Erha and Erha North Development: Automatic Ultrasonic Inspection of Corrosion-Resistant Alloy Clad Steel Catenary Risers
Authors: Sutherland J.B. , Nolop N.C. , Weir M.S. , Hoyt D.S. , Prentice G. , Tang Z. , Montbarbon S. 
Published: October 2008

The Erha and Erha North Project utilizes a steel catenary riser (SCR) system to connect the floating production, storage, and offloading vessel (FPSO) to the subsea infrastructure. The system is a hybrid, composed of both steel and clad corrosionresistant alloy (CRA) pipe, with the CRA clad pipes located in the touch-down and hang-off areas of the SCRs. The CRA clad pipe sections are required to obtain satisfactory fatigue life due to the presence of CO2 and potential H2S in the production stream. In total, four production and two test lines were required, with a total of 1,120 m of CRA clad pipe installed.

Ultrasonic inspection of the welds joining the CRA clad pipes proved to be technically challenging due to the bimetallic features of the weldment and the small flaw size allowable for this fatigue-dominant application. Further complicating this effort was the need for reliable flaw detection on a real-time basis to maintain the pipelay rate. To meet these challenges, Shaw Pipeline Services (SPS) was enlisted to develop an automatic ultrasonic testing (AUT) system capable of reliable inspection of the weldments.

This paper discusses the AUT system qualification program, product implementation, and lessons learned. Included in these discussions are technological challenges and respective approaches for metallurgically bonded and weld overlaid clad girth weld ultrasonic inspection, as well as detailed sizing accuracy data on seeded (i.e., intentionally planted) defects during the qualification stage and sizing accuracy data on production welds with naturally occurring flaws.

Introduction

The Erha and Erha North development, located in 1,200 m of water approximately 97 km offshore Nigeria, consists of three subsea drill centers tied back to an FPSO. Drill Centers East and West (DCE and DCW) are located approximately 1.5 km from the FPSO, and Drill Center North (DCN) is an 8-km subsea tieback to DCE. The subsea production system includes trees and manifolds connected to 6-in. and 10-in. insulated steel flowlines that tie back to the FPSO via steel catenary risers (SCRs). Injection fluids are transferred from the FPSO via the SCRs and flowlines for subsea injection at the drill centers. The FPSO is a new-build, spread-moored vessel with installed capacity for 210,000 barrels per day (bpd) oil processing, 150,000 bpd water injection, and 340 million standard cubic feet per day (Mscfd) of gas injection. Processed crude is stored in the FPSO and regularly offloaded to tankers through two 22-in. offloading lines and catenary anchor leg mooring (CALM) offloading buoy. Figure 1 shows the field layout.

Figure 1. Erha and Erha North Field Layout (available in full paper)

During the early phases of the Project detailed design, it was realized that conventional AUT techniques would not suffice for CRA clad weldments and that new inspection methodologies would need to be developed and qualified. The qualification program validated the AUT system's capability to operate consistently and to detect minute critical defects. The end result was a new AUT system that was qualified and successfully implemented in time to maintain the overall Project schedule. The systematic approach to development and qualification has resulted in robust, reliable hybrid SCRs.