NO20150754A1 - System for deploying an instrument at a seafloor - Google Patents

Abstract

A system (100) for deploying an instrument (200) at a seafloor (2) comprises a positioning tool (110) configured to move the system (100) through a body of water above the seafloor (2). The system (100) further comprises an instrument unit (120) with a protective housing (120; 123, 124) around the instrument (200) and associated equipment (210). A base unit (130) provides a stable platform for the instrument (200). First (115) and second (125) couplings Connect the positioning tool (110), the instrument unit (120) and the base unit (130). The couplings (115, 125) may be releasable, e.g. for removing the positioning tool (110) during measurements. The positioning tool (110) is purpose built for the system (100) and controlled from any surface vessel (10) with a ship's crane (11) capable of lifting the system (100). This eliminates the need for a specialized and expensive survey vessel, an ROV, large lifting equipment etc.

Description

BACKGROUND

Field of the invention

[01] The present invention concerns a system for deploying an instrument at a seafloor.

Prior and related art

[02] Generally, exploration and monitoring of underground formations involves 2D, 3D and 4D-measurements. Examples include seismic, magnetic and gravimetric surveys, monitoring subsidence or formation of cracks on an oil or gas field, etc. Marine exploration and monitoring offshore reservoirs may involve deploying sensors and instruments on a seafloor. Such sensors may involve hydrophones and geophones capable of detecting P-waves and S-waves from seismic events and various other instruments to provide additional information.

[03] Some sensors are relatively inexpensive, and are conveniently deployed as nodes connected to a rope or cable. Advances in technology, especially in MEMS-technology, low power electronics and battery technology, enable robust autonomous nodes to be deployed and remain on the seafloor for some time, e.g. during a seismic survey. This reduces the cost of the survey significantly. Of course, other instrument packages benefit from the same advantages. After deployment, nodes and instruments may be retrieved for uploading data, recharging, maintenance and calibration as required. As known in the art, cables providing power and communication are still preferred in many applications, e.g. in permanent reservoir monitoring.

[04] Some measurements involve instruments that are too sensitive and/or expensive to be deployed in large numbers on a field. One example is a gravimeter used to measure small changes in Earth's gravity due to density changes in an underground reservoir. Such measurements may require accuracy in the order of 10-50 ng, i.e. 10"<8>x9.8m/s<2>, or less.

[05] Historically, cost and complexity have limited gravimetric techniques. For example, in 2006 a gravimeter with sensitivity in the order of ng/-v/Hz came with a price tag of 100 000 - 500 000 USD, and took about a year to build. Deploying several such instruments was therefore too expensive for many practical purposes, including monitoring all but the most valuable reservoirs. Relatively high power consumption and limited battery capacity added to the cost, as the devices needed power supply from the surface for extended deployment.

[06] Traditionally, general purpose remotely operated vehicles, RO Vs, have been used to deploy various equipment, including instruments, on the seafloor. However, RO Vs are designed for a wide variety of tasks, and typically have some functions that are superfluous and other functions that are inadequate for a particular application. For example, a robot arm capable of handling a variety of objects may not be required in simple applications. On the other hand, positioning provided by general purpose navigation means and visual input from a camera may be too inaccurate for some applications. In addition, using an ROV may involve hiring a surface vessel with appropriate equipment and trained personal, which adds operational cost.

[07] An instrument package deployed at or on a seafloor is exposed to pressure and may be disturbed by strong currents that may influence some measurements. For example, the accuracy and data quality of gravimetric measurements may be affected by strong water currents that induces vibrational noise, moves sand and other loose material and/or reduces visibility such that exact positioning becomes difficult.

[08] The objective of the present invention is to provide a system that solves, or at least reduces, some or all of the problems above while retaining the benefits of prior art.

SUMMARY OF THE INVENTION

[09] This is achieved by a system according to claim 1 and a method for using the system according to claim 10.

[010] In a first aspect, the invention concerns a system for deploying an instrument at a seafloor. The system comprises a positioning tool configured to move the system through a body of water above the seafloor; an instrument unit comprising a protective housing containing the instrument with its associated equipment, and a base unit configured to provide a stable platform for the instrument. The system further comprises a first coupling between the positioning tool and the instrument unit, and a second coupling between the instrument unit and the base unit. The positioning tool is purpose built for the system and controlled from a surface vessel.

[011] The positioning base unit and protective housing minimises the effect of water currents and other disturbances on the instrument and its associated equipment. This equipment typically includes electronics, data storage and power supply, but may also include orientation sensors and other devices, all of which are adapted to the instrument. The purpose built positioning tool has enough power and functionality to move the system at a desired speed, but lacks many of the functions and/or power required for a general purpose vessel, e.g. an ROV or a tool designed for heavier loads.

[012] In some embodiments, the first coupling is releasable for releasing and removing the positioning tool from the instrument unit during measurements. The second coupling may still be fixed, e.g. for exploration.

[013] In some applications, e.g. field monitoring, the second coupling is releasable such that the instrument may be moved between several permanently deployed base units if desired. In addition, different base units for different types of seafloor, e.g. one base unit for solid rock and another for loose sand, may be coupled to one instrument unit at different times. The second coupling may optionally be releasable by remote control such that the instrument unit can be released and retrieved independent of the base unit.

[014] In some embodiments, the instrument unit comprises an instrument frame with the first and second couplings and brackets for attaching at least one protective housing. The instrument frame facilitates deployment of different protective housings, for example, a protective housing for a seismometer with associated electronics, data storage and power supply may have a different shape and size than a protective housing for a gravimeter with different electronics, data storage and power supply. Both housings may be mounted in the same instrument frame at different times or at the same time by means of the brackets.

[015] In embodiments with an instrument frame, the brackets may be releasable. Manually releasable brackets facilitates removing and attaching protective housings. The brackets may optionally be releasable by remote control such that the protective housing can be released and retrieved independent of the instrument frame.

[016] Typically, the protective housings are mounted in the instrument frame and/or between the positioning tool and the base unit, on the deck of a vessel. Accordingly, the protective housings are preferably provided with lifting ears and/or other devices to facilitate handing on a boat deck at sea.

[017] In preferred embodiments, the positioning tool is connected to a buoyancy system. This typically comprises ballast tanks to control the vertical position in the body of water, and decouples the positioning tool from motions on the sea surface. In turn, this reduces or eliminates the need for dynamic heave compensation on a surface vessel. The base unit may be set firmly on the seafloor by releasing the positioning tool with the buoyancy element.

[018] In some embodiments, the protective housing comprises internal releasing means configured to support the instrument during transport, and to decouple the instrument from the protective housing during measurement. For example, the instrument within the protective housing may be set directly on the stable base unit during measurements, so that no vibrational noise is transmitted to the instrument through the protective housing.

[019] The base unit may comprise a large mass for providing stability. This embodiment is particularly suited for a firm or solid seafloor.

[020] Alternatively or in addition, the base unit may comprise a suction caisson for providing stability. A suction caisson is particularly suited for a loose seafloor.

[021] For failsafe recovery, the system may comprise a receiver for receiving a signal from the surface and a control unit configured to release an inflatable buoy upon reception of the signal. The optional inflatable buoy with its associated receiver and control circuit and, by necessity, a gas flask and actuators, is typically installed in an instrument frame or protective housing in order to retrieve the instrument if other parts of the system fails. However, such a buoy and associated equipment may also be installed in the positioning tool for similar reasons.

[022] In a second aspect, the invention concerns a method for using the system summarised above. The method comprises the steps of: connecting the instrument unit to the base unit; releasing the positioning tool from the instrument unit; acquiring measurement data using the instrument; and connecting the positioning tool to the protective housing.

[023] In accordance with the description of the system, connecting the instrument unit to the base unit and the positioning tool may involve attaching the protective housing to an instrument frame and attaching the instrument frame to the positioning tool and base unit.

[024] The steps of the method encompass an embodiment wherein the base unit is moved along with the protective housing, e.g. during exploration, and embodiments where one or more base units is/are permanently deployed on the seafloor, e.g. for the lifetime of an oilfield or for a long time compared with a measurement period.

[025] The latter case, e.g. performing a time-lapse series at a fixed location or moving the instrument between several permanently deployed base units require the additional step of releasing the protective housing from the base unit. If the protective housing is mounted in an instrument frame, this step involves releasing the instrument frame.

[026] Regardless of whether the base unit is moved along with the protective housing or permanently installed, the instrument may be released from the protective housing, e.g. to be placed in direct and firm contact with the base unit for precise measurements.

[027] The steps above, including the required step of releasing the instrument frame from the base unit, may be repeated according to a round-robin scheme in a time-lapse series.

[028] Further features and benefits will become clear from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[029] The invention will be described in greater detail by means of examples with reference to the accompanying drawings, in which:

Figure 1 illustrates a system according to the invention,

Figure 2 illustrates an alternative embodiment of the system,

Figure 3 shows a detail from figure 2,

Figure 4 illustrates locating a base unit,

Figure 5 illustrates cleaning a base unit and

Figure 6 illustrates failsafe recovery.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[030] The drawings are schematic and intended to illustrate the principles of the invention. Hence, they are not necessarily to scale, and numerous details known to the skilled person are omitted for clarity.

[031] Figure 1 shows a system 100 controlled from a surface vessel 10 floating on the surface 1 of a body of water.

[032] The surface vessel 10 is typically a small general purpose vessel with a ship's crane 11 capable of lifting the system 100. There is no need for systems or specialists, e.g. for supporting an ROV or a diver. A drum 12 for an umbilical 15 can be temporarily mounted on a deck of the surface vessel 10. Other support systems, e.g. a control system for the system 100, can also be temporarily provided aboard the surface vessel 10.

[033] The system 100 comprises a positioning tool 110, a releasable instrument unit 120 and a base unit 130, and is intended for use in exploration and/or as a part of a permanent monitoring system.

[034] The instrument unit 120 comprises an instrument 200 to be deployed, typically an accurate and/or expensive geophysical instrument such as a seismometer or a gravimeter, and equipment 210 associated with the instrument, for example electronics, data storage and power supply adapted to the instrument 200. In this embodiment, the instrument unit 120 is just a cylindrical protective housing with couplings 115 and 125.

[035] The positioning tool 110 is purpose built, i.e. specialised for handling the instrument unit 120 and the base unit 130. Thus, the positioning tool 110 is provided with a minimum of functionality required to put the instrument unit 120 and/or the base unit 130 in a desired position at the seafloor 2. Releasable and preferably standardised couplings 115 connect the positioning tool 110 to the instrument unit 120 during transport. Other embodiments may comprise fixed couplings 115. As shown in Fig. 1, the positioning tool 110 can be disengaged and moved away from the instrument unit 120 during measurement periods. Thus, the positioning tool 110 and a releasable coupling 115 replace a robotic arm and a carrier frame associated with a general purpose ROV.

[036] An umbilical 15 connects the positioning tool 110 to the surface vessel 10, and provides power and/or communication. A steel wire or synthetic rope for lifting the system 100 may also be included in the umbilical 15, and water currents in different directions at different depths are likely to affect the umbilical. As known in the art, buoyancy elements and/or fairings may be attached to the umbilical to compensate for the associated forces.

[037] In principle, the system 100 may be maintained at a predetermined depth by a dynamic heave compensation system associated with the ship's crane 11 and/or the drum 12. Such heave compensation systems are commercially available, and may comprise a dynamic element inserted into a lifting line between the crane 11 and the load. However, the weight of a steel rope, the stretching of a synthetic rope and/or forces applied by underwater currents quickly make heave compensation less useful as the depth increases.

[038] Instead, the positioning tool 110 may be provided with a buoyancy system to control the vertical position of the system 100 in the body of water. Thereby, the system 100 becomes essentially decoupled from the surface vessel's vertical motion on the surface 1 due to waves and swell. Such buoyancy systems typically include permanent buoyancy elements and ballast tanks with associated equipment. Suitable buoyancy systems are commercially available, and therefore not described further herein.

[039] Partial autonomous operation of the positioning tool 110 may be considered in some applications, e.g. in permanent monitoring systems with a guidance system for autonomous vehicles.

[040] The thrusters 112 and 114 in figures 1 and 2 represent motors and actuators required for propulsion and steering. Normally, hydrodynamic forces are small due to low speeds during positioning, so there is litrJe or no need for streamlining, and control surfaces with fins are of limited value.

[041] Figure 2 shows an alternative embodiment, where protective housings 123 and 124 are mounted in an instrument frame 121 provided with brackets 122 for attaching one or more protective housings. Each protective housing 123 and 124, as well as the cylindrical instrument unit 120 in figure 1, protects instruments and equipment within from pressure, currents and other factors in the subsea environment that may affect the measurements. The brackets 122 and protective housings 123, 124 are preferably standardised to facilitate change of protective housings 123, 124, and thereby instrumentation, for different applications.

[042] As illustrated by Fig. 2, the releasable coupling 115 between the positioning tool 110 and the instrument unit 120 may be placed physically on the instrument frame 121. Similarly, the releasable coupling 125 between the instrument unit 120 and the base unit 130 may be placed on the instrument frame 121.

[043] Embodiments wherein different instruments 200 are inserted into a protective housing 123 at different times are anticipated. For example, a protective housing 123 may contain a seismometer in one application and a gravimeter in another regardless of whether the protective housing 123 is coupled directly to the positioning tool 110 and the base unit 130 as shown in Fig. 1 or if an instrument frame 121 is provided as in Fig. 2. If different instruments 200 are inserted into the protective housing 123 at different times, the protective housing 123 may be permanently attached to an instrument frame 121, i.e. so that the brackets 122 need not be releasable.

[044] The main function of the base unit 130 is to provide a stable platform, e.g. for sensitive instruments.

[045] In some applications, e.g. permanent monitoring of a reservoir, there is a need or desire to perform measurements at the same position at different times. In such applications, the base unit 130 is preferably left permanently in the required position, and the instrument frame is attached to the base unit 130 at exactly the same spot and preferably with the exact same orientation in every measurement during a time-lapse series. For this, the second coupling 125 provides firm attachment, and optionally a predetermined orientation of the base unit 130 relative to instrument unit 120.

[046] In some applications, e.g. exploration, the base unit 130 need not be releasable. Hence, a releasable coupling between the instrument unit 120 and the base unit 130 is optional, such that the base unit 130 may be permanently attached in some applications.

[047] In either type of application, the base unit 130 may simply comprise a large mass capable of providing the required stability. In this case, materials with high density may be preferred to minimise volume, and thereby the effects of strong water currents at the seafloor. Alternatively, the base unit 130 may comprise a suction caisson, which may provide a stable measurement platform in loose materials. In either case, it is important that the base unit 130 does not sink into the seafloor, and that it remains stable over time.

[048] Figure 3 shows the protective housing 123 from figure 2 partly cut through. The protective housing 121 is shown as a cylinder, but may have any suitable shape. The protective housing 123 is firmly connected to the stable base unit 130 through the bracket 122 and the instrument frame 121 (Fig, 2). This reduces influence of water currents on the seafloor to a minimum. Still, vibrational noise transmitted through the protective housing 123 may affect some measurements, e.g. the gravimetric measurements of the previous example. This may be overcome by decoupling an instrument 200 from the protective housing 123, e.g. by placing the instrument 200 with a gravimeter or some other sensitive instrument directly on the base unit 130 as shown in figure 3.

[049] In figure 3, releasing means 150 configured to support the instrument 200 during transport, and to decouple the instrument 200 from the protective housing 123 during measurement, are represented by grippers. This embodiment of the releasing means 150 can be pivoted between a holding position, where they support the instrument 200 during transport, and the released position illustrated in figure 3, where the instrument 200 is in firm contact with the stable base unit 130, e.g. during measurements.

[050] Figure 4 illustrates locating a base unit 130. Due to local conditions, e.g. strong local currents and a sandy seafloor around the base unit 130, the base unit 130 and/or its associated coupling means 125 may be partly or fully covered.

[051] In some applications, one or more rods 132 may mark the site. However, such rods 132 are impractical to handle on a boat deck, e.g. when lifted by the crane 11, and on the seafloor 2, e.g. when landing the instrument unit 120 on the base unit 130.

[052] Thus, in most instances, a subsea vessel 400 emitting some signal 401 to locate the base unit 130 will be preferred. The subsea vessel 400 can be the positioning tool 110 or a completely different device provided with equipment suitable for the application at hand. Equipment based on acoustic or electro-magnetic signals 401 for locating buried objects are generally known, for example, acoustic locators and ultrasonic tranceivers are non-limiting acoustic devices that may be used. A geo-radar, an RFID-transceiver and a Hall-detector are non-limiting examples of electro-magnetic devices that may be used.

[053] A signal 133 emitted by the base unit 130 illustrates that the base unit may comprise similar locating means, e.g. a radio beacon, a transponder, an active or passive RFID-tag etc. Locating the base unit 130 as such is not part of the invention, and any suitable present and future method or means for locating the base unit 130 can be used with the invention.

[054] Figure 5 illustrates cleaning of the base unit 130 by a cleaning tool 500. The base unit 130 may be treated, e.g. with a coating to prevent fouling in shallow waters as known in the art. The cleaning tool 500 shown in figure 5 is a bottom vessel, e.g. a modified trencher, with a shovel 501 to remove most of the sand or debris, a rotary brush 502 to remove more sand and a water jet 503 to flush any remaining deposits from the coupling means 125 and the surrounding area. The shovel 501, brush 502 and water jet 503 are optional. A real embodiment may comprise one or more of these elements and other known means, e.g. for suction and/or digging.

[055] A continuous track 510 provides a large ground area and good traction on a soft seafloor 2, and is therefore used for propulsion in this example. Alternatively, the cleaning tool 500 may be a subsea vessel similar to the subsea vessel 400, for instance a positioning tool 110 equipped with a motorised brush 502 and pumps and nozzles for the water jet 503. Cleaning the base unit 130 as such is not part of the invention, and any suitable present and future method or means for cleaning the base unit 130 can be used with the invention.

[056] If the positioning tool 110 cannot be used for some reason, there is a need for failsafe recovery to retrieve the expensive instrument within the instrument unit 120. Failsafe recovery usually involves retrieving the protective housing 123 to prevent damage to the instrument, and optionally involves the base unit 130.

[057] In such cases, a secondary subsea vessel, e.g. an ROV, or in shallow waters even a diver, may be sent down to attach a lifting cable, e.g. to a lifting ear on the instrument unit 120, so that a suitable surface vessel may retrieve the instrument unit 120 with the instrument.

[058] However, as discussed above, vessels with equipment and personnel to operate or support an ROV or a diver are expensive and may not be readily available, so preferably any surface vessel 10 with sufficient crane capacity should be able to retrieve the instrument.

[059] Figure 6 illustrates failsafe recovery without a need for a secondary subsea vessel or a diver. The instrument unit 120 to be retrieved may comprise an instrument frame 121 as in figure 2. There may be a desire for failsafe recovery of the positioning tool 110 for similar reasons, so the claims specify that the system 100, i.e. not necessarily the instrument unit 120, may comprise an inflatable buoy 600.

[060] A signal 602 from a surface vessel triggers the failsafe recovery. Upon receiving the signal 602, a control circuit releases the inflatable buoy 600. The signal 602 could be any signal capable of propagating from the surface to the seafloor, e.g. electromagnetic signals. However, acoustic transmitters and receivers and their associated control units are used for so-called acoustic release devices used for subsea seismic surveying. As the associated devices are commercially available and well proven for a wide variety of depths, an acoustic signal 602 may be preferred in many applications.

[061] In some embodiments, the buoy 600 provides sufficient buoyancy to lift the instrument unit 120 and/or the base unit 130 to the surface, where it can be retrieved by the surface vessel.

[062] Alternatively, the buoy 600 may be just large enough to lift a pilot line 601, e.g. made of synthetic fibres with density similar to that of water so that the line 601 has a buoyancy nearly equal to its weight. Thereby, a long reel ed up line 601 does not add significant buoyancy during normal operation, and it does not add significant weight as the length of line 601 suspended from the buoy 600 increases during failsafe recovery. Once the buoy 600 is retrieved, the pilot line 601 is used to guide a lifting rope to the instrument unit 120. Similar to the pilot line 601, the lifting rope is preferably a synthetic rope that is nearly neutral in water, this time to avoid excessive loads from e.g. a steel wire suspended from the surface vessel 10. A funnel shaped guide at the instrument unit 120 may suffice to guide a lifting hook to a lifting ear on the instrument unit 120. Alternatively, the technical field of fishery may be searched for methods and devices for retrieving hooks stuck to the bottom, and scale up an associated device for use with the present invention.

[063] The skilled person will recognize that there is a point above which the cost associated with a reel ed up pilot line 601 exceeds the cost associated with a larger buoy 600 for lifting the instrument unit 120, and hence one of the above methods will be more costly than the other dependent on the application at hand.

[064] An third alternative involves releasing a relatively small buoy 600 with a relatively short line 601 and pick up the buoy 600 and/or line 601 with a suitable device towed at, i.e. on or somewhat above, the seafloor 2. The skilled person will be able to configure the control unit, which is required to release the buoy 600, to release the instrument unit 120 from the base unit 130 or the protective housing 123 from the instrument frame 121. Thus, trawling or dragging may be a feasible alternative to the above methods for failsafe recovery.

[065] In some applications, it may be possible to pull the instrument unit 120 and/or the base unit 130 directly off the base unit 130 or seafloor 2, respectively, i.e. without using a buoy 600 and associated line 601. However, such direct pulling involves large forces that are potentially harmful to the instrument(s), and is hence unsuitable for many applications.

[066] Any suitable method may be used for failsafe recovery. For example, long or telescopic members for attaching a lifting rope to an instrument unit 120 may be used in shallow waters. However, such members quickly become unsuitable in deeper waters due to problems of steering them to the instrument unit 120, their weight and/or the added cost of buoyancy elements.

[067] Thus, all three preferred methods of failsafe recovery involves releasing an inflatable buoy 600, optionally attached to a line 601, and any surface vessel with a crane or winch capable of lifting the required equipment is suitable for use in these methods.

[068] While the invention has been described with reference to specific embodiments, numerous modifications and alternatives are obvious to one skilled in the art. Thus, the scope of the present invention is defined by the following claims.

Claims (14)

1. A system (100) for deploying an instrument (200) at a seafloor (2), comprising: a positioning tool (110) configured to move the system (100) through a body of water above the seafloor (2); an instrument unit (120) comprising a protective housing (120; 123, 124) containing the instrument (200) with its associated equipment (210); a base unit (130) configured to provide a stable platform for the instrument (200); a first coupling (115) between the positioning tool (110) and the instrument unit (120); and a second coupling (125) between the instrument unit (120) and the base unit (130),characterised in thatthe positioning tool (110) is purpose built for the system (100) and controlled from a surface vessel.

2. The system according to claim 1, wherein the first coupling (115) is releasable.

3. The system according to claim 1 or 2, wherein the second coupling (125) is releasable.

4. The system according to any preceding claim, wherein the instrument unit (120) comprises an instrument frame (121) with the first and second couplings (115, 125) and brackets (122) for attaching at least one protective housing (123, 124).

5. The system according to claim 4, wherein the brackets (122) are releasable.

6. The system according to any preceding claim, wherein the positioning tool (110) is connected (111) to a buoyancy system.

7. The system according to any preceding claim, wherein the protective housing (120; 123, 124) comprises internal releasing means (150) configured to support the instrument (200) during transport, and to decouple the instrument (200) from the protective housing (123) during measurement.

8. The system according to any preceding claim, wherein the base unit (130) comprises a large mass for providing stability.

9. The system according to any preceding claim, wherein the base unit (130) comprises a suction caisson for providing stability.

10. The system according to any preceding claim, further comprising a receiver for receiving a signal (602) from the surface (1) and a control unit configured to release an inflatable buoy (600) upon reception of the signal (602).

11. A method for using the system according to any preceding claim, comprising the steps of: connecting the instrument unit (120) to the base unit (130); releasing the positioning tool (110) from the instrument unit (120); acquiring measurement data using the instrument (200); and connecting the positioning tool (110) to the instrument unit (120).

12. The method according to claim 11, further comprising the step of releasing the instrument unit (120) from the base unit (130).

13. The method according to claim 11 or 12, further comprising the step of releasing the instrument (200) from the protective housing (123).

14. The method according to claim 12 or 13, further comprising repeating the steps according to a round-robin scheme in a time-lapse series.