WO2025191243A1

WO2025191243A1 - Alert system for a subsea infrastructure

Info

Publication number: WO2025191243A1
Application number: PCT/GB2025/050475
Authority: WO
Inventors: Andrew Jaffrey; Neil Gordon; Robert Thompson
Original assignee: Sentinel Subsea Ltd
Current assignee: Sentinel Subsea Ltd
Priority date: 2024-03-11
Filing date: 2025-03-10
Publication date: 2025-09-18
Anticipated expiration: 2026-09-11
Also published as: GB2639206A; GB202403465D0

Abstract

An alert system for use with a subsea infrastructure is presented. The alert system 400 includes a pressure detection apparatus 410 connectable to the subsea infrastructure 405; a signalling device; and a release mechanism 420 operable between a locked state in which the signalling device is attached to the alert system, and an unlocked state in which the signalling device is released from the alert system. The alert system 400 releases the signalling device upon detection of a pressure variation at a point of interest on the subsea infrastructure above a first threshold value when the pressure variation is due to an increase in pressure, or below a second threshold value when the pressure variation is due to a decrease in pressure. Also presented is a method of monitoring a subsea infrastructure using the alert system.

Description

ALERT SYSTEM FOR A SUBSEA INFRASTRUCTURE

Technical Field

The present disclosure relates to an alert system for use with a subsea infrastructure, such as a subsea well. The present disclosure also relates to a method of monitoring a subsea infrastructure using the alert system.

Background

At the end of the productive life of a subsea oil or gas well, there is a requirement to make the well safe and to remove production-related infrastructure from the seabed. Several techniques exist and are under development for the sequence of activities generally referred to as decommissioning a well, including Plugging and Abandonment (P&A). These activities might include creating multiple barriers (plugs) between the subsurface reservoir (which might be a considerable distance below the seabed) and the wellhead location on the seabed through which previous drilling and production activities have been performed.

However, although multiple barriers are created during decommissioning between a reservoir and the external environment, there are multiple potential leak paths that can result from processes including corrosion of the metalwork such as the casing and/or deformation caused by geological movement. Examples of potential leak paths include micro-annuli at the interface between casing and cement caused by channels and the presence of materials such as wax, scale, oil and dirt; connected pores, cracks and channels caused by permeability in cement; and through perforation of casing caused by corrosion and mechanical deformation. Such leak paths may occur at any depth within the well arrangement and give rise to the propagation or migration of hydrocarbons through the sub-surface geology. Upon completion of well decommissioning activities, it may therefore be desirable to monitor the environment around a well site for an extended period to provide reassurance that a well's integrity is secure and that hydrocarbons from the reservoir are not leaking into the subsea environment. Several conventional systems are currently available to address this requirement, using approaches including active acoustics, bio sensors, capacitance, fibre optics, fluorescence, optical sniffers, optical cameras and passive acoustics, but these have various limitations associated with them. These limitations may include complex deployment requirements, high cost, interference with fishing activities, the need for regular maintenance or intervention, e.g. changing batteries used in active electronic systems, and data recovery requirements e.g. communications infrastructure, which might be part of a fixed installation or mobile in nature.

A further limitation of existing systems is a general inability to distinguish between hydrocarbons (such as biogenic or thermogenic methane) that are released through natural means from those emerging from a reservoir of a decommissioned well that has been isolated from the external environment. The difficulty in distinguishing between hydrocarbons may lead to false alarms being generated if a leak is notified in error.

Early notification of an increase in bore or annulus pressure is important as it gives the responsible party more time to consider how to deal with the situation. If this includes hiring a rig for intervention work, delay can result in significant financial increases.

Alternative means of monitoring, such as Pressure and Temperature (PT) gauges, are available, but have many limitations. If they are not already installed in the tree or well then expensive intervention is required to install them. If the tree is partially decommissioned (without power or data communication lines) then operating PT gauges is much more difficult and often relies on irregular acoustic communication with suitably equipped surface vessels to recover data. Such data are clearly not obtained in a timely manner, meaning that the advantages of early notification are lost. Ongoing management of battery packs is another issue that adds considerable expense to this form of monitoring.

Patent document GB2573661 describes a passive detection system for monitoring the integrity of abandoned, suspended, and/or decommissioned subsea wells. The system utilises a passive detector that reacts to the presence of a predetermined chemical to release a buoyant beacon. In such a system, a well fluid should be allowed to escape as part of the monitoring scheme. Figure 1A is a schematic view of an example drilling system including a sectional sub-surface representation as shown in GB2573661. Figure IB is a diagram of a schematic cross-sectional representation of example sub-surface post-decommissioning leak paths associated with a loss of well integrity as shown in GB2573661.

It is an object of the disclosure to address one or more of the above mentioned limitations, including , but not limited to, the need to contain all fluids to avoid any uncontrolled discharge.

Summary

According to a first aspect of the disclosure there is provided an alert system for use with a subsea infrastructure, the system comprising a pressure detection apparatus connectable to the subsea infrastructure; a signalling device; and a release mechanism operable between a locked state in which the signalling device is attached to the alert system, and an unlocked state in which the signalling device is released from the alert system; the alert system being configured to release the signalling device upon detection of a pressure variation at a point of interest on the subsea infrastructure above a first threshold value when the pressure variation is due to an increase in pressure, or below a second threshold value when the pressure variation is due to a decrease in pressure.

For instance, the subsea infrastructure may be a subsea well or a carbon capture and storage (CCS) infrastructure.

Optionally, the pressure variation is a positive pressure difference between a pressure at the point of interest on the subsea infrastructure and a hydrostatic pressure at a seabed where the subsea infrastructure is located.

For instance, the point of interest may be a production wing valve PWV or the annulus wing valve AWV of a subsea tree.

Optionally, the pressure variation is a negative pressure difference associated with a drop in pressure of an energy storage device of the subsea infrastructure.

For instance, the energy storage device may be a pressure based storage device. The energy storage device may be designed to power a safety mechanism such as the safety mechanism of a blowout preventer.

Optionally, the pressure detection apparatus comprises a hydraulic arrangement having an input port to connect the alert system to the point of interest on the subsea infrastructure, and an output port coupled to the release mechanism.

For instance, the input port may comprise a coupling device for coupling the hydraulic arrangement to the point of interest. For example the coupling device may be quick-release coupling device such as a Hot stab and receptacle. Optionally, the input port is coupled to the output port via a control channel comprising a control valve to control the release mechanism.

For instance, the control valve may comprise a pressure relief valve.

Optionally, the pressure variation is due to an increase in pressure, and the control valve is normally closed and is configured to open when a pressure at its input increases to become equal to, or greater than, the first threshold value.

Optionally, the pressure variation is due to a decrease in pressure, and the control valve is normally closed and is configured to open when a pressure at its output decreases to become equal to, or less than, the second threshold value.

Optionally, the control channel comprises a first safety valve provided between the input port and the control valve.

Optionally, the control channel comprises a second safety valve provided between the control valve and the release mechanism.

Optionally, the control channel comprises a third safety valve provided between the second safety valve and the release mechanism.

Optionally, the alert system comprises an accumulator coupled to the control channel.

Optionally, the alert system comprises a pressure compensator coupled to the release mechanism. For instance a channel may connect control channel to the pressure compensator which may be used to relieve pressure from the release mechanism to the pressure compensator .

Optionally, the alert system comprises a plurality of control channels and a plurality of release mechanisms; wherein each control channel extends between the input port of the first control channel and a channel-specific output port.

For instance, each release mechanism may be coupled to a beacon, wherein each beacon is individually coded and assigned to a known pressure set point.

Optionally, each control channel comprises a control valve configured to change state at a different threshold value.

Optionally, the release mechanism comprises a hydraulic cylinder operable between the locked state in which a rod is extended to lock the signalling device and the unlocked state in which the rod is retracted to release the signalling device.

Optionally, the signalling device comprises at least one beacon configured to transmit an alert signal, wherein the beacon comprises internal or external buoyancy.

Optionally, wherein the signalling device is configured to communicate with a satellite network; wherein the satellite network is configured to relay the alert signal to a party monitoring the subsea infrastructure.

Optionally, wherein the alert system is mounted on a frame.

For instance the frame may be a skid. According to a second aspect of the disclosure, there is provided a method of monitoring a subsea infrastructure, the method comprising providing an alert system as claimed in any one of the preceding claims; coupling the alert system to the subsea infrastructure; and releasing the signalling device upon detection of a pressure variation at a point of interest on the subsea infrastructure above a first threshold value when the pressure variation is due to an increase in pressure, or below a second threshold value when the pressure variation is due to a decrease in pressure.

Optionally, the method comprises pre-charging the alert system prior to coupling it to the subsea infrastructure.

Description of the drawings

The disclosure is described in further detail below by way of example and with reference to the accompanying drawings, in which: figure 1A is a diagram of a drilling system for subsea; figure IB is a diagram of a decommissioned well showing leak paths; figure 2 is a diagram of a conventional vertical subsea tree; figure 3 is a flow chart of a method for monitoring a state of a subsea well according to the disclosure; figure 4 is a diagram of an alert system according to the disclosure; figure 5 is a diagram of another alert system provided with a plurality of beacons; figure 6 is a diagram of yet another alert system; figure 7A is a see through perspective view of a beacon assembly, including a release mechanism for releasing a beacon; figure 7B is a sectional view of the beacon assembly of figure 7A; figure 8A is a perspective view of a skid including two alert systems; figure 8B is a front view of the skid of figure 8A; figure 8C is a left side view of the skid of figure 8A; figure 8D is a back view of the skid of figure 8A; figure 8E is a right side view of the skid of figure 8A; figure 8F is a top side view of the skid of figure 8A.

Description

Figure 2 is a simplified diagram of the key features of a conventional vertical subsea tree. Figure 2 is a vertical subsea tree as shown in Khalifeh, M., Saasen, A. (2020). General Principles of Well Barriers. In: Introduction to Permanent Plug and Abandonment of Wells. Ocean Engineering &

Oceanography, vol 12. Springer, Cham. https://doi.org/10.1007/978-3-03Q-

39970-2 2 This chapter is licensed under the terms of the Creative

Commons Attribution 4.0 International License

(http://creativecommons.Org/licenses/by/4.0/). Credit to original author Bellarby, Jonathan. Well completion design. Elsevier, 2009. Changes have been made to add references to the main valves.

The subsea tree 200 is attached to the subsea wellhead 205 and provides access for well intervention operations and performing well-control and flow-control functions. It provides connection to the annulus of the well (that is any void between any piping immediately surrounding the well) via annulus conduit 220; and to the well borehole via borehole conduits 230.

The tree is used to direct flow through a series of valves. The annulus conduit 220 includes the annulus swab valve 221, the annulus wing valve 222, the annulus master valve 223. Similarly the borehole conduit 230 includes the production swab valve 231, the production upper master valve 232, the production lower master valve 233, and the production wing valve 234. A high pressure cap 210 is provided on top of the tree. At various times in the lifecycle of a well plugs might be set to isolate vertical sections of the bore (and in the case of full abandonment, also the annuli). There are many materials and techniques used for setting plugs, including creating cement barriers that might be many tens of meters long. Setting plugs might be performed during shut-in, suspension and decommissioning work, but the integrity of such plugs is of prime importance to prevent the flow of fluids between isolated sections or to the environment. Plugs are illustrated in the simple schematic presented in Figure IB where items 80, 95 and 110 are packers which provide a bridge on which the plugs are set and 85, 100 and 115 are the plugs.

The Plugging and Abandonment (P&A) of oil and gas wells is very expensive, and operators are increasingly looking for new techniques and tools that reduce the time and cost associated with the practice. Testing new techniques to verify that they are successful is necessary to ensure all stakeholders (including shareholders and regulators) and the general public, that this is the case. The system and method of the disclosure is advantageous as it can be implemented at a low cost, and proposes a low risk means of monitoring new plugging techniques and materials.

Failure of a plug results in the flow of well fluids and an increase in pressure in one, other, or both of the production bore or the Annulus; these being accessible for fluidic connection via the production wing valve (PWV) 234 and the annulus wing valve (AWV) 222, respectively.

It is important that the party responsible for the well is alerted at the earliest opportunity of the failure of the plug(s) and the attendant increase in pressure.

For a well that is partially decommissioned, the various control systems, umbilicals, power supplies and other equipment normally associated with a subsea tree are disconnected and/or inactive. This means that the indicators of a pressure increase that might otherwise have been used on a day-to-day operational basis are unavailable.

Figure 3 is a flow chart of a method for monitoring a subsea infrastructure according to the disclosure.

At step 310 an alert system is provided. The alert system includes a pressure detection apparatus connectable to the subsea infrastructure; a signalling device; and a release mechanism operable between a locked state in which the signalling device is attached to the alert system, and an unlocked state in which the signalling device is released from the alert system. The alert system is configured to release the signalling device upon detection of a pressure variation at a point of interest on the subsea infrastructure above a first threshold value when the pressure variation is due to an increase in pressure, or below a second threshold value when the pressure variation is due to a decrease in pressure .

At step 320, the alert system is coupled to the subsea infrastructure.

At step 330 the signalling device is released upon detection of the pressure variation above the first threshold value or below the second threshold value.

The first threshold value and the second threshold value are pre-determined values which may be set by a user of the system.

The method may further include the step of pre-charging the alert system prior to coupling it to the subsea infrastructure. This may be achieved by filling a network of channels/conduits of the alert system with a liquid such as mineral oil or water-based control fluid. The signalling device may be configured to communicate with a satellite network. The satellite network can then relay the alert signal to a party monitoring the subsea infrastructure.

For instance the subsea infrastructure may be a subsea well such as an oil and gas well, however the method may be used with other infrastructures where electrical power and data communications are not readily available. For instance, the alert system and method may be used for monitoring changes in pipe-in-pipe annuli; detecting pressure changes in the lower stack of a blowout preventer (BOP) when the power, control and communications in the lower marine riser package (LMRP) are disconnected; and advising of an unacceptable rise in pressure where a pressure-retaining debris cap is fitted to a wellhead or mandrel.

In another example, the subsea infrastructure may be a carbon capture and storage (CCS) infrastructure, such as a repurposed well or reservoir for CCS. Recharging the reservoir by pumping in CO2 could lead to a pressure increase at the monitoring cap if the bore is not properly plugged. The proposed alert system could be used to monitor such a CCS infrastructure.

Figure 4 is a schematic diagram of one possible example of an alert system according to the disclosure. The alert system 400 includes a pressure detection apparatus 410 coupled to a release mechanism 420 for releasing a signalling device (not shown). The pressure detection apparatus 410 is configured to control the release mechanism 420 in response to a variation in pressure (in this case an increase in pressure) beyond a specified threshold value, detected at a point of interest such as the PWV or AWV of the tree.

The release mechanism 420 is operable between a locked state in which the signalling device is attached to the alert system, and an unlocked state in which the signalling device is released from the alert system. In this example the release mechanism 420 is provided by a hydraulic cylinder Cl for releasing an alert beacon (not shown). The control channel 411 is coupled to the front chamber 422 of the cylinder Cl, so that an increase in pressure in the chamber 422 pushes on the cylinder’s piston and its attached rod 421. The alert beacon is designed to have buoyancy and may also be referred to as buoyant beacon.

The pressure detection apparatus 410 includes a hydraulic arrangement having an input port provided with an input valve labelled V-03 adapted to connect the hydraulic arrangement to a point of interest . In this example the subsea infrastructure includes a subsea tree 405 having a production wing valve (PWV) and annulus wing valve (AWV). The valve V-01 represents the PWV valve or the AWV valve. The valve V-01 is connected to the input valve V-03 via one half of a quick-release coupling labelled V-02.

The valves V-02 and V-03 may be implemented as so-called receptacle and hot stab. Hot Stabs and Receptacles are used to power hydraulic tools, transfer fluid, perform chemical injections, and to monitor pressure. There are two American Petroleum Institute (API) specifications that cover the design and manufacture of ROV Hot Stabs and Receptacles: API 17H (also referred to as ISO 13628-8) and API 17D.

A control channel 411, also referred to as control conduit, couples the input valve V-03 to the release mechanism 420 via a control valve V-04 for controlling the release mechanism 420.

The control valve V-04 is implemented as a pressure relief valve. The valve V-04 is normally closed (NC) and is configured to open when the pressure P4 received at its input increases to become equal to, or greater than, a predetermined threshold value which may be set by the operator. Here the threshold value is set to the differential pressure API. The direction of flow when the valve V-04 opens is from left to right as indicated by the arrow of the pressure relief valve symbol. So the input of V-04 is on the left side at node N1 with pressure P4, and the output of V-04 is on the right side before V-05 with pressure P5. Therefore the control valve V-04 has its input directed towards the input port of the control channel 411, and its output directed towards the output port of 411.

The control channel 411 may also include several safety valves. In this example three safety valves are provided: a first safety valve labelled SV1, a second safety valve V-05 and, a third safety valve SV2.

The safety valves are provided to protect individual components from over pressure or to protect the monitoring system 400 from excessive pressure that might rupture a part, tube of fitting, and cause a discharge of well fluids to sea.

The first safety valve SV1, also referred to as main safety valve, is provided at the input port after the connection valve V-03 to protect the system downstream and prevent a major leak coming out of the well. The valve SV1 may be implemented as a pressure-controlled shut-off valve.

The second safety valve V-05, may be implemented as a pressure relief valve. The valve V-05 is normally open (NO) and is configured to close when the pressure P6 at its output increases to become equal to or greater than a predetermined threshold value which may be set to correspond to a limit pressure above which the hydraulic cylinder Cl may be damaged.

The third safety valve SV2, is provided between V-05 and Cl to further protect the system downstream. The valve SV2 may be implemented as a pressure-controlled shut-off valve. The control channel 411 is coupled to an accumulator Al via channel 412 between nodes N1 and N4 and channel 413 between nodes N2 and N4. The channel 412 includes valve V-09 and the channel 413 includes valve V-10. A valve V-ll is provided between N3 and N4, and valve V-12 is provided at node N3.

The valve V-05 is coupled to a pressure compensator PCI via channel 414 that includes automatic shut-off valve SV3. The pressure compensator PCI is coupled to another valve V-08. The circuit formed by channel 414 may be used to relieve pressure from the cylinder Cl to the pressure compensator PCI.

The hydraulic cylinder Cl is also coupled to the accumulator Al, also referred to as hydraulic accumulator, via channel 415. The control channel 411 may be made of different sections. For instance the section between the safety valve SV1 and the input port may be provided by a flexible conduit or flexible hose. The remining section between SV1 and Cl may be provided by a solid conduit.

All hydraulic components are selected according to the pressure rating of the subsea tree to ensure that the maximum pressure is in line with the well control equipment.

The hydraulic arrangement is initially pre-charged, meaning that the network of channels/conduits forming the hydraulic arrangement is filled with a liquid such as mineral oil or water-based control fluid. The pre-charge step is performed using the charge and bleed port CBP1. For instance, the port may be provided with a spring loaded non return (NR) valve.

Once pre-charged the pressures along the control channel 411 are all equal so that P3 =P4 =P5 =P6 = P23; and P23 is low enough such that Cl is in the locked position. The isolation valves V-01, V-06, V-09, V-12, V-10 and V-08 shown in black are normally closed. The isolation valve V-ll is normally open.

The valves V-02 , V-03 of the receptacle / hot stab are normally closed and only open when the two parts of the union are connected.

The control valve V-04 is normally closed (NC), while the safety valve V-05 is normally open (NO).

The valves V-01, V-09, V-10 and V-ll may be operated remotely subsea by an ROV. The valves V-06, V-12, V-08 are manually operated at the surface, i.e. not subsea.

Once the pressure detection apparatus 410 is connected to the tree 405, the valve V-01 (shown as closed) is then opened. The valves V-02 and V-03 are also opened.

In normal circumstances, that is when the well is in a stable state, the pressure Pl at the valve V-01 should be equal to the hydrostatic pressure at the seabed Pseabed. Therefore, there should be zero differential between Pl and Pseabed and the pressures should be in equilibrium.

When the well becomes unstable, the pressure Pl at the valve V-01 may increase. If Pl rises to reach or exceed the differential pressure API set by the operator, then the valve V-04 (initially closed) opens, allowing flow through V-05 (normally open) and SV2 to the driving side of the piston of the hydraulic cylinder Cl. The hydraulic pressure is communicated to one side of the piston in the cylinder. The force imbalance causes the piston and its attached rod 421 to move from the locked position to the unlocked position (piston moves towards the left against the resistance of the spring which by default holds the rod in the fully extended position). The rod 421 is withdrawn from an attachment point which forms part of the beacon assembly, thus allowing the buoyant beacon to rise toward the surface.

If the pressure Pl keeps increasing and P6 increases beyond the maximum threshold pressure set for the valve V-05, then the valve V-05 shuts down to protect Cl.

If the driving force associated with API is insufficient to move the piston of Cl, the hydraulic arrangement can be modified to include an additional accumulator (not shown) and appropriate control valves (not shown) to provide such additional force as is necessary to ensure the cylinder operates correctly.

The system 400 may be provided with multiple pressure gauges at various locations, allowing subsea and in-air visual inspections of the pressures in the system. For instance, pressure gauge G4 displays the pressure P4 at node Nl.

The system 400 is fully contained and in correct operation does not allow any fluid from the well or from the control circuitry to be vented subsea. Multiple safeguards may be included to allow management of pressure within the monitoring/alert system such that prior to and subsequently upon recovery to the surface all residual pressures can be bled-off in a safe and controlled manner.

The system 400 is designed to monitor a single connection point. Multiple systems can be deployed to monitor two or more points of interest. For instance, a first system may be deployed for monitoring the PWV valve and a second one for monitoring the AWV valve. The system 400 can be extended to include a plurality of release mechanisms each connected to a signalling device to provide an incremental response to pressure increases.

Figure 5 is a diagram of an alert system provided with a plurality of safety beacons. The alert/monitoring system 500 includes three hydraulic arrangements labelled 510a, 510b and 510c coupled to hydraulic cylinders 520a, 520b and 520c, respectively.

The hydraulic arrangement 510a is implemented in the same way as the hydraulic arrangement 410, and includes the channels 511, 512, 513, 514 and 515 corresponding to channels 411-415 in figure 4. In this case the channels 512 and 513 extend from nodes N1 and N2 respectively to connect the hydraulic arrangements 510a, 510b and 510c in parallel.

The channel 515 connects the accumulator Al to Cl via node N5, to C2 via node N5’, and to C3 via node N5”. As a result, the connection to the accumulator Al is shared between the three hydraulic arrangements.

The operation of the system 500 is similar to the operation of the system 400, however in this case additional beacons can be released incrementally as the pressure Pl keeps increasing beyond the normal value.

In this example, the control valves V-04 of the first control channel 511a, V-13 of the second control channel 511b, and V-15 of the third control channel 511c are set with incremental threshold values of differential pressure APla=50 psi, APlb =100 psi and APlc=150 psi, respectively. Of course, different thresholds may be chosen.

In this embodiment, a first beacon attached to Cl 520a is released in response to a first increase in the pressure Pl by 50 psi. If the pressure Pl keeps increasing, then a second beacon attached to C2 520b is released when APlb reaches 100 psi and a third beacon attached to C3 520c is released when APlc reaches 150 psi.

The benefits of this approach are that the responsible party not only knows that there is an integrity issue with the well from the release of the first beacon, but also the rate of change of pressure within the monitored space. Each beacon is individually coded and assigned to a known pressure set point. In this way a simple cross-reference to the beacon identity reveals the minimum detected pressure in that part of the monitoring system. The interval between the receipt of satellite signals from each beacon is used to estimate the rate at which the pressure Pl is increasing.

The provision of multiple pressure gauges gives an operator detailed insights into the current state of the well control equipment at the time of inspection I investigation, for example, after the release of one beacon but before the release of another. For instance, several pressure gauges can be provided to view the pressures P4, P13, P18 which can be viewed subsea by a remotely operated vehicle (ROV).

The system 500 could be extended further to include additional hydraulic arrangements and additional beacons.

It is anticipated that upon receipt of a signal from an alert beacon the responsible party will take steps to investigate the monitored tree. This will be in accordance with the party’s operating procedures and could include local or remote reading of pressure and temperature gauges installed in the equipment in order to better understand the circumstances. Fundamentally, however, notification means that one or more of the barrier plugs has been compromised and the responsible party needs to assess what kind of intervention is required. Figure 6 is a diagram of another alert system that can be used to alert a critical drop of pressure. An example application would be to signal a loss of pressure in a stored energy system such as an accumulator bank used to drive safety critical systems. If such systems are operating without remote power and data connections it is essential to know if there is a change that might compromise performance in an emergency.

The monitoring/alert system 600 includes a pressure detection apparatus 610 coupled to a release mechanism 620 for releasing a signalling device. The pressure detection apparatus 610 is configured to control the release mechanism 620 in response to a decrease in pressure.

The pressure detection apparatus 610 includes a hydraulic arrangement having an input port provided with an input valve labelled V-03 adapted to connect the hydraulic arrangement to a well structure. In this example the well structure is a blowout preventer (BOP) 605 having a valve V-01. The valve V-01 is connected to the input valve V-03 via another valve labelled V- 02.

A control channel 611 couples the input valve V-03 to the release mechanism 620 via an isolation valve labelled V-04 and a control valve labelled V-05. The valve V-05 is provided between nodes N1 and N2. The direction of flow when the valve V-05 opens is from the right to the left as indicated by the arrow of the pressure relief valve symbol. So the input of V-05 is on the right side at node N2 with pressure P5, and the output of V-05 is on the left side at node N1 with pressure P4. Therefore the control valve V-05 has its input directed towards the output port of the control channel 611, and its output directed towards the input port of 611.

The control channel 611 is coupled to the back chamber 622 of the cylinder Cl, so that a decrease in pressure in the chamber 622 reduces the force on the left side of the piston to the point that the spring force on the right of the piston is sufficient to drive the piston and rod 421 to the release position.

The nodes N1 and N2 are connected via channel 612 provided with the valves V-06, V-07 and V-08. The hydraulic cylinder Cl is coupled to a pressure compensator PCI via channel 613 that includes a non-return valve (NR1).

Like in the previous examples the hydraulic arrangement is initially pre-charged. The pre-charge step is performed using the charge and bleed ports CBP1 and CBP2. For instance, the ports may be provided with spring loaded NR valves.

Once pre-charged the pressures along the control channel 611 are all equal so that P3 =P4 =P5; and P5 is high enough such that Cl is in the locked position, hence retaining the beacon.

The valve V-04 is closed during deployment of the equipment subsea and opened after the connection is made to the pressure system being monitored 605. By deploying 610 with V-04 closed the pre-charge in 610 is better protected from a brief pressure transient as the union between V-02 and V-03 is made-up. The aim is to ensure that the small volume of fluid that might be displaced during the connection does not trigger the release of the beacon.

The control valve V-05, is normally closed and is configured to open when the pressure P4 at N1 (output of V-05) decreases to become equal to, or less than, a predetermined threshold value which may be set by the operator. Here the threshold value is set to the differential pressure API corresponding to a decrease in pressure. If Pl decreases to reach the differential pressure API set by the operator, then the pressure in the chamber 622 decreases, the rod moves left and releases the beacon.

In normal circumstances, the BOP 605 stores energy to drive critical safety systems in case of emergency. In this example access to this energy storage is controlled by the valve V-01. When the BOP is in a stable state, the pressure Pl at the valve V-01 is equal to a predetermined value. If the energy storage pressure at Pl starts decreasing, this drop in pressure is communicated along the control channel 611.

Figure 7A is a see-through perspective view of a simplified beacon assembly (many details are omitted for clarity). Figure 7B is a sectional view of the beacon assembly of figure 7A. The beacon assembly 700 includes a release mechanism 720 coupled to a buoyant beacon 730. The release mechanism 720 includes hydraulic cylinder 722 (which is item Cl / 420 in Figure 4) part of which is a retractable rod 721 which is movable between a first position in which the buoyant beacon 730 is held in place and a second position in which the buoyant beacon 730 is released. The buoyant beacon 730 has two parts: the beacon 731 and a buoyant jacket 732 allowing the beacon 731 to rise to the surface of the water upon release.

The alert systems as described above can be mounted on a skid for convenient transport and simple lowering to the seabed in close proximity to the tree to be monitored.

All components would normally be selected or designed for long-term submersion subsea (typically 25 years) but in the first instance the system described might only be needed for periods of one to five years before a decision is made regarding the efficacy of a new plugging technique and the monitoring withdrawn as part of the well decommissioning process. Figure 8A is a perspective view of an example skid including two, independent, alert systems. The skid 800 has a frame that forms three chambers 810, 820 and 830. The two monitoring systems are mounted inside the chamber 830.

The chamber 810 is provided with a flexible hose 812 connecting the hot stab

811 to the first alert system. Similarly, the chamber 820 is provided with a flexible hose 822 connecting the hot stab 821 to the second alert system.

Figure 8B is a front view of the skid of figure 8A. A control panel 811 is provided to control the first monitoring system. In this example the first monitoring system is directed at monitoring the AWV. The panel 811 shows the pressure gauges with pressures Pl to P6. Three controls are provided for operation of the valves V-09,V-10 and V-ll which may be operated by an ROV.

Figure 8C is a left side view of the skid of figure 8A.

Figure 8D is a back view of the skid of figure 8A. A control panel 821 is provided to control the second monitoring system. In this example the second monitoring system is directed at monitoring the PWV.

Figure 8E is a right side view of the skid of figure 8A. Figure 8F is a top side view of the skid of figure 8A.

Once landed on the seabed a remotely operated vehicle (ROV) may be used to follow normal industry procedures to connect a flexible hose, for instance

812 or 822 , or both 812 and 822, from the equipment skid to the tree. The interface is typically in accordance with API 17H although other configurations or standards may apply. Although the method described in this application discusses connections via wing valves, other points of access to a tree apply equally well, e.g. through a crown plug. The skid of figure 8 may be used to deploy one or more alert systems, and to connect the alert systems to the tree or energy storage equipment as required.

The skid is deployed onto the seabed prior to the operation/monitoring phase. When it has been decided that the alert system can be disconnected, subsea pressure relief operations are performed by ROV manipulation of control valves. The system is then recovered to the surface where final disarming and venting can be carried out in a safe and controlled manner.

It will be appreciated that additional features may be provided to the system. For instance, various sensors, such as pressure sensors, may be added to the hydraulic circuits (for both increasing and decreasing pressure) along with data loggers, power supplies and remote communication capability (such as through-water acoustic modem). By adding such equipment with appropriate control software, the equipment skid illustrated in figure 8 then includes the ability to record the pressure gradients before and after the release of the beacon and, on recovery to the surface or by remote interrogation (surface vessel, ROV or AUV) to download the recorded data for review and analysis. This also allows the operator to establish the current status of the equipment once in range but before getting an underwater vehicle to the monitoring site, i.e. pressure updates are possible in real-time even if the beacon has already been recovered by the surface vessel.

The alert system of the disclosure may also be used in combination with other alert systems. This may be achieved by connecting the release mechanism to an additional control channel. For instance, in figure 4, the hydraulic cylinder Cl may be connected to two control channels: the control channel 411, and another control channel from the detection system described in GB2573661. For instance, the detection system of GB2573661 may be augmented to trigger a hydraulic pressure upon the detection of a leak. This hydraulic pressure may be communicated via the additional control channel to Cl to release the beacon.

One means of implementing the alternative trigger signal is by the use of an accumulator to provide the necessary pressure, this flow being controlled by a valve which is held closed by an apparatus incorporating the passive (chemical detection and degradation) features described in GB2573661. Upon degradation of the chemical restraint by the fluid of interest the valve opens and allows flow from the dedicated accumulator to the additional control channel connected to 411.

As an example, in the situation where a WellSentinel™ Coral-type solution is appropriate, e.g. a cap is placed over a wellhead, then two means of releasing a single beacon are provided. Should either monitoring condition of increased pressure or detection of a fluid of interest be met, the beacon is released.

A skilled person will therefore appreciate that variations of the disclosed arrangements are possible without departing from the disclosure. Accordingly, the above description of the specific embodiments is made by way of example only and not for the purposes of limitation. It will be clear to the skilled person that minor modifications may be made without significant changes to the operation described.

Claims

1. An alert system for use with a subsea infrastructure, the system comprising a pressure detection apparatus connectable to the subsea infrastructure; a signalling device; and a release mechanism operable between a locked state in which the signalling device is attached to the alert system, and an unlocked state in which the signalling device is released from the alert system; the alert system being configured to: i] release the signalling device upon detection of a pressure variation at a point of interest on the subsea infrastructure above a first threshold value when the pressure variation is due to an increase in pressure; and/or: ii) release the signalling device upon detection of a pressure variation at a point of interest on the subsea infrastructure below a second threshold value when the pressure variation is due to a decrease in pressure; wherein: the pressure detection apparatus comprises a hydraulic arrangement having an input port to connect the alert system to the point of interest on the subsea infrastructure, and an output port coupled to the release mechanism; and the input port is coupled to the output port via a control channel comprising a control valve to control the release mechanism.

2. The alert system as claimed in claim 1, wherein the pressure variation is a positive pressure difference between a pressure at the point of interest on the subsea infrastructure and a hydrostatic pressure at a seabed where the subsea infrastructure is located.

3. The alert system as claimed in claim 1, wherein the pressure variation is a negative pressure difference associated with a drop in pressure of an energy storage device of the subsea infrastructure.

4. The alert system as claimed in claim 1, wherein the pressure variation is due to an increase in pressure, and wherein the control valve is normally closed and is configured to open when a pressure at its input increases to become equal to, or greater than, the first threshold value.

5. The alert system as claimed in claim 1, wherein the pressure variation is due to a decrease in pressure, and wherein the control valve is normally closed and is configured to open when a pressure at its output decreases to become equal to, or less than, the second threshold value.

6. The alert system as claimed in any one of the claims 1 to 5, wherein the control channel comprises a first safety valve provided between the input port and the control valve.

7. The alert system as claimed in any one of the claims 1 to 6, wherein the control channel comprises a second safety valve provided between the control valve and the release mechanism.

8. The alert system as claimed in claim 7, wherein the control channel comprises a third safety valve provided between the second safety valve and the release mechanism.

9. The alert system as claimed in any one of claims 1 to 8, comprising an accumulator coupled to the control channel.

10. The alert system as claimed in any one of claims 1 to 9, comprising a pressure compensator coupled to the release mechanism.

11. The alert system as claimed in any one of the claims 1 to 10, comprising a plurality of control channels and a plurality of release mechanisms; wherein each control channel extends between the input port of the first control channel and a channel-specific output port.

12. The alert system as claimed in claim 11, wherein each control channel comprises a control valve configured to change state at a different threshold value.

13. The alert system as claimed in any of the preceding claims, wherein the release mechanism comprises a hydraulic cylinder operable between the locked state in which a rod is extended to lock the signalling device and the unlocked state in which the rod is retracted to release the signalling device.

14. The alert system as claimed in any of the preceding claims, wherein the signalling device comprises at least one beacon configured to transmit an alert signal, wherein the beacon comprises internal or external buoyancy.

15. The alert system as claimed in claim 14, wherein the signalling device is configured to communicate with a satellite network; wherein the satellite network is configured to relay the alert signal to a party monitoring the subsea infrastructure.

16. The alert system as claimed in any one of the preceding claims, wherein the alert system is mounted on a frame.

17. A method of monitoring a subsea infrastructure, the method comprising providing an alert system as claimed in any one of the preceding claims; coupling the alert system to the subsea infrastructure; and releasing the signalling device upon detection of a pressure variation at a point of interest on the subsea infrastructure above a first threshold value when the pressure variation corresponds to an increase in pressure, or below a second threshold value when the pressure variation corresponds to a decrease in pressure .

18. The method as claimed in claim 17, comprising pre-charging the alert system prior to coupling it to the subsea infrastructure.