US3698348A - Method and apparatus for subsurface towing of flowlines - Google Patents

Abstract

This disclosure is directed to a method of transporting a flowline or flowline bundle for installation in an offshore petroleum field wherein the fabrication of the flowline is land based, after which the full-length flowline is towed (at a sufficient depth below the surface of the water) to the installation site, so as not to be affected by adverse sea conditions while in tow. Each free end of the line is adapted with a connecting apparatus for connecting to submerged connecting assemblies or structures. When the flowline is drawn down to the connecting assemblies, connection is made thereto just prior to the final settlement of the flowline to the ocean floor.

Description

,XR 3,69 ,348 5R nnn'etrstat es Patent Morgan METHOD AND APPARATUS FOR SUBSURFACE "rowmc o FLOWLINES I Inventor: George W. Morgan, Anaheim, Calif.

Assignee: Subsea Equipment Associates Limited, Hamilton, Bermuda Sept. 21, 1970 Filed:

Appl. No.:

US. Cl. .L .1...l 14/235 B Int. Cl. ..B63b 21/00 Field of Search ..l 14/235 R, 235 8, 0.5 R;

References Cited UNITED STATES PATENTS 3/1965 Wittgenstein ..l i4/235 R X Buller et al. ..l 14/235 ll/i968 Oct. 17, W72

Primary Examiner-Trygve M. Blix Attorney-Allan Rothenberg, Richard F. Carr and Richard L. Gausewitz 1 {57] ABSTRACT This disclosure is directed to a method of transporting a flowline or flowline bundle for installation in an offshore petroleum field wherein the fabrication of the flowline is land based; after which the full-length flowline is towed (at a sufficient depth below the surface of the water) to the installation site, so as not to be affected by adverse sea conditions while in tow. Each free end of the line is adapted with a connecting apparatus for connecting to submerged connecting assemblies or structures. When the flowline is drawn down to the connecting assemblies, connection is made thereto just prior to the final settlement of the flowline to the ocean floor.

14 Claims, 11 Drawing Figures Pmzmiassnmzz' "3.698 348 L SBEEIlflFG INVENTO F/ 6. Z GEORGE w. MORAN ATTORNEY PATENTEDUU-l 1912 CURRENT FLOW sum 3 [1F 6 CURREIiT FLOW INVENTOR. GEORGE W. MORGAN I BY ATTORNEY PATENTEDncr 1 m2 3.698.348

sum 9 0F 6 INVlz'N'I'OR. GEORGE w MORGAN ATTORNEY PMENTEDUCI 11 1972 saw 5 or 6 INVENTOR. GEORGE W. MORGAN PATENTEDHBI 17 1972 SHEET 5 OF 6 FIG. /0

, INVENTOR.

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A r TORNfY METHOD AND APPARATUS FOR SUBSURFACE TOWING OF FLOWLINES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method of transporting varying offshore operations for the production of hydrocarbons, such as oil and/or gas at depths of 100 feet or more. There have been several methods used for the production of wells in offshore oil fields at depths of lot) feet or less and these depthsare compatible with the use of divers. However, where there is production in waters of greater depths than lOO feet, the use of divers becomes dangerous and quite often impractical. Therefore, problems still continue to arise in the area of oil well productions where the depth is over 100 feet and the distance to the producing field from land is such that operations become economically unfeasible. More feasible means for installing flowlines are required so that these wells may be economically produced for future requirements. Due to the ever inditions to withstand a rough surface of the ocean,

whether it be by bad weather or just adverse conditions such as current flow. Third, there is the method of using a barge to transport and lay pipe at an installation site. it has been found by experience that laying pipe from a lay barge is not only costly but cannot be done where the bottom of the ocean is at such a depth that the flowline cannot physically maintain its proper I conditions in which the length of the pipe run is only creasing demand for hydrocarbons such as oil and gas,

facilities, the pipe and/or flowlines are assembledonshore and literally dragged along the ocean floor to the production stations and there connected. Others have been assembled at sea and laid into position by what is known as a lay barge. And still other flowlines have been floated on the surface of the water where the distance of the producing well from the shore line is not too great a distance. A major problem to a flowline is protecting the line from damage, particularly kinking of the pipe.

There are three particular methods in use for moving pipe and connecting the pipe or flowline to submerged structures. One method is to drag the assembled pipe along the floor of the ocean to its connecting point as it is assembled on shore. However, this becomes impractical where there is considerable distance involved due to adverse conditions of the terrain of the ocean floor which will cause serious damage to the pipe or pipe bundles. A second method of floating a flowline bundle has been used in various operations. However, the

flowlines are restricted in length and to the number of pipes that may be used in a bundle system. The floating of the flowlines exposes the complete assembly to various unstable sea conditions and especially'to bad weather which causes the surface of the ocean to become unduly rough. To be economically useful, flowlines in the future will have to be assembled at of lines per bundle. Substantial lengths of flowlines of three to four times the depth of the water, and the installation of flowline bundles are limited to not more than three to four lines.

SUMMARY OF THE'INVENTION The present invention may be briefly described as a method of transporting an assembled flowline or flowline bundle to an offshore installation and submerging said flowline for connection with submerged petroleum producing structures for the purpose of oil production.

In this method the flowline or flowline bundle is fabricated on shore and is progressively launched into the water tn u=9nqiii9ns ofsqntrqlls tsss sn. and floatation. The flowline bundles are assembled on tracked dollies, using guide rollers at the shore line to guide the bundles into the sea. Since steel flowline bundles have a negative buoyancy (as is generally the case), a series of releasable buoys may be intermittently attached along the length of the bundle so as to provides slight amount of uniformly distributed positive buoyancy. During transportation, the ends of the line bundles are attached to the terminal ballasts which are sufficiently heavy to submerge the total assembly.

The mogntisubmergence of .the total assembly is controlled by theleii'gth'aiitfanglefofthffto v cable's which are attached toth e forward and aft terminal ballastyyhen the flowline assembly reaches the installation area, it is connected to barges known as a drawerdown barge. The flowline bundle is lowered by means of a drawer-down cable which passes through previously lowered J-tube fixtures.

Each free end of the line is adapted with a connecting means for mating with a complementary connecting fixture. As an example, one fixture is an integral part of a submerged structure and another connecting fixture is an integral part of a second structure, which are to be interconnected by said flowline bundle. At the time of connecting the bundle, at critical damage potential exists. To reduce damage potential, the flowline is congreater lengths than they are now presently being as N sembled and the bundles will require a larger number two miles or more and bundles ranging from four or more pipelines are not capable under the present con- -plished by the use of a mid-line barge controlling th e rate of descent of thernidportion of the flowline:

' Generally, the line has a length longer than the distance between each underwater structure to which it is to be connected. This, thereby, allows the flowline to be lowered to the ocean floor in a gentle, arcuate curve.

3 l. OBJECTS OF THE INVENTION A general object of this invention is to provide a method of transporting a flowline or a flowlinefbundle flowline bundles which are subjected to control tension 10 loads as required to maintain bundle control during the fabrication, transportation, and placement phases thereof.

Another object of the invention is to proyide a shown in FIG. 1 and H6. 2 a land-based installation Sidering this example, skilled persons will understand Meof-bycontrol means.

that many variations may be made without departing from the principles disclosed and it is contemplated that the employment of any structures, arrangements or modes of operation are properly within the scope of the appended claim.

BRIEF DESCRIPTION OF THE DRAWINGS Referring to the drawings which are for illustrative purposes only:

FIG. I is a diagrammatic plan view showing a landbased assembly of a flowline bundle;

FIG. 2 is a diagrammatic side view of that shown in FIG. 1;

FIG. 3, is a diagrammatic view illustrating the flowline being towed under the surface of the water;

FIG. 4 is a sectional view of a typical flowline bundle having a releasable buoy attached thereto;

FIG. 5 is also a sectional view similar to FIG. 4 using a continuous tube-type of buoy arrangement;

FIG. 6 is a top plan view showing the flowline being towed under ideal conditions;

FIG. 7 is a view similar to FIG. 6 but showing the flowline subjected to a transverse current flow;

FIG. 8 is a diagrammatic view illustrating the general relationship of all major items previous to connecting and drawing down of the flowlines;

FIG. 9 illustrates the flowline attached to draw-down cables;

FIG. 10 shows the flowline connected at an end thereof to an underwater facility; and

FIG. II shows the completion of the underwater connections and the release of the buoy system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring more particularly to the drawings, there is site indicated generally at 10. The purpose of the landbased site is to fabricate flowlines or flowline bundles 12 on shore which is generally indicated at '14. Then the bundle is simultaneously and progressively launched into the water 16 under conditions of controlled tension and floatation. Each flowline bundle 12 is assembled on tracked dollies 18, using guide rollers 19 at the shore line 14 to guide the bundle into the sea.

From this point, the free end 20 of the line 12 is at-' tached to a floating facility or barge 22 by controlled tensioning means such as a cable and winch generally indicated at 23.

Where steel flowline bundles are used (as is generally the case), there is created a negative buoyancy which causes the line to sink to the ocean floor. As the flowline assembly is extended out from the land base, a

buoyancy means 24, shown as a series of releasable buoys, is intermittently attached to the bundle so as to providea slight amount of uniformly distributed positive buoyancy. This can readily be seen in the diagrammatic view of FIG. 2. Therefore, during the assembly phase of the line bundle wherein the line is extended into the ocean, it is necessary for the line to have a positive buoyancy. This is accomplished, as mentioned above, by a buoyancy means 24. However, when a completed unit is assembled for towing to the production site, that is during transportation thereof, each respective free end of the line bundle is attached to a terminal ballast 26 which is sufficiently heavy to submerge the total assembly, as shown in FIG. 3. Therefore, it can be seen that controlling the means for floating the line in conjunction with the means for submerging the line, the floating facilities or tugs 22 can transport the lines submerged at any predetermined depth below the surface of the ocean 16.

In FIGS. 4 and 5 there is shown two different means for creating a buoyancy condition of the flowline bundle. Referring now particularly to FIG. 4, there is illustrated a buoyancy means comprising a ball-like buoy 25 which is releasably attached to the line bundle 12. A plurality of the ball-like buoys will be placed longitudinally along the bundle and sufficiently spaced apart whereby the entire line may be supported. Also indicated in FIG. 4 is the configuration of the pipeline assembly which comprises a plurality of various size pipes as indicated by reference numeral 27. These pipes are fixed in a spaced relationship to each other by a support wall 28.

Referring now to FIG. 5, there is illustrated an enlarged tube or pipe 29 made of lightweight material, such as aluminum, which is used as a-means for positive buoyancy along the length of the flowline. This pipe is adapted to be attached along the full length of the flowline bundle and pressurized for a positive buoyancy condition. However, this pressurization is notcritical and can range from O to 5 pounds per foot. This then will develop a residual upward tendency in floatation and provide an inverted catenary bridgerTherefore, the combination of pressurizing and flooding of this particular type of buoyancy means becomes an effective way to control the depth at which the flowline may be submerged during towing operations.

In addition, however, the amount of submergency for the total assembly can be controlled by the length and the angle of the tow cables 30 which are attached to the forward and aft terminal ballast 26. For example, if the tugs pay out cables and separate themselves so as to maintain some similar angle (say 45) for the fore and mately similar angles of towline inclination), the

overall flowline assembly will rise. The amount of tug separation, of course, will vary with the tug surging. The distance between tugs can be, continuously monitored by radar and the average separating distance is then used to evaluate the general position of the flowline bundle. Constant tension winches on the tow cables can be used to minimize theeffects of tug heave and surge upon the flowline bundle. in addition, pressure transducers on the terminal ballast could be used to monitor the actual depth of the terminal ballast.

Referring now to F I08. 3, 6 and 7, there is shown an auxiliary barge or tug-31 positioned intermediate to the ends of the bundle 12 for control of undue bending under adverse conditions such as shown in FlGS. 6 and 7. H6. 6 illustrates a possible use of two forward tugs along with an optional intermediate tug when current results in a lateral oscillation.

HO. 7 illustrates a possible configuration of an overall system when subjected to transverse current and an auxiliary tug is needed as mentioned above. As one example of an area of subsea production of gas and oil, I now refer to FIG. 8 in which there is illustrated a surface facility 32 connected to a, subsea manifold 33 by means of a riser 34.

There is means for submerging the flowline 12 to the ocean floor which comprises a guideline 35 for placement of a .l-tube fixture 36. The guideline 35 is suspended from the surface facility. 32 enabling a draw down barge 38 to string a draw-down cable 40 therethrough. As the subsurface flowline assembly is towed into the area, the draw-down cable is then attached to the end of the flowline bundle 12 as shown in F K]. 9. A similar activity will be occuring at the aft end of the flowline bundle, as shown in FIGS. 10 and 11, in which the aft end is being connected to a satellite producing unit 42.

After connection is made at both ends of the bundle to each of the subsea units, the flowline is then permitted to settle completely on the ocean floor. That is, the flowline l2 issuspended above the bottom of the ocean while the connections between each subsea satellite in it is made. The tug 31, as shown in FlGS. l and 11, then allows the flowline to be lowered gently into place.

When relatively smooth and compatible bottom conditions are encountered, the buoyancy means 24 may be remotely released, coming to the surface in a connected configuration for future use. if the intermittent buoying system is replaced by the continuous line system, as shown in FIG. 5, it too may be releasably secured. However, it is contemplated that this type of buoy means will generally be used as an aid to submerging the flowline by controlled flooding of the system with sea water.

Although the present invention has been described with respect to a preferred form of the present invention witha certain degree of particularity, it is to be urtderstood that thescope of the invention should not be limited thereto, but should be afforded the full scope of the following claims.

I claim:

. 6 methodoftransporting an underwater flowline from an onshore assembly site to an offshoreinstalla' lign site havingsubsea units comprising the steps off E fabricatin g said flowline,

launching said flowline into a body of water, floating said flowline with a buoyant means attached thereto, su bmerging saidflowline at a predeterrn i negzLglepjth 53mm Surface sis .liftdibfl iistl EFFEElL igw ss s i ssbne s s n satixelabsqyant .flsuxlinsw tg said offshore installation.,

2. A method of transporting an underwater flowline as recited in claim 1 wherein said flowline is fabricated on said onshore assembly site, and said flowline is progressively launched from said onshore assembly site into a body of water; and including attaching positive buoyant means for floating said flowline.

3. A method asrecited in claim 2 wherein said step of submerging includes the steps of:

securifig ne'gative 566955615 for submerging i said assembly to a predetermined depth between the surface of the water and the ocean floor; and towing said assembly flowline below the surface of tliemefiosaid offshore installation site by means 3f cables exerting upwardly directed tensile forces.

"lat opposite ends otl said negatively buoyant as? "ssts d l A method as recited in claim 3 including the steps of: I

submerging said flo line to a de ti: adjacent said subsea unitsiand connecting each end of said flowline to said subsea units. 5. A method as recited in claim 4 including the step of:

. 'lHEJP? w e *9 .9 ?ss 99is s as liibseaunitsiiiii 6. An apparatus for underwater towing in combination comprising:

V a flowline bundle;

a positive buoyant means attached .to said flowline along the length thereof; v a negative buoyant means attached to said flowline Newt-M..- s a 1 whereby said flowline may be positioned at a predetermined depth for towing below the surface of the water, and tow cables connected to forward rearward ends of said bundle for exerting fores ftn..s siteathssss ti ss'islss of saidbundle. 7. An apparatus as recited in claim 6 wherein said positive buoyant means comprises a plurality of releasable buoys.

3. An apparatus as recited in claim 7 wherein said negative buoyant means comprises a plurality of terminal ballasts attached to each end of said flowline.

9. An apparatus as recited in claim 8 wherein said flowline bundle comprises a plurality of different diameter pipes, said pipes being arranged in a horizontal spaced relationship with each other and a plurality of support walls longitudinally spaced along said pipes.

10. A method of transporting an underwater flowline comprising the steps of floating said flowline with a buoyant means attached thereto, weighting both of the ends of said flowline with a negative buoyancy sufficient to provide the assembly of flowline, buoyant means and weights with a negative buoyancy, and towing said assembly beneath the surface of the water, said last-mentioned step comprising the steps of connecting the forward end of said flowline to a forward towing surface vessel by means of a forward tow cable, connecting the rear end of said flowline to a rearward towing surface vessel by means of a rear tow cable, said forward surface vessel being positioned forwardly of said forward end and said rear vessel being positioned to the rear of said rearward end, whereby said forward and rear tow cables will counter the negative buoyancy of said assembly. 7 11. The method of claim 10 including the step of [controlling depth of submergence of said assembly, 1 said last mentioned step comprising adjusting depth of i submergence of said flowline by changing the effective length of both of said forward and rearward tow cables and simultaneously changing the distance between said a forward and rearward vessels.

i 12. The method of claim 11 including the steps of are completed and other portions are being fabricated, said steps of towing and adjusting depth of submergence of said flowline assembly being performed after completion of the full length of said flowline.

13. The method of towing a positive buoyancy flowline beneath the surface of the water comprising the steps of weighting forward and rearward ends of said flowline in an amount to provide the weighted flowline with a net negative buoyancy,

countering negative buoyancy at the forward end of the flowline by an upwardly and forwardly directed tensile force exerted on the flowline at such forward end, and

countering negative buoyancy at the rearward end of the flowline by exerting a rearwardly and upwardly directed tensile force at the rearward end of the flowline.

14. The method of claim 13 wherein said steps of countering negative buoyancy are achieved by means of a forward cable connected between the forward end of the flowline and a forward surface vessel and a rearward cable connected between the rearward end of the flowline and a rearward surface vessel, and further including the step of controlling the depth of submergence of the flowline as it is towed by varying the effective length of said forward and rearward cables and the distance between said surface vessels.

I t i t t

Claims (14)

1. A method of transporting an underwater flowline from an onshore assembly site to an offshore installation site having subsea units comprising the steps of: fabricating said flowline, launching said flowline into a body of water, floating said flowline with a buoyant means attached thereto, submerging said flowline at a predetermined depth under the surface of said body of water by means of weights sufficient to provide a net negative buoyancy, and towing said submerged negatively buoyant flowline to said offshore installation.

2. A method of transporting an underwater flowline as recited in claim 1 wherein said flowline is fabricated on said onshore assembly site, and said flowline is progressively launched from said onshore assembly site into a body of water; and including attaching positive buoyant means for floating said flowline.

3. A method as recited in claim 2 wherein said step of submerging includes the steps of: securing negative buoyant means for submerging said assembly to a predetermined depth between the surface of the water and the ocean floor; and towing said assembly flowline below the surface of the water to said offshore installation site by means of cables exerting upwardly directed tensile forces at opposite ends of said negatively buoyant assembly.

4. A method as recited in claim 3 including the steps of: submerging said flowline to a depth adjacent said subsea units; and connecting each end of said flowline to said subsea units.

5. A method as recited in claim 4 including the step of: lowering the flowline to the ocean floor after each end of said flowline is connected to each of said subsea units.

6. An apparatus for underwater towing in combination comprising: a flowline bundle; a positive buoyant means attached to said flowline along the length thereof; a negative buoyant means attached to said flowline whereby said flowline may be positioned at a predetermined depth for towing below the surface of the water, and tow cables connected to forward and rearward ends of said bundle for exerting forces thereon to counter the negative buoyancy of said bundle.

7. An apparatus as recited in claim 6 wherein said positive buoyant means comprises a plurality of releasable buoys.

8. An apparatus as recited in claim 7 wherein said negative buoyant means comprises a plurality of terminal ballasts attached to each end of said flowline.

9. An apparatus as recited in claim 8 wherein said flowline bundle comprises a plurality of different diameter pipes, said pipes being arranged in a horizontal spaced relatIonship with each other and a plurality of support walls longitudinally spaced along said pipes.

10. A method of transporting an underwater flowline comprising the steps of floating said flowline with a buoyant means attached thereto, weighting both of the ends of said flowline with a negative buoyancy sufficient to provide the assembly of flowline, buoyant means and weights with a negative buoyancy, and towing said assembly beneath the surface of the water, said last-mentioned step comprising the steps of connecting the forward end of said flowline to a forward towing surface vessel by means of a forward tow cable, connecting the rear end of said flowline to a rearward towing surface vessel by means of a rear tow cable, said forward surface vessel being positioned forwardly of said forward end and said rear vessel being positioned to the rear of said rearward end, whereby said forward and rear tow cables will counter the negative buoyancy of said assembly.

11. The method of claim 10 including the step of controlling depth of submergence of said assembly, said last mentioned step comprising adjusting depth of submergence of said flowline by changing the effective length of both of said forward and rearward tow cables and simultaneously changing the distance between said forward and rearward vessels.

12. The method of claim 11 including the steps of fabricating said flowline on land and simultaneously launching fabricated portions thereof as such portions are completed and other portions are being fabricated, said steps of towing and adjusting depth of submergence of said flowline assembly being performed after completion of the full length of said flowline.

13. The method of towing a positive buoyancy flowline beneath the surface of the water comprising the steps of weighting forward and rearward ends of said flowline in an amount to provide the weighted flowline with a net negative buoyancy, countering negative buoyancy at the forward end of the flowline by an upwardly and forwardly directed tensile force exerted on the flowline at such forward end, and countering negative buoyancy at the rearward end of the flowline by exerting a rearwardly and upwardly directed tensile force at the rearward end of the flowline.

14. The method of claim 13 wherein said steps of countering negative buoyancy are achieved by means of a forward cable connected between the forward end of the flowline and a forward surface vessel and a rearward cable connected between the rearward end of the flowline and a rearward surface vessel, and further including the step of controlling the depth of submergence of the flowline as it is towed by varying the effective length of said forward and rearward cables and the distance between said surface vessels.

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