LETS Global Life Extension Procedure consist of a number of steps to allow for the repeatability of the fatigue life improvement in every treated detail in an offshore installation.
LETS Global Life Extension procedure has been applied in a number of offshore installations around the world. The procedure contains a number of steps to ensure the optimal application as well as the maximum benefit can be achieved for every weld connection treated.
NDT inspection previous to ultrasonic peening treatment
The selected weld joints for treatment on an operating structure have normally been in service at least half of its design life before we carry out the ultrasonic peening treatment. As a result we need to ensure that if any fatigue crack would be found, their size(s) would be such that the treatment will restore its fatigue life an/or reset the clock to zero as a minimum value for that specific weld location. If the crack found shows to be too deep in relation to plate thickness the crack would need to be repair previous to ultrasonic peening treatment. The inspection is carried out in selected high stressed areas, which are found with the information given by the local FEA model. The inspection can be carried out with MPI but the use of ACFM technique is desirable because it is easier to apply and it does not require any coating removal.
Preparation of weld toes previous to ultrasonic peening
The weld toe and weld reinforcement must be cleaned previous to the treatment using sand blast or needle gun. This is to complete remove the coating and other corrosion surface impurities and to ensure the peening working tool have free access to the entire surface of the weld. If for some reason this cleaning operation can not be carried out the ultrasonic peening treatment will simply take longer time. After the described cleaning operation is completed the preparation of weld toe area can start with a pin tool of 3.0 mm diameter. This is to achieve a complete removal of all cold laps, lack of fusion defects, weld spatter, etc. which are located at weld toe.
Weld Groove Formation
Weld groove formation is done with a 4 mm pin tool diameter to achieve the reduction of geometrical stress concentration at weld toe. The final radius at weld toe transition to plate will be 2.0 mm and groove 0.5 mm deep. Furthermore, the metal surface at the weld toe groove will be completely clean from any impurity or weld spatter. It is also important that the weld toe groove follows the contours of all weld toes including eventual start and/or stop located in the weld. The same requirement will apply for inter-pass weld toes.