inspection, repair & maintenance ... Automatic benefits
  from: Offshore Engineer
  Tuesday, June 01, 2004

North Sea operator Shell UK (Shell) in Aberdeen teamed up with Shell Global Solutions* in an effort to reduce dependence on divers for the inspection of fire- and service-water caissons beneath offshore platforms. OE kicks off this three-part review of recent advances on the IRM front with a look at the eventual fruits of this joint endeavour - an ROV-based system that uses pulsed eddy current (PEC) technology to map the wall thickness of the steel tubulars in the vulnerable area around the enclosed pump.

Fire and service water is raised on most offshore platforms through steel pipes, or caissons, that extend between 20m and 50m down into the sea. Each caisson encloses an electrical pump that normally sits about 15m below the surface. Although not themselves classed as safety critical, the caissons obviously perform a vital role on the platform, and if they are out of commission it is quite likely that oil and gas production will have to be interrupted.

Monitoring the condition of these caissons is not easy, since any significant corrosion that occurs tends to affect the inside of the pipe as a result of galvanic action between the noble metal components of the pump and the nearby steel wall.

For a long time, Shell regularly employed divers using ultrasonic techniques to check the wall thicknesses of the 70 or so caissons on its various platforms in the North Sea. This was not the ideal answer though, as Graham Dillaway, one of the company's most experienced inspection engineers, explains: 'In many instances we had to take caissons out of service during inspection to ensure the diver's safety. And it was an unpopular task as, working close to the surface, the diver was buffeted by wave action and suffered from the effects of constant pressure fluctuations. Ultrasonic inspection had its drawbacks too. It was notably timeconsuming, requiring the diver to clean the caisson surface thoroughly before a reading could be taken. Also, ultrasonic measurements are highly focused, which is not really what we want in this case as we are looking to map the wall thickness of the caisson over quite a large area in the vicinity of the pump.'

Dillaway first turned to Shell Global Solutions in 1998, after he saw one of its pulsed eddy current (PEC) tools being used in platform topside and splash-zone applications. (Shell Global Solutions had been pursuing PEC technology since around 1995 and by 1998 had advanced a range of land-based and offshore applications.) A glance at the key features of the tool suggested to him that PEC technology had the potential to be taken subsea using a remotely operated vehicle (ROV) - and so began a programme of development that has continued until today.

Shell Global Solutions took responsibility for adapting one of its existing PEC tools for use underwater. Robust housings were designed for both the probe and the associated instrumentation. The tool was adapted so that it could be powered from the ROV (and measures taken to shield it from electromagnetic interference) and a suitable remote control system and software were developed.

Dillaway's team handled the ROV side. To begin with, in the 1998 inspection season, the PEC tool's probe was installed inside a crude V-frame attached to the front of an ROV. The vehicle was simply driven up to the caisson and its thrusters were used to push the probe against the pipe. According to Shell's George Hogg, who led the ROV development programme, the system worked reasonably well and certainly provided proof of concept. Monitoring a reasonably large area around a caisson was time-consuming, however, as the ROV had to be accurately manoeuvred between each single reading. Over the following couple of seasons, Hogg concentrated on devising a more manageable way of taking multiple readings with the aim of speeding up the entire inspection process.

The system being used today was built to Shell's design specification by a local specialist engineering company, Sub- Atlantic, and features a rather more sophisticated semicircular frame, which has clamps to hold it onto the pipe (see Figure 1 ). Within the frame there are now two probes, fixed to separate arms. Between them, with the aid of vertical and rotational sliders, they can access points on the surface of the pipe around its entire circumference over a height of about 0.8m. By this means the wall thickness of the caisson may be mapped over an extended area. Figure 2 illustrates the sort of output routinely obtained.

It is also worth noting that the frame is now connected to the ROV by a controllable, articulated joint. The ROV delivers the frame to the pipe with the joint locked and then, once the frame is firmly clamped into position, the joint is unlocked. This permits the ROV to float reasonably freely while still attached to the frame. Consequently, forces on the ROV arising from wave action are not transmitted to the frame when readings are being taken, even under adverse weather conditions.

'We have succeeded in upgrading the system during every inspection season,' says Hogg. 'Our latest ROV-PEC is being used for regular monitoring of caissons on all of the platforms in Shell's Central and Northern fields. It helps us to identify problems before they become critical, and we can arrange to carry out any repairs during planned shutdowns.'

The ROV-PEC system is capable of mapping the wall thickness of a caisson in the area a metre or two above and below the enclosed pump in less than an hour. Before he had the ROV-PEC, Dillaway needed a diving support vessel at $175,000/day, and this allowed him to inspect up to three caissons in 24 hours. Now he uses a simpler vessel at $70,000/day, ties up only one of the four ROVs it normally carries, and completes six to seven inspections during an eighthour shift.

Dillaway and Hogg are both proud of the system and the cost savings they have achieved with Shell Global Solutions' help. They describe the tool as 'worldclass' with regard to its reliability, effectiveness, and ease and speed of use.

Hans van der Steen, who has led the development from Shell Global Solutions' side, feels the same way. 'To the best of our knowledge, nobody else has tried to marry PEC technology with an ROV. It is certainly very innovative. It has been a great team effort from everybody involved.'

The collaborators are still looking to make further improvements to the ROV-PEC system. A reduction of the probe's footprint and automatic data interpretation out on the platform are key goals - data from the tool are currently transmitted to Shell Global Solutions in the Netherlands, for overnight analysis. Also high on the agenda is extending the range of applications to the subsea inspection of oil and gas riser pipes. OE

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