UHB Design Approach for Multiple Pipelines Installed in Shared Trench

Upheaval buckling (UHB) mitigation for trenched and buried pipelines can constitute a substantial cost element for offshore field development. There appears to have a variety of reasons for dual or more pipelines and umbilicals to be considered for installation inside the same trench. A single shared trench has been used for multiple pipelines not only for cost saving, but especially when constrained and driven by route corridor challenges. The common practice for dual pipeline trenching and UHB design is to either perform UHB design independently without due consideration of the pipelines in the proximity, potentially resulting in a compromised UHB mitigation design, or simply combine the uplift resistance required for each individual pipeline in the proximity to obtain the overall backfill/rock dumping to account for pipeline interactions. This paper re-examines the rationale of the normal practice and some fundamental aspects of UHB design for dual pipelines installation inside the same trench. The proximity effect on the uplift resistance is investigated with respect to pipeline spacing and burial depth. Its impact on the UHB mitigation is considered by a detailed analysis and a series of parametric simulations with respect to pipeline dimensions and gaps. The sensitivity of the soil slip failure angle and the dilatancy is also performed. Based on the theoretical analysis and FEA modelling, a model solution is formulated and proposed for evaluating uplift resistance reduction for multiple lines. The formulae are extended to deal with multi-layered soil and rockdump. A number of pipeline configurations have been discussed including a piggyback arrangement. A robust UHB mitigation and reduced optimum rockdumping can be achieved by considering the proximity effect through challenging the industry norms and common approach.