CA1194410A - Subsea caissons - Google Patents
Subsea caissonsInfo
- Publication number
- CA1194410A CA1194410A CA000420988A CA420988A CA1194410A CA 1194410 A CA1194410 A CA 1194410A CA 000420988 A CA000420988 A CA 000420988A CA 420988 A CA420988 A CA 420988A CA 1194410 A CA1194410 A CA 1194410A
- Authority
- CA
- Canada
- Prior art keywords
- caisson
- soil
- wall
- suction head
- base
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
- 239000000463 material Substances 0.000 claims abstract description 15
- 238000000034 method Methods 0.000 claims abstract description 9
- 239000002689 soil Substances 0.000 claims description 39
- 238000009434 installation Methods 0.000 claims description 15
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims description 14
- 239000004020 conductor Substances 0.000 claims description 8
- 230000002265 prevention Effects 0.000 claims description 7
- 230000002706 hydrostatic effect Effects 0.000 claims description 5
- 230000002093 peripheral effect Effects 0.000 claims 1
- 238000007789 sealing Methods 0.000 claims 1
- 239000004927 clay Substances 0.000 description 12
- 238000005553 drilling Methods 0.000 description 7
- 230000009471 action Effects 0.000 description 4
- 230000035699 permeability Effects 0.000 description 4
- 239000004568 cement Substances 0.000 description 3
- 239000012530 fluid Substances 0.000 description 3
- 239000000203 mixture Substances 0.000 description 3
- 230000035515 penetration Effects 0.000 description 3
- 230000000750 progressive effect Effects 0.000 description 3
- 239000004576 sand Substances 0.000 description 3
- 230000014509 gene expression Effects 0.000 description 2
- 230000004048 modification Effects 0.000 description 2
- 238000012986 modification Methods 0.000 description 2
- 239000002245 particle Substances 0.000 description 2
- 230000008569 process Effects 0.000 description 2
- 238000005086 pumping Methods 0.000 description 2
- 229910000831 Steel Inorganic materials 0.000 description 1
- 238000005299 abrasion Methods 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- NEHMKBQYUWJMIP-UHFFFAOYSA-N chloromethane Chemical compound ClC NEHMKBQYUWJMIP-UHFFFAOYSA-N 0.000 description 1
- 238000010276 construction Methods 0.000 description 1
- 238000007599 discharging Methods 0.000 description 1
- 238000005755 formation reaction Methods 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- 230000007246 mechanism Effects 0.000 description 1
- 238000009991 scouring Methods 0.000 description 1
- 239000013535 sea water Substances 0.000 description 1
- 239000010959 steel Substances 0.000 description 1
- 230000000007 visual effect Effects 0.000 description 1
- 239000011800 void material Substances 0.000 description 1
Classifications
-
- E—FIXED CONSTRUCTIONS
- E21—EARTH OR ROCK DRILLING; MINING
- E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
- E21B7/00—Special methods or apparatus for drilling
- E21B7/18—Drilling by liquid or gas jets, with or without entrained pellets
- E21B7/185—Drilling by liquid or gas jets, with or without entrained pellets underwater
-
- E—FIXED CONSTRUCTIONS
- E21—EARTH OR ROCK DRILLING; MINING
- E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
- E21B33/00—Sealing or packing boreholes or wells
- E21B33/02—Surface sealing or packing
- E21B33/03—Well heads; Setting-up thereof
- E21B33/035—Well heads; Setting-up thereof specially adapted for underwater installations
- E21B33/037—Protective housings therefor
Landscapes
- Life Sciences & Earth Sciences (AREA)
- Engineering & Computer Science (AREA)
- Geology (AREA)
- Mining & Mineral Resources (AREA)
- Physics & Mathematics (AREA)
- Environmental & Geological Engineering (AREA)
- Fluid Mechanics (AREA)
- General Life Sciences & Earth Sciences (AREA)
- Geochemistry & Mineralogy (AREA)
- Earth Drilling (AREA)
Abstract
ABSTRACT
A subsea caisson or well head silo is disclosed together with a method for installing it. The caisson includes a conical suction head located near the bottom of the caisson wall, the suction head being provided with cutting heads fixed to its lower surface and being rotatable so as to cut into sea bottom material of hard consistency.
A subsea caisson or well head silo is disclosed together with a method for installing it. The caisson includes a conical suction head located near the bottom of the caisson wall, the suction head being provided with cutting heads fixed to its lower surface and being rotatable so as to cut into sea bottom material of hard consistency.
Description
IMPROVEMENTS IN SUBSEA CAISSONS
Field of the Invention This invention relates to subsea caissons or silos for housing wellhead equipment and to protect that equipment from damage by ice, anchors, fishing gear or any other object in contact with the sea bed. This application is related to Canadian Application Serial No. 411,343 filed September 14th, 1982 and Canadian Application Serial No. 417,258 filed December 8th, 1982 both of which are entitled Subsea Caisson .
Background of the Invention Canadian Applications 411,343 and 417,258 disclose several forms of subsea caisson construction and methods for securing the caisson structures in a sea bed. While the use of subsea caissons of small and large diameter have been tried in the past for several underwater areas including the Artic area, these have been generally unsuccessful mainly due to installation pro~lems. The practice of dredging glory holes- is currently used but is expensive and does not protect equipment from scour depris. Conventionally, subsea wells have the wellhead at the sea bed and have connected to them either a blow out preventer during the pulling phase or a valve block assembly and flowline connector during the production phase~ The wellhead and associated e~uipment project somewhat above the sea bed and are thus vulnerable -to damage. Additionally, problems are also encountered in the installation of conventional caissons when the sea bed consists of clay or other hardpan formations.
Summary of the Inventlon The subsea caisson or ~ellhead silo is an enclosure that allows a conventional wellhead to be set below the mudline.
It is used when protection is required from damage by scouring of ice, icebergs, fishing gear or anchors. Installation is done from a work boat prior to the arrival of a drill rig on the site. When installed, a caisson can be large enough to contain a blow-out preventer and has a base complete with guide posts.
Installation of the caisson is made using the caisson's own weight and hydrostatic pressure created by reducing pressure below the baseO Thus, soil is removed fr~m beneath the caisson as it penetxates the sea bed by using the base as a rotatable cutter with fluid jets and a suction pipe~
The latter provides a means for creating the hydrostatic pressure and of removing a soil below the structure.
The permanent base of the caisson prevents fluidi~ed soils from filling the caisson from underneath due to the head of soil on the outside~ With cutters on the base and pro~ision for the base to rotate, hard soils, particularly clay, can be removed during the caisson installation.
Accordingly, caissons according to the present invention and to those disclosed in applications 411,343 and 417,258 can be installed in a light a sandy or silty soils; clay; or in sea beds with layers of both sand and clay.
After the caisson is in p~ace, the installation equipment is removed and is re-usable. This equipment includes the suction and jetting pumps, suction head and piping, the motor and dri~e means for rotating the base and the top cover of the caisson.
The improvements disclosed in the present invention provide means for lowering the caisson into the sea bed including a rotatable cutter head mounted on the inside of the caisson wall adjacent the lower end thereof. The caisson consists of a large diameter, elongated cylinder having a fluid jetting manifold ring and a conical suction, cutter head mounted adjacent the lower end of the cylinder The suction cutter head is supported on radiall~ located bearing-means on the inner perimeter of the cylinder so as to allow the suction head to rotate relative to the wall of the caisson.
Seal means prevents excessive leakage of pressur and material from the suction cylinder portion of the caisson to the back fill c~linder portion.
Radially spaced steel cutting heads are Eixed to the bottom of the conical cutter head sur~ace in a spiral pattern to affect the cutting and transporting action to the sea bed material, directing it to the center of the caisson. A back fill tube mandrel and conductor guide tube is provided at the center of the suction head to which a backfill prevention tube is connected. This tube prevents material from below the head from backfilling the caisson and acts as a torque tube to transmit power rom motor means located on the caisson installation tool.
Brief Description of the Drawings The invention is illustrated by way of example in the accompanying drawings in which Figure 1 and 2 are cross sectional views of a caisson being lowered into the sea bed to a required depth and illustrating the interior mechanism for affecting the lowering;
Figures 3 and 4 show the cover and suction equipment being removed from the caisson;
Figure 5 shows the installation of the drilling means for the outer conductor;
Figure 6 shows the drilling means being recovered from the silo;
Figures 7 and 8 show the backfill prevention tube being removed and a hole being drilled for the inner conductor;
Figure 9 shows the blowout preventer in place in the completed well;
Figure 10 shows the well with the tree and flow lines installed; and, Figure 11 is a prespective view of the completed well.
De~.ailed Description Referring to Figures 1 and 2, the subsea caisson indicated generally at 10 consists of an elongated cylindrical body with a continuous curved wall 12 f an open bottom 14 and a detachable top closure 16. The caisson 10 has a bottom wall in the form of a conical suction head 18 which in~ludes connectionsto a dredge pump 20 located on the top of the caisson.
When the caisson is moved to the site, it is gradually flooded and lowered to the sea bedO Upon contact, the caisson will penetrate the sea bed under its own weight and may initially penetrate the sea bed, depending on the soil condition, up to a depth of approxlmately 4 feet where the suction head or base 1~ engages the sea bo-t-tom.
Under pressure is then created through the use of the retrievable dredge pump 20 situated on the top of the caisson, the pump being designed to be capable of supplying a low head while at the same time discharging a large volume of a soil-water mixture. The pump 20 is started when the caisson has reached its initial penetration of approxi~ately 4 feet. Water jets which are spaced equally around the circumference of the inner wall 12 are directed radially towards to a direct soil material losened thereby towards the central suction pipe. A combination of the jetting and the suction removes the soil beneath the base and the caisson sinks further into the soil due to its self weight plus the force supplied by the under pressure. The lower part of the wall 12 of the caisson extends substantially below the base area to provide a skirt 22 to reduce the chances of seepage of surrounding soil into the confines of the lower caisson wall when the soil thereunder is ~eing removed, The suction head 18 which constitutes the base of the caisson is, as shown in Figures 1-3, conical in configuration and located adjacent the bottom of the caisson wall so that when it is installed in the sea bottom by means of lowering the pressure below the suction head 18, the upper volume of the caisson above the suction head 18 remains void of sea bottom soil. Accordin~ly, any material or equipment in the upper re~ion of the caisson interior is not sub~ect to damage by abrasion or fouIing by soil being stirred up and transported by the jetting and/or suction process. The skir-t portion 22 of the caisson wall is made of material of suitable thickness and strength to withstand a full hvdrostatic head of the suction process. However, the wall portion 12 above the suction head 18 can be made of a thinner or lower strength material suitable only to withstand the soil pressure gradient.
The suction head 18 is provided on its lower surface with a plurality of radially spaced, s-teel cutters ~4 which are secure~ to the bottom surface of the head 18 :in a spiral pattern so as to cut and transport soil material ~owards the center of the head 18 assisted by the action of the inwardly directed fluid je~s 26~ The suction head 18 is mounted for rotation on a plurality of beariny means 28 located on the inner perimeter of the lower area of the caisson wall 12. This perimeter area includes a seal to prevent excessive leakage of pressure and material from ~he suction cylinder poxtion of the caisson to the backfill cylinder portion.
The suction head 30 is connected via a backfill prevention tube 32 which prevents material from below the head from backfilling the caisson and which also acts as a torque tube to transmit power from hydraulic motor means 34 detachably connec~ed to a collar 36 on the upper end of the tube. It would be appreciated that actuation of the motor means 34 will rotate the backfill prevention tube 32 and the suction head 18 to which it is attached. As seen in Figures 1 and 2, the suction pump 20 and jets 26, together with the rotary action of the head 18 removes the soil from beneath the caisson and directs it upwardly to the pump 20 and away from the caisson. As shown in Figure 3 and 4, the cover 16 and the suction pump 20 and its related equipment are then removed from the interior of the caisson and the backfill prevention tube 32. Standard ~adius guide posts 40 are provided on the top surface of the suction head 18 to subsequently allow a standard marine wellhead system to be run into the interior of the caisson.
Turning now to Figures 5 and 6, drilling through the caisson follows normal practice except that the a temporary guide base is not required as the caisson already contains the permanent guide base 18 and drilling and cement returns must be directed out of the caissonO
First, the guide cables 42 are connected to the guide posts 40 on the base 18 and a drill string 44 (Figure 5) is lowered to the caisson with a returns di~erter pipe 46 around the drill string and which rests on the top o~ the backfill preventor tube 32. The diverter in this way is locked ~o the base 18 and the drill string proceeds through the tube 32 and commences drilling as shown in Figure 5.
The returns, instead of normally spilling onto the sea bed, will continue up the tube 32 and a diverter 46 and, once above the caisson, go through a side extension 48 to be dumped on the sea bed. Subsequently, as shown in Figure 6, the diverter returns to the drill rig with the drill string 44.
A conductor is run in the normal way but without the permanent guide base. For cementing, a pipe is provided up the outside of the silo (not shown) to allow cement returns to reach the surface. This is needed not only for visual observation but also to permit extra cement to be pumped in case of channelling.
As shown in Figure 8, drilling the hole for the casing is carried out normally with the riser 50 locking onto the conductor housing 52.
With the casing and wellhead in place, a blowout preventer 54 is run onto the wellhead inside the caisson. The large diameter of the caisson allows diver access all around the blowout preventer and drilling latter then follows regular practice. Whenever the caisson is not being actively used, a cover can be placed over the top to prevent it from being illed with scoured depris.
A dredge pump used with this invention must be capable of pumping the sea water obtained from 3 sources:
(i) ~ater trapped below the caisson (say 400 gpm).
(ii) Water supplied by the jets ~2000 gpm).
(iii) Water from seepage through the soii around the sides of the caisson.
Seepage flow is a function of soil permeability and the hydraulic gradient. For clean sand to silty sand mixtures, the coefficient of permeability ranges rom 10-2 to 10-4 in/sec.
A flow net constructed to estimate the seepage of water given the hydraulic gradient of 5 ft. water over 10 ft. soil and soil permeability of K=10- in/sec. gives seepage on the order of 25 gpm.
A dredge pump capable of pumping 2500 gpm of soil/water mixture (S.G. = 1.35) to create an underpressure of 5 psi is required. ~n example is the Mobile Dredge Pump -size 8' x lO"x 27 1/2'~AA. The motor supplies 150 hp at 720 rpm. The pump is to be retrievable to the sur~ace after the silo has been installed.
Two Cornell progressive cavity pumps, t~pe 1200 gpm at 300 psi wi~h 325 hp diesel drives, are proposed to supply 16 water jets.
''Quicking-' occurs when the seepage force on soil particles by upward flowing water is equal to or greater than the submerged ~eight of the particles. For typical sands, the critical hydraulic gradient at which quicking begins ranges from 0.~ to 1.2. Calculations show that for an underpressure of 5 psi, quicking will not occur except at local areas near the tip of the silo skirt.
Piping failure is thought to be likely if quicking occurs under the silo base. Under a sufficiently high hydraulic gradient, fine grains are washed out of the soil. Small channels or pipes are created which increase the hydraulic gradient across the remaining intact soil. The resulting seepage forces can now dislodge larger grains. Failure is therefore progressive, starting at the outlet of water flow~
working back towards -~he source. Complete piping failure during silo installation would result in the dredge pump ineffectively circulating water with no underpressure (or overpressure) able to be developedO
Both quicking and piping failure ~ust be avoided during silo installation. The silo shell is provided with a 4~ft.
skirt to increase the length of soil through which seepage must travel. Underpressures are also restricted to less than 5 psi to lessen the possibility of failure.
In clay soils, the action of the jets alone will not be enough to remove the clayq When the silo reaches the clay layers, the cutters on the rotating base break up the soil and direct it towards the central suction pipe. ~his is aided by the jets. Because clay has negligible permeability, higher suction pressures can be used without the danger of a piping failure. I'his higher pressure is needed to overcome the a~hesion of the clay against the silo walls.
To install an 18-ft. diameter silo in clay, the following procedure will be followed.
Initial Penetration Under its own weight, the 18-ft. diameter silo is estimated to penetrate medium stiff clay soil to a depth of 16 ins. The initial penetration creates a seal, allowing underpressure to be applied. ~ dredge pump creating an underpressure oE 10 psi will create a force of 360,000 lbs., driving the silo into the soil after the silo skirt has reached a depth of about 6 feet~ the silo base will contact the sea floor. Soil must be removed from below ~he silo before installation can continue.
Soil Cutting , Installation of th~ silo in stiff clay soil requires the use of the cutting devices. Preliminary calculations show that the force required to cut and displace a 12-in.
swath of stiff clay 1~2-in. deep is about 300 lbs~ With the aid of jetting to direct cut soil towards the central suction pipe, it is determined that a hydraulic motor supplying 3000 ft. lbs. is required. The motor will be retrievable to the surface after installation is complete.
A dredge pump is required to create an underpressure of up to 30 psi and to remove 5000 gpm of 1.35 S.G. water/soil rni~ture~ An example is -the Mobile Dredge Pump - size 12 x 14-- x 34-- - AA. The motor supplies 350 hp at 600 rpm.
Three Cornell progressive cavity pumps, each supplying 1200 gpm at 300 psi can be used to supply 24 jetsO Each pump is equipped with a 325 hp diesel drive.
A Staffa B200 hydraulic motor can be used to drive the 6-ft. radius gear which rotates the silo base and soil 35 cutters. This motor supplies 5000 ft.lbs. torque at 50 rpm for 1500 psi input pressure. Under direct drive, the silo base rotates at ahout 4 rpm, moving the outside cutters at a speed of about 3 ft/sec.
While the invention has been described in connection with a specific embodiment thereof and in a specific use, various modifications thereof will occur to those skilled in the art without departin~ from the spirit and scope of the invention as set forth in the apended claims.
The terms and expressions which have been employed in this disclosure are used as terms of description and not of limitation and there is no intention in the use of such terms and expressions to exclude any equivalence of the features shown and described or portions thereof but it is recognized that various modifications are possible within the scope of the invention claimed.
Field of the Invention This invention relates to subsea caissons or silos for housing wellhead equipment and to protect that equipment from damage by ice, anchors, fishing gear or any other object in contact with the sea bed. This application is related to Canadian Application Serial No. 411,343 filed September 14th, 1982 and Canadian Application Serial No. 417,258 filed December 8th, 1982 both of which are entitled Subsea Caisson .
Background of the Invention Canadian Applications 411,343 and 417,258 disclose several forms of subsea caisson construction and methods for securing the caisson structures in a sea bed. While the use of subsea caissons of small and large diameter have been tried in the past for several underwater areas including the Artic area, these have been generally unsuccessful mainly due to installation pro~lems. The practice of dredging glory holes- is currently used but is expensive and does not protect equipment from scour depris. Conventionally, subsea wells have the wellhead at the sea bed and have connected to them either a blow out preventer during the pulling phase or a valve block assembly and flowline connector during the production phase~ The wellhead and associated e~uipment project somewhat above the sea bed and are thus vulnerable -to damage. Additionally, problems are also encountered in the installation of conventional caissons when the sea bed consists of clay or other hardpan formations.
Summary of the Inventlon The subsea caisson or ~ellhead silo is an enclosure that allows a conventional wellhead to be set below the mudline.
It is used when protection is required from damage by scouring of ice, icebergs, fishing gear or anchors. Installation is done from a work boat prior to the arrival of a drill rig on the site. When installed, a caisson can be large enough to contain a blow-out preventer and has a base complete with guide posts.
Installation of the caisson is made using the caisson's own weight and hydrostatic pressure created by reducing pressure below the baseO Thus, soil is removed fr~m beneath the caisson as it penetxates the sea bed by using the base as a rotatable cutter with fluid jets and a suction pipe~
The latter provides a means for creating the hydrostatic pressure and of removing a soil below the structure.
The permanent base of the caisson prevents fluidi~ed soils from filling the caisson from underneath due to the head of soil on the outside~ With cutters on the base and pro~ision for the base to rotate, hard soils, particularly clay, can be removed during the caisson installation.
Accordingly, caissons according to the present invention and to those disclosed in applications 411,343 and 417,258 can be installed in a light a sandy or silty soils; clay; or in sea beds with layers of both sand and clay.
After the caisson is in p~ace, the installation equipment is removed and is re-usable. This equipment includes the suction and jetting pumps, suction head and piping, the motor and dri~e means for rotating the base and the top cover of the caisson.
The improvements disclosed in the present invention provide means for lowering the caisson into the sea bed including a rotatable cutter head mounted on the inside of the caisson wall adjacent the lower end thereof. The caisson consists of a large diameter, elongated cylinder having a fluid jetting manifold ring and a conical suction, cutter head mounted adjacent the lower end of the cylinder The suction cutter head is supported on radiall~ located bearing-means on the inner perimeter of the cylinder so as to allow the suction head to rotate relative to the wall of the caisson.
Seal means prevents excessive leakage of pressur and material from the suction cylinder portion of the caisson to the back fill c~linder portion.
Radially spaced steel cutting heads are Eixed to the bottom of the conical cutter head sur~ace in a spiral pattern to affect the cutting and transporting action to the sea bed material, directing it to the center of the caisson. A back fill tube mandrel and conductor guide tube is provided at the center of the suction head to which a backfill prevention tube is connected. This tube prevents material from below the head from backfilling the caisson and acts as a torque tube to transmit power rom motor means located on the caisson installation tool.
Brief Description of the Drawings The invention is illustrated by way of example in the accompanying drawings in which Figure 1 and 2 are cross sectional views of a caisson being lowered into the sea bed to a required depth and illustrating the interior mechanism for affecting the lowering;
Figures 3 and 4 show the cover and suction equipment being removed from the caisson;
Figure 5 shows the installation of the drilling means for the outer conductor;
Figure 6 shows the drilling means being recovered from the silo;
Figures 7 and 8 show the backfill prevention tube being removed and a hole being drilled for the inner conductor;
Figure 9 shows the blowout preventer in place in the completed well;
Figure 10 shows the well with the tree and flow lines installed; and, Figure 11 is a prespective view of the completed well.
De~.ailed Description Referring to Figures 1 and 2, the subsea caisson indicated generally at 10 consists of an elongated cylindrical body with a continuous curved wall 12 f an open bottom 14 and a detachable top closure 16. The caisson 10 has a bottom wall in the form of a conical suction head 18 which in~ludes connectionsto a dredge pump 20 located on the top of the caisson.
When the caisson is moved to the site, it is gradually flooded and lowered to the sea bedO Upon contact, the caisson will penetrate the sea bed under its own weight and may initially penetrate the sea bed, depending on the soil condition, up to a depth of approxlmately 4 feet where the suction head or base 1~ engages the sea bo-t-tom.
Under pressure is then created through the use of the retrievable dredge pump 20 situated on the top of the caisson, the pump being designed to be capable of supplying a low head while at the same time discharging a large volume of a soil-water mixture. The pump 20 is started when the caisson has reached its initial penetration of approxi~ately 4 feet. Water jets which are spaced equally around the circumference of the inner wall 12 are directed radially towards to a direct soil material losened thereby towards the central suction pipe. A combination of the jetting and the suction removes the soil beneath the base and the caisson sinks further into the soil due to its self weight plus the force supplied by the under pressure. The lower part of the wall 12 of the caisson extends substantially below the base area to provide a skirt 22 to reduce the chances of seepage of surrounding soil into the confines of the lower caisson wall when the soil thereunder is ~eing removed, The suction head 18 which constitutes the base of the caisson is, as shown in Figures 1-3, conical in configuration and located adjacent the bottom of the caisson wall so that when it is installed in the sea bottom by means of lowering the pressure below the suction head 18, the upper volume of the caisson above the suction head 18 remains void of sea bottom soil. Accordin~ly, any material or equipment in the upper re~ion of the caisson interior is not sub~ect to damage by abrasion or fouIing by soil being stirred up and transported by the jetting and/or suction process. The skir-t portion 22 of the caisson wall is made of material of suitable thickness and strength to withstand a full hvdrostatic head of the suction process. However, the wall portion 12 above the suction head 18 can be made of a thinner or lower strength material suitable only to withstand the soil pressure gradient.
The suction head 18 is provided on its lower surface with a plurality of radially spaced, s-teel cutters ~4 which are secure~ to the bottom surface of the head 18 :in a spiral pattern so as to cut and transport soil material ~owards the center of the head 18 assisted by the action of the inwardly directed fluid je~s 26~ The suction head 18 is mounted for rotation on a plurality of beariny means 28 located on the inner perimeter of the lower area of the caisson wall 12. This perimeter area includes a seal to prevent excessive leakage of pressure and material from ~he suction cylinder poxtion of the caisson to the backfill cylinder portion.
The suction head 30 is connected via a backfill prevention tube 32 which prevents material from below the head from backfilling the caisson and which also acts as a torque tube to transmit power from hydraulic motor means 34 detachably connec~ed to a collar 36 on the upper end of the tube. It would be appreciated that actuation of the motor means 34 will rotate the backfill prevention tube 32 and the suction head 18 to which it is attached. As seen in Figures 1 and 2, the suction pump 20 and jets 26, together with the rotary action of the head 18 removes the soil from beneath the caisson and directs it upwardly to the pump 20 and away from the caisson. As shown in Figure 3 and 4, the cover 16 and the suction pump 20 and its related equipment are then removed from the interior of the caisson and the backfill prevention tube 32. Standard ~adius guide posts 40 are provided on the top surface of the suction head 18 to subsequently allow a standard marine wellhead system to be run into the interior of the caisson.
Turning now to Figures 5 and 6, drilling through the caisson follows normal practice except that the a temporary guide base is not required as the caisson already contains the permanent guide base 18 and drilling and cement returns must be directed out of the caissonO
First, the guide cables 42 are connected to the guide posts 40 on the base 18 and a drill string 44 (Figure 5) is lowered to the caisson with a returns di~erter pipe 46 around the drill string and which rests on the top o~ the backfill preventor tube 32. The diverter in this way is locked ~o the base 18 and the drill string proceeds through the tube 32 and commences drilling as shown in Figure 5.
The returns, instead of normally spilling onto the sea bed, will continue up the tube 32 and a diverter 46 and, once above the caisson, go through a side extension 48 to be dumped on the sea bed. Subsequently, as shown in Figure 6, the diverter returns to the drill rig with the drill string 44.
A conductor is run in the normal way but without the permanent guide base. For cementing, a pipe is provided up the outside of the silo (not shown) to allow cement returns to reach the surface. This is needed not only for visual observation but also to permit extra cement to be pumped in case of channelling.
As shown in Figure 8, drilling the hole for the casing is carried out normally with the riser 50 locking onto the conductor housing 52.
With the casing and wellhead in place, a blowout preventer 54 is run onto the wellhead inside the caisson. The large diameter of the caisson allows diver access all around the blowout preventer and drilling latter then follows regular practice. Whenever the caisson is not being actively used, a cover can be placed over the top to prevent it from being illed with scoured depris.
A dredge pump used with this invention must be capable of pumping the sea water obtained from 3 sources:
(i) ~ater trapped below the caisson (say 400 gpm).
(ii) Water supplied by the jets ~2000 gpm).
(iii) Water from seepage through the soii around the sides of the caisson.
Seepage flow is a function of soil permeability and the hydraulic gradient. For clean sand to silty sand mixtures, the coefficient of permeability ranges rom 10-2 to 10-4 in/sec.
A flow net constructed to estimate the seepage of water given the hydraulic gradient of 5 ft. water over 10 ft. soil and soil permeability of K=10- in/sec. gives seepage on the order of 25 gpm.
A dredge pump capable of pumping 2500 gpm of soil/water mixture (S.G. = 1.35) to create an underpressure of 5 psi is required. ~n example is the Mobile Dredge Pump -size 8' x lO"x 27 1/2'~AA. The motor supplies 150 hp at 720 rpm. The pump is to be retrievable to the sur~ace after the silo has been installed.
Two Cornell progressive cavity pumps, t~pe 1200 gpm at 300 psi wi~h 325 hp diesel drives, are proposed to supply 16 water jets.
''Quicking-' occurs when the seepage force on soil particles by upward flowing water is equal to or greater than the submerged ~eight of the particles. For typical sands, the critical hydraulic gradient at which quicking begins ranges from 0.~ to 1.2. Calculations show that for an underpressure of 5 psi, quicking will not occur except at local areas near the tip of the silo skirt.
Piping failure is thought to be likely if quicking occurs under the silo base. Under a sufficiently high hydraulic gradient, fine grains are washed out of the soil. Small channels or pipes are created which increase the hydraulic gradient across the remaining intact soil. The resulting seepage forces can now dislodge larger grains. Failure is therefore progressive, starting at the outlet of water flow~
working back towards -~he source. Complete piping failure during silo installation would result in the dredge pump ineffectively circulating water with no underpressure (or overpressure) able to be developedO
Both quicking and piping failure ~ust be avoided during silo installation. The silo shell is provided with a 4~ft.
skirt to increase the length of soil through which seepage must travel. Underpressures are also restricted to less than 5 psi to lessen the possibility of failure.
In clay soils, the action of the jets alone will not be enough to remove the clayq When the silo reaches the clay layers, the cutters on the rotating base break up the soil and direct it towards the central suction pipe. ~his is aided by the jets. Because clay has negligible permeability, higher suction pressures can be used without the danger of a piping failure. I'his higher pressure is needed to overcome the a~hesion of the clay against the silo walls.
To install an 18-ft. diameter silo in clay, the following procedure will be followed.
Initial Penetration Under its own weight, the 18-ft. diameter silo is estimated to penetrate medium stiff clay soil to a depth of 16 ins. The initial penetration creates a seal, allowing underpressure to be applied. ~ dredge pump creating an underpressure oE 10 psi will create a force of 360,000 lbs., driving the silo into the soil after the silo skirt has reached a depth of about 6 feet~ the silo base will contact the sea floor. Soil must be removed from below ~he silo before installation can continue.
Soil Cutting , Installation of th~ silo in stiff clay soil requires the use of the cutting devices. Preliminary calculations show that the force required to cut and displace a 12-in.
swath of stiff clay 1~2-in. deep is about 300 lbs~ With the aid of jetting to direct cut soil towards the central suction pipe, it is determined that a hydraulic motor supplying 3000 ft. lbs. is required. The motor will be retrievable to the surface after installation is complete.
A dredge pump is required to create an underpressure of up to 30 psi and to remove 5000 gpm of 1.35 S.G. water/soil rni~ture~ An example is -the Mobile Dredge Pump - size 12 x 14-- x 34-- - AA. The motor supplies 350 hp at 600 rpm.
Three Cornell progressive cavity pumps, each supplying 1200 gpm at 300 psi can be used to supply 24 jetsO Each pump is equipped with a 325 hp diesel drive.
A Staffa B200 hydraulic motor can be used to drive the 6-ft. radius gear which rotates the silo base and soil 35 cutters. This motor supplies 5000 ft.lbs. torque at 50 rpm for 1500 psi input pressure. Under direct drive, the silo base rotates at ahout 4 rpm, moving the outside cutters at a speed of about 3 ft/sec.
While the invention has been described in connection with a specific embodiment thereof and in a specific use, various modifications thereof will occur to those skilled in the art without departin~ from the spirit and scope of the invention as set forth in the apended claims.
The terms and expressions which have been employed in this disclosure are used as terms of description and not of limitation and there is no intention in the use of such terms and expressions to exclude any equivalence of the features shown and described or portions thereof but it is recognized that various modifications are possible within the scope of the invention claimed.
Claims (8)
1. A method securing a subsea caisson in a sea bed, said caisson having an elongated cylindrical body with a detachable top cover, a rotatable bottom located inwardly of the lower end of the cylindrical body and cutting means on the lower surface of the rotatable bottom; said method comprising the steps of a) lowering the caisson to the sea bed surface so that the peripheral bottom wall of the body engages and sinks into the sea bed;
b) rotating said bottom wall and its cutters to cut into the sea bed and simultaneously actuating water jetting means directed downwardly and inwardly from the bottom wall to fluidize the sea bed material;
c) removing said fluidized material via suction means located centrally of said bottom cover whereby said caisson is lowered into said sea bed by hydrostatic pressure.
b) rotating said bottom wall and its cutters to cut into the sea bed and simultaneously actuating water jetting means directed downwardly and inwardly from the bottom wall to fluidize the sea bed material;
c) removing said fluidized material via suction means located centrally of said bottom cover whereby said caisson is lowered into said sea bed by hydrostatic pressure.
2. A subsea caisson for installation in a sea bed comprising, an elongated cylindrical housing having a removable top and a base mounted for rotation inwardly of the lower end of the cylindrical body, said base having a plurality of cutters on its lower surface for engaging the surface of the sea bed; bearing means on the inner wall of said caisson mounting said base thereto; a water jet manifold ring and a plurality of jet means mounted on the caisson wall below said base.
3. A subsea caisson according to Claim 2 including a backfill tube and conductor guide located centrally of the base and a backfill prevention tube connected thereto and extending upwardly and centrally of the caisson to prevent material from below the base from backfilling the silo.
4. A caisson for installation in a hole in a seabed, said caisson comprising:
(a) an elongated cylindrical housing having a detachable top cover and an open bottom;
(b) a removable, self-contained soil-fluidizing and removal means located within the confines of said cylindrical housing and comprising (i) a plurality of jetting means spaced circumferentially inside of the caisson wall adjacent a lower end thereof; (ii) a suction head located substantially central of the lower end of said caisson;
(iii) jetting means associated with said suction head; and a bottom wall mounted in he caisson adjacent the lower end thereof for rotation with respect to the caisson; and a plurality of cutting means on the lower surface of said bottom wall;
(c) said circumferentially spaced jetting means and said jetting means associated with said suction head combining, when actuated with said suction head, to fluidize the subsea soil in the areas adjacent a lower inner edge of the caisson wall and centrally of the caisson for removal of the subsea soil from within confines of the caisson through said suction head;
(d) said removable self-contained soil-fluidizing and removal means simultaneously forming said hole in said seabed and causing said caisson to sink therein by hydro-static pressure to a depth below a predetermined line; and (e) means on the inner wall of said caisson for locking and sealing a permanent base therein.
(a) an elongated cylindrical housing having a detachable top cover and an open bottom;
(b) a removable, self-contained soil-fluidizing and removal means located within the confines of said cylindrical housing and comprising (i) a plurality of jetting means spaced circumferentially inside of the caisson wall adjacent a lower end thereof; (ii) a suction head located substantially central of the lower end of said caisson;
(iii) jetting means associated with said suction head; and a bottom wall mounted in he caisson adjacent the lower end thereof for rotation with respect to the caisson; and a plurality of cutting means on the lower surface of said bottom wall;
(c) said circumferentially spaced jetting means and said jetting means associated with said suction head combining, when actuated with said suction head, to fluidize the subsea soil in the areas adjacent a lower inner edge of the caisson wall and centrally of the caisson for removal of the subsea soil from within confines of the caisson through said suction head;
(d) said removable self-contained soil-fluidizing and removal means simultaneously forming said hole in said seabed and causing said caisson to sink therein by hydro-static pressure to a depth below a predetermined line; and (e) means on the inner wall of said caisson for locking and sealing a permanent base therein.
5. A caisson according to claim 4 wherein the circumferentially spaced jetting means of the soil-fluidizing and removal means are directed angularly inwardly of said caisson wall.
6. A caisson according to claim 4 including a bottom closure for said caisson, said bottom closure provided with a central aperture therein so as to permit passage there-through of the suction head of said soil-fluidizing and removal means and a centrally located well conductor.
7. A caisson according to claim 6 including a conductor guide located centrally of the base and a backfill prevention tube connected thereto and extending upwardly and centrally of the caisson to prevent material from below the base from backfilling the caisson.
8. A method of sinking and securing a caisson in a hole in a seabed, said caisson having an elongated cylindrical body, a detachable top cover, a bottom edge, a removable, self-contained soil-fluidizing and removal means including a plurality of jetting means spaced circumferentially inside of a wall of said caisson adjacent a lower end of the caisson and a suction head located centrally of and adjacent the lower end of the caisson; a bottom wall mounted in the caisson adjacent the lower end thereof for rotation with respect to said caisson; and a plurality of cutting means on the lower surface of said bottom wall; said method comprising the steps of:
(a) lowering the caisson to the seabed so that said bottom edge engages said seabed;
(b) simultaneously (i) rotating said bottom wall and said cutters with respect to said caisson to cut into the surface of said seabed, (ii) actuating said plurality of jetting means to fluidize said soil in the areas adjacent the lower edge of caisson wall, the cutters of the bottom wall and the suction head; and removing said fluidized soil via the suction head from the confines of the caisson;
(c) simultaneously forming said hole and sinking said caisson therein by said self-contained soil-fluidizing and removal means and said rotatable bottom to a depth below a predetermined level of said seabed by utilizing hydrostatic pressure; and (d) removing said top cover and said soil-fluidizing and removal means.
(a) lowering the caisson to the seabed so that said bottom edge engages said seabed;
(b) simultaneously (i) rotating said bottom wall and said cutters with respect to said caisson to cut into the surface of said seabed, (ii) actuating said plurality of jetting means to fluidize said soil in the areas adjacent the lower edge of caisson wall, the cutters of the bottom wall and the suction head; and removing said fluidized soil via the suction head from the confines of the caisson;
(c) simultaneously forming said hole and sinking said caisson therein by said self-contained soil-fluidizing and removal means and said rotatable bottom to a depth below a predetermined level of said seabed by utilizing hydrostatic pressure; and (d) removing said top cover and said soil-fluidizing and removal means.
Priority Applications (5)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000420988A CA1194410A (en) | 1983-02-04 | 1983-02-04 | Subsea caissons |
| US06/531,682 US4558744A (en) | 1982-09-14 | 1983-09-13 | Subsea caisson and method of installing same |
| NO83833291A NO833291L (en) | 1982-09-14 | 1983-09-14 | Submersible container for wellhead equipment as well as methods for using the container |
| DK419583A DK419583A (en) | 1982-09-14 | 1983-09-14 | DRINKING BOX FOR DRILLING EQUIPMENT AND PROCEDURE ON ITS APPLICATION ON THE SEA |
| GB08324660A GB2133060B (en) | 1982-09-14 | 1983-09-14 | Subsea caisson |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000420988A CA1194410A (en) | 1983-02-04 | 1983-02-04 | Subsea caissons |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1194410A true CA1194410A (en) | 1985-10-01 |
Family
ID=4124503
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000420988A Expired CA1194410A (en) | 1982-09-14 | 1983-02-04 | Subsea caissons |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1194410A (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2001065050A1 (en) * | 2000-02-29 | 2001-09-07 | Harald Strand | Foundation for suction in installation of conductor casing |
| WO2015118348A1 (en) * | 2014-02-07 | 2015-08-13 | Enovate Systems Limited | Wellbore installation apparatus and associated methods |
| WO2017048941A1 (en) | 2015-09-15 | 2017-03-23 | Noble Drilling Services Inc. | Method for excavating a mud line cellar for subsea well drilling |
-
1983
- 1983-02-04 CA CA000420988A patent/CA1194410A/en not_active Expired
Cited By (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2001065050A1 (en) * | 2000-02-29 | 2001-09-07 | Harald Strand | Foundation for suction in installation of conductor casing |
| US6692194B2 (en) | 2000-02-29 | 2004-02-17 | Harald Strand | Method for installing a conductor casing through a suction substructure |
| WO2015118348A1 (en) * | 2014-02-07 | 2015-08-13 | Enovate Systems Limited | Wellbore installation apparatus and associated methods |
| US10253569B2 (en) | 2014-02-07 | 2019-04-09 | Enovate Systems Limited | Wellbore installation apparatus and associated methods |
| WO2017048941A1 (en) | 2015-09-15 | 2017-03-23 | Noble Drilling Services Inc. | Method for excavating a mud line cellar for subsea well drilling |
| EP3353368A4 (en) * | 2015-09-15 | 2019-05-01 | Noble Drilling Services, Inc. | Method for excavating a mud line cellar for subsea well drilling |
| US10364609B2 (en) | 2015-09-15 | 2019-07-30 | Noble Drilling Services Inc. | Method for excavating mud line cellar for subsea well drilling |
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