The paper deals with the design methodology to define the design loads and determine the maximum allowable size of girth weld defects. The motivation for this work is reduced intervention costs obtained by opening all free spans as these are governing for rock infill volumes. 20–30% reduction of the intervention work is obtained. Structural integrity of the pipeline related to the interference with fishing gear is an important design scenario. Trawling in free span, pull-over loads with clump weight as an ALS condition is the main issue. REINERTSEN observed during detail design a lack of acceptance criteria for ALS conditions in the DNV OS-F101 design code, Ref.  for interference between trawl gear and subsea pipelines with low D/t ratio. Curvature in the trawl pull-over point as a function of time is found approximately constant while trawl load is increasing. The membrane forces carry most of the trawl load a few seconds after the trawl impact while bending moment decreases. This is in accordance to the philosophy that the strain and the curvature will be nearly constant for increased loading. The global load bearing mechanism is membrane and less bending. This means that we have control on the strain and that the pipeline system maintains its stiffness against loading for this high axial capacity of the flowline. These observations leads to a deformation controlled trawl load approach where an ECA of the flowline can be used to document structural integrity. Engineering Criticality Assessment (ECA) analysis is applied to evaluate the integrity of the flowlines with respect to risk for unstable fracture in girth welds due to impact from trawl equipment. The fatigue load effects from installation, temporary and operational phases are included in the ECA analysis. Geometric effects and external/internal pressure are included using the tailormade softwares LINKpipe, Ref.  and Crackwise4, Ref. . The residual capacity of the flowlines is calculated with emphasis on fatigue during operation after the trawl pull-over. The fatigue life should be within the inspection interval, reflecting the Integrity Management Scheme.
- Ocean, Offshore and Arctic Engineering Division
Using ECA in Structural Integrity of Submarine Pipelines for Optimalization of Intervention Work
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Igland, RT, & Lamvik, T. "Using ECA in Structural Integrity of Submarine Pipelines for Optimalization of Intervention Work." Proceedings of the ASME 2010 29th International Conference on Ocean, Offshore and Arctic Engineering. 29th International Conference on Ocean, Offshore and Arctic Engineering: Volume 5, Parts A and B. Shanghai, China. June 6–11, 2010. pp. 387-393. ASME. https://doi.org/10.1115/OMAE2010-20349
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