NO346161B1

NO346161B1 - Method for deployment of ocean seabed node

Info

Publication number: NO346161B1
Application number: NO20200051A
Authority: NO
Inventors: Erik Godøy
Original assignee: Shearwater Invest As
Priority date: 2020-01-15
Filing date: 2020-01-15
Publication date: 2022-03-28
Also published as: NO20200051A1; WO2021145775A1

Description

Method for Deployment of Ocean Seabed Node

Field of Invention

This invention relates to the field of marine seismic exploration, more specifically to a method for deployment of at least one ocean seabed node using a surface vessel comprising:

- positioning the vessel in a position for deploying a node;

- connecting the node to a deployment tool including a release hook, and

- lowering the node towards the seabed by paying out the correct length to position the node close to the seabed;

Prior and Related Art

There are presently several known systems for deploying and positioning seismic nodes. The first system is generally known as Node-on-a-Rope (NOR). US Patent 10,209,382 describes system and method for NOR. In this system multiple nodes are attached along a rope at a given interspace which is lowered to the seabed from a ship. The same nodes can then be recovered by pulling the rope back onto the ship. This system allows for relatively fast deployment and recovery. However, it is difficult to position the nodes with the required accuracy on the seabed. It is also a costly method, generally requiring a dedicated vessel and is less suitable in very deep waters (beyond 1 000 – 1 500 meters).

In a second known system nodes are deployed and recovered from a ship using a ROV (Remote Operated Vehicle). Such a system is for example described in patent application WO 2018/204084. The ROV is normally equipped with a “basket” which can hold multiple nodes in order to improve deployment and recovery speed. This system allows for deployment and recovery of nodes in almost any water depths. The nodes can also be positioned very accurately. However, this is a very costly system requiring a dedicated vessel and the use of ROV’s. It is also relatively slow.

In a third deployment system the nodes can simply be dropped from the surface vessel and find their own way to the seabed. This system is commonly used in shallow waters (10-50 meter) This is a very efficiency deployment method both in regarding operational speed and equipment cost but a major drawback is lack of positioning accuracy since one can’t know for sure exactly where the node will land on the seabed.

In a fourth deployment system the nodes used are so-called “free-swimming” nodes. Patent applications US 2015/0151819 and US 2017/0075014 describe such nodes and methods for deployment and recovery of these. These nodes are launched from the surface, either from a vessel or platform or even from a landbased station. In these systems the nodes are equipped with steering mechanisms (wings, steering flaps) and are able to find their position on the seabed by themselves. Recovery is typically done in reversed order where the nodes will “swim” from the seabed to the surface where they can be picked up by a ship. However, deployment and recovery can only be achieved by using relatively complex steering mechanisms and control functions which has to be integrated into purpose made nodes or in some way the same functionality has to be added onto conventional “dumb” nodes.

US Patent No. 4516227 discloses a system having at least one sub ocean bottom seismic device and a deployment unit. When the deployment unit is lowered it selectively implants the seismic device at predetermined locations in the ocean bottom. The system also includes a signal cable. The signal cable is capable of retrieving the implanted seismic device and is connected to an anchored buoy containing a communications unit.

NO Patent Application No. 20150754 discloses a system for deploying an instrument at a seafloor comprising a positioning tool configured to move the system through a body of water above the seafloor. The system further comprises an instrument unit. First and second couplings connect the positioning tool, the instrument unit and a base unit. The couplings may be releasable, e.g. for removing the positioning tool (110) during measurements.

EP 1593987 discloses an arrangement for the deployment of seismic sensor units, such as sensor nodes, on the seabed, with a frame structure that is adapted to carry a container (11) for containment of a data registration unit and additional auxiliary equipment for the sensor node. The arrangement has supports that can rest against the seabed, and an attachment means for a lifting hook, for raising and lowering by means of a crane. There is a holder for the sensor node that shall be positioned on the seabed and which is connected to the container through a signal cable. It comprises a releasable attachment means, which holds the sensor node in a downwardly extending position with respect to the supports. The attachment means is located with a horizontal distance from the container. A latch mechanism for the attachment means is adapted to release the sensor node.

US Patent 9,523,780 describes a typical autonomous seabed node.

This invention only describes an improved system and method for deployment of seabed nodes. Recovery of the same nodes from the seabed can be done using ROV. As an alternative the nodes can be brought to the surface by themselves where they can be picked up by a ship. In this case the node is made positive buoyant by releasing a ballast weight. A release mechanism is typically triggered by a timer or via an acoustic link. These described node recovery methods are already well described in prior art.

The described deployment method is most of all suitable when:

a) Deploying nodes in relatively shallow water, typical 50-200 meter. At these water depths nodes dropped from the surface can normally not be positioned with acceptable accuracy and deployment using ROV will be relatively slow.

b) When deploying sparse node networks with long distances between each node. In such case both NOR and ROV will become inefficient deployment methods.

c) When deploying a relatively small number of nodes where it will be fairly costly to mobilize heavy equipment required for NOR or ROV node deployment.

Summary of the Invention

This invention provides a method for deployment of individual nodes without the use of NOR or ROV.

● It provides better deployment speed compared with ROV deployment.

● It provides similar accuracy of seabed positioning of nodes as for ROV deployment.

With the described method nodes must still be recovered from the seabed by using ROV. But since deployment can be done without ROV and also done faster than node deployment using ROV the operational efficiency and cost is improved.

The primary object of the invention is therefore to provide a method for seismic node deployment from a surface vessel, in a more efficient way. This is achieved by:

-stopping the node a short distance above the seabed;

determining the exact position of the node;

- activating the deployment tool to release the node causing the node to drop to the seabed.

In one aspect of the invention the method comprises an echosounder or similar instrument to measure the water depth.

In another aspect the release hook is activated by acoustic signal or by electric signal.

In another aspect an acoustic transponder is used for measuring the actual distance of the node from the seabed as the node is lowered through the water column.

In another aspect the ship is moved in order to bring node into correct position, accounting for any deviations due to currents etc.

In another aspect of the invention thrusters on the deployment tool are used to move the deployment tool.

In another aspect of the invention the deployment tool is brought back onboard the ship to prepare for next node to be deployed.

In another aspect of the invention a cable having means to transmit signals is provided.

In another aspect of the invention the node can be retrieved using an ROV.

In yet another aspect of the invention the node is retrieved by releasing a ballast weight.

Description of the Invention

The deployment will now be described in detail referring to the enclosed figure As shown in Fig. 1 the system consists of a vessel (ship) 1 floating on the surface 2. The vessel is equipped with a winch 5 for deploying nodes. The winch may be a standard type hydraulic or electric winch is used for lowering/hauling the rope. The winch will preferably have heave compensation for better control when lowering the node to the sea bed, eliminating vertical “yo-yo effects” caused by wave generated vessel motion (pitch/roll/heave).

The vessel (ship) used should preferably have dynamic positioning (DP) allowing for better manoeuvrability when positioning nodes on the seabed.

A node 10 is lowered using a rope 6 that is run from the winch and over a sheave 7. The rope may be a synthetic fibre rope or steel wire. The node 10 is attached to the rope by a release mechanism comprising a hook 8.

In an alternative embodiment the node deployment system will use a cable rather than rope/wire for lowering the node to the seabed. This will allow for illumination (lights) and cameras to be used for monitoring position of the node on the seabed feeding live signals through the cable back to the vessel. If a cable allowing for transmission of signals is used the “deployment tool” could be outfitted with sonar, laser or other measuring device giving a more accurate distance to the sea floor

The release hook will then be triggered by electrical signals through the cable rather than via acoustic signals.

An acoustic transponder/transduced will still be used for determining the position of the node.

In another version the deployment tool will be outfitted with thrusters allowing for more accurate positioning of nodes. Even with thrusters implemented such a deployment tool will be far less complex and mode compact than ROVs normally used for node deployment and recovery.

Node deployment is done with the following steps:

1. The ship is positioned on coordinates where the node is to be deployed.

2. Node is connected to the acoustic release hook.

3. Echosounder or similar instrument onboard ship is used to measure water depth.

4. Winch pays out correct length of rope positioning the node and acoustic release close to the seabed. The acoustic transponder can also be used for measuring actual distance of node from the seabed as the node is lowered through the water column.

5. With node lowered to correct depth the acoustic transponder is used to determine exact X-Y position of node. The ship can if necessary be moved in order to bring node into correct position, accounting for any deviations due to currents etc.

6. With transponder in correct position, the acoustic release is activated to drop the node.

7. Release hook is brought back onboard the ship to prepare for next node to be deployed.

For recovery of the nodes an ROV is used, picking up each node in accordance with standard practice.

Claims

1. Method for seismic node deployment from a surface vessel (1), comprising the following steps:

- positioning the vessel (1) in a position for deploying a node (10);

- connecting the node (10) to a deployment tool including a release hook

- lowering the node (10) towards the seabed; characterized by

- stopping the node (10) a short distance above the seabed;- determining the exact position of the node;

- activating the deployment tool to release the node (10) causing the node to drop to the seabed.

2. Method according to claim 1, where an echosounder or similar instrument is used to measure the water depth.

3. Method according to claim 1, where the release hook is activated by acoustic signal.

4. Method according to claim 3, where the release hook is activated by electric signal.

5. Method according to any of the preceding claims, further comprising the steps of using an acoustic transponder for measuring the actual distance of node (10) from the seabed as the node (10) is lowered through the water column.

6. Method according to any of the preceding claims, further comprising the step of moving the vessel (1) in order to bring node (10) into correct position, accounting for any deviations due to currents etc.

7. Method according to any of the preceding claims, further comprising the step of using thrusters on the deployment tool.

8. Method according to any of the preceding claims, further comprising the steps of bringing the deployment tool back onboard the vessel (1) to prepare for next node (10) to be deployed.

9. Method according the any of the preceding claims, further comprising using a cable having means to transmit signals.

10. Method according to any of the preceding claims, further comprising the steps of retrieving the node (10) using an ROV

11. Method according to any of the preceding claims, further comprising the steps of retrieving the node (10) by releasing a ballast weight to achieve positive buoyancy.