Abstract

Multiphase product transportation from offshore is facing the challenges of deepwater exploration and long-distance tieback. Prior predictions of the multiphase flow behavior, both steady state and transient, are crucial for successful design and operation of offshore facilities. The pressure surge and thermal stresses caused by transient multiphase flow during emergency shut down (ESD) are critical design criteria for pipeline systems.

Numerical methods have been developed to simulate the subsea HIPPS/ESD shut down and the ensuing system transient behaviors. These methods are able to predict pressure and temperature response during and after ESD valve closure. The analytic predictions are used in the design of deepwater pipeline and riser systems.

This paper demonstrates the use of transient multiphase flow analyses in the design of a deepwater subsea pipeline and riser systems. The pressure and temperature responses, and the pressure and temperature profiles from wellheads to the platform are investigated. The system design criteria are validated. The study improves the understanding of transient process in multiphase flow pipelines, and will benefit the deepwater production system design.

Introduction

Over the past decade, strong market demand has triggered deepwater exploration and production. With the increase in water depth, the risk and cost of offshore activities have become critical concerns. Safety considerations have always had high priority for the offshore operations. One of the methods to mitigate the risk and reduce the flowline cost is the use of subsea high integrity pressure protection system (HIPPS). The application of HIPPS now is not only optimizational, but also regulational (Onshus, et al., 1995).

HIPPS is a system of ultra reliable valves (such as ESD), sensors and controls, with which can perform process shut down. Process shut downs generally involve process-related variables that exceed preset limits. The pressure in the production tubing provides the actuating power when the valve has to be closed. HIPPS can shut off the wellstream in less than two seconds, if it senses that the pressure has become above the setting point. This system protects the downstream components against a high potential shut-in pressure from any one of the oil wells. As applied at subsea manifold, HIPPS enables design of flowlines according to flowing pressure, rather than well shut-in pressure and thereby gains considerable savings on flowline costs. By allowing flowline design pressure to be the maximum allowable operation pressure (MAOP) and designing short jumper lines to sustain the well shut-in pressure, subsea HIPPS gives a substantial saving on capital expenditure, especially for sour fields with high pressure and long offsets (Lund, et al., 1995). The malfunction of an ESD represents a threat to the safety of downstream flowline and riser because it imposes additional loads on where the lower rated components are designed.

An ESD shutdown sequence is an inherently dynamic effect resulting in large and rapid changes in pressure and temperature throughout the system. The pressure surge and thermal stresses caused by transient multiphase flow during ESD shut down are also critical design factors for pipeline systems, specially for the valve selection and flowline thermal expansion criteria. The transient process of multiphase flow during and ensuing ESD valve shutdown has not been well understood and current design methods are limited and cannot address a variety of issues in pipeline design (Bratland, 1995). Current numerical analysis technology provides the possibility to model such a complicate dynamic process (Rygg and Ellul, 1991).

In this study, a novel method is developed to simulate the transient processes in pipeline systems. This method is able to predict pressure and temperature response during and after ESD valve closure. The analytical predictions are used to design deepwater pipeline and riser systems.

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