Dead-leg inspection for onshore and offshore facilities

Overview

A natural gas supplier sought TWI’s assistance and expertise to undertake a dead-leg integrity management programme for their onshore and offshore facilities in the Middle East.

Dead-legs are components of a piping system that normally have minimal or zero flow. They can be found in oil and gas production facilities, chemical plants, and refineries, and pose particular issues for integrity management as they are prone to corroding at a higher rate than the rest of the piping system.

Each plant or facility will typically have a large number of dead-legs, which are either present as a result of operations, design or modifications.

Objectives

The client asked TWI to help define an integrity management programme for the dead-legs at a gas plant and offshore platform.

The objective was to outline the strategy for the identification and management of corrosion in dead-legs in a hydrocarbon process system, as well as providing a risk assessment in order to determine dead-leg criticality and how to mitigate the risk

This required a risk assessment of the existing facilities alongside a strategy for future in-service inspection of dead-legs.

Risk increases with time without inspection

Solution

TWI began by producing a dead-leg integrity management strategy, in consultation with the client, to define the different stages of the process and provide guidance on the required tasks for the implementation of this strategy.

This involved a risk-based approach to identify and register dead-legs using operational, physical and historic inspection data, before creating a mitigation plan which was implemented before further inspection determined whether further work was required.

The approach was based guidance provided by API 581 on the risk-based assessment of dead-legs. However, a more in-depth methodology was developed using additional parameters, such as; the type of system, the dead-leg design, dead-leg orientation, the length to diameter ratio, flow rate and the corrosion rate of the main line.

Suitable NDT techniques were highlighted for this work based on the type of equipment to be inspected, the surface condition, operating temperatures, the damage that was expected to have occurred and the size and accessibility of components.

This information was used to create the inspection/mitigation recommendations, with each dead-leg location receiving a unique identification number for reference in a dead-legs register.

The register allowed for risk assessment details to be compiled in consideration of damage mechanisms and the consequence of failure. This, in turn, allowed for the prioritisation of inspection based on risk categorisation for each item in the register.

Conclusion

The project found that, while most of the dead-legs were medium or low risk, there were some that were in danger of failure and would need re-inspection within 12 months. Most of the dead-legs that were identified were small bore pipes with a diameter of under 3” NPS.

Based on the findings it was recommended that high-risk dead-legs should be removed or, if that was not immediately possible, they should be re-inspected within 1 year. Those dead-legs determined to be medium risk should be inspected again within 1 to 3 years, while low risk dead-legs wouldn’t need inspecting until 3 years had passed.

Finally, it was determined that any inspection results should be used to update the dead-leg register with details of corrosion remaining life and risk assessments should be repeated for each dead-leg.

Summary of recommended inspection interval of dead-legs based on risk category

Chi-Ming Lee Senior Corrosion Engineer

Dr Chi-Ming Lee is a Senior Corrosion Engineer, working in the Material Performance and Corrosion Department at TWI Ltd, based in Cambridge, UK. His responsibilities include management of consultancy related projects, from a corrosion and asset integrity prospective, for a range of clients from the oil and gas industry and other sectors.

Prior to joining TWI Ltd, Dr Chi-Ming Lee worked in the field of corrosion and pipeline cathodic protection at several companies, including Clariant, PIM and Cathodic Protection Service Ltd. Notable is his 3 year period at Clariant, where his role was a member of the corrosion awareness campaign team within BP, tasked to reduce the cost of corrosion, across all business units.

Dr Chi-Ming Lee received a degree in Metallurgy and Material Science, and PhD on the topic of pitting corrosion from University of Nottingham, UK. He is also a Chartered Engineer and a Member of The Institute of Materials, Minerals and Mining (IOM3).

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