Abstract: A method of measuring biaxial stress in an object of a ferromagnetic material in which material in a region (10) in the vicinity of a surface of the object is subjected to a conditioning method by application of a conditioning magnetic field that is at least initially at a high field strength. Values of biaxial stress within the said region are measured with an electromagnetic measuring probe (14) in at least two different orientations, the electromagnetic measuring probe (14) using an alternating measuring magnetic field that is at a field strength well below saturation. The conditioning may subject the region (10) to a low frequency alternating magnetic field (38, 58) initially at a high field strength, and gradually reducing the strength to zero over a decay time period at least equal to the time for many cycles of the low frequency magnetic field. Conditioning the material enables the stress to then be measured more accurately, and enables ambiguities in biaxial stress to be resolved.

Abstract: A flexible elongate structure, such as a flexible riser (10) for connecting oil and gas wells to floating production platforms, comprising at least one layer (20) of steel wires near the surface which extend at least partly along the length of the structure, can be monitored by inducing a magnetic field in the steel wires using an electromagnetic coil, and monitoring the magnetic flux density near the surface of the structure so as to detect if any wires have broken. Measurements are made at two different frequencies, the lower frequency giving an output dependent both on stresses and on the number of adjacent wires in the layer (20), and the higher frequency giving an output primarily dependent on the number of these wires. By comparing these two measurements a corrected output parameter (P) may be obtained that is indicative only of stress. A break in a wire can be expected to change the stress in that and adjacent wires.

Abstract: A method of measuring biaxial stress in an object of a ferromagnetic material in which material in a region (10) in the vicinity of a surface of the object is subjected to a conditioning method by application of a conditioning magnetic field that is at least initially at a high field strength. Values of biaxial stress within the said region are measured with an electromagnetic measuring probe (14) in at least two different orientations, the electromagnetic measuring probe (14) using an alternating measuring magnetic field that is at a field strength well below saturation. The conditioning may subject the region (10) to a low frequency alternating magnetic field (38, 58) initially at a high field strength, and gradually reducing the strength to zero over a decay time period at least equal to the time for many cycles of the low frequency magnetic field. Conditioning the material enables the stress to then be measured more accurately, and enables ambiguities in biaxial stress to be resolved.