SEISMIC SURVEYING 


Six nodes are placed in a basket which is then deployed overboard down to working depth of approximately 40-50 metres above the seabed. Te basket is a rectangular frame lowered to working depth by means of a winch and wire on a central pivot. Te basket winch is fitted with an active heave compensator (AHC), providing a stable work platform for the ROV operation. Te 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. Te ROV removes nodes individually from the basket. Tis ‘mid-water pick’ is a skilled operation. Te 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. Te tilt values (X and Y axis) are read via the acoustic


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. Te basket is deployed to working depth


Fig. 2. The sensor unit.


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


and the active heave compensator engaged. Trough 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. Te 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. Te 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 16 or ✔ at www.engineerlive.com/ihss


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


Fig. 3. Winch preparing to deploy a basket containing six nodes.


IHSS 16


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