|NDT.net - October 2002, Vol. 7 No.10|
The Alternating Current Field Measurement (ACFM) technique has now been in use for over ten years both in the on and offshore industrial sectors. As the technique has developed and more specialised probes and equipment have been produced the number of applications has increased. More inspection companies have used the technique and have reported cost benefits arising from using the technique instead of magnetic particle inspection. The initial example was on a North Sea platform installation when ACFM was used for the subsea inspection of nodal weld joints and the cost savings justified the change. In this case the cost savings were made up of reduced cleaning, reduced installation and commissioning time, greater available subsea inspection periods and negligible pollution. When the technique was used to inspect the pressurised and some civil and mechanical engineering systems the cost savings were different. They were a combination of ease of access, no need for external coating removal, no need for internal process product produced coatings, no requirement to dispose of grit/shot blast by- product and the production of defect sizing information that could be used by the structural engineers.
Since then the ACFM technique has been used for many other petrochemical associated applications and inspection companies and owner operators of plant have reported the cost benefits.
The ACFM technique is a non-contacting electromagnetic technique for the detection of surface breaking defects in conducting materials. The ACFM probe induces a uniform electric current into the material to be inspected which then produces a magnetic field which will be disturbed and flow around the edges of a defect if present. Small detectors or sensors are built into probes, which are used to detect these magnetic field disturbances. Two components of the magnetic field are measured these are the Bx and Bz, the former for to estimate crack depth and the latter to estimate crack length. These measurements together with software algorithms are used to determine the accurate length and depth of the defect.
|Fig 1: ACFM presentation.|
Magnetic particle inspection of a 24” diameter tubular weld can take up to 200 minutes, if the time for cleaning is included then this can increase to 4 hours and 30 minutes. If defect depth measurements are also required then this can increase again up to 5hours and 30 minutes. This is a substantial reduction from the 10-12 hours required prior to 1991. ACFM inspection of the same weld can take 60 minutes.
In 1991 initial subsea trials carried out comparing the inspection times between magnetic particle inspection and ACFM inspection on 10 node welds showed that there was an 85% saving in time and money. (1). During the following two years improvements were made in the inspection procedure used with magnetic particle inspection but even with these improvements ACFM still had a 75% saving in time and money. The offshore Gas Company carried out further trials with continued savings.
|Fig 2: Diver inspecting weld using the underwater ACFM equipment.|
The additional advantages of using ACFM compared to magnetic particle inspection are: If the node is coated then ACFM can be used directly to inspect through the coating. If the node is uncoated and requires some cleaning because of weed growth and hard calcareous layers, then a smaller area is required to apply the ACFM technique. Because the ACFM system requires shorter time to deploy and to be rigged on to the node, more welds can be inspected during the dive period.
The ACFM technique can be used during any lighting conditions, either day or night so that there are no restrictions to use. Fluorescent inks usually require a limited light value for operating practice.
If a defect is located the ACFM technique can be used to determine the length and the depth of the defect in an uncoated or coated weld and does not require the use of another technique.
Since these initial trials in the early 1990’s more than 30 operators around the world are now using ACFM for subsea weld inspection including in 2001, operators in Russia, Scandinavia, Western Australia, BP in the UK and SISTAC and Petrobras in Brazil as well as Pemex in Mexico.
These last four examples have all been unique in that in Scandinavia, the inspection was carried out on a jack-up rig whilst still on station. This was instead of inspecting the rig in a dry dock or near shore thus saving the owner both considerable time and money, not taking the rig off stream and the cost of a dry dock. A similar exercise was carried out in Brazil for Petrobras where the savings were $100,000 when ACFM was used instead of MPI and reduced the time for inspection by two days.
The work for Australia was carried out with a Remote Operated Vehicle deploying an ACFM array. The Australian offshore industry has been trying to reduce the use of diver involvement because of industrial relations problems and the problems of shark attack. In this case the ROV was able to inspect the weld required after the work had been planned using ROV and manipulator modelling software developed by GRL. This method of planning illustrates the degree of success prior to the actual inspection and demonstrates the amount of access that the manipulator/ROV combination will get from a particular docking position on the platform.
BP had a particular problem in that they required welds to be inspected at a water depth of 490 metres (1592’). In this case they used a Triton ROV and a mini pencil probe. Although a special deployment frame had been built to deploy the probe and had proven satisfactory during trials the mini probe was also deployed directly with the manipulator.
|Fig 3: An ACFM mini probe attached to a manipulator used to inspect a subsea weld.|
The use of the ROV saved the cost of saturation divers with all of the ancillary costs associated with special dive boats. (2)
Further work in Brazil was on semi-submersible rigs requiring shallow dives only. Instead of using the specially developed subsea ACFM system used by both divers and ROV’s, SISTAC used a standard AMIGO portable system together with special underwater probes having 50 metres (162’) of cable. Offshore operators with fixed platforms in shallow water i.e. less than 30 metres (97’) water depth have been considering this combination for inspection from the spider deck, boat inspection and riser inspection in the splash zone inspection. None of these applications would require diver support vessels and the additional cost. Over 15 platforms have been inspected using the standard underwater ACFM system by Petrobras instead of MPI and savings of $1,800,000 have been achieved. (3).
The first major topside inspection was carried out on 12 platforms from two offshore installations. The inspection was to be carried out on pressure vessels and process pipework. Previously the inspection had been carried out using magnetic particle inspection, cleaning the plant prior to inspection, using scaffolding to gain access to the areas to be inspected and using calendar based inspection. It was decided to change this operation to the use of ACFM, use rope access trained NDE technicians and also to introduce risk based target inspection.
The risk based target inspection philosophy can be applied to the inspection of a platform in order to produce an inspection programme that can be used to detect the onset of fatigue cracking at its most economical stage. This would include the early detection and assessment of fatigue cracking in high-risk areas and also eliminate the cost of unnecessary inspection in low risk fatigue areas of the platform. This can also be applied to pressure systems where the system is separated into high, low and medium sensitivity areas. The minimum inspection interval corresponding to the detectable crack size of the detection system used is determined from the crack growth data. It is essential to detect the crack before it reaches the repair stage. Past and in the case of some companies the current practice suggests that the inspection interval remains the same independent of the fatigue sensitivity of the joint i.e. prescriptive inspection. This means that some joints are receiving too much attention and some are being neglected, whereas a rationalised inspection-scheduling programme would ensure that only those welds located in high-risk areas would receive adequate and reliable inspection. This practice can only perform efficiently if the sensitivity i.e. the accurate sizing of length and depth, of the inspection technique is of the correct level for the detection of the minimum size detectable defect. The application of the technique and the inspection intervals scheduled must be optimised in relation to the inbuilt redundancy of the structure and the consequence of failure.
The above approach targets joints where cracking is most likely to occur, and schedules inspection when the crack can be detected by the inspection technique applied. In areas of high-risk i.e. high sensitivity areas, inspection techniques are required that will determine the length and the depth of the defect detected. In medium risk areas the inspection technique will have to detect and accurately determine the length, whereas in low risk areas the defect will only be required to be detected. Past and sometimes current practice has been to use one inspection technique for detection and length sizing and then to apply a separate technique to determine the depth of the defect. This has occurred in the high- risk areas. In the medium risk areas only one inspection technique has been used to determine the length of the defect detected and in the low risk areas a macro technique has been used for detection. In all of these applications extensive cleaning would have been required. The ACFM technique was developed so that it could perform all of these tasks with the same sensitivity and can be used for detection and sizing without the requirement for extensive cleaning.
The above risk based target inspection was also introduced to the inspection of the offshore installation. This reduced the number of inspections to achieve a given level of reliability by extending the periods between inspections. This surprisingly only reduced inspection costs by 25%.
Introducing ACFM instead of MPI gave a cost saving of 66% on the cost of inspection. Instead of using scaffolding, rope access NDE technicians were used on structures, cranes, pipe work and pressure vessels. This gave a saving of 50%.
|Fig 4: Rope access technician using ACFM to inspect bulkhead weld.|
Overall the cost of inspection was reduced by 67% with the introduction and combination of the use of ACFM, rope access NDE technicians and target inspection. In terms of cost this was $500K/year.
In a recent offshore inspection seven ACFM crews were used to inspect 28 vessels in three weeks. The cost of the ACFM operation was $300,000 but the client saved $500,000 by not having to sandblast and not having to cleanup the debris produced by the sandblasting operation. Using a combination of laptop computers and the Amigo ACFM system no hot work permits are required and with the use of long cables access into hazardous areas is possible.
The ACFM probes are designed to operate at temperatures up to 200°C and have been used for on stream inspection.
In a recent conference two inspection companies presented cost savings data. (4).
In one paper direct comparisons were given for the use of ACFM compared with wet fluorescent magnetic particle inspection for both time and cost.
For a 6’ x 6’ vessel there was a 62% savings on cost and a 63% savings on time.
For a 10’x 50’ vessel the results were 65% and 58% and for a 20’ x 85’ tower the results were 61% and 57%.
A very detailed analysis of the inspection tasks was carried out and in all cases the biggest saving was in the difference between the times for surface preparation and for the larger vessels the other major saving was in the time taken to carry out the inspection. The general comment was that conditionally ACFM was 60% faster than wet magnetic particle inspection.
One client had problems with a shaft in which an unscheduled stoppage had occurred due to a shaft failure on a gas compression station. The shaft had failed on a change of section, which was difficult to inspect. A special ACFM probe was produced to examine this area and is now being used. The client has stated it would cost the company $250,000 for every unscheduled shutdown so that every time a defect is detected the company saves $250,000. (5).
|Fig 5: Specially designed shaft probe.|
A pressure vessel 150”x 74” was located in a refinery. A fire had occurred in the refinery and a pipe bundle had buckled under the heat and had been damaged subsequently damaging the adjacent pressure vessel. The damaged area of the vessel was removed and a patch was welded onto the shell from the inside. No post weld heat treatment was carried out. Because the vessel was operating under a hydrogen sulphide environment and the weld on the patch had not been heat-treated it was decided to inspect the internal welds of the vessel after six months. The conditions on the inside of the vessel were dirty and corrosive and instead of using MPI, ACFM was used. The ACFM inspection took three hours whereas past inspections with MPI had taken one day. The scanning rate was six metres in 1.5 minutes. Four cracks were located and sized, These were removed by grinding and ACFM was used to monitor the grinding operation with satisfactory results. (6).
A new 347 stainless steel pipeline was being fabricated and in the past the process has been to produce the root weld, allow the weld to cool down then inspect with dye penetrant. Any repairs were then carried out and the next pass laid down after re-heating. At each inter-pass stage the weld was allowed to cool, then inspected and then reheated to allow welding to continue. The ACFM system together with a pencil probe was used instead of the dye penetrant inspection as a quality control tool. No cooling below the re- heating temperature was required and the weld production increased. The specialised welding time had been reduced from 12 hours /weld to 2 hours /weld because of the reduced inspection time and heat cycle time. No repairs were necessary during the fabrication. It was estimated that $2,000,000 had been saved on this particular application. (7).
Various service companies have reported the cost benefits and have confirmed that above statement that ACFM is typically 2-3 times faster than MPI and up to 5 times faster for large inspection jobs on a painted structure. One example given was that of 600’ of weld on a painted structure was inspected in 1.5 days instead of 6 days with MPI. Cost comparisons have been given for the inspection of 100 x 3’ welds with defect removal.
One man inspecting a painted structure with MPI would cost $1500 compared to $750 with ACFM. If the same scenario was used with a combination of MPI and scaffolding the cost of inspection would be $5250 compared to $2250 using a 2 man ACFM team.
The ACFM technique has now been used for over ten years for the inspection of welds in a number of conducting materials in the petrochemical and non-petrochemical industry. In the petrochemical industry it is used throughout the production chain from E and P and the inspection of offshore structures to the refinery, where fractionating columns, pressure vessels and coke drums are inspected. In all of these inspections the replacement of MPI by ACFM during these applications has resulted in substantial cost savings as well as other associated benefits. In some countries national training schemes have been set up by the major users, such as Petrobras, in others training schemes used in other countries have been accepted. Close co-operation between the users of ACFM and the major Certifying Authorities has meant that ACFM is accepted as an acknowledged inspection technique.
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