Ocean bottom nodes in operation

Paul Boughton
Ocean bottom nodes are used to good effect in situations where accuracy of sensor placement is most important, as in time lapse surveys. Brian Anderson and Christi Todd describe how that accuracy is achieved, using the node, an ROV, and navigational systems.

In August of 2010, SeaBird Exploration FZ-LLC completed a base line 4D OBN (ocean bottom nodes) seismic survey for Chevron North Sea Limited in the Rosebank field west of the Shetlands, using node vessel M/V Hugin Explorer and source vessel M/V Kondor Explorer. ROV operations were provided by FUGRO - Rovtech Ltd, Aberdeen. The nodes used were SeaBird's patented CASE Abyss nodes. At the end of the project 765 nodes had been deployed. Two of these were deployed outside of 5m due to trenches across the preplot which appeared to have been caused by a dragged anchor. The remaining nodes were positioned within 5m. The mean radial distance of as-laid position from preplot was 1.17m.

The CASE Abyss node system consists of the following main components: Data Acquisition Unit (DAU); Sensor Unit (SU); Acoustic Modem; Battery Units; Plastic Shell; Aluminum frame.

The Data Acquisition Unit (DAU) carries out most of the control and communication tasks in the system. It stores digitized sensor data and status data onto files, contains the reference clock, and the system power supply.

The Sensor Unit (SU) contains the primary seismic sensors (three geophones and a hydrophone). It also contains a hydrophone pre-amplifier with programmable gain. A dual-axis inclinometer measures the SU verticality with great accuracy. The SU is connected to the DAU via a short, highly flexible cable, to ensure good acoustic decoupling between the SU and the rest of the CASE Abyss unit.

The Acoustic Modem is connected to the DAU and offers a control and data link between the individual CASE Abyss unit and the sea surface. By way of the acoustic link one can control the operation of individual CASE Abyss units and retrieve status information and data files.

Six nodes are placed in a basket which is then deployed overboard down to working depth of approximately 40-50 metres above the seabed. The basket is a rectangular frame lowered to working depth by means of a winch and wire on a central pivot. The basket winch is fitted with an active heave compensator (AHC), providing a stable work platform for the ROV operation.

The basket, fitted with a beacon, is visible on the navigation screen beyond a depth of 50m, allowing safe and accurate navigation of the ROV while the vehicle travels to collect the node. The ROV removes nodes individually from the basket. This 'mid-water pick' is a skilled operation.

The ROV is guided to location and the node positioned using a combination of Kongsberg HAIN positioning system and Winfrog navigation software. As the ROV approaches a target location the pilot uses a combination of ROV altitude sensors and camera images to determine the gradient of the slope in the planting area.

When in the intended position on the seabed, the sensor unit is removed from the node and planted in the seabed. The tilt values (X and Y axis) are read via the acoustic modem and the node adjusted by use of the ROV arm to ensure near vertical values. With tilt values confirmed, node recording is initiated and the ROV releases the sensor.

At each deployment station video and still images are captured, and details of the planting quality are recorded. A fix of the node is taken to ensure installation positioning accuracy when compared to preplot and for repeatability and recovery.

The basket is deployed to working depth and the active heave compensator engaged. Through a modem on the ROV, the node recording is stopped, and a status report from the node retrieved. Interrogation of the node is done while the ROV flies towards the unit. The ROV moves onto the node, grabs it, and moves away from the node location in a single movement, or series of near-continuous movements. As the ROV picks the node up from the seabed, the sensor is pulled out of the seabed using its safety line. The sensor is left to float freely while the ROV transports the node to the basket and latches it into a recovery slot. With the respective nodes secured in place, the basket is recovered back onboard and the nodes removed.

Enter √ at www.engineerlive.com/ihss

Brian Anderson and Christi Todd are with SeaBird Exploration Americas Inc, Houston, TX, USA. www.sbexp.com

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