GB2627730A

GB2627730A - Improved drilling arrangement

Info

Publication number: GB2627730A
Application number: GB2302474.8A
Authority: GB
Inventors: Weltzin Tore
Original assignee: Equinor Energy AS
Current assignee: Equinor Energy AS
Priority date: 2023-02-22
Filing date: 2023-02-22
Publication date: 2024-09-04
Also published as: GB202302474D0

Abstract

A deep-water drilling riser assembly comprising an outer riser 27, an inner riser 28 arranged co-axially within the outer riser, wherein the inner riser is suitable for receiving a drill string 29 co-axially within it. An annulus between the inner and outer risers may be filled with seawater as a buffer fluid. One or more spacers may be provided between the inner and outer riser, the spacers may comprise bow springs. The return flow from the bottom of the well can be selectively flowed through the annulus between the risers or the annulus between the inner riser and drill string.

Description

Improved drilling arrangement The invention relates to drilling operations, in particular deep water drilling operations, and improved drill string arrangements.

Background

Drilling operation are carried out when creating a producing well, for example for the purpose of hydrocarbon production or water production. The tools used in the drilling operation include a drill string and a drill bit, alongside many other components. The drill bit includes rotating abrasive elements arranged to remove soil and formation. The drill bit may be driven by rotation of the drill string and/or by hydraulic pressure from fluid being pumped through a bore of the drill string. The fluid may also be used for removing cuttings from the formation, and for removing heat from the drill bit due to the rotational friction. The fluid flow is directed into the well through the bore of the drill string, and back up around the annulus between the drill string and formation, or between the drill string and a riser, thereby taking drill cuttings back up. The drill string provides a fluid channel for the drilling fluid, as well as torque for the drill bit, and further provides weight to drive the drill bit into the formation. The drill bit is part of the bottom hole assembly (BHA), which further includes heavy drill collars to add further weight on the drill bit, reamers, jars, shock subs, and drill string stabilizers.

The drilling assembly further includes a blowout preventer, or BOP. A BOP is typically installed above the mudline for deepwater drilling setups. The BOP is intended to prevent a sudden flow of pressurised combustible well fluids up the wellbore, which could have disastrous consequences for personnel and the environment.

Drilling risers are used to surround the drill string. The longitudinal axes of the drilling riser and the drill string are generally arranged co-axially, although during a drilling operation there is a significant amount of relative movement. The drilling riser does not rotate with the drill string. Once the drilling operation has finished, the well is completed and will be prepared for production. The BOP will be removed, and will be replaced with a Christmas tree including various entry points and shear rams. The drilling riser is removed together with the drill string once the drilling operation has been completed. A production riser is installed for the production operation. Production risers and drilling risers are different types of risers, with different technical specifications and layouts. A drilling riser does not need to withstand the full well-pressure because the BOP is arranged below the drilling riser, while the production riser can withstand the full well-pressure. Production risers are installed permanently, rather than the temporary drilling risers, and need to withstand wave motion for prolonged periods of time without experiencing fatigue. Production risers are also arranged between the water surface and the seabed with one or more S-bends created by way of buoys to absorb the wave motion, while drilling risers preferably avoid bends and approach the seabed generally straight from the drilling platform to reduce friction between the drill string and the drilling riser.

Statement of invention

According to a first aspect of the invention, there is provided a deep water drilling riser assembly, the assembly comprising: an outer riser; an inner riser arranged within the outer riser; wherein the inner riser is suitable for receiving a drill string; wherein the outer riser and the inner riser are arranged co-axially with respect to each other, and, when the drill string is received within the inner riser, the drill string and the inner riser being arranged co-axially with respect to each other.

The annulus between the inner riser and the outer rise may filled with water, in particular seawater. The water or seawater acts as a buffer fluid.

In a particular example, the inner diameter of the outer riser is 48cm. In a further example, the inner diameter of the inner riser is 31cm. The inner diameter of the drill string may be 10cm. These examples can be used in combination or independent of one another. The depth of the deep water may be around 2km, and the length of the riser assembly may therefore have a similar dimension of 2km..

One or more spacers may be arranged between the inner riser and the outer riser. The one or more spaces may comprise bow springs.

According to a second aspect of the invention, there is provided a method of arranging a drilling assembly, the method comprising: providing an outer riser, arranging an inner riser within the outer riser, arranging a drill string within the inner riser, wherein the outer riser, the inner riser, and the drill string are arranged co-axially with respect to each other.

The method may further comprise filling the annulus between the inner riser and the outer rise with a buffer fluid, in particular seawater.

The method may further comprise filling the annulus between the inner riser and the outer rise with seawater.

The method may further comprise pumping drilling fluid into the drill string downhole, and back to the surface through the annulus between the drill string and the inner riser.

The method may further comprise pumping drilling fluid into the drill string downhole, and back to the surface by selecting the annulus between the drill string and the inner riser, and/or the annulus between the inner riser and the outer riser.

Drawings Some embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a graph illustrating a drilling window; Figure 2 is a vertical cross section through a schematic of a riser assembly; Figure 3 is a flow diagram, and Figure 4 is a schematic perspective drawing of a spacer.

Specific description.

A drilling assembly comprises a drill string extending from the surface to the wellbore. The drill string is received within a drilling riser. The distance between the top of the wellbore and the water surface is much larger in deep waters than in shallow waters.

The expression deep water is a term of the art, referring to bodies of water such as the ocean or seas with a water depth of 1 to 2 km, or even more, with the most advanced systems extending to a depth of 3500m. The drilling riser and drill string therefore have similar length, to bridge the distance between a platform at the water surface and the wellhead. This large length of riser and string poses particular challenges. A vertical riser of around 2km will have a large weight, which is sometimes alleviated by attaching buoyancy modules to the outside of the riser. The riser and the string are not only subject to the gravitational forces that will pull the material at the top, and compress the material at the bottom, but also experience significant environmental strains. Even a small flow of the surrounding water volume will create a large drag on the outer surface of the riser due to the total exposed surface area being large.

Another problem posed by the large distance is the increased risk of a leak. A larger surface area itself increases the risk of a leak, because there are more joints and more exposed areas to cause failure. The larger forces described previously also increase the risk of a failure. The consequences of a failure to the environment are larger for a large column of fluid, possibly including well fluids. Even if the BOP or other safety mechanisms are activated to stop the inflow of fluids if there is a failure, the column of fluids already present in the riser can still leak out.

A further challenge posed by a riser with a large length is the inertia of the fluid column within the riser. Although the pressure of a static fluid column only depends on the height of the column, and not the diameter, the fluid dynamics of a large volume is different from that of a smaller volume. The fluid pressure within a drilling system to drive the fluid column needs to be contained within carefully managed bounds. If the pressure is too low, the column cannot be displaced and there is a risk of breakout, but if the pressure is too high, the formation can fracture. The presence of a large fluid column therefore narrows the operational window.

Fig. 1 illustrates a drilling window within a well. The horizontal axis is pressure and the vertical axis is depth. The operational drilling window A is limited on one side by fracture pressure B, which increases in an irregular manner with increasing depth, and on the other side the drilling window is limited by pore pressure C which is lower than the fracture pressure at a particular depth. The pore pressure is the naturally existing fluid pressure within pores of the formation. If the pressure is lower than the pore pressure, a breakout D will occur and well fluids will enter into the wellbore. If a large static column of drilling fluid is present, the well pressure may be close to the fracture pressure, and it may not be possible to increase the pressure further to displace the column without exceeding the threshold for fracture pressure.

An example of an inner diameter of a drilling riser is 19 inch, or 48 cm. The corresponding inner volume of a 2km long riser, if no other parts occupy the volume within the riser, is 366 m3. The large inner diameter corresponds to a relatively fluid velocity of 0.2 m/s, for example, or 2000 litres per minute. If a drill string is included within the riser, the effective inner volume of the riser will be reduced by the volume of the drill string. The inner diameter of the drill string may be 3.9 inch, or 10 cm.

The inventors have appreciated that the operational window can be widened by the use of an inner riser, arranged between the (outer) drilling riser and the drill string. Fig. 2 illustrates the arrangement. The inner riser has a smaller diameter than the outer riser, but a larger diameter than the drill string. The inner diameter may be 12.25 inch, or 31cm.

Fig. 2 illustrates a drilling rig 21 floating on the water surface 22. The drilling rig is connected to a riser assembly 23 by way of a slip joint 24 and tensioners 25. The riser assembly comprises a plurality of joints for connecting individual riser sections together to form a long riser extending from the surface 22 to a BOP 26. The riser comprises an outer riser 27, and inner riser 28, both surrounding a drill string 29. A flex joint 30 is included above the BOP to absorb lateral movement of the riser. An annular connector 31 connects the flex joint and the BOP. The BOP is arranged just above the mud line 32.

In a particular example, the inner riser has a 12.25 inch, or 31 cm inner diameter. This size riser can be arranged co-axially within the 19 inch, or 48 cm outer riser. The outer diameter corresponding to a 12 % inch (31cm) internal diameter is 13 3/8 inch, or 33.97cm. A 2000m long inner riser would therefore have an outer volume of 181 m3.

The inner volume of the inner riser for a length of 2000m is 152 m3. The outer volume is 181m3, and the difference in volume with the outer riser, which is the volume of the annulus is 184m3 is 214 m3. The maximum fluid velocity within the inner volume can be increased by 50% to 2000 litres per minute.

The riser displacement, which is the volume divided by the fluid flowrate, is 73 min for a 366m3 riser volume, compared to 30 min for the 152m3 riser volume, at 5000 litres per minute. Riser displacement refers to operations where displacing or changing the fluid in the riser becomes necessary, for example before disconnection the riser from the BOP. In that example, the internal volume of the riser is then displaced to (or replaced with) water, and the water can in turn be discharged to the sea before pulling the riser to the rig.

It should be noted that these numerical examples are only provided by way of illustration, and that other diameter risers may also be used.

An advantage of the inner riser is that the annulus around the drill string is reduced in volume, enabling a larger operational range because the overall weight of the fluid column within the inner riser is smaller when compared to a fluid column contained within the outer riser. The energy required to affect movement of the column is smaller for a smaller fluid column, even if the length is not reduced.

Besides improving the operational envelope, the inner riser can be used to reduce the risk or impact of failures. The annulus between the outer riser and the inner rise may be filled with a static column of water, in particular seawater, or other suitable buffer fluids. A leak in the wall of the outer riser would therefore only cause a leak of the buffer fluid into the environment. A leak in the wall of the inner riser would cause drilling fluid to leak into the annulus between the inner riser and the outer riser, but not into the environment. If there is a leak both in the inner riser and the outer riser, the drilling fluid would still leak into the environment, but the risk of that scenario occurring is smaller when compared to an arrangement with a single drilling riser.

The outer riser and inner riser terminate at the BOP. The flow of fluids through the first annulus between the drill string and the inner riser, and through the second annulus between the inner riser and the outer riser, is controlled at the BOP and at the top. In one embodiment, fluid flows through the bore of the drill string into the well, and selectively flows back through the first annulus or the second annulus, or both. The selective control enables displacement of the fluids contained within one or both of the annuli, for example if the riser assembly needs to be recovered to the surface after the drilling operation has been completed and the annuli need to be flushed to prevent any pollution of the surrounding seawater. The selective control can also be used for emergency relief if one of the annuli is blocked by debris.

The key steps discussed above are also illustrated in Fig.3, with steps S1 of providing an outer riser, S2 arranging an inner riser within the outer riser, and S3, arranging a drill string within the inner riser.

Optionally, spacers can be arranged between the inner riser and the outer riser to maintain a fixed separation between the inner riser and the outer riser. However, given that the inner riser and outer riser do not rotate with respect to each other, the spacers may also be omitted. If the annulus between the inner riser and the outer riser is filled with a buffer liquid, such as seawater, it may not be a problem if the inner riser is not co-axial with the outer riser, or even if the inner riser rests against the outer riser for part of the way. If, on the other hand, the annulus between the inner riser and the outer riser is used for fluid flow, it is preferable to have a regular and symmetric annulus, especially if it is used for drilling fluids with possible debris that may get stuck at narrow sections.

The spacers may be resiliently deformable elements such as bow spring spacers, provided symmetrically at three or more positions around the outer circumference of the inner riser. A set of spacers may be repeated at one or more longitudinal positions of the riser assembly. Fig. 4 illustrates schematically bow string spacers 41 arranged between bands 42 and around a riser 43.

Although the invention has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.

Claims

CLAIMS: 1. A deep water drilling riser assembly, the assembly comprising: an outer riser; an inner riser arranged within the outer riser; wherein the inner riser is suitable for receiving a drill string; wherein the outer riser and the inner riser are arranged co-axially with respect to each other, and, when the drill string is received within the inner riser, the drill string and the inner riser being arranged co-axially with respect to each other.
2. The assembly of claim 1, wherein the annulus between the inner riser and the outer rise is filled with a buffer fluid, in particular water or seawater.
3. The assembly of claim 1 or 2, wherein the inner diameter of the outer riser is 48cm.
4. The assembly of any one of the preceding claims, wherein the inner diameter of the inner riser is 31cm.
5. The assembly of any one of the preceding claims, wherein the inner diameter of the drill string is 10cm.
6. The arrangement of any one of the preceding claims, wherein the length of the drilling riser assembly is substantially 2km.
7. The arrangement of any one of the preceding claims, further comprising one or more spacers arranged between the inner riser and the outer riser.
8. The arrangement of claim 7, wherein the one or more spaces comprise bow springs.
9. A method of arranging a drilling assembly, the method comprising: providing an outer riser, arranging an inner riser within the outer riser, arranging a drill string within the inner riser, wherein the outer riser, the inner riser, and the drill string are arranged co-axially with respect to each other.
10. The method of claim 9, further comprising filling the annulus between the inner riser and the outer rise with a buffer fluid, in particular seawater.
11. The method of claim 9, further comprising filling the annulus between the inner riser and the outer rise with seawater.
12. The method of claim 9, further comprising pumping drilling fluid into the drill string downhole, and back to the surface through the annulus between the drill string and the inner riser.
13. The method of claim 9, further comprising pumping drilling fluid into the drill string downhole, and back to the surface by selecting the annulus between the drill string and the inner riser, and/or the annulus between the inner riser and the outer riser.