When production starts through a pipeline, internal temperature and pressure will rise. The temperature increase will lead to thermal expansion of the steel. A pipeline will be restrained variously along the routing due to soil friction, and the temperature rise will result in axial compressive forces in the pipe. As a response to the longitudinal compressive force interacting with local curvature of the pipe, global buckling may occur.
A pipeline can buckle downwards in a free span, sideways on the seabed or upwards for buried pipelines. Vertical buckling of a pipeline is called upheaval buckling, and the direction of the buckle is upwards because this is the way of least resistance. If a vertical buckle leads the pipe into exposure on the seabed, this is a severe problem. An expensive and time consuming operation is needed to re cover the pipe at this location. If the buckle damages the pipeline, this part must be replaced before re covering takes place.
Figure describes upheaval buckling on buried pipe.
For upheaval buckling to occur, the pipeline must first have an initial imperfection. Imperfections are typically due to the pipeline being laid over a boulder or due to irregularities in the seabed profile.
Figure below illustrates a sequence of events which initiates buckling in a buried pipeline:
The pipeline is laid across an uneven seabed (a) and later trenched and buried (b). The trenching and burial operations modify the profile of the foundation on which the pipe is resting, so that it is not precisely the same as the original profile. Trenching may smooth the profile overbends, but may also introduce additional imperfections, if, for instance, a lump of bottom soil falls under the pipe.
The occurrence of an upheaval buckle is highly depending on the smoothness of the seabed profile. According to the DNV-RP-F110 (Global Buckling of Submarine Pipelines), it gives criteria to avoid upheaval buckling from occurring by designing sufficient cover providing enough resistance for pipelines to remain in place. Therefore, upheaval buckling is considered as an ultimate limit state (ULS) in the RP.
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