Global Buckling of Pipe-in-Pipe: Structural Response and Design Criteria

Abstract

Pipe-in-pipe is a flowline concept that shows an increasing popularity when certain flow assurance aspects are to be dealt with. U-values in the order of 1W/m2K are achievable, ensuring that the well fluid temperature can be kept high for longer time after shut-down and over longer distances. This two layer steel construction also has some mechanical properties that can be beneficial for the design such as a stronger, more robust cross section, better protection against 3rd party interference and increased stability as a few examples. Disadvantages such as high weight causing higher bending moments during installation or in free spans should also be mentioned. The global response of a pipe-in-pipe will be very much similar to a conventional single pipeline. It will expand due to pressure and temperature loads, longitudinally or in combination with lateral motion (global buckling), always in the direction of the lowest resistance. However, there are also some important differences compared with conventional pipelines. Interaction between the inner and outer pipeline is secured using rigid bulk heads at each end. Along the pipeline the separation of the two pipes can be ensured by e.g. rollers, plates, spacers, insulation and additional intermittent bulk heads. In the most extreme case the spacing between intermittent buck heads is down to every pipe joint. At present there is no public pipeline code that in detail addresses pipe-in-pipe. One reason is of course that there is not only one pipe-in-pipe concept but a large number of variants. This paper’s focus is on mechanical behaviour of a pipe-in-pipe in relation to expansion and global buckling. Both the friction between to two pipes and soil friction influence on the global buckling pattern is discussed. An introduction is given to where pressure and temperature loads are acting and how these loads raises forces in the pipeline (load effects).