Hydrostatic Collapse Of Deepwater Pipelines, A Rigorous Analytical Approach
Authors: Guarrancino Federico , Fraldi Massimiliano , Freeman Richard Ward , Slater Simon 
Published: May 2011

Abstract
The demand for pipe materials that can withstand increasing hydrostatic pressures continues to grow as oil and gas exploration takes us into deeper and deeper waters. As water depth increases, failure by hydrostatic collapse becomes more likelyas the limiting failure mode for pipeline design. In general, pipes for deepwater applications possess a diameter to thickness ratio in a region where failure is dominated by both instability and plastic collapse. This implies that, prior to failure, the compressive yield strength of the material must be exceeded, followed by ovalisation and further local yielding.
To enable effective design work to be performed, and proven during service, it is necessary to ensure that the mechanical and dimensional properties of the pipe material are controlled within very strict boundaries and reliable and accurate formulae are required to take into account these factors. However, mechanical equations for this simple failure mode have not been satisfactory established. Current existing formulations use basic mechanical expressions that are calibrated to some limited test data. However none of these expressions examine the actual mechanics of the failure mechanism.
This paper presents an investigation into the mechanics of this specific problem and permits a simple assessment of the effects of geometrical and material data on the collapse pressure of moderately thick tubes. An analytical expression has been generated and correlated to extensive test data and finite element models, proving the accuracy of the expression. The resulting formula also allows a deeper understanding of the effect pipe shape has on collapse and opens to the door for manufacturers to further enhance pipe material performance. Despite the complexity of the collapse mechanisms this work presents a concise yet rigorous and effective analytical approach.
The work presented in this paper will form the basis of future investigations that will enhance the understanding of this failure mode in pipelines. This will lead to increased pipeline safety while allowing the boundaries of pipeline application to be challenged, enabling hydrocarbon recovery in deeper and deeper waters.

Introduction
Since the early 1990's the export of hydrocarbons through deepwater trunklines has been seriously considered. In deep and ultra-deeep waters the diameter of trunklines coupled with the hydrostatic pressure tends to lead to failure of the pipeline by external collapse. This failure mode is an instability phenomenon that is governed by the geometry of the pipeline and its material properties. Failure of a pipeline section is through a subtle combination of its properties and as such the exact limiting collapse pressure is very difficult to determine through simple mathematics. Finite element techniques can allow the estimation of collapse where the actual geometry and material properties are known but in reality shape and properties vary along the pipe length, around the circumference as well as through the wall thickness.
Despite these complications relationships do exist that provide adequate prediction of collapse pressure and these are discussed in the next section. However, each of these relationships has its route in the same fundamental, simple mathematics with expressions generated to blend transitions between elastic collapse for thin walled structures and plastic collapse for thicker sections. None of these relationships actually seeks to understand the actual mechanics of this failure mode however, their accuracy and applicability is generally improved through calibration of the output with actual test data.