Pipeline Response to Trawl Pull-over

Abstract

Structural integrity of production pipelines with small diameter is analyzed considering large lateral displacements due to interaction loads from trawl pull-over. The offshore standard provided by Det Norske Veritas, DNV-OS-F101, dictates the design practice and allows limited utilization of the cross section due to the load controlled condition criterion. Thick walled pipelines with D/t ratio less than 20 can sustain large plastic strains prior to onset of local buckling. In addition, introduction of membrane forces is assumed to carry an increasing part of the pull-over load for increasing lateral displacements. Use of a strain based criterion is therefore desired.

Typical global response of a 12-inch production pipeline subjected to trawl pull-over is obtained by using pipe elements in non-linear Finite Element analyses in accordance with the recommended practice DNV-RPF111.Parametric studies regarding friction between pipe and sea bed, operational conditions i.e. internal pressure and temperature, and material model are performed. Local response of the cross section due to acombined loading of bending moment and internal pressure is investigated using a shell model.

Results from the global analyses of the pipeline indicate that typical structural response is dominated by a combination of plastic bending and membrane forces. This is based on observations where the cross sectionwas partly penetrated by plasticity and where the growth in membrane forces increased due to increasingdisplacement. The cross section with D/t ratio of 15 was observed to develop a full plastic mechanism prior tolocal buckling in the local analyses. Maximum moment capacity occurred at minimum 5 % tensile strain andeven larger compressive strain. This is over five times higher than the typical strain values obtained from theglobal analyses. Based on these results it is suggested that use of a strain based criterion should be takeninto consideration regarding design of pipelines subjected to trawl pull-over.