Dropped Object Risk Assessment

This article delineates a typical flowchart illustrating the risk-based methodology for evaluating the adequacy of subsea pipeline protection against accidental external dropped loads within the safety zone of offshore installations, predominantly spanning from the subsea installation connection point to the subsea manifold piping.

A typical failure mode of subsea pipelines resulting from impact loads includes indentation or puncturing of the pipe wall. The pipeline's resilience against impact loads depends on factors such as local pipeline geometry (e.g., size/design and stiffness) and the nature of the load (e.g., impact energy, energy absorption by the object).

This methodology facilitates the evaluation of the sufficiency of concrete weight coating thickness and/or bare subsea pipe against the impact energy of dropped objects from platforms and towers during routine operations and maintenance activities at pipeline approaches.

Aligned with industry standards, DNV-RP-F107 (Risk Assessment of Pipeline Protection) and IOGP Report 434-8 (Mechanical Lifting Failures), this approach can be extended to umbilicals and other subsea installations with necessary modifications suggested in the standard/guidance. Detailed calculations and guidance for each step can be referenced from these standards. The approach assumes that both the pipeline and its coating conservatively absorb all available kinetic energy from impacting objects.

In the event of an automated process, a calculation routine can be employed wherein the drop location is distributed across a grid of multiple drop points within the drop zone, with equal weighting based on grid density, rather than relying on a single drop point and manual calculations. IOGP reports offer recommended failure data, such as dropped object probabilities for mobile units and fixed installations per lift. Note: The risk assessment for topside locations is not addressed in the flowchart.

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