Testing and optimisation using stress tests
May 11, 2007
The TÜV SÜD stress test constitutes of integral strength testing to eliminate critical defects. This method optimises pipeline operation, extends the service life and enhances the safety and cost-effectiveness of pipelines.
As a genuine strength and load test, the stress test imposes a load approximating the actual yield point on all pipe sections of a pipeline. In a quantified (interrupted) stress test with "training effect" (pre-stressing to increase fatigue strength), steps are taken to prevent unacceptable permanent deformations in the pipes. This is achieved through statistical evaluation of the actual values of pipe strength and wall thickness, measuring the volume of water pumped into the pipe, and what is known as the 'training effect of the material", which refers to the observation that the fatigue strength of a material can be increased through the correct application of pre-stressing.
The water pressure test carried out according to the stress test method is performed on pipeline sections laid in the ground which have been filled with water to exclude air if possible. During the filling process, pigs (pipeline inspection gauges) run against a certain supply pressure. The water or air supply pressure must be high enough to ensure that the water column between pig and filling point remains dense throughout the entire filling process and does not break at local peaks.
A quantified (interrupted) stress test with "training effect" can be seen in Figure 2 . This type of stress test is used when the objective is to achieve particularly high pressures while minimising permanent expansion in circumferential direction in order to eliminate defects. Further areas of application refer to pipelines made of materials with specific expansion properties or pipeline sections involving major height differences. The figure shows that this method allows higher pressures to be reached with lower permanent pipe expansion.
The pressure load imposed on the pipeline is intended to eliminate faults in the pipe wall. This is achieved if a load approximating the material's cohesive strength which corresponds to its tensile strength occurs at the edges or across the remaining cross sections of faults. There is negligible growth in the sheet's direction of rolling of localised faults which cause leaks when stress is imposed during the pressure test. Longer faults result in rupture when tearing.
We know from experience that a 100 per cent circular pipe does not exist. Manufacturing causes certain deformations which,according to the standard, are still acceptable and, to some extent, may occur simultaneously. Soil pressure and the degree of compression of the pipeline after laying also cause pipeline deformations and stresses.
The load cycle performance of any faults and deformations withstanding the loads applied during the stress test is clearly improved. Around crack-tips and locations with high stress concentrations in particular, yielding will occur due to high internal pressure loads. This, in turn, will improve pipe geometry leading to lower stress concentration factors (SCF) and negative compression stress during pressure relief.
One of the prerequisites of stress test effectiveness is that test stresses act in the same direction as the stresses occurring during pipeline operation and that stress levels differ sufficiently. In pipelines with pitting corrosion, however, stress tests are not sufficient as the sole rehabilitation procedure. In this case, defects caused by corrosion must be tracked down with the help of other suitable test instruments such as "intelligent pigging systems" and removed if they overstep a critical size.
Stress tests, as high-pressure tests, offer the possibility of optimising new pipelines and rehabilitating existing ones that display signs of stress corrosion cracking or groove corrosion.
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Contact
Dipl.-Ing. (FH) Hans-Joachim de la Camp
Munich (Germany)
Phone:
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