Quick Estimating Pipeline Walking Anchor Force - It is Much Less Than the Full-Length Mobilization

Abstract

Abstract An analytical methodology for estimating walking anchor force is presented. Currently, anchor force calculation requires substantial FEA which is not compatible with early phase design schedules. At such stage, anchor force is typically assumed to correspond to full pipeline length mobilization. This is very conservative, in many cases by an order of magnitude, and can hence mislead important early design decisions. The force on a walking anchor increases with cycles of pipeline operation, eventually approaching an asymptotic limit value, often of the order of 1 MN. Robustly identifying this limit requires extensive FEA, incorporating detailed bathymetry, simulating tens to hundreds of operational cycles, and covering numerous sensitivities to input parameters, which often yield counterintuitive results. The proposed analytical method does not require repetitive calculations. It derives the limit value straightforwardly, based on fundamental principles, and has been implemented in a spreadsheet. Providing immediate results, it permits quick assessment of the effects of many parameters such as soil resistance, operating loads, as well as the number and location of global buckles, on the system's ultimate response. Results are shown to agree very well with extensive FEA results, and yield required anchor loads which are much lower than the full-length mobilization approach. Having such results early in design allows better informed decisions, for example making end anchors viable, and thus avoiding much more complicated mid-line ones. Pipeline ends can be anchored by either adopting piled foundations or by connecting them to a separate anchor pile. Mid-line anchors mobilize lower loads, however in most cases require additional attachments or structures, which bring additional concerns in terms of through-life integrity, and installation cost, safety and schedule risks. The proposed tool permits proper estimation of anchor loads at early design stages, and can make otherwise too expensive concepts viable. Its capability of quickly assessing multiple sensitivity cases also allows early identification of the relevant aspects to be tackled later in the project, in a value engineering approach.