Global Buckling

Temperature and pressure effects create expansion effective forces which may cause a pipeline to buckle globally. Pipelines installed on the seabed and left exposed have a potential to buckle globally and change configuration while a buried pipeline is designed to stay in place being restricted by the surrounding soil reaction forces.

The driving force for global buckling of the pipeline is the effective axial force, S, which represents the combined action of pipe wall force, N, and internal and external pressures, see Sec.5.2.2.The effective force for a restraint straight pipe, S0, constitute an upper bound axial load and is discussed in Sec.5.3.1.

For a certain expansion force, the pipeline will buckle globally. For a partially displacement controlled condition, this implies that it will find a new equilibrium by moving perpendicular to the pipe axis at the same time as the pipe will move axially, feed-in, from both sides towards the buckle. The level of axial force to initiate this global buckling depends on:

• ·         pipe cross section properties
• ·         lateral resistance
• ·         out-of-straightness in the pipeline
• ·         lateral triggering force (e.g. trawling)

A straight column will buckle according to the classical Euler buckling formulation. As the out-of-straightness in the column increases, the level of axial force required to buckle it will be reduces. This effect, away from the buckle, is illustrated in Figure 2-1.

The out-of-straightness may be caused by:

• ·         small imperfections on the seabed like the pipeline resting on rocks
• ·         global imperfections as uneven seabed

• ·         curvature in the horizontal plane purposely made or random from installation

To illustrate the global buckling of a section in a pipeline, the following idealised sequence of a pipeline with free end expansion can be used :

1.       Prior to applying pressure and temperature, the effective force will be limited to the residual lay tension. The effective force variation will be tri-linear; from zero at the pipeline ends with a linear increase proportional to the axial resistance to the soil, until it reach the residual lay tension H. It will then be constant until it reaches the decay from the other side, see lower curve of Figure 2-2.

2.       When the temperature or pressure increase the compressive effective force will increase to maximum S0, This will vary along the pipeline as the temperature and pressure decrease along the line. At the pipe ends, the load will still be zero, see upper curve of Figure 2-2. A snap shot from a short section is now selected for closer examination in Figure 2-3.

The buckling development is described in Figure 2-3.

Note also that the post-buckling load, point B above, may not be reached directly but through a continuous development. This may imply that higher force close to the buckle is achieved prior to reaching B, that may trigger another buckle.

Source: Recommended Practice DNV-RP-F110 October 2007