This paper discusses the effects of local deformation, dent, and strain hardening properties on strain capacity in compression of a line pipe.
Compression tests were conducted using two pipes with the nominal diameter of 400mm. These pipes had roundhouse type stress-strain curve, and correspond to L290 grade in Spec 5L of API (American Petroleum Institute) standards. One pipe was a plain pipe without dent, The other was a dented pipe. The depth of the dent was about 3% of the diameter. The test results explain that the strain capacity can be reduced by 25% due to the effect of dent.
A series of finite element analyses were conducted to investigate the compression behaviors. The strain capacity in compression was defined as the longitudinal critical remote strain whose strain distribution was free from the effects of a dent. At first, that finite element analyses were verified that they could reproduce the results of compression tests. Next, the size of dent were changed on that finite element analyses model, some different case were analyzed in order to investigate the changes of the strain capacity in compression. The strain capacity, the longitudinal critical remote strain, decreased to about a half in case of 3%-depth dent, compared with a plain pipe.
Seismic integrity of the pipeline with a dent is discussed in accordance with the seismic design guideline issued by Japan Gas Association. In case of the strong earthquake, “Ground Motion Level-1”, the dented gas pipeline is safe, even if the depth of the dent is 10% of the diameter. In case of the maximum earthquake, “Ground Motion Level-2”, the gas pipeline might buckle longitudinally in soft ground.