EMERGENCY DISCONNECT SYSTEM

Abstract

A control system for controlling disconnection of riser system that extends between a vessel and a subsea location that comprises a wellbore, the riser system configured to receive a string in use, the control system being configured to disconnect the riser system according to a sequence of operations, the sequence of operations comprising disconnecting the riser system prior to or simultaneous with cutting the string and sealing the well bore.

Claims

1. A control system for controlling disconnection of a riser system that extends between a vessel and a subsea location that comprises a wellbore, wherein the control system is configured to, when a loss of a power source is detected, enter an emergency disconnect monitoring procedure in which a predetermined parameter of the riser system is monitored, the predetermined parameter has a critical condition; and wherein the control system, in the emergency disconnect monitoring procedure, is configured to: determine a position of the vessel, determine drift of the vessel, determine a value of the predetermined parameter, determine whether a disconnect condition has been met based on the determined position of the vessel, the drift of the vessel, and the value of the predetermined parameter of the riser system, and disconnect the riser system when the disconnect condition has been met.

2. The control system according to claim 1, wherein the control system, in the emergency disconnect monitoring procedure, is configured to detect whether the power source is still lost, and to discontinue the emergency disconnect monitoring procedure when a loss of the power source is no longer detected.

3. The control system according to claim 1, wherein the control system is determining drift of the vessel by monitoring the position of the vessel over time.

4. The control system according to claim 1, wherein the control system is determining drift of the vessel by determining a rate, rate of change and direction of movement of the vessel.

5. The control system according to claim 1, wherein the control system is configured to disconnect the riser system prior to or simultaneous with cutting a string accommodated in the riser system.

6. The control system according to claim 1, wherein the control system is configured to disconnect the riser system prior to or simultaneous to sealing the well bore.

7. The control system according to claim 1, wherein the critical condition of the predetermined parameter is set to protect a BOP stack, an Xmas tree or a wellhead from damages due to drift of the vessel.

8. The control system according to claim 7, wherein the predetermined parameter comprises a bend angle of the riser system.

9. The control system according to claim 2, wherein the control system, in the emergency disconnect monitoring procedure, is configured to allow the power source to be restored prior to the disconnect condition has been met and thereby avoid an unnecessary disconnecting of the riser.

10. The control system according to claim 1, wherein the control system is configured to determine a disconnection threshold specifying how far the vessel can drift from its target location before the critical condition is reached.

11. The control system according to claim 10, wherein the control system is configured to operate with an emergency disconnection scheme associated with an indicative time for the riser to be disconnected, and wherein the control system is configured to determine whether the vessel drifting at the determined drift will reach the disconnection threshold in a time less than the indicative time for the riser to be disconnected.

12. A method of controlling disconnection of a riser system that extends between a vessel and a subsea location that comprises a wellbore, the method comprises detecting a loss of a power source, entering, when a loss of a power source is detected, an emergency disconnect monitoring procedure in which a predetermined parameter of the riser system is monitored, and in the emergency disconnect monitoring procedure determining a position of the vessel, determining drift of the vessel, determining a value of the predetermined parameter, determining if a disconnect condition has been met based on the determined position of the vessel, the drift of the vessel, and the value of the predetermined parameter of the riser system, and disconnecting the riser system when the disconnect condition has been met.

13. The method according to claim 12, wherein the method comprises detecting, in the emergency disconnect monitoring procedure, whether the power source is still lost, and discontinuing the emergency disconnect monitoring procedure when a loss of the power source is no longer detected.

14. The method according to claim 12, wherein the method comprises determining drift of the vessel by monitoring the position of the vessel over time.

15. The method according to claim 12, wherein the method comprises disconnecting the riser system prior to or simultaneous with cutting a string accommodated in the riser system.

16. The method according to claim 12, wherein the method comprises determining a disconnection threshold specifying how far the vessel can drift from its target location before the critical condition is reached.

17. The method according to claim 12, wherein the method comprises operating with an emergency disconnection scheme associated with an indicative time for the riser to be disconnected, and wherein the control system is configured to determine whether the vessel drifting at the determined drift will reach the disconnection threshold in a time less than the indicative time for the riser to be disconnected.

18. A computer program product, the computer program product is embodied in a non-transitory tangible computer readable storage medium and adapted for controlling disconnection of a riser system that extends between a vessel and a subsea location that comprises a wellbore, the computer program product comprising computer instructions for: detecting a loss of a power source, entering, when a loss of a power source is detected, an emergency disconnect monitoring procedure in which a predetermined parameter of the riser system is monitored, and in the emergency disconnect monitoring procedure determining a position of the vessel, determining drift of the vessel, determining a value of the predetermined parameter, determining if a disconnect condition has been met based on the determined position of the vessel, the drift of the vessel, and the value of the predetermined parameter of the riser system, and instructing the riser system to disconnect when the disconnect condition has been met.

19. A vessel comprising a riser system that extends between a vessel and a subsea location that comprises a wellbore and comprising a control system for managing an emergency disconnect monitoring procedure in case of loss of a main power source, wherein the control system is configured to, when a loss of a power source is detected, enter an emergency disconnect monitoring procedure in which a predetermined parameter of the riser system is monitored, the predetermined parameter has a critical condition; and wherein the control system, in the emergency disconnect monitoring procedure, is configured to: determine a position of the vessel, determine drift of the vessel, determine a value of the predetermined parameter, determine if a disconnect condition has been met based on the determined position of the vessel, the drift of the vessel, and the value of the predetermined parameter of the riser system, and disconnect the riser system when the disconnect condition has been met.

20. The vessel according to claim 19, wherein the control system comprises a computer on which software on a computer program product according to claim 18 is run.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0058] These and other aspects of the present disclosure will now be described, by way of example only, with reference to the accompanying Figures, in which:

[0059] FIG. 1 is a schematic of a drilling arrangement;

[0060] FIG. 2 is a schematic of a blowout preventer and lower marine package for use in the drilling arrangement of FIG. 1 ;

[0061] FIG. 3 is a flowchart illustrating method steps of an emergency disconnection scheme;

[0062] FIG. 4 is a schematic of an example of a drilling arrangement involving a drilling vessel;

[0063] FIG. 5 is a schematic of a control system for controlling disconnection or riser system supported by a drilling vessel, such as those shown in FIG. 1 or FIG. 4;

[0064] FIG. 6 is a flowchart illustrating a method of controlling disconnection of riser system supported by a vessel, such as those shown in FIG. 1 or 4.

DETAILED DESCRIPTION OF THE DRAWINGS

[0065] Various aspects and examples of the present disclosure relate to methods, systems and apparatus for the dynamic position control of offshore vessels. Any vessel may be considered, but for the purposes of the exemplary description provided below, a semi-submersible drilling vessel or “rig” is presented.

[0066] FIG. 1 shows a semi-submersible 5 that is connected to, and supports one end of, a riser system comprising a riser 10, such as a drilling riser. The riser 10 extends from the vessel 5, underwater to connect to a blow-out preventer (BOP) stack 11 comprising a lower marine riser package (LMRP) 12 and a lower blow-out preventer (BOP) stack 15 at a fixed subsea location on the sea bed 20. The lower marine riser package (LMRP) 12 is coupled to the riser and is connected in a selectively releasable manner to the blow out preventer (BOP) stack 15. The riser 10 is optionally provided with a pair of flex joints 25, one at the surface end and one at the subsea end. The vessel 5 can be moved, e.g. due to currents, waves and wind, and is maintained in a target position using a dynamic positioning system, as is well known in the art. The vessel comprises a propulsion system that comprises a plurality of propulsion devices such as thrusters 30. The thrusters 30 are configured to propel the vessel 5 in different directions and are selectively controllable to adjust the position of the vessel 5.

[0067] The vessel shown in FIG. 1 is a drilling rig, but it will be appreciated that the vessel is not limited to this and can equally be a ship, or indeed any other suitable vessel or vehicle, such as a drilling vessel that can perform drilling of all sections of the well. Furthermore, although the riser 10 is connected at the subsea end to a lower marine riser package 12 which is in turn releasaby connected to the lower BOP stack 15, it will be appreciated that the riser could be connected to any other suitable wellhead apparatus, such as a Xmas tree, e.g. via the BOP stack 11. In addition, although an example comprising a drilling riser is shown, it will be appreciated that the present concept could be used for other functions such as completion, in which case the riser 10 would be a completion riser.

[0068] The dynamic positioning system of the vessel 5 is configured to determine the position of the vessel 5, compare the determined position of the vessel 5 with a target position and determine corrective control actions for specific thrusters 30 to, as far as possible, maintain the vessel 5 substantially in the target position. However, there may be situations, such as a blackout or loss of power by the vessel 5, where it could be desirable to perform an emergency disconnect of the riser 10. For example, since the riser 10 is suspended between the vessel 5 and a fixed subsea position 12, 15, if the vessel moved too far off the target location, then damage to the riser 10 and/or the subsea components 11, 12, 15 could occur, e.g. due to bending of the riser 10 past a critical angle.

[0069] The time available to disconnect the riser 10 in an emergency situation can vary. Operations in shallow water provide much less time to act, as the shallower the water the less the distance that the vessel 5 can move off station before the riser 10 is bent to its critical angle. As such, having an emergency disconnection scheme that allows a quick disconnect can be highly advantageous, particularly for shallow water operations. Emergency disconnection of the riser 10 can comprise several steps and the choice of steps and the sequence in which they are implemented can have a significant effect on the time taken for the riser 10 to be disconnected.

[0070] FIG. 2 shows an example of the BOP stack 11, comprising the lower marine riser package 12 and the lower BOP stack 15, shown in FIG. 1 . A string 27 (e.g. a drill string) runs through the lower marine riser package (LMRP) 12 and lower BOP stack 15. The drill string 27 supports the drill bit (not shown) and serves to convey drilling fluid from the surface to the drill bit, as is well known in the art. The lower BOP stack 15 comprises a wellhead connector 35 for connecting to the wellhead 40, pipe rams 42 for sealing around a drill pipe, casing shear rams 45 for cutting the string 27 without sealing, blind shear rams 50 for sealing the wellbore and an upper connector 51 for connecting to the LMRP 12, the upper connector 51 being distal to the wellhead connector 35. The LMRP 12 comprises a lower connector 52, and lower and upper annular BOPs 53, 54. The LMRP 12 is coupled at an end distal to the lower BOP stack 15 to the lower flex joint 25 for connecting to the riser 10. It will be appreciated that the BOP stack 11 would typically comprise a number of other well-known components such as variable bore rams (VBRs) and the hydraulic systems and valves required to operate the various rams but that these are not shown only for clarity and simplicity. Similarly, it will also be appreciated that the BOP stack 11 could come in a range of forms and comprise one or more of each ram type and the present disclosure is not limited to the BOP stack 11 shown. For example, the BOP stack could alternatively be, or comprise one or more features of, a BOP stack described in US2012/0197527, which is hereby incorporated by reference in its entirety as if set out in full herein.

[0071] The lower marine riser package 12 is releasably coupled to the lower BOP 15 stack such that it can be selectively released to decouple the lower marine riser package 12 and the riser 10 from the lower BOP stack 15 to thereby disconnect the riser 10 from the fixed subsea location. When disconnected, the LMRP 12 remains coupled to the riser 10 and is free to move away from the lower BOP stack 15, which remains fixed at the wellhead (optionally via a Xmas tree).

[0072] One emergency disconnection scheme for releasing the riser 10 would be to initially cut the string 27 using the casing shear rams 45, then subsequently seal the wellbore with the blind shear rams 50 before disconnecting the riser 10 by switching the connector 55 of the lower marine riser package 12 into the released configuration. This sequence provides good wellbore sealing and minimises loss from the wellbore but takes a relatively long time before the riser 10 is disconnected. In one example, implementation of this sequence resulted in a 76 second delay between the sequence being initiated and the riser being released. However, it will be appreciated that the actual time to disconnect may vary in different systems.

[0073] Another emergency disconnection scheme would be to cut the string 27 using the casing sheer rams 45, then disconnecting the riser 10 using the connector 55 of the lower marine riser package 12 before sealing the wellbore with the blind shear rams 50. This emergency disconnection scheme has a much shorter delay between initiation of the emergency disconnection scheme and the release of the riser 10, with an example under equivalent conditions to the example given above resulting in a delay in the region of 39 to 44 seconds between the sequence being initiated and the riser being released. However, it will be again appreciated that the actual time to disconnect may vary in different systems.

[0074] If approximately 30 seconds is added to these delays in disconnecting the riser to account for human reaction and thinking time in a manually operated system, then there is a real risk that, for shallow water operations, these emergency release sequences won't result in release of the riser 10 before damage to the riser 10 or other components occurs.

[0075] In such cases, a particularly beneficial emergency disconnection scheme, as shown in FIG. 3, comprises, in step 305, initially disconnecting the riser 10 using the connector 55 of the lower marine riser package 12 before, in step 310, then cutting the string 27 using the casing sheer rams 45 and subsequently, in step 315, sealing the wellbore with the blind shear rams 50. In this case, in an equivalent example to those given above, the delay between the emergency disconnection scheme being started and the release of the riser 10 was found to be in the order of 25 to 29 seconds. In situations where every second could potentially count, such as during shallow water operations, this faster emergency disconnection scheme could make the difference between the riser 10 being successfully released or damaged.

[0076] The emergency disconnection schemes described above are implemented by a suitable control system 60 (see FIGS. 4 and 5). This could be part of an existing control system or a dedicated stand-alone system. Regardless, it may be possible to retrofit the emergency disconnection schemes in existing vessels by suitably reprogramming an existing control system or by installing the stand-alone system.

[0077] The control system 60 could be configured to simply implement a single, pre programmed emergency disconnection scheme, which could be any of those described above, or the control system could be configured such that it is possible to switch between emergency disconnection schemes, e.g. by manual selection, to suit the particular operation being performed.

[0078] Beneficially, the electronic control system 60 is configured to automatically initiate the emergency disconnection scheme. As indicated above, when the emergency disconnection scheme is manually initiated, a further delay of around 30 s must be factored in for human intervention“thinking time”. This further delay can be significantly reduced by having the control system 60 dynamically monitor the vessel 5 in use and automatically initiate the emergency disconnection scheme.

[0079] Examples of a vessel and control system for implementing this are shown in FIGS. 4 and 5.

[0080] FIG. 4 shows a vessel 5′ provided with a dynamic positioning system that comprises a control system in the form of an emergency disconnection system (EDS) controller 60 that is in communication with a plurality of location sensors 65A-65G used for determining the location of the vessel 5′ and a plurality of riser sensors 70A, 70B used for monitoring properties of the riser 10. The controller 60 is configured to provide dynamic and automatic control of emergency disconnection of the riser 10 based on the location of the vessel 5′ determined using the position sensors 65A-G and/or the properties of the riser 10 determined using the riser sensors 70A, 70B and is also operative only whilst a blackout or other loss of power or positioning ability is detected.

[0081] As in the example of FIG. 1 , the vessel 5′ supports an end of a riser 10′ that extends between the vessel 5′ and a wellhead 50 on the seabed 20. The wellhead 50 is coupled to the BOP stack 11 that comprises the lower marine riser package 12 and the BOP stack 15 as shown in FIGS. 1 and 2.

[0082] A detailed schematic of the controller 60 is shown in FIG. 5. The controller 60 comprises a processor 75, a communications system 80 and a data store in the form of a memory 85. The communications system 80 is configured to communicate via wires or wirelessly with a plurality of location sensors 65A-65G used for position determination and a plurality of riser sensors 70A, 70B to receive data signals therefrom. The wireless communications could comprise acoustic communications, e.g. along the riser 10. In this way, the controller 60 can receive data from the plurality of location sensors 65A-65G, which is processed by the controller 60 to determine the position of the vessel 5, 5′.

[0083] Although an example of a controller 60 is shown in FIG. 5, it will be appreciated that other controller configurations could be used. For example, the controller 60 may comprise a plurality of processors 75 and/or a plurality of data stores 85, which may be contained in a single unit or distributed, e.g. over several systems, some of which may be remote.

[0084] The actions performed by the controller 60 are described in relation to FIG. 6.

[0085] In the present embodiment, the goal of the automatic emergency disconnect monitoring procedure is to automatically disconnect the riser optionally 10 before the riser 10 or another component such as the BOP stack 11, Xmas tree or wellhead reaches a critical condition in which it could be damaged. One example of a critical condition could be when the riser 10 reaches a predetermined critical bend angle, e.g. 6°. However, the automatic emergency disconnect monitoring procedure is also configured such that, if possible without risking damage to the riser 10, time is allowed for power to be restored in order to avoid unnecessary disconnecting of the riser 10. Within this procedure, reducing the time between initiation of an emergency disconnection scheme that disconnects the riser 10 and the LMRP 12 from the lower BOP stack 15 and the actual disconnection of the riser 10 and LMRP 12 (e.g. by using the method of FIG. 3), allows more time in which the power can be restored, increasing the likelihood of unnecessarily disconnecting the riser 10.

[0086] In step 605 of FIG. 6, the controller 60 determines if the main power is available on the vessel 5, 5′. If the power is available, an automatic emergency disconnect monitoring procedure lies dormant and no automatic emergency disconnection of the riser 10 takes place. The controller 60 then continues to check the main power of the vessel until it detects that the main power is not available. If the controller 60 determines that the main power of the vessel 5, 5′ has been lost, then it implements the automatic emergency disconnect monitoring procedure. Although a detection of the main power is described above, detection of other power loss or loss of positioning conditions could be additionally or alternatively performed.

[0087] In step 610, the controller 60 monitors the location of the vessel 5, 5′ via the location sensors 65A-65G and in step 615 determines the degree, rate, rate of change and direction of drift of the vessel 5, 5′ from its target location and/or from the location of the vessel before the loss of power.

[0088] In step 620, the controller 60 determines a disconnection threshold that specifies how far the vessel 5, 5′ can drift from its target location (e.g. above the wellbore/subsea location, or its location when the power failed), before the critical condition is reached (e.g. an operating margin before the lower flex joint 25 or other component would reach its critical bend angle). For example, the controller 60 could be provided with a determined or predetermined disconnection threshold or the controller could determine the disconnection threshold based on parameters such as the depth of the sea at the current location, the length of the riser 10, the amount of flex in the flex joint(s) 25 and so on, e.g. using geometry, an algorithm or modelling.

[0089] Each emergency disconnection scheme is associated with an indicative time for the riser to be disconnected using that emergency disconnection scheme (which may also include an additional operating margin or represent a “worse case” scenario). The controller 60, in step 625, determines if the vessel 5, 5′ drifting at the determined or predicted rate of drift, in the determined drift direction, from the determined location would reach the disconnection threshold in a time that is with an operation margin of, the same or less than the indicative time for the riser to be disconnected and if so initiates the disconnection of the riser 10 using the emergency disconnection scheme (e.g. the emergency disconnection scheme shown in FIG. 3, or any of the other emergency disconnection schemes described above, amongst other possibilities).

[0090] If the controller 60 instead determines that the vessel 5, 5′ drifting at the determined or predicted rate of drift, in the determined drift direction, from the determined location would not reach the disconnection threshold in the indicative time for the riser to be disconnected, it instead reverts the process back to step 605 and re-checks if the main power is available on the vessel 5, 5′. In this way, if the main power is restored before the emergency disconnect scheme is initiated, then the automatic emergency disconnect procedure is effectively aborted and returned to the dormant state until the controller 60 once again determines that the main power is lost.

[0091] In this way, the controller 60 is configured to automatically initiate the emergency disconnect scheme to disconnect the riser if the main power of the vessel is lost (i.e. blackout) shortly before the vessel 5′ reaches the point that it would drift far enough for the critical condition to be reached (e.g. the riser 10 being bent past its critical angle) before the riser 10 could be disconnected using the current emergency disconnection scheme. In this way, if the riser 10 needs to be disconnected quickly (e.g. if the vessel is in shallow water operations or if the sea or weather state is such that the vessel 5, 5′ will drift off station quickly) then the controller 60 would act quickly to automatically initiate the emergency disconnect scheme. However, in more benign conditions, e.g. in summer when the weather and sea state is favourable or if the water depth is greater, then the initiation of the emergency disconnect scheme can be delayed until shortly before the drift of the vessel would result in the critical condition being reached before the riser could be disconnected. In this way, the controller 60 provides a chance for the vessel power to be restored, thereby potentially avoiding an unnecessary riser 10 disconnection.

[0092] It will be appreciated that alternatives to the above method are possible.

[0093] Although the example described above describes an automatic emergency disconnect monitoring procedure that seeks to automatically disconnect the riser 10 before the critical condition is reached, where the critical condition comprises the riser 10 reaching a predetermined critical bend angle, the present disclosure is not limited to this. For example, in an embodiment, the automatic emergency disconnect monitoring procedure could comprise automatically initiating the emergency disconnection sequence immediately upon loss of power blackout, or after a determined or predetermined period of time with loss of power/blackout.

[0094] In another embodiment, the automatic emergency disconnect monitoring procedure could comprise initiating only a subset of steps in the emergency disconnect sequence immediately upon loss of power/blackout or after a determined or predetermined amount of time of loss of power/blackout and then initiating the remainder of the steps of the emergency disconnect sequence once the critical threshold is reached or approached or after a further determined or predetermined period of time. For example, the predetermined amount of time could be the time a black-out recovery should normally take, such as 5 minutes or less, such as 2 min or less, such as 1 min or less, e.g. 30 s or less or 20 s or less. The controller 60 is optionally configured to initiate or bring forward the remainder of the steps of the emergency disconnect sequence if there is a failure to complete any of the subset of steps in the emergency disconnect sequence. The subset of steps could include, for example, steps that do not include physically disconnecting the riser and/or which are easily reverted, such as closing various valves, closing annular BOPs etc. The remainder of the steps may comprise steps that involve disconnecting the riser and shearing, e.g. shearing the string, sealing the wellbore, sealing the string, and/or the like.

[0095] It will also be appreciated that other critical conditions could be used and that the critical condition need not be a condition of the riser but could be a critical condition of the wellhead, BOP stack, Xmas tree or other component. For example, the critical condition could be a critical wellhead bending moment.

[0096] For example, the above method determines if the current drift of the vessel would result in it reaching the critical condition (e.g. where the riser 10 is bent to or beyond its critical angle, including the operational margin) only using the determined location of the vessel 5, 5′.

[0097] However, it will be appreciated that other metrics for determining whether or not the drift would result in the critical condition (e.g. the bending of the riser to or beyond its critical angle) could be used instead of, or in addition to, the determined location of the vessel 5, 5′. For example, riser sensors 70A, 70B, optionally located at the flex joints 25, could be used to directly measure the bend angle and/or the rate of change of bend angle of at least part of the riser 10, 10′, which could be used instead of or in addition to the location / drift of the vessel 5, 5′. Strain, stress or other properties of the riser, flex joint or any other critical component could also be used. Furthermore, additional data, such as tide data, sea state data and/or weather forecast or measurement data could be used to increase the accuracy of the determination.

[0098] The controller 60 can determine the location of the vessel 5 from data from the plurality of sensors 65A-65G used for position determination, which could include a variety of different sensor types. In this particular example, the sensors 65A-65G used for position determination include at least two satellite navigation sensors 65A, 65B provided on the vessel 5, a wire sensor 65C on the vessel 5 that determines a tension or force on a taut wire that is suspended between the vessel 5 and a fixed external point, e.g. on the sea bed and a plurality of motion sensors 65D, in this example in the form of four gyroscopic sensors, which are configured to measure motion of the vessel. The sensors 65A-65G used for position determination in this example further include a plurality of beam position sensors 65E, 65F, including a surface based beam sensor 65E that sends a beam to and/or receives a beam from a reference point, e.g. on land or on another vessel or other sea structure. Similarly, the vessel 5 could comprise a plurality of underwater beam sensors 65F, such as sonar or other sonic sensors, that are each in communication with a plurality of reference points located on the sea bed using sonic or sonar signals and can be used to determine changes in the location of the vessel 5 using the relative timing and/or strength of the sonic or sonar signal received from each reference point. The sensors used for position determination in this example further include condition sensors 65G, which include weather sensors such as wind sensors, and also include water current sensors to determine current direction and speed of water currents.

[0099] The controller 60 is in communication with all of the sensors 65A-65G used for position determination and uses the data collected by the sensors 65A-65G used for position determination to determine a position of the vessel 5, e.g. according to a predetermined algorithm or other suitable relation. However, it will be appreciated that the above are just examples, and only one or more or any combination of the above location sensors, or indeed entirely different location sensors could be used.

[0100] In an optional example, each end of the riser 5′ is provided with a flex joint 25′ and each flex joint is provided with at least three of the sensors 70A, 70B that monitor at least one property of the flex joint 25′ that changes when the vessel 5′ is moved. Suitable properties of the flex joint include tilt, inclination, angle, orientation, flex, bend, stress or strain, or other suitable property of the respective flex joint 25′ or other part of the riser 10. The sensors 70A, 70B need not be located only at the flex joint 25′ and one or more sensors may be provided on or along the riser 10′, in addition to or as an alternative to the sensors 70A, 70B at the flex joints 25′. The controller 60 can optionally be configured to at least partially determine if the emergency disconnection procedure should be initiated at least partly based on the determined values of the at least one property of the riser 10

[0101] Although, specific examples are described above in relation to the Figures, it will be appreciated that variations on the above examples are possible.

[0102] For example, although a drilling riser 10 and drilling arrangement are described above, the same concepts could also be applied to completion and production arrangements e.g. using completion or production risers. Indeed, the present disclosure could also be used in relation to other objects, lines or general infrastructure that extends between a vessel and a subsea location.

[0103] Furthermore, although certain specific examples of sensors 65A-65G used for position determination are given above, it will be appreciated that different sensors or combinations of sensors could be used instead.

[0104] In addition, the vessel 5 need not be a drilling rig and other vessels such as ships could be used instead. For example, the vessel 5 could be a drill-ship, semi-submersible or other floating drilling unit. The vessel could be any suitable vessel that can drill and utilizes dynamic positioning and is preferably not moored. In the event of a completion/work-over riser, the vessel could also be a drilling work-over platform.

[0105] Although examples are given above where the riser system 10 is disconnected by disconnecting the LMRP 12 that is coupled to the riser 10 from the lower BOP stack 15, the present disclosure is not limited to this. The disconnection of the riser system 10 may comprise other means for disconnecting the riser system 10 subsea, e.g. at or proximate the subsea location and at or towards an end of the infra-structure that is at towards the subsea location and away or distal from the vessel, in use. The disconnection of the riser system 10 may comprise disconnecting the riser system 10 from the subsea location such that it remains supported by the vessel. The control system may be configured to directly or indirectly control a latching mechanism located at or proximate the subsea location, e.g. comprised in the lower marine riser package (LMRP) or a completion riser unlatch mechanism. The latching mechanism may be selectively switchable between a latched configuration in which the riser system is latched and secured at the subsea and an unlatched configuration in which the riser system is disconnected from the subsea location. The control system may be configured to disconnect the riser system by switching the latching mechanism into the unlatched configuration, e.g. from the latched configuration.

[0106] Although one possible example of control system 60 is descried in relation to FIG. 5, the control system 60 configuration is not limited to this. Method steps of the invention can be performed by one or more programmable processors of the control system 60 executing a computer program to perform functions of the invention by operating on input data and generating output. Method steps can also be performed by special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application-specific integrated circuit) or other customised circuitry of the control system 60. Processors suitable for the execution of a computer program include CPUs and microprocessors, and any one or more processors. Generally, a processor will receive instructions and data from a read-only memory or a random access memory or both. The essential elements of a computer are a processor for executing instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto-optical disks, or optical disks. Information carriers suitable for embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, e.g. EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in special purpose logic circuitry.

[0107] As such, the present invention is not limited by the examples shown in the drawings but only by the claims.