When an underwater pipeline is buckled in the presence of sufficient external pressure, a propagating buckle is initiated, and the buckle front will propagate along the pipeline until a region of much less external pressure is reached. This paper describes the results of experimental research on methods of arresting the advancement of a propagating buckle along the pipeline. The arresting capacities of three types of buckle arrestors conceived during this program, free ring, welded-ring, and heavy-walled section arrestors, are described in detail. Results of propagating pressure-experiments are also presented.
The term "propagating buckle" describes a phenomenon in which a buckle in an offshore pipeline changes its geometry from a transverse dent or crease to a .longitudinal buckle of long wave length and propagates along the pipe collapsing it along its length. The most insidious nature of this buckle(1,2) is that the pressure required to maintain the propagating buckle (the propagation pressure,-Pp) is less than that required to initiate it (the initiation pressure, Pi)' Therefore, should a pipe be buckled while subjected to an external pressure in excess of the initiation pressure, a propagating buckle will be formed that will advance along the pipe until a. zone of less pressure, P, is reached, such that P < Pp '.
After encountering this phenomenon in early 1970 in laboratory experiments, conditions under which the propagating buckle would occur were investigated. During this work, it became apparent that to design offshore pipelines such that the initiation pressure of the line was not exceeded would become very costly for deep-water pipelines. Therefore, a need was apparent for concepts to arrest the advancement of the propagating buckle, should it occur. In December, 1971, Battelle proposed to the member companies of the Offshore Pipeline Group (see Acknowledgements) to investigate the ability of three methods of arresting the propagating buckle through performing full- and small scale experimental studies. The three methods investigated were heavy-walled cylinder arrestors, free-ring arrestors, and welded-ring arrestors (Figure 1).
A heavy-walled cylinder arrestor is a length of pipe whose inside diameter is approximately equal to that of the pipeline, but whose wall thickness is greater than that of the basic pipeline. Its length varies but generally is greater than two to three pipe diameters.
A welded-ring arrestor is also a stiffening ring slipped over. a pipe, as in the free-ring arrestor, but this ring is completely welded to the pipe which increases cross-sectional moment of inertia of the pipe. This stiffening action causes an increase in the propagation pressure at and adjacent to the buckle arrestor.
In this paper is contained the results of the experimental study of the buckle-arresting abilities of the above three devices.
