Thorough Investigation Allows Mitigation of Corrosion-Resistant Alloy Pipeline Issues
Authors: Carpenter Chris 
Published: April 2021

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 30149, “Corrosion Resistant Alloy Pipeline: Creating Knowledge Behind the Scenes,” by M. Hasbi A. Razak and Nur Izyan Mukhtar, Petronas, prepared for the 2020 Offshore Technology Conference Asia, originally scheduled to be held in Kuala Lumpur, 2-6 November. The paper has not been peer reviewed. Copyright 2020 Offshore Technology Conference. Reproduced by permission.

High-pressure/high-temperature (HP/HT) environments in offshore gas fields with a significant presence of carbon dioxide (CO2) and hydrogen sulfide (H2S) in the full well stream demand the use of corrosion-resistant alloy (CRA) pipelines. This pipeline system comprises a metallurgically bonded CRA layer, produced by roll bonding and known as metallurgical cladded pipe (MCP), and a mechanically expanded and fitted CRA layer in a backing steel known as mechanical lined pipe (MLP). The complete paper outlines specific issues, mitigation steps, and lessons learned during the development phase of the CRA pipeline.

Pipeline System and Materials

To achieve the lowest life-cycle cost, CRA pipeline designed to handle a maximum partial pressure of 18 bar of CO2 and 13 mbar of H2S at a maximum temperature of 150°C was selected for an offshore gas field. The pipeline system consists of two types of CRA line pipe, MCP and MLP. The pipeline system is divided into Zone 1 and Zone 2. The areas of application of MLP and MCP are summarized in Table 1.

MLP Challenges

The MLP challenges experienced are subdivided in the complete paper according to the development phase of the pipeline during which they were experienced (i.e., during the manufacturing or installation stages).

Undulation Appearance on the Internal Surface of the MLP. Undulation, in this context, refers to a wavy appearance of the internal surface of the CRA pipe, which was found after the mechanical expansion process. Fig. 1 shows a sample of undulation in the line pipe.

Several factors were suspected to have caused the undulation in the internal CRA surface after the hydroforming process. One such factor was the manufacturing process of the seamless carbon-steel (CS) backing pipe, which involves a pipe-piercing process using a conical device to pierce billets to create hollow, seamless pipes. To determine the actual cause, additional inspection and testing on MLP were performed.

Visual inspection found that the undulation appearance on the CRA liner is similar to the CS backing-pipe contour, proving that undulation in the CRA liner was caused by the liner being plastically deformed and following the contour of the CS backing pipe.

Wall-Thickness Measurement of CS Backing Pipe. Thickness measurement was performed at selected areas in circumferential and longitudinal directions of the pipe. The circumferential thickness measurement determined that significant thickness variations existed in the CS backing pipe. Some of the measured wall thicknesses already exceeded the maximum positive tolerance of the seamless pipe-wall thickness.