Ultra Heavy Wall Linepipe X65: Full Scale Severe Straining Sequences of Pipeline Strings

Tenaris and Centro Sviluppo Materiali (CSM) are carrying out a Joint Industrial Project aimed at developing heavy wall line pipes. The suitability for very severe applications, involving high service pressures and temperatures, the latter causing large strain fluctuations, in presence of an aggressive sour environment, is analyzed both theoretically and experimentally, including small and full pipe models and tests. The full project program aims at developing a new generation heavy wall product, supported by a comprehensive laboratory analysis of the material response under severe mechanical loading in aggressive environment and a full scale testing program, including both pipe and girth weld. Both investigations are mainly addressed to basic understanding of impact on design criteria of interaction between severe loading and aggressive environment. Four papers, [2], [3], [4] and [5], have been already presented, in previous OMAE conferences, on this project. The present paper focusses on the full scale testing performed on strings of pipeline consisting of different pieces of pipe. In particular, two full scale testing have been performed applying the severe straining sequence defined as extreme in term of resistance against ratcheting, also involving plastic straining, as deduced in a previous work inside the project. The loading sequence was applied in global strain control, averaged on the whole string length, but necessarily the local distributions of strain differs in the three pieces of pipe. Analytical considerations are done about the expected straining behaviour and experimental results confirmed the theoretical considerations. The high strain hardening capability of the X65 steel pipes, metallurgically designed for strain based applications, guaranteed a good recovery of any non-homogeneity in straining, both during cycling that in larger axial deformation of the string. During severe cycling in elastic-plastic regime in presence of pipe internal pressure, the strings demonstrated good resistance to ratcheting. In fact, due to the material capability to redistribute the cycling strains along the whole sample length, any section experienced limited ratcheting with unreversed circumferential expansion, during cycling, well within limits of tolerability. It is worth noting that, even in presence of severe cycling conditions, both on-shore type girth welds (SAW for double joint) and off-shore type (GMAW in Narrow Groove Bevel Preparation) preserved their integrity with no cracking or other damage.