Exploration • Drilling • Field Services
Magnetic field nulling or degaussing? It depends...
What’s best for stopping magnetic arc blow on your job? Dr Steve Foulds reports. M
agnetic arc blow causes welding job delays, poor weld quality, frustration and unexpected costs – particularly irritating when everything is in place yet the
project is completely stalled because of magnetism. Why can’t you just degauss the whole pipe? Tis question is often voiced but it is simply not a sensible option. Pipes are large and the coils required to demagnetise them are similarly large and must be passed over the pipe. Te process is difficult, expensive, power hungry and slow. Worst of all, a demagnetised pipe may have become magnetised again by the time it reaches the pipeline destination. Two solutions are commercially available and commonly used. Te first is generation of a magnetic field that exactly cancels the field in the weld preparation due to the inherent magnetism in the pipe – commonly referred to as field nulling. Te second is removal of the magnetism from the material around the weld preparation – referred to as joint degaussing when carried out at fit up or pipe end degaussing if carried out before fit up.
Field nulling Magnetism in pipes appears in two distinct ways. One, referred to as longitudinal magnetism, results in a magnetic field across the weld preparation
that is of similar magnitude and sign around the circumference. In contrast, zone magnetism results in a magnetic field that changes magnitude and sign around the pipe circumference. Diverse’s Zeromag ZM100A provides field nulling with two modes: manual and automatic. Manual nulling is an excellent solution for all cases of longitudinal magnetism. Tere are no restrictions on the number of weld positions and no field probe needs to be deployed during welding. For cases of zoned magnetism, manual nulling is not preferred due to the need to continually stop the welder to check the field and readjust the nulling field. Automatic nulling works well for all types of magnetism but is particularly useful with zoned magnetism. In this mode the field is actively nulled at the weld position as the welder progresses. Te drawback with active nulling and zoned magnetism is that only one weld position can be effectively nulled, in this case a second welder is not able to weld the other side of the pipe. Fig. 1 shows how the field generated by coils concentrates in the pipe walls and the weld preparation region between the pipes.
Degaussing
Although degaussing complete pipes is not practical, degaussing the area of the joint at fit up or pipe-ends before fit-up is possible in some cases.
Fig. 1. Magnetic field in pipes.
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