure will sit too
high in the water when in its transit condition (with the second
compartment means empty), and the pontoon means would be larger
than is necessary.
According to a third aspect of the invention, there is provided
a semi-submersible structure adapted to carry consumable solid
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or liquid materials to a site of use, the structure comprising
pontoon means having first compartment means for containing
consumable material, second compartment means for containing
ballast, sensing means fox detecting the amount of consumable
material in the first compartment means and ballast control
means connected to the sensing means and arranged to introduce
ballast Water to the first compartment means to compensate for
the loss of mass due to the removal of consumable material.
Preferably, the first compartment means comprises a plurality of
tanks, and the ballast control means is arranged to add ballast
to one or more selected tanks that no longer contain consumable
material.
The invention also lies in a method of ballasting a semi-
submersible structure having pontoon means which include a
plurality of tanks containing consumable liquids, which are
progressively extracted from the tanks during use of the
structure, the method comprising determining which, if any, of
the tanks has been substantially emptied of consumable liquid
and introducing ballast into that tank or at least one of those
tanks.
Preferably, if a plurality of tanks containing the same type of
consumable liquid are partially full, the method further
comprises determining whether the volume of consumable liquid
remaining in one of the tanks is less than or equal to the spare
capacity of the other tank or tanks, and if it is transferring
the liquid to said other tank or tanks to enable said one tank
then to receive ballast.
35
Preferably the structure is an offshore drilling rig.
Preferably, the pontoon means comprises a pair of elongate,
parallel, spaced apart pontoons.
These and other features of the invention, preferred embodiments
and variants thereof, possible applications and advantages will
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become appreciated and understood by those skilled in the art
from the following detailed description and drawings.
g DRAWINGS
FIG. 1 is a perspective view of the rig, when in its semi-
submerged condition;
FIG. 2 is a side elevation of part of the rig;
FIG. 3 is a partially sectioned, partially cut away plan view
of the rig, the section being taken along the plane of
the water line indicated in FIG. 2;
FIG. 3 is a cut away plan view of a pontoon for the rig;
FIGs. 4 to 10 are sectional end views of the pontoon taken along
the lines V-V to X-X respectively;
FIG. 11 is a sectional plan view taken along the line XI-XI of
FIG. 6;
FIG. 12 is a sectional view taken along the line XII-XII of
FIG. 10;
FIG. 13 is a schematic view of the system for controlling the
ballast for the rig; and
FIG. 14 is a table giving examples of sizes of various parts
of the rig structure..
MODES) FOR CARRYING OUT THE INVENTION
~nlith reference to FIG. 1, a semi-submersible drilling rig
comprises a platform 2 mounted on a pair of elongate, parallel
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pontoons 4 and 8 through columns 10. 12, 14 and 18. In order to
provide additional rigidity to the structure, the rig includes
fore and aft lateral bracing members 20 and 22, extending
between the columns 10 and 12 and 14 and 18 respectively and
cross-bracing members 24 extending between diametrically opposed
pairs of the columns. Each of the pontoons 4 and 8 is provided
with a respective pair of thrusters, three of which are shown in
FIG. 1 at 26, 28 and 30 driven by electric motors housed in
pontoons 4 and 8. The azimuth of each of the thrustera is
adjustable to enable the thrusters to be used in the dynamic
positioning of the rig, ie to counteract any tendency of the rig
to drift away from a drilling site on the sea floor. In
addition, when the rig is in its transit condition, the columns
10, 12, 14 and 18 are substantially entirely above the water
line and the thrusters can be used to transport the rig to a
site of use. The fourth thruster is shown at 27 in FIG. 2.
The rig includes a derrick 32 mounted at its aft end, and
racking (not shown) for drilling tubulars. ie drill pipes,
drilling casings and drilling risers, (not shown) is situated
adjacent the derrick 32. As can be seen from FIG. 2, the aft
columns 18 and 14 are of a larger cross-sectional area than the
fore columns 10 and 12. When the rig is in use, this reduces
any pitch caused by change of load at the aft end of the rig
since any such change would tend to be counteracted by the
increased change in displacement caused by the columns 18 and
14.
Each of the columns 14 and 18 also includes a tank for
containing potable water. The tank in column 12 is denoted by
the reference numeral 38.
Each of the fore columns 10 and 12 includes containers, such as
are denoted by reference numerals 34 and 36 (and 34' in FIG.
10), for solid powder which is to be mixed with brine or
drilling water to form mud. The columns 10 and 12 each house a
respective chain locker (not shown), whilst all four columns
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include elevators to facilitate access to the interior of the
pontoons 4 and 8.
As can be seen from FIG. 3, each of the pontoons 4 and 8 is
divided into a number of compartments. The arrangement of
compartments is substantially identical, and only the
compartments of the pontoon 8 will therefore be described with
reference to FIG.s 4-10.
The pontoon 8 includes two aft ballast tanks 40, 42 situated
adjacent a thruster chamber 44 for housing a 7,000 or B.OOOkw
electric motor 46 for the thruster 27. Adjacent the tanks 40, 42
and the chamber 44 is a winch room 48 located beneath the base
of the column 14. The winch room 48 contains a winch 50 for
anchor cable for the rig. A further dedicated ballast tank 52
is situated at one side of the chamber 48 opposite a tank 54 for
base oil. A further ballast tank 56 is located beneath the
floor of the chamber 28 (see FIG. 6).
In front of the chamber 48, the pontoon contains a pair of drill
water tanks 58 and 60 adjacent which a pair of fuel oil tanks 62
and 64 are situated. Each of the tanks 58. 60, 62 and 64 abuts
a respective one of four reserve mud tanks 66, 68, 70 and 72.
Each of the reserve tanks includes a respective level sensor and
a pair of pumps for pumping liquids into and out of the reserve
mud tanks.
Two of the level sensors (for the tanks 68 and 70) are shown,
referenced 152 and 154 in FIG. 13. That FIG. also shows the
pair of pumps. referenced 114 and 133, for the tank 68, and the
corresponding pumps 116 and 130 for the tank 70.
Each of the reserve mud tanks is fitted with a respective one of
four stirrers, 74, 75. 76 and 78, and is provided with a
respective access hatch, for example as is shown at 80 and 82 in
FIG.s 7 and 8.
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Three brine tanks 84, 86 and 88 are interposed between the
reserve mud tanks and the outer skin of the pontoon 4. An
electrical control room 90 situated immediately in front of the
tanks 64 and 78 contains power distribution switch gear 92 for
distributing power generated by three prime movers situated in
the next compartment 94. Each of the prime movers, referenced
96, 98 and 100 comprises a 4.4 megawatt diesel engine connected
to a respective electrical generator.
With the prime movers 96, 98. 100 located iri the pontoon 8, it
is necessary to convey, treat and in some instance store all of
the operating consumables (fuel, air, lube oil, etc) to the
machinery spaces in the pontoon 8.
Fuel is loaded at the levels of the main deck via a fuel loading
system (not shown), and stored in the main fuel tanks 62 and 64.
From these tanks, it is treated/filtered and then pumped to and
stored in a secondary tank 200 periodically as required. From
these secondary tanks, the treated fuel is pumped to the engines
on demand, via a series of supply lines which may be fitted with
inline filters. In the event that the fuel pumping system is
inoperable, the fuel is transferred to the engines on demand via
gravity transfer. This is achieved via a secondary fuel supply
line (not shown) which bypasses any inline filters, when a
normally closed electric solenoid valve is de-energised either
through loss of power (black out). on command from the
engine/power management system (automatic, or operator
requested), or via manual intervention. Fuel delivered to the
engines in excess of the demand is returned to the secondary
storage tank via a fuel return line (not shown). All fuel tanks
ate vented at the main deck level.
Air required for combustion is supplied to the prime movers via
a series of coarse filters, fine filters and any secondary
treatment to preheat, cool or assist with the conveyance
(fan/blower) of the air via duct work as required. The ductwork
is fitted with sufficient means to isolate the engine intakes
from the ductwork in the event of downflooding, or gas ingress.
The duct intakes) are located at the main deck level, and are
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situated along the side of the deck box in such a way as not to
interfere with the main deck space, and draw air from a safe
area (aon classified space). These intakes are fitted with the
necessary equipment to ensure that wind/wave borne water spray
does not invade the intake ductwork. The intakes are also
fitted with the equipment required to insure the watertight
integrity of the unit should the duct intake become submerged.
Ventilation for the machinery spaces) in the pontoon 8 is
forced, and provided via a series of coarse filters, fine
filters. and any secondary treatments required, and ductwork.
The air is supplied in sufficient quantity to adequately remove
any excess/waste heat generated by the prime movers and related
machinery, as well as all ancillary and propulsion equipment and
machinery. The ductwork is fitted with sufficient means to
isolate the machinery spaces from the ductwork in the event of
downflooding, or gas ingress. The duct intakes) are located at
the main deck level, and are situated along the side of the deck
box in such a way as to not interfere with the main deck space,
and draw air from a safe area (non classified space). These
intakes are fitted with the necessary equipment to ensure that
wind/wave borne water spray does not invade the intake duct
work. The intakes are also fitted with the equipment required
to insure the watertight integrity of the unit should the duct
intake become submerged. The ventilation ductwork is arranged
is a configuration which ensures adequate air movement within
each space to prevent the accumulation of heat, vapours, and
moisture. (no isolated pockets of air). The ventilation exhaust
is returned to surface via ductwork and exhausted to a safe
area. The exhaust duct work is fitted With the same facilities
for ensuring isolation and water tight integrity as the intake
duct work. The exhaust air is not treated or filtered.
Prime mover exhaust is conveyed to the main deck level via
shielded exhaust manifolds 156 and ductwork. The shielding is
designed to minimise the heat radiated into the machinery
spaces. void spaces, columns etc in the vicinity of the
manifolds and ductwork. The exhaust ductwork is fitted with
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sufficient means to isolate the primemover exhaust manifolds in
the event of downflooding. Additionally, the exhaust ductwork
can be fitted with spark arresting facilities.
Prime Mover control is provided via an engine and power
management system, which incorporates facilities for automatic,
remote and manual operation of the engines. as well as alarm and
supervisory functions. The system is designed to selectively
start or shut down primemovers as required, as well as initiated
emergency shutdown ensuring that the vessel integrity is
maintained by isolating fuel, air and exhaust ducting as
required.
Ancillary vessel services, such as compressed air for engine
starting, air tools and fresh/potable water, etc are provided
via the respective distribution networks (not shown) with
sufficient takeoff connection points to enable efficient
operations.
Further information on housing the prime movers in the pontoon
is given in the UK Patent Application entitled Semi-submersible
Structure filed concurrently herewith under case reference
694/S.
Compartment 94 is situated ad3acent the base of the column 12,
which overlies a further ballast tank 102. A further,
substantially C-shaped ballast tank I04 extends behind the
floor, ceiling and one wall of the room 94.
The front of the pontoon 8 houses another 7,000 or 8,OOOkw
thruster motor 104 for the thruster 26 and two fore ballast
tanks 106 and 108 similar to the tanks 40 and 42.
The first tank means of the pontoon 8 is constituted by the
drilling water tanks 58 and 60, the reserve mud tanks 66, 68, 70
and 72 and the brine tanks 84, 86 and 88.
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Each of these tanks accordingly is connected to inlets for its
consumable liquid, ie drilling water. mud or brine, and for
ballast water. Similarly, the tanks are connected to outlet
conduits either for ballast water to be expelled and for the
consumable liquid to be used. The ballast tanks 40, 42, 52, 54,
102, 104, 106. and 108. however, only accept ballast, and only
have outlets for ballast discharge. Accordingly, these are
dedicated ballast tanks which constitute the second compartment
means of the pontoon 8. The remaining compartments/spaces in
the pontoon 8 constitute the third compartment means.
In the present example, the pontoon 8 has a total volume of
approximately 11,000 m3. the dedicated ballast tanks, ie the
second compartment means, have a combined capacity of
approximately 4,500 m3, whilst the drill water, brine and
reserve mud tanks have a total capacity of approximately 1,700
m3. Thus the capacity of the dedicated ballast tanks amounts to
approximately 40°~ of the volume of the pontoon 8. The
capacities of the individual tanks are set out in the table of
FIa. 14.
In a paper entitled "The Design Process of the Semi-Submersible
Drilling Vessel "DSS-20'"' P J Shepman, J A van Santen and C Y Ho
discuss the various factors influencing the design of the
pontoons and the columns of a semi submersible rig.
From these discussions, it can be deduced that. with the rig in
its semi-submerged condition, the volume of the submerged
columns should constitute around 30~ of the total volume of
water displaced by the rig, whilst the volume of the (submerged)
pontoons therefore amounts to 70~ of the total submerged volume.
Accordingly, at least 40~ of the volume of the pontoons must be
capable of being flooded with ballast water in order to
counteract the buoyancy of the columns, so that the rig may move
from its transit condition into the semi-submerged condition.
However this is not achieved if the drilling water, reserve mud
and brine tanks are empty and no other consumables are being
carried by the rig. Consequently, when one of the those tanks
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has been emptied of its consumable liquid, it is important for
it to be available for ballast. FIG. 13 shows an example of one
ballast control system for achieving this.
The ballast control system will only be described in relation to
the reserve mud tanks 75 and 76, although it will be understood
that the system operates on the other tanks constituting the
first compartment means in a similar fashion.
Each of the tanks 75 and 76 has a respective inlet 110 and 112
for recirculated drilling mud or ballast. Each inlet is
associated with the respective pump 114 and 116 for filling the
tank with whichever liquid is required. The inlet to each pump
can be selectively connected to any one of a mud supply conduit
118, a ballast water supply conduit 120 or a mud transfer
conduit 122 by means of valves 123 to 128. A similar
arrangement of pumps, 130 and 132 and valves 133 to 138 can be
used to selectively supply liquid from each tank via a
respective outlet to a selective one of a ballast water
discharge conduit 140, a mud outlet conduit 142 or a transfer
conduit 144 and 156 which is in turn connected to the inlet of
another of the reserve mud tanks.
The operation of the valves and motors is all controlled by a
control unit 148. The controller is also connected to the level
sensors 75 and 76 and, in use, monitors the level of drilling
mud in the reserve mud tanks. If mud is required from any of
those tanks, a mud supply system (not shown) sends an
appropriate signal to the controller 148, which supplies mud
along the conduit 140 using either the pump 132 or the pump 130
(or both). As this happens, the controller 148 continues to
monitor the level of mud in each of the tanks, and when one of
the tanks is empty, closes the valve connecting the outlet of
that linked to the conduit 142, and opens the valve between the
inlet of the tank and the ballast conduit 120. The pump 114 or
116 is then operated to urge water into the reserve mud tank
(which now no longer contains mud) if necessary in order to
maintain the desired draft and orientation of the structure. If
16.044 PCT ca o23iooi2 2ooo-os-ns ; ; ~; , ': ' '
- 16 -
one of the reserve mud tanks is almost empty, and the other has
spare capacity, the controller 148 rnay open the valve 135 or 138
and 128 or 123 and thus allowing mud to be pumped from one of
the reserve mud tanks to the other, thus freeing the first of
those tanks to receive ballast.
The brine and drilling water tanks can similarly be filled with
ballast once emptied of their consumable liquids.
The controller 148 controls all of the tanks which can receive
ballast (in both pontoons), and is programmed so that the
moments about the structure's centre of gravity caused by the
removal of liquid from any of the tanks are balanced by the
moments caused by the introduction of compensating ballast to
other tanks. Thus, referring to FIG. 3, the removal of m kg
of mud from the tank 60 produces a moment of m x a and m x b
about the centre of gravity 150. In order to compensate for
this, the liquid introduced into the other tanks must be equal
and opposite moments about the centre of gravity 150. This
requirement can be expressed in the form of series of
simultaneous equations (which may have a plurality of
solutions). In addition, the controller may also be connected
to equipment for detecting the pitch or roll in the structure
and to add or remove ballast to compensate for this movement.
The structure can be modified so that they form part of the
first compartment means, and can be used either to store brine
or hold ballast.
The storage provided in the pontoons 4 and 8 enable the rig to
have a very large bulk and liquid mud storage capacity for its
size: the rig can store 4,800 barrels of mud at the platform 2,
whilst the pontoons have a capacity for 2,650 barrels of reserve
mud, 2,400 billion barrels of base oil and 1,925 barrels of
brine. This allows for two or three separate mud systems to be
operated on the rig concurrently, which saves time and cost when
changing over mud systems considered no longer necessary to stop
to clean pits.
~~~~~r.'~ii~~fs :'s~P°E ~.
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Additionally, pontoon mud tanks can be served by dedicated
supply and discharge piping thereby minimising the risk of
product contamination. The mud pumping and circulating system
does not form part of the subject matter of the present
invention.
In addition to the equipment and processes described above, the
rig incorporates equipment and facilities which are configured
to integrate the handling, storage, preparation, use and
disposal of all drilling, completion and work over fluid which
may be used during the well construction process. These include
the equipment required to maintain multiple fluid systems on
board concurrently. Additionally, certain equipment items,
processes and controls have been configured to enable dual use
(ie cement and mud mixing and pumping using common equipment).
The integrated fuel management system incorporates local, remote
and manual operation and controls facilities which enable the
fluid system to be configured remotely by a driller from a dog
house on the platform 2, locally from a fluid process control
centre or manually at each tank/manifold. The drilling fluid
process control room is located in the mud house and houses all
of the equipment required for fluid process management
monitoring and alarm functions, pump control, test and analysis.
The decision to locate the prime movers and related power
generation and distribution switchgear and machinery in the
pontoons of the vessel was taken to achieve a number of design
objectives including, but not limited to:
1. Reducing the steel weight of the vessel for a given
displacement
2. Reducing the machinery deck space requirements
3. Satisfying the dual independent engine room
requirements for DPS3 classification
4. Reducing the noise and vibration exposure at main deck
level
CA 02310012 2000-OS-15
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5. Lowering the Centre of Gravity of the vessel which
increases the variable deckload capacity
6. Reducing the amount of ballast required to lower the
rig to operation draft
To enable the safe and efficient operation of the prime movers
in this configuration, the rig is arranged to:
1. Provide adequate ventilation and cooling of the
machinery spaces
2. Condition the engine and machinery space intake air
3. Remove radiant heat from the machinery space
4. Convey a combustion exhaust to surface
a0 5. Provide for prime mover cooling via closed loop heat
exchangers, using a combination of salt water/fresh
water coolant heat exchangers in conjunction with pipe
rune within the ballast tanks.
6. Provide gravity fuel feed in the event that fuel
transfer/pumping is interrupted.
Rig Construction
The rig structure is based on a series of simple boxes, and
rectangular hull forms. This feature facilitates prefabrication
and final assembly. The modular nature of the construction
means that the individual modules could be pre-assembled and
outfitted prior to final assembly in a different location, which
minimizes overall construction time.
