Buckle Arrestors for Deepwater Pipelines

Abstract

Abstract Progress has been made in the design of buckle arrestors, or more precisely collapse arrestors, for deepwater pipelines. Empirical relationships have been developed for the design of both integral ring and grouted sleeve arrestors, forming the basis of a simple and straightforward design procedure. The good agreement between the latest design formulas and the crossover pressure data obtained from large scale tests by Shell E&P Technology Company and by Professor Kyriakides at U.T. Austin over the past few years, should result in more efficient and reliable buckle arrestors for deepwater pipelines. Introduction An offshore pipeline which has been damaged locally may fail progressively over long distances by a propagating collapse failure driven by the hydrostatic pressure of the seawater. The pressure required to propel a propagating collapse is much smaller than the pressure required to initiate collapse of an undamaged pipe. For deepwater pipelines it is often uneconomical to design the pipeline with sufficient strength to prevent a propagating collapse failure. Such pipelines are designed to prevent buckling and collapse failures due to normal combined bending and external pressure loads, but are left vulnerable to propagating collapse failures initiated under extraordinary circumstances. In such cases, it is feasible to install buckle arrestors, such as thick-wall rings, at intervals along the pipeline. A series of such arrestors, each sufficiently strong to stop a propagating collapse failure, will limit the extent of damaged pipe in event of a mishap. In general, the distance between buckle arrestors is selected to enable repair of the flattened section of pipeline between two adjacent arrestors, at "reasonable" cost. For pipelines installed by J-Lay, the buckle arrestors also serve as pipe support collars. In this case the distance between arrestors is simply the length of each J-Lay joint. Three types of buckle arrestors are in common use, namely Grouted Sleeve arrestors, Integral Ring arrestors, and Thick Wall Pipe Joints. Grouted Sleeve arrestors are steel sleeves that are slid over the ends of selected pipe joints andare grouted in place, as shown in Figure 1, before being installed offshore. Grouted Sleeve arrestors are preferred, where feasible, because of their low cost. However, this type of arrestor has limited usefulness in deep water because, as external pressure increases, a collapsed pipe will transform from its normal flat "dogbone" cross section into a C-shaped cross section which then passes through the arrestor. Hence, for sufficiently deep water, even an infinitely rigid Grouted Sleeve arrestor is ineffective. Integral Ring arrestors are thick-wall rings that are welded into selected pipe joints, as illustrated in Figure 2, before being installed offshore. Integral Ring arrestors are used for pipelines in which the strength of sleeve type arrestors is not adequate, and for J-Lay applications that require a support collar on each pipe joint. These arrestors are very efficient in terms of strength for a given amount of steel, but are more expensive than sleeve arrestors because of the additional welding required. Thick Wall Pipe Joint arrestors are special pipe sections, each designed to prevent collapse propagation, that are welded into a pipeline at intervals. A Thick Wall Pipe Joint is essentially a very long integral ring arrestor, but is much less efficient in the amount of steel used.