Detecting failures in flexible multistrand steel structures

A method for monitoring a flexible elongate structure with a generally cylindrical surface, the structure comprising at least one layer of steel ligaments near the cylindrical surface, the steel ligaments extending at least partly along the length of the structure, the method comprising inducing an alternating magnetic field less than saturation in the steel ligaments using an electromagnet with spaced-apart poles adjacent to the cylindrical surface, and monitoring the alternating magnetic flux density near the said cylindrical surface in the vicinity of the said electromagnet, from the detected flux density determining a parameter sensitive to stress in the ligaments, and from any variation of the said parameter as measured adjacent to different ligaments detecting if any ligaments have broken, wherein the alternating magnetic field is induced at at least two different frequencies at every measurement location, and the said parameter is determined from the values of flux density detected at those different frequencies.