ROV in residence

Elaine Maslin

October 1, 2016

Elaine Maslin details Statoil’s journey towards the resident ROV – using an existing ROV design.

The Merlin UCV. Photo from IKM Subsea.

The concept of the subsea resident remotely operated vehicle (ROV) has been around for some time, mostly modeled on new types of vehicles.

A new project in Norway looks to take a different approach, which could see an ROV parked subsea permanently – or at least for periods of time, which will be extended – as early as Spring next year.

The project is being run by IKM Subsea under contract to Statoil and will see a resident ROV, or RROVs stationed underwater at the Snorre B platform offshore Norway, plus a more traditionally deployed ROV at the Visund facility.

By “parking” ROVs subsea, for drilling-related work, operational time could be increased, as deployment wouldn’t be weather dependent. Furthermore, offshore staffing could be reduced – the vehicle could be piloted from shore. For other subsea work, the need for support vessels could also be reduced.

“The idea is it would always be available for operations because you’re not dependent on weather anymore because it is constantly submerged – there is no splash zone,”says Jan Vegard Hestnes, operations director, IKM Subsea, based south of Stavanger. “It just goes back to its station. You will only bring it to surface for planned maintenance.”

The approach being taken by IKM and Statoil will build on a trend towards the electrification of ROVs, a move which helps increase energy efficiency, reduces hydraulic support functions and fluid use, and helps to reduce the size of umbilicals. It also creates the opportunity to use the ROV as an extension cord for other subsea electric power requirements, i.e. when you’re not flying, you can divert power to subsea pumps or dredgers, etc.

While electric ROVs are in use, the resident ROV is still a goal – but perhaps not for much longer. Under a 10-year contract with Statoil, with a 15-year extension option, IKM will provide ROV and subsea services for the Visund and Snorre B facilities, including introducing the RROV (residential ROV) based on IKM’s Merlin ultra-compact vehicle (UCV). The contract also covers ongoing RROV technology development. The vehicle will be parked close to its respective platform and in such a way that it could be brought up to the platform for any maintenance needs.

Hestnes says that docking will not be an issue. “For subsea docking, to have it down there, we don’t need to actually develop any new technology, it is more using existing technology in a new way.” More details are not yet available, however.

IKM’s Merlin UCV, measuring 2.5m-long by 1.5m-wide and high. It’s a 2750kg, 200HP, 3000m water depth rated vehicle with seven electrical thrusters with Schilling manipulators. Its pulling force is 8kN, forward, aft, vertical and lateral. It’s an electrically powered vehicle, with interfacing for electric and hydraulic tooling, and auto heading, depth, altitude functions with dynamic positioning system.

All the thrusters are independently powered, so if one fails, the rest will continue to work and allow the vehicle to continue operating.

IKM designed the vehicle to be smaller but with the same capabilities as a larger work class type ROV, building on its Merlin WR200, which also has all electric propulsion. In comparison, the WR200 is 2.8m-long, 1.8m-wide and 1.7m-high and weighs 2800kg. The first Merlin UCV was contracted to Shell to work on the Draugen field offshoreNorway last year, off a support vessel.

Because it is electric there are fewer components subsea, less hydraulic oil, and fewer fittings and connections, Hestnes says. “We think the future is in electric and we saw demand in the market for a smaller ROV, which still needs to have the same capability as a large ROV,” he says. “Operators are working on subsea structures, which are narrow and difficult to access, so they need smaller vehicles. So, we looked at the WR200 and used the same technology but shrank everything. It’s smaller, but has the same capability.”

When deployed Snorre B, the Merlin UCV could be operated either from the platform on from an onshore operations center, Hestnes says.

While the vehicle will initially be tethered, and joined by a standard work class ROV and a standard UCV, and able to work out to an 800m radius, the idea is it will eventually drop its tether. “In the future we hope and foresee a RROV that is tetherless,”Hestnes says. “More and more rigs will have this. It will help reduce persons onboard and make operations more cost effective. But it is important to do this step by step. First, we will have a submerged ROV, over long periods of time, and then move over to tetherless.”

To achieve a full, tetherless work class ROV would also need to see battery technology developed, as today’s batteries would be too big and make the RROV too big. “You can’t take a Tesla battery and put it in an ROV,” Hestnes says.

Another piece of this is the development in machine vision and machine learning technology, where the ROV will have situational awareness and the operator will be able to give it a command, instead of the complex, difficult manual positioning the industry has had. It’s not yet capability the Merlin UCV has, but it’s something IKM is looking at closely, Hestnes says.

A number of operators already have their own projects, most starting from a less capable machine, like an autonomous underwater vehicle (AUV) with the aim of increasing functionality as the design matures. Eni, for example, has been working on its AUV technology, Clean Sea, which the firm says is conceptually similar to a drone, but in the marine environment. OE has also covered Subsea 7’s autonomous inspection vehicle (AIV) development, Total’s inspection AUV concept (OE: October 2014) and Statoil’s work with Oceaneering using the eNovus.

Snorre B, from Statoil.

Snorre B

Snorre B is a semisubmersible integrated drilling, process and accommodation facility, which came onstream in 2001, in the Norwegian North Sea. It produces, then pipes oil 45km to the facility Statfjord B for storage and export.

Statoil and its partners on Snorre, which also comprises the Snorre A facility, are evaluating redevelopment concepts for the field (the Snorre 2040 project – initially comprising a new platform, but revised to a subsea development early 2016). A plan for development and operation is expected to be submitted to the authorities at the end of 2017, according to the Norwegian Petroleum Directorate.

The Visund A facility is also a floating production, drilling and accommodation unit. It came on stream in 1999, in the Norwegian North Sea.

Oil is piped to Gullfaks for storage and export. Visund began producing gas in 2005.

Tether life

Carbon fiber, from bobbin to umbilical. Photos from Nexans.

ROV tether cables have a hard life. The tether, the physical connection between the ROV and its tether management system (TMS), supplies power and communications to the ROV.

The cable is installed onto a relatively small drum on the TMS, and spooled in an out, subject to strain, sharp movements, axial loading, hydrostatic pressure, not to mention repetitive bending across multiple planes.

With operations moving into 4000m water depth and beyond and excursion length reaching 1500m, the wear and tear of tether cables is continuously increasing.

The strength member in tether cables are typically fiber yarns, typically aramid, such as Kevlar, with the fibers used in contra-helically stranded layers around an electro-optical core. The core and the fiber yarns are coated with an outer sheath made of thermoplastic elastomer.

Now, the Norwegian ROV cable manufacturing business of France’s Nexans is moving away from traditional fiber yarns. Nexans and Dutch firm DSM Dyneema worked together to create a new higher strength, fatigue resistant ROV tether, called ENABLE, using bending optimized Dyneema, called XBO Technology, made with ultra-high molecular weight polyethylene (UHMWPE) fiber, also known as high modulus polyethylene (HMPE).

“We are constantly trying to improve our designs,” says Karin Vaslestad, sales and marketing manager, Nexans. While the market is tough, introducing improved products with longer service life should be a focus for the industry, she says. “It is something we have wanted to do for years but we have not found fibers with the unique and improved characteristics as the new XBO Technology fibers.”

Comparative bend cycle testing between tethers with Dyneema (SK78 XBO) and aramid, showed that Dyneema outperformed aramid with regards to fatigue, and retained constant physical characteristics over time, while aramid shows a significant loss of strength due to friction abrasion.

To extend service life, conventional tethers require multiple re-terminations during their service life. The cable sees a lot of load and compression forces leading to internal friction and wear, resulting in loss of strength.

The number of re-terminations needed for conventional tethers is dependent on the application, however, a rough estimate would be 5-10 re-terminations per 1800 hours, according to Nexans and DSM. For the ENABLE trial cable (operated by Fugro Subsea Services), no re-termination was needed up to 1100 hours.