Forward Looking Directions
Number of nodes handled per day is a key challenge in seabed seismic node operations. In order to speed up deployment and recovery of nodes, we need to be handling more nodes in a shorter timeframe. This can be addressed by a combination of carrying an increased number of nodes per dive and faster operations when the node handling vehicle is deploying and recovering the nodes at the seabed. The efficiency gains are relevant across all water depths. Though particularly evident for deep water operations, where the dive time is larger, it is also relevant in shallower waters, where the number of nodes to be deployed on the seabed per unit area is typically higher.
Seismic nodes can be placed on the seabed using a variety of underwater vehicles. The subsea vehicle typically has neutral buoyancy in water or slightly positive buoyancy (to ensure the vehicle is not lost in case of electrical failures) and can carry some tens of nodes either in a skid or a tray. The more nodes in the tray/skid, the more weight is added to the vehicle, and this naturally impacts the natural buoyancy of the vehicle. There is a limit to the maximum payload (and thereby change between positive and negative buoyancy) a subsea vehicle can handle. This loss of natural buoyancy is often compensated for by the use of vertical thrust, but this solution has several drawbacks. Firstly, the vehicle loses forward momentum as thrust power is utilised to achieve buoyancy. The use of vertical thrust over loose sediments/silt also creates a disturbance on the seabed which impacts the visibility and thereby also the efficiency of the operations. This latter drawback is particularly critical upon retrieval of nodes, as visibility is crucial to finding the nodes on the seabed.
Shearwater has for a long time looked at how one can improve the efficiency in handling seismic nodes in the water. Three major efficiency gains can be achieved by:
- 1.bringing more nodes to and back from the seafloor in the same operation,
- 2.being able to use the thrust in the node handling vehicle for forward speed rather than to compensate for buoyancy, and
- 3.automation in handling the nodes during deployment and recovery.
In order to be able to carry more nodes, the buoyancy issue needs to be addressed. The payload of a node handling vehicle will vary depending on the number of nodes in the vehicle at any time, which means that buoyancy change cannot be offset with a fixed buoyancy system. Thus, the buoyancy needs to be adjusted as the weight of the vehicle changes and, to complicate this further, this adjustment needs to happen remotely at high water depths.
By using long standing proven technology from the subsea industry, Shearwater has been able to integrate an in situ adaptable buoyancy system in our next generation node handling vehicle.[1] An adaptable buoyancy system changes the buoyancy of the device to make the vehicle positively or negatively buoyant so that it will move up or down through the water column, typically by changing the vehicle’s total mass, or by changing its volume. In other words, the buoyancy system ensures that a desired level of buoyancy can be maintained at all times independent of the weight of the payload or the density of the water. By integrating an adaptable buoyancy system in our node handling vehicle, the need for vertical thrust to maintain buoyancy in the water column is reduced, which allows us to focus on thrust for forward motion. It also allows the vehicle to take more nodes (payload) without jeopardizing visibility on approach to the seabed for deployment or recovery and provides flexibility to balance buoyancy according to the number of nodes currently in the vehicle.
With buoyancy related limitations solved, the next challenge is to move faster over the seabed. This is done by changing the hydrodynamic shape of the vehicle. This change, in combination with the addition of more thrust and the re-allocation of thrust to forward speed, will allow the vehicle to move at far greater speeds than what is possible today.
Our final step on the journey to get an efficient node handling operation, is to add autonomy to the vehicle. This is done by integrating state of the art in underwater navigation, autonomous detection of nodes, and combining these to drive manipulators and sensors to automatically pick up and place nodes on the seabed. Shearwater is inter alia developing tools to aid in the remote sensing of deployed nodes in-situ on the seabed and/or buried below the seabed/water interface, such as AI based visual recognition, remote detection through ferrous material, and active pinging of deployed units.
With this node handling platform, Shearwater manifest its position as the industry leader in seabed seismic technology.
[1] Examples of which is described in e.g. M. Elkolali, A. Al-Tawil, A. Alcocer, “Design and Testing of Miniature Variable Buoyancy System for Underwater Vehicles”, IEEE Access Vol. 10 2022 and M. Worall, A. J. Jamieson, R. D. Neilson, M. Player, P. M. Bagley, “A variable buoyancy system for deep ocean vehicles”, IEEE Xplore July 2007 (Conference Paper)