Chapter 6 - Collapse of Dented and Corroded or Worn Pipes Under External Pressure

Abstract

Offshore pipelines are large-scale structures installed and operated in hostile environments. This chapter considers two scenarios that result in degradation of their collapse pressure. The first is damage in the form of dents caused by impact, and the second is wall thinning due to corrosion or erosion arising from transporting corrosive fluids or ones with suspended particles. The chapter addresses both problems through experiments by denting or thinning sectors of tubular specimens in controlled manners. The damage introduced is carefully characterized and the specimen is collapsed under external pressure. In the case of dents, the ovalization of the most damaged cross-section is the main parameter that controls the collapse pressure. For wall thinning, both the angular span and amount of thickness reduction are important factors. The chapter also presents finite element formulations and results from simulations for both problems. In the first, the denting process is simulated first, followed by collapse under external pressure. Excellent agreement with the experiments is demonstrated. Additionally, given the dent ovalization, the collapse pressure can also be closely estimated using a Universal Collapse Resistance Curve that is generated numerically from the descending branch of the pressure-ovalization curve of the tube with an initial uniform ovality of 0.5%, thus relieving the need for three-dimensional analyses. For wall thinning, the numerical results also show excellent agreement with experimentally measured collapse pressures. The results show that the length of the thinned region ceases to be a factor if it is greater than 10 pipe diameters. The methods and results in this chapter are important for operators to consider when estimating the severity of a damage in a pipeline and deciding on a repair strategy.