The increasing needs of natural gas, foreseen for the next years, makes more and more important the type of transportation chosen, both from strategic and economic point of view. The most important gas markets will be Northern America, Europe, Asia and Russia but the demand shall be fulfilled also by emerging producers as Kazakhstan, Turkmenistan and Eastern Siberia that at the moment are developing their resources in order to be competitive on Gas market. In this way producers and customers will be placed at greater and greater distances implying realization of complex gas transportation pipeline network, when use of LNG tankers is impossible or uneconomic. On the base of these considerations in 1997 began a feasibility study on X100 steel, given that, comparing different design approaches, it has been observed that consistent savings could be obtained by means of using high grade steel and high pressure linepipes. In this multi-sponsored project (Eni group, European Community of steel and Carbon, CSM, Corus and Europipe) CSM and Corus group were involved in the laboratory and full-scale pipes testing, Europipe was the pipes producer and Snam Rete Gas was involved in field weldability and technical coordination. No technical breakthrough, but only improvements in the existing expertise were involved in the X100 production; consequently, the production window is very narrow. However optimized steelmaking practices and processes enabled the material to reach the desired properties: strength, toughness and weldability. This paper is intended to present the general results arising from this project, in terms of steel properties (chemical composition, mechanical properties), ductile and brittle fracture resistance (results of full scale burst tests, West Jefferson tests) and field weldability, but above all the know-how stored till now on high grade steel and its possible use from a Gas company and a Pipe maker point of view.

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Actually, the increase in natural gas needs in the European market, foreseen for the beginning of the next century, compels to develop new solutions for the exploitation of gas fields in remote areas. For natural gas transportation over long distances the hypothesis of a large diameter high-pressure pipeline, up to 150 bar (doubling of the actual one) has been found economically attractive, resulting in significant reduction of the transportation cost of the hydrocarbon. In this contest the interest amongst gas companies in the possible applications of high-grade steels (up to API X100) is growing. A research program, partially financed by E.C.S.C. (European Community for Coal and Steel), by a joint co-operation among Centro Sviluppo Materiali (CSM), S.N.A.M. and Europipe in order to investigate the fracture behaviour of large diameter, API X100 grade pipes at very high pressure (up to 150 bar) has been carried out. This paper presents: the current status of technology of API X100 steel with respect to the combination of chemical composition, rolling variables and mechanical properties the results obtained from West Jefferson tests, in order to confirm the ductile-brittle transition behaviour stated from laboratory tests (DWTT), the results obtained concerning the control of long shear propagating fracture and in particular the results of a full scale crack propagation test on line operating at very high hoop stress (470 MPa). Besides, in order to investigate the defect tolerance behaviour of the pipe with respect to axial surface defect, burst tests with water as pressurising medium have been carried out and the relative results are presented and discussed.