Background
Flooded Member Detection is an inspection regime designed to screen subsea platform structural members for defects. A cracked weld, for example, can allow the ingress of sea water into the member which may create a point of failure for the structure. Operators therefore inspect these members on a regular basis to gain confidence that there has been no sea water ingress.
Techniques
There are two recognized non-destructive testing techniques that allow for Flooded Member Inspection; the use of a radioactive source (commonly referred to as the Gamma Technique), or using Ultrasound.
Inspection using a radioactive source is effective and can be fast – as the source does not need to physically touch the Member and therefore no cleaning or surface preparation is required.
But Cygnus Instruments has embarked on a full development and trials cycle because the UK-based ultrasonic specialist believes that a fully developed, professional UT system can now play a much greater role in FMD in the future.
This is because while some degree of surface preparation is needed for Ultrasonic Testing (UT) of Members, that perceived disadvantage can very often be outweighed by two distinct advantages: operational flexibility and cost savings.
Because the Gamma Technique uses a radioactive source, the equipment is almost always provided as a service by a third-party contractor. To operate the equipment one, or often two, specialist technicians must come with the equipment – and they will then take up precious bed-spaces offshore. The Cygnus UT equipment is extremely easy to understand and simple to operate; therefore, existing personnel from the diving or ROV team will perform the inspection work themselves – creating a good cost saving for the operator.
In addition, as the Cygnus system is able to sit on a vessel without incurring personnel costs, it means that an operator and its inspection contractor can perform small packets of FMD inspection work as and when it is convenient; a vessel is not tied to meeting a third-party inspection service provider offshore at a certain time in a certain place. With the Cygnus system, work can be done as and when it suits.
Cygnus Equipment
The Cygnus UT FMD system is the only system available to buy that is designed by dedicated ultrasonic non destructive testing specialist. To that end, when the Cygnus system presents a Flooded / Not Flooded result, that result is backed up with a true UT A-Scan. This allows for more interpretation of a result and that the technician is properly informed.
Other considerations have also been taken properly in to account; there is a 4-way gimble built around the Cygnus transducer that ensures correct transducer alignment and the system has a transducer contact sensor – to let the topside technicians know the ROV or diver has made good contact.
“Over the past decade, we feel that FMD with UT has not gained as much respect from the offshore inspection industry as it should have done”, says Cygnus Managing Director Lucinda Crosthwaite-Eyre. “So we knew if we were to change people’s minds, the system would need to be very good indeed. While all operators want to ‘save money’ – that is not enough on its own to justify taking a decision to use one piece of equipment or technique over another. That equipment has to work brilliantly as well. After exhaustive development work, our system’s advanced signal processing and low noise data acquisition will work on thick-walled members and pipes (up to 75mm thick), on heavily corroded members and on buoyancy tanks”.
Cygnus has been in the underwater inspection business for nearly 4 decades. And Cygnus Business Development Director, Graham Haines, recalls that “we have been asked directly for a UT FMD solution many dozens of times over the years as a result of a lack of reliability from other UT offerings. But the simple fact was we were always busy on another project. In 2015 we gave ourselves some time and space to assess the market and quickly saw that there was a clear space where Cygnus could play”.
And Cygnus customers are agreeing.
Dan Jones, Inspection Team Lead for Bibby Offshore in Aberdeen is an early adopter and helped with trialling the new UT FMD system in 2016 “We are always looking to use new and innovative equipment – especially if it helps reduce costs to our customers. We were proactively involved with Cygnus during their trials of the ultrasonic FMD system in 2016 and we were impressed with the system. It proved to be robust and simple to use, without the need for specialist personnel. The ROV deployment had clearly been given a lot of time and thought. We plan to utilize the system this year on more inspection campaigns.”
