WO2015039667A2 - A floating offshore rig for drilling in a hydrocarbons containing formation and having a converted ballasting tank, and a method of operating the same - Google Patents

A floating offshore rig for drilling in a hydrocarbons containing formation and having a converted ballasting tank, and a method of operating the same Download PDF

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Publication number
WO2015039667A2
WO2015039667A2 PCT/DK2014/050290 DK2014050290W WO2015039667A2 WO 2015039667 A2 WO2015039667 A2 WO 2015039667A2 DK 2014050290 W DK2014050290 W DK 2014050290W WO 2015039667 A2 WO2015039667 A2 WO 2015039667A2
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WO
WIPO (PCT)
Prior art keywords
drilling mud
tank
rig
holding tank
mud
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Application number
PCT/DK2014/050290
Other languages
French (fr)
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WO2015039667A3 (en
Inventor
Kaj KRISTENSEN
Original Assignee
Semco Maritime A/S
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Application filed by Semco Maritime A/S filed Critical Semco Maritime A/S
Publication of WO2015039667A2 publication Critical patent/WO2015039667A2/en
Publication of WO2015039667A3 publication Critical patent/WO2015039667A3/en

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Classifications

    • E—FIXED CONSTRUCTIONS
    • E21—EARTH DRILLING; MINING
    • E21B—EARTH DRILLING, e.g. DEEP DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
    • E21B21/00—Methods or apparatus for flushing boreholes, e.g. by use of exhaust air from motor
    • E21B21/01—Arrangements for handling drilling fluids or cuttings outside the borehole, e.g. mud boxes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
    • B63B—SHIPS OR OTHER WATERBORNE VESSELS; EQUIPMENT FOR SHIPPING 
    • B63B1/00—Hydrodynamic or hydrostatic features of hulls or of hydrofoils
    • B63B1/02—Hydrodynamic or hydrostatic features of hulls or of hydrofoils deriving lift mainly from water displacement
    • B63B1/10—Hydrodynamic or hydrostatic features of hulls or of hydrofoils deriving lift mainly from water displacement with multiple hulls
    • B63B1/107—Semi-submersibles; Small waterline area multiple hull vessels and the like, e.g. SWATH
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
    • B63B—SHIPS OR OTHER WATERBORNE VESSELS; EQUIPMENT FOR SHIPPING 
    • B63B35/00—Vessels or similar floating structures specially adapted for specific purposes and not otherwise provided for
    • B63B35/44—Floating buildings, stores, drilling platforms, or workshops, e.g. carrying water-oil separating devices
    • B63B35/4413—Floating drilling platforms, e.g. carrying water-oil separating devices
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
    • B63B—SHIPS OR OTHER WATERBORNE VESSELS; EQUIPMENT FOR SHIPPING 
    • B63B39/00—Equipment to decrease pitch, roll, or like unwanted vessel movements; Apparatus for indicating vessel attitude
    • B63B39/02—Equipment to decrease pitch, roll, or like unwanted vessel movements; Apparatus for indicating vessel attitude to decrease vessel movements by displacement of masses
    • B63B39/03—Equipment to decrease pitch, roll, or like unwanted vessel movements; Apparatus for indicating vessel attitude to decrease vessel movements by displacement of masses by transferring liquids
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
    • B63B—SHIPS OR OTHER WATERBORNE VESSELS; EQUIPMENT FOR SHIPPING 
    • B63B1/00—Hydrodynamic or hydrostatic features of hulls or of hydrofoils
    • B63B1/02—Hydrodynamic or hydrostatic features of hulls or of hydrofoils deriving lift mainly from water displacement
    • B63B1/10—Hydrodynamic or hydrostatic features of hulls or of hydrofoils deriving lift mainly from water displacement with multiple hulls
    • B63B1/12—Hydrodynamic or hydrostatic features of hulls or of hydrofoils deriving lift mainly from water displacement with multiple hulls the hulls being interconnected rigidly
    • B63B2001/128—Hydrodynamic or hydrostatic features of hulls or of hydrofoils deriving lift mainly from water displacement with multiple hulls the hulls being interconnected rigidly comprising underwater connectors between the hulls

Definitions

  • a floating offshore rig for drilling in a hydrocarbons containing formation and having a converted ballasting tank, and a method of operating the same The present invention relates generally to an improvement of floating offshore drilling rigs of the type comprising a deck structure with a derrick supporting a drill string and a substructure including ballasting tanks and a number of legs or columns extending into the water from the deck, the deck structure having a drilling mud pit supplying drilling mud to the drill string, at least one drilling mud holding tank being located on the substructure and being connected with the drilling mud pit via piping for transfer of the drilling mud between the mud holding tank and the drilling mud pit, a vent line extending from a top portion of the drilling mud holding tank to a vent port located at the level of the deck structure at a certain height above a bottom of the drilling mud holding tank.
  • the invention aims to allow for an increase in the capacity for holding drilling mud onboard such a rig and, hence, for an increased operation efficiency of the rig by allowing drilling staff to optimize procedures involved in the preparation of drilling mud.
  • the ballasting tanks in the leg or columns and/or in the pontoons have been converted to hold a liquid other than seawater, such as brine or oil for use on the rig, after a suitable rearrangement of the onboard piping. Conversion of a ballasting tank for use for storing drilling mud gives rise to challenges.
  • the ballasting tanks are conventionally sized to have a specified seawater capacity, and the steel bulkheads defining the ballasting tanks are dimensioned to provide a corresponding required structural strength.
  • drilling mud which typically has a specific gravity that is higher than that of seawater and that may vary greatly compared to that of oil
  • the maximum volume of that drilling mud that may be contained in the tank is less than the volume of the tank. If this maximum volume is exceeded the hydrostatic pressure at the tank bottom will generally exceed a value corresponding to the structural strength of the tank, which will lead to tank rupture.
  • Drilling mud generally has a specific gravity SG of up to about 2.7, compared to a SG of about 1.025 of seawater. Filling a ballasting tank with a liquid having a known SG of 2.7 would mean that only 1.025/2.7, i.e.
  • the tank capacity may be used if tank rupture is to be prevented. This is the typical way of determining the maximum allowable filling of a ballasting tank. In other words, even for a specific gravity less than 2.7 filling is halted once the measured volume of liquid pumped into the tank reaches 34 % of the tank volume. So, the tank may in fact have a true capacity higher than 34 % but is nevertheless not filled to this true capacity which could be significantly higher than 34 %, taking structural tank strength into account.
  • a drilling mud holding tank in particular a tank having served originally as a rig ballasting tank converted after installing suitable mud pit piping, comprises at least one pressure sensor for registering a hydrostatic pressure in the drilling mud tank at or near the level of said bottom, it becomes possible to fill a converted ballasting tank to a higher degree than hitherto, without the need to calculate the tank strength, which is often not readily known.
  • the specific gravity of the drilling mud is unknown it may be calculated using a level sensor as defined in claim 8, and on this basis the volume to which the tank may be filled can be calculated as 1.025/SG, where SG is the calculated specific gravity.
  • a control device will preferably be configured to halt filling when the level to which the tank is filled reaches a vent line for venting the tank. This is for preventing the drilling mud from rapidly raising upwards in the narrow vent line.
  • vent port will normally be located at the level of the rig deck, to prevent ingress of seawater into the vent line.
  • FIG. 1 shows schematically of offshore drilling rig seen from the side
  • Fig. 2 shows schematically a section of the rig of fig. 1.
  • Fig. 1 is a schematic side view showing an example of an offshore rig 10 floating in a body B of water and used for drilling in a formation below the body B of water.
  • the rig 10 has, as is conventional, a number of columns 20 extending between submerged pontoons P and a platform or deck 15, with a derrick 22 on the deck 15 supporting a drill string (not shown).
  • the columns 20 are part of the rig 10 substructure and are respective steel structures having a central passage with anchoring devices 5 for anchoring the rig 10 in a fixed position above the seabed.
  • the rig 10 has a plurality of ballasting tanks 30, some of which are in the pontoons P, that are flooded with seawater to provide for a ballasting when the rig 10 is in the drilling position.
  • ballasting tank 30 may have a capacity in the order of 300 m 3 , and ballasting tanks may typically be interconnected by appropriate piping.
  • the ballasting tanks 30 each have a vertical vent line extending up to a respective vent port 34, the latter being arranged at the level of the deck 15 to prevent seawater from ingressing through the vent port 34 into the ballasting tank 30. It will be understood that for a ballasting tank 30 in the pontoons the corresponding vent line will be relatively longer than for a ballasting tank 30 in one of the columns 20.
  • one such vent line extending from a ballasting tank 30 located in one of the columns 20 is shown schematically by reference nu- meral 32.
  • ballasting tanks 30 in the columns 20 and/or in the pontoons P have been converted to hold a liquid other than seawater, such as brine or oil for use on the rig, after a suitable rearrangement of the onboard pip- ing.
  • drilling a liquid known as drilling mud When drilling a liquid known as drilling mud is circulated in the drill string.
  • the drilling mud is conventionally prepared on the rig 10 by mixing additives, such as barite and bentonite, to desalinated seawater, base oil or other liquids, and is normally stored in a mud pit 13 located centrally on the deck 15 near the derrick 22.
  • the drilling mud is pumped from the mud pit 13 and to the drill bit in the well through the drill string.
  • the drilling mud is then circulated back to the mud pit 13 on the rig deck 15 by flowing up in the annulus between the well bore/marine riser and the drill string, bringing cuttings removed from the formation by the drill bit along with it.
  • the composition of the drilling mud is critical to the drilling for several reasons, inter alia to prevent well blowout and to stabilize the well bore, and dependent on the formation use is made of drilling mud of a specific density or specific gravity (SG) with respect to water.
  • the specific gravity of drilling mud typically is between 0.8 and 2.7.
  • the operators planning the drilling will prepare a batch of drilling mud having a desired specific gravity SGi and store it in a corresponding drilling mud holding tank 12, ready for being pumped to a mud pit 13 and then circulated to the drilling string. Batches, each having different SG will be stored in other drilling mud holding tanks.
  • the previously used "old" drilling mud of specific gravity SG2 is returned to a drilling mud holding tank and replaced in the formation by the al- ready prepared drilling mud having the other specific gravity SG I and originating from its drilling mud holding tank 12.
  • ballasting tanks 30 for storing the drilling mud of different specific gravities, in a condition ready for being pumped to the active mud pits 13 and then to the drill bit on demand.
  • the ballasting tanks of such a rig can also be used for storing "old" drilling mud or slops that was previously backloaded to a vessel. Conversion of a ballasting tank 30 for use for storing drilling mud gives rise to challenges.
  • ballasting tanks 30 are conventionally sized to have a specified seawater capacity, such as 300m 3 , meaning that the steel bulkheads defining the ballasting tanks 30 on existing rigs are dimensioned to provide the cor- responding required structural strength.
  • Seawater has a specific gravity SGsw in the order of 1.025 which is decisive for the required structural strength; if another liquid having a higher SG is filled into such a converted ballasting tank 30 the capacity, i.e. the maximum volume of that other liquid that may be contained in the tank 30, must be correspondingly less to prevent the hydrostatic pressure at the tank bottom from exceeding a value corresponding to the structural strength of the tank, which would lead to tank rupture.
  • filling a ballasting tank 30 with a liquid having a known SG of 2.7 would mean that only 1.025/2.7, i.e. about 34 %, of the tank capacity may be used if tank rupture is to be prevented.
  • This would be a typical way of determining a maximum allowable filling of a ballasting tank 30 with a liquid having a known SG higher than that of seawater.
  • filling is halted once the measured volume of liquid pumped into the tank reaches 34 % of the tank volume, which is already known.
  • the specific gravity of the drilling mud pumped into the tank is not known in advance, or may vary over time as the drilling mud is pumped into the converted ballasting tank 30. This is especially the case where the tank is filled with "old" drilling mud. This means that the determination of the maximum allow- able filling needs to be done on a worst case assumption, typically based on the assumption that the drilling mud has a specific gravity SG in the order of 2.7, and so the tank may in fact have a true capacity higher than 34 % but is nevertheless not filled to its true capacity which could be significantly higher than 34 % taking structural tank strength into account.
  • one or more hydrostatic pressure sensors S are installed in a converted ballasting tank 30 at the bottom thereof, preferably together with one, preferably at least two for safe- ty reasons, level sensors L registering the level to which the converted tank 30 is filled at any given time.
  • the pressure sensors S and the level sensors L are connected to a control device C (see fig. 2) connected to valves and pumps at the ballasting tank 30 inlet and having a computer for storing parameters/values reflecting a maximum permitted hydrostatic pressure (which may conveniently be indicated as meters of water column) and a maximum permitted liquid level, and for comparing the stored parameters/values with the values measured by the sensors S, L.
  • the hydrostatic pressure sensor S may be way of example be a diaphragm type pressure sensor which is simply submerged into the converted ballasting tank 30.
  • this value is stored in the computer. Where it is not known the determination of this value may simply be done on the basis of a on-site determination of the vertical distance or height H between the tank bottom and the aforementioned vent port 34.
  • the control device halts filling already when a level sensor L signals filling up to the level of the lower end of vent line or pipe 32, i.e. to the top of the tank 30, to avoid any drilling mud from standing in the vent line 32.
  • a level sensor L signals filling up to the level of the lower end of vent line or pipe 32, i.e. to the top of the tank 30, to avoid any drilling mud from standing in the vent line 32.
  • the control device halts filling already when a level sensor L signals filling up to the level of the lower end of vent line or pipe 32, i.e. to the top of the tank 30, to avoid any drilling mud from standing in the vent line 32.
  • the cross-sectional dimension of the vent line 32 is so small that liquid which fills the tank 30 beyond the latter level will rise along the vent line 32 at a very high speed, with a corresponding rapid increase in the hydrostatic pressure at the tank bottom, and so as to avoid sudden discharge of drilling mud at the vent port 34 caused by delays in the halting
  • the value of the specific gravity of the drilling mud in the tank 30 may also be determined, by simply dividing the measured hydrostatic pressure with the measured height level of the liquid above the bottom of the tank 30.
  • Shown in fig. 2 is an enlarged sectional and schematic view of such a ballasting tank 30 converted for use as a mud holding tank and provided with a hydrostatic pressure sensor S.
  • the pressure sensor S which may be of the Vegabar 66/67-type by Ohmart Vega, is in the shown embodiment suspended from a position above the bottom of the tank 30, by appropriate wiring such that the sensor head is located close to the tank 30 bottom, at which position it may be mechanically fixed by fixture F or held by the weight of the sensor S.
  • the sensor S may be of the type having a membrane M and is preferably suspended such that the membrane M faces downwards towards the tank bottom, or in any other way that preferably ensures that when the tank 30 is emptied, by pumping out the con- tents, liquid that previously was in contact with the membrane M is allowed to drip off the membrane M.
  • FIG. 2 shows a conduit 2 for pumping drilling mud to the tank by pump 1 which may be reversed for discharging the drilling mud by having in addition a suction pipe (not shown) reaching to the tank bottom. Also shown is electrical wiring Wl, W2 connecting sensor S with the computer C and pump 1, respectively.
  • the sensor L by way of example, use may be made of Vegapuls 62 radar type transmitters or similar sensors that are unaffected by vapour, gas composition, pressure and temperature.
  • an agitator device (not shown) arranged in the tank 30; such an agitator device may include an array of preferably movably mounted nozzles and may conveniently share pump with a separate tank 30 washing system.
  • the pump When the pump is used for agitating the content of the tank 30 is recirculated and discharged via the nozzles; on the other hand, when in tank washing mode the pump may conveniently circulate production water discharged via dedicated washing nozzles.

Abstract

The invention relates to a floating offshore rig (10) for drilling in a hydrocarbons containing formation below a body (B) of water, said rig (10) comprising: a deck structure (15) with a derrick (22) supporting a drill string, and a sub- structure including ballasting tanks (12) for said rig (10) and a number of leg or columns (20) for extending into said body (B) of water from said deck (15), said rig (10) including on said deck structure (15) a drilling mud pit (13) for supplying drilling mud to said drill string, at least one drilling mud holding tank (30) being located on said substructure and being connected with said drilling mud pit (13) via piping for transfer of said drilling mud between said mud holding tank (30) and said drilling mud pit (13), a vent line (32) extending from a top portion of said drilling mud holding tank (30) to a vent port (34) located at the level of said deck structure (15) at a height (H) above a bottom of said drilling mud holding tank (30), said drilling mud holding tank (30) com- prising at least one pressure sensor (S) for registering a hydrostatic pressure in said drilling mud tank (30) at or near the level of said bottom.

Description

A floating offshore rig for drilling in a hydrocarbons containing formation and having a converted ballasting tank, and a method of operating the same The present invention relates generally to an improvement of floating offshore drilling rigs of the type comprising a deck structure with a derrick supporting a drill string and a substructure including ballasting tanks and a number of legs or columns extending into the water from the deck, the deck structure having a drilling mud pit supplying drilling mud to the drill string, at least one drilling mud holding tank being located on the substructure and being connected with the drilling mud pit via piping for transfer of the drilling mud between the mud holding tank and the drilling mud pit, a vent line extending from a top portion of the drilling mud holding tank to a vent port located at the level of the deck structure at a certain height above a bottom of the drilling mud holding tank. More particularly, the invention aims to allow for an increase in the capacity for holding drilling mud onboard such a rig and, hence, for an increased operation efficiency of the rig by allowing drilling staff to optimize procedures involved in the preparation of drilling mud. In some existing offshore rigs the ballasting tanks in the leg or columns and/or in the pontoons have been converted to hold a liquid other than seawater, such as brine or oil for use on the rig, after a suitable rearrangement of the onboard piping. Conversion of a ballasting tank for use for storing drilling mud gives rise to challenges. The ballasting tanks are conventionally sized to have a specified seawater capacity, and the steel bulkheads defining the ballasting tanks are dimensioned to provide a corresponding required structural strength. If drilling mud, which typically has a specific gravity that is higher than that of seawater and that may vary greatly compared to that of oil, is filled into such a ballasting tank the maximum volume of that drilling mud that may be contained in the tank is less than the volume of the tank. If this maximum volume is exceeded the hydrostatic pressure at the tank bottom will generally exceed a value corresponding to the structural strength of the tank, which will lead to tank rupture. Drilling mud generally has a specific gravity SG of up to about 2.7, compared to a SG of about 1.025 of seawater. Filling a ballasting tank with a liquid having a known SG of 2.7 would mean that only 1.025/2.7, i.e. about 34 %, of the tank capacity may be used if tank rupture is to be prevented. This is the typical way of determining the maximum allowable filling of a ballasting tank. In other words, even for a specific gravity less than 2.7 filling is halted once the measured volume of liquid pumped into the tank reaches 34 % of the tank volume. So, the tank may in fact have a true capacity higher than 34 % but is nevertheless not filled to this true capacity which could be significantly higher than 34 %, taking structural tank strength into account.
By the invention as defined in the appended claim 1 where a drilling mud holding tank, in particular a tank having served originally as a rig ballasting tank converted after installing suitable mud pit piping, comprises at least one pressure sensor for registering a hydrostatic pressure in the drilling mud tank at or near the level of said bottom, it becomes possible to fill a converted ballasting tank to a higher degree than hitherto, without the need to calculate the tank strength, which is often not readily known. Where the specific gravity of the drilling mud is unknown it may be calculated using a level sensor as defined in claim 8, and on this basis the volume to which the tank may be filled can be calculated as 1.025/SG, where SG is the calculated specific gravity. Since sometimes the SG of drilling mud may be less than 1.025 a control device will preferably be configured to halt filling when the level to which the tank is filled reaches a vent line for venting the tank. This is for preventing the drilling mud from rapidly raising upwards in the narrow vent line.
Alternatively, no calculation as mentioned above need be performed when an estimate of the tank strength has previously been made based on the height between the tank bottom and a vent port of the vent line, which will correspond to a maximum hydrostatic pressure for which the ballasting tank will originally have been designed. The vent port will normally be located at the level of the rig deck, to prevent ingress of seawater into the vent line.
The invention will now be discussed in further details with reference to an embodiment shown in the drawings where Fig. 1 shows schematically of offshore drilling rig seen from the side, and
Fig. 2 shows schematically a section of the rig of fig. 1. Fig. 1 is a schematic side view showing an example of an offshore rig 10 floating in a body B of water and used for drilling in a formation below the body B of water. The rig 10 has, as is conventional, a number of columns 20 extending between submerged pontoons P and a platform or deck 15, with a derrick 22 on the deck 15 supporting a drill string (not shown). The columns 20 are part of the rig 10 substructure and are respective steel structures having a central passage with anchoring devices 5 for anchoring the rig 10 in a fixed position above the seabed. As is conventional the rig 10 has a plurality of ballasting tanks 30, some of which are in the pontoons P, that are flooded with seawater to provide for a ballasting when the rig 10 is in the drilling position.
By way of example one such ballasting tank 30 may have a capacity in the order of 300 m3, and ballasting tanks may typically be interconnected by appropriate piping. The ballasting tanks 30 each have a vertical vent line extending up to a respective vent port 34, the latter being arranged at the level of the deck 15 to prevent seawater from ingressing through the vent port 34 into the ballasting tank 30. It will be understood that for a ballasting tank 30 in the pontoons the corresponding vent line will be relatively longer than for a ballasting tank 30 in one of the columns 20. In fig. 1 one such vent line extending from a ballasting tank 30 located in one of the columns 20 is shown schematically by reference nu- meral 32.
For some existing rigs the ballasting tanks 30 in the columns 20 and/or in the pontoons P, have been converted to hold a liquid other than seawater, such as brine or oil for use on the rig, after a suitable rearrangement of the onboard pip- ing.
When drilling a liquid known as drilling mud is circulated in the drill string. The drilling mud is conventionally prepared on the rig 10 by mixing additives, such as barite and bentonite, to desalinated seawater, base oil or other liquids, and is normally stored in a mud pit 13 located centrally on the deck 15 near the derrick 22. The drilling mud is pumped from the mud pit 13 and to the drill bit in the well through the drill string. The drilling mud is then circulated back to the mud pit 13 on the rig deck 15 by flowing up in the annulus between the well bore/marine riser and the drill string, bringing cuttings removed from the formation by the drill bit along with it.
The composition of the drilling mud is critical to the drilling for several reasons, inter alia to prevent well blowout and to stabilize the well bore, and dependent on the formation use is made of drilling mud of a specific density or specific gravity (SG) with respect to water. The specific gravity of drilling mud typically is between 0.8 and 2.7.
On modern rigs the operators planning the drilling will prepare a batch of drilling mud having a desired specific gravity SGi and store it in a corresponding drilling mud holding tank 12, ready for being pumped to a mud pit 13 and then circulated to the drilling string. Batches, each having different SG will be stored in other drilling mud holding tanks. When the formation requires use of drilling mud of specific gravity SGi the previously used "old" drilling mud of specific gravity SG2 is returned to a drilling mud holding tank and replaced in the formation by the al- ready prepared drilling mud having the other specific gravity SG I and originating from its drilling mud holding tank 12. This option is typically available on modern rigs only since older rigs only carry a mud pit arrangement that requires the "old" drilling mud to be back loaded to a vessel moored by the rig 10 or to be dumped overboard, via the mud holding tank. On such older rigs this discharge procedure requires the drilling operation to be halted until "new" drilling mud has been prepared and stored in the drilling mud tank.
Having mud holding tanks 12 on the deck 15 takes up much deck space and increases the vertical center of gravity and so it has been suggested to use of some of the ballasting tanks 30 for storing the drilling mud of different specific gravities, in a condition ready for being pumped to the active mud pits 13 and then to the drill bit on demand. The ballasting tanks of such a rig can also be used for storing "old" drilling mud or slops that was previously backloaded to a vessel. Conversion of a ballasting tank 30 for use for storing drilling mud gives rise to challenges. As mentioned, ballasting tanks 30 are conventionally sized to have a specified seawater capacity, such as 300m3, meaning that the steel bulkheads defining the ballasting tanks 30 on existing rigs are dimensioned to provide the cor- responding required structural strength. Seawater has a specific gravity SGsw in the order of 1.025 which is decisive for the required structural strength; if another liquid having a higher SG is filled into such a converted ballasting tank 30 the capacity, i.e. the maximum volume of that other liquid that may be contained in the tank 30, must be correspondingly less to prevent the hydrostatic pressure at the tank bottom from exceeding a value corresponding to the structural strength of the tank, which would lead to tank rupture.
By way of example, filling a ballasting tank 30 with a liquid having a known SG of 2.7 would mean that only 1.025/2.7, i.e. about 34 %, of the tank capacity may be used if tank rupture is to be prevented. This would be a typical way of determining a maximum allowable filling of a ballasting tank 30 with a liquid having a known SG higher than that of seawater. In other words, for a SG of 2.7 filling is halted once the measured volume of liquid pumped into the tank reaches 34 % of the tank volume, which is already known.
Sometimes, the specific gravity of the drilling mud pumped into the tank is not known in advance, or may vary over time as the drilling mud is pumped into the converted ballasting tank 30. This is especially the case where the tank is filled with "old" drilling mud. This means that the determination of the maximum allow- able filling needs to be done on a worst case assumption, typically based on the assumption that the drilling mud has a specific gravity SG in the order of 2.7, and so the tank may in fact have a true capacity higher than 34 % but is nevertheless not filled to its true capacity which could be significantly higher than 34 % taking structural tank strength into account.
To determine the point where filling of a converted ballasting tank must be terminated one might install strain gauges monitoring deformations in the ballasting tanks 30 at the bottom thereof. However, this would require knowledge of the maximum allowable tank deformations, information which is not always readily available. Alternatively it is necessary to perform complicated final element calcu- lations and subsequently install reinforcements where required to accommodate for a 100% filling of the tank 30 with liquid having the maximum prevalent SG of about 2.7. However this would require knowledge of the original global stress analysis, information which is not always readily available. Also, in many cases will reinforcements not be practical.
With the present invention one or more hydrostatic pressure sensors S, preferably at least two for safety reasons, are installed in a converted ballasting tank 30 at the bottom thereof, preferably together with one, preferably at least two for safe- ty reasons, level sensors L registering the level to which the converted tank 30 is filled at any given time. The pressure sensors S and the level sensors L are connected to a control device C (see fig. 2) connected to valves and pumps at the ballasting tank 30 inlet and having a computer for storing parameters/values reflecting a maximum permitted hydrostatic pressure (which may conveniently be indicated as meters of water column) and a maximum permitted liquid level, and for comparing the stored parameters/values with the values measured by the sensors S, L. The hydrostatic pressure sensor S may be way of example be a diaphragm type pressure sensor which is simply submerged into the converted ballasting tank 30.
Where the maximum permitted hydrostatic pressure reflected by the structural design strength of the ballasting tank 30 is directly known in advance from rig data sheets, this value is stored in the computer. Where it is not known the determination of this value may simply be done on the basis of a on-site determination of the vertical distance or height H between the tank bottom and the aforementioned vent port 34. This height H is the height of a column of seawater (SGsw = 1.025) that the tank 30 must originally have been dimensioned to accept, at the time the rig was designed. In operation, when the converted ballasting tank 30 is filled to level I and the pressure sensor(s) S registers a hydrostatic pressure (which may be indicated in a unit height, such as meter/cm, of water column) at the tank 30 bottom corresponding to the maximum permitted hydrostatic pressure (which may be indicated as meter/cm desalinated water column, such as H/1.025), filling of the tank 30 with drilling mud is stopped by the control device C. This will typically occur where the liquid level I in the tank 30 is less that H since drilling mud pumped into the ballasting tank 30 most typically has a specific gravity SG higher than 1,025, which is the specific gravity SGsw of seawater. Preferably, such as where the height H is used for determining the maximum permitted hydrostatic pressure the control device halts filling already when a level sensor L signals filling up to the level of the lower end of vent line or pipe 32, i.e. to the top of the tank 30, to avoid any drilling mud from standing in the vent line 32. This is because the cross-sectional dimension of the vent line 32 is so small that liquid which fills the tank 30 beyond the latter level will rise along the vent line 32 at a very high speed, with a corresponding rapid increase in the hydrostatic pressure at the tank bottom, and so as to avoid sudden discharge of drilling mud at the vent port 34 caused by delays in the halting of the filling pump.
Based on the signals from the level sensors L, which may by way of example be laser level sensors L, the value of the specific gravity of the drilling mud in the tank 30 may also be determined, by simply dividing the measured hydrostatic pressure with the measured height level of the liquid above the bottom of the tank 30. Shown in fig. 2 is an enlarged sectional and schematic view of such a ballasting tank 30 converted for use as a mud holding tank and provided with a hydrostatic pressure sensor S. As shown, the pressure sensor S, which may be of the Vegabar 66/67-type by Ohmart Vega, is in the shown embodiment suspended from a position above the bottom of the tank 30, by appropriate wiring such that the sensor head is located close to the tank 30 bottom, at which position it may be mechanically fixed by fixture F or held by the weight of the sensor S. The sensor S may be of the type having a membrane M and is preferably suspended such that the membrane M faces downwards towards the tank bottom, or in any other way that preferably ensures that when the tank 30 is emptied, by pumping out the con- tents, liquid that previously was in contact with the membrane M is allowed to drip off the membrane M. This ensures that no drilling mud will remain on the membrane M and then dry out to form a mud cake on the membrane M surface. Such a mud cake remaining as a barrier between the membrane M and drilling mud subsequently filled into the tank 30 could give rise to incorrect pressure readings. Fig. 2 shows a conduit 2 for pumping drilling mud to the tank by pump 1 which may be reversed for discharging the drilling mud by having in addition a suction pipe (not shown) reaching to the tank bottom. Also shown is electrical wiring Wl, W2 connecting sensor S with the computer C and pump 1, respectively. As level the sensor L, by way of example, use may be made of Vegapuls 62 radar type transmitters or similar sensors that are unaffected by vapour, gas composition, pressure and temperature.
Where necessary, to keep the drilling fluid (such as Barite) suspended use may be made of an agitator device (not shown) arranged in the tank 30; such an agitator device may include an array of preferably movably mounted nozzles and may conveniently share pump with a separate tank 30 washing system. When the pump is used for agitating the content of the tank 30 is recirculated and discharged via the nozzles; on the other hand, when in tank washing mode the pump may conveniently circulate production water discharged via dedicated washing nozzles.
Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous. It is noted that for the purpose of the conversion of the ballasting tank 30 a shortening of the vent line 32 may be carried out, although this is normally not envisaged.

Claims

A floating offshore rig (10) for drilling in a hydrocarbons containing formation below a body (B) of water, said rig (10) comprising :
- a deck structure (15) with a derrick (22) supporting a drill string, and
- a substructure including ballasting tanks (12) for said rig (10) and a number of leg or columns (20) for extending into said body (B) of water from said deck (15),
- said rig (10) including on said deck structure (15) a drilling mud pit (13) for supplying drilling mud to said drill string, at least one drilling mud holding tank (30) being located on said substructure and being connected with said drilling mud pit (13) via piping for transfer of said drilling mud between said mud holding tank (30) and said drilling mud pit (13), a vent line (32) extending from a top portion of said drilling mud holding tank (30) to a vent port (34) located at the level of said deck structure (15) at a height (H) above a bottom of said drilling mud holding tank (30), said drilling mud holding tank (30) comprising at least one pressure sensor (S) for registering a hydrostatic pressure in said drilling mud tank (30) at or near the level of said bottom.
The rig of claim 1, said pressure sensor (S) being a membrane type pressure sensor mounted in said mud holding tank (30).
The rig of claim 2, said membrane type sensor (S) being mounted with the membrane (M) thereof facing downwards, to substantially prevent any drilling mud from remaining on said membrane (M) when said drilling mud tank (30) is empty.
The rig according to any of the previous claims, said substructure including one or more submerged pontoons (P) for stabilising said rig (10).
The rig according to the previous claim, at least one of said pontoons (P) including one or more of said mud holding tanks (30).
The rig according to any of the previous claims, at least one of said legs
(20) including one or more of said mud holding tanks (30).
The rig according to any of the previous claims, including a control device
(C) for terminating drilling mud filling of said mud holding tank (30) on the level of drilling mud in said drilling mud holding tank (30) reaching a given level (I).
8. The rig according to the previous claim, said mud holding tank (30) further comprising at least one level sensor (L) for registering the level of drilling mud in said drilling mud holding tank (30) and being connected to said control device (C).
9. The rig according to the previous claim, said control device (C) terminating filling of said mud holding tank (30) on said level sensor (L) registering a predetermined level (I) of drilling mud in said drilling mud holding tank (30).
10. The rig according to the previous claim, said predetermined level (I) being below said vent line (32).
11. The rig according to the previous claim, said predetermined level (I) being where said vent line (32) extends from said mud holding tank (30).
12. The rig according to any of claims 7-11, said control device (C) being connected with said pressure sensor (S) and configured for terminating said drilling mud filling of said mud holding tank (30) on said pressure sensor
(S) registering a predetermined pressure.
13. The rig according to the previous claim, said control device (C) being configured for terminating said filling when one of said predetermined pressure or said predetermined level (I) is reached.
14. The rig according to the previous claim, said control device (C) including information stored relating to a correspondence between the value of registered level of drilling mud and the volume of said drilling mud contained in said drilling mud tank (30) in that level.
15. The rig according to the previous claim, said control device (C) being con- figured to determine a specific gravity SG of said drilling mud in said drilling mud tank (30) based on said correspondence.
16. A method of controlling the filling with drilling mud of a rig (10) ballasting tank converted for use as a drilling mud holding tank (30), said rig (10) being as defined in any of the previous claims 12-15, comprising the step of i) obtaining knowledge of said height (H), ii) determining the value of a maximum allowable hydrostatic pressure in said drilling mud tank (30) on the basis of said obtained knowledge, iii) storing said value in said control device (C), iv) filling said drilling mud holding tank (30) with drilling mud having a specific gravity while registering the hydrostatic pressure at the bot- torn of said mud holding tank (30), v) comparing said stored value with said registered hydrostatic pressure, and vi) terminating said filling at the latest on said registered hydrostatic pressure corresponding to or essentially to said stored value.
17. The method of claim 16, wherein said filling is terminated already when said predetermined level (I) of filling is reached.
18. The method of claim 17, including the step of selecting said predetermined level as the level where said vent line (32) extends from said mud holding tank (30), such that raising of said drilling mud into said vent line is prevented.
PCT/DK2014/050290 2013-09-23 2014-09-17 A floating offshore rig for drilling in a hydrocarbons containing formation and having a converted ballasting tank, and a method of operating the same WO2015039667A2 (en)

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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2018164572A1 (en) * 2017-03-06 2018-09-13 Itrec B.V. Semi-submersible drilling vessel with a mud treatment and circulation system
CN109209267A (en) * 2018-10-16 2019-01-15 四川宏华石油设备有限公司 A kind of mud device
DE102019104178A1 (en) * 2019-02-19 2020-08-20 GICON GROßMANN INGENIEUR CONSULT GMBH Oil rig and / or production rig for the search, extraction, processing and / or further transport of crude oil or natural gas
NL2023604B1 (en) 2019-08-02 2021-02-23 Itrec Bv Semi-submersible drilling vessel with a mud treatment and circulation system.

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US4043193A (en) * 1976-08-03 1977-08-23 Bailey Mud Monitors Inc. Method and apparatus for measuring volume and density of fluids in a drilling fluid system
US4476715A (en) * 1983-03-11 1984-10-16 Murphy Jack L Mud metering tank monitoring system
GB9424930D0 (en) * 1994-12-09 1995-02-08 Kazim Jenan Marine system which retains hydrostatic stability even when fully submerged
GB9724063D0 (en) * 1997-11-15 1998-01-14 Sofitech Nv Improvements in and relating to semi-submersible structures

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2018164572A1 (en) * 2017-03-06 2018-09-13 Itrec B.V. Semi-submersible drilling vessel with a mud treatment and circulation system
NL2018472B1 (en) * 2017-03-06 2018-09-21 Itrec Bv Semi-submersible drilling vessel with a mud treatment and circulation system.
US11346166B2 (en) 2017-03-06 2022-05-31 Itrec B.V. Semi-submersible drilling vessel with a mud treatment and circulation system
CN109209267A (en) * 2018-10-16 2019-01-15 四川宏华石油设备有限公司 A kind of mud device
DE102019104178A1 (en) * 2019-02-19 2020-08-20 GICON GROßMANN INGENIEUR CONSULT GMBH Oil rig and / or production rig for the search, extraction, processing and / or further transport of crude oil or natural gas
WO2020169581A1 (en) 2019-02-19 2020-08-27 GICON GROßMANN INGENIEUR CONSULT GMBH Drilling and/or production platform for locating, producing, processing and/or transporting crude oil or natural gas
NL2023604B1 (en) 2019-08-02 2021-02-23 Itrec Bv Semi-submersible drilling vessel with a mud treatment and circulation system.

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