A new product from TSC for measuring stress underwater

The TSC StressProbe has been developed for measurements of stress in threads, stress in tubular joints, residual stress in welded components (pipes, risers), and residual stress in forgings. In particular special purpose probes have been designed for measuring bolt loads in underwater repair clamps.

Two approaches have been developed so far for threads that either measure stress directly in a loaded thread or, alternatively, determine the stress by comparison between the loaded section and the free end. Both approaches have been successfully tried in laboratory tests and underwater on 34mm and 40mm high strength bolts. The TSC StressProbe has now been used for inspection of subsea clamps.

TSC are now in a position to provide the StressProbe, or conduct service work, for pipes, risers, tubular welded joints, subsea clamps and FPSO hulls., so that stresses can be measured and reported. StressProbe is available for diver use as well as ROV deployment. Stress measurement can be made through coatings (up to 5mm) as the system is non-contacting.

The StressProbe, previously known as the "ACSM system", received type approval from Bureau Veritas in July 1998.

The measurement of mechanical stress in a body is important in general terms as it is an indicator of residual strength or possible fatigue damage. In many cases the original stress in a component may change during service. It is important to be able to measure the stress periodically during the service life, or, in some cases, monitor continuously.

For repairs, it is also an indicator of whether the repair is effective. This is particularly true of bolted/clamped repairs where bolt tightness is required at all times. In other situations, where clamps are used for restraint or support, bolt tightness is also important. A non-destructive measure of the load (stress) being a component, structure or bolt would be very advantageous and the TSC StressProbe can be used for this purpose.

Mechanical stress can influence the magnetic domain distribution in ferro-magnetic metals. This feature, known as piezo-magnetism can be utilised to determine changes in the stress state on the surface of metal. Magnetic permeability changes of this sort can be measured using a.c. field measurement devices. These devices have improved recently to the point where very small changes in permeability and hence stresses are detectable. Thus changes in stress of the order of a few percent of the zero to yield stress range have been measured in the laboratory for structural steels. The very latest a.c. field measurement devices can now be used for non-contacting induced field measurements and this has provided the opportunity of developing the TSC StressProbe for use in the service.

In order to exploit this stress measurement capability a new non-contacting probe system has been designed and tested. This system uses multiple induced fields and sensor coils together with materials data and software interpretations based on the theoretical analysis of anisotropic permeability. A completely new, purpose made, stress measurement device, the TSC StressProbeis now available for a wide range of applications. The work was partly conducted using a DTI SMART award and was completed in August 1996. It received Millennium Product Status in 1999.

For further information please contact: