A view from the inside

Subsea pipelines are often designed to withstand movement up to a certain magnitude caused by factors such as thermal expansion or challenging metocean conditions. However, excursions from intended operating limits, extreme environmental conditions, or accidental damage through external force can all result in pipeline movement that exceeds any design limits and therefore requires some form of intervention, be it further assessment, more frequent inspection, or repair.

It is a regulatory requirement in many countries to regularly monitor subsea pipelines for signs of external damage or movement so that any issues can be identified early and addressed before an ultimate limit state is reached and potentially catastrophic failure occurs.

This article will discuss the common causes of movement in subsea pipelines, how a pipeline operator may identify, measure and monitor outof-straightness and what actions may be required upon the discovery of movement outside of design limits.

Out-of-straightness in subsea pipelines

Out-of-straightness in a subsea pipeline may be part of the original design and therefore does not pose a threat to pipeline integrity. The following are examples of out-of-straightness that have their origins in the design and installation stages:

• Directional changes due to pipeline routing and installation conditions.
• Seabed topography and features (e.g. potholes, boulders and coral outcrops).
• Gravity pull where a pipeline lacks support (e.g. freespans and catenary risers).
• Crossings of existing infrastructure (e.g. pipelines and cables).
• Crossings of buckle trigger structures.
• Tie-in to pipeline inline or end termination structures.
• Crookedness of the pipejoints as per fabrication.

Many of the above upfront expected out-of-straightness situations introduce static, and in some cases dynamic, loads to a pipeline that need to be compared to stress or strain-based limits, as well as other limit states such as fatigue. Providing the pipeline is operated within its design limits, these loads should not impact the integrity of a pipeline during its design life, but may require further consideration if the life of the pipeline needs to be extended.

However, other sources of out-of-straightness may be unexpected and associated with pipe movements due to operational or environmental loadings or external impact. Examples include:

• On-bottom instability in inclement metocean conditions (e.g. severe currents during storms).
• Soil movement (e.g. by soil transportation, scour, slope instabilities and seismic activities).
• Impact by trawl boards, anchors and icebergs.
• Lateral or upheaval buckles caused by thermal expansion.
• Pipe walking (also referred to as pipe ratcheting) due to cyclic pressure and temperature loading, and gravitational pull or axial tension from, for example, catenary risers.

As pipeline movement during operation is a known phenomenon, pipeline integrity management programmes should include measures for identifying, measuring and monitoring pipeline out-of-straightness where such a threat exists …

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Read the article online at: https://www.worldpipelines.com/special-reports/20102022/a-view-from-the-inside/