Underwater ACFM Crack Microgauge Model U21
- Reliable crack detection and crack sizing without the need for calibration, ie Crack LENGTH and Crack DEPTH
- Ruggedised topside and subsea units successfully proven in the North Sea
- Reduced cleaning requirements with no need to clean to bare metal
- Capability of inspection through metallic and non metallic coatings
- State of the Art software operating under Windows 95 for ease of operation and compatibility with other reporting software packages
- Replay' facility for review of data
- Fully automated back-up facilities for data storage
- Rapid scanning around weld toes using diver held probe
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Access to a wide range of geometries by using TSC's range of underwater probes
When deployed by ROV the system provides:
- Fully integrated communications via the ROV umbilical
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Rapid inspection without the need for weld scanning
BACKGROUND TO ACFMAlternating Current Field Measurement (ACFM) is an electromagnetic inspection technique and relies on the fact that an alternating current flowing in a component will be disturbed by the presence of a crack. The ACFM probe introduces an electric current locally into the structure and measures the associated electromagnetic fields close to the surface. The presence of a defect disturbs the associated fields and the information is graphically presented to the system operator. The ends of a defect are easily identified to provide information on defect location and length. The significance of a defect, in terms of structural integrity, generally depends on the depth of the defect. Using mathematical models, the system also provides the depth of the defect, thus allowing an immediate evaluation of the significance of the indication.This is a major advantage compared to other techniques that only give detection or at best, length information. ACFM systems have been used for a range of applications where rugged inspection systems are needed. The speed of scan provides a rapid and cost effective means of inspecting welded connections with reduced cleaning requirements compared to conventional techniques.
The probes have been developed to minimise signals from features that are not cracks, eg corrosion, undercut, HAZ, material property changes, etc, thus overcoming the problems often associated with other electromagnetic systems and minimising false calls.
The ACFM technique has been approved for subsea weld inspection by major authorities around the world, including Lloyds, ABS, Bureau Veritas, DNV and OCB Germanischer Lloyd.
SYSTEM LAYOUT - Diver Deployed (U21-Diver)
FIGURE 2 : Model U21 - Diver Deployed
Figure 2 shows the system layout for diver deployment. The system comprises a topside PC, which acts as the system controller as well as a data acquisition unit, a topside unit providing all power to the system, a 350m umbilical and a subsea unit. The complete system is operated from a single 110V supply to ensure maximum safety for the operator both subsea and topside.
The topside computer is a standard PC and requires no additional cards. The computer is provided with full battery back-up facilities to ensure that in the event of a power failure no data is lost, and operation can continue for at least an hour to allow data to be reviewed / replayed if required.
SYSTEM LAYOUT - ROV Deployed (U21-ROV)
FIGURE 3 : Model U21 - ROV Deployed
Figure 3 shows the system layout for ROV deployment. The ROV system makes use of an ACFM array probe which avoids the need for any scanning around the structure welds. Like the diver version, the system is controlled from the surface by a PC. The major differences between the diver system and the ROV system are that the ROV system communicates via the ROV umbilical and has the capability for a number of array probes to be connected simultaneously.
SYSTEM SPECIFICATION
Standard
Special
Maximum operating depth
300 m
500 m
Umbilical Length
350 m
1000 m
Operating temperature range
Subsea Unit
0-250
Operating temperature range
Topside Unit
0-350
Maximum power consumption
1 A @ 110 V
Electrical supply
110 V (+10%)45-65Hz
SYSTEM FEATURES - Model U21
Hardware (General)
- Rugged Topside Unit with illuminated system checks
- Earth Leakage Trip protection for complete system
- Rugged subsea housing
Diver System
- Subsea connections allowing probe change out underwater
- Grab rails for easy handling and protection against damage
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Range of probe configurations to cover different geometries:
- Standard probe for general weld inspection
- Tight access probe for angles down to 350 m
- Pencil probes for difficult access of grinds
- Edge effect probes
ROV System
- Up to three array probes simultaneously connected
- Easy interfacing to ROV for power and communications
Software (General)
- Windows 3 or 95 software environment supporting simultaneous running of word processor package / spreadsheet, etc, and interface to all common printers
- Graphical display of raw data
- Graphical display of processed data for crack detection
- Multiple screen facility for comparing consecutive scans
- Real time adjustment of trace position on screen
- Moveable cursors for use during data review
- Crack depth calculation including recording of crack size on screen
- Multiple page facility in a single file
- Automated back-up at end of session onto diskette and hard disk
- Full system status reports for hardware checking
- Graphical printout of user screens
- Data transfer into standard spread sheets eg Excel
Diver System
- Real Time data display
- Clock points and marking to indicate position on scan
- Variable speed time base to suit application
- Replay facility of recorded data
ROV System
- Colour contour plots of probe area
- Free format text input associated with each scan
- Computer aided calculation of crack sizes
- Automated crack length measurement
- Data merging facilities to combine scans of adjacent areas of weld
OPERATOR TRAINING FOR DIVER DEPLOYMENT
The diver is required to move the probe around the component under inspection, under instruction from the topside operator. The diver has no display of data from the instrument and thus does not require any training in terms of interpretation or operation of the system. However, moving the probe correctly is a requirement of the system and the topside operator should be trained to ensure that the diver is performing the probe movement correctly.
The topside operator is therefore responsible for all aspects of the inspection and TSC recognise that quality in training leads to quality in results. TSC have therefore co-operated with TWI who have prepared a training course for topside operators, part of which includes training on how to train the diver.
The one week course is run at regular intervals at TWI North’s diving facility in Middlesborough. It involves practical and theoretical training of all aspects of ACFM inspection. An examination is given at the end of the training and those passing obtain a certificate. The course is approved by Lloyds Register. Training and examinations are also being arranged within the CSWIP and PCN schemes. TWI also run courses, on request, in different parts of the world and will provide training at customer sites if required.
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