The paper deals with design philosophy for small diameter HP/HT flowlines on uneven seabed with major emphasis on controlled snaking expansion design of free spanning flowlines subject to fishing gear interaction loads. The response from trawl pull-over is given major attention as part of the overall design procedure including ECA (Engineering Criticality Assessment), covering high and low cycle crack growth. The background for the design procedure is reference projects in the Norwegian sector, where REINERTSEN performed the flowlines detail engineering. Iceberg plough marks make an irregular seabed with partly sharp and deep valleys crossing the flowline routes, resulting in free spans. The bathymetry and extent of unevenness varies for the lines, resulting in different behavior and design approaches, especially for trawl pull-over. The major criterion for a robust design is strong sectioning by intermittent rock berms. For operation loads these serve as locking points and as such they are termed ARBs (Anchoring Rock Berms), whereas for trawl pull-over axial motion through the berms is allowed, depending on seabed profile, and as long as the pipeline deforms without unacceptable crack growth in girth welds. Design criteria for the ARBs are presented, covering the expansion phases of buckle initiation, post-buckle and shutdown, respectively. For trawl pull-over separate control of the intermittent rock dumps is shown, covering the cases of section interaction and complete locking, respectively. Criteria for the choice of trawl design approach are evaluated. Traditional design handles the trawl impact capacity of pipelines by a local buckling criterion, in which axial force and bending moment are the response parameters. For expansion control this is a relevant approach, however for a concentrated and curvature. Applying the latter criterion is illustrated by numerical example in view of displacement- or load controlled behaviour. A summary on rock saving potential is made for a typical reference project from a traditional design involving infill of spans to the new approach where free spans are allowed also for trawl pull-over. The potential cost saving for seabed intervention work by rock dumping is found to be well above 5000 NOK/m (>1000 $/m) for very irregular seabed, while typically in the order of 1000 $ for normal uneven seabed.

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