The aim of this project was to implement a large-scale monitoring solution for detection of time dependent degradation - corrosion and fatigue cracking - in sub-sea pipelines and risers using ultrasonic guided waves. Such waves have the capability to travel long distances in metals (many tens of metres) so that long lengths of pipe may be examined from a limited number of test locations. Furthermore, by generating a circular wave travelling in the pipe wall from an encircling transducer tool, 100% of the volume of the pipe wall may be examined. Consequently this approach is highly attractive for examination of inaccessible areas, as only a limited number of access points are needed and no prior estimation needs to be made of the most likely location of degradation, as the whole pipe is monitored.

The main tasks in the project were:-

• To specify the requirements for the test system,
• To develop sensors and the associated electronics to meet those requirements,
• To enhance the techniques and procedures for performing long range ultrasonic testing (LRUT) for improved performance,
• To demonstrate the capabilities in the laboratory,
• To demonstrate the feasibility of performing such tests sub-sea by means of underwater demonstration of a prototype system and tests under high pressure to simulate depths down to 2,000 metres.

This project addresses the emerging need to monitor offshore pipelines, including the steel catenary risers (SCRs) used in oil and gas production from deep water fields, for degradation in service which, if left undetected, could cause catastrophic failure of the riser. The main objectives are:

• To develop new and novel long-range ultrasonic testing (LRUT) technology for the detection of corrosion in sub-sea flow lines and of fatigue cracks and corrosion in sub-sea risers and SCRs,
• To develop the world's first technology able to monitor and inspect deepwater SCRs in the ocean continuously.

Most of the new finds of oil and gas offshore are now in very deep water, up to 3,000m deep. There are considerable technical and engineering difficulties in production in deep water environments, and a major factor is the potential for degradation of the riser pipes bringing the products from the sea floor to the production platform, which is normally floating. Failure of such a riser from corrosion or fatigue cracking would have severe consequences, in terms of both pollution and in economic losses for the field operator. More operators of deep-water fields are now considering that some form of in-service inspection will be required during the service life of these components. The availability of inspection systems for monitoring will be a major benefit to the offshore oil and gas production industry.

The project will take LRUT technology forward in the following ways:

• Develop the application of a marinised LRUT system to the inspection of ultra-deepwater assets. (Whilst the deployment techniques will differ for very deep oceanic applications to those for shallow seas, the marinised flaw detector and transducer array developed for deep seas will be applicable to shallow ones).
• Develop methods of data capture and transmission to the surface from deepwater locations.
• Increase range, particularly in situations where ultrasound is severely attenuated by viscoelastic coating.
• Enable discrimination between deep/narrow and wide/shallow flaws and introduce some flaw sizing capability.
• Improve sensitivity -
  1. by formalising the difference technique mentioned above and
  2. by utilising the phased array focusing approach.

The main technical developments are:

• Ultrasonic beam focusing and steering for improved sensitivity and resolution,
• Increased test range and compensation for heavy coatings,
• Condition monitoring procedures,
• Flaw severity assessment,
• Marinisation of system components - flaw detector and sensors,
• Field demonstration of system operation sub-sea.
• A prototype sub-sea test system for long range UT monitoring of risers and flow lines, incorporating advance procedures for condition monitoring and assessment of flaws detected.