Fatigue Aspects of CRA Lined Pipe for HP/HT Flowlines
Authors: Sriskandarajah Thurairajah , Roberts Graeme , Rao Venu 
Published: May 2013

Abstract
Hydrocarbon exploration and production is moving into deep and ultra-deepwater to meet global energy demands. The industry is having, as a result, to face up to field developments with great challenges, including designing for a HP/HT product whilst also meeting sour service requirements. It has been a common practice in the past two decades to use pipelines with metallurgically bonded corrosion resistant alloys for such field developments.
CRA lined (mechanically bonded) pipes are, however, a viable alternative. The viability of using CRA lined pipes depends largely upon the behaviour of the welds associated with the CRA liner under fatigue loading. Pressure and temperature loading that varies cyclically can be expected to result in plasticity induced fatigue in the CRA liner as a function of the local radial gap at the liner/weld overlay interface (the seal weld). This fatigue behaviour is important in the context of the different manufacturing processes adopted by linepipe manufacturers and the need for consistency in the local geometry of the seal weld overlay when pipe joints produced in large quantities.
Conventional S N methods are not adequate to estimate the fatigue damage at seal weld. This paper describes an FE based analytical approach to estimate strain range and thus the fatigue damage at the seal weld for Low Cycle Fatigue (LCF). The study shows the fatigue damage variation as a function of the magnitude of the local radial gap between the liner and the backing steel, when the fatigue damage is caused by axial plastic strain ratcheting and elastic plastic stress strain hysteresis.
The paper discusses the effects of ‘low cycle’ fatigue of HP/HT flowlines at the seal weld using an analytical approach. The paper also discusses the importance of the radial gap between the CRA layer and the backing steel in general and at the seal weld in particular as a key parameter to be considered at the manufacturing stage.

Introduction - Why CRA Lined Pipes?
An increasing share of world's oil & gas production is obtained from offshore areas in deep and ultra deepwater. Many deepwater hydrocarbon fields are required to be designed for sour service due to the presence of carbon dioxide and/or hydrogen sulphide either in the early or later part of design life. Development of such fields needs equipment with high reliability and low maintenance costs. The material selection philosophy forms an integral part of the development process. There are surveys indicating that 60% of all maintenance costs in oil & gas exploration and production are related to corrosion maintenance and hence providing cost effective solutions become imperative.
The corrosive environments favour usage of corrosion resistant alloy materials in stainless steels (standard martensitic (13%Cr), austenitic (e.g.316L), duplex/super duplex (22%Cr, 25%Cr), nickel based alloy materials (e.g. Alloy 825, Alloy 625) and cupronickles). The cupronickel pipe is technically suitable for seawater, but less satisfactory when exposed to sulphide-containing oil. The austenitic steels are prone to pitting and crevice corrosion when exposed to chloride containing seawater. Thus the pipes made of solid CRA materials are technically not suitable for corrosion sensitive applications and they are also expensive to implement for a large application. The metallurgically bonded CRA clad pipes with C Mn backing steel to provide strength and corrosion resistance against seawater and incorporating 3mm thick thin layer of CRA clad material to provide the corrosion resistance against the presence of CO2/H2S offer a technical alternative to solid CRA pipes. Mechanically bonded lined pipes are the most cost effective alternatives to solid / clad CRA pipes for corrosion resistance applications by a factor of 3 to 4. Recent studies [3] have demonstrated that strength of CRA layer can be considered together with the strength of backing steel for pressure containment calculations and thus provide competitive advantage for lined pipes compared to clad pipes.