LETS Global Life Extension Procedure
The LETS Global Life Extension Procedure combines the FE analysis of the structure with verification of the results by means of offshore inspections. These two combined with the extensive experience of LETS Global personal and the selective application of ultrasonic peening treatment would enable operators to increase the resistance against structural fatigue of any offshore installation without disturbing the production. The LETS Global Ultrasonic Peening Treatment does not produce any heat, and is therefore safe to use on a producing installation or a drilling rig.
Early signs of degradation: coating damage in high stressed connections
One of the most common signs of early degradation in a vessel is the visual appearance of coating damage in weld connections subjected to high stresses. It is however important to differentiate the different types of coating damages which could be encountered during an inspection of a ballast tank. Many of the coating damage issues could be motivated by different causes other than high stresses and hence these will have a limited significance for the structural integrity of the installation.
Damage coating in longitudinal connections iwo web frames and iwo bulkheads
For an experienced Naval Architect it would not be difficult to find these “compromised” locations from the point of view of potential fatigue damage. One of these specific locations which from tanker to tanker shows coating damage are the longitudinal connection in the way of frames and in the way of transversal bulkheads. Subsequent to the inspection and the photographic documentation of the coating damage a FE sub-modeling will confirm that indeed these coating damage is originated by the high- stresses present at this location.
Why coating cannot stand the high-stresses and what are the consequences
The two main reasons could be the degree of adhesivity, not enough roughness or to much roughness as in a welded surface, or a limited strain capacity of the coating. Nevertheless, a coating damage will give way to an early corrosion if the cathodic protection system for one or other reason is not working accordingly. As the scheduled ballast tank inspection for a specific tank could take two or more years to happen it is crucial that all the protection systems are reliable and work accordingly to prevent any structural integrity degradation to happen.
Elements used to categorize coating anomalies encountered during offshore inspections
• The seriousness of the location; i.e. on the cargo tank bulkhead is potentially more serious than elsewhere due to both the safety consequences – hydrocarbon gas in a non-inerted space, and the difficulty/cost of repair.
However, any method is never exact and the judgment and experience of a LETS Global Inspection Engineer will be a valued contribution.
LETS Global Life Extension Procedure is a Pro-active Solution
The LETS Global Life Extension Procedure is a pro-active solution for structural integrity challenges that operators of ageing offshore installation face. The general condition of the installation is documented and assessed during the offshore inspections. The different types of coating damage would need to be analyzed and quantified in relation to its influence on the structural integrity of the installation. Certain type of damage would not be dangerous for structural integrity while other would need to be carefully considered. Coating damage on high stressed areas will be of special importance. LETS Global Ultrasonic Peening Procedure will be applied on weld connections in these areas in order to increase their resistance against premature fatigue cracking and in order to prevent fatigue cracks from developing.
Life Extension Concept: locating of fatigue hot-spots in the offshore installation.
Offshore inspections will confirm locations showing stress concentrations possibly already detected in the FE coarse model. Moreover, the inspection will be the basis for a FE fine mesh model of any specific location and will give advice about any other fatigue cracking sensitive area which for some reason have not been detected in the FE model. Subsequently this FE fine mesh model will be used for spectral fatigue analysis and henceforth the remaining service life until cracking could be estimated. Offshore inspections would also be useful to detect any discrepancies in the construction drawings which could be of importance from the structural integrity point of view.
The condition of the cathodic system would need to be checked and confirm it is working accordingly. As this is done normally during the scheduled inspections it could be combined with the ultrasonic peening treatment of selected weld connections.
Fatigue Cracks Detected During Offshore Inspections
If fatigue cracks are detected during an offshore inspection it would need to be decided if a weld repair needs to be carried out previous to the application of the ultrasonic peening. The application of MPI (Magnetic Particle Inspection) must be complemented by UT (Ultrasonic Testing). If the fatigue cracks detected are long cracks along the weld toe normally depth 1-2 mm the weld could be treated with ultrasonic peening and the crack would not cause any harm. However if the crack is a deep the UT must be implemented to measure the crack dimensions. If the cracks are too deep in relation to plate thickness then weld repair is necessary previous to the application of ultrasonic peening. In any circumstance the weld repair procedure would need to be complemented with the subsequent ultrasonic peening treatment.