Reeled Clad SCR Weld Fatigue Qualification
Authors: Jones Richard Lloyd , Karunakaran Daniel N. , Mair John , Wang Howard 
Published: May 2011

SCR's in deepwater developments can be subject to severe environmental loading during service and the fatigue performance is normally limited by the girth weld. The need for transporting corrosive constituents demands the use of clad pipe. Installation of SCR's by the reel lay method has been demonstrated to be cost effective. However to date, reel lay installation of clad pipe risers has been impeded by the lack of fatigue data. Therefore the overall aim of the project was to develop and qualify a suitable welding procedure for internally clad pipe for SCR service and to demonstrate that the required fatigue life could be achieved after installation by the reel lay method.

A welding solution was developed based on the Auto Hot Wire PGTAW process using Alloy 625 filler wire. A clad pipe weld procedure was qualified in accordance with the requirements of DNV OS F101 - 2007 and typical industry SCR specifications. AUT was selected as the primary NDE method. A validation of the AUT inspection procedure was performed to ensure that planar flaws of concern could be reliably detected and sized. Clad pipe strings comprising representative butt welds were then manufactured and inspected. These were subjected to a full scale simulated reeling and installation procedure which was representative of pipeline installation by the Seven Oceans reel lay vessel. These butt welds were subsequently subject to full scale fatigue testing using the high frequency resonance testing technique. The fatigue performance exceeded expectations, with most test samples failing in the parent material with fatigue levels close to the Class B mean curve. This paper provides a summary of the work carried out and the results obtained.

Introduction

Steel catenary risers have been an attractive choice for deep water field development. One of the major design and fabrication challenges for SCRs is the achievement of the required fatigue performance at highly stressed regions. Additionally, the need for the transmission of corrosive constituents has demanded the use of corrosion resistant alloy linepipe materials. To date the most attractive pipe material option is the use of metallurgically bonded clad pipe. Installation of such SCRs by the reel lay method is a cost effective approach. Up to the present time, reel lay installation has been limited to carbon steel SCRs. The lack of fatigue test data to demonstrate the suitability of installation of clad SCRs by the reel lay method has impeded the utilisation of this technique.

Therefore the overall aim of the project was to develop and qualify a suitable welding procedure for internally clad pipe for SCR service and to demonstrate that the required fatigue life could be achieved after installation by the reel lay method. The three primary criteria that would verify the suitability of reeling included:–Confirm by mechanical and corrosion testing that the girth welds made in Incoloy 825 clad pipe meet the relevant industry standards.–Confirm by physical testing that such girth welds are reelable.–Confirm that the fatigue performance of welds made in clad pipe that have been reeled will meet or exceed the target levels of fatigue Class C/2.