GB2142286A

GB2142286A - Pressurized liquid filled tendons

Info

Publication number: GB2142286A
Application number: GB08416341A
Authority: GB
Inventors: Gerald E Burns
Original assignee: Chevron Research and Technology Co; Chevron Research Co
Current assignee: Chevron USA Inc
Priority date: 1983-06-28
Filing date: 1984-06-27
Publication date: 1985-01-16
Also published as: NO842599L; ES8601469A1; GB2142286B; FR2548367A1; YU112684A; YU46143B; ES533817A0; CA1220039A; FR2548367B1; GB8416341D0

Abstract

Pressurized liquid filled tubular tendons (6) for a tension leg offshore platform (1) provide a means for detecting leaks therein. Filling the tendon (6) with a liquid having a specific gravity less than that of sea water provides increased buoyancy and reduces the weight supported by the buoyant offshore structure (1). The use of a corrosion inhibiting liquid reduces the corrosion of the interior tendon wall. <IMAGE>

Description

SPECIFICATION Pressurized liquid filled tendons This invention relates to tension leg platform tendons and is concerned with pressurized liquid filled tendons for detecting leaks, providing buoyancy and resisting corrosion. A change in pressure denotes a structural deficiency. An increase in tendon buoyancy reduces the weight supported by the buoyant structure.

In deep water, the use of bottom-founded structures for oil well drilling and production operations is cost prohibitive due to the expense of fabrication and installation of such large structures.

For water depths in excess of 1,000 feet (305 metres), buoyant offshore structures moored to the sea floor can be used to perform drilling and production operations cost effectively.

As water depth excees 1 ,000 feet (305 metres), the tension leg platform (TLP) concept can be introduced to perform oil drilling and production operations. A TLP consists of a buoyant offshore structure moored to fixed sea floor anchor points with vertical tension legs; also referred to as tendons. Drilling, producing and processing equipment as well as crew's quarters are contained in or on the buoyant offshore structure.

Tendon designs include both cable and tu bular leg elements, see for example U.S.

Patent 4,285,615 which discloses a mooring apparatus for a structure floating on a body of water, comprising a corrosion resistant cable system, including a multistrand cable, having voids between adjacent strands, and U.S.

Patent 4,226 555 which discloses a mooring system for a tension leg platform, comprising a tension leg, including a plurality of tubular leg elements having threaded connections between adjacent leg elements.

The use of pre-tensioned vertical mooring elements prevents vertical motion but permits lateral motion of the floating structure during the passage of waves. Pretensioning is accomplished by deballasting the buoyant offshore structure after the tendons are connected between the buoyant structure and fixed sea floor anchor bases.

Tendon inspection is necessary as both a maintenance expenditure and safety precaution. Tendon repair and replacement are both very expensive and laborious operations.

Cracks and corrosion due to exposure to sea water decrease the failure load and working lifetime of the tendon. The desirability of minimizing tendon corrosion has been recognized in the art. The aforementioned U.S.

Patent 4,285,615 discloses an invention for providing a corrosion resistant design for a tension leg cable which isolates the steel wire cable from the sea water environment.

The present invention provides an apparatus and method for detecting structural deficiencies in a tubular tendon, increasing its buoyancy and extending its useful life.

According to one aspect of the invention, there is provided an apparatus for detecting a leak in a tension leg platform tendon, comprising: a fluid tight tubular tendon; means for supplying liquid to said tendon; means for pressurizing said liquid; means for monitoring pressure, said means monitoring variations in liquid pressure in said tendon; means for connecting an upper end of said tendon to a buoyant offshore structure; and means for connecting a lower end of said tendon to an anchor means connected to the sea floor.

According to another aspect of the invention, there is provided a method of detecting a leak in a tension leg offshore platform tendon, comprising the steps of: connecting a tendon between a buoyant offshore platform and an anchor means connected to the sea floor at an offshore location, said tendon being a fluid tight tubular member; supplying a liquid to said tendon; pressurizing said liquid; and monitoring liquid pressure in said tendon to detect leaks therein.

According to a further aspect of the invention, there is provided a tension leg offshore platform having anchor means connectable to the sea floor; at least one tendon connectable between said platform and said anchor means, said tendon being a fluid tight tubular member; means for supplying liquid to said tendon; means for pressurizing said liquid; and means for monitoring pressure, said means being capable of monitoring variations in liquid pressure in said tendon.

The present invention provides a method and means for detecting leaks in a tubular tendon, increasing its buoyancy and extending its useful life. A plurality of tendon segments, each consisting of a tubular element and sealable couplings, are joined to provide a single elongated tubular tendon. The tubular tendon is filled with a liquid which is preferably a corrosion inhibiting liquid, having a specific gravity less than that of sea water. A compressor is utilized to pressurize contents of the tubular tendon and pressure gauges monitor variations in pressure.

The corrosion inhibiting liquid protects the interior tendon wall from salt water corrosion.

The liquid having a specific gravity less than that of sea water increases the buoyancy of each tendon, thereby reducing the weight supported by the buoyant offshore structure.

Variations in pressure indicate cracks or punctures through the tendon or an inadequate coupling seal.

The object of the present invention is to provide a method and means for detecting leaks in a tubular tendon, increasing its buoyancy and extending its useful life. A method and means for detecting leaks indicating structural deficiencies promotes safety and reduces routine maintenance expenditures. Increasing the buoyancy of the tendon reduces the weight supported by the buoyant offshore structure; permitting a more efficient design.

Increased tendon life provides more cost effective deep water drilling by reducing maintenance, repair and replacement of the tendons.

Another object of the present invention is to provide an improved design for a tension leg platform incorporating the invention described herein.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made to the accompanying drawing in which: Figure 1 is an elevation schematic view, partially in section, of a tension leg platform; Figure 2 is an enlarged detailed view of part of a tendon of Figure 1; and Figure 3 is a section of the tendon of Figure 2 taken along the line 3-3.

Referring now to the drawing, Figure 1 shows an elevation schematic view, partially in section, of a tension leg platforirr(TLP) 1 deployed at an offshore drilling site. A lower platform 2 is provided on which may be mounted crew's living quarters, well test equipment and processing equipment. An upper platform 3 is provided on which may be mounted a pilot house, cranes, the drilling derrick, skid base, the drill string and a helicopter landing site. Similar conveniences as are known to those skilled in the art of oil exploration and production may also be stored on the lower and upper platforms. Platforms 2 and 3 are supported by a plurality of annular support columns 4. When the TLP is in its illustrated buoyant condition, columns 4 and pontoons 5 extend beneath the surface of the water.A plurality of tendons 6 extend from each support Column 4 to anchor means consisting of a foundation template 7 secured to the sea floor 8 with friction piles 9, thereby restricting movement of the structure. A drill string 10 and risers 11 extend from platform 1 or 2 between pontoons 5 to the sea floor 8 during drilling and producing operations. Well template 12 maintains the risers in a stationary position relative to the sea floor 8.

Referring to Figure 2, an enlarged detailed view of a part of tendon 6 depicts the tendon as a tubular element. A plurality of tendon segments, each consisting of a tubular element and sealable couplings, are joined to provide a single elongated tubular tendon.

The tubular element typically has a relatively thin wall compared to its overall diameter. A tubular element has been designed utilizing inside and outside diameters of 18 inches (45.7 cm) and 20 inches (50.8 cm), respectively. Figure 3 shows a section view of the tendon of Figure 2 taken along the line 3-3.

In accordance with the present invention, a liquid, preferably corrosion inhibiting and having a specific gravity less than that of sea water, enters the tendon through a conduit located at its upper end. The liquid may be water or a hydrocarbon. The liquid may contain a corrosion inhibitor. Thus the liquid can be a mixture of fresh water and hydrazine.

Subsequent to the introduction of liquid to the tendon, compressor 1 3 supplies pressure through the conduit to the tendon's contents.

A pressure in excess of the maximum hydrostatic pressure exerted by the sea water on the tendon is recommended to avoid the instance where the pressure inside the tendon is equal to the sea water pressure at the same elevation. A positive net internal pressure is utilized to detect a leak. A valve is closed to retain the pressurized contents. Pressure gauges monitor the pressure therein. Reductions in pressure, in excess of a predetermined value, activate a signal to inform crew members of a deficient tendon.

The corrosion inhibiting liquid protects the interior walls of the tubular tendon from exposure to sea water. A liquid having a specific gravity less than that of sea water provides buoyancy and reduces the tendon weight supported by the offshore buoyant structure. Barring any pressurizing malfunctions, a change in pressure indicates a leak in the tendon attributable to a crack or puncture through the tendon or an inadequate coupling seal.

Variations in the embodiment described with reference to the drawing will be readily apparent to those skilled in the art.

Claims

1. Apparatus for detecting a leak in a tension leg platform tendon, comprising: a fluid tight tubular tendon; means for supplying liquid to said tendon; means for pressurizing said liquid; means for monitoring pressure, said means monitoring variations in liquid pressure in said tendon; means for connecting an upper end of said tendon to a buoyant offshore structure; and means for connecting a lower end of said tendon to an anchor means connected to the sea floor.

2. Apparatus as claimed in Claim 1, wherein said liquid is water.

3. Apparatus as claimed in Claim 1, wherein said liquid is a hydrocarbon.

4. Apparatus as claimed in Claim 1, 2 or 3, wherein the liquid includes a corrosion inhibitor mixed therewith.

5. Apparatus as claimed in any preceding claim, wherein said liquid has a specific gravity less than that of sea water.

6. Apparatus as claimed in any preceding claim, wherein in use the liquid pressure in said tendon is in excess of the maximum hydrostatic pressure exerted on said tendon by the sea water in an offshore location.

7. A tension leg offshore platform having anchor means connectable to the sea floor; at least one tendon connectable between said platform and said anchor means, said tendon being a fluid tight tubular member; means for supplying liquid to said tendon; means for pressurizing said liquid; and means for monitoring pressure, said means being capable of monitoring variations in liquid pressure in said tendon.

8. A tension leg platform as claimed in Claim 7, wherein said liquid is water.

9. A tension leg platform as claimed in Claim 7, wherein said liquid is a hydrocarbon.

10. A tension leg platform as claimed in Claim 7, 8 or 9, wherein said liquid includes a corrosion inhibitor mixed therewith.

11. A tension leg platform as claimed in Claim 7, 8, 9 or 10, wherein said liquid has a specific gravity less than that of sea water.

1 2. A tension leg platform as claimed in any one of Claims 7 to 11, wherein in use the liquid pressure in said tendon is in excess of the maximum hydrostatic pressure exerted on said tendon by the sea water in an offshore location.

1 3. A method of detecting a leak in a tension leg offshore platform tendon, comprising the steps of: connecting a tendon between a buoyant offshore platform and an anchor means connected to the sea floor at an offshore location, said tendon being a fluid tight tubular member; supplying a liquid to said tendon; pressurizing said liquid; and monitoring liquid pressure in said tendon to detect leaks therein.

1 4. A method according to Claim 13, wherein said liquid is water.

15. A method according to Claim 13, wherein said liquid is a hydrocarbon.

16. A method according to Claim 13, 14 or 15, whereinsaid liquid includes a corrosion inhibitor mixed therewith.

17. A method according to Claim 13, 14, 1 5 or 16, wherein said liquid has a specific gravity less than that of sea water.

1 8. A method according to any one of Claims 1 3 to 17, wheren said liquid maintains a pressure in excess of the maximum hydrostatic pressure exerted on said tendon by the sea water at said offshore location.

19. A tension leg offshore platform substantially as hereinbefore described with reference to, and as shown in, the accompanying drawing.

20. A method of detecting a leak in a tension leg offshore platform tendon, substantially as hereinbefore desribed with reference to the accompanying drawing.