GB2579352A

GB2579352A - System, method and station for subsea monitoring

Info

Publication number: GB2579352A
Application number: GB1818826.8A
Authority: GB
Inventors: Ravndal Ola; Eriend Bustnes Trond
Original assignee: Equinor Energy AS
Current assignee: Equinor Energy AS
Priority date: 2018-11-19
Filing date: 2018-11-19
Publication date: 2020-06-24
Also published as: GB201818826D0

Abstract

A subsea monitoring station 4 for monitoring one or more subsea installations comprising at least one sensor for sensing at least one variable associated with the one or more subsea installations, a receiver configured to receive an interrogation signal 8, and a transmitter configured to transmit a response signal 10 indicative of a status of the one or more subsea installations. The sensor may be a hydrophone, which makes an audio recording of the subsea installation and compares the recording to comparison data. The response signal may indicate a status of the installation based on the comparison. The station may be interrogated by a ship including a second transmitter 7 and second receiver 9. A plurality of monitoring stations may be used to relay signals between one another. Data storage may be provided so that the station may function as a black box recorder.

Description

SYSTEM METHOD AND STATION FOR SUBSEA MONITORING

The present invention relates to a system and a station for monitoring subsea installations. The invention also extends to corresponding methods of monitoring subsea installations.

Subsea installations are commonplace, particularly in the oil and gas industry. Such subsea installations may be pipelines, active and operational production wells, and temporarily plugged and abandoned wells to name but a few. It is often required that subsea installations be monitored to ensure that they'are operating as intended and that the subsea installations have not been damaged.

For instance, in the case of a temporarily plugged and abandoned well, monitoring may be required to ensure the quality of the seal in the temporarily plugged and abandoned well is maintained, and that no damage has been inflicted to the temporarily plugged well that may allow for undesirable leakage. Similarly, in the case of a mooring line associated with a floating, offshore installation, monitoring may be required to ensure that the mooring line remains operational and has not been damaged or snapped.

A number of systems and devices are known to enable monitoring of subsea installations_ One such system involves direct inspection of the subsea installation through use a remotely operated underwater vehicle (ROV). A vessel carrying the ROV will typically travel to the site of the subsea installation and deploy the ROV such that it may carry out inspection of the subsea installation to check for damage of the site and/or improper functioning. Inspection of the subsea installation by a ROV will typically be carried out periodically. The ROV may also be able to carry out any necessary repair work that is required to the subsea installation.

However, deployment and inspection using ROVs is costly due to the equipment, resources and labour that is required to get the ROV to the subsea installation, Moreover, in instances where the subsea installation is positioned in remote, hard to reach locations, for instance locations where sea conditions are typically rough, it can prove challenging to deploy an ROV in order to carry out the necessary inspection of the subsea installation.

A number of monitoring systems that are at least not completely reliant on ROVs for monitoring and inspection of subsea installations have also been employed. For instance, it is known to provide a number of combined sensor and alarms systems for monitoring subsea installations. Such systems often comprise sensors that are configured to be positioned in the proximity of the subsea installations in order to monitor certain variables associated with the subsea installation. Each of these sensors is typically associated with an'alarm system, with each alarm system either positioned subsea alongside the sensor, or tethered as a floating buoy at sea level above the sensor.

In cases where the alarm system is provided subsea, the alarm will comprise means for propulsion to the sea surface when it is desired to communicate the data recorded at the sensor to a user. Once at sea level, the alarm system will transmit the recorded data, typically by means of satellite communication, in order to inform the relevant user of the recorded data. In cases where the alarm system is provided at the sea surface, tethered as a buoy, the sensor will communicate its recorded data by means of its tethered attachment with the alarm system. Upon receiving said communication, the alarm system will then transmit the recorded data via a satellite communication to the relevant user. It can then be ascertained from the data received by the user whether further action/inspection is required in respect of the subsea installation, e.g. whether repair work using an ROV may be required.

However, there are drawbacks associated with the combined monitoring and alarm systems discussed above. In the case of the subsea alarm system, whereby the alarm comprises means for propulsion to the sea surface, until the alarm has transmitted its signal at the surface, there is no way of knowing whether the sensorlalarm system is functioning as intended and monitoring the subsea installation as required. Moreover, once each alarm has propelled to the surface and transmitted its data, it is rendered redundant. Thus, each alarm system will either need to be replaced, or a plurality of alarm systems will need to be provided in order to provide continued monitoring of the subsea installation, which comes at considerable expense. Even in scenarios where a plurality of alarm systems are provided subsea, there are still only a finite number of alarm systems and so these will also eventually need to be replaced if, continued monitoring is desired.

Regarding those combined monitoring and alarm systems that rely on an alarm comprised in a tethered buoy, whilst such systems overcome some of the drawbacks associated with the subsea alarm system, they have their own unique drawbacks associated therewith. For instance, as such systems require tethered connection between the alarm and its respective sensor, it is not practical to use such systems in deep sea conditions and/or in locations where sea conditions are particularly rough (and may cause damage to the relevant tether).

Further monitoring systems for subsea installations have also been employed, wherein a subsea sensor is in cabled communication with a shore based site to enable continuous monitoring of the subsea installation. However, providing permanent, cabled connection to a subsea monitoring station can be costly, particularly when the subsea monitoring station is positioned in a remote, hard to reach location. Moreover, if the subsea installation is in a particularly remote location, it may not be possible to provide cabled communication thereto at all.

WO 03/100453 Al discloses a subsea monitoring system comprising hydrophones for recording a soundtrack associated with a subsea installation The monitoring system is configured to be placed in the proximity of the subsea installations in order to detect sounds in the region of the subsea installation. The sounds that are detected by the subsea monitoring system may indicate whether the subsea installation is damaged and/or operating correctly. The recorded soundtrack is stored at the subsea monitoring system and is subsequently transferred via cabled connection, either periodically or continuously, to a vessel or to shore such that the recorded soundtrack may be further analysed to ascertain if any of the recorded sounds may indicate possible damage and/or improper functioning to the subsea installation of interest.

It will be noted that, as in the case of some of the systems described above, a direct, cabled connection with the system of WO 03/100453 Al must be achieved before any data from the system indicative of:the conditions of the subsea installation can be obtained. As such, in cases where the recorded soundtrack is only downloaded periodically from said system, there is no way of knowing between each download whether the, monitoring system itseif is properly functioning and obtaining the necessary data in respect of the subsea installation. Moreover, given that a cabled connection is required to download the data from the system of WO 03/100453 Al, the system is not suitable for use in remote, hard to reach locations, where providing cable connection thereto, whether temporarily or permanently, is difficult or commercially unviable to achieve.

Thus, improvements in monitoring of subsea installations are desired. According to a first aspect of the invention, there is provided a subsea monitoring station for monitoring one or more subsea installations, the subsea monitoring station comprising: at least one sensor for sensing at least one variable associated with the one or more subsea installations; a receiver configured to receive an interrogation signal; and a transmitter configured to transmit a response signal indicative of a status of the one or more subsea installations; wherein upon receipt of the interrogation signal at the receiver, the transmitter is configured to transmit the response signal indicative of the status of the one or more subsea installations.

Thus, there is provided a subsea monitoring station with at least one sensor for monitoring one or more subsea installations. When the subsea monitoring station receives an interrogation signal (at a receiver), it transmits (from a transmitter) a response signal which is indicative of the status of the one or more subsea installations. Tnus, the response signal may be transmitted on receipt of the interrogation signal (at a receiver). The response signal may be based on, or depend on, (at least partially, for example) the at least one variable associated with the one or:more subsea installations sensed by the sensor. Such a station can thereby allow monitoring of subsea installations to be performed without the need for a cabled connection to sea-level (or the shore), for example, and without the need for the transmission of large amounts of data, for example. The transmission of a simple response signal, based, for example, on the at least one sensed variable associated with the one or more subsea installations, may provide an indication of the status of the one or more subsea installations.

The:status of the one or more subsea installations may refer to whether the one or more subsea installations are operating as intended, whether the one or more subsea installations are:operating within tolerable limits and/or whether the one or more subsea installations are damaged or not, For instance, the status of the one or more subsea installations may refer to whether a temporarily plugged and abandoned well is leaking or not. Equally, the status of the one or more subsea installations may refer to whether hydrocarbon fluid flow through a pipeline is as intended.

The status of the one or more subsea installations, and thereby the response signal, is preferably determined (or at least partially determined) based on the at least one sensed variable associated with the one or more subsea installations. For example, the response signal may be determined based on the at least one sensed variable associated with the one or more subsea installations using comparative or reference data for the one or more subsea installations, e.g. as described below.

The subsea installations may be or comprise any installation or apparatus installed in a subsea location, e.g, whose status it is desirable to monitor. For example, the subsea installations may be or comprise one or more pipelines, one or more active and operational production'wells, and one or more temporarily plugged and/or abandoned wells, Sound can be a particularly useful variable for monitoring the operation of subsea installations as particular components of subsea installations often have distinctive, characteristic sounds associated therewith, e.g. the snapping of 'a mooring line typically will emit sound at a particular frequency and/or amplitude.

Additionally, damage occurring, at subsea installations often creates anomalous, unexpected sounds that can be easily detected, for instance a rupture in a pipeline caused by undesired impact can cause a sound of large amplitude. Similarly, leaking at a temporarily plugged and abandoned well can typically have a distinctive, characteristic sound associated therewith, Thus, in preferred embodiments the at least one sensor is configured to detect sound, i.e. the at least one sensed variable is a sound variable (e.g. comprising amplitude and/or frequency data) or a soundtrack comprising the sounds sensed over a period of time by the at least one sensor. Accordingly, the at least one sensor preferably comprises a hydrophone.

The at least one sensor may comprise additional and/ or alternative sensors, such as accelerometers, temperature sensors, chemical sensors, voltmeters, ammeters, ohmmeters, LCR meters, capacitance meters and/or conductance meters. Accordingly, the at least one variable may be a degree of vibration, temperature, identification of the presence or absence of a chemical marker, current, potential difference (voltage), conductance, resistance and/or capacitance. Other sensors and/or variables may also or alternatively be used. The response signal may be any suitable signal, for instance a radio frequency signal or an infrared signal However, it is preferred that the response signal is a hydroacoustic signai due to its long range.

The response signal preferably comprises an identifier. For example, the identifier may be a characteristic (or part) of the response signal that allows for the origin of the response signal (i.e. which subsea installation the response signal relates to) to be identified. The identifier of the response signal is preferably unique to its point of origin (i.e. unique to each subsea installation to which the response signal relates). For example, the identifier could be a reference, number, code, or -6 -letter (or plurality of letters), corresponding to such an identifier which had previously been assigned to a subsea installation, thereby allowing that subsea installation to be identified.

Alternatively, the transmitter may be configured to transmit a signal comprising the identifier separately from, for example before (e.g., immediately before) the response signal The response signal may be (or comprise, e.g. in addition to the identifier described above) either a first response signal or a second response signal, such that the response signal is (or comprises) preferably a binary or Boolean signal.

The first response signal may correspond to the status of the subsea installation to which it corresponds being "OK" (e.g. operating as intended and/or operating within tolerable limits and/or not damaged), and the second response signal may correspond to the status of the subsea installation to which it corresponds not being "OK" (e.g. not operating as intended and/or not operating within tolerable limits and/or damaged), or not being known, and/or requiring further investigation, for example (or vice versa). For example, a response signal of "1" may correspond to the status of the subsea installation to which it corresponds being "OK", and a response signal of "0" may correspond to the status of the subsea installation to which it corresponds not being "OK", or not being known, and/or requiring further investigation, for example (or vice versa).

Additionally, or alternatively the interrogation signal may similarly be any suitable signal, for instance a radio frequency signal or an infrared signal.

However, it is preferred that the interrogation signal is a hydroacoustic signal due to its long range.

The interrogation signal may be a simple "blip", "ping" or "1" signal, indicating a request for a response signal to be transmitted.

The receiver is configured to receive the interrogation signal. Any type of receiver suitable for receiving the interrogation signal underwater could be used. For example, the receiver could be a receiver for receiving radio wave signals (a radio receiver), a receiver for receiving infrared signals (an infrared receiver) or, preferably, a receiver for receiving hydroaccustic signals (a hydroacoustic receiver). The receiver may be the same component as the sensor. For example, the receiver may be arranged to receive (or detect) both the interrogation signal and at least one variable associated with the one or more subsea installations. -7 -

The transmitter is configured to transmit the response signal (e.g, comprising a binary (or Boolean) response signal and/or an identifier, as described above) indicative of the status of the one or more subsea installations. Any type of transmitter suitable for transmitting such a response signal underwater could be used. For example, the transmitter could be a transmitter for transmitting radio wave signals (a radio transmitter), a transmitter for transmitting infrared signals (an infrared transmitter) or, preferably, a transmitter for transmitting hydroacousfic signals (a hydroacoustic transmitter).

The subsea monitoring station preferably comprises a memory, The memory may be configured for storing comparative data. The comparative data may be data that is indicative of one or more characteristic variables associated with the one or more subsea installations and/or subsea conditions at which the subsea monitoring station is disposed. For instance, the comparative data may comprise data associated with typical background noise in the region of the subsea monitoring station. Equally, the comparative data may comprise data indicative of one or more characteristic variables associated with the functioning and operating conditions of the one or more subsea installations. For instance, the comparative data may comprise data indicative of one or more characteristic sounds associated with particular events of interest. the comparative data could, for example, comprise one or more threshold values for or associated with the at least one sensed variable.

The memory may be configured for storing the at least one sensed variable associated with the one or more subsea installations. Alternatively, a further memory may be provided for storing the at least one sensed variable associated with the one or more subsea installations.

The subsea monitoring station preferably comprises a processor or processing means (e.g. one or more processors). For example, the processor or processing means may be configured to compare the comparative data stored in the memory with the at least one sensed variable. The status of the one or more subsea installations may be determined based on the comparison between the comparative data and the at least one sensed variable. As such, the response signal, indicative of the status of the one-or more subsea installations, may correspond to, or, preferably, be based on, the result of this, comparison.

The comparison may be a live comparison, i.e. the comparison may take place as soon as the at least one variable has been sensed by the at least one -8 -sensor and, optionally, before the at least one sensed variable is stored in the memory. In such cases, the (or a) memory may (additionally or alternatively) store the result of the comparison. Alternatively, the comparative data stored in the memory may be compared with the at least one sensed variable that has also been stored in the (or a) memory, and thus the comparison may not be'a live comparison. The result of the comparison is preferably stored in the memory or in a further memory.

The processor (or processing means) is preferably in communication with the transmitter and/or the receiver in the subsea monitoring station. As such, the receipt of an interrogation signal at the receiver may be communicated, preferably immediately or nearly immediately, to the processor (or processing means). On receipt of such an indication (i.e. that an interrogation signal has been received at the receiver), the processor (or processing means) may be configured to then control the transmitter to transmit the response signal indicative of the status of the one or more subsea installations.

The subsea monitoring station (e.g. Its processor or processing means) may be configured to determine the response signal to be transmitted based on the comparison between the comparative data and the at least one sensed variable.

The'subsea monitoring station (e.g. its processor or processing means) may be configured to determine that a first response signal is to be transmitted when the subsea monitoring station (e.g. its processor or processing means) does not identify, by virtue of the comparison, an anomalous, unexpected signal in the at least one sensed variable and/or a signal in the at least one sensed variable indicative of a particular event of interest associated with one of the one or more subsea installations.

Additionally, or alternatively, the subsea monitoring station (e.g. its processor or processing means) may be configured to determine that the second response signal is to be transmitted when the subsea monitoring station (e.g. its processor or processing means) identifies, by virtue of the comparison, an anomalous, unexpected signal in the at least one sensed variable and/or a signal in the at least one sensed variable indicative of a particular event of interest associated with one of the one or more subsea installations.

The subsea monitoring station preferably comprises a battery or power source. The battery or power source may provide power to each of the components of the subsea monitoring station, such as the processor, the memory, the transmitter, the receiver and/or the at least one sensor. The battery may be configured to supply the necessary power requirements to the subsea monitoring station for around or up to 10 years, for example.

The subsea monitoring station may be configured to relay signals, such as interrogation and/or response signals, to/from other subsea monitoring stations. As such, a plurality or series of subsea monitoring stations may be provided, e.g. in the form of a relay, which can relay interrogation and/or response signals between them, thereby facilitating the monitoring or covering or a larger area of subsea installations.

The plurality of other subsea monitoring stations may be in accordance with the subsea monitoring station described above. Additionally, or alternatively, the plurality of other subsea monitoring stations may be configured to relay the interrogation signal. This relay may communicate the interrogation signal from, for example, a shore or vessel (ship) based transmitter/receiver to the receiver of a subsea monitoring station. Further additionally or alternatively, the plurality of other subsea monitoring stations may be configured to relay the response signal transmitted by the transmitter on the subsea monitoring station. This relay may communicate the response signal from the transmitter of the subsea monitoring station to the shore or vessel (ship) based transmitter/receiver.

The subsea monitoring station of the above statements may be comprised in a subsea monitoring system. Thus, in accordance with a second aspect of the invention there is provided a subsea monitoring system, the subsea monitoring system comprising: the subsea monitoring station of any of the preceding statements; a second transmitter, the second transmitter being configured to transmit the interrogation signal; and a second receiver, the second receiver being configured to receive the response signal indicative of the status of the one or more subsea installations.

As described above, the response signal may be any suitable signal, for instance a radio frequency signal or an infrared signal. However, it is preferred that the response signal is a hydroacoustic signal due to its long range.

The response signal may be (or comprise) either a first response signal or a second response signal, such that the response signal is preferably a binary (or Boolean) signal, e.g. as described above.

The response signal may comprise an identifier. For example, the identifier may De a characteristic (or part) of the response signal that allows for the origin of -10 -the response signal (i.e. which subsea installation the response signal relates to) to be identified. The identifier of the response signal is preferably unique to its point of origin (i.e. unique to each subsea installation to which the response signal relates). Alternatively, the transmitter may be configured to transmit a signal comprising the identifier separately from, for example before (e.g., immediately before) the response signal.

Additionally, or alternatively the interrogation signal may be:any suitable signal, for instance a radio frequency signal or an infrared signal. However, it is preferred that the interrogation signal is a hydroacoustic signal.

The interrogation signal may be a simple 'blip", "ping" or "1" signal, indicating, a request for a response signal to be transmitted.

The second receiver is configured to receive the response signal. Thus, any type of receiver suitable for receiving such a response signal could be used as the second receiver. For example, the second receiver could be a receiver for receiving radio wave signals (a radio receiver), a receiver for receiving infrared signals (an infrared receiver) or, preferably, a receiver for receiving hydroacoustic signals (a hydroacoustic receiver).

The second transmitter is configured to transmit the interrogation signal. Any type of transmitter suitable for transmitting such an interrogation signal, e.g. as described above, could be used as the second transmitter. For example, the second transmitter could be a transmitter for transmitting radio wave signals (a radio transmitter), a transmitter for transmitting infrared signals (an infrared transmitter) or, preferably, a transmitter for transmitting hydroacoustic signals (a hydroacoustic transmitter).

The subsea monitoring system may comprise a processor or processing means associated with tne second transmitter and the second receiver. This processor or processing means may be provided in addition to the optional processor or processing means in the subsea monitoring station. This processor or processing means may be in communication with the second transmitter and the second receiver. The processor or processing means associated with the second transmitter and the second receiver may control the second transmitter to transmit the interrogation signal. It may be communicated to the processor or processing means when the second receiver receives the response signal. The processor or processing means associated with the second transmitter and the second receiver may be configured, upon receipt of the response signal at the second receiver, to determine whether further action in respect of the one or more subsea installations and/or the subsea monitoring station is required. Additionally or alternatively, if after sending the interrogation signal no response signal is received at the second receiver (e.g. within a particular, predefined, period of time), the processor or processing means associated with the second transmitter and the second receiver may be configured to determine that further action is required in respect of the subsea monitoring station and/or the one or more subsea installations.

The further action may comprise exploration, analysis and/or investigative tests of the subsea monitoring station and/or one or more subsea installations, to investigate the status and/or condition of the subsea monitoring station and/or to investigate the status and/or condition of the one or more subsea installations. For instance, such further action may comprise deployment of a remotely operated underwater vehicle to investigate the status and/or condition of the subsea monitoring station and/or to investigate the status and/or condition of the one or more subsea installations.

The second transmitter may be disposed on a vessel, for instance on a floating oil rig. However, in a preferred embodiment the second transmitter is disposed on a snip or boat configured to pass in a region of the subsea monitoring station. The second receiver may also be disposed on a vessel, for instance on a floating oil rig. However, in a preferred embodiment the second receiver is disposed on a ship or boat configured to pass in the region of the subsea monitoring station. In a preferred embodiment, the second receiver and the second transmitter are on the same vessel (e,g. oil rig, ship or boat).

It is preferred that the second transmitter and the second receiver are retrofitted to the vessel (e.g. ship), such that the second transmitter and the second receiver may be disposed on any type of vessel (i.e. there are no special requirements associated with the vessel for the second transmitter and second receiver to be fitted thereto).

The second transmitter may be configured to transmit the interrogation signal periodically. For instance, the second transmitter may be configured to transmit the interrogation signal hourly. Alternatively, the second transmitter may be configured to transmit the interrogation signal less frequently, e.g. daily, weekly or monthly. The second transmitter may be, additionally or alternatively, configured to transmit the interrogation signal as and when instructed by a user, on demand.

Further, the second transmitter may be configured, additionally or alternatively, to transmit the interrogation signal as the ship passes in the region of the subsea monitoring station. Determination of when the ship is in the region of the subsea monitoring station may be achieved via any suitable means. For instance, it may be determined using a GPS, or other orienteering/positioning technique.

In, certain embodiments, the second receiver and the second transmitter may be comprised in a shore based transmitter/receiver.

The subsea monitoring system may further comprise a plurality of other subsea monitoring stations. The plurality of other subsea monitoring stations may be in accordance with the subsea monitoring station described above in relation to the first aspect of the invention. Additionally, or alternatively, the plurality of other subsea monitoring stations may be configured to relay the interrogation signal. This relay may communicate the interrogation signal from a vessel or shore based transmitter/receiver to the receiver of the subsea monitoring station, and/or it may relay the interrogation from another source, such as a ship comprising, an appropriate transmitter and receiver. Further additionally or alternatively, the plurality ofother subsea monitoring stations may be configured to relay the response signal transmitted by the transmitter on the subsea monitoring station. This relay may communicate the response signal from the transmitter of the subsea monitoring station to the shore based transmitter/receiver to, and/or it may relay the response to another destination, such as a vessel comprising an appropriate transmitter and receiver.

In a third aspect of the'invention, there is provided a method of monitoring a subsea installation with a subsea monitoring station, the method comprising the steps of sensing at the subsea monitoring station at least one variable associated with the subsea installation; receiving at the subsea monitoring station an interrogation signal; and transmitting from the subsea monitoring station a response signal indicative of a status of the subsea installation.

The, subsea monitoring station used in the third aspect of the invention may be the subsea monitoring station as described above in relation the first aspect of the invention.

The method may comprise monitoring one or more subsea installations, The entire method of the third aspect is preferably performed (or performable) subsea.

In a fourth aspect of the invention, there is provided a method of monitoring a subsea installation'with a subsea monitoring system, the subsea monitoring system comprising a subsea monitoring station, the method comprising the steps of: sensing at the subsea monitoring station at least one variable associated with the subsea installation; transmitting to the subsea monitoring station an interrogation signal; receiving the interrogation signal at the subsea monitoring station; and transmitting a response signal indicative of a status of the subsea installation from the subsea sea monitoring station.

The subsea monitoring system used in the fourth aspect of the invention may be the subsea monitoring system as described above in relation to the second aspect of the invention. The subsea monitoring station of the first aspect of the invention may be, used with the method of the fourth aspect of the invention.

The method may comprise monitoring one or more subsea installations.

The various aspects of the invention described above may comprise any of the features, e.g. optional or preferred features, of the other aspects of the invention.

15== Preferred embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which.: Figure 1 is a schematic view of a subsea monitoring station positioned on the seabed proximate a first, a second and a third subsea installation; Figure 2 is schematic view of the subsea monitoring station and its constituent components; Figure 3 is a schematic view of a subsea monitoring system comprising the subsea monitoring station of Figure 1, and a transmitter and receiver disposed on a vessel at sea level; Figure 4 illustrates a remotely operated underwater vehicle (ROV) that has been deployed from the vessel as depicted in Figure 2 and connected with the subsea monitoring station; and Figure 5 is a schematic view of two subsea monitoring stations in relay communication with one another and with a shore based transmitter/receiver. Figure 1 illustrates a subsea monitoring station 4 positioned on the seabed.

The subsea monitoring station is positioned proximate a first subsea installation 1, a second subsea installation 2 and a third subsea installation 3 and is configured to monitor each of the subsea installations 1, 2, 3 as will be described in more detail below.

The first subsea installation 1 is a pipeline 1 that acts as a conduit for the transit of hydrocarbons. The second subsea installation 2 is a temporarily plugged -14 -and abandoned well 2. The third subsea installation 3 is an active and producing well 3 from which hydrocarbons are recovered.

The subsea installations 1, 2, 3 described above are merely examples and, of course, in other embodiments a different number of subsea installations may be present, and/or they may be of different types or different combinations of types of subsea installations.

As seen in Figure 2, the subsea monitoring station 4 comprises a hydrophone 42 configured to detect sound on the seabed in the proximity of the subsea monitoring station 4. In particular, the hydrophone 42 detects sounds that are emitted from each of the pipeline 1, the temporarily plugged and abandoned well 2 and the well 3. In some embodiments, the subsea monitoring station 4 also comprises one or more other sensors 43, e.g. temperature sensors, accelerometers etc., configured to monitor a variety of other variables associated with the subsea installations 1, 2, 3. These other sensors 43 may be used, in the context of the invention, in addition to, or as an alternative to the described hydrophone 42.

However, it is particularly preferred that hydrophones be employed on the subsea monitoring station 4 and thus the following description is given in relation to hydroacoustic monitoring.

Since the subsea monitoring station 4 is configured to detect sound via the hydrophone 42 from each of the pipeline 1, the temporarily plugged and abandoned well 2 and the well 3, the subsea monitoring station 4 is positioned within close proximity of each of the pipeline 1, the temporarily plugged and abandoned well 2 and the well 3, as shown in Figure 1. This is due to the tact that the range of hydroacoustic, communication, whilst long in comparison to all other types of underwater communication, is still limited. In an embodiment, the subsea monitoring station 4 is positioned within 40 nautical miles (74 km) or less of each of the pipeline 1, the temporarily plugged and abandoned well 2 and the well 3. Thus, as depicted in Figure 1, the subsea monitoring station 4 is located 10 nautical miles (19 km) from the pipeline 1, 20 nautical miles (37 km) from the temporarily plugged and abandoned well 2, and 3 nautical miles (6 km) from the well 3. In embodiments where the subsea monitoring station 4 comprises other sensors 43, for instance a temperature sensor, it may be required that the subsea monitoring station 4 is situated much closer to at least one of the subsea installations 1, 2, 3 that it is configured to monitor. Such distances may be in the range of 0 metres to 100 metres. It may also be desired to position the subsea monitoring station 4 at a -15 -distance of 0 metres to 100 metres from the subsea installations 1, 2, 3 even when hydroacoustic monitoring is used.

The subsea monitoring station 4 comprises a battery 41 that provides the necessary power to the subsea monitoring station 4 and its components. The battery 41 may store enough power to keep the subsea monitoring station 4 active for up to 10 years.

The subsea monitoring station 4 further comprises a memory 44. The memory 44 is configured to store a soundtrack comprising the sounds detected over time by the hydrophone 42. The memory 44 also stores comparative data indicative of characteristic sounds associated with the subsea conditions at which the subsea monitoring station 4 is disposed (e.g. background noise) and data indicative of characteristic sounds associated with the functioning and operating conditions of the subsea installations 1, 2, 3. For instance, the memory 44 can store comparative data relating to the frequency of the sound that is emitted when4 the temporarily plugged and abandoned well 2 leaks.

The subsea monitoring station 4 further comprises a transmitter 46 and a receiver 47.

The receiver 47 is configured to receive a signal, known as an interrogation signal (discussed in more detail below). The interrogation signal configured to be received by the receiver 47 is typically a hydro-acoustic signal. However, the receiver 47 may also (or alternatively) be configured to receive other signals such as radio or infrared frequency signals.

The interrogation signal that is configured to be received by the receiver 47 can be transmitted from any suitable transmitter, with said transmitter being attached to any suitable entity, for instance a floating of rig. However, as will be described below in relation to Figures 3 and 4, it is preferred that the receiver 47 and transmitter 46 of the subsea monitoring station 4 communicate with a transmitter 7 and receiver 9 fitted to a ship 6 passing above (or in close proximity) to the position cf the subsea monitoring station 4 on the seabed.

The transmitter 46 of the subsea monitoring station 4 is configured to transmit a signal, known as a response signal (discussed in more detail below) after receipt of the interrogation signal at the receiver 47. The response signal is a hydroacoustic signal and is configured to be detected at a receiver positioned on the same entity from which the interrogation signal emanated (e.g. receiver 9 on the ship 6 as described in more detail below with reference to Figures 3 and 4). -1,6-

A processor 45 is also comprised within the subsea monitoring station 4. The processor 45 is configured to compare sound that is received at the hydrophone 42 (soundtrack) with the comparative sound data stored in the memory*' 44. This comparison can involve the comparison of variables such as frequency, amplitude, intensity etc. of the soundtrack with the comparative data. The processor 45 is configured, by virtue of this comparison, to identify anomalous, unexpected signals within the recorded soundtrack, For example, the processor 45 may identify signals in the recorded soundtrack that have an amplitude larger (i.e. are louder) than those signals comprised in the comparative data which are indicative of the typical, background noise for the subsea location of the subsea monitoring station 4.

Equally, by virtue of this comparison, the processor 45 can identify sounds that are indicative of particular events of interest associated with one of the subsea installations 1, 2, 3. For instance, the processor 45 can identify a particular frequency in the soundtrack that is indicative of a mooring cable snapping by virtue of a comparison with the reievant signal comprised in the comparative data Equally, the processor 45 can identify a particular frequency in the soundtrack that is indicative of the temporarily plugged and abandoned well 2 leaking.

The processor 45 also processes the interrogation signals received at the receiver 47 of the subsea monitoring station 4 and once received, controls the transmitter 46 to send one of two response signals back to the entity (e.g. ship 6) that sent the initial interrogation signal, the response signal being indicative of a status of the one or more subsea installations. The selection of which of the two response signals to be sent is dependent on the comparison carried out at the processor 45, which identifies the status of the one or more subsea installations.

For instance, if the processor 45 has not identified an anomalous, unexpected sound and/or a sound indicative of particular event of interest associated with one of the subsea installations 1, 2, 3, then the processor 45 controls the transmitter 46 to transmit a first, positive (1) response signal indicating that status of the one or more subsea installations is "LW. Alternatively, if the processor 45 has identified an anomalous, unexpected sound and/or a sound indicative of a particular event of interest associated with one of the subsea installations 1, 2, 3, then the processor 45 controls the transmitter 46 to transmit a second, negative (0) response signal indicating that the status of at least one of the one or more subsea installations is "not OK". The first and second response signals that can be transmitted by the -17 -transmitter 46 make up a binary (Boolean) response signal. Such a binary (Boolean) signal is an example of a low bit signal, and thus is suitable for relatively long range hydroacoustic communication.

An identifier is sent with each response signal to identify to which subsea installation the response signal corresponds. The identifier is a reference, number, code, or letter (or plurality of letters) corresponding to the subsea installation to which the response signal corresponds and thereby allowing the subsea installation to which the response signal corresponds to be identified. A unique identifier is provided for or assigned to each subsea installation within the system. During normal operation, the subsea monitoring station 4, by virtue of the hydrophone 42, records a soundtrack that is made up of sounds emitted from each of the pipeline 1, the temporarily plugged and abandoned well 2 and the well 3 over a period of time and stores said soundtrack in its memory 44. The processor 45 in the subsea monitoring station 4 continuously monitors the incoming, recorded soundtrack and compares it with the comparative data stored in the memory. During the comparison between the soundtrack and the comparative data, if the processor 45 does not identify an anomalous, unexpected sound and/or a sound indicative of a particular event of interest associated with one or more of the pipeline 1, the temporarily plugged and abandoned well 2 or the well 3 then the processor 45 instructs the transmitter 46 to transmit a positive (1) response signal, indicating that status of the one or more subsea installations is "OK", upon receipt of the next interrogation signal at the receiver 47. Alternatively, if the processor 45 identifies an anomalous, unexpected sound and/or a sound indicative of a particular event of interest associated with one or more of the pipeline 1, the temporarily plugged and abandoned well 2 or the well 3 then the processor 45 instructs the transmitter 46 to transmit a negative (0) response signal, indicating that the status of at least one of the one or more subsea installations is "not OK" upon receipt of the next interrogation signal at the receiver 47.

The subsea monitoring station 4 as described in relation to Figures 1 and 2 forms part of a subsea monitoring system 100 as depicted in Figures 3 and 4. In addition to the subsea monitoring station 4, the subsea monitoring system 100 comprises a transmitter 7 and a receiver 9 disposed on a ship 6. The transmitter 7 and receiver 9 are configured to communicate with the receiver 47 and transmitter 46 of the subsea monitoring station as the ship 6 passes at sea level in the proximity of the subsea monitoring station 4. The transmitter 7 is controlled to -1,8-transmit the hydroacoustic, interrogation signal 8 by virtue of a processor 11 on the ship 6. The purpose of the interrogation signal 8 is to evoke a response signal 10 from the subsea monitoring station 4 when the ship 6 passes nearby. The ship's processor 11 may control the transmitter 7 to transmit the interrogation signal periodically, which is particularly beneficial when there are a plurality of subsea monitoring stations 4 on the seabed positioned below or proximate the planned shipping path. For instance, the signal may be sent out once per hour.

Alternatively, the processor may control the transmitter 7 to transmit the interrogation signal 8 when it comes into proximity of the subsea monitoring station 4, which can be determined by GPS or otherwise. The interrogation signal 8 emitted from the transmitter 7 on the ship 6 is received by the receiver 42 at the subsea monitoring station 4. Once the interrogation signal 8 has been received, the processor 45 in the subsea monitoring station 4 controls the transmitter 46 to transmit the response signal 10 back to the ship 6. It will be noted that transmitting the response signal 13 only after an interrogation signal 8 has been received is more power efficient, and thus saves on stored battery power, as compared to a system in which the response signal 10 is transmitted periodic,ally/constantly.

The response signal 10, as discussed above, will either be a positive (1) signal, indicating that the status the one or more subsea installations is "OK", or a negative (0) signal, indicating that the status of at least one of the one or more subsea installations is "not OK", and is determined based on the identification of an anomalous, unexpected sound and/or 'a sound indicative:of a particular event of interest associated with one or more of the pipeline 1, the temporarily plugged and abandoned well 2 or the well 3. If no anomalous, unexpected sound and/or no sound indicative of a particular event of interest has been identified by the processor 45 at the subsea monitoring station, then the processor 45 controls the transmitter 47 to send a positive (1) response signal 10 back to the ship 6, which is picked up at the receiver 9. The receiver 9 is connected to the processor 11, and the processor is configured to process the received response signal 10. As the response signal 10 received at the ship 6 is positive, the processor 11 on the ship 6 identifies that there are no issues with the pipeline 1, the temporarily plugged and abandoned well 2 or the well 3 and that each of *these subsea installations 1, 2, 3 are within normal, tolerable conditions -i.e. that the status of the one or more subsea installations is "OK". The positive response signal 10 received by the receiver 9 on the ship -6 also indicates that the subsea monitoring station 4 is operational (i.e. that the subsea monitoring station 4 is not faulty or damaged and that the power stored in the battery 41 has not been depleted). Therefore, the processor 11 on the ship 6, after receiving the positive response signal 10, can identify that no furtner action is required and the ship 6 can continue along its planned course.

If an anomalous, unexpected sound and/or a sound indicative of a particular event of interest has been identified by the processor 45 (e.g. the temporarily plugged and abandoned well 2 leaking), then the;processor 45 controls the transmitter 47 to send a negative (0) response signal 10 back to the ship 6, which is picked up at the receiver 9. As tne response signal 10 received at the ship 6 is negative, the processor 11 on the ship 6 identifies that a sound is being/was emitted in the region of the subsea monitoring station 4 that may indicate an issue with one of the pipeline 1, the temporarily plugged' and abandoned well 2 and/or the well 3 -i.e. that the status of at least one of the one or more subsea installations may be 'not OK". For instance, a sound caused by impact damage to one of the subsea installations 1, 2, 3 that requires repair may be responsible tor the generated negative response signal. On receipt of the negative response signal 10, the processor 11 on the ship 6 identifies that further action is required to determine the origin and reason behind the negative response signal 10. It should be noted that the negative response signal 10 also indicates to the ship 6 that the subsea monitoring station 4 is operational.

If, after sending an interrogation signal 8 to the subsea monitoring station 4, no response signal 10 is received at the receiver 9 (i.e. neither a positive nor a negative response signal 10), this indicates to the processor 11 that the subsea monitoring station 4 is not operational, and that further action is required to determine the origin:and reason behind the absence of a response signal 10.

As shown in Figure 4, such further action (either in response to a negative response signal 10 or no response signal at all) may involve the deployment of a remotely operated underwater vehicle (ROV) 12 from the ship 6 to investigate the status and/or condition of the subsea monitoring station 4, and/or the soundtrack stored in the memory 44 of the subsea monitoring station 4.

For example, the processor 11 on the ship 6 can alert the crew on the ship 6 (or an automated system) to deploy the ROV 12. The ROV 12 can travel down from the ship 6 to the location of the subsea monitoring station 4 and connect thereto via a wired connection such that the soundtrack stored in the memory 44 of -20 -the subsea monitoring station 4 can be transferred into an internal memory within the ROV 12. When subsequently brought back to the ship 6, the soundtrack can then be downloaded from the ROV 12 for further analysis to determine the origins of a negative response signal 10. For instance, upon analysis of the soundtrack, it may be found that part of the soundtrack exhibits a frequency corresponding to a signal emitted from a snapping mooring line or leaking from the temporarily plugged and abandoned well 2. Therefore, the ship 6, or another ship or vessel, can travel to the site of the temporarily plugged and abandoned well 2 in order to carry out the necessary investigations and repairs that are deemed necessary upon close inspection.

After being sent down to the subsea monitoring station 4 and downloading the soundtrack therefrom, the ROV 12 can also carry out a preliminary investigation of the subsea installations 1, 2, 3 to review any likely source in the subsea installations 1, 2, 3 of the generated negative response signal 10. For instance, the ROV 12 may undertake a preliminary investigation of the temporarily plugged and abandoned well 2 to ensure there is no leakage.

Whilst in Figure 4 it is shown that immediate further action (i.e. deployment of the ROV 12) is carried out by the ship 6 after receipt of the negative response signal 10 (or no response signal), it will be appreciated that the further action may not be immediate. For instance, the ship 6 may not have the capacity to carry out the required further action. In such a scenario, the ship's processor 11 may merely organise communication with another ship or a shore based user to inform them that a negative response signal 10 has been received from the subsea monitoring station 4 and that further action is required. The shore based user and/or the other ship can then orchestrate to have the necessary further, investigative action carried out on the subsea monitoring station 4 and the subsea installations 1, 2, 3 which it monitors.

As alluded to above, and as depicted In Figure 5, the subsea monitoring system 100 may comprise a plurality of subsea monitoring stations, for instance a first subsea monitoring station 4a and a second subsea monitoring station 4b, may be positioned on the seabed. Each subsea monitoring station 4a, 4b can monitor various, respective subsea installations and communicate with a ship much in the same way as the subsea monitoring station 4 described above in relation to Figures 3 and 4 does. As such, each subsea monitoring station 4a, 4b may comprise substantially the same components as the subsea monitoring station 4 as depicted -21 -in Figure 2. Additionally or alternatively, instead of communicating a response signal 10 with a ship 6, the first and second monitoring stations 4a, 4b can work on a relay system and relay interrogation and/or response signals to a receiver/transmitter 14 based on shore 16. In such an embodiment, the subsea monitoring system 100 may not require an equivalent to transmitter 7 and receiver 9 on ship 6. However, it is preferred that a ship 6 with a receiver 9 and a transmitter 7 is also comprised in the system 100 to provide the necessary redundancy should one or more of the subsea monitoring stations 4a, 4b fail out of operation. The receiver/transmitter 14 can then communicate the information back to a shore based user, typically via cabled and/or satellite communication, who can subsequently orchestrate the necessary further action to be taken at the relevant subsea installations dependent on the response signal that is received from either of the first and/or second monitoring stations 4a, 4h.

In instances where the subsea monitoring system 100 works on the basis of a relay system, each of the plurality of subsea monitoring stations 4a, 4b may still have substantially the same constituent components as the subsea monitoring station 4 described above. However, in order to effectuate the relay system, the transmitter of each subsea monitoring station 4a,4b can communicate with a receiver on another of the subsea monitoring stations 4a, 4b and vice versa.

In the relay system depicted in Figure 5, the first subsea monitoring station 4a is positioned at:a location further from shore 16 than the second subsea location 4b. The distance between the second subsea monitoring station 4b and the shore 16 is small enough to allow for direct hydroacoustic communication therebetween. Equally, the distance between the first subsea monitoring station 4a and the second subsea monitoring station 4b is small enough to allow for direct hydroacoustic communication therebetween. However, the distance between the first subsea monitoring station 4a and the shore 16 means that the first subsea installation 4a is, due to the limited range of hydroacoustic communication, unable to communicate directly with the receiver/transmitter 14. In this instance, the first subsea installation 4a communicates with the receiver/transmitter 14 via the second subsea monitoring station 4b.

The first subsea monitoring station 4a monitors a set of subsea installations. As described above in relation to the subsea monitoring station 4, after identification of an anomalous, unexpected sound and/or a sound indicative of a particular event 35= of interest associated with the first set of subsea installations, a processor in the -22 first subsea monitoring station its determines that a negative response signal should be emitted therefrom -indicating that the status of at least one of the one or more subsea installations is not OK". The negative response signal from the first subsea monitoring station 4a may be transmitted from a transmitter after an interrogation signal sent from the receiver/transmitter 14, relayed by the second subsea monitoring station 4b, has been received at a receiver of the first, subsea monitoring station 4a. Additionally or alternatively, the first subsea monitoring station 4a may send out its response signal periodically, or periodically under certain conditions such as when the controller in the first subsea monitoring station 4a determines that, a negative response signal is to be sent.

The response signal sent from the first subsea monitoring station 4a will comprise an identifier, such that when the response signal is eventually received at the shore based receiver/transmitter 14, it can be identified that the first subsea monitoring station 4a is the origin of the negative;response signal. This identifier will be maintained in the response signal as it is relayed through the second subsea monitoring station 4b (as well as any additional subsea monitoring stations that might be present) such that the origin of the response signal can always be identified.

The response signal from the first subsea monitoring station 4a is transmitted and subsequently received by a receiver on the second subsea monitoring station 4b. Upon receipt of the negative response signal, the second subsea monitoring station 4b is configured to relay the negative response signal to the transmitter/receiver 14, either as part of its periodically emitted signal or after the necessary interrogation is received from the transrniller/receiver 14. This thereby allows communication of the response signals emitted from the first subsea monitoring station 4a to reach the transmitter/receiver 14 despite the fact that the distance between the first subsea monitoring station 4a and the receiver/transmitter 14 falls outside the range of direct hydroacoustic communication.

Once received at the shore-based transmitter/receiver 14, the negative response signal can be communicated to a shore based user who can orchestrate the necessary investigative analysis and repair work that may be, needed to the set of subsea installations associated with the first subsea monitoring station 4a. Wh'i*.st the relay, subsea monitoring system 100 as depicted in Figure 5 has been described above in relation to only first and second subsea monitoring stations 4a, -23 - 4b, the skilled person will recognise that any number of subsea monitoring stations may be provided in a relay system to the same effect as described above.

Claims

-24-Claims 1. A subsea monitoring station for monitoring one or more subsea installations, the subsea monitoring station comprising: at least one sensor for sensing:at least one variable associated with the one or more subsea installations; a receiver configured to receive an interrogation signal; and a transmitter configured to transmit a response signal indicative of a status of the one or more subsea installations; wherein upon receipt of the interrogation signal at the receiver, the transmitter is configured to transmit the response signal indicative of the status of the one or more subsea installations.
2. The subsea monitoring station of claim 1, wherein the at least one sensor 15== comprises a hydrophone, and/or wherein the at least one variable is a sound or soundtrack comprising the sounds sensed over a period of time by the at ieast one sensor.
3. The subsea monitoring station of claim 1 or 2, further comprising a memory for storing comparative data and/or tne at least one sensed variable associated with the one or more subsea installations.
4. The subsea monitoring station of claim 3, wherein the comparative data comprises data that is indicative of characteristic variables associated with the subsea conditions at which the subsea monitoring system station is disposed and/or that is indicative of characteristic variables associated with the functioning and operating conditions of the subsea installation.
5. The subsea monitoring station of claim 3 or 4, further comprising a processor, wherein the processor is configured to compare the comparative data with the at least one sensed variable.
6. The subsea monitoring station of any of claims 3 to 5, wherein the subsea monitoring station is configured to determine the response signal to be -25'_ transmitted based on a comparison between the comparative data and the at least one sensed variable.
7. The subsea monitoring station of any preceding claim, wherein the response signal is either a first response signal or a second response signal, such that the response signal is preferably a binary signal.
8. The subsea monitoring station of claim 7, wherein the subsea monitoring station is configured to determine that the first response signal is to be transmitted when the subsea monitoring station does not identify, by virtue of a comparison, an anomalous, unexpected sianal in the at least one sensed variable and/or a signal in the at least one sensed variable indicative of a particular event of interest associated, with one or more of the one or more subsea instaliations; and/or wherein the subsea monitoring station is configured to determine that the second response:signal is to be transmitted when the subsea monitoring station identifies, by virtue of a comparison, an anomalous, unexpected signal in the at least one sensed variable and/or a signal in the at least one sensed variable indicative of a particular event of interest associated with one or more of the one or more subsea installations.
9. The subsea monitoring station of any preceding claim, wherein the response signal comprises an ioentifier that allows for the origin of the response signal to be determined.
10. A subsea monitoring system, the subsea monitoring system comprising: the subsea monitoring station of, any of claims 1 to 9: a second transmitter, the second transmitter being configured to transmit the interrogation signal; and a second receiver, the second receiver being configured to receive the response signal indicative of the status of the one or more subsea installations.
11. The subsea monitoring system of cla;m 10, further comprising a processor associated with the second transmitter and the second receiver; wherein, upon receipt of the response signal at the second receiver, the processor associated 26 -with the second transmitter and receiver is configured to determine whether further action in respect of the subsea installation is required.
12. The subsea monitoring, system of claim 11, wherein the further action comprises deployment of a remotely operated underwater vehicle to investigate the status and/or condition of the subsea monitoring station and/or to investigate the status and/or condition of the subsea installation.
13. The subsea monitoring system of any of claims 10 to 12, wherein the second transmitter and the second receiver are disposed on a vessel preferably a ship configured to pass in a region of the subsea monitoring, station
14. The subsea monitoring system of any of claims 10-13, wherein the second transmitter is configured to transmit the interrogation signal periodically
15. The subsea monitoring system of claim 13, wherein the second transmitter is configured to transmit the interrogation signal as the ship passes in the region of the subsea monitoring station.
16. The subsea monitoring system of any of claims 10 to 12, wherein the second receiver and the second transmitter are comprised in a shore based transmitter/receiver.
17. The subsea monitoring system of claim 16, further comprising a plurality of other subsea monitoring stations which are configured to relay the interrogation signal from the shore based transmitter/receiver to the receiver of the subsea monitoring station.1.
The subsea monitoring system of claim 17, wherein the plurality of other subsea monitoring stations are configured to relay the response signal transmitted4 by the transmitter on the subsea monitoring station to the shore based transmitter/receiver.-27 -
19. The subsea monitoring system of claim 17 or 18, wherein the plurality of other subsea monitoring stations are in accordance with the subsea monitoring station of any of claims 1 to 9.
20. A method of monitoring a subsea installation with a subsea monitoring station, the method comprising the steps of: sensing at the subsea monitoring station at least one variable associated with the subsea installation; receiving at the subsea monitoring station an interrogation signal; and transmitting from the subsea monitoring station a response signal incEcative of a status of the subsea installations.
21. The method of claim 20, wherein the subsea monitoring station is the subsea monitoring station of any of claims 1 to 9.
22, A method of monitoring a subsea installation with a subsea monitoring system, the subsea monitoring system comprising a subsea monitoring station, the method comprising the steps of sensing at the subsea monitoring station at least one variable associated with the subsea installation; transmitting to the subsea monitoring station an interrogation signal; receiving the interrogation signal at the subsea monitoring station; and transmitting a response signal indicative of a status of the subsea installation from the subsea sea monitoring station.
23. The method of claim 22, wherein the subsea monitoring system is the subsea monitoring system of any of claims 10 to 19