The Dynamic Two-Phase Modelling of Offshore Live Crude Lines Under Rupture Conditions
Authors: Rygg O.B. , Ellul I.R. 
Published: October 2008

An important factor in the exploitation of the world's oil and gas resources is the mode of transportation away from the exploration and production facilities to regions where processing can be performed. Commonly, pipelines are employed providing annual transport capability for billions of dollars worth of crude oil, natural gas and associated products.

This paper focuses on the offshore transportation of crude oil via inter-platform and platform-shore flowline systems, In particular, the study addresses the dynamics of riser-flowline configurations under conditions generated by the topsides rupture of one of the risers. The problem is of a complex dynamic multiphase nature, necessitating the use of a reliable mathematical tool. Here, the OLGA code was used to provide predictions of the transient phenomena occurring in the system. Field data relating to operating conditions and fluid properties were used throughout the simulations.

The investigations focus on the system dynamics subsequent to a rupture with the aim of predicting the mass release from the flowline-riser system. Various factors are seen to contribute to the resulting predictions. These include the oil bubble point pressure, flowline topology as well as modelling assumptions.

Introduction

Over the past recent years, events within the petroleum industry have contributed to a substantial increase, among pipeline operators, in the awareness of safety and environment related issues. As a result of this, increased efforts are now being made to employ the latest technology in pipeline integrity monitoring techniques. Offshore production platform scenarios, in particular; provide for a higher order of complexity in aspects of safety and loss prevention. The Cullen Report[l] calls for measures to mitigate the effects of any accident that may occur, one of which relates to the minimisation of hydrocarbon inventory on the platform and in risers and pipelines and isolation of pipelines. This may be achieved by the installation of valves that shut off flow. These may include topside emergency shut down valves (ESD) as well as subsea valves, whether a non-return valve (NRV), or a subsea isolation valve (SSIV).

The effect of installat ion of a SSIV in a gas transport flowline is clear in that the maximum inventory available for evacuation through a rupture is bounded by the SSIV. In the case of an oil line the hydrocarbon release may be said to be governed by a mobility effect determined by, among other factors, the oil volatility. The merits for installation of a SSIV in an oil line must therefore be investigated on a case by case basis.

Such investigations necessitate an appreciation of the phenomena associated with the dynamics of pipelines under leaking or rupture conditions. This paper shall address the latter topic, reporting the application of the OLGA model to the rupture of crude oil lines. A brief approach to pipeline rupture calculations will first be presented followed by a description of OLGA. The application of the model to cases of riser rupture is presented next with model predictions subsequently detailed and discussed.