The job of placing ocean bottom nodes for seismic acquisition could be set for a revolution. Elaine Maslin looks at Autonomous Robotics Limited’s concept.
A single node, nose on. Images from Autonomous Robotics Ltd.
Imagine a scenario where you do not need to individually deploy ocean bottom nodes (OBN) for seismic surveys using remotely operated vehicles (ROVs). Instead, they fly and accurately land themselves on the seabed, autonomously.
It’s an attractive idea, especially when you are talking about deploying hundreds or thousands at a time in deepwater offshore the likes of the Gulf of Mexico, Brazil or West Africa. It’s also not that far-fetched. Autonomous Robotics Limited (ARL), a subsidiary of Thalassa Holdings, has been working hard on the idea since 2013, after buying GoScience, a technology company which had developed its own concept for flying nodes. The firm is now presenting the concept.
Nodes in containers.
Put simply, the concept is a fleet of autonomous underwater vehicles (AUVs) operating as OBNs, which ARL call “flying nodes.” Stored on the deck of a vessel in racks, they are deployed in groups of 72 via a cage, which is lowered into the water using an onboard launch and recovery system. Each exits the cage in turn under its own propulsion then flies to its pre-programmed position on the seabed. Once their work is done, and they’re given a signal, each flies back up, back into the cage, before being stored back on deck where data is downloaded and they are recharged.
“Each flying node is effectively a combined AUV and OBN,” says Dave Grant, ARL’s CEO, and former managing director at SAAB Seaeye. The company has used, where possible, already tried and tested technology, or a minor variant of it, such as using ultra-short baseline (USBL) positioning and lithium ion batteries, although battery technology is fast improving and changing, so these could well change and continue to change, Grant says.
Node vessel (3).
According to simulation work performed by ARL, deployment speed using the firm’s flying nodes would be far greater than ROV-placed nodes, with some 1200 able to be deployed in one day, compared to 50-80 a day using an ROV. “This is more than an order of magnitude greater,” Grant says. “Even with the automation coming into ROVs today, it would not be much faster and ROVs are currently the only option for deep water OBNs at the moment. What is holding ROV-placed nodes back isn’t the quality of the data, it is the cost, which is excessive. If we are able to reduce the survey cost of deepwater ocean bottom nodes compared with ROV placed nodes there will be a major justification for this solution.”
If you were surveying a 96sq km area, with nodes placed every 200 x 200m, with one node vessel and one source vessel, the costs would be 20% cheaper than using an ROV-deployed system, Grant says. If you go to the next stage and have two source vessels and one node vessel, you could save 66%, he says, by being able to shoot the survey faster.
While ARL bought GoScience’s IP, it has pretty much started from scratch with the concept. GoScience had been developing a “ring wing” autonomous underwater vehicle (AUV), Grant says, and it wasn’t a configuration ARL thought would work for OBN seismic. ARL has looked at the challenge, taking into consideration cost of manufacture, and ability to handle high numbers from one vessel.
Cage being deployed.
What they have come up with are more rounded, disc-shaped AUVs. The shape was chosen both for ease of handling and storage on deck and in the deployment cage, but also to lower drag as it travels through the water.
For navigation, established USBL underwater positioning systems, using acoustic signals, used on vessels today for guiding AUVs and ROVs has been used. But, for its flying nodes, ARL uses two USBL systems, one on the node vessel and a second on an unmanned surface vessel – another technology, which, while new to the oil and gas industry, is established in the defense and environmental monitoring space.
“Using two is necessary to accurately position the node on the sea bed,” Grant says. “As the vessel is moving, by the time the node is on the sea bed you wouldn’t be able to accurately position it. The accuracy will be equivalent to ROV-placed nodes.”
For power, the firm is currently looking at lithium ion batteries, but this may change. “We are looking at a number of new technologies and the battery technologies used will likely change as we go forward as technology improves. But, it’s just a battery pack and battery management at the end of the day,” Grant says.
Node recovery into cage.
The flying nodes have been designed to work down to 3000m. While ARL isn’t revealing the unit’s power consumption, Grant says in a 60-day deployment at 3000m, about half the battery power would be used on the seabed, with the rest used to descend and return the node to its cage. In shallower waters, they would of course use less power during deployment, which means they could be left for longer, Grant says.
With deep ocean bottom currents mostly less than half a knot, or even a quarter of a knot, there is not much to deal with in terms of disturbance on the seabed, he says. However, ARL would measure the current profile through the water column and on the seabed before deployment in order to develop a deployment plan.
And this is where the flying nodes get interesting. ARL has developed the flying nodes to have variable buoyancy, which will aid their descent as well as how they rest on the seabed, with a mixture of pre-programmed and live control via acoustic communications.
“The nodes will have neutral buoyancy when they are deployed to 20-30m beneath the vessel,” Grant says. “When we want them to descend we can change the mass distribution and they fly down to the seabed at about 60°. Near the seabed, the mass distribution is again changed so they descend vertically to the seabed. On the seabed they are heavy to give the pressure needed between the coupling plate [a type of metal castellated plate on the OBN’s base] and the seabed to be able to give a good coupling to transmit the sounds waves in to the OBN’s sensors.” The rounded disc-shape also means each unit has a uniform weight distribution around the center of the sensor plate, which helps the coupling. Most of these features, the variable buoyancy and mass distribution are pre-programmed before deployment, but, certain features during the operation will be controlled, so a low data rate, bi-directional system has been built in to the USBL system, between the node and the node vessel, so there is status information and you can send control information back to the node when necessary. The USBL system has also been adapted, because of the high numbers of nodes being deployed.
“It is quite a challenge trying to fit all these features and capability in to a volume about 580mm diameter and 280mm high,” Grant says. “And we want to get this down another few millimeters.”
So far most of the design work has been based on studies and simulation, as well as work with technology suppliers, such as the USBL system. But, the company is moving towards the end of concept stage and hopes to get the first prototype node, which will initially be tethered, into the water in Q1 2016. “We expect to go into detailed design some time in 2016,” Grant says.