Introduction

ACFM was first developed for the North Sea oil and gas industry as an alternative to ACPD for sizing defects underwater. It is now the most widely used subsea crack detection technique worldwide.

ACFM is routinely used to inspect welded tubular intersections as well as subsea pipelines, manifolds, templates, risers, wellheads, valve assemblies and other metal structures and components. Although usually diver-deployed, ACFM can also be deployed by remotely-operated vehicle (ROV), and by atmospheric diving suit (ADS).

As well as subsea inspection, ACFM is regularly used for inspecting topside areas of offshore platforms, including process pipework (carbon steel and duplex), flame sprayed flare booms, cranes and pedestals, pressure vessels, hydrocyclones, knocK-out drums, gas trains and compressors and wellhead components. Live lines can also be inspected at operating temperature using specialised high temperature probes.

U31 instrument

Underwater standard weld probe 5KHz

Straight long nose probe 5KHz

Underwater right angle mini probe 5KHz

Underwater straight mini probe 5KHz

Inspection, Repair and Maintenance (IRM) is a core requirement for offshore oil and gas operators in all parts of the world. TSC provide a range of specialist IRM services worldwide, using both diver and remote intervention methods.

TSC Inspection Systems have participated in successful long-term IRM campaigns, working from Dive Support Vessels and fixed installations, for major operators in the UK, Dutch, Danish and Norwegian sectors of the North Sea. TSC technicians, many of who are trained and experienced commercial divers and highly experienced in offshore environments, work closely with all members of the team - project managers, diving superintendents and supervisors, inspection controllers, data recorders, divers and deck crews.

On-site integration, installation and hand over, development and initiation of ACFM inspection campaigns and ACFM project management are just some of TSC's areas of expertise.

StressProbe instrument

Custom made inspection jig

Custom made stress probes

The original strain gauge system, fitted to locking pins on a fixed jack-up platform in the North Sea, to retain the clamps, had failed. Interrogation of these pins was required. It is necessary to know that the pins experience their design load at all times. The TSC StressProbe was investigated to show it can measure the tensile load in the locking pins.

StressProbe makes measurements of an electromagnetic field close to the surface of a material. There is an induced input field and sensors measure particular components of the magnetic field. The magnetic field is affected by the stress level in the material so as the stress changes, the system responds.

The client delivered to the NDE Centre, University College, London, a spare locking pin for the calibration work. This allowed the StressProbe interrogation to provide a zero stress value on a locking pin in addition to a calibration curve to provide an accurate measure of stress.

U31 instrument

Underwater standard weld probe 5KHz

Straight long nose probe 5KHz

Underwater right angle mini probe 5KHz

Underwater straight mini probe 5KHz

During February 2006, a North Sea gas platform experienced a serious explosion and associated fire. The resulting damage to the topside structure, plant and equipment was assessed and structural integrity assured.

Alternating Current Field Measurement (ACFM) inspection was carried out on designated subsea welds on the platform.

All welds inspected were located subsea, using air / saturation diver intervention from a Dive Support Vessel. ACFM was used to determine the presence or absence of surface breaking defects in specified areas.

U19 AMIGO instrument

Standard weld probe 5KHz

Straight long nose probe 5KHz

Right angle mini probe 5KHz

Straight mini probe 5KHz

40mm 8 channel flat array probe

TSC Inspection Systems and BP have worked together for many years on BP assets throughout the world, providing support for both underwater and topside inspections. A great deal of ACFM inspection has been carried out on BP's North Sea assets.

Examples of routine ACFM inspection projects include platform LP booster suction coolers, high pressure vessels and compressors, post vibration analysis of welds, wellhead, christmas tree and flowline inspections, some of which were carried out at operating temperature.

Stud thread system

External scanner system

Internal scanner system

General purpose flat array probe

Elliptical transition probe

Lifting groove probe

Photograph of stud sample broken at cracked thread.

TSC in association with the Norwegian State oil company, designed and manufactured a complete ACFM inspection system for a titanium riser used for offshore drilling. The system was designed to allow full circumference, automated inspection of the outside wall of the riser sections. Full circumference internal wall inspection through neoprene coating, lifting groove and elliptical transition inspection and stud thread inspection.

Riser sections were cycled periodically after offshore use and all inspection was carried out onshore in controlled conditions.

During one ACFM inspection of a set of 6 titanium studs with visible thread damage were found to contain small ACFM crack indications. The indications were too small to size, but the studs were nevertheless withdrawn and labeled as faulty.

The studs were sectioned at University College London using standard procedures normally used for determining crack shapes in welded joints in ferritic steel offshore structures. A small coupon was first cut out of the stud around the defect. This was then machined down to reduce the thickness, while maintaining good clearance between the machined surface and the bottom of any defect. This is done to minimize the remaining thickness of metal that needs to be broken. The coupon size was approximately 75mm long by 35mm across by 10mm thick. The first few threads on one end up to the cracked thread were machined down in order to allow a better grip in a vice.

In order to embrittle the metal before fracture, the coupon was immersed in liquid nitrogen before being placed in the vice. The free end was struck with a hammer to break the coupon at the crack site.

When the two revealed ends of the broken coupon were inspected, a shallow defect was visible running around the thread root, as shown in the photograph. The defect measures approximately 10mm long by 0.5mm deep.

The depth observed for the defect (0.5mm) is well below the target detection threshold set for the overall system. This reflects the fact that the background signals are very stable for the stud system, allowing it to detect shallower defects than the array probes used on the pipes. The fact that the defect is quite clear on the stud system, compared to similar sized slots used during the development, also indicates that signals from the sharp geometry of a fatigue crack may be stronger than anticipated from the earlier results on slots.