An offshore 100km long gas export pipeline in the North Sea is subject to periodic inspection to monitor seabed interaction and freespan development. Recent multibeam echo sounder (MBES) surveys identified localised seabed change and the emergence of new freespans at several locations. Jee was commissioned to carry out a detailed comparative assessment of historical and latest survey data to understand and evaluate seabed changes and freespan development behaviour and inform future integrity management requirements.
The challenge was to align and normalise MBES data from a number of successive surveys and identify and quantify the changes in seabed topography over time and the resultant impacts on freespan development. From this it becomes possible to determine whether identified freespans present not just an immediate short-term threat to integrity but also a longer-term threat as they grow in the future. Identification of areas of most rapid seabed change allow focus on future inspection requirements and intervention, and determination of rates of change feeds directly into inspection planning and the optimal intervals between inspections.
Jee completed a structured two stage assessment using MBES data from three inspection campaigns (2018, 2021, and 2024). These data were imported into Navimodel and normalised to a common spatial reference. After cleaning and filtering, the datasets were overlaid to allow direct comparison of seabed elevation changes and pipeline geometry. An initial high-level comparison was used to screen the full pipeline length and highlight areas of change. This was followed by a detailed comparative assessment of the most onerous spans and those showing notable development.
Span length and seabed profiles were compared across surveys to identify growth mechanisms. Particular focus was placed on spans that had newly formed due to localised scouring. Growth rates were calculated to provide a projection of future span length and these were benchmarked against allowable limits. The analysis was supported by calibration of growth rates informed by neighbouring span behaviour and historical trending of seabed behaviour.
Benefits and outcomes
The assessment showed that most of the pipeline length exhibited limited change with the majority of existing freespans remaining unchanged over time. Two newly developed spans were identified as having experienced the greatest change since the previous inspection. While conservative projections suggested these spans could exceed allowable lengths if linear growth continued, the wider evidence indicated that rapid growth was associated with initial formation and was not expected to persist.
Previously remediated sections were also reviewed and early signs of seabed mobility were identified allowing these locations to be flagged for closer monitoring.
The study provided clear differentiation between stable and higher risk spans enabling the operator to focus attention where it was genuinely required. By demonstrating how span growth rates are non-linear and tend to reduce as the span length increases, the likely development of newly formed freespans could be predicted, avoiding unnecessary conservatism while maintaining a robust integrity position. The findings supported retention of the existing inspection strategy with targeted focus areas defined for the next survey.
For more information, visit www.jee.co.uk/integrity-management
To contact Jee’s Technical Authority, Graham Wilson, email Graham.Wilson@jee.co.uk.