WO2012041844A1

WO2012041844A1 - Marine seismic surveying assembly and method

Info

Publication number: WO2012041844A1
Application number: PCT/EP2011/066757
Authority: WO
Inventors: Robert Van Es; John Wei-Khin Voon
Original assignee: Shell Internationale Research Maatschappij B.V.
Priority date: 2010-09-28
Filing date: 2011-09-27
Publication date: 2012-04-05

Abstract

A remotely operated seismic source assembly comprising an Autonomous Submersible Vessel comprising a remotely controlled navigation system, a refueling probe, a combustion engine for propelling the vessel through a body of water, a compressed air tank and a snorkel assembly for providing air to the combustion engine and the compressed air tank during at least part of a period when at least a substantial part of the vessel floats at a distance between 0 and 20 m below the water surface; and a substantially neutrally buoyant airgun array which is connected to the vessel and the compressed air tank by a towing line.

Description

MARINE SEISMIC SURVEYING ASSEMBLY AND METHOD

BACKGROUND OF THE INVENTION

The invention relates to a marine seismic surveying assembly and method.

Such an assembly and method is known from International patent application WO 02/25315.

In the known method a plurality of seismic sources, such as airguns, are towed by a plurality of towing vessels, which may also tow a set of seismic sensors, contained in

streamers that comprise a string seismic signal receivers, such as hydrophones, which are sensitive to reflections and refractions from sound impulses emitted by the seismic sources .

The known method utilizes a multiple sampling technique which is known as the Common Mid-Point (CMP) method wherein the addition of multiple signals from the same sub-surface reflection point results in an increase of the information signal with a cancellation of the noise.

The known method utilizes one or more secondary towing vessels which may be manned, unmanned and tethered, or not, to a host vessel that all follow a similar path and with equal cruising speed.

International patent application WO2010/136142 discloses a system for generating pressure waves in an underwater environments using swarms of autonomous submarine vessels that are propelled by battry powered electric engines and that comprise compressed air which is cyclically transmitted to striker pistons, which generate acoustic vibrations in the hull of the vessels.

It will be understood that the acoustic vibrations in the hull of the submarine vessels generate acoustic pulses that have a lower acoustic bandwidth and strength than acoustic pulses generated by airgun assemblies which transmit air pulses into the water.

UK patents 2460073 and 2425597 disclose other marine seismic surveying methods using submarine vessels that are

configured to operate at large waterdepths .

There is a need for an improved seismic surveying

assembly and method with improved secondary towing vessels comprising dedicated hull structures to be semi submersible and thus less dependent to weather and season, that can operate in different and variable directions, draught and cruising speed with respect to the other secondary towing vessels or host vessels and may tow a single or multiple naturally buoyant strings comprising air guns or alternative sources at various depths amongst each other or to the other towing vessels.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a remotely operated marine seismic source assembly comprising:

- an autonomous submersible vessel comprising a remotely controlled navigation system, a refueling probe, a

combustion engine for propelling the vessel through a body of water, a compressed air tank and a snorkel assembly for providing air to the combustion engine and the compressed air tank during at least part of a period when at least a substantial part of the vessel floats at a distance between

0 and 20 m below the water surface; and

- a substantially neutrally buoyant airgun array which is connected to the vessel and the compressed air tank by a towing line.

The vessel may comprise a torpedo-shaped hull which is configured to float at a distance between 1 and 10 m below the water surface and the airgun array may comprise a depth control and stabilizer assembly which is configured to maintain the airgun array during normal use thereof at a distance between 0 and 50 m below the water surface, while the airgun array is induced to transmit a sequence of seismic shots.

The vessel may comprise a remotely controlled navigation system and collision avoidance and mammal detection

equipment and may be configured to induce the airgun array to regularly transmit seismic shots when the vessel floats at a distance between 0 and 5 m below the water surface, whilst the vessel may temporarily dive during a period of up to 10 minutes to a larger waterdepth to avoid floating obstacles like logs or ice. For refueling and air inlet at sea the snorkel may be mounted on a probe at a tower

construction on the vessel that reaches till above the water surface and which may be closed water tight. Air inlet is required for providing air to at least one combustion engine that is needed for vessel propulsion and for generating compressed air required for the airguns of the naturally buoyant seismic source. Compressed air tanks are present to overcome temporarily shortage of air either for the airgun array and/or for providing air to the combustion engines if the snorkel assembly is temporarily closed.

The torpedo-shaped hull of the vessel may be configured to float typically at a distance between 0 and 20 m below the water surface and to incidentally reach depths of 50 m and may cruise at speeds, draught and directions and may tow the airgun arrays at depths that can be non-related to each other or the other vessels that make part of the seismic operation. The seismic source assembly may comprise an airgun array provided with a depth control and stabilizer assembly which is configured to maintain this the airgun array during normal use thereof at a distance between 0 and 50 m below the water surface. It may be made possible to reel in the seismic source into the vessel for easy

retrieval and increased maneuverability and cruising speed at times the seismic source is not required.

In accordance with the invention there is furthermore provided a marine seismic surveying method, wherein use is made of at least one autonomous seismic source assembly according to the present invention.

In this specification and claims the autonomous seismic surveying source assembly according to the invention is also identified as a Remotely Operated Airgun Platform or a ROAP and the Semi-Submersible Autonomous Surface Vessel is also identified by the abbreviation SSASV.

These and other features, embodiments and advantages of the method according to the invention are described in the accompanying claims, abstract and the following detailed description of preferred embodiments disclosed in the accompanying drawings in which reference numerals are used which refer to corresponding reference numerals that are shown in the drawings .

BRIEF DESCRIPTION OF THE DRAWINGS

FIG.l is a schematic top view of the first preferred embodiment of the method according to the invention wherein a principal towing vessel is at each side flanked by a pair of remotely operated auxiliary towing vessels, which are shown as four Remote Operated Airguns Platforms ROAP 1-4;

FIG. 2 is a schematic vertical sectional view of the multiple towing vessel configuration shown in FIG.l, which illustrates how this configuration increases the covered subsurface area from Al = 700 m to A2 = 2100m;

FIG.3 is a schematic top view of a the second preferred embodiment of the method according to the invention, wherein a principal towing vessel is at one side thereof flanked by two pairs of remotely operated airgun vessels ROAP 1-4; and FIG.4 is a schematic longitudinal sectional view of one of the Remote Operated Airgun Platforms ROAP 1-4 shown in FIG.l and FIG.3.

FIG 5 illustrates a third preferred embodiment of the method wherein sensors are positioned at the sea bottom that requires the ROAPs to cruise irrespectively of the principal vessel .

DETAILED DESCRIPTION OF THE DEPICTED EMBODIMENTS

FIG.l is a schematic top view of a multiple towing vessel configuration according to the invention, wherein a

principal towing vessel 1 tows an acoustic source 2 of any known type, such as an airgun or alternative source and an array of twelve seismic streamers S₁-S₁2, which each comprise a string of seismic signal receivers, such as hydrophones Hi-H_x.The seismic streamers typically have a Maximum Spread

(MS) of about 1.5 km.

The principal towing vessel 1 moves in a substantially straight direction 3 through a body of water 4 is at each side thereof flanked by a pair of Remotely Operated Airgun Platforms ROAP 1 and 3 and ROAP 2 and 4, which for this survey are instructed to move in similar directions 3 and at similar speeds as the towing vessel 1 but all individually may tow the source arrays at different depths.

ROAPs 1 and 3 move at the left side of, and at a lateral distance L of about one to two kilometers from, the

principal towing vessel 1 and ROAPs 2 and 4 move at the right side of, and at a lateral distance R of about one to two kilometers from, the principal towing vessel 1.

Furthermore a chase boat 5 sails between the principal towing vessel 1 and the second ROAP 2.

FIG. 2 is a schematic vertical sectional view of the multiple towing vessel configuration shown in FIG.l, which illustrates how this configuration increases the covered subsurface area from Al = 700 m to A2 = 2100m. This increase is also schematically illustrated by the lines Al and A2 and the shaded areas Normal Coverage and Increased X-line

Coverage in FIG.l.

FIG.3 is a schematic top view of an alternative multiple towing vessel configuration according to the invention. In FIG.l and 3 similar components of the multiple towing vessel according to the invention are illustrated by similar reference numeral.

In the embodiment shown in FIG.3 a principal towing vessel

1, similar to that shown in FIG.l also tows an acoustic source 2 of any known type, such as an airgun or alternative source, and an array of twelve seismic streamers S₁-S₁₂, which each comprise a string of seismic signal receivers, such as hydrophones Hi-H_x. The seismic streamers typically have a Maximum Spread (MS) of about 1.5 km.

In the embodiment shown in FIG.3 the principal towing vessel 1 moves in a substantially straight direction 3 through a body of water 4 is at one side thereof flanked by two pairs of Remotely Operated Airgun Platforms ROAP 1 and 3 and ROAP 2 and 4, which each move in similar directions 3 and at similar speeds as the principal towing vessel 1.

ROAPs 1 and 3 move at the right side of, and at a lateral distance L of about two to four kilometers from, the

principal towing vessel 1.

ROAPs 2 and 4 also move at the right side of, and at a lateral distance R of about one to two kilometers from, the principal towing vessel 1.

Furthermore a chase boat 5 sails near the centre of the area covered by the ROAPS 1-4.

FIG.4 is a schematic longitudinal sectional view of one of the Remote Operated Airgun Platforms ROAP 1-4 shown in FIG.l and FIG.3. The Remotely Operated Airgun Platform ROAP of which an generic illustration is shown in FIG.4 comprises a

substantially neutrally buoyant annular port air gun array 40 which is connected via a depth control and stabilizer assembly 41 and towing cable 42 to a ROAP vessel 43.

However, use of an alternative source instead of airguns may be optional.

The ROAP vessel 43 is a Semi-Submersible Autonomous

Surface Vessel (SSASV) comprises a semi-submersible hull 44 to which a steering and propulsion propeller assembly 45 and a vertical snorkel assembly 46 are connected.

The snorkel assembly comprises a refueling probe 47 and contains an antenna, air intake, vertical stabilizer and access hatch assembly. The hull of the SSASV may be designed with a tower construction to reinforce the snorkel assembly.

The semi-submersible hull 44 contains horizontal

stabilizer and compressor assemblies 48, fuel and compressed air tanks 49 and a diesel engine 50.

FIG. 5 illustrates a third preferred embodiment of the method wherein Ocean Bottom Sensor (OBS ) nodes 51 are

positioned at the sea bottom 52 that requires the ROAPs 53 to cruise irrespectively of the principal vessel 54, which is used for data recording and other operational activities. In different shooting patterns the ROAP seismic sources 55 may operate individually or by groups in directions and

SSAVs 53 may tow the airgun or alternative seismic source arrays 55 at depths that can be unrelated to each other and to the principal vessel 54.

It is observed that, as illustrated in FIG.5, one or more remotely operated ROAP vessels 53 and substantially

neutrally buoyant seismic source arrays 55 according to the present invention may be operated together with one or more Ocean Bottom Seismic sensors (OBS) 51, that are mounted at or near the bottom 52 of an ocean, sea, lake or other body of water 56 by a Remotely Operated Vehicle (ROV) 57 and are configured to receive seismic signals transmitted by at least one substantially neutrally buoyant seismic source assembly 55 according to the present invention.

It will be understood that each ROAP vessel 53 and substantially neutrally buoyant seismic source array 55 may be configured to be fully submerged below the water surface 58 during an extended period of time, for example if a seismic survey is undertaken underneath ice, icebergs and/or other floating obstacles or in rough seas.

For operational easiness the ROAPs can be stored on deck, launched and retrieved by the principal seismic vessel 54, a participating seismic vessel, a cargo vessel, a

participating vessel or in cases like land locked seas, even by trucks. It is also possible to tow the ROAPs to their work area by using one or multiple tug boats.

One or more Remotely Operated Airgun Platform (ROAP) assemblies 53,55 according to the invention, may be deployed in combination with multi and simultaneous source

applications by high quantities in combination with seismic Wide Azimuth (WAZ) techniques used for Common Mid Point (CMP) streamer, Ocean Bottom Sensors (OBS)51 nodal sensor surveys or Ocean Bottom Cable surveys.

Eventually, after an initial start-up of five

experimental ROAP systems 53,55 according to the invention, seismic acquisition techniques may use dozens of ROAP systems 53,55 according to the present invention.

Consequently, for this a source system needs to be provided that keeps the overall costs realistically in balance as compared to current streamer surveys. Therefore the ROAP system 53,55 according to the invention is based on unmanned source systems that can be stored and launched by a service vessel 54.

The ROAP systems 53,55 according to the invention may be transported or towed by a vessel 54 to the survey location, which could either be the same streamer vessel or the same vessel for OBS (Ocean Bottom Sensor) 51 applications.

Alternatively, the ROAPs 53,55 may be located on a platform (not shown) . The invention comprises a purpose build semi- submersible autonomous surface vessel (SSASV) or ROAP 53 that will tow a semi-buoyant airgun or alternative source array 55 at deeper depth and on the overall will be cheaper in use as compared to manned vessels. For deck storage, a complete ROAP system 53,55 needs to be as small as possible. The ROAP system 53,55 according to the present invention requires development and synergetic optimization of various equipment and machinery to become feasible for use for seismic surveys.

The ROAP assembly, its SSASV 53 and seismic source array 55 according to the invention addresses several challenges like snorkel fuelling, collision avoidance control,

navigation and route planning, deployment and launch systems from the service vessel and off deck attach and detach systems of the airguns or alternative sources for

maintenance purposes.

Furthermore, the SSASV 53 and ROAP assembly 53,55

according to the present invention optionally addresses the following challenges:

-Provision of a dedicated SSASV 53. Currently available SSASVs are not designed for seismic airgun or alternative source applications. A maneuverable SSASV 53 that at the same time can be stored on a service vessel for transport needs to be specially designed. -Propulsion. To keep the SSASV 53 as small as possible, small powerful engines need to be implemented to the design. For further optimization the same engine may be used for the compressor .

-A purpose build compressor unit for air supply to the air guns. The physical size of the compressor merely determines the size of the SSASV. This is related to the minimum required seismic source energy for which several studies and field trials are being implemented.

-Combined engine use for compressor and propulsion. To optimize the design of the SSASV it will be investigated to combine the same engine for air compression and propulsion.

- A substantially neutrally-buoyant airgun or alternative source array 55. Conventional seismic sources are kept at the specified depth by surface floats and supplied by air hoses outside the guns. With the ROAP 53 the airguns or alternative sources 55 are towed deeper and thus surface floats cannot be used. Also surface floats would impose to much drag to the entire system such that it would become inefficient. The semi buoyant airgun or alternative source array 55 may use special guns with the airhoses and the control wires going thought the middle of the guns. Special floats may be integrated within the source arrays to achieve sufficient uplift to become semi-buoyant and at the same time limit drag.

-Airgun or alternative source array steering and depth control. To keep the source arrays 55 behind the SSASV 53 at the right depth and following the SSAVS 53, special wings and controls are being implemented to the ROAP system.

-Snorkel fuelling. To be able to keep the SSASV 53 going for longer time, fuelling out at sea will be a necessity.

Special gear need will be required to adhere to regulations for work at sea and for HSE . Similarly to allow for the required air-inlet a special designed snorkel will be required .

-Collision avoidance control. Dedicated sonar techniques and analytic interpretation algorithms will be required to avoid the ROAP assembly 53,55 to collide into unexpected ob ects .

-Positioning systems and route planning. Special

equipment will be installed for positioning tracking and autonomic steering according to a downloaded route path which will constantly interact with the collision avoidance control equipment and for finding back its original path after a possible emergency route correction.

-Service vessel handling equipment. For servicing the source arrays or SSASVs 53 by the surveying vessel 54 special equipment will be designed for alongside source array detach and retrieve or to retrieve a full ROAP system 53, 55.

For deck launching of ROAP 53 and for systems for

launching from a parenting vessel airgun detachment systems may be provided. Airguns need to be regularly maintained even within a single survey, special handling equipment will be designed for detaching and re-attaching the airgun arrays

55 from the SSASVs 53.

-Emergency Remote Death Mans Handle. An emergency system will be required to get each ROAP 53 to a complete stop or to have it immediately returned to the service vessel 54.

This system needs to adhere to all the marine international regulations .

In general the SSAVS 53 and seismic source arrays 55 according to the present invention need to comply with safety and seismic standards which are necessary for WAZ data and multi source applications.

Claims

C L A I M S

1. A remotely operated marine seismic source assembly comprising :

combustion engine for propelling the vessel through a body of water, a compressed air tank and a snorkel assembly for providing air to the combustion engine and the compressed air tank during at least part of a period when at least a substantial part of the vessel floats at a distance between 0 and 20 m below the water surface; and

2. The assembly of claim 1, wherein the vessel comprises a torpedo-shaped hull which is configured to float at a distance between 1 and 10 m below the water surface and the airgun array comprises a depth control and stabilizer assembly which is configured to maintain the airgun array during normal use thereof at a distance between 0 and 50 m below the water surface, while the airgun array is induced to transmit a sequence of seismic shots.

3. The assembly of claim 1 or 2, wherein the snorkel assembly comprises a water lock which prevents water to flow into the combustion engine and the compressed air tank and which allows the vessel to dive below floating ice, logs and other floating obstacles.

4. A marine seismic surveying method, wherein use is made of at least one remotely operated marine seismic source assembly according to any one of claims 1-3.

5. The method of claim 4, wherein a plurality of seismic sources are towed by separate towing vessels, the method comprising :

- providing a principal towing vessel which is connected by towing lines to a primary seismic source and an array of seismic streamers, which each carry a string of seismic sensors;

- providing a plurality of secondary towing vessels which each comprise a remotely operated submersible vessel according to claim 1 and which secondary towing vessel tows a secondary seismic source provided by a neutrally buoyant airgun array according to claim 1 ;

- inducing the principal towing vessel to tow the primary seismic source, streamers and seismic sensors along a selected towing path through a body of water;

- inducing each secondary towing vessel to tow each

secondary seismic source such that each secondary seismic source is maintained in a predetermined position relative to the principal towing vessel;

- inducing the primary and each secondary seismic source to transmit seismic signals through the body of water and water bottom; and

- inducing the seismic sensors to receive and record

reflections of seismic signals transmitted by the primary and each secondary source and reflected by subsurface earth layers .

6. The method of claim 5, wherein an array of secondary towing vessels is provided, which maintain an array of secondary seismic sources in a predetermined pattern

relative to the primary seismic source and to the towing path .

7. The method of claim 6, wherein the array comprises a first pair of auxiliary towing vessels, which tow a first pair of auxiliary seismic sources and a second pair of auxiliary towing vessels, which tow a second pair of auxiliary seismic sources, such that the first and second pairs of auxiliary seismic sources are maintained in a predetermined horizontal pattern.

8. The method of claim 7, wherein the first and second pairs of auxiliary seismic sources are maintained in a substantially square horizontal pattern at predetermined lateral and longitudinal positions relative to primary seismic source and to the selected towing path of the principal towing vessel.

9. The method of claim 8, wherein the first pair of auxiliary sources is maintained in a substantially lateral position relative to the primary seismic source and to the selected towing path of the principal towing vessel, when seen in the direction of said towing path.

10. The method of claim 9, wherein the second pair of auxiliary sources is maintained at a selected distance behind the first pair of auxiliary sources and behind the array of seismic streamers, when seen in the direction of the towing path.

11. The method of claim 10, wherein the array of seismic streamers has a substantially square outer periphery with four horizontally spaced corners and a central point which is located at substantially equal distances from said corners, which central point substantially coincides with or is maintained at a substantially lateral position relative to a midpoint of the square horizontal pattern of auxiliary seismic sources.

12. The method of claim 11, wherein the central point substantially coincides with said midpoint and the

substantially square outer periphery of the array of seismic streamers has a smaller length and width than the

substantially square horizontal pattern of auxiliary seismic sources .

13. The method of claim 10, wherein the midpoint of the substantially square horizontal pattern of auxiliary seismic sources is maintained at a predetermined substantially lateral position relative to the central point and at a predetermined lateral distance from the towing path such that the substantially square horizontal pattern of

auxiliary seismic sources does not overlap with the

substantially square outer periphery of the array of seismic streamers .

14. The method of claim 4, wherein one or more remotely operated seismic surveying assemblies according to any one of claims 1-3 are operated together with one or more ocean bottom seismic sensors, that are mounted at or near the bottom of a body of water and are configured to receive seismic signals transmitted by at least one substantially neutrally buoyant seismic source assembly according to any one of claims 1-3.

15. The method of any one of claims 4-14, wherein the reflections of seismic signals received and recorded by the seismic sensors are used to make a seismic map of subsurface earth layers which contain hydrocarbon assets and said hydrocarbon assets are produced on the basis of said seismic map .