Uplift Capacity of Buried Offshore Pipelines

El-Gharbawy, Sherif (INTEC Engineering)

OnePetro 

ABSTRACT

The increase in the number of small diameter, in-field subsea pipelines transporting-high pressure and high-temperature hydrocarbons significantly raised the importance of geotechnical engineering in the pipeline system design process. When a pipeline is buried and operated at higher than ambient temperature, it will try to expand. If it is axially restrained, a compressive axial force is produced, leading to potential upheaval buckling. The resulting pipeline response to such upheaval buckling might be unacceptable in terms of vertical movement or excessive yielding of the material. The risk of upheaval buckling must be mitigated by appropriate design of the pipeline backfill material. The prediction of trenching and burial performance, and the resulting physical and geotechnical properties of the pipeline, trench, and backfill have become critical.

Scale-model tests of a buried pipeline were carried out to investigate particular issues related to burial in very loose silty sand. The main results of this testing program have shown that the uplift behavior of buried offshore pipelines is governed by a combination of at least two mechanisms: wedge failure and soil flow-around. The dominance of one mechanism over the other depends on basic parameters such as the depth-to-diameter ratio and soil relative density. The wedge-failure mechanism alone, which forms the basis of existing models does not capture the potential for soil to flow around the pipe, which may provide an added component to uplift resistance or initiate a separate mechanism under certain conditions, and therefore the choice of uplift factor must rely on empirical factors. A new uplift model is presented that accounts for both failure mechanisms and is dependent on typical soil parameters such as the angle of internal friction and the Rankine coefficient of passive earth resistance and can be shown to better replicate test results.