This paper presents the main findings resulting from the upheaval buckling design challenges associated with a real industry application of a 32" subsea pipeline, buried in a trench produced by a jet trenching operation. The uplift mobilization distance is regarded as a critical parameter influencing the upheaval buckling behaviour of a pipeline. This parameter is highly uncertain, and its adequate consideration is essential to ensure a reliable UHB design. It is typically understood to be a conservative approach of applying the upper estimate value in the analysis. However, for the project application presented in this paper a conservative backfill cover failure load (based on effective axial compression) has been predicted when the lower estimate of the uplift mobilization distance is applied. The uplift limit criterion appears to govern largely over the critical buckling load criterion. It may therefore be of importance for similar future pipeline projects to give due consideration of this aspect. The fundamental principle of this phenomenon is explained in this paper and the opportunity of a potentially cost optimization regarding rock dumping requirement is discussed with focus on the choice of the uplift mobilization distance, relaxation of the design criterion, load factors as well as the uplift ratcheting limit.
Offshore submarine oil and gas pipelines will occasionally be trenched and buried in order to fulfill a set of different design criteria related to flow assurance, on-bottom stability and protection against third party loads, for example trawl gear interaction and anchor drag. While it is a common industry practice to consider burying relatively small diameter pipelines (except deep water applications), typically a size up to around 16", there are exceptions where pipelines of a significant size are decided to be buried as well. One critical design aspect for these buried pipelines operating at high pressure/high temperature (HP/HT) condition is to avoid upheaval buckling (UHB), which may potentially cause severe failure / leakage of a pipeline system and associated significant economical loss. The UHB phenomenon of a pipeline buried in a trench is illustrated in Fig.1.
