Influence of preliminary deformation on micromechanisms of failure of offshore gas pipeline material Pavlo Maruschak 1,a , Iryna Danyliuk, 1,b , Lyubomyr Poberezhnyi 2 , Taras Pyrig 2 , Sergey Panin 3,4 1 Ternopil Ivan Pul’uj National Technical University, Ternopil, Ruska 56 str., 46001, Ukraine 2 Ivano-Frankivsk National Technical University of Oil and Gas, Ivano-Frankivsk, Karpatska 15 str., 76000, Ukraine 3 Institute of Strength Physics and Materials Science SB RAS, Tomsk, pr. Akademicheskii, 2/4, 634021, Russia 4 National Research Tomsk Polytechnic University, Tomsk, pr. Lenina, 30, 634050, Russia a Maruschak.tu.edu@gmail.com (corresponding author) Keywords: pipe laying, fatigue, fracture, in-service properties, damage, plastic deformation Abstract. The S and J - methods of pipe laying tend to increase the brittle fracture component in the material, however, they have practically no effect on the residual life-time of metal under low amplitude cyclic loading (260 MPa). An increase in the stress range up to 320 MPa enhances the negative effect of preloading, which is more pronounced in case of using the S - method of pipe laying as compared to the J - one. Within the high amplitude range (420 MPa) the process of preliminary deformation causes the activation of relaxation processes and longitudinal splitting in the material, giving rise to an abnormal increase in the cyclic life of the plastically deformed material. Introduction Offshore pipelines are one of the strategic parts of gas transportation systems, which have significant prospects for development [1]. This determine the necessity in investigating the effect of the pipe laying technology onto the in-service properties of the pipe, in particular, the cyclic life and resistance to vibration related to the impact of sea waves (heaving) [2]. Such investigations allow optimizing the pipe laying technology and ensuring a reliable operation of gas pipelines during their service life [3]. Several pipe laying methods are known, in particular: the S - method of pipe laying, whose name is derived from the shape of the pipe, which it acquires during the laying process. The main disadvantage of this method is the impossibility for a precise regulation of deformation in bending points which are S-like sections [4]. The J - method of pipe laying was developed as an alternative to the previous one, however, it is in fact its add-on [4]. The peculiarity of this method consists in the possibility of pipe laying at deep sea regions, where the pipe gets the J-like shape, which is achieved by its practically vertical immersion. In this case, the probability of its bending and significant deformation is decreased [5]. This method allows the pipe-laying vessels to regulate the pipe bending deformation more precisely. As a consequence, the accumulation of technological strains in the pipe wall decreases. Such strains may have a significant effect onto the service life of the pipe [6], especially in case of the S-method of pipe laying. However, there are practically no works dedicated to the influence of the preliminary deformation processes on the regularities in the accumulation of microdamages and fatigue life of steels of the offshore gas pipelines. The stress- strain state of the pipe wall under the analyzed modes is calculated before its laying. The calculation results are introduced into a technological map for pipe laying, as well as into the standards of the allowable defectiveness for field circumferential joints of the pipe. However, the effect of pipe damage caused by the process of laying on its cyclic life remains unattended by engineers and needs Applied Mechanics and Materials Vol 770 (2015) pp 304-309 Submitted: 2015-03-12 © (2015) Trans Tech Publications, Switzerland Accepted: 2015-03-18 doi:10.4028/www.scientific.net/AMM.770.304 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (ID: 5.58.26.137-28/04/15,12:52:57)