Structural Integrity of Submarine Pipelines Subjected to Large Strains Caused By Trawl Pull-Over

Hval, Morten (Reinertsen AS, Trondheim, Norway) | Brækstad, Lars Løberg (Reinertsen AS, Trondheim, Norway) | Olsø, Erlend (SINTEF Materials and Chemistry, Trondheim, Norway)

OnePetro 

For submarine pipelines installed in an area with high fishing activity, special attention must be drawn to the structural integrity of the pipelines related to interference with fishing gear such as trawl equipment. In a worst case scenario the impact/pull-over by trawl equipment will result in damage of the pipeline with possible fracture or leakage as a result. This paper deals with the design methodology developed for verifying the integrity of flowlines installed by the reeling method. The design methodology includes determination of design loads and resulting strains as a function of material stress-strain properties and Engineering Criticality Assessment (ECA) of pipeline girth welds including defects equal to maximum allowable size according to the welding fabrication acceptance criteria. For the ECA analyses the FEM program LINKpipe was applied. This program is well suited for analyses of pipelines including defects subjected to axial load, bending moment and internal/external pressure, which is typical for submarine pipelines during operation.



INTRODUCTION


Ormen Lange Southern Field Development (OLSFD) is a part of the phase 2 development of the Ormen Lange gas field located about 120 km offshore the coast of Norway. The OLSFD includes an 8 slot template, two 16 inch infield flowlines, one 6 5/8 inch MEG line and one umbilical located at about 850 m water depth. The design philosophy with respect to possible interference with fishing equipment was initially adopted from the Phase I development; The MEG and the umbilical line shall be fully protected by means of trenching and subsea rock installation, while the production line shall be left exposed on the seabed designed to withstand loads from intervention with fishing gear such as trawl impact/pull-over and hooking. However as the OLSFD infield flowlines are reduced to 16” compared to the 30” production lines from phase 1, the utilization in the pipe was significantly increased by adopting this philosophy.