Join OEdigital on Facebook Join OEdigital on LinkedIn Join OEdigital on Twitter
 

Starting from the (ocean) bottom

Written by  Elaine Maslin Wednesday, 01 March 2017 00:00

A seismic shift – pun intended – has been made in ocean bottom node technology. It is being made more efficient and cheaper to deploy, bringing it closer to challenging the high-end towed streamer market. Elaine Maslin reports.

Miniaturization in practice. Magseis’ latest node.  Photo from Magseis.

While ocean bottom seismic (OBS) is not new, it has, until now, been something of a niche within the wider seismic, towed streamer dominated technology market.

The technology puts seismic sensors on the seafloor, where they collect high-quality, full azimuth data, often in 4D mode, for reservoir monitoring. Ocean bottom node (OBN) technology offers a flexible, high-quality seismic solution, but, its higher cost has held the technology back, until now.

Huge strides have been made in reducing node size and weight, and in developing automated handling systems – for both remotely operated vehicle (ROV) and cable/rope (node on a rope) deployments. This enables far faster surveys, thereby reducing costs.

OBS firms are adding to their inventory and mobilizing additional crews, as they expect their market to grow – and not just in the reservoir monitoring market. They think this technology could also be cost-effective for exploration surveys, which traditionally cover wider areas.

“We believe [OBS] is going to take a bigger part of the exploration market,” says Petter Steen-Hansen, vice president, sales and marketing at Norway’s Magseis. “Already, a year ago, we did our first multi-client survey in the Barents Sea for exploration purposes. The feedback was very encouraging. [Operators] see it as having potential for certain areas where there is complex geology. As we increase efficiency and reduce cost, we think there is definitely scope for more 4D operations and certain exploration areas.”

“People now see that acquisition cost is coming down to a cost level that is competitive with traditional towed streamer technology,” says Vidar Hovland, CEO of inApril, a relatively new Norwegian OBN system supplier. “We saw in the 1990s, when the 3D streamer vessels started towing more streamers, and the cost came down, that the market responded fairly quickly. We believe that we will see the same now, that OBN will take a portion of the streamer market as costs come down closer to streamer seismic.”

2017 could be the year things rebound for OBS. “We expect that, if the oil companies maintain their current seismic budgets, the 2017 OBS market will be back up to that of 2014,” says Chris Walker, chief geophysicist at Seabed Geosolutions (a CGG, Fugro joint venture). Industry estimates put the 2014 market at about US$850-900 million, while it had dropped to an estimated $390 million in 2016.

Taking seismic to the seafloor. Magseis’ MASS system deployed using an ROV.  Image from Magseis.

Starting at the bottom

To date, OBNs, which usually contain four components (i.e. 4C: one hydrophone and three geophones), have mostly been used for reservoir characterization and monitoring. Thousands of nodes are laid on the seafloor, using remotely operated vehicles (ROVs) or cable on grids up to a few hundreds of meters apart. Vessels towing the seismic source sail over and the nodes collect the reflected data (instead of receivers on streamers towed behind vessels). The data is downloaded when the nodes are retrieved and the batteries recharged or replaced.

By putting the receivers on the seabed, instead of on long cables behind source vessels, the signal to noise ratio is reduced. Seabed nodes can also gather both shear and pressure waves, and offer full azimuth coverage, all of which adds up to very high quality data and an ability to see where other technologies struggle. Nodes can also be deployed where streamers cannot go, i.e. around existing infrastructure.

ROV positioning accuracy enables accurate repeat surveys for 4D surveys. Cable, or node-on-a-rope, positioning is generally seen as getting less accurate as water depth increases – although this is being challenged.

Seabed Geosolution’s Manta node in ROV and node on a rope guide. Image from Seabed Geosolutions.

Past issues

In the past, issues with nodes have been around battery life, size (largely due to the large batteries), and slow, inefficient deployment and recovery. In some cases, this meant having to do a survey in chunks, retrieving and redeploying nodes multiple times in order to cover a set area, while source vessels waited.

How it’s done, by Seabed Geosolutions.  

In the past, a typical deepwater node setup would see nodes deployed 400m apart, with source shot over them on a 50m grid, on X and Y axis, Walker says. Sensors were deployed sparsely, to keep down ROV vessel costs, with denser shot grids (from the cheaper source vessels) to compensate. Even then, because of the time it took to place the nodes, using ROVs, the source vessels would have to wait 2-3 days after node laying started before they could start working. Node spreads would possibly need to be laid, shot over, then picked up and moved to the adjacent area, but with overlap, and then another shot run, which was also inefficient.

“Historically, even with two ROVs, you would be deploying 80-100 nodes a day, maximum, on a 400m node grid,” Walker says. “With smaller new generation nodes, with comparable node spacing, you are able to, at a minimum, double that deployment rate, which means we can handle nodes fast enough to allow rolling receiver spreads with two dual source shooting vessels operating simultaneously.”

Previously, laying nodes was the normal project critical path activity, but now it’s the source vessels, says Martin Hartland, executive vice president, performance, at Seabed Geosolutions. “As multiple source vessels start shooting simultaneously, thanks to faster node handling, that’s when you can get three-, four-fold improvements in production and a significant reduction in the overall project cost. That’s where we are,” Hartland says.

Miniaturization

A mid-water transfer of Seabed Geo-solutions’ Case Abyss Nodes, while sailing between two node deployment lines at 0.7 knots.  Images from Seabed Geosolutions.

Reducing node size has been key. The size and weight reduction has been aided by using micro-components and micro-electronics, helping to reduce power consumption, as well as advances in rechargeable battery technology, for those who use rechargeable batteries. Node designs have also been optimized, which, together with the introduction of automated handling systems, has allowed for greater numbers of nodes to be stored, charged, data downloaded and deployed again faster.

“The small size of our MASS I nodes, at about 8kg, allows us to create a much more efficient and portable system and virtually means we can carry an unlimited number of sensors,” says Magseis’ Steen-Hansen.

The firm’s 65-day battery life Marine Autonomous Seismic System (MASS I) nodes, weigh 8.35kg, are 227mm-long, 160mm-wide, 88mm-tall,rated for 3000m water depth, and can be deployed by ROV or by inserting the sensor capsules into casings on Magseis’ own automated cable system. A few years ago, nodes were weighing in excess of 60kg. Magseis also recently launched a MASS III node with 150-day battery life.

“Everything is containerized and fully automated, which means they could be deployed on ROV vessels already on hire by a client within a field,” Steen-Hansen says. “Robots change the batteries, dock them and download data. The small size enables us to do that.”

Magseis’ MASS nodes were designed around using an atomic clock, for timing, and positioning accuracy. They have a 32-bit analogue to digital resolution, Steen-Hansen says. “Some of the technical specifications are leading edge sensor electronics,” he says. “The company has been inspired by miniaturization from the mobile phone industry and does a lot of research and development in Sweden. We are raising the bar of what’s achievable and it doesn’t stop. It’s all about more inventory, fast deployment and reducing overlap and source acquisition,” he adds.

Magseis has mostly deployed its nodes with its automated cable system. But, this year it will do its first full-scale ROV deployed project, with ConocoPhillips in the Norwegian sector of the North Sea. Some 3000 MASS I nodes will be deployed from a vessel of opportunity – another first for Magseis, which has been using its cable spread on the Artemis Athene.

The nodes, because they’re so small, will be deployed in large batches via a skid on an ROV, reducing the time needed for the ROV to travel through the water column and eliminating the need for a subsea basket from which to collect nodes, a technique others also use. Two ROVs will work in parallel from the one vessel, which will traverse between the middle of two receiver lines. “Because our nodes are very small, we can carry a lot more so that we don’t need a basket solution,” says Steen-Hansen, although he wouldn’t give away numbers. “Each ROV can carry significantly more nodes on each run and, therefore, increase the deployment efficiency.”

Smart systems

Tightly packed: inApril’s node system.  Photo from inApril.

Seabed Geosolution’s latest generation node, the 3000m water depth-rated Manta, weighs 15kg in the water (a previous generation weighed 65kg) and now measures 350mm-wide, 350mm-deep and 75mm-high. It can be deployed by ROV or on a rope/wire, and will work for 75+ days in -5°C to 40°C.

Some 900 Manta units can be stored in a standard 20ft container, which, along with the deployment system, can be deployed on a vessel of opportunity, for deployment by ROV or, with the addition of an additional attachment and rope module, it can become a node-on-a-rope system.

This year, Seabed Geosolutions is due to launch its first large-scale 4C/4D Manta OBN crew, with the nodes due to be delivered in Q2.

“The size reduction allows a single ROV, generally working in tandem with a second ROV, to carry a tray of 21 nodes at a time, instead of one-eight (node type dependent), we have seen in the past,” says Hartland. The trays are retrieved from a subsea basket containing two, 21-node trays, keeping reload requirements down.

To avoid any turbidity created by putting the basket on the seafloor, the basket is suspended in the water column near the seabed and a mid-water transfer takes place. Using pixel or image recognition software, the ROV will dock and lock on to the basket automatically, removing the need for difficult manual ROV manipulation and allowing the surface vessel to maintain a steady 1.5 knots. All of this automation is commanded from an integrated navigation and control system, which connects the ROVs (and node positioning), their respective tether management systems (TMS), basket position, the ROV vessel position, and the source vessel position, through one integrated command and control system, Hartland says.

“We can also control the seismic source vessels, ROV and other navigation systems from one integrated navigation control system,” says Hartland. “That’s the enabler, automating the node laying system. It is a big step forward. Laying nodes on the seafloor is a very repetitive activity. The ship goes along a sail line, two ROVs, 200m each side of the ship lay nodes every 200-300m. It’s very repetitive and, today [in current generation systems], that’s done by people.”

Software

Light packaging: inApril’s node and node system. Photos from inApril.

Control systems and simulation are also playing their role in making these systems more efficient. Seabed Geosolutions is using Fugro DeepWorks, a simulation program, which helps train pilots ahead of a job, and actually also helps plan out the job, using physics engine software. “Using time domain physics-based simulation, you can fly missions in a simulated environment using the actual ROV console,” Hartland says. “We’ve taken that and adapted it to two vehicles at once. When we plan projects, we put in the seafloor from multi-beam echo sound data or Lidar data, then fly the mission over the same seafloor terrain.”

This can include a floating facility’s mooring system, risers etc. or an extreme terrain, which would help plan out the optimal height to position the TMS and enable greater understanding and control of umbilicals’ and tethers’ catenaries, reducing wear and tear, and operational risk and highlighting areas for further operational optimization. Because it’s a physics engine, it can model the whole operation, including active heave compensation effects and current profiles. “These tools and systems allow us to be more productive and more certain of our performance before entering an area,” Hartland says.

On the ropes

Automation is also improving node-on-a-rope operations, where previously a human had to hook each node onto the “rope” as it was deployed. Now the process is hands free and automated, from storage to the wire and to the seafloor. Because the node is on a rope, or wire, and deployed continuously, node-on-a-rope deployment is much faster than ROV deployment, especially with dense inline configurations, making it potentially more attractive for exploration work.

In 2016, Magseis laid 350km of continuous cable with its MASS I nodes, for a project with BGP Arabia in the Red Sea (in close to 0-1100m variable water depth), which continues into this year. Magseis’ latest cable spread, due on the market this year, will have 1000km of cable with 10,000 MASS I nodes – all to be deployed from one vessel using an automated system and down to 2000m water depth.

It’s quite a step-change. Steen-Hansen wouldn’t give deployment rate figures, partly due to the variability that these could be, depending on water depth and seabed topography, but he said the firm was confident it was market-leading.

Furthermore, Steen-Hansen says that the nodes, when inserted into their casings along the cable, have a shape and weight density that enables improved positioning accuracy, offering the potential for 4D work, where repeat positioning accuracy is needed, even as the system goes into deeper waters, in excess of the 1100m, achieved in the uneven seafloor topography of the Red Sea.

inApril is also chasing fast node-on-a-rope deployment speeds. The firm is looking to prove deployment speeds up to 6 knots this year, and Hovland thinks that node-on-a-rope can be efficient in up to 1500m water depth.

inApril was set up to be an equipment supplier, with the same node and system for shallow and deepwater, whether deployed by rope or by ROV. The firm says the system can work down to 3000m and that 10,000 nodes could be handled from a vessel of opportunity via node-on-a-rope using an inApril automated system, or via ROV with the contractors’ own methodology.

inApril’s Venator (hunter in Latin) A3000 nodes are 300mm x 300mm, 110mm high, weigh 22kg and have a 100-day rechargeable battery life. The firm’s automated system tracks all nodes’ statuses and selects deployment sequence, as well as controlling the speed and the node spacing along the rope.

The firm has built and tested all the critical components for its system and ran a commercial trial in the northern Caspian Sea in November for Geo Energy Group. A more extensive 50-node test is due to start early this year, to demonstrate a 6 knot deployment with an oil company in the Norwegian sector of the North Sea.

MASS en-MASS

As part of a joint development project with Shell, Magseis is working on a system to deploy MASS technology in ultra-deepwater with positioning accuracy for 4D work, using a towed subsea vehicle, from which the nodes are automatically deployed. A full-scale pilot test in >1200m water depth was completed in Q3 in a Norwegian fjord. Steen-Hansen says that first commercial operations are planned for 2018.

“We recognize that for deepwater and certain types of deployment, ROV could be a good solution,” Steen-Hansen says. “But, if we really want a step change, something else is needed.”

Flying nodes

There are also systems in development that wouldn’t need any type of vehicle or cable for deployment – so-called flying nodes. OE covered Autonomous Robotics Ltd’s (ARL) flying nodes concept last year (OE: February 2016). The concept stage for ARL’s system is complete and a tethered version of the first prototype flying node will start in water testing during 1H 2017. Seabed Geosolutions is working on a similar product, dubbed spicerack nodes, with Saudi Aramco.

Another alternative is using nodes for long-term, or permanent reservoir monitoring. However, with the increased efficiency of today’s node laying operations, Walker thinks permanent systems will be an inefficient use of a capital asset.

Retrievable systems, compared to permanent systems, offer more flexibility and scalability, Steen-Hansen says. “You can have flexibility in survey design, if you want it bigger or smaller, depending on how the reservoir develops and scale it accordingly,” he says. “You’re also not locking yourself into one technology for a long period of time.”

Either way, OBN technology seems here to stay. “The future is on the seafloor,” Walker says. “I joke that, when people join seismic vessels in the future, people will say to them ‘you know they used to tow sensors.’”

Read more:

Making nodes fly

Read 4003 times