Project: Development of an advanced medium range ultrasonic technique for in-water inspection of mooring chains

The Need: _x000D_ The forecasted large scale deployment of offshore renewable energy systems will rely on floating platforms in deep water which are moored to the seabed via chains. 14-17% of Europe's 2011 total electricity demand (40Gw) will be coming from offshore, deepwater platforms. As offshore oil/gas exploration and production goes further afield into deeper waters, operations must be conducted from floating platforms. Mooring lines are safety critical on floating and semi-submersible platforms and designed to last for the productive life of the field, usually 20-25 years. Despite fatigue design recommendations such as API (API, 1993) and DNV (DNV-OS-E301, 2004), probability of failure in much shorter time scales is high. The Noble Denton Mooring Integrity Joint Industry Project (JIP) has shown that serious chain failures have occurred in the past and that even modern designs can deteriorate to unacceptable levels well before their design lives are reached. This JIP concluded that expecting mooring systems to last 20 years or more without an active inspection and COtenance program is unrealistic. In the North Sea, where high quality statistics are available, a drilling semi-submersible can expect a mooring system failure once every 4.7 years, every 8.8 years for a Floating Production Storage and Off-loading platform (FPSO) and 9 years for a production semi-submersible. Indicative costs of a single mooring line failure are of the order of €2.4M for a medium sized FPSO producing 50,000 bpd and €12.6M for a large FPSO producing 250,000 bpd, both based on shutting down for two days. Most of the cost is due to deferred production. Failure of a single mooring line can go unnoticed, which greatly increases the probability of multiple failures and catastrophic consequences. An example is the failure of 4 out of 10 mooring chains on the FPSO Gryphon Alpha in February 2011 which then had to be shut down. The platform was exposed to 9m waves, wind speeds of 53 Knots and experienced a heel angle of 12 degrees. The Coastgaurd removed 74 workers from the platform. _x000D_ _x000D_ Limitations of existing inspection procedures: _x000D_ Chains can be inspected in air or in the water however with the exception of drilling semi-submersibles floating platforms (FPS) are on location for the producing life of the field , so mooring lines need to be inspected in-situ. Mooring lines can be recovered part way through field life, but: 1) The lines may damaged during recovery or re-installation and 2) The operation is expensive because anchor handling and heading control tugs will be required for a period of days. For these reasons there is a strong motivation to develop in water inspection methods which can be used without stopping production. Diver inspections are hazardous due to the movement of the chains and are seriously depth limited. Although stand-alone robotic systems have been developed they are too cumbersome for off-shore use. ROV deployed mechanical calliper systems have enjoyed limited success as they are easily knocked out of true and require re-calibration between measurements. Welaptega Marine have developed what is effectively an 'optical calliper' and this is the most established system. Existing in-water inspection systems are limited to performing dimensional measurements to indicate wear, they are not capable of detecting and sizing fatigue damage. Cracks are difficult to detect because they tend to occur where they are obscured by the adjacent link. _x000D_ _x000D_ The Solution: _x000D_ Ultrasonic guided waves (UGW) are an established technique for screening underwater pipelines for anomalies (especially corrosion) but it has never been applied to mooring chains. In this project we propose medium range guided wave ultrasonic techniques with frequencies ranging from 100 kHz to 250 kHz. In order to propagate longitudinal guided waves in the chain links it would be necessary to couple the transducers to the chain link. For piezoelectric transducers this requires good surface contact, however chains will be corroded and fouled by marine growth. To overcome this we will trial electromagnetic acoustic transducers (EMATs) which generate the ultrasound directly in the test piece rather than the transducer. EMATs can accomodate surface roughness. To deploy the inspection technology we will develop a remote manipulator that can 'climb' along the chain to perform the inspection. _x000D_ The Pship: Our Pship consists of Tecnitest, Baugh&Weedon, Vicinay and Single Buoy Moorings. Baugh&Weedon has capacity for engineering solutions for using EMAT technology in a ROVs. Vicinay manufacture mooring chains and will supply undamaged chain links and technical expertise to help validate the inspection procedure. Tecnitest will be developing the transducers and software in order to realize the inspection technology. Single Buoy Moorings will provide ex-service samples and facilitate in-water testing.

Acronym LinkInspect (Reference Number: 6633)
Duration 21/10/2011 - 20/04/2014
Project Topic The LinkInspect project will develop a medium range ultrasonic guided wave technique to screen mooring chains for fatigue damage and corrosion. By means of a remote manipulator it will be possible to perform the non-destructive test both in-water and in-air.
Project Results
(after finalisation)
For floating and semi-submersible platforms in deep water which are moored to the seabed via chains, the mooring lines are safety critical and therefore should be designed to last for the productive life of the field, usually 20-25 years. Despite fatigue design recommendations, the probability of failure in much shorter time scales is high. Therefore expect that a mooring system has a duration of 20 years or more without an active inspection and COtenance program is unrealistic._x000D__x000D_The costs of a line failure are very high. Further the failure of a single mooring line can go unnoticed, which greatly increases the probability of multiple failures and catastrophic consequences._x000D__x000D_Therefore it is important to carry out tests to detect the presence of defects before the failure occurs. However when mooring lines aren't inspected in-situ._x000D__x000D_1) The lines may damage during recovery or re-installation and_x000D_2) The operation is expensive because anchor handling and heading control tugs will be required for a period of days._x000D__x000D_For these reasons there was a strong motivation to develop in water inspection methods which could be used without stopping production. So, LinkInspect project was defined with the CO objective of developing an ultrasonic guided wave technique in order to detect fatigue damage in mooring chains._x000D__x000D_Throughout the project's development three CO challenges were achieved_x000D__x000D_1.-Regarding the probes_x000D_Piezoelectric transducers need a good contact surface, however chains are corroded and fueled by marine growth. To overcome this, electromagnetic acoustic transducers (EMAT) have been used. This type of probes generates the ultrasound directly, therefore using EMAT's it's possible to adapt to the surface roughness._x000D__x000D_2.-Regarding the transducer collar_x000D_A marinised transducer collar suitable for inspecting typical size ranges of mooring chains was designed. The scanner is able to move through 180 °, after which, a B-scan is obtained that allows the detection of defects in inner area where fatigue cracks appear._x000D__x000D_3.-Regarding the remote manipulator_x000D_The inspection system has to be assembled on a remote manipulator able to work in marine environments and to climb the chain, above and below the water-line (up to 25m). Climbing is complicated by the fact that the plane of each chain link is at 90° degree to the next and because the movement of the chain could damage the inspection system._x000D__x000D_Finally, the ultrasonic inspection technique thus defined allows:_x000D_• Detection and sizing of flaws in inner area of the link_x000D_• Study of crack growth up to 50% of the diameter_x000D_• Inspect the chain in areas close to the surface with an angle of inclination up to 45_x000D_• Inspection in automatic mode_x000D_• Inspection remotely_x000D_• To optimize time and costs.
Network Eurostars
Call Eurostars Cut-Off 6

Project partner

Number Name Role Country
5 Baugh and Weedon Ltd. Partner United Kingdom
5 Innovative Technology & Science Ltd Observer United Kingdom
5 Tecnitest Ingenieros SL Coordinator Spain
5 VICINAY CADENAS S.A. Partner Spain
5 SBM Offshore Observer Switzerland