WO2006024956A1

WO2006024956A1 - Methods of placement of nodes and seismic sea exploration, corresponding basket and node

Info

Publication number: WO2006024956A1
Application number: PCT/IB2005/003157
Authority: WO
Inventors: Hubert Lecomte
Original assignee: Compagnie Generale De Geophysique
Priority date: 2004-08-30
Filing date: 2005-08-30
Publication date: 2006-03-09
Also published as: FR2874705A1; FR2874705B1

Abstract

This invention relates to a method for placing seismic acquisition nodes (2) on the seabed, using a basket (1) with a plurality of nodes, and suspended from a boat (3). The method comprises the following steps (a) a plurality of nodes (2) are arranged in the basket on. board the boat (3), the arrangement being such that the nodes are held in place in the basket by respective holding elements (11), against the action of gravity; (b) the basket (1) is positioned close to a desired placement position on the seabed (6); (c) a node is released from the basket under the effect of gravity by remote controlled release of the corresponding holding element; (d) steps (b) and (c) are repeated for each required placement position. It also relates to a seismic survey method comprising such a placement method, and associated baskets and nodes.

Description

METHODS OF PLACEMENT OF NODES AND SEISMIC SEA EXPLORATION, CORRESPONDING BASKET AND NODE

General technical field

This invention relates to a method of placing seismic acquisition nodes on the seabed. It also relates to a seismic exploration method using such a placement technique. It also relates to transport baskets for use of these methods, and the associated acquisition nodes.

State of the art

Techniques for the placement and recovery of seismic acquisition nodes to be placed on the seabed are known in prior art. In the present application, the term "nodes" as usually used by those skilled in the art, refers to seismic acquisition units that will be placed on the seabed. In this respect, a node comprises at least one sensor, a recorder and a battery.

For example, document WO 03/083514 discloses a transport basket provided with compartments in which nodes are arranged horizontally. The basket is suspended from a crane installed on a boat. A submarine ROV (Remote Operated vehicle) controlled from the boat unloads the nodes one by one from the basket and places them on the seabed. The ROV then acts on a handle to separate a sensor from the rest of the node. The acoustic positioning of each node is controlled from the boat using a USBL or LUSBL technique. This technique requires action by a ROV to place nodes on the seabed, and is long and tedious.

Document EP 1217390 describes nodes comprising each a pointed end portion containing sensors. The nodes are released from the boat and fall by gravity. They are provided with inflatable buoys triggered by the reception of an acoustic signal to bring the nodes to the surface so that they can be recovered after the measurements.

This technique does not enable precise positioning of nodes on the seabed. Recovery of units and of data is more difficult, particularly in deep water (100 m and more) .

Presentation of the invention

One purpose of the invention is to enable the efficient and economic placement of large numbers of acquisition nodes (of... the oxder of a hundred or several hundred nodes) .

To this effect, there is provided according to the invention a method for placing seismic acquisition nodes on the seabed, using a basket carrying a plurality of nodes, and suspended from a boat, comprising the steps of :

(a) arranging a plurality of nodes in the basket on board the boat, the arrangement being such that the nodes are held in place in the basket against the action of gravity by respective holding elements;

(b) positioning the basket close to a desired placement position on the seabed;

(c) releasing a node from the basket under the effect of gravity by remote controlled release of the respective holding element; (d) repeating steps (b) and (c) for each of the desired placement positions.

List of the drawings

Other characteristics, purposes and advantages of the invention will become clear from the following description that is given as being purely illustrative and non- limitative, and that should be read with reference to the attached drawings in which:

Figure 1 shows a diagrammatic view of a basket on the seabed, the boat being on the surface;

Figures 2 and 3 show side and top views of a basket according to the invention;

Figure 4 shows a diagrammatic view of a basket compartment according to the invention;

Figures 5 and 6 show a ROV picking up a buoy; Figure 7 shows the ROV putting the node into a basket; and

Figures 8 and 9 show a system for loading baskets on the deck of the boat.

Similar elements are marked with the same reference numerals on all drawings.

Detailed description of an embodiment

Figures 1 to 3 show that nodes 2 are loaded on the deck of a ship 3 on the surface 7 of the sea, in a basket 1 comprising a series of n compartments 17, each of which can contain several nodes 2. For example, the basket 1 comprises about ten compartments 17, each compartment 17 being designed for example to contain four nodes 2.

The preferred shape of the basket 1 is a horizontal circular or polygonal section, or any other equivalent shape.

The basket 1 includes a vertical rotation axis 18 with compartments 17 arranged radially with respect to the said axis 18.

The compartments 17 form ramps 19 inclined downwards towards the periphery of the basket 1, at their top end.

Figure 4 also shows that the basket 1 has a "roof" that is conically shaped in the case of a basket with a circular section.

Each node 2 comprises a seismic sensor assembly 23 connected by a cable 24 to a housing 22 containing a rec.ox.der.- and a battery.

The housing 22 is suspended from a buoy 20 by a flexible sling 21. An element is provided on housing 22 to hitch the sling to the housing in a simple and robust manner, which element can have any appropriate shape.

The sling 21 between the buoy 20 and the node may be 1 to 3 meters long, the housing 22 may be 1 to 2.5 meters long, and hence the total height of the basket 1 (at the bottom of the roof) can range between 2.5 and 6.5 m (allowing 0.5 to I m for the basket structure) . The slope of the roof may be 30 to 45°. Obviously, these figures are only given as order of magnitude and are not in any way limitative. The buoy 20 is designed to form an acoustic reflector. It is provided for this purpose with metallic paint or any- other equivalent means on its surface.

The nodes 2 are put into place vertically in the compartments 17 in the basket 1 up to the furthest available position in the periphery of the basket. When a. node 2 is in position in the basket 1, the buoy 20 projects from the basket 1 above the ramp 19 that forms the top end of the compartment 17. Holding elements 11 are provided at the ramps 19, with the same number of elements as the number of nodes 2.

Once a node 2 is loaded in a compartment 17, an element 11 is actuated to hold it in place in the compartment 17 and to enable placement of another node 2, until the compartment 17 is full in the radial direction.

Each holding element 11 may be actuated for displacement between a blocked position in which the holding element 11 engages the buoy 20 of the node 2 and a released position in which the holding element 11 no longer retains the buoy 20, which has the effect of releasing the node 2 from the basket 1 by gravity.

Once the node 2 is blocked by a holding element 11, it is suspended in the basket 1 under the buoy 20.

Figures 8 and 9 show an advantageous system for loading nodes 2 in the basket 1 on the deck of the boat 3.

The system comprises mainly a running track 15 installed approximately at the same height as the ramps 19 of the basket 1 arranged on the deck of the bridge 3. One of the ends of the track 15 is located at the basket 1, the other end being located on the inside of a workroom 16. Advantageously, the end of the track 15 is located between two baskets in which nodes 2 are accommodated.

The workroom 16 is used for preparation of nodes 2 before a seismic acquisition session, and for recovery of nodes 2 after an acquisition session. Room 16 also comprises a mechanical device for actuation of a chain along the rolling track 15. For example, the chain is provided with hooks that can grip the buoys 20 of nodes 2, and move the nodes 2 between the room 16 and the basket 1 on the running track 15.

When the basket 1 is being loaded, an operator is placed on a platform 14 superelevated above the deck of the boat 3, so as to be at a suitable height for making manipulations. The operator grasps a node 2 from the room 16 and places it in a housing 17 of the basket 1, as shown by arrow 110 in Figure 9.

Obviously, it is desirable to load a large number of nodes 2 in the basket 1. All compartments 17 are loaded with nodes 2 by rotating the basket 1 about its rotation axis 18, the rotation being 360°/n every time. As shown in Figure 9, the basket 1 may be suspended from a crane 4 or a winch, or it may be placed on a platform on the deck of the boat 3. The operator aligns the compartment 17 in which the nodes 2 are to be placed with the running track 15 on the deck of the boat.

The holding elements 11 are articulated on the ramps 19. Figure 4 shows that elements 11 may comprise a metal plate 12 pivotally mounted about an axis 13, for example in a plane transverse to the compartment 17. The plate 12 then acts at the level of the buoys 20, or at the level of the sling 21. Obviously, other designs can be used for elements 11. For example, a hook-like design can be used.

The elements 11 are actuated either by acoustic remote control, or directly. This is done by connecting the basket 1 to the boat 3 through an electrical, hydraulic or electro-hydraulic umbilical cable 40, or directly by a submarine remote operated vehicle (ROV) controlled for example from the boat.

The basket 1 comprises a collector to actuate elements 11 individually to release the nodes 2 one by one from the basket 1.

For the placement of nodes 2, the boat 3 provided with a dynamic positioning (DP) device positions itself above the desired position for node 2. The basket 1 is lowered to a few meters above the seabed 6 using a crane 4 or a winch.

When an element 11 is actuated, a node 2 is released and moves by gravity along the inclination of the ramp 19, and falls onto the seabed 6.

During placement, each node 2 is precisely identified and positioned on the seabed 6 using a ROV 5 remote- controlled from the boat 3. The ROV 5 is not normally involved in releasing the nodes 2 from the basket 1, but is used to ensure that the node is placed in a right position on the seabed. If it is not, the ROV 5 may be operated to put the node in a right position, the ROV including an articulated manipulator arm 50 shown particularly in Figures 5 and 6.

The ROV 5 gives the position of the node 2 by giving its own geographic position using standard means: acoustic system (short or long base) and if necessary an inertial system on the ROV 5 if high precision is required.

The ROV 5 reads the node serial number, for precise identification of node 2. The seismic acquisition is then made by activating one or several seismic sources, typically located on the surface and towed by boat to take successive positions . The memories of the nodes 2 located in housings 22 are programmed to start recording at a predetermined time and then record continuously until the end of the acquisition session.

To recover the nodes 2, the ROV 5 moves to the position of node 2, locates the position of the node 2 using its acoustic sonar that emits waves that are reflected by the reflecting surface of the buoy 20, and then makes a visual approach.

Figures 5 and 6 show that at least one of the arms 50 of the ROV comprises a fork 51 used to lift the node 2 by engaging its buoy 20, and then as shown in Figure 7 by arrow 90, to insert it into a compartment 17 of a recovery basket 1, approximately in the same way as for insertion in the placement basket. The buoy 20 holds the sling 21 vertically in the water to facilitate gripping of the node by the ROV 5. In the case of a fork 51, the vehicle grips the node under the buoy as shown by the arrow 70. Other gripping means such as clamps are also possible.

Once the node 2 is placed in the recovery basket 1, a holding element 11 is actuated from the boat 3 or the ROV to hold the node in place in the basket 1. To save time, it is appropriate to use at least two baskets per boat 3. One of the baskets is loaded on the deck of the boat 3 while the other places nodes 2 on the seabed 6. Similarly, for recovery, one basket is unloaded on the deck of the boat 3 while the other is in the water.

When seismic acquisition operations are performed in several sessions each corresponding to a different set of node positions, the steps to be carried out to explore different areas of the subsoil include placement of all nodes, followed by the seismic firing campaign, recovery of the nodes,, extraction of the data, and reinitialisation of the nodes, followed by placement of the nodes once again for a second acquisition session, and so on. Therefore it is desirable to minimize the duration of node recovery, reinitialisation and redeployment operations.

The sequence of operations then consists of using at least two baskets to perform several operations at the same time: while one of the baskets is on the seabed to recover nodes, the other is unloaded or loaded on the deck of the boat. During this time, other nodes on board are reinitialised until there are enough nodes to be redeployed.

This makes it possible to limit the number of nodes onboard the boat, so that it becomes easier to handle them.

Claims

1. Method for placing seismic acquisition nodes on the seabed, using a basket carrying a plurality of nodes, and suspended from a boat, comprising the steps of :

(a) arranging a plurality of nodes in the basket on board the boat, the arrangement being such that the nodes are held in place in the basket against the action of gravity by respective holding elements ;

(b) positioning the basket close to a desired placement position on the seabed ; (c) releasing a node from the basket under the effect of gravity by remote controlled release of the corresponding holding element ;

(d) repeating steps (b) and (c) for each required placement position.

2. Method according to claim 1, in which the holding elements engage buoys respectively connected to the nodes.

3. Method according to claim 1 or 2, further comprising the step of identifying and precisely determining the position of each node on the seabed, using a ROV remotely operated from the boat.

4. Seismic exploration method using acquisition nodes placed on the seabed, comprising a placement method according to claims 1 to 3. 5. Method according to claim 4, comprising the step of retrieving nodes using a remotely operated vehicle provided with gripping means to lift a buoy connected to each node and insert the node into a compartment of a recovery

5 basket.

6. Method according to claim 5, in which the remotely operated vehicle is provided with a fork to lift said buoy.

10_. 7. Method according to any one of claims 5 and 6, in which the remotely operated vehicle is remotely operated from the boat.

8. Method according to any one of claims 5 to 7, 15 further comprising the step of determining the position of each node on the seabed by means of the remotely op_er.ated_ vehicle using acoustic means to locate its buoy.

9. Basket for carrying seismic acquisition nodes to be 20 placed on the seabed, comprising compartments for receiving a plurality of nodes and forming inclined ramps, and holding elements that may be actuated to be displaced between a position in which the nodes are held in said compartments and a position in which nodes are released. 25

10. Basket according to claim 9, having a rotation axis, said compartments being arranged radially with respect to said rotation axis.

11. Seismic acquisition node designed to be placed on the seabed, comprising an acquisition unit, said unit comprising a buoy and a sling connecting said unit to said buoy.

12. Node according to claim 11, in which said buoy forms an acoustic reflector.