Corrosion-erosion monitor set for subsea

Non-invasiveness, good repeatability and high coverage are listed among the key subsea selling points for ClampOn's new subsea corrosion-erosion monitor (CEM), the prototype of which will be on display for the first time at Houston's OTC show next month.

Previously installed topside, the CEM technology is considered ready for deployment subsea. The prototype has been tested subsea and field testing is scheduled this month.

The technology is based on dispersion of ultrasonic guided wave modes. By using electromagnetism these waves can be transmitted through the pipe wall without the sensor being in direct contact with the metallic surface. ‘It is installed on the outer pipe wall to produce realtime wall thickness information, not as a spot measurement, but as a unique average path wall thickness,' explains ClampOn.

The CEM utilizes acoustic guided lamb waves (AGLW), a technology that gives an average wall thickness reading for larger pipe sections. The transducers are fixed at pre-determined spots on the pipe to monitor the wall thickness loss in larger stretches of the pipe, typically up to 2m. The absence of any transducer movement or mechanical motion adds a high degree of robustness to the instrument. As it is permanently installed and needs no recalibration, ClampOn believes the CEM will be both more cost effective and reliable than other ROV-controlled methods.

Various system configurations are possible, ranging from stand-alone monitoring stations with data logging to full real-time integration into existing data infrastructure. Subsea installation avenues already identified by the developers include a retrofitted ROV solution and a solution where the system is installed in advance of subsea deployment.

This is how ClampOn describes system operation: ‘The CEM system consists of up to eight transducers and an electronics unit which handles all signal acquisition and processing. Two and two transducers operate consecutive in a pitch/catch mode of operation giving the average wall thickness of the area between the transducers, which can be up to 2m in length. By choosing the transducer positions with care, normally unavailable areas can be monitored, eg buried parts of a pipeline.

‘Requirements imposed by important factors such as mode separation and spurious arrivals place limits on the maximum and minimum distance between transducers. These limits are functions of pipe thickness and diameter, and need to be decided for each installation in order to be able to maximize coverage area. In most cases the transducers will be placed on two rings around the pipe, and set up to monitor the area between the rings and, if possible, the area along the ring.

‘Because of wave diffraction the covered area stretches beyond the physical dimension of the transducers,' ClampOn adds. ‘Clearly, with the transducers being permanently installed, the coverage area of the system will be a certain fraction of the area over which the system is deployed. Typically, this fraction will be greater than 0.65, or 65%, and can reach almost 90%. OE

TITAN TICKS: Maritime satellite communications
provider Marlink recently confirmed delivery and
installation of its innovative Sealink VSAT system
on the Atlantic Oilfield Services oil & gas support
unit KS Titan 2. The rig, chartered by ExxonMobil,
is the second Atlantic Oilfield Services unit to
install Sealink.
‘Since its installation, the crew have been very
satisfied with Sealink VSAT,’ comments Atlantic
Oilfield Services’ IT manager Partha Rajkumar. ‘The
system provides always-on, reliable connectivity
which is essential to meet the modern demands
of the digital oilfield. Our positive experience with
Marlink led us to extend our contract to include
the KS Titan 2, with a view to further expanding
the capabilities of the satellite communications
network in the near future.’
The C-band VSAT system will provide the
rig with up to 256Kbps bandwidth and eight
telephone lines for a multitude of critical offshore
applications, including real-time data management
and data sharing with sites ashore.


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