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Structural integrity assessment of well conductors

As life extension projects see many oil wells being used beyond their design life, ensuring the structural integrity of critical components to prevent unexpected failures has become more important than ever.

TWI provided a comprehensive structural integrity assessment for a major offshore oil and gas operator, providing peace of mind that around 100 oil well conductors were safe to continue service. As well as carrying out the assessment according to a custom fitness-for-service (FFS) methodology, TWI also provided the operator with an inspection strategy for the future, to ensure the most efficient use of resources and reduced inspection costs going forward.

High-value, high-risk components

In offshore wells, the outermost well casing – the conductor – protects the surface casing from the aggressive marine environment throughout its life. Conductors are subject to aging-related issues including excessive wall loss and cracking due to corrosion and many are already operating beyond their design life.

Due to the high cost of replacement and severe consequences of failure, there is a growing need to closely monitor the integrity of these components throughout their lifecycle to prevent them from leaking, buckling or collapsing.

TWI began the project by carrying out structural integrity assessments of around 100 well conductors, involving the development of an FFS assessment methodology that considered all possible loading scenarios: both axial compression and global bending. The next step was to develop a bespoke risk assessment model. This informed an inspection

and mitigation plan which highlighted any particularly high-risk conductors and defined target dates for proposed inspection or mitigation (Figure 1).

Past research in this area

There are a limited number of research programmes addressing the assessment of corrosion defects in pipeline structures subject to global bending, compressive loading or a combination of the two. The majority of this work has been carried out under internal pressure or longitudinal tension loading conditions. Because code-based FFS approaches such as BS 7910 or API 579-1/ASME FFS-1 are not directly applicable, TWI instead used a finite element analysis (FEA)-assisted assessment.

Previous work has investigated the structural behaviour of corroded pipelines subjected to compressive loading and bending movement as a result of temperature gradient. Research has also investigated the reliability of corroded pipelines using full-scale testing and FEA to calculate pipeline failure pressure, including the effect of longitudinal compressive loading. The recommended practice DNV RP-F101 is the outcome of such work.

Another approach TWI considered is a reliability assessment method, which is a level 1 assessment developed by Benjamin and Andrade (2003) and is a modified version of the RSTRENG method. However, these methods do not take into account the effect of global bending or longitudinal compressive loading on the failure of the corroded structure, and their predictions of failure pressure are quite conservative compared to full-scale tests (Benjamin, 2013).

Figure 1. Illustration of risk-based remaining life on risk matrix
Figure 1. Illustration of risk-based remaining life on risk matrix

TWI’s approach

The initial, high-level screening was based on a conservative effective area calculation and corrosion rate (CR) estimation, which established a remaining life (RL) for each conductor. TWI then selected an appropriate FFS approach for each conductor, depending on whether it was subject to global bending, compressive loading, or a combination of the two. This assessment was facilitated by a stability check, based on international standards, of the well conductors.

TWI worked in consultation with the operator to determine the risk (Probability of Failure) target, with the remaining time to that target considered as the risk-based remaining life (RL) (as shown in Figure 1).

This determines the point at which an inspection is recommended to be carried out to better quantify the damage state. 

TWI also performed the conductor assessment with the inclusion of any imperfections, even when the RL at the assessment target date was greater than zero. If the RL was equal to or below zero at the assessment date due to localised corrosion, TWI assessed the conductor, taking into consideration the presence of corrosion defects using FEA. 

TWI then carried out a risk assessment to consider the likelihood (with limit state equation as the second moment of inertia) and consequence of failure, before putting forward suitable risk-mitigating actions.

Reducing the cost of inspection

TWI provided the results to the operator in terms of risk, as the product of probability and consequence of failure. This provided the company with inspection recommendations based on the strength response of the corroded conductors. These mitigation actions will determine the acceptability of the conductor to remain in service within the acceptable risk. Through this comprehensive approach to structural integrity assessment, the operator’s cost of inspection was substantially reduced.

References and credits

  • API RP 579-1 / ASME FFS-1 (2007). “Fitness-For-Service”, Second Edition, American Petroleum Institute and The American Society of Mechanical Engineers.
  • BS 7910:2013 (2013): “Guide to methods for assessing the acceptability of flaws in metallic structures”, British Standards Institution, London.
  • Benjamin, A.C. (2013). “Prediction of the failure pressure of corroded pipelines subjected to a longitudinal compressive force superimposed on the pressure loading”, The Journal of Pipeline Engineering, pp301.
  • Benjamin, A.C. and Andrade, E.Q. (2003). “Modified method for the assessment of the remaining strength of corroded pipelines.” October.

For further information please email contactus@twi.co.uk.

Figure 2. FEA of corroded conductors under combined loading
Figure 2. FEA of corroded conductors under combined loading
Avatar Payam Jamshidi Manager – Asset Integrity Management

Payam joined TWI in 2013 as a Chartered Engineer in the Asset Integrity Management section, and since then he has managed a range of projects related to asset integrity management. His main area of work is on Risk Based Inspection (RBI,) and reliability assessment of oil and gas assets operating in offshore or onshore environments. Payam is also product leader for TWI’s Risk-Based Inspection (RBI) software RiskWISE®.

Prior to working for TWI, Payam obtained undergraduate and master degrees, and a PhD at University of London, in Materials Science and Engineering. He also held the role of Chief Technical Officer for the National Composites Certification and Evaluation Facility (NCCEF), University of Manchester.

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