Exploration • Drilling • Field Services
Fig. 2. Classic degaussing removes the magnetic field by down cycling.
Fig. 3. Magnetic removal performance is similar to the down cycle method.
magnetisation of the material). Te high field requires high currents and over the time to degauss this is energy intensive. Penetration depth into the steel when running at line frequency of 50/60Hz is insufficient to degauss thick walled pipes completely. Te solution is to use a degaussing system with high current and relatively low frequency of operation. Since in practice it is the ends of the pipe where the welding is to take place, it is only here that the magnetism needs to be removed. Consequently a degaussing scheme working at high current and low frequency over just the end of the pipe, will provide a solution. Te Zeromag demagnetiser can be used with the ZM150 degauss controller to provide this function. Pipe end degauss usually takes 3-5 minutes. Unfortunately, although the field is significantly
reduced by this method, the rest of the pipe will still be magnetised and that magnetism will, like some magic spell, slowly creep back into the material at the end of the pipe. Te time taken for this is typically 1-2 hours. Tis therefore gives a time horizon for this process - both pipe ends and at the very least the root pass must be completed before the bad black witch of magnetism returns. Although this may seem to be inappropriate for some applications, it is used in production scenarios, for example spool base or lay barge where the production rate is high and the weld time is short. Te above discussion suggests that it would be good to be able to degauss quickly and, if necessary, degauss again mid-job if the magnetism returns. To speed the process, the degauss controller can be configured to use a novel single cycle demagnetising scheme. Te idea is to identify a critical point in the B-H characteristic of the material, which, when the demagnetising current is returned to zero, the
remnant magnetic field also returns to zero- (Fig. 3). Te magnetic removal performance is similar to the down cycle method. Using this technique, the pipe end can be degaussed in a single cycle taking less than 80 seconds. When this is combined with clam coils, which allow the demagnetising cable to be deployed in a few seconds, a highly efficient methodology unfolds. Te process is as follows:
1. Fit up the butt joint. 2. Check the field in the weld prep. 3. Apply the clam coils. 4. Undertake a single loop demagnetisation of the joint.
5. Remove the clam coil, check field in the weld prep.
6. Weld. 7. If the magnetism returns, then repeat the demagnetising process as required.
Tis method is fast! In fact so fast there is virtually no impact on job completion times, no matter what the level of magnetism. If your material or job geometry preclude this method, you can always fall back to magnetic field nulling.
Summary Te arc blow problem can cast a spell over a project causing severe project delays, so having the right demagnetising equipment available is seen by many leading fabricators as very good insurance, especially when the cost of delay is high. Using one of the active nulling or degaussing methodologies described can get a project going at full speed again without the need for magic wands. Abracadabra! ●
Enter 16 or ✔ at
www.engineerlive.com/iog
John Anderson is a magnetic technical specialist at Diverse Technologies, Shelford Cambridge, UK. www.diverse–
technologies.net
16
www.engineerlive.co
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