Technical Field

The present invention relates to a method for performing subsea well interventions, particularly in deep water.

Background

Performing subsea well intervention with a riser is known and involves connecting a riser to the Christmas Tree (XT) at the seabed which allows access to the well. The riser runs from the well to the vessel or rig on the surface of the water and is pressurised with the same pressure that is experienced by the well (up to 15000 psi). A subsea lubricator and a pressure control head (PCH) cap the riser at the top of the riser, on the vessel and enable tools to be passed into the pressurised riser and transferred to the well. This method can be complex and costly, because the riser must be manufactured to withstand the extreme levels of pressure that are in the well, as well as tension variations along its length underwater, which makes it very heavy. Additionally, there is the risk of having a highly pressurised pipeline so close to a manned vessel. Finally, the assembly of the riser must be done piece by piece as it is lowered to the subsea well which is very time consuming.

To combat the risk and cost of riser well interventions, riserless interventions were developed in which a well control package (WCP) and subsea lubricator, with a PCH, are installed on top of the XT and a wireline is used to pass tools, such as a bottom hole assembly (BHA), into the well. This method is well established and used in shallow water (e.g. 300 - 500m depth), enabling the omission of a riser, which reduces the cost, time, and risk of performing subsea well interventions. Sometimes it is desirable for the wireline (WL) to deliver a precise downward force through the wireline tool, in a procedure called ‘’jarring”. Such a force is delivered by raising the wireline tool by a predetermined amount, and letting it fall back down to impact the desired target (jarring). While there are no or less issues in shallow water, issues arise when performing riserless well interventions in deep water (e.g. >2000m depth). As shown in Figure 1 , current in the water can cause wireline deflections in the wireline, meaning that raising the top of the wireline by a predetermined amount will not necessarily raise the BHA by the same amount. As a result, it is difficult to determine the height to which the BHA is raised, as well as the jarring force that the Wire Line operator will apply to the Wire Line tool, and the jarring procedure may not have the proper response.

Summary

The invention provides a method of performing subsea well interventions, and a pressure control head, in accordance with the accompanying claims.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings.

Brief Description of the Drawings

Figure 1 shows wireline deflections caused by underwater currents during a riserless well intervention;

Figure 2 shows a method of installing a well control package and a subsea lubricator onto a vertical Christmas tree using a riser in an embodiment of the invention;

Figure 3A shows the lowering of a bottom hole assembly and a pressure control head through the riser of Figure 2;

Figure 3B shows the passing of the bottom hole assembly into the well, and the pressure control head on top of the subsea lubricator; Figure 4 shows an enlarged view of the top of the subsea lubricator when the bottom hole assembly and the pressure control head land on the top of the lubricator; and

Figure 5 shows the riser with a thruster and a buoyancy aid, the riser also being tethered to the seafloor with anchors.

Description of preferred embodiments

As shown by Figure 2, a riser 2 may be used to run a subsea lubricator 4 and a well control package (WCP) 6 from a vessel 8 on the surface of the water to a vertical Christmas tree (VXT) 10 that is installed on top of a subsea well 12. The riser 2 is connected to the vessel 8 and may be a drill pipe riser, but could also be a marine riser or the like as long as it has a sufficient inner diameter (ID) as discussed later. The vessel 8 may be an oil rig, or a drill ship or any other suitable vessel. It is not necessary that the WCP 6 and the subsea lubricator 4 be installed using the riser 2, and many other methods may be employed. Once installation of the subsea lubricator 4 and the WCP 6 is complete, the riser is positioned at a predetermined distance from the subsea lubricator 4. The distance the riser 2 is positioned from the subsea lubricator 4 may vary depending on various conditions such as weather conditions, underwater currents, and the type of intervention to be performed, but is typically between 50m and 1000m. The distance could be shorter than 50m, however, if the distance is too small there is a risk of the riser 2 hitting the subsea lubricator 4, the WCP 6, or the VXT 10 due to the vertical motion of the vessel 8 to which the riser 2 is connected while it floats on the surface of the water. Therefore the distance should be large enough to avoid this risk.

After the riser 2 has been positioned away from the subsea lubricator 4, a bottom hole assembly (BHA) 14 and a pressure control head (PCH) 16 are then lowered on a wire to the subsea lubricator 4 through the riser 2 as shown in Figure 3A. The lubricator 4 has valves 17 which are pumped with grease to enable a pressure-tight seal between the PCH 16 and the Wire Line (WL). The ID of the riser 2 must, therefore, be wide enough to accommodate both the PCH 16 and the BHA 14. Alternatively, the outer diameter (OD) of the PCH 16 and the BHA 14 must be less than the ID of the riser 2. For example, when the riser 2 is a drill pipe riser, a PCH 16 with an OD less than the ID of the drill riser 2 (e.g. ID < 5”) may be used. As shown in Figure 3B, the BHA 14 enters the subsea lubricator 4 and the PCH 16 sits on top of the subsea lubricator 4 to enable the subsea lubricator 4 to pressurise and convey the BHA 14 into the well 12 as will be discussed below. When the PCH 16 has an OD that is less than the ID of the riser, there may arise a situation in which the OD of the PCH 16 is less than the ID of the subsea lubricator 4. In such a scenario, rather than the PCH 16 sitting on top of the subsea lubricator 4 acting as a cap, it may instead sit within a receiving end 24 of the subsea lubricator 4 and act as a plug as shown in steps 2 and 3 of Figure 4 and described below.

As shown in steps 1 , 2 and 3 of Figure 4, the BHA 14 and the PCH 16 are lowered through the riser 2 toward the subsea lubricator 4 using a wireline 18. It is preferable that the wireline 18 is preinstalled through the PCH 16 and holding the BHA 14 before they are fed through the riser 2. The subsea lubricator 4 has a storage compartment 20 and a hydraulic operated latch 22. The storage compartment 20 is vertically below the receiving end 24 of the subsea lubricator 4, and the hydraulic operated latch 22 is located in the receiving end 24 of the subsea lubricator 4. This is a typical latch that will "stay as is” if hydraulic latch pressure is lost since it is critical that this latch does not fail. The ID of the storage compartment 20 is more than the ID of the receiving end 24. The OD of the BHA 14 is less than the ID of the receiving end 24 of the subsea lubricator 4 such that when the BHA 14 is lowered, it passes through the receiving end 24 of the subsea lubricator 4 and into the storage compartment 20 where it is stored. The PCH 16 is then lowered by the wireline 18, and while step 1 of Figure 4 shows a vertical space between the BHA 14 and the PCH 16, this is not necessary. The PCH 16 stops being lowered when it is in the receiving end 24 on top of the storage compartment 20 and aligned with the hydraulic operated latch 22. The latch 22 then locks the PCH 16 in place and pumps grease into the valves 17 of the receiving end 24 to enable it to withstand the pressure of the well 12 and seal the PCH 16 around the wireline 18. After the PCH 16 is secured, the subsea lubricator 4 opens valves on the end of the subsea lubricator 4 that is connected to the well 12 and once the pressure in the storage compartment 20 has equalised with the pressure of the well 12, the BHA 14 is lowered into the well 12.

When the PCH 16 and the BHA 14 are in place, the wireline 18 connecting them to the vessel 8 remains surrounded by the riser 2, except for the portion of the wireline 18 that is between the riser 2 and the subsea lubricator 4. Because most of the wireline 18 is surrounded by the riser 2, most of the underwater currents are prevented from affecting the wireline 18. Indeed, it will only be the portion of the wireline 18 between the bottom of the riser 2 and the top of the subsea lubricator 4 that could be affected by currents, and deep-water currents are not as fast as those closer to the surface. Therefore, when a precise jarring force is desired, the riser 2 ensures that there are minimal wireline deflections affecting the vertical height that the BHA 14 is raised.

The disclosed method is advantageous for several other reasons. As the riser 2 is not sealed to the well 12, it is not pressurised, and so does not need to be reinforced to withstand the pressure of the well 12. Therefore, manufacturing costs are reduced. The riser 2 will also be lighter in weight, and so can be used on a larger variety of vessels 4 and deployed in a shorter time. Since the PCH 16 and the subsea lubricator 4 are disposed on the VXT 10, instead of on the vessel 8, the vessel 8 is in less danger should either of these two components malfunction, as the riser 2 connecting to the vessel is not pressurised.

As the underwater currents will mostly act upon the riser 2 instead of the wireline 18, it is likely that the bottom of the riser 2 will move out of alignment vertically above VXT 10, however, there are many methods to prevent such motion or account for it. For example, the bottom of the riser 2 may be connected to the sea floor with anchors 26 to reduce the motion of the riser 2 caused by underwater currents. These anchors 26 can be fixed or flexible, and may for example be cables or chains. Alternatively, or additionally, a transponder 28 or similar device may be used to send the position of the bottom of the riser 2 relative to the VXT 10, and when the position is known to be out of alignment, the vessel 8 to which the riser 2 is attached can move the required amount to compensate for the motion of the bottom of the riser 2 away from the VXT 10. Another method to prevent the riser 2 from moving out of alignment with the VXT 10 is to increase the weight at the bottom of the riser 2 which would then require more force to cause the riser 2 not to stay substantially vertical in the length direction. Other methods of keeping the riser 2 substantially vertical in the length direction include using one or more underwater thrusters 30 along its length to allow the position and the angle of the riser 2 to be corrected. The thrusters 30 can be used to adjust the position of the bottom of the riser 2 in relation to the well 12, thereby enabling alignment of the riser 2 with the well 12 to be maintained. These thrusters 30 could be operated manually or automatically, and can be used in tandem with the transponder 28 that detects the position of the bottom of the riser 2 in relation to the well 12. The transponder 28 may also detect the angle of the longitudinal axis of the riser 2 to the vertical and operate the one or more thrusters 30 to minimise said angle. Preferably, the longitudinal axis of the riser is within 20 degrees of vertical and more preferably within 10 degrees of vertical. The advantage of the riser 2 being substantially vertical in the length direction is that the PCH 16 and tools such as the BHA 14 can be lowered through the riser 2 without the need for specialised equipment to push them along the riser 2. In other words, the vertically of the riser 2 means that the PCH 16 and tools can be lowered through the riser 2 by their own weight and without the need to apply any additional downward forces on the PCH 16 or tools. As a result, specialised equipment for pushing tools through risers are not required and simpler vessels without this equipment may be used. Additionally, the riser 2 may have one or more buoyancy aids 32 along its length. These buoyancy aids 32 would aid in supporting the weight of the riser 2 underwater resulting in the vessel to which the riser 2 is connected supporting less of the weight of the riser 2. An example of buoyancy aids 32 that could be used are buoys. As a result of the buoyancy aids 32, smaller vessels can be used for risers 2 that would normally be too heavy to be supported by the vessel. Additionally, if the buoyancy aid(s) provide sufficient buoyancy, once a riser 2 is deployed and positioned as described above, it can be left unattended, i.e. “suspended and parked” by the initial vessel 8 used to deploy the riser. The buoyancy aid(s) 32 can result in the riser 2 being neutrally buoyant in the water and prevent the riser 2 from sinking or rising, and if necessary, anchors 26 can also be used to prevent the riser 2 from drifting. As a result, after the initial vessel 8 leaves the riser 2 suspended and parked, a second vessel 8 can then be used for the well intervention. As the second vessel 8 does not need to deploy the riser 2, it can be lighter and does not require as much specialised equipment, allowing a wider range of vessels to perform the well interventions of the present method.

Figure 5 depicts a riser 2 with the additional optional features described above, in particular, the one or more buoyancy aids 32, the one or more thrusters 30, the transponder 28, and the anchors 26 are shown. These features are beneficial but not essential for the method of performing a subsea well intervention described above.

In scenarios where the OD of the PCH 16 is too big to go through the riser 2, the wireline 18 may be installed through the PCH 16 once it has been attached to the subsea lubricator 4. Alternatively, the PCH 16 and installed wireline 18 could be attached to the subsea lubricator 4 such that the other end of the wireline 18 is also on the sea floor, and then the wireline 18 could be pulled back through the riser 2. A further advantage of the described embodiments is that no modification is required for any of the equipment described above. The method can use any type of riser already in use for deep water interventions as well as less reinforced risers as described above. Similarly, the wireline 18 that is used in the present method does not need to be modified from known openwater wirelines already in use. The same applies for any other mentioned piece of equipment such as the PCH 16, the subsea lubricator 4, the VXT 10, the BHA 14, the WCP 6 and even the vessel 8. Additionally, the method of deploying the riser 2 can be any suitable method that is known, for example a piece by piece assembly of the riser 2 as it is lowered into the water. Such known methods of riser deployment are compatible with the well intervention method described above by omitting the step of connecting the riser 2 to the well 12 and, therefore, not pressurising the riser 2. As a result, standard vessels 8 capable of riser 2 deployment for deep water interventions can be used for the method of the present disclosure. Finally, because the riser 2 is not attached to the subsea lubricator 4 which is connected to the WCP 6 and the VXT 10, there is less risk of damaging the subsea well if the vessel 8 drifts too far or rolls due to waves.

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.