GB2170250A

GB2170250A - Installing an offshore structure at a site having a strong current

Info

Publication number: GB2170250A
Application number: GB08601910A
Authority: GB
Inventors: Robert Hanks Rothberg
Original assignee: Exxon Production Research Co
Current assignee: ExxonMobil Upstream Research Co
Priority date: 1985-01-29
Filing date: 1986-01-27
Publication date: 1986-07-30
Also published as: BR8600226A; GB8601910D0

Abstract

Hauldown lines (36) are used to pull a buoyant platform jacket (40) down to a touchdown base (10) anchored to the sea floor (16). Male latch members (32, Fig. 3) articulatedly mounted on the jacket (40) engage with female latch members (14, 15) on the touchdown base (10) to secure the jacket (40) to the sea floor until jacket piles can be driven. The female latch members (14, 15) are mounted on anchor piles driven through the base (10). <IMAGE>

Description

SPECIFICATION Installing offshore structures in high current areas The present invention relates to the installation of offshore structures. More particularly, the present invention relates to the installation of offshore platform jackets in high current areas.

In the petroleum industry, wells are commonly drilled from offshore platforms to recover hydrocarbons from subsea petroleum formations. Offshore platforms are bottomfounded structures having a platform jacket and a platform deck which is supported by the jacket. Platform jackets are fabricated on land from hollow structural members and are transported by barge to the offshore sites where they are installed. Once at the site, the platform jacket is usually lauched from the barge in a buoyant condition. The buoyant condition is made possible by the hollow structural members and by buoyancy tanks attached to the jacket. In the buoyant condition, the jacket floats on its side. With the platform jacket floating over the installation site, it is gradually ballasted by flooding portions of the structural members and buoyancy tanks with sea water.This is done in a manner which causes the platform jacket to upend and descend to the sea floor. Mud mats at the bottom of the platform jacket prevent the platform jacket from sinking too far into sea floor sediments. Piles are then driven through structural members of the jacket or through pile skirts attached to the perimeter of the jacket to secure it to the sea floor.

Correct piacement of the platform jacket at the installation site is very important, as is maintaining proper control of the jacket during ail installation steps. For this reason, jacket installation is preferably carried out during periods when the sea is calm. However, if the installation site is located in an area where there are strong prevailing currents, merely having calm seas does not suffice. This is because existing approaches to installation are generally not capable of dealing with strong currents. During the substantial period of time between the launch of the platform jacket and its descent to the sea floor, strong currents can displace the jacket away from the installation site and can also create problems for the proper unpending-of the jacket.Even after the jacket has settled to the sea floor, it can be displaced or even toppled by strong currents during the substantial period of time it takes for the piles that secure the jacket to be driven into the sea floor.

As petroleum reservoirs located in benign environments become depleted, the need to develop reservoirs located in hostile offshote environments, such as those were strong currents exist, will become ever more urgent.

Thus, it would be highly advantageous to have an approach for installing platform jackets in high current areas.

According to the invention from one aspect there is provided a method for installing an offshore structure at an installation site having a strong current, said method comprising the steps of: (a) positioning a subsea template on the sea floor at said installation site, said template having an opening extending therethrough, said opening being adapted to receive an anchor pile; (b) running a messenger line through a first latch member, said first latch member being attached to said anchor pile; (c) lowering said anchor pile through said opening in said subsea template; (d) driving said anchor pile into the sea floor; (e) connecting said messenger line to a hauldown line, said hauldown line being attached to a second latch member, said second latch member being attached to said offshore structure and being adapted to engage with said first latch member;; (f) pulling said messenger line and said hauldown line through said first latch member; (g) launching said offshore structure proximate said installation site; and (h) pulling said hauldown line through said first latch member until said first and second latch members engage, thereby securing said offshore structure to the sea floor at said installation site.

According to the invention from another aspect there is provided an apparatus for installing an offshore structure at an installation site having a strong current, said apparatus comprising: (a) a touchdown base comprising a subsea template and an anchor pile which penetrates into, or is for penetration into, the sea floor at said installation site; (b) a first latch member, said first latch member being attached to said touchdown base and having a passageway extending therethrough; and (c) a second latch member, said second latch member being attached to, or for attachment to, said offshore structure and attached to a line passing through said passageway of said first latch member for drawing second latch member into engagement with said first latch member to secure said offshore structure to the sea floor at said installation site.

According to a preferred way of performing the invention, a subsea template having a number of openings extending vertically therethrough is lowered to the sea floor from a floating vessel. A number of anchor piles are then driven into the sea floor through the openings in the template to secure the template to the sea floor. Each anchor pile has a female latch member attached thereto which remains above the level of the template. Each anchor pile also has a lug which cooperates with a slot in each opening of the template to orient the piles, thereby azimuthally orienting the female latch members. Extending down from the vessel, through each female latch member, and back up to the vessel, is a messenger line. The subsea template, anchor piles and female latch members together form a touchdown base for the platform jacket.

The platform jacket is transported by barge to the installation site. Male latch members are attached to one or more structural members near the bottom of the jacket by articulating connections which permit the male latch members to pivot with respect to the jacket.

The spacing of the male latch members on the jacket corresponds to the spacing of the female latch members on the touchdown base. Each male latch member is attached to one end of a hauldown line. When the jacket is ready to launch, the other end of each hauldown line is attached to one end of a messenger line. The other end of each messenger line is then pulled in from the vessel. This causes each hauldown line to be pulled through a female latch member on the touchdown base and up to the vessel. The female latch members have bending shoes which permit the hauldown lines to be pulled up to the vessel under high tension.

With the vessel upcurrent and the barge downcurrent from the subsea template, the jacket is launched in a buoyant condition with its bottom end pointing toward the vessel.

The hauldown lines are then pulled in by the vessel so that the jacket upends and descends slowly toward the sea floor. The jacket is pulled down to the touchdown base in this manner until the male latch members engage with the female latch members. This secures the jacket to the sea floor in the proper orientation. The jacket is then completely ballasted and piles are driven in a conventional manner to further secure the jacket to the sea floor.

The invention will be better understood from the following description, given by way of example and with reference to the accompanying drawings, wherein: Figure 7 is a side view showing a touchdown base and an underwater pile driver.

Figure 2 is a side view, partly in section, showing one of the female latch members of the touchdown base.

Figure 3 is a perspective view showing a male latch member connected to a structural member of a platform jacket.

Figure 4 is a side view showing the platform jacket immediately after launch.

Figure 5 is a side view showing the platform jacket being pulled down to the sea floor by hauldown lines extending from a floating vessel through the female latch members of the touchdown base.

Figure 6 is a side view showin the platform jacket after it has been secured to the touchdown base.

Figure 7 is a perspective view showing the bottom of the platform jacket after it has been secured to the touchdown base.

Referring to Fig. 1, a touchdown base used to install a platform jacket in a high current area can be seen. The touchdown base serves to guide the platform jacket to the installation site, to properly orient the jacket, and to secure the jacket to the sea floor. Touchdown base 10 comprises subsea template 11, anchor piles 12 and 13, and female latch member 14 and 15. The female latch members are attached to the anchor piles. As will be explained in more detail below, the female latch members on the touchdown base engage with male latch members of the platform jacket to secure the jacket to the sea floor. Fig. 6 shows how platform jacket 40 looks after it has been secured to touchdown base 10, and also shows floating vessel 41 which is used to install the touchdown base and platform jacket.

Referring again to Fig. 1, subsea template 11 is shown as having already been lowered to the sea floor from the floating vessel. The subsea template is heavy and can be accurately lowered to the sea floor despite the existence of a strong current due to its relatively high weight to surface area ratio.

Acoustic beacons 17 on the template enable the vessel to accurately position the template at the installation site in a desired azimuthal orientation through the use of well known sonar techniques. The template has two openings 18 and 19 extending vertically therethrough. It is through the openings that the anchor piles are driven. Guide funnels 20 and 21 facilitate entry of the anchor piles into the openings. Anchor pile 13 is shown in the process of being lowered from the floating vessel by underwater pile driver 23. Messenger line 22 extends downward from the vessel, through female latch member 15, and back up to the vessel. Underwater pile driver 23 is controlled from the vessel to drive anchor pile 13 through opening 19 and into the sea floor.

Underwater pile drivers are well known to those skilled in the art. Like the subsea template, the anchor pile and underwater pile driver can be accurately lowered through the strong current due to their relatively high weight to surface area ratios. Entry of the anchor pile intoe the opening in the template and operation of the underwater pile driver is observed using an underwater camera (not shown) deployed on a remotely operated vehicle (not shown). Remotely operated vehicles and underwater cameras are commonly used in offshore petroleum recovery operations.

While retaining both ends of messenger line 22 on the floating vessel, anchor pile 13 is lowered until it passes through guide funnel 21 and opening 19. Lug 24 on the anchor pile cooperates with slot 25 in the guide funnel to azimuthally orient female latch member 15 in the proper direction. Once the anchor pile has entered the opening, underwater pile driver 23 is activated to drive the anchor pile into the sea floor. This secures the female latch member to the sea floor in a fixed position. Follower head 26 on anchor pile 13 prevents the underwater pile driver from damaging the anchor pile or the female latch member. When the anchor pile has been fully driven, collar 27 contacts guide cone 21. Anchor pile 13 will then look like previously driven anchor pile 12, and female latch members 14 and 15 will both point in the same direction.After the anchor piles have been driven, the underwater pile driver is retrieved by the vessel. Both ends of messenger lines 22 and 37 are retained on the vessel until the platform jacket is ready for installation. However, if the vessel needs to leave the installation site before the platform jacket is ready for installation, the messenger lines can be connected to a buoy (not shown) for later retrieval.

Referring to Fig. 2, a close-up side view, partly in section, of one of the female latch members can be seen. Female latch member 15 is attached-to anchor pile 13 and has frustoconically shaped end 28 which facilitates entry of a male latch member. Extending horizontally through the female latch member is passageway 29. Disposed inside the passageway is bending shoe 30, which enables a hauldown line to be pulled through the passageway and up to the vessel while the hauldown line is under high tension. As will be explained below, the hauldown line is attached to a male latch member on the platform jacket. The hauldown line is used to pull the jacket down to the touchdown base and to pull the male latch member into the female latch member until the two engage.Spring loaded latch pins 31 inside the passageway of the female latch member engage with a latch groove on the male latch member to secure the platform jacket to the touchdown base.

Referring to Fig. 3, one of the male latch members can be seen. Male latch member 32 is shaped to slide into the passageway of the female latch member so that the latch pins of the female latch member can engage with latch groove 33. One end of male latch member 32 is attached to structrual member 34 of the platform jacket by articulating connection 35, while the other end is attached to hauldown line 36. Structural member 34 is a horizontal structural member located near the bottom of the platform jacket. The articulating connection permits the male latch member to pivot with respect to the platform jacket to accommodate the varying attitudes of the jacket as it descends from the sea surface to the touchdown base.Another male latch member (not shown) is attached to structural member 34 at a distance from male latch member 32 which is equal to the distance between the female latch members on the touchdown base.

Following installation of the touchdown base on the sea floor, the vessel used to install the touchdown base is relocated to a position upcurrent from the touchdown base. The platform jacket is transported on a barge by tugboats to the installation site, and the tugboats are used to position the barge downcurrent from the touchdown base with the bottom end of the platform jacket pointing upcurrent toward the vessel. As will be recalled, the ends of the messenger lines which extend through the female latch members on the touchdown base have been retained on the vessel. One end of each messenger line is transferred from the vessel to the barge.

There, each messenger line is connected to one of the hauldown lines attached to the male latch members on the platform jacket.

The messenger lines are then pulled through the female latch members from the vessel.

This causes the hauldown lines to be pulled through the female latch members and up to the vessel. Thus, each hauldown line will extend from a male latch member on the platform jacket, through a female latch member on the touchdown base, and up to the vessel.

When the hauldown lines have been pulled up to the vessel, they are each hooked up to a linear winch on the vessel. As will be explained below, the linear winches are used to pull the platform jacket down to the touchdown base. Linear winches are well known to those skilled in the art, and one suitable linear winch is the Lucker Linear Winch manufactured by Lucker/Amhoist Corporation of Pennsylvania. Although linear winches are preferred due to capacity and control, other means for pulling in the hauldown lines can be employed. Once the hauldown lines have been hooked up to the linear winches, the platform jacket is launched from the barge in a buoyant condition, with its bottom end facing upcurrent toward the vessel.

Referring to Fig. 4, the position of the platform jacket following launch can be seen. The direction of the strong current at the installation site is indicated by arrow 39. Platform jacket 40 floats downstream from touchdown base 10 with its bottom end is pointing toward vessel 41. Although the platform jacket is made of heavy steel, it is able to float due to its hollow structural members and due to buoyancy tanks 42. Hauldown lines 36 extend from structural member 34, which is a horizontal structural member at the bottom of the platform jacket, through female latch members 14 and 15 on touchdown base 10, and up to linear winches 43 on vessel 41. The hauldown lines prevent the platform jacket from being swept away from the installation site by the strong current. Upper control line 44 is run from tugboat 38 to a connection point near the top of platform jacket 40.With the upper control line, the tugboat is able to keep the floating platform jacket properly oriented.

Following launch of the platform jacket in a buoyant condition, the jacket is made less buoyant by partially flooding the buoyancy tanks and some of the structural members with sea water. As is well known in the art, this is done by remote control in a manner which causes the platform jacket to upend. In conventional jacket installation methods, the platform jacket would be ballasted until it is no longer buoyant in order to cause the jacket to descend on its own to the sea floor. However, with the method described herein, the platform jacket is kept positively buoyant to a slight extent and the hauldown lines are used to pull the platform jacket down. This can be seen in Fig. 5, which shows platform jacket 40 being pulled downward by hauldown lines 36 to touchdown base 10.As the bottom of the platform jacket is pulled closer to the touchdown base, tension in the hauldown lines increases. This is because the angle between the downstream portion of each hauldown line and the longitudinal axis of the horizontal passageway through each female latch member increases. This increasing angle creates more friction between each hauldown line and each female latch member, thus increasing tension. However, as described above, each female latch member has a bending shoe which reduces friction by providing a curved path which gradually redirects the hauldown line as it passes through the female latch member. This in turn reduces tension on the hauldown lines and enables them to be pulled in by the linear winches on the vessel.

Once the platform jacket has been pulled almost all the way down to the sea floor, upper control line 44 is transferred from tugboat 38 to drum winch 45 on vessel 41 and is pulled in to completely upright the jacket.

Then, the hauldown lines are further pulled in until the male latch member at the end of each hauldown line engages with the female latch member through which the hauldown line extends. This is the point in time depicted in Fig. 6. At this point, the bottom of the platform jacket will be contacting or almost contacting the sea floor. The platform jacket is then further ballasted to make it non-buoyant.

This causes the platform jacket to fully settle.

Mud mats (not shown at the bottom of the platform jacket prevent the jacket from sinking into sea floor sediments. If there are no substantial sediments at the installation site, mud mats of course would not be needed.

A close-up view of the touchdown base and the bottom of the platform jacket following engagement of the latch members and settling of the jacket can be seen in Fig. 7. The male latch members are attached to structural member 34 of platform jacket 40 by articulating connections 35 and are engaged inside of female latch member 14 and 15. With the male and female latch members engaged and the platform jacket in a nonbuoyant condition, the platform jacket will be secured against being displaced or toppled by the strong current until conventional platform jacket piles (not shown) can be driven into the sea floor to further secure the jacket. Securing the platform jacket to the touchdown base as described above also prevents the jacket from being displaced or toppled by large waves generated by storms which might occur before the jacket piles can be driven.Once the platform jacket is fully installed, hauldown lines 36 can be dropped from the vessel to the sea floor for use in future removal of the jacket from the installation site. if the jacket is to be removed, the the male and female latch members would be adapted for detachment either by remote control or by a remotely operated vehicle. If the hauldown cables are not to be used for future removal of the jacket, they can be cut near the female latch members and retrieved by the vessel. Cutting can be accomplished for example with a remotely operated vehicle using shaped explosive charges or an abrasive wheel.

The technique described above enables one to guide a platform jacket to its installation site, to properly orient the jacket, and to secure the jacket to the sea floor despite the existance of a strong current. In this manner, an approach is provided for installing platform jackets in high current areas which should make it possible to recover hydrocarbons from offshore petroleum fields which cannot be developed using existing platform installation methods. The same approach can also be used to install other types of offshore structures, such as subsea production systems, in high current areas.

Inasmuch as the present invention as defined by the appended claims is subject to many variations, modifications and changes in detail within the scope of those claims, it is intended that all subject matter described herein be interpreted as illustrative and not in a limiting sense. For example, any number of latch members can be used, and they can be mounted on the subsea template rather than on the anchor piles. In addition, the latch members could be arranged in a nonlinear fashion if more than two are used. Such variations, modifications and changes in detail are included within the scope of the present invention as defined by the following claims.

Claims

CLAIMS:

1. A method for installing an offshore structure at an installation site having a strong current, said method comprising the steps of: (a) positioning a subsea template on the sea floor at said installation site, said template having an opening extending therethrough, said opening being adapted to receive an anchor pile; (b) running a messenger line through a first latch member, said first latch member being attached to said anchor pile; (c) lowering said anchor pile through said opening in said subsea template; (d) driving said anchor pile into the sea floor; (e) connecting said messenger line to a hauldown line, said hauldown line being attached to a second latch member, said second latch member being attached to said offshore structure and being adapted to engage with said first latch member;; (f) pulling said messenger line and said hauldown line through said first latch member; (g) launching said offshore struxcture proximate said installation site; and (h) pulling said hauldown line through said first latch member until said first and second latch members engage, thereby securing said offshore structure to the sea floor at said installation site.

2. A method according to claim 1, wherein steps a, b, c, d, f and h are performed from a floating vesel, wherein said floating vessel is maintained upcurrent from said subsea template in step h, and wherein said structure is launched downcurrent from said subsea template in step g.

3. A method according to claim 1 or 2, wherein said method is performed to install an offshore platform hacket at said installation site.

4. A method according to claim 3, wherein said offshore platform jacket is launched in a buoyant condition in step g and wherein said offshore platform jacket is made less buoyant during step h.

5. A method according to claim 4 wherein said offshore platform jacket is made nonbuoyant after step h.

6. An apparatus for installing an offshore structure at an installation site having a strong current, said apparatus comprising: (a) a touchdown base comprising a subsea template and an anchor pile which penetrates into, or is for penetration into, the sea floor at said installation site; (b) a first latch member, said first latch member being attached to said touchdown base and having a passageway extending therethrough; and (c) a second latch member, said second latch member being attached to, or for attachment to, said offshore structure and attached to a line passing through said passagway of said first latch member for drawing second latch member into engagement with said first latch member to secure said offshore structure to the sea floor at said installation site.

7. An apparatus according to claim 6, wherein said subsea template has a plurality of openings extending therethrough, wherein said touchdown base has a plurality of said anchor piles, each of said anchor piles extending through one of said openings in said subsea template, and wherein said apparatus further comprises a plurality of said first latch members and a plurality of said second latch members.

8. An apparatus according to claim 7, wherein said first latch members are spaced apart from each other in an arrangement, and wherein said second latch members are spaced apart from each other in an arrangement which corresponds to the arrangement of said first latch members.

9. An apparatus according to claim 7, wherein each of said first latch members is attached to one of said anchor piles.

10. An apparatus according to claim 9, wherein said anchor piles have orientation means which serve to azimuthally orient said first latch members.

11. An apparatus according to claim 9 or 10, wherein said first latch members are attached to said subsea template.

12. An apparatus according to any one of claims 6 to 11, wherein the or each forst latch member is adapted to permit said line to be pulled through said passageway at an angle to the longitudinal axis of said passageway while said line is under high tension.

13. An apparatus according to any one of claims 6 to 12, wherein the or each second latch member is attached to said offshore structure by an articulating connection.

14. An apparatus according to any one of claims 6 to 13, wherein the or each first latch member is a female latch member and wherein the or each second latch member is a male latch member.

15. An apparatus according to any one of claims 6 to 14, wherein said offshore structure is an offshore platform jacket.

16. An apparatus for installing an offshore platform jacket at an installation site having a strong current, said apparatus comprising: (a) a plurality of male latch members, said male latch members being attached to said platform jacket in a linear arrangement by articulating connections, each of said male latch members being attached to a line; (b) a subsea template having a plurality of openings extending vertically therethrough, said openings being in a linear arrangement which corresponds to said linear arrangement of said male latch members; (c) a plurality of anchor piles adapted to be driven through said openings and into the sea floor at said installation site; and (d) a plurality of female latch members, each of said female latch members being attached to one of said anchor piles, said female latch members each having a passageway extending therethrough through which passes one of said lines, said female latch members being adapted to permit said lines to be pulled through said passageways at an angle to the longitudinal axis of said passageways while said lines are under high tension, for drawing one of said male latch members into each female latch member for engagement therewith.

17. A method of installing an offshore structure at an installation site having a strong current, substantially as hereinbefore described with reference to the accompanying drawings.

18. Apparatus for installing an offshore structure at an installation site having a strong current, substantially as hereinbefore described with reference to the accompanying drawings.