DK202070679A1

DK202070679A1 - Marine seismic acquisition with a support vessel

Info

Publication number: DK202070679A1
Application number: DKPA202070679A
Authority: DK
Inventors: GODØY Erik
Original assignee: Polarcus Dmcc; Erik Godoey
Priority date: 2014-05-13
Filing date: 2020-10-07
Publication date: 2020-10-12
Also published as: RU2673219C2; DK201600678A1; RU2016148629A3; WO2015174849A1; NO337643B1; RU2016148629A; NO20140603A1

Abstract

The present invention relates to a system for towing a tow (12) comprising seismic sensor cables (2) in completely or partly ice-covered water (8), where a towing vessel (1) tows the tow, and the tow at one side comprises a deflector (4) pulling sideways away from the tow (12), where a support vessel (7) is connected to and tows the deflector (4). The invention also relates to a method for towing a tow (12) comprising seismic sensor cables (2) in completely or partly ice-covered water (8), where a towing vessel (1) tows the tow, and the tow on one side comprises a deflector (4) pulling sideways away from the tow (12), where a support vessel (7) is connected to the deflector (4), and the deflector is towed by the support vessel.

Description

DK 2020 70679 A1 1

MARINE SEISMIC ACQUISITION WITH A SUPPORT VESSEL The present invention relates to a system for towing a tow comprising seismic sensor cables in completely or partly ice-covered water, and a method for towing a tow comprising seismic sensor cables in completely or partly ice- covered water. It is known to use sensor cables with hydrophones, accelerometers and electromagnetic sensors for retrieving geological information from the sea floor. Such investigations are normally based on reflection of acoustic energy when it approaches different geological structures of the earth’s crust. The acoustic energy it most often generated by a source such as air guns or some other form of acoustic source. The sensor cables and the acoustic source are most often towed by a towing vessel, which is also storing and optionally processing the retrieved information. If two or more sensor cables are used, these often have a mutual distance, typically of 50-200 m. In order to achieve this distance between the sensor cables, so-called deflectors or paravanes are used on each side of the tow. The deflectors have inclined wings, which by movement in the water, lead the deflectors away from the tow, and pull the sensor cables sideways apart from each other. Such deflectors are described in US 4130078, US 4729333 and US 5357892. Deflectors most often have surface pontoons regulating vertical position and draught of the deflector. When operating in completely or partly ice-covered waters such surface pontoons can normally not be used, as collision with ice may cause damage to the pontoon, but most importantly because such collisions cause a marked increase of the towing resistance, which again reduces the distance between the sensor cables. US5532975 describes a deflector with a submerged pontoon. US8593905 describes many submerged deflectors for operations in ice-covered waters. The solutions described in the patents above utilise to a large degree active guidance and control functions, which require signal transmission, power transmission and relatively complicated mechanical devices. Such solutions

DK 2020 70679 A1 2 are expensive to acquire and often implies operating problems.

US6234102 describes retrieval of seismic data in water with a towing vessel towing a number of parallel seismic sensor cables.

A deflector pulling sideways away from the tow is located furthest out on the tow and is connected to both the towing vessel and the sensor cables.

The deflector comprises connecting means for connection to a support vessel adapted to pulling the deflector in a direction parallel to the moving direction of the support vessel.

A cable connects the support vessel and the deflector for transmission of electric or hydraulic effect, in addition to control signals.

US6234102 does not mention ice-covered water.

Figure 1 shows a typical known towing of sensor cables in ice-free water 13. A towing vessel 1 tows in a towing direction 10 a tow 12 with sensor cables 2, here with a number of four, via connection cables 3, which also transmit electric power to the sensor cables 2 and transmit electrical and/or optical signals between the towing vessel 1 and the sensor cables.

A deflector 4 at each side of the tow 12 pulls the outermost sensor cables 2 outwards, so that a sideways distance is maintained between the sensor cables.

The shown deflectors 4 are towed by separate towing lines 5 from the towing vessel 1, but they can also be towed by the outermost connection cables 3. By towing sensor cables 2 in ice-covered water, the towing vessel 1 may go in the open channel behind an icebreaker, or it may be an icebreaker itself.

The deflectors 4 may be subsea deflectors, so that they avoid impact with ice.

The ice may be fractured or solid.

The sensor cables may be towed only in the open channel after the towing vessel, or they may be towed in the open channel and to the side of the open channel.

The sensor cables may be towed in the water surface, or they may be towed under water, so that they avoid impact with ice.

DK 2020 70679 A1 3 In certain situations, it will occur that the icebreaker must stop for a shorter or longer period because of the ice conditions or because of other circumstances.

Even with relatively short loss of the propulsion, it will be difficult to maintain control of depth and sideways distance between the sensor cables.

This will often result in the sensor cables getting tangled, which implies damage and lost time of operation.

The purpose of the invention is to provide a system and a method utilizing a simpler and more robust technology and thereby solving the problems above related to towing of sensor cables in completely or partly ice-covered waters where it must be expected that the towing vessel must stop for a shorter or longer period of time.

Further purposes and benefits of the invention will appear from the description.

The purposes will be achieved with the features given in the description and the claims.

In a first aspect, the invention thus relates to a system for towing a tow comprising seismic sensor cables in completely or partly ice-covered water, where a towing vessel tows the tow, and the tow at one side comprises a deflector pulling sideways away from the tow.

According to the invention, a support vessel is connected to and tows the deflector.

During a possible stop of the propulsion of the towing vessel, the deflector will thereby not drift towards the tow, but because of the connection to the support vessel, a distance is maintained from the tow.

One thereby avoids that the sensor cables and other lines and cables included in the tow drifts towards each other and get tangled.

The support vessel may be indirectly connected to the deflector, that is, connected to the deflector via lines and cables forming the tow.

However, it

DK 2020 70679 A1 4 is preferred that the support vessel is directly connected to the deflector, so that the most predictable and efficient transmission of forces from the support vessel to the deflector is achieved when towing the deflector.

It may be beneficial that the support vessel is only connected to the deflector, that is, that the support vessel is not at the same time connected to the tow in another way, as this gives a beneficial and simple control of the position of the deflector.

The towing vessel as well as the support vessel may go in the open channel behind icebreakers. However, it is preferred that the towing vessel and the support vessel are themselves ice-breaking vessels, as this is cost saving by avoiding separate icebreakers.

The tow may also at its other side comprise a deflector pulling sideways away from the tow, and a support vessel towing also this deflector, in the same way as the deflector mentioned first. This is particularly beneficial where wide tows are used.

In another aspect, the invention relates to a method for towing a tow comprising seismic sensor cables in completely or partly ice-covered water, where a towing vessel tows the tow, and the tow at one side comprises a deflector pulling sideways away from the tow. According to the invention, a support vessel is connected to the deflector, and the deflector is towed with the support vessel.

The support vessel may be connected indirectly to the deflector, that is, connected to the deflector via lines and cables forming the tow. However, it is preferred that the support vessel is connected directly to the deflector, so that a most predictable and efficient transmission of forces from the support vessel to the deflector are achieved when towing the deflector. The deflector

DK 2020 70679 A1 may also be connected to the towing vessel.

The towing vessel may thereby also contribute to towing of the deflector, which may be beneficial in situations where the support vessel must reduce its velocity, undertake evasive maneuvers or stop because of ice or other circumstances. 5 The tow may also on its other side comprise a deflector pulling sideways away from the tow, and in that case it is preferred that also this deflector connects to a support vessel, and that also this deflector is towed with the support vessel.

This is the preferred solution for operations in ice-covered waters where a larger number of sensor cables shall be towed.

The invention allows that when towing sensor cables in ice-covered waters one may essentially use already existing equipment and operational methods normally used under conventional operations in ice-free waters, which implies a gain by rationalisation.

In addition, the invention reduces the risk of the sensor cables and other equipment of the tow getting tangled if propulsion through the water is lost.

Existing patents and other known solutions for seismic operations in ice- covered waters most often describe the use of fully submerged deflectors and similar devices with wings in order to maintain sideways distance between the sensor cables.

It is known that these devices will cease to function even with relatively short loss of the propulsion through water, which most likely will lead to technical problems and operating down time, as described above.

The invention will, to a large degree, eliminate these problems by that the deflectors at any time may be maneuvered to the desired position by means of the support vessel.

The invention will now be further described with reference to the accompanying drawings, where:

DK 2020 70679 A1 6 fig. 1 shows towing of a tow comprising seismic sensor cables in completely or partly ice-covered water according to prior art; fig. 2 shows towing of a tow comprising seismic sensor cables in completely or partly ice-covered water according to a first embodiment of the invention; and fig. 3 shows towing of a tow comprising seismic sensor cables in completely or partly ice-covered water according to a second embodiment of the invention.

Fig. 2 shows an ice-breaking towing vessel 1 towing a tow 12 with four sensor cables 2 in a towing direction 10 in partly ice-covered water 8. Connection cables 3 transmit electric effect and electrical and/or optical signals between the towing vessel 1 and the sensor cables 2. A deflector 4 is connected to one side of the tow 12 with a transversal line 14. Relative movement between the deflector 4 and the water 8 implies that inclined surfaces of the deflector pull the deflector sideways outwards, away from the tow 12. The deflector 4 thereby pulls, via the transversal line 14, the outermost sensor cable 2 sideways outwards, so that a sideways distance is maintained between the sensor cables. An ice-breaking support vessel 7 is connected to and tows the deflector 4 in a towing direction 11 via a towing line 6. The deflector 4 is thereby towed in the open channel formed behind the support vessel 7. Accumulations of ice 9 may complicate the towing because the towing vessel 1 as well as the support vessel 7 may have to reduce its velocity or stop. The relative movement between the deflector 4 and the water 8 will then be reduced or cease, and the deflector 4 will no longer pull the outermost sensor cable 2 sideways outwards. However, the towing line 6 between the support vessel 7 and the deflector 4 secures that the deflector is held approximately in its position in relation to the tow 12, which prevents that the sensor cables 2, the connection cables 3 and optional other lines and cables of the tow are drifting towards each other and get tangled.

DK 2020 70679 A1 7 The towing vessel 1 and the support vessel 7 primarily tows in parallel directions, that is, the towing directions 10, 11 are substantially parallel. This gives a simple control with predictable movement of the deflector 4.

As an alternative to reducing velocity or stop, the towing vessel 1 as well as the support vessel 7 may sometimes undertake evasive maneuvers in order to avoid impact with accumulations of ice. It has been found that the invention is beneficial also in these cases, since it is possible to guide the deflector 4 in a far better way than when it is not connected to a support vessel. The support vessel 7 is directly connected to the deflector 4 via the towing line 6. This ensures that the support vessel 7 may exert a large degree of control of the position of the deflector 4. It is seen that the towing vessel 1 is also connected to the deflector 4, but that this is an indirect connection via the connection cables 3 and the transversal line 14. The towing vessel 1 may thereby also exert a certain control of the movement of the deflector 4, but to a lesser degree than the support vessel 7. Optionally the towing vessel 1 may also be directly connected to the deflector 4, in the same way as for the prior art shown in fig. 1, where the towing vessel 1 is connected to the deflector 4 with a towing line 5. It is also possible that the support vessel 7 is indirectly connected to the deflector 4, that is, the support vessel 7 is connected to the tow, and indirectly connected to the deflector 4 via the transversal line 14. The deflector 4 may thereby be connected to the towing vessel 1 and the support vessel 7 via a combination of direct and indirect connections, so that the desired control of the position of the deflector may be maintained in case the towing vessel 1 or the support vessel 7 has to reduce its velocity or stop. However, it is found that it may be a benefit that the support vessel 7 is only connected to the deflector 4, that is, that the support vessel is not at the same time connected to the tow 12 in another way, as this gives a beneficial and

DK 2020 70679 A1 8 simple control of the position of the deflector 4. For the same reason, and also with consideration of the best possible distribution and utilisation of the towing capacity of the towing vessel and the support vessel, it is preferred that the largest part of the force for propulsion of the deflector 4 comes from the support vessel 7, and preferentially that all the force for propulsion of the deflector comes from the support vessel.

The shown deflector is a surface deflector.

It is preferentially of a robust type of reliable materials and with a reliable design enabling it to withstand impact with ice.

Figure 2 shows a preferred solution for operations in ice-covered waters where a smaller number of sensor cables shall be towed.

Then, as shown, it is utilised an asymmetrical towing arrangement with only one deflector which is towed by an ice-breaking support vessel.

With this asymmetrical solution, only one support vessel is required.

Figure 3 shows the invention with a symmetrical towing arrangement where it is used a deflector 4 and a support vessel 7 at each side of the tow 12. This is preferred for operations in ice-covered waters where a larger number of sensor cables shall be towed.

The deflectors 4 at each side of the tow 12 pulls the outermost sensor cable 2 at each side of the tow 12 sideways outwards in the same way as explained with reference to fig. 2, so that it is maintained a sideways distance between the sensor cables 2. The support vessels 7 are each connected to and tows one deflector 4 in towing directions 11 via towing lines 6, in the same way as explained with reference to fig. 2. The towing arrangement of fig. 3 is particularly beneficial when retrieving 3D geophysical data, where a towing vessel tows many, typically 4-20 sensor cables.

The sensor cables 2 are towed in symmetry around the extension of the centre line of the towing vessel 1, with the same number of sensor cables

DK 2020 70679 A1 9 at each side, in the same way as shown in Figure 1. At each side of the tow it is used a deflector pulling the sensor cables sideways outwards to a pre- set mutual distance, typically 50-200 m.

Claims

DK 2020 70679 A1 1 Claims

1. A system for towing a tow (12) comprising seismic sensor cables (2) in completely or partly ice-covered water (8), where a towing vessel (1) tows the tow, and the tow at one side comprises a deflector (4) pulling sideways away from the tow (12), characterised in that a support vessel (7) is directly connected to and tows the deflector (4).

2. The system according to claim 1, wherein the support vessel (7) is only connected to the deflector (4).

3. The system according to any of the previous claims, wherein the deflector (4) is also connected to the towing vessel (1).

4. The system according to any of the previous claims, wherein the deflector (4) is manufactured to withstand impact with ice (9).

5. The system according to any of the previous claims, wherein the largest part of the force for propulsion of the deflector (4) comes from the support vessel (7).

6. The system according to any of the claims 1-5, wherein all the force for forward movement of the deflector (4) comes from the support vessel (7).

7. The system according to any of the previous claims, wherein the tow also on the other side comprises a deflector (4) pulling sideways away from the tow (12), and a support vessel (7) is also towing this deflector.

8. A method for towing a tow (12) comprising seismic sensor cables (2)

DK 2020 70679 A1 2 in completely or partly ice-covered water (8), where a towing vessel (1) tows the tow, and the tow on one side comprises a deflector (4) pulling sideways away from the tow (12), characterised by connecting a support vessel (7) directly to the deflector (4), and towing the deflector with the support vessel (7).

9. The method according to claim 9, wherein the deflector (4) is also connected to the support vessel (1).

10. The method according to claim 9 or 10, wherein the support vessel (7) is an ice-breaking vessel, and the deflector (4) is towed in an open channel behind the support vessel.

11. The method according to any of the claims 9 — 11, wherein the towing vessel (1) and the support vessel (7) are towing mainly in parallel directions (10, 11).

12. The method according to any of the claims 9 — 12, wherein the largest part of the force for forward movement of the deflector (4) comes from the support vessel (7).

13. The method according to any of the claims 9 — 12, wherein all the force for forward movement of the deflector (4) comes from the support vessel (7).

14. The method according to any of the claims 9 — 14, wherein, since the tow (12) also at the other side comprises a deflector (4) pulling sideways away from the tow (12), also this deflector (4) is connected to a support vessel (7), and the deflector is towed with the support vessel.