Bending Strain and Pipe Movement Services

Hazard identification – HazID – is based on measurements of curvature and associated bending strain and pipe movement (changes in curvature) using in-line inspection (ILI) tools equipped with an on-board Inertial Measurement Unit (IMU). IMUs are frequently paired with Magnetic Flux Leakage (MFL) or high-resolution geometry tools.

Geometry measurements also indicate load sources that deform the pipe wall and thus can provide a crucial additional source of information on the presence of geohazards. Although on-board IMUs are normally used to map the pipeline route providing pipe centerline coordinate data, a true strength of this unit is its ability to quantify the bending strain and detect pipe movement. Virtually all pipeline operators request coordinate data, but most do not take advantage of post-ILI bending strain calculations and review of strain patterns to identify areas of concern. Once data from an IMU has been collected, it can be used later – even years later – to determine changes in the pipeline loading condition.

ROSEN has unsurpassed experience in using IMU inspection and has identified many landslide features that operators were previously unaware of.

Once active loading has been identified, ROSEN uses structural analysis with finite element methods to evaluate the complete pipeline condition. This requires expertise in soil-pipe interaction and understanding pipeline performance limits. A key part of the process is the integration of the predictions from the modelling with the measurements from the in-line inspection. This ensures that the results are representative of the field condition – reducing reliance on uncertain assumptions.

For hazard control, the outcome of the modelling is a relationship between the ground movement source – such as a landslide – and the strain in the pipeline. This includes predictions of the future development of the load, by understanding the most likely future movements and when limit conditions are reached.

This involves the detection and listing of bending strain events. Incidents of bending strain may indicate construction or installation loads – such as hilly topography or an uneven trench. It is possible, however, to interpret inspected strains for indications of external loading, thus confirming the presence of local ground, seabed or riverbed movement.

The Level 2 assessment identifies the limit strain values for pipelines for tensile fracture and compressive local buckling. Measured strains are compared to the limit values for acceptability. The assessment can take account of any pipe body anomalies that have been identified on the pipeline within the area of high bending strain (e.g. metal loss and girth weld anomalies). The Level 2 assessment also includes an evaluation of the potential cause of loading on the pipeline, which contemplates horizontal and vertical pipeline profiles and bending strain profiles. Ground topography studies and aerial imagery showing ground surface features are aggregated with relevant environmental or geographic data – including locations of crossings for roads, railroads and rivers – to help in painting a full picture.

This third level modelling approach is based on integrating the applied movement loads with ILI mapping data. This method provides excellent assurance on the modelling inputs.

Here, structural modelling is carried out using finite element methods and can include non-linear material behavior. Displacements or loads are applied – theoretically – to the pipeline structure, and the measured bending strain profile in the pipeline is reproduced in finite element modelling. Once this is achieved, the FE model presents a reliable picture of the pipeline and can be acted on with confidence.

The goals of the assessment are to establish the limit strains for the onset of failure, the current pipeline integrity based on predicted strain levels, and how strains develop towards the limit values with further movement.

Using this modelling, decisions can be taken on the urgency and timing of any necessary interventions and remediation.