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).