ACFM – Alternating Current Field Measurement

ACFM – Alternating Current Field Measurement

For 30 years, alternating current field measurement (ACFM®) electromagnetic inspection technology has been the method of choice for detecting and sizing subsea surface-breaking cracks. Recognized and approved by many certification bodies, including DNV, ABS and Lloyds, the technique has been used successfully to supersede traditional non-computerized and more user-dependent methods, such as magnetic particle inspection (MPI). As today’s industry is looking to scale the use of auditable NDT methods to all their assets, faster, more flexible, remotely deployed inspections and better reporting capabilities are required.

TSC Subsea’s ACFM® was originally developed as a way for divers to find and depth-size fatigue cracking in jacket structures, so it is no surprise that the technique has become a benchmark in subsea crack detection. We use the U41 true ACFM® subsea inspection system, which is recognised as the industry-standard platform for diver- and ROV-deployed inspections where high PoD and low false calls are key factors in the determination of offshore asset integrity. The ability to accurately depth-size defects produces the information required to allow engineering decisions that protect and extend the life of these assets.

How it works

ACFM® remotely deployed via our NodeScanner™ and MagCrawler™ equipment provides external inspection of subsea asset welds and other crack-like indications. The ACFM® introduces an alternating current into the surface of a component to detect surface-breaking cracks. The presence of a crack disturbs the electromagnetic field and the return signal is instantaneously converted by advanced mathematical techniques so that operators are alerted to the presence of defects.

Immediate defect sizing and recording is a major benefit compared to other NDT methods. Results from independent testing show that ACFM® matches MPI performance when inspecting underwater structural welds. The amount of missed and spurious signals is significantly lower with ACFM® compared to MPI and conventional eddy current testing. And, with ACFM®’s lower cleaning requirements and fewer false calls, inspections are significantly shorter, maximising inspection campaigns.

The Technology – ACFM

The True Alternating Current Field Measurement (ACFM®) is described in the original patents and described in many technical papers from the origin of the technique at University College London.

Only the True ACFM® Technique has been recognized over the years by authorization bodies.

To be True ACFM®, as so described, a system has to feature: 

  • No on-site calibration – sizing is based on a mathematical solution of the field-crack interaction
  • A uniform field inducer
  • A constant current field drive
  • Orthogonal sensors in the Bx and Bz axes
  • Ability to sample the sensors at a discrete phase relative to the field inducer
  • Large inducer-surface height compared with sensors-surface height

ACFM Independent Approvals

Non-Destructive Testing of Hull Welds, February 2014. Section 6, ACFM Technique.

ASME V Article 15: ACFM Technique, 2006

Standard Practice for Examination of Welds using the ACFM Technique (E2261-03), latest revision E2261-12.

Standard Practice for Examination of Drill String Threads using the ACFM Technique (E2928-13)

Accepted for in-service examination of structural welds. TSC is also an LR-certified ACFM Inspection Services Provider.

Classification Note No. 7, March 2012. Section 2.1, Detection of Surface-Breaking Cracks.

BS EN ISO 19902: Petroleum and natural gas industries. Fixed steel offshore structures.

Approved technique for surface-breaking crack detection

North Sea standards (NS-2), 2012 revision: Accepted alternative to MPI for drill string thread inspection