In the course of project planning for one of Europe‘s largest onshore gas pipeline, questions araised if alternative technical approaches, which - in terms of dimensioning - provide parity for the same high-level state of safety like EN1594.
Line pipe as a ‚delivery item‘ will normally have higher actual yield strength limits and wallthicknesses than required by applicable standards. This potential can be technically assessed and used for safe pipeline operation. This approach results in an techno-econommically optimum of pipeline dimensioning without diminishing any aspect of safety. In accordance with the corresponding set of rules, dimensioning of line pipe has to apply EN1594. As basic prerequisite, DIN/EN10208-2 states more stringent requirements on line pipe, particularly with regard to tolerances of ordered wallthickness, yield ratio, elongation at break and impact strength. For determination of strength values K x s , a number of line pipes per each production lot will be mechanical-technological analysed. Applying the ‚Gaussian Distribution‘ shall ensure min. 97.5% of all pipes are calculational fit for purpose, i.e. suitable for desired design pressure and all actual material properties as per Inspection Certificates do correspond to the stringent requirements (i.e. EN10208-2).
Pipelines will be hydrostatically tested, i:e. in accordance with requirements by „Stress-Test-Verfahren“ (VdTÜV1060). There have to be taken into account the actual mechanical properties verified by tensile tests (uniaxial stress state, determination of theoretical test pressure), and the actual load conditions within a pipe (mulitaxial stress state). During stress testing, a certain test pressure level must be achieved at the test section’s high point, which, depending on the actual ‚new‘ design pressure and maintaining the required safety factor, has to prove surplus stress allowance of min. 5% multiaxiality proportion. Therefore, stress testing enables to compare the uniaxial stress state of the tension specimen with the actual multiaxial stress state of the pipe wall. A caliper survey will be performed after pressure testing to examine for inadmissible deformation, i.e. dents, ovalities, enlarged diameter. The admissible threshold value for deformations is determined by pipe material, diameter and wallthickness. Thereby a verification will be given that all pipes withstand required load scenarios. After in-service experience, the operating pressure may be increased based on the aforementioned verification of the ‚new‘ design pressure.