Zero-Radius Bend Method to Trigger Lateral Buckles

Abstract

By deliberately triggering lateral buckles in a subsea pipeline at a sufficiently low axial load, one can ensure that the distance between lateral buckles is sufficiently small, so that the thermal expansion to be absorbed by each buckle is not excessive. A method of triggering such buckles by a zero-radius bend (ZRB) on a trigger that elevates the pipeline above the seabed is described. This method is more effective than other methods that have been proposed and/or used. The use of triggers designed to work only in the vertical direction by laying the line over a linear support aligned at 90° to the pipeline route, is already known. For the ZRB method, a post is added at one side of the trigger to prevent the pipe of being pulled off the trigger on that side. Once the pipe is firmly seated on the trigger, but not yet touching down onto the seabed on the downlay side of the trigger, the laybarge moves laterally without paying out more pipe, thereby pulling the pipe against the post on the trigger, i.e., the movement of the laybarge is as if it was laying a bend of zero radius, but because of the stiffness of the pipe the radius of curvature remains finite and can be kept well within the elastic range. This paper describes the method, a simple approximate analysis method used to assess the performance of the approach and the pipe stresses due to pulling the pipe around the post, and experiences from application of this method.