Physical and Numerical Modeling of a Sub-Surface Caisson for Protection of Subsea Facilities against Scouring Icebergs

Abstract

A simple, rectangular sub-surface caisson was evaluated using physical and numerical modeling to develop a method for protecting subsea installations against scouring keels of icebergs on the Grand Banks. This protection strategy was developed to assess the feasibility of the structure as an alternative to costly excavations used to protect subsea equipment on the Grand Backs. The scenario considered was an ice keel (i.e., iceberg) scouring over a buried caisson, avoiding direct contact but with minimal clearance. A centrifuge testing program, consisting of five tests, formed the basis for calibration of a finite element model. The first four centrifuge tests were carried out with water as the pore fluid in order to simulate drained conditions during the keel-soil-structure interaction. In the field, fully drained conditions may not exist; hence the fifth centrifuge test was conducted using a viscous pore fluid to simulate the partially drained conditions. The numerical analyses were conducted using the commercially available finite element software package ABAQUS. This study demonstrated that the Coupled Eulerian Langrangian (CEL) technique is capable of modeling slow keel-soil-structure interaction events in sand. The numerical model performed satisfactorily in simulating the centrifuge tests. The results indicated that such a system can potentially provide the protection needed for subsea installations in ice infested waters. Although this work was based on the conditions encountered on the Grand Banks, the concept would be applicable to other regions. Future work should involve evaluating various ice keel-soil-structure interaction scenarios and attack angles, development of Inspection, Repair and Maintenance (IRM) strategies as well as a detailed assessment of construction and installation issues.