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Subsea acoustics on the upgrade

Posted on 09.08.2011 - 13:00 UTC in SURVEY NEWS by Rons_ROV_Links

Subsea acoustics on the upgradeWith its advanced Acoustic Digital Spread Spectrum Signalling (branded ADS²) technology and operating from bases in Australia, the US and UK, Nautronix targeted subsea acoustics applications in both the defence and oil & gas sectors in its early days.


"In early 2000, Royal Australian Navy submarines equipped with Nautronix NAS-HAIL through-water communications were able to outmanoeuvre the opposition in exercises," recalls CEO Mark Patterson. "The company then won a two-year FCT (Foreign Comparative Testing) trial programme from the US Navy which resulted in a proposed fleet wide fit of the technology to US nuclear submarine fleet. In the critical area of silent, undetectable operations, submarine test ranges equipped with NASPAR (Nautronix Portable Acoustic Range) were recognised as amongst the best in the world, with projects in Japan, Australia and the UK," he adds.

In 2006 the Nautronix defence business was sold to a leading US defence contractor, L3 Communications, a move which would allow the technology to be used in top secret US Navy work where a Scottish company would be unable to operate. Following this sale, Nautronix relocated its headquarters to the current premises in Aberdeen to concentrate on the development and production of subsea acoustics for the oil & gas industry. Its systems are today deployed globally and right across the water depth spectrum from shallow to ultra-deep.

NASNet Developments

Recognising the need for a multi-user, fast update rate, deepwater acoustics network for both positioning and communications offshore, Mark Patterson elected to focus development activity on the two products he saw as 'very exciting' - NASNet and NASBOP. In his view, had NASNet, with its unlimited multi-user capability, been in operation in the Macondo field at the time of last year's Deepwater Horizon disaster 'much more could have been achieved much faster' in the aftermath.

Undeterred by potential accusations of '20:20 hindsight', Mark Patterson notes: "It has been accepted that one of the major limitations of subsea activity, following this tragic event, was the lack of acoustic channel availability using traditional acoustic systems. The oldest of these systems are analogue and, worryingly, it is estimated that over 80% of offshore vessels of various types are still operating with analogue acoustic back-up systems in 2011. These need to change to digital as soon as possible. Even then, most systems are limited by the fundamental concept employed for signalling. They may perform better than when they were analogue but there are still limits to the number of systems which can operate in close proximity, and using'two way' signals they still have much slower update rates than a passive, truly multi-user system such as NASNet."

"The positioning challenges at Macondo were posed by the number of close proximity simultaneous operations (SIMOPS) due to the number of vessels, subsea vehicles and assets," adds Mark. "Acoustics are now a must on every vessel with ROV and DP, and it is when you have a group of these vessels together that the problems arise. Traditional LBL systems are severely limited by the amount of frequencies that can operate in the same area at the same time. One ROV support vessel could easily use all the available channels. Therefore, with traditional technology - analogue or digital - there is a tremendous amount of field management required and in busy situations that can mean a lot of vessel and asset downtime. This can waste a great deal of money, but in the case of an emergency, such as Macondo, the losses can extend far beyond that."

"In the past, decisions to not install NASNet have in the main been financially based, but in the future we anticipate that the value of having such a system in place, for normal operations and emergency planning, will now be fully recognised," says Mark.
Another area outwith normal field operations where he sees NASNet providing 'truly outstanding' service in future is in handling the disruption to GPS satellite signals caused by 'scintillation' during periods of solar activity. "This cyclical phenomenon occurs every 11 years and we are now in year nine and nearing the peak of the cycle," explains Mark. "The last time it happened there were only around five vessels with DP in Brazil which could have lost position. There are now 35 vessels, and in three years this could be 50 vessels potentially affected by the loss of GPS signals during scintillation. At the height of the next new cycle (2025) there could be hundreds of DP vessels affected if Petrobras achieve their ambitions for all the newbuilds they are planning."

"Rigs and vessels which are equipped with NASNet DPR deployed will be able to continue working seamlessly through scintillation because the information update rate of every second is fast enough for DP systems to recognise as reliable positioning data and therefore to continue operating. Without such a system there is a high risk of having to suspend operations during scintillation, with the resultant cost impact."

Life-of-field requirements

According to Alan Nicol, Director of Operations at Nautronix, NASNet is 'at its best' during field installation/development when multiple vessels are operating in close proximity. "In this scenario, both operational and financial benefits of NASNet are significant," he says, citing the following examples of potential multi-vessel requirements:

  • Fast track projects to achieve first oil
  • Simultaneous drilling and construction operations
  • Any simultaneous operations (SIMOPS) requiring vessels - for drilling, construction support, pipelay, heavy lifting and the like - that command high day rates while waiting on basic frequency management.

"It seems clear that a true unlimited multi-user subsea positioning system offers real benefits over traditional systems in normal field operations - especially in deepwater," he adds.

Alan says it would be ideal if a complete NASNet network were deployed at the start of field development, but in reality this is normally achieved in stages. "NASNet stations can be set out where the first activities are to take place, and then stations can be added as the field expands," he explains.

"It is important to recognise that NASNet is much more than a positioning system. It is a through-water communications system which enables a wide range of data to be transmitted across large fields," says Alan. "For example, well condition, riser and environmental monitoring can be measured and sent to the host platform. This can offer potentially huge benefits in field management as well as safety and redundancy scenarios."

Mark Patterson adds: "Macondo has taught the industry a lot of lessons, not least of which is to review some of the parameters on which investment decisions are based. A NASNet MkII network might cost a little more to install initially, but the life-of-field benefits and potential savings greatly outweigh installation costs."

Cascade-Chinook contract

A 2009/2010 contract from Petrobras Americas Inc saw Nautronix deploy a NASNet network for the deepwater Gulf of Mexico Cascade-Chinook development which, in just under 3000m of water, is the world's deepest production subsea field development yet.

The initial scope of supply was 14 NASNet stations and 22 NASNet MS (Mini Station) units with all the associated vessel equipment for a two-year period. Petrobras installed the system and then made the positioning signals available free of charge to the field installation contractors. "The system was used by multiple contractors simultaneously to great success," reports Mark, quoting the following observations on NASNet performance from the survey and ROV crews on a Cascade-Chinook support vessel:

"Positioning has been excellent throughout the project" and "Time saved during umbilical lay due to having good positioning throughout project." Offshore ROV team

"Far superior to LBL for large field positional coverage to <1m accuracy during tracking;" "far simpler to use for vehicle tracking and sub-meter accuracy at depth" and "once the array is installed it has a longer lifespan subsea in comparison to a constantly interrogated Sonardyne Mk 5 LBL array." Offshore survey team

The company received similarly impressive feedback following earlier large-scale deployments of NASNet in the Agbami field, offshore West Africa, and Frade, offshore Brazil, reports Mark. "Significant cost savings have been achieved through each deployment," he adds.

So what's next for NASNet and Nautronix? Mark sees the system's usage continuing to grow, particularly in subsea oil & gas operations. "NASNet has been proven, it works, and it brings significant benefits over traditional subsea acoustics, especially when introduced at the development stage of a field and in multi-user situations," he says. But he admits that it is never easy bringing new, game-changing technology to market. He reckons Nautronix has already invested over £20 million in developing NASNet and the ADS² technology behind i t and is currently committing several million pounds more to further development and enhancement of the technology.

As well as expanding geographically - the company recently opened a Brazil office to back up the work of its Aberdeen and Houston facilities - Nautronix continues to grow its product portfolio. The latest additions include NASMUX (Nautronix Acoustic Subsea Multiplex), developed in collaboration with Cameron. Afforded OTC 'Spotlight on New Technology' status at OTC 2009, this 128-channel acoustic control and command system was hailed as enabling full acoustic primary control of a subsea BOP for the first time.

"These are just two of our current products, based around ADS² technology, which offer major safety benefits to subsea oil and gas operations, especially in deep water and difficult conditions," he concludes. "We position Nautronix as global leaders in through water communication and positioning technology for the offshore industry. I would like to think we can continue in that role into the foreseeable future."

How NASNet works

"The operating principles of NASNet are easy to understand, hence the analogy to 'underwater GPS'," explains Mark Patterson. "NASNet stations are laid in a grid on the seabed - across a whole field or in a specific area. Set at distances of up to 4km apart, depending on the topography of the seabed, they form a network from a minimum of three stations. These stations are constantly broadcasting ADS² acoustic signals and any vessel or asset fitted with a NASNet receiver and the topside equipment is able to accurately measure its position - irrespective of how many other vessels or assets may be operating in proximity."

"Therefore not only does NASNet offer this genuine multi-user capability, its update times are so much faster than traditional LBL. Speed of sound in water is approximately 1500m/sec, therefore in a water depth of 4500m a traditional two-way signal takes a minimum of six seconds to update. A lot can happen in that time. With a NASNet MkII passive system the rapid, one-second update rate can be beneficial to ROVs and even a drilling vessel if the GPS signal were to fail."

Launching NASNet MkII proved irresistible late last year when, as Nautronix Chief Surveyor Sam Hanton puts it, "we reached a stage when there were so many real improvements we had developed from the original system."

Key upgrades cited by Sam include:

  • Development of the GeoLine planning software, allowing maximum optimisation of the NASNet station layout and improving array performance and efficiency.
  • NASNet MS (Mini Stations) and NASNet stations can now operate together in combined arrays, providing much more flexibility
  • NASNet DPR, a new facility for drilling rig DP input providing the same NASNet MkII benefits, can be used either with an existing large NASNet array or a small Mini Station array accompanying the rig wherever it operates.
  • Field management is now fully automatic with information on NASNet Station buoy positions, timing etc all being automatically by the seabed array itself.
  • MkII stations have a battery life now extending up to four years' continuous use, regardless of how many users are operating in the area.

According to Sam, the NASNet MkII user interface has also been significantly enhanced and is more user friendly thanks in large part to customer feedback. "It is now extremely comprehensive, but much easier to use and more intuitive in operation."

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