Finite Element Analysis of Flexible Pipe Tension Loads Due to Iceberg Interaction

Abstract Offshore Newfoundland and Labrador, Canada, development costs associated with iceberg protection pose significant challenges in terms of project execution and economics for marginal field subsea tie-backs. The current standard practice is to assume that if an iceberg makes contact with a subsea flowline, the flowline is dragged indefinitely imparting significant load to the connections at each end. To isolate flowlines from downstream and upstream assets, weak links are installed in the flowline that are designed to separate once a specified level of tension is reached. This prevents damage to wellheads and other subsea equipment and eliminates the possibility of uncontrolled hydrocarbon release. However, the weak links are very costly and possess inherent risk of failure, which can lead to an uncontrolled release of hydrocarbons. This paper addresses the requirement of weak links by analyzing the flowline tensions transmitted due to iceberg-flowline-soil interaction events. The assumption that an iceberg drags a flowline indefinetly imparting significant tension on the end connections can be challenged. This paper seeks to estimate the tension loads developed in an untrenched flexible flowline due to interaction with free-floating and gouging icebergs. Large deformation finite element analysis is utilized to simulate the iceberg-flowline-soil interaction scenario. The iceberg keel is idealized with shape and dimensions based on analysis of recent iceberg profiles. A sensitivity study is conducted to assess the influence of keel size, gouge width and depth on flowline tension developed throughout the flowline resting on very dense sand. The sand constitutive behavior is modelled using a user subroutine accounting for the effects of mean effective stress and relative density on the soil strength and volumetric response. The ice-flowline-soil interaction mechanisms are detailed for free-floating and gouging interaction events. During interaction with free-floating icebergs, the flowline is typically depressed into the seabed, and the keel rides over the flowline. The gouging interaction scenario simulates the complex interaction between the frontal soil mound developed during the gouging process and the untrenched flowline. This paper provides new insight into the iceberg-flowline-soil interaction scenario that has not been examined previously. Based on the analysis results presented, an alternative strategy to mitigate tension transfer to downstream and upstream assets is discussed.