WO2002063106A1 - A method for installing submerged oil and gas wells, and apparatus for the same - Google Patents

Abstract

The inventions related to submerged oil and/or gas wells and particularly to a method and apparatus specially suited for use in shallow waters where the surroundings gives a risk of drifting ice destroying well equipment positioned on the seabed. According to the present invention the wells are positioned in a hollow structure (1) which is driven down into the seabed in a way so that the wells are situated in a distance below the seabed. The hollow structure (1) comprises a foundation to make it possible to use a multiwell template (5). The foundation is in a form of a base slab (4) on which the template are positioned.

Description

A method for installing submerged oil and gas wells, and apparatus for the same

The invention pertains to submerged oil and gas wells and, more particularly, to a method and apparatus having particular utility in the context of submerged oil and gas wells in shallow waters where floating ice may destroy structures placed on the seabed and where conventional semi-submersible drilling rigs with pontoons cannot normally be utilised and in which the equipment associated with the wellhead during drilling of the well and during subsequent operation of the well is housed in a silo-like structure disposed below the bottom of the water body.

In this kind of environment protection of subsea wellheads is very important. The use of large diameter subsea caissons have been tried in the past for Arctic areas, but have been generally unsuccessful mainly due to installation problems. The practice of dredging "glory holes" is currently used but is expensive and does not protect equipment from scour debris. Piled frameworks have been used for protection against fishing gear and anchors but again they are large, difficult to install and expensive.

Conventionally, therefore, subsea wells have the wellhead at the sea bed and have connected to them either a blowout preventer during the drilling phase or a valve block assembly and flow line connector during the production phase. The wellhead and the associated equipment project somewhat above the sea bed and are thus vulnerable to damage.

All equipment on the ocean floor is at risk due to a variety of hazards. In offshore hydrocarbon developments to date, the major hazards to subsea equipment are from trawlboards used in commercial fishing and from anchors used by supply boats, laybarges, drill ships, etc. For hydrocarbon development in cold water regions, a new hazard which must be considered is ice scour of the sea bottom. For example in the Beaufort Sea area, ice scouring occurs in the region beyond shelf ice where ice ridges are formed and frequently scour the seabed. Off the east coast of Canada very deep scours can occur when an iceberg grounds out. Therefore there is a requirement for a well completion design that will maintain control over the well even in the extreme event of an ice scour shearing off the top of the well.

One conventional solution consists of placing pressure control devices (such as master valves, DHSVs, etc.) and the wellheads with their sealing systems below the scour zone together with suitable shear devices to insure their integrity. There are many alternate ways to implement such a solution. One method is to excavate a large glory hole with sufficient depth to place all well control devices below the scour line. The glory hole approach is a good technical solution but is not economically attractive. The other end of the spectrum of solutions, is to design compact Xmas trees that can be installed inside the well bore to the required depth with sacrificial equipment above the well control devices. The insert tree has been developed to minimize equipment heights above the mudline but it is unknown whether the equipment between the surface and the well control devices is sacrificial.

However, between the two extremes described above, there are a number of other solutions available. They basically consist of installing a caisson large enough to allow a conventional subsea tree to be installed inside the caisson. The success of this approach depends on the feasibility of mstalling the caissons.

To overcome some of these problems, it has been proposed to make a caisson large enough to contain the blowout preventer. U.S. Pat. Nos. 3,344,612 and 3,796,273 describe methods of installing such a caisson type. U.S. Pat. No. 3,344,612 uses a jetting technique for caissons installed in a soft sea bed and U.S. Pat. No. 3,796,273 uses a rotary drilling technique for a hard sea bed. In this latter arrangement, the base of the caisson has cutting teeth on its surface and the complete caisson is rotated thereby boring its own hole. Although these techniques appear feasible, they require a drilling rig which is very expensive and must rely on cementing to ensure their soundness since the surrounding soil is highly disturbed. In the case of the rotating caisson, the feasibility of easily rotating such a large body when it nears its full penetration is questionable since there is a large surface area at a large radius.

U.S. Patent no. 3,344,612 describes a caisson useful during the drilling and subsequent operation of a well at an offshore location from a floating drilling vessel. A principal purpose of the caisson described in this patent is to artificially lower the bottom of the body of water over the well site so that the blowout preventers, normally associated with the wellhead during the drilling operation, may be disposed clear of the bottom of the drilling vessel from which drilling operations are carried out. A blowout preventer has substantial height and is required in the drilling of offshore wells. A blowout preventer is a mechanism for sealing off the well in the event that high pressure oil or gas pockets are encountered in the earth formation being drilled. Floating drilling vessels conventionally have a centerwell through the hull thereof through which the drilling operations are performed from a drilling rig mounted to the vessel over the centerwell. In areas where shallow water exists, the height of the blowout preventer may be greater than the vertical distance between the wellhead and the bottom of the floating drilling vessel; where this situation exists, the floating drilling vessel is captive to the blowout preventer during drilling operations and cannot be removed from the well site during a storm or in the event of an emergency. U.S. Patent no. 3,344,612 teaches that, in such shallow water conditions, the bottom of the body of water is artificially lowered by the use of a caisson so that the distance between the wellhead, located at the bottom of the caisson, and the bottom of the drilling vessel exceeds the height of the blowout preventer. Thus, it is possible for the floating drilling vessel to be removed from the well site in the event of emergency prior to installation of a production valve assembly (known in the industry as a «Christmas tree») in place of the blowout preventer on completion of the well drilling operation. The principal purpose of the caisson shown in this patent is merely to lower the bottom of the body of water sufficiently that the blowout preventer is located below the hull of the drilling vessel; the top of the blowout preventer stack, as connected to the wellhead during drilling operations, may project above the bottom of the body of water.

U.S. Pat. No. 4, 189,255 discloses a sea-floor cellar that includes a substantially cylindrical retainer wall assembly having a hollow interior, an upper rim portion forming a top opening and a lower rim portion forming a bottom opening. Attachment means are mounted with the upper rim structure for attaching the retainer wall assembly to a suitable crane for lowering the sea-floor shoring cellar to the sea-floor from a drilling vessel. An annular air ejector is mounted within the retainer wall assembly adjacent the lower rim structure, the annular air ejector including means for connecting the air injector to a source of air under pressure. The annular air ejector includes a plurality of air nozzles directed generally vertically upwardly. A plurality of discharge tubes are mounted substantially within the interior of the retainer wall assembly and include an upper, exit portion extending outwardly from the retainer wall assembly and a bottom inlet portion positioned at least partly over the nozzles for directing air, water and solids received upwardly and outwardly of the retainer wall assembly. The sea-floor shoring retainer wall assembly disclosed in this patent is installed by first drilling a hole at approximately the centre of the designated area on the ocean floor to be cleared. A jetting tool is then utilised to jet compressed air or other gas or drilling fluid in the area of the drilled hole to enlarge the hole to sufficient size to receive the retainer wall assembly. A T-bar jetting tool can also be used within the partly cleared hole to rotate therein and direct outwardly through a plurality of nozzles a jetting fluid to loosen up the soil of the subsea bottom, which soil is then removed by the suction action applied into the discharged tubes mounted on the retainer wall assembly.

These inventions all deals with traditional drilling and completion, e.g. drilling single wells where a conductor pipe is cemented into the ground and a wellhead attached thereto. However, it has in recent years been a growing interest for using templates, where several wells can be drilled from the same location. A template however, requires a good foundation and must be stabilised before drilling. None of the above solutions can readily be used with a template-type completion.

Another problem concerns the connection of flowlines and umbilicals to the wellhead. These have traditionally been connected to the top of the Christmas tree and therefore been in danger of being snagged and destroyed by the moving ice. To counter that, existing solutions have incorporated so-called «weak links» so that the flowline or umbilical will break at a designed location, usually at the connector, if the line is snagged by ice.

The present invention provides an effective, simple, economic and reliable method and apparatus for the installation of a silo-like, hollow structure, as a sea-floor cellar to be used in conjunction with a drilling template. The silo-like hollow structure will further also be referred to as the silo. Specifically, the invention provides an improved shallow water silo which may be installed by simple hammering or vibrating means. With shallow water it is understood water depths less than approximately 10 meter and specifically below 5 meter.

In terms of method, the silo-like hollow structure, the silo, is disposed in a substantially vertical attitude. The silo is then lowered until the lower end of the silo is resting on the seabed, and then hammered into the ground until the upper end of the silo is disposed proximate to the top of the earth formation, i.e. the bottom of the body of water. The silo is then emptied for rock and/or soil and a concrete slab is formed in the bottom of the silo. A drilling template is placed on the concrete flooring and the desired wells are then drilled in the earth formation through the drilling templates drilling guides.

The apparatus includes a silo-like structure, in the form of a hollow structure with open top and bottom. The interior of the silo defines a chamber sized to receive wholly therein a drilling template with associated equipment for several wells, such as manifold piping. A concrete slab is positioned in the floor of the silo to provide a foundation for a drilling template. Christmas trees, manifold and other associated equipment is secured to the template. Flowlines and/or umbilicals are entered into the silo through a hole in the side wall.

The above mentioned and other features of the present invention are more fully set forth in the following detailed description of presently preferred embodiments of the invention, which description is presented with reference to the accompanying drawings, wherein: Fig. 1 is an exploded schematic view illustrating the main components of the sea-floor cellar;

Fig. 2 is a view of the cellar as installed below the seabed; Fig. 3 is a diagrammatic sequence drawing of the installation method.

Referring to fig. 1 there is shown a sea-floor cellar in the form of a silo-like hollow structure 1. The hollow structure 1 is preferably in the shape of a cylinder, but may also be of other shapes, for example rectangular, octagonal or even an asymmetric shape. The hollow structure 1 is preferable made of steel but if the soil condition permits, it can be made of concrete. The hollow structure 1 may be made up of several smaller structures to make up the necessary height. The hollow structure is further referred to as the silo 1.

A lid 3, forming a protection roof, may be placed on top of the silo 1, closing the upper opening of the silo 1 to protect the wellhead equipment housed inside the silo 1.

A base slab 4 preferably made of concrete is designed to be placed inside the silo 1. This concrete slab 4 will preferably be cast in situ after the silo 1 has been emptied of material. The slab 4 provides a foundation for a drilling template 5. The slab 4 is preferably cast in situ so that it will provide a level base for the template 5 and by this eliminating the need for levelling of the template. The procedure for levelling the template 5 in a conventional installation are extensive and time-consuming. By providing a levelled base for placing the template on, this procedure is simplified or even eliminated. A base slab 4, especially one which is cast in situ, will prevent water from seeping into the silol, during the installation and drilling procedure or later during intervention work. This enhances the possibility to perform work on the well in a principally dry environment. A base slab 4 will also, when the see floor consists of a loose material as mud, sand or similar, give a firm base in the working environment. The drilling template 5 includes guide housings for guiding a drillstring during drilling and for placing wellheads and Christmas trees 8 and associated manifold piping 6.

Flow lines or umbilicals will preferably be laid buried in the seabed, to avoid damage by ice. To tie the flow line to the manifold piping it is therefore preferred to extend the hne through a hole 9a in the silo wall. In a preferred embodiment, a tie-in plug 9 is placed in the hole 9a to seal the opening and permit passage of the flowline or umbilical to the inside of the silo 1. Alternatively, the tie-in plug 9 may include connectors both inside and outside. The flow hne or umbilical is tied into the plug on the outside, while a short spool connects the plug with the manifold on the inside of the silo 1.

Also as shown on fig. 1 hollow extensions 7 may be added to the top of the silo structure, to extend the silo to above the water hne. In this manner, the water inside the silo may be pumped out so that work, for example intervention work, can be carried out in a dry environment.

As shown in fig. 2, when the complete installation is made, the well equipment is housed in a safe environment inside the silo and therefore the well equipment is protected from the actions of floating ice or fishing equipment that may otherwise damage the wells.

The sea-floor cellar system is intended to be used with a special drilling vessel 10 that is able to be lowered over the silo 1 and to seal the silo 1 and the vessels moon-pool 12 from the outside water. This makes it possible to install and connect equipment in a dry environment.

This installation process is shown in fig. 3 by diagramatic sequence drawings. When installing the sea-floor cellar, the silo 1 is brought out on a barge or similar vessel 10 and lifted into a vertical position. The silo 1 is then first lowered until it sits vertically on the seabed 2. To drive the silo 1 into the seabed, preferably hammer or vibratory apparatus is used. This is well known in the art and therefore not described further. These steps are not shown in fig. 3.

In fig. 3(1) the situation where the silo 1 has been hammered into the seabed 2 is shown. After the silo 1 is driven into the seabed, the material inside the silo is dredged or dug out (fig. 3(2)), forming an excavated area 11. After this, the drilling vessel 10 is moved over the silo 1 and lowered so that the vessel moon-pool 12 sits on the seabed 2. As is shown in fig. 3(3), the moon-pool 12 can be equipped with protruding skirts that will penetrate a short distance into the seabed 2 to enable water to be pumped out of the moon-pool 12 and the inside of the silo 1. Next, a base slab 4 is positioned in the silo, either as a pre-made concrete slab lowered down from the vessel or preferably by concrete poured into the silo 1 to make a base slab 4 and a foundation for the drilling template 5. As it is now possible to operate in a dry environment the casting of the concrete can be done in such a way to provide a level foundation. The drilling template 5 can now be lowered into the silo. It may be anchored to the concrete by any standard means. Drilling can now be carried out utilising the template drillguides in the conventional manner. After drilling, manifold piping 6 is installed on the template 5 and Christmas trees 8 are installed on each well (fig- 3(4)).

If necessary, the silo 1 internal walls can be equipped with vertical guides (not shown in the drawings), so that the template 5 is guided into its correct position on the slab. Also, during casting of the concrete, anchoring or fastening means can be embedded, for secure fastening of the template.

After drilling and completion activities are finished the roof element, the lid 3 is lowered into place to make the equipment in the silo 1 safe against ice or other outer factors (fig. 3(5)).

During the installation of the template 5 and manifold equipment, a pipe can be installed that extends from the manifold 6 to the tie-in plug 9 in the silo wall, for later connecting of the flowline.

During completion, or if there is later a need to do workover operations on the well, the lid 3 will be removed and hollow extension rings 7 (fig. 1) may be stacked on top of the silo, extending above the surface of the water and making it possible to do the operations in a dry environment.

Claims

1. A method for installing wells in shallow waters, where there is a risk for ice drift comprising the steps of a) a vessel (10) brings a hollow structure (1) to an installation site, and lower it down to the seabed (2) in an upright vertical position, b) the structure (1) is driven down in the seabed (2) by hammering or vibration means, until an upper end of the structure (1) is essentially level with the seabed, c) material is removed from within the structure (1), d) the interior of the structure (1) is filled with concrete, thereby forming a base slab (4) and the slab (4) is levelled, e) a drilling template (5) having drilling guides is placed on the slab (4), and f) then a desired number of wells are drilled and completed through the drilling template (5) drilling guides.

2. A method of claim 1, wherein the hollow structure's (1) upper end is closed with a lid (3).

3. A method of claim 1, wherein a hole (9a) is formed in the wall of the structure (1), and flow fines and/or umbiUcals are drawn from the outside to the interior of the structure (1) through the hole (9a).

4. An apparatus for installation of wells in shallow waters, where there is a risk for ice drift, comprising a hollow structure (1) with open ends and adapted to be driven down in the seabed (2), and designed to accommodate a template (5) with associated equipment, as manifold (6) and Christmas trees (8) for several wells, wherein it further comprises a base in form of a cast slab (4) within the hollow structure (1), which slab (4) serves as a foundation for the template.

5. An apparatus according to claim 4, wherein the hollow structure (1) comprises a hole (9a) in the side wall for inducting a flow line or similar.

6. An apparatus according to claim 4 or 5, wherein the cast base slab (4) comprises anchoring or fastening means for the template (5).

7. An apparatus according to any of the claims 4-6, wherein the hollow structure (1) comprises a lid (3).

8. An apparatus according to any of the claims 4-7, wherein the hollow structure (1) comprises guides arranged to the internal wall to guide the template into its correct position on the slab (4).

9. An apparatus according to any of the claims 4-8, wherein the hollow structure (1) is cylindrical.

10. An apparatus according to any of the claims 4-9, wherein the hollow structure (1) is polygonal.