MagSens TM Theory of Operation & Interpretation Guide
Magnetic Flux Leakage General Theory MagSens TM LF cage assemblies
Background The System was adapted from the Off Shore Drilling Industry The cables this head checks are 5 inches in diameter and made from ferritic steel
Magnetic Flux Leakage Inspection Is very effective when used in conjunction with a visual inspection and a knowledge of the wire ropes history It has been used for approximately 60 years in the oil and mining industries The MagSens TM system was developed in cooperation with Longview Inspection and CanMet
Paramagnetic Austenitic Stainless Steel The rescue hoist wire rope is made from 302/304 austenitic staineless steel 302/304 stainless is non-magnetic when annealed The steel becomes paramagnetic as a result of cold working to form the wires The cold working imparts strength The loss of paramagnetic properties indicates a loss of strength in the wires
MagSens Magnetic Circuit Lines of induced magnetic flux Flux Bar Magnet Magnet LMA pole piece LMA pole piece Wire Rope LMA pole piece Magnet LMA pole piece Magnet LF Cage LF Hall Effects Sensors (4) Flux Bar LMA Hall effects sensors (4) yellow
Two signals are recorded LMA- Loss of Metallic Area- i.e. indicates abrasion or stretching of the cable LF- Local Fault- i.e. indicates broken wires, welds, broken strands, corrosion etc The LF signal is the signal used to detect even a single broken wire
New Wire Rope Baseline This cable has no defects
Different New Wire Rope Baseline S/N0003 Each cable has its own distinctive magnetic signature Note this cable has been run through the head by hand to check it before installation
Same rope loaded to 300 lbs after installation This measurement can be used to verify the cable is the correct length Extending from hoist Retracting onto hoist
Same rope loaded to 600 lbs
Same rope at 40 hrs inspection
Theory of Operation BROKEN WIRE NORTH POLE SOUTH POLE WIRE ROPE DIRECTION OF TRAVEL As the broken wire passes the LF section, the leading end produces a negative voltage in the hall sensor and then the trailing end produces a positive voltage. LF SENSOR A Hall effect device generates a voltage when subjected to a Magnetic Field
When the cable is extending the trace is blue EXTERNAL NICK INTERNAL BROKEN WIRE
When the cable is retracting the trace is red BACKWARDS
MagSens find Internal Defects This damage is caused by the crushing and abrasion of the wires upon themselves at the turnarounds of the hoist drum
Finds internal dynamic overload This damage is caused by shock loading the cable
Allows checking rope end The cable can be fed through the MagSens at the start and the end of the inspection to check the rope end. A thorough visual inspection of the ball end is still required.
Sample Defect Indications The following defects were intentionally inflicted on cables in order to show how the MagSens trace varies with the size and type of defect
One nicked wire
Test Cable with one broken wire
Broken wire
Multiple broken wires
Two broken wires
External cut strand S/N0003
Cut Strand
1 volt scale
Internal Cut Strand S/N0003
Internal cut strand Most of the wires and strands are inside the cable and not visible or detectable when damaged
2 Nicked wires in center strand S/N0003
Cut Internal Wire Rope Core
1 volt scale
Indications Found in Service The following indications are actual defects found in the field since the MagSens TM was first fielded in 2005. A large database is being accumulated as users continue to inspect the cable and document the results
One defect The event marker tool can be used to label indications
S/N048 defect
SEM pictures Internal crushing and abrasion Material removed estimated @ 10%
Two indications S/N0003 Defects should be monitored for growth over time
Same cable at 120 hrs inspection
Marking spikes prior to cable replacement Run the cable back and forth through the head to isolate the defect for a visual inspection
Severe Internal Pitting and Crushing on Internal Strands
Internal Damage Due to crushing and abrasion at the turnaround
New uninstalled wire rope S/N0003 Brand new wire ropes are routinely shipped with welds in them
Another weld indication on new wire rope This cable was cut up by the customer who found a large weld in the center wire of the center strand
Center wire weld The center wire is normally perfectly straight
Pits in weld
Effects of welds on wire rope Welding anneals 302/304 stainless Soft wires break first Welding 302/304 leads to Carbide Precipitation Carbide Precipitation leads to localized corrosion Localized corrosion leads to loss of wire rope strength
Carbide precipitation Sensitization is a concern if temperatures between 500-800 degrees Celsius are reached. Chromium rich carbides form, leaving areas with less Chromium. Chromium depleted areas are more susceptible to corrosion attacks. Low carbon steels avoid this problem, but if a regular 304 is used it is a concern.
Heat Affected Zone/Carbide Precipitation The precipitation of carbides in the Heat Affected Zone (HAZ) is a phenomenon that can reduces stainless steels' resistance to corrosion. The carbide precipitates from the steel, depending upon two factors: temperature reached in the metal (usually between 425 C and 800 C), and the percentage of carbon present. During welding, when the temperature falls between the aforementioned precipitation range the carbide will precipitate preferentially along grain boundaries, within the ferrite phase, or along slip planes in cold-worked material. (Lancaster 181) The carbide that most commonly forms in stainless steel is chromium carbide (Cr23C6). The presence of the carbide along the grain boundaries can lead to intergranular corrosion, which will deteriorate the material along the weld.
Corroded Wires Welded wire corroded away
External Welds This indication was found by a customer who sent in the data and a picture of the defect
External Weld
Strand was welded
Broken Strand This broken strand was found in a brand new cable Date of test November 1 st 2006
Weld in New Wire Rope Date of test November 2 nd 2006
Broken Internal Strand Strand broke at the turnaround between the 3 rd and 4 th layer of a Breeze Hoist after many cycles
Broken Internal Strand
Extreme Abrasion LMA indication LF indication
Extreme Abrasion This abrasion was caused by a cable being pulled off the drum during a 600 lbs load check
Multiple Broken Wires
Broken Wires These wires broke after what is estimated to be a thousand or more cycles
Sometimes when the breaks are close together the indication shows up as one larger indication
Broken wires due to abrasion
Welds corroding
Many times there are many welds grouped into one area as a result of manufacturing practices
Welds corroding This weld was in an area subjected to severe salt water intrusion
More welds
Welded wire corroded away
Approximately 20 welds in less than 20 feet
Trend monitoring Evolution of faults monitored over time The cable used in the following tests has been in use for over four years on the Zephyr test frame used to test RHGSE and MagSens TM systems prior to shipment
RTTC s/n 12 Internal damage due to levelwind turnaround
Air Affairs s/n14
Phoenix s/n 15
LASD s/n 16
GFS run 1 s/n 17 Developing wire defect
GFS run 2 Note new break
GFS run 3 Break opens up on retract with 300 lbs load
GFS run 4 Break now obvious when extending and retracting
GFS run 5
Monitoring Therefore it can be seen that one can track internal faults as they progress from abrasion to actual wire breaks over time. Also one can monitor manufacturing welds to determine if corrosion develops or the soft wires break in time
Same cable one year later New damage Gain is increased Defect locations the same
June 20, 2008 2 broken wires June 27, 2007 Note average signal strength variation due to gain variation
2 Broken wires Due to repeated cycling over 4 years
Guidelines for interpretation Wire breaks that have not separated may not produce a recognizable signal Wire breaks with a gap as small as.08 will produce a signal Welds produce a signal that may be as large as 1.5 volts or as small as.7 volts Welds sometimes occur in groups as a result of manufacturing practices Abrasion may produce a signal larger then a wire break Metallic material embedded in cable may produce a signal Broken strands produce signals in general that are much larger then broken wires Corrosion of soft wires will produce a larger signal than a broken wire Kinks may not produce a signal unless a wire breaks Two or more broken wires in the same area will add up to produce a larger signal Variation in gain will cause a variation in signal size Gain should be set to approximately +/-.25 volts Contact Zephyr for instructions to adjust gain
Guidelines for rejection According to the hoist OEM s one broken wire is cause for rejection But they implicitly mean a broken wire that is on the outside diameter of the cable Therefore if an indication is found that looks like a broken wire but it is not visible, there is no cause for rejection, but the frequency of inspections should increase If increasing the load produces a larger signal then the cable should be replaced Knowledge of the original condition of the cable allows one to discern between a weld and a broken wire or broken strand defect
Guidelines for rejection Indications that appear to be welds are not cause for rejection However welds that show an increasing signal over time should be monitored An increasing signal should be cause for rejection Weld indications usually differ from a broken strand indication
Comments relative to cable strength A single broken wire has no impact on the cable static breaking strength Isolated corrosion due to welds has limited impact on static breaking strength Broken strands have an adverse impact on static breaking strength Saline residuals leading to corrosion has an adverse impact on static breaking strength Severe abrasion has an adverse impact on static breaking strength High heat has an adverse impact on static breaking strength Cable loosening has an adverse impact on static breaking strength
Comments relating to rejection criteria Rejection criteria has been established over the years to protect the users and the OEMs Many different concerns lead to establishment of rejection criteria Cable fouling on drum Personnel safety (cut hands from broken wires) Accident prevention Zero tolerance
Summary The database is still growing 52 MagSens TM systems are in use worldwide as of October 2010 We are always available for comments and request additional findings of indications and pictures to enlarge the database