Fit for service, fit for life – part one

Published by Nicholas Woodroof, Editor
Oilfield Technology, Wednesday, 18 March 2020 16:00

In the first part of a two-part article, Pieter van der Vyver, Oceaneering, explains how to ensure assets remain safe and sustainable through quantitative engineering analysis.

As the world continues to demand energy sources, there is mounting pressure on hydrocarbon producers to find new reserves and extract more from existing assets. In recent years, the industry has made noticeable advancements in recovery techniques, using efficient technology to extend the life of mature fields.

As these facilities are extended beyond their original design life and the burden for integrity verification and assurance steadily increases, it is essential to demonstrate continued safety and integrity of ageing assets.

Any infrastructure which has been in service for an extended period, whether a pressure vessel, pipeline or machine component, has the potential to degrade until it no longer meets the original design requirements. Therefore, the first step in reviewing redevelopment programmes on ageing fields is to assess the condition of existing infrastructure and its ability to handle current operational loads. If a condition assessment indicates concerns, then further analysis is required to determine the appropriate remedial action to ensure continued, safe operations.

Fitness for service - what is it?

Fitness for service (FFS) provides a quantitative engineering evaluation to demonstrate the integrity of a component to continue to operate under a specific set of conditions, potentially in the presence of a defect or degradation mechanism. It translates inspection results into quantifiable operational and safety risks, enabling informed integrity management decisions.

FFS provides a basis for engineers to distinguish between acceptable and unacceptable defects and conditions, with principles based on internationally recognised procedures. Although many industry standards address some form of fitness for service assessment, the American Petroleum Institute (API) compiled the best practices into a single, modular assessment standard (API 579-1), which has become the authoritative publication on FFS.

Clear understanding of risk

The benefits of carrying out FFS assessments are clear: improved safety, reduced downtime, proactive maintenance, all the elements necessary for keeping continued operations, as safe, compliant and efficient as possible. However, the question often arises as to when FFS should and must be applied.

Identifying the point of deviation from design intent is often more complex than expected due to the absence of detailed design information, changes in operating environments, and multiple or complex loading scenarios. An FFS assessment should therefore be considered as soon as any reported defects exceed the design code limits. For example, defect size exceeding the limit stipulated in the original fabrication quality control standard, metal loss exceeding the design corrosion allowance, material property degradation to below the material specification limits, or exposed to pressures and temperatures outside of original operating boundaries.

In oil and gas, degradation is often dominated by metal loss as a result of corrosion. Operators tend to use the minimum allowable wall thickness (MAWT) as guideline to initiate FFS. For piping components there is also a significant reliance on the API 574 guidelines for minimum structural wall thickness; although these do not consider material grade, span length, operating medium or support arrangements.

For example, a 6 in. schedule 40 pipe system has a nominal thickness of 7.11 mm, inclusive of a potential 12.5% thickness under tolerance. If this is specified to have a 1.5 mm corrosion allowance, it results in a minimum design wall thickness of 4.72 mm. The API 574 Default Minimum Structural Thickness for a 6 in. carbon and low-alloy steel pipe is 2.8 mm. If designed for internal pressure of up to 50 bar, the MAWT for pressure retention could be 1.21mm (depending on material grade).

If MAWT is used to initiate FFS assessment, there would be no possibility of a successful outcome for this hypothetical scenario, the remaining wall thickness would not satisfy the API 579 Limiting Thickness criteria. Similarly, if the API 574 structural thickness is used for initiating FFS assessment and the pipe system is operating at temperatures above 149°C, it could experience local thermal pipes stress levels, well in excess of the structural thickness. For pressure vessels it is even more complex due to changes in geometry, localised reinforcement zones, major structural discontinuities and loading complexities.

Simply considering components based on thickness for pressure retention could leave operators exposed to a substantial risk. Performing at least basic FFS once the design requirements are no longer satisfied can reduce the risk and provide valuable insight into operating boundaries and future degradation, as well as highlight future requirements for advanced FFS and potential repair.

Decoding degradation

The first step of assessing any defect is identification of the damage type. The assessment procedures are damage specific, with the API 579-1 standard providing assessment methods for 12 different types of damage. Understanding the damage is also important for predicting the progression and determining the remaining safe, operating life.

For each damage type there is a subset of assessment methods, each with specific applicability and limitation criteria that needs considering. There are also different levels of assessment, with progressively increased accuracy and reduced conservatism, accompanied by an increase in required accuracy of input information:

Level 1 - very basic and aimed at quick screening of defects in simple components, normally considering pressure retention only.
Level 2 - intermediate, for more complex components with additional loads, increased accuracy enables a reduction in design.
Level 3 - advanced assessment of complex components or severe degradation using detailed mathematical modelling to determine structural stability.

Component classification

API 579-1 uses an alpha-numeric classification system based on component complexity and loading conditions, to determine the appropriate minimum level of assessment:

Type A - is the most basic component, with a simple geometry and equation relating thickness to pressure, and simple loading conditions dominated by pressure. Type A components are perfectly suited for Level 1 assessment.
Type B class 1 - have similar basic geometries and thickness equations to Type A components but requires consideration of additional loading conditions due physical size and/or exposure temperature. Type B class 1 components requires level 2 assessment as a minimum.
Type B class 2 - are more complex components with thickness interdependencies requiring procedural design evaluation rather than simple thickness. Type B class 2 components requires Level 2 assessment as a minimum.
Type C - have the most complex geometries and load distribution normally causing significant local structural or stress discontinuity requiring advanced mathematical analysis by means of the Level 3 assessment.