Offshore-Pipeline and -Riser Integrity - The Big Issues
Authors: Denney Dennis 
Published: February 2015

This article, 1,000th written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 125060, ’Offshore-Pipeline and -Riser Integrity - The Big Issues,’ by Jonathan Marsh and Phil Duncan, J.P. Kenny and Ionik Consulting, and Ian MacLeod, MCS, prepared for the 2009 SPE Offshore Europe Oil & Gas Conference & Exhibition, Aberdeen, 8-11 September. The paper has not been peer reviewed.

The dominant corrosion and integrity issues affecting offshore rigid and flexible pipelines and risers in the North Sea were examined. For many aging rigid pipelines, extension of life beyond the design criteria is increasingly common, and the increasingly dominant integrity issues are associated with internal and external corrosion. The use of flexible pipes is increasing exponentially, and by definition they are aging. Over the coming years, an increasing number of flexible pipes will reach the end of their design life; therefore, prudent operators should focus on understanding the integrity status of their flexible pipes.    

Introduction

Some of the earliest pipelines installed in the North Sea have experienced a wide variety of issues that required intervention. One dominant issue was pipeline instability. The early 1990s saw issues with hot high-pressure pipelines and upheaval or lateral buckling. Failure of an early pipe-in-pipe oil-line construction provided design lessons that have benefited subsequent designs.

More recently, with the use of corrosion-resistant alloys, risk from hydrogen-induced stress cracking from overprotection by cathodic-protection systems has become a major issue. However, this issue can now be resolved at the engineering stage by compliance with design guidelines.

Compared to rigid pipelines, the operational experience of flexible pipes is relatively limited. The complex composite nature of flexible-pipe construction means that there is a large range of failure modes that occur in service. A key conclusion from various studies was that although there was a spread of failure modes occurring in service, breaches to the outer sheaths of flexible pipes was the most common issue. 

Internal Corrosion of Rigid Pipelines and Risers

The primary internal-corrosion threats vary for different pipeline types. Broadly, these threats can be divided into water-injection pipelines, multiphase oil- and gas-production pipelines, and oil- and dry-gas-transport lines.

Water-Injection Pipelines. The two primary mechanisms for internal corrosion of water-injection pipelines are oxygen corrosion from poor deaeration practice and microbial-influenced corrosion (MIC) from poor hypochlorite and biociding treatment. From experience, MIC appears much more of a threat than oxygen corrosion. In a study of 23 pipelines, nine failed. The failures were all attributed to MIC, and the average life was approximately 8 years, with some as short as 4 years. The typical corrosion rate was 2 mm/a. As a result, some operators have installed high-density-polyethylene water-injection pipelines.

Multiphase-Production Pipelines. CO2 Corrosion. Despite the best efforts of the chemical companies, various aspects of CO2 corrosion continue to be an issue. Where corrosion-inhibitor availability is maintained, general corrosion of pipelines usually is well controlled.