ROVs go electric, AUVs go to work

Published November 12th, 2000 - 02:00 GMT

Underwater inspection, repair and maintenance will become more efficient by using electricity. Site surveys and pipeline route studies will be more cost-effective by using pre-programmed robots. Bob Barton reports the latest developments in underwater vehicles.  

 

With all the excitement about the imminent entry into mainstream E&P activity of cable-less autonomous underwater vehicles (AUVs) - of which more later - it's easy to overlook some of the striking developments that are being made with the more conventional cable-controlled remotely operated vehicles (ROVs).  

 

Foremost of the ROV developments is the move towards electrically powered vehicles for heavy-duty deepwater oilfield work. Until now this has been the preserve of the big electro-hydraulically powered units: only these, said their operators, had the power and manoeuvrability to perform complex manipulative tasks at depth.  

 

The proponents of electrical power thought otherwise. Chief among these is Alstom Schilling Robotics of the US which earlier this year launched its all-electric Quest vehicle and backed this up with the announcement that it had sold no fewer than 10 systems to Houston-based ROV operator Canyon Offshore.  

 

Quest represents a fundamental rethink of ROV design and incorporates: multiple uses for individual components (for example, there is only one cable/ connector type for all accessories and telemetry equipment); electric ring thrusters, each of which has just one moving part; a 27mm diameter umbilical that saves 22,000lbs of weight on a 3000m system; and so on. A key feature, however, is the electric propulsion system and Tyler Schilling, president and CEO of Alstom Schilling Robotics discussed this with OE.  

 

'ROV efficiency is affected by the number and type of power conversions that take place between the initial application of power at the surface and the performance of useful work (such as thrust production) at the ROV,' he says.  

 

'In an electric ROV, electric power from the surface is applied to an electric motor that spins the thruster blades. In a hydraulic ROV, electric power is converted to hydraulic power by the hydraulic power unit (HPU); this power runs a hydraulic motor that in turn rotates thruster blades.'  

 

This means that hydraulic ROVs intrinsically require several more power conversions than electric vehicles and the system pays an efficiency price in the form of waste heat with each conversion.  

 

In fact, hydraulic systems generate so much waste heat that their on-deck operating times must be strictly limited to avoid damage from overheating.  

 

'In contrast,' Tyler Schilling points out, 'an electric ROV is so efficient that it can remain out of the water indefinitely with the power on. The difference in efficiency between hydraulic and electric systems varies depending on details of each design, but 30 percent is a conservative estimate.'  

 

This difference in efficiency has its greatest impact not in reduced electricity consumption but in reduced umbilical size and weight. With a comparable level of vehicle performance, the umbilical for an electric system can be 55 percent lighter than that of a hydraulic system.  

 

This difference is critical for deepwater systems where the weight of the umbilical dwarfs the weight of the ROV and requires the presence of enormous, heavy, on-deck handling systems.  

 

'Reliability is another consideration: any of the same factors that affect ROV efficiency also affect reliability,' explains Tyler Schilling.  

 

The substantial waste heat created by hydraulic ROV systems is principally caused by the large number of moving parts in the system. Heat indicates the presence of friction, which means that wear is occurring on the moving parts.  

 

(Hydraulic fluid is one of the moving parts in a hydraulic propulsion system, and its contribution to waste heat and wear increases substantially as hydrostatic pressure grows in deepwater applications.)  

 

Hydraulic systems are also less reliable because they have shaft seals, hydraulic hoses, and compensation systems that are vulnerable to leaks. Seawater intrusion from leaks is the most common cause of thruster malfunction in hydraulic ROVs.  

 

An electric ROV can have 15 to 20 times fewer moving parts than an equivalent hydraulic system simply because the power conversion is done by electrical devices such as transformers that do not have moving parts.  

 

This simplicity makes electric ROVs less expensive to build and maintain, and allows reduced deck space for service operations and spares storage.  

 

Electric ROVs have a significantly lower environmental impact than hydraulic ROVs because they require much smaller volumes of hydraulic fluid (which is typically used only to operate hydraulic tools or compensate electronics cavities).  

 

Vegetable-based fluids, which pose little environmental threat, are affordable and suitable for use in electric ROV applications.  

 

Finally, there's control. Electrically driven thrusters provide improved control accuracy because they do not suffer from the fluctuations in power that result in hydraulic systems from HPU saturation.  

 

Saturation can occur either because demand for fluid exceeds the available flow capacity (with a resulting drop in fluid pressure) or because the HPU's large electric pump cannot respond quickly enough.  

 

Electric motors, which have a greater dynamic range, can respond almost immediately. This more precise and predictable control will allow increasingly sophisticated control modes that improve system productivity.  

 

Great gains in productivity will result from automating certain operator functions such as automatically keeping the vehicle on station or automatically following a programmed track.  

 

Other operators, notably Oceaneering, are also looking closely at electric ROVs as are some of the rapidly amalgamating band of independent manufacturers such as Perry Slingsby.  

 

Deepwater demands: 

Meanwhile, there has been a flurry in the development of autonomous underwater vehicles (AUVs). This has been prompted by demands from deepwater operators for better data from pre-drilling site surveys.  

 

At present these rely on instrumented 'fish' towed many thousands of metres behind and below a surface craft at slow speeds (OE May 1999). Positioning of the fish cannot be made to required accuracy and ship time is wasted by the long turns that have to be made at the end of each survey line.  

 

Deepwater operators - notably BP Amoco and Shell - are now actively encouraging the development of AUVs. BP Amoco, for example, is the first operator to use the first AUV specifically built for deepwater commercial survey work.  

 

This is the Kongsberg Simrad-designed-and-built Hugin 3000 recently delivered to survey company C&C Technologies Inc of Lafayette, Louisiana, initially for work in the deepwater Gulf of Mexico but capable of being airfreight-transported worldwide to 'golden triangle' E&P zones such as West Africa.  

 

BP Amoco has been particularly concerned about the lack of productivity in deepwater surveys. The company calculates that as much of two-thirds of site survey time is unproductive because of the turns made by the ship at the ends of lines. 

 

And while it acknowledges that AUV rates will be higher than those of conventional surveys these will be offset by increased productivity: at the moment, BP Amoco typically operates on a 300m¥900m grid for a deepwater Gulf of Mexico site survey; this could be reduced to 200m¥500m using an AUV; the speed of the survey would be increased from two to four knots.  

 

BP Amoco also looks for survey improvements by using AUVs to produce full-route terrain models on pipeline route surveys; in getting hi-res seabed imagery; and in achieving ultra hi-res profiling data of the top 30 or so metres of sediments, digitally cross-referenced to the seabed terrain data.  

 

C&C, not surprisingly, concurs with BP Amoco's thoughts. The company has gained considerable AUV experience by working with the US Navy over the past five years and has drawn up its own commercial cost comparisons.  

 

On a Gulf of Mexico pipeline route hazard survey, for example, in depths from 400m to 2200m with a total survey line distance of 600km at 300m line spacing, C&C calculates a saving of 59 percent using an AUV compared to a deeptow survey using two vessels and ultra short baseline positioning (USBL).  

 

Off West Africa, C&C points out, a 26km¥17km survey in 1500m with 100m¥250m line spacing requires a total line kilometre survey distance of 6274km. A two-vessel deep-towed system with USBL would have a day rate of $54,000.  

 

Total cost for 96 days at 2.5 knots would be $5,184,000. An AUV with single vessel USBL would have a higher day rate ($55,000) but would complete the work in 58 days at a speed of 3.5 knots to give a total cost of $3,190,000. This represents a saving of $1,994,000 or 39 percent. In addition there is the bonus of better, more accurately referenced data.  

 

The C&C Technologies Hugin 3000 is 5.2m long, 0.96m diameter with a depth rating of 3000m. It is powered by a unique aluminium oxygen fuel cell developed in conjunction with the Norwegian Defence Establishment to give the vehicle an endurance of 40 hours.  

 

Normal speed is four knots. Survey systems include a Simrad EM2000 multibeam and bathymetry system, EdgeTech chirp sidescan sonar operating at 120kHz or 410kHz, Edgetech chirp 2-16kHz sub-bottom profiler, Seabird CTD and a magnetometer.  

 

Underwater positioning and vehicle attitude are by a Kalman filter-aided inertial navigation system integrating data from an inertial measuring unit, Doppler speed log, fibre optic gyro, depth sensor, altitude/forward-looking sensor and Simrad USBL HiPap positioning.  

 

There has been some debate during the development of commercial AUVs about the degree of communication that there should be between the vehicle and the surface support vessel. Should the vehicle be launched on its pre-programmed run and 'trusted' to surface again, days later, packed with survey data? Or should there be a communications link - necessarily acoustic - between vehicle and the surface? C&C says that in the strictest sense an AUV with an acoustic link to the surface is classified as an Unmanned (or Untethered) Underwater Vehicle (UUV) and not an AUV, which is fully autonomous.  

 

'Although Hugin is capable of operating fully autonomously it would be foolish to do so except under extreme or unusual circumstances,' says C&C. 'Losing days of deepwater survey data for the sake of fully autonomous operation is a risk we are not willing to take.'  

 

So Hugin 3000 has an acoustic 'tether' to give its operators a degree of control. Sub-sampled data are transmitted to the surface so that should, for example, hazards be located during a pipeline route survey the mission can be altered to interject developmental survey lines.  

 

Changes can be made to the volumetric weighting of the sensor data to maximise system performance. Full-density data are downloaded by fibre optic cable at the end of each mission.  

 

Shell, too, has been looking closely at the possible benefits of AUVs and has come up with some remarkable figures.  

 

Nick van der Veen of Shell's Hague-based EP Technology Applications & Research unit polled Shell managers worldwide and as a result claims that the use of AUVs could save the company $100 million E&P costs over five years. This is how the $100 million works out.  

 

* Survey AUVs. Between 2001 and 2006, survey class AUVs of the Hugin 3000 type would give direct operational cost savings of $9 million and further savings of $50 million by reducing design conservatism. ('Design conservatism' in this context refers to the costly tendency to over-design exploration programmes and field development facilities either because of lack of data on which to base engineering decisions or because of mistrust of the accuracy of available survey data.)  

 

A 'survey' vehicle in the Shell definition has a depth capability of over 4000m, endurance of at least 24 hours, a speed of up to four knots and a payload comprising multibeam echo sounder/ swath sonar, sidescan sonar, sub-bottom profiler and possibly a magnetometer.  

 

It makes continuous measurements but does not have manipulators and is not required to hover or land on the seabed. Its main tasks are site surveys and pipeline route studies, but it can also be equipped for oceanographic data collection which is at present expensive over large areas and is therefore often limited or avoided altogether.  

 

This leads, says Shell, to 'unnecessary (and costly) design conservatism'. Survey AUV use could see savings of $15 million over a five-year period.  

 

* Hybrid AUV/ROV. On a new subsea field a hybrid vehicle will give operational savings of between $0.5 million and $12 million over five years. This is equivalent to a total Shell saving (based on anticipated number of new developments) of over $22 million between 2002 and 2007.  

 

A hybrid vehicle is a physical combination of a workclass ROV and AUV module. The AUV module provides control and power to move the vehicle to a target on a subsea docking station, hover and then dock its tether management system (TMS) onto the structure.  

 

The structure has power and control cables relayed from a host platform to enable the hybrid to be controlled from that host in the same way as a normal ROV - unreeling its tether and using standard ROV tool packages and/or manipulators.  

 

Shell is confident that this is feasible within five years. One of its attractions is that it is a low-risk system: the package can have standard ROV tools and manipulators so that subsea hardware need not be modified. So if the hybrid system fails the inspection, repair and maintenance (IRM) may be carried out conventionally from an ROV support vessel.  

 

A workclass AUV navigates from its host surface structure or from an underwater 'garage' then docks on an AUV-friendly structure. Its tool packages enable it to perform IRM and construction tasks.  

 

Moves towards the development of a hybrid system seemed to be under way late last year with the announcement by Fugro-UDI of an involvement with Coflexip Stena Offshore (CSO) for a hybrid system called AutoRov which envisaged an AUV acting as a shuttle for an electric ROV.  

 

There has been little news of the project since the key players in Fugro-UDI - Mark Vorenkamp, Jan Garmulewicz and Ian Valentine - left the company to form Century Subsea, an Aberdeen-based ROV/AUV independent operator.  

 

Other, more incidental, suggestions for AUV applications obtained by Shell's poll included:  

 

* Subsea construction under ice or in busy shipping lanes  

 

* Supply mode for a subsea drilling rig  

 

* Deploying and retrieving ocean bottom seismometers in 4D seismic.  

 

* Acting as a hydrocarbon 'sniffer'.  

 

A further area of market potential for AUVs is in monitoring the touchdown point on the seabed of a pipeline as it comes off the laybarge. In deep water this touchdown point is obviously some distance behind the laybarge and currently requires a dedicated ship/ROV spread.  

 

An AUV would monitor touchdown but it would require constant position monitoring and at least one-frame-per-second through-water video. Cybernetix SA (a subsidiary of Comex SA, Marseilles) has developed Spider, an autonomous subsea crawler for pipelay monitoring.  

 

Other survey companies who have announced their intention to join the AUV race include Fugro Geoservices in the US and Racal Survey in the UK.  

 

Fugro has an agreement with International Submarine Engineering, Canada, to build a 6m long, 5000m depth-rated vehicle with 430km range at 2.0m/s and fitted with the standard suite of survey systems. Navigation is by Kearfott INS, DGPS, RDI Doppler log, LBL and USBL. There was no news on delivery date as OE went to press.  

 

Early 2001 delivery to the Gulf of Mexico is scheduled for the first of Racal Survey's 3000m depth capability vehicles, completing at Bluefin Robotics in the US. Racal has another vehicle on order with options for a further six. Racal Survey is, of course, already a major operator of ROVs with over 40 self-built vehicles in its worldwide fleet.  

 

Meanwhile, Denmark's Maridan A/S has completed a 600m depth capability vehicle to go diamond prospecting off South Africa for De Beers Marine (Pty). A 3500m vehicle with a duration of 48-72 hours, with 'novel titanium technology', is due for completion in early 2001 and there are tie-ups with survey companies worldwide to operate 600m vehicles.  

 

A further development is a proposed joint venture with Halliburton Subsea, the UK's Defence Evaluation & Research Agency (DERA) and the Southampton Oceanography Centre (SOC).  

 

Little information is available regarding the agreement, other than that the intention is to build vehicles using SOC technology and DERA sonar capability.  

 

SOC is, of course, the operator of the UK's Autosub AUV which this summer became the first (or at least the reported first) vehicle to justify the operator's nightmare of putting the thing in the water, going back to pick it up a day or so later and - nothing happens! SOC gave this frank account of what happened:  

 

'Mission 240 began much like a score of previous unescorted runs for Autosub. It was 0155 UTC on Saturday 17 June and the task for the 22-hour mission was to study the physics of the overflow from the Eastern to the Western Mediterranean Sea in the Strait of Sicily, following the terrain at a height of 30m above the seafloor.  

 

Autosub would be making measurements from two salinity probes, upward and downward looking acoustic Doppler current profilers and a nose-mounted turbulence probe.  

 

Following launch, Autosub engineers on the RV Urania verified that the vehicle was working correctly and on track to the first waypoint. Urania then left the vehicle to do its job while it carried out a complementary CTD survey over the sill - all part of the new way of working made possible by autonomous vehicles.  

 

Urania was first to the rendezvous point - two hours early. At 0000 UTC on 18 June, Autosub should have arrived. It didn't. After two hours, the 'vehicle-failed-to-rendezvous' procedure began with a local search using satellite, radio and acoustic navigation aids. With no contact obtained, the search was extended along the planned track of Autosub.  

 

At 0725 UTC on Monday 19 June contact was made with two of the acoustic transponders on the vehicle - just in time to reach the SOC director's desk before he read the note about the 'loss'!  

 

The acoustic navigation system showed that the vehicle was on the bottom, at a depth of about 340m. An echosounder survey revealed a steep 150m high cliff rising from 450m with a nearly perpendicular slope just about where Autosub seemed to be.  

 

It was established that the best option for recovering Autosub was to use an ROV and the job went to Sonsub using the Polar Prince equipped with two Innovator ROVs.  

 

At the site, one of the ROVs located Autosub some 40m west of the estimated position. Not only was the cliff vertical - there was an overhang. Autosub was stuck beneath the overhang - with a damaged nose, but otherwise in good shape.  

 

It had been able to cope with the scarp, it had tried valiantly to deal with the vertical face of the cliff, but the overhang had been too much. For the ROV the recovery was a simple job - grab the bracket near the rudder, pull gently to drag Autosub clear of the overhang and then let it rise slowly by itself to the surface.'  

(oilonline)  

By Bob Barton 

© 2000 Mena Report (www.menareport.com)

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