Assessment of Pipeline Damages Caused by Ship Anchor-Chain Drag Incidents

Abstract Offshore pipelines are at increased risk of damage from dropped objects such as anchors, chains, and dumped rock debris from fishing or other activities. Depending on the complexity of vessel movements and the duration, it can lead to varying magnitudes of global and local damage to the pipeline. Pipeline global damage includes buckling and pipeline bending, which leads to a high tensile stress/strain state that could cause girth weld failure. The local damage is often formed by contact between the anchor/chain and pipe, causing pipe crushing, dents, buckle/wrinkles, gouges, cracks, or a combination of these, which could trigger immediate or delayed failures. Sometimes, vortex-induced vibration (VIV) may occur at the displaced pipe (free-span) segment, which requires evaluation and mitigation. Sub-sea pipeline damage could involve environmental consequences such as a spill, asset loss, and possibly even casualty. For this reason, relevant investigations should include realistic engineering assessments and, if possible, mitigate prior to consequential damages. A comprehensive assessment methodology was developed here and is presented in some detail. The ductile failure damage methodology has been applied to dent assessments in pipelines, thermal fatigue, and expandables. However, the application of this to large displacement problems, such as anchor drag, is novel. Further, this is the only approach that uses plasticity and does not use a fracture mechanics parameter. Some key assessment elements include understanding global pipeline displacement, material properties, local damage assessment using the plastic strain limit damage model such as the Ductile Failure Damage Indicator (DFDI), local stresses and strains, and fatigue analysis utilizing the actual operational pressure fluctuation data. One anchor-chain drag incident was modeled, and the observations were predicted through finite element modeling and analysis (FEA). The methodology was verified using full-scale testing of the local dent damage. The results from two different anchor drags are discussed in this paper.