Installation of conductors for offshore well platforms

Abstract: A method of installing a well conductor in a marine environment comprises sealing a well conductor (16) with a watertight plug (28), submerging the conductor (16) from an elevated platform (12), adding additional conductor lengths (16) to the said conductor (16) as required, thereby forming a conductor string (20), adjusting the buoyancy of the said string (20) to control the lowering of the string to the seafloor, and drilling through the plug (28) after the conductor string (20) has achieved the desired penetration depth. Due to the buoyancy of the conductor string (20) the load upon the platform (12) is significantly reduced. A smaller crane than the usual derrick crane (14) is able to lower the conductor string (20) so that the derrick crane (14) may be more efficiently used.

Description: This invention relates to installation of well conductors associated with offshore platforms. In an offshore environment, well conductors are installed soon after the platform has been secured in place so as to provide support for well casings or other drilling equipment which is subsequently inserted through the conductors. Well conductors are normally large tubes having a diameter of about 0.5 metres (20 inches) or more. One of the following methods is generally used to install these conductors. A first method (which has lost favour in the industry due to recent improvements in equipment) involves the welding of stops or padeyes to the outer surface of each conductor. These stops bear on framing members (which may be either permanent or temporary) which are designed to support the entire conductor string hanging from the top of the platform until the string becomes self-supporting. Consequently, the number of conductor strings which can be worked simultaneously is limited by the strength of these framing members and the overall ability of the platform to resist such loading. During installation, as additional conductor lengths are needed, a crane is used to lift each individual conductor length from a supply barge, upend it, and vertically stab it in place. Afterwards, when the new length is securely added to the string, the crane lifts the entire string (a feat in itself!) so that the lower stops can be removed in order to lower the string by the length of the new member. Stops secured to the upper end of the new member then engage the framing members and the whole process starts over again. As can be imagined, this method is very slow and time consuming, and is costly in terms of labour, required crane capacity, and crane time since the same crane that upends the new length must also lift the entire string, a separate smaller crane being unable to handle either procedure. Further, the cost of fabricating stops and the cost associated with removing stops is often significant. An improvement of this method involves the use of specialised external and internal grippers which grab and hold the conductors in lieu of the aforementioned stops and/or padeyes. The internal gripper is generally secured to the crane while the external gripper is generally secured to the platform. The crane uses the internal gripper to hoist the conductor length and place it onto the conductor string for subsequent welding. The external gripper, which supports the string during this operation, is deactivated only when it is desired to allow the new conductor length to slide through it, the crane supporting the entire conductor string during this operation. While this method is quicker than that previously described in that there is no need continuously to add and then remove stops and/or padeyes, it still requires the addition of framing members to the platform to support the string until it becomes self-supporting, and it still requires a very large crane for both upending each new member and lowering the entire conductor string. In order to reduce the required crane time, a system has been developed which utilises two external grippers, one being movable relative to the other by means of a series of jacks. With this system, a large crane and the internal gripper or padeyes and slings would still be used to lift the new conductor length and align it with the string for welding as before. Afterwards, however, one external gripper (which is in a raised position) would be lowered by means of the jacks toward the other external gripper, this lower gripper being deactivated so as to allow the string to slip through it. In this way, the entire conductor string is always supported by one or both of the external grippers and not by the crane. While this method eliminates the need for large crane tonnage, it is a very slow process due to the leisurely pace and small stroke of the jacks. Additionally, framing members are still needed, and the platform itself must still be designed so as to withstand the temporary imposition of large installation loads. According the invention there is provided a method of installing a well conductor in a marine environment, which method comprises: sealing a well conductor with a watertight plug; lowering the conductor from an elevated platform; adding additional conductor lengths to the said conductor as required, thereby forming a conductor string; adjusting the buoyancy of the said string to control the lowering of the string to the sea-floor, and eliminating the plug from the conductor string after the conductor string has achieved the desired penetration depth. It will be seen that an end region, for example, of a conductor may be plugged so that it will achieve the desired degree of buoyancy when submerged. Afterwards, additional conductor lengths can be affixed to this first conductor and also submerged. Additional buoyancy or ballasting of the conductor string may be provided as needed so as to control the rate of sinking and to limit the load applied to the platform. Upon achieving self-support, the plug or plugs in the conductor string may be either removed directly, or may be left in place until completion of driving and later removed, for example by drilling, in preparation for the insertion of a well casing or other drilling equipment. The method of the invention, at least in preferred forms, can provide the following advantages over known methods of installing conductors in an offshore environment: reduction of the need for large crane tonnages; substantial elimination of the need for additional framing members; reduction of the installation or construction loading on the platform, thereby permitting a more efficient and lighter structure to be built, and faster and easier installation of well conductors. The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings in which: Figure 1 is a front pictorial view of an offshore platform illustrating a conventional method of installing a conductor in a marine environment; Figure 2 is a plan pictorial view of the apparatus shown in Figure 1; Figure 3 is a front pictorial view of an offshore platform illustrating one method embodying the invention for installing a conductor in a marine environment; Figure 4 is a plan pictorial view of the apparatus shown in Figure 3, and Figure 5 is a pictorial view, partially broken away, of a conductor plug for use in a method embodying the invention. Figures 1 and 2 show a conventional method of installing a conductor in a marine environment. In accordance with this method, a derrick or supply barge 10 is anchored or otherwise positioned alongside a platform 12 prior to conductor installation. Because of the long lengths of the conductors, which are normally anywhere from about 15 metres (50 feet) to over 60 metres (200 feet), a derrick crane 14 on the derrick barge 10 is used to uplift and stab each conductor 16 within its guides on the platform 12. Generally, an internal gripper 18 is used by the derrick crane 14 to lift a conductor 16 off the supply barge 10 and position it as needed. Once the conductor 16 is properly installed and secured to the top of the conductor string 20, the derrick crane 14 lowers the string 20 until the addition of another conductor 16 is required. A series of external grippers 22 on jacks could support the conductor string 20 rather than the crane 14. However, the crane 14 will still be needed to lift and stab the conductors 16 as shown. The crane 14 may also be needed to balance and stabilise the conductors 16, and while being occupied by this task it is being vastly underutilised. In any event, due to the heavy weight of each conductor 16, which typically ranges from 5080 Kg (5 tons) to 30482 Kg (30 tons) depending on length, it does not take very many conductor lengths to amount to a sizable load upon the platform 12, and such platforms are sometimes 300 metres (a thousand feet) or so above the ocean bottom. Consequently, it becomes necessary to install additional framing members 24 on the platform 12 to withstand the loading and to transfer this loading to the legs of the platform 12. This additional construction or installation loading will remain until the conductor string 20 becomes self-supporting. The weight of the conductor string 20 normally necessitates the use of large cranage or sophisticated jacking equipment to lower the string 20. Furthermore, there are often thirty or so conductor strings 20 installed on a single platform (see Figure 2), but, by necessity, they are installed one, or only a few, at a time. This makes it important to develop a quick and reliable method of installing each conductor string 20 so as to save both time and money. Obviously, the size of the group of conductors 16 which can be installed simultaneously, and the amount of time required to install each conductor string, is dependant upon the installation pace. Figures 3, 4 and 5 show apparatus for installing conductors 16 in accordance with a preferred method embodying the present invention. An additional crane 26 is employed so as to free the derrick crane 14 from tasks which would result in it being inefficiently utilised. The additional crane 26 may be a platform mounted crane or another crane located on the barge 10. In any case, by utilising the method described hereinafter the conductor string loading is significantly reduced, thereby enabling the smaller crane 26 to lower the conductor string 20 to the sea-floor. There is often no, or only a slight, increase in expense associated with using the crane 26, or other suitable cranage, during conductor installation. However, this added expense is quickly recouped by the reduced need for the larger derrick crane 14. A conductor plug 28 (Figure 5) is employed within one or more of the conductors 16 of the conductor string 20. The plug 28 seals an end of the conductor 16 thereby making it watertight, and should its strength and diameter to wall thickness ratio be in the proper range, the conductor 16 may actually achieve a positive buoyancy when submerged without collapsing. Strategic placement of the plug 28 greatly reduces the loading upon the platform 12 by effectively eliminating the excessive weight of the string 20. Should additional ballast be needed to submerge the string 20, water can be allowed to enter the string 20 thereby weighing it down so that it will sink rather than float. Thus, by adjusting the water level in the conductor string 20, the desired buoyancy can be achieved. Figure 5 shows a typical plug 28 which is described in more detail in US Patent No. US-A-4 804 018 (Carr et al). Alternative designs may also be used, such as modifications of those manufactured by Davis-Lynch Inc or others in this field. Furthermore, plugs comprising grout, cement polymer materials, rubber based materials or inflatable bladders are equally suitable since after drilling, the interior of the conductor 16 must be free of permanent obstructions. About the only requirement is that the plug 28 is water-tight to the point of self-support, and that it can be drilled after the string 20 is installed or removed prior to the driving of well casings. The plug 28 is preferably capable of being installed at a variety of locations along the string 20 so as to prevent or control the flooding of the string 20. A sealed steel tubular member achieves neutral or positive buoyancy when the ratio of the outside diameter to wall thickness is approximately thirty to one. This parameter is often referred to as "the D/t ratio". Sealed tubulars with a D/t ratio greater than approximately 30:1 will float, while those with a D/t ratio less than approximately 30:1 will sink in water. The actual numerator of the neutral point ratio will vary according to the density of the fluid medium in which the tubular is immersed. However, regardless of the D/t ratio, sealing or plugging of the conductor string 20 to prevent flooding will reduce the negative buoyancy of the conductor string 20 due to the displacement of water, and thus reduce its weight and the associated load upon the crane 26. The plug 28 is installed in the usual fashion, either within the lowest conductor 16 or at some other location depending upon the amount of positive buoyancy desired. (As an alternative, the conductor 16 could be submerged before the plug 28 is installed, but this may necessitate de-ballasting of the submerged conductor 16 after the plug 28 is set). After the plug 28 has been installed, the conductor string 20 is lowered under gravity, either by means of the smaller crane 26 (which can handle such smaller loads) or by a series of external grippers supported on jacks (not shown in Figure 3). Should the positive buoyancy of the string 20 become too great, it can be flooded so that the string 20 can once again be lowered under its own weight. In this way, the derrick crane 14 is used solely to upend and stab the individual conductor lengths 16 in place. Because of the great height of the derrick crane 14, it may be possible for two or more such conductor lengths to be combined on the supply barge 10 before being upended. This will cut in half the (already reduced) length of time required to install each conductor string 20. It is also possible for two or more conductor strings 20 to be installed simultaneously. During this procedure, while the derrick crane 14 is upending and stabbing conductors for one conductor string 20, the platform crane 26 is lowering the other conductor string 20. Thus, when the stabbing operation is completed, it is likely that the lowering operation is also completed, so that the derrick crane 14 can now upend a conductor length 16 for the string just lowered while the platform crane 26 lowers the string 20 that has just been stabbed. It is also plausible for three or more strings 20 to be installed simultaneously following a similar procedure. Once the string 20 has achieved self-support by either self-penetration or by being driven, both of the cranes 14 and 26 become free to initiate the installation procedure for another conductor string 20. It also becomes possible to remove or drill out the plug 28 as required since the platform 12 will not be incurring any significant additional load. After the conductor strings 20 are installed and the plugs 28 drilled out or otherwise removed, casings or wells may be inserted through the string for future undersea development. A direct result of the buoyancy provided for the string 20 is the elimination of the need for additional or strengthened framing members 24 to support the string 20 during installation. Additionally, because the construction load upon the platform 12 is significantly reduced, the platform 12 may be designed without taking these excessive forces into consideration (ie a lighter structure will result). Also, smaller stops or padeyes can be used since the forces on these devices are significantly reduced. The same can be said for external grippers 22 if these are used since these need only support a load that is a fraction of what they are conventionally required to support. It will be appreciated that by installing a plug 28, or some other suitable temporary seal, inside the conductor string 20, either at the bottom end of the string 20, or at one or more predetermined locations, the effective weight of the string 20 can be significantly reduced. This will achieve the benefits referred to above by reducing the load on the platform 12. Also, by utilising one or more supplemental cranes 26, the derrick crane 14 can be used solely to lift and stab the additional conductors 16 in place (where the height provided by such a crane is needed), while the smaller crane or cranes 26 can be used for balancing and stabilising the stabbed conductor 16. Thus, the installation procedure will be faster and the time required to install each of the thirty or so conductor strings 20 will be greatly reduced. An added benefit of the reduced load of the string 20 is that larger batches of conductors (ie pre-assembled lengths of conductors) can be hung so as to speed the installation process even further. Where only small conductor lengths 16 are used, it is possible for the smaller platform crane 26 to perform the conductor installation without the need for the much larger derrick crane 14. Although using only the smaller platform crane 26 will increase the amount of time needed to install the various conductor strings 20, the cost of installation will be drastically reduced since the expense associated with use of the derrick crane 14 will not be incurred. One benefit of choosing to plug the bottom end of the first conductor 16 and driving the conductor string 20 to the desired penetration depth below the sea-bed (or refusal if penetration cannot be achieved) is that drilling survey tools can then be deployed immediately inside the conductor string 20 to establish its bearing and inclination without first having to drill or jet out the cored soil which would exist in an open-ended conductor 16. This will further reduce the time and expense normally required to achieve a working or producing platform.

Claims: A method of installing a well conductor in a marine environment, which method comprises: sealing a well conductor (16) with a watertight plug (28); lowering the conductor (16) from an elevated platform (12); adding additional conductor lengths (16) to the said conductor (16) as required, thereby forming a conductor string (20); adjusting the buoyancy of the said string (20) to control the lowering of the string to the sea-floor, and eliminating the plug (28) from the conductor string (20) after the conductor string has achieved the desired penetration depth. A method according to claim 1, wherein said elimination of the plug (28) comprises drilling through the plug. A method according to claim 1 or claim 2, wherein the said conductor (16) and the conductor string (20) are lowered by a platform crane (26) under their own weight. A method according to claim 1, claim 2 or claim 3, including upending each said additional conductor length (16) by means of a derrick crane (14) and using the derrick crane to stab each additional length (16) through the said platform (12). A method according to any one of the preceding claims, wherein the said conductor (16) has a diameter-to-wall thickness ratio such that the conductor (16) will achieve a desired degree of positive buoyancy. A method according to claim 5, wherein the said ratio is approximately thirty to one, the ratio being varied in dependence upon the amount of positive buoyancy and strength desired. A method according to any one of the preceding claims, including selecting the position of the plug (28) along the conductor string (20) in dependence upon the desired degree of buoyancy. A method according to any one of the preceding claims, wherein the buoyancy is adjusted by installing one or more additional plugs (28) in the conductor string (20) as required and/or by selectively flooding the conductor string (20) as required. A method according to any one of the preceding claims, including installing two or more conductor strings (20) simultaneously. A method according to any one of the preceding claims, including joining two or more said conductor lengths (16) prior to adding these to the said conductor (16).

Inventors: HOUSER DANIEL MAJOR, HOUSER, DANIEL MAJOR
Applicants: MCDERMOTT INT INC, MCDERMOTT INTERNATIONAL, INC.
Application year: 1991
Main classification: E21B17
IPC codes: E21B7, E21B19, E21B17