Cool-Down-Temperature Overshoot Phenomenon in Subsea Flowline and Riser Systems
Authors: Chin Y. Doreen , Kvaerner Aker 
Published: October 2008

During shut-in of insulated pipeline and riser systems carrying multiphase flow, the temperature at pipeline segments with large geometrical variations may decrease below the local ambient temperature. The low temperatures at these locations increase risk of hydrate formation during system cool-down. Susceptibility of a system for "cool-down temperature overshoot" should be assessed during design stage and operation procedures should be developed to manage it.

This phenomenon is caused by the mass transfer between gas-liquid interface, the phase re-allocation, and inner energy re-distribution due to the combined effects of gravitational and buoyancy forces. Pressure, mass flowrate, fluid compositions, flow orientation, and external and internal heat transfer during system cool-down are the key factors for the phaseredistribution between gas and liquid, and along the system.

This paper presents an explanation of the governing mechanisms of this phenomenon for a wide range of system parameters based on a series of theoretical and numerical studies of gas/condensate and oil/gas flows in subsea flowline and riser system after system shutdown.

Introduction

The exploitation of deepwater discoveries is increasingly being achieved with the use of lengthy tie-backs to existing platforms. The considerable capital cost associated with the construction and installation of a new platform may require production from a number of wells, some distance from the platform. Deepwater subsea tiebacks rely on subsea flowline and riser systems to bring the produced fluids from subsea wells to the platform.

In subsea flowline/riser system, oil very often is produced along with the gas and water. The amount of liquid holdup varies according to the local temperature and pressure, as determined by the phase equilibrium envelope of the fluid, resulting complex multiphase flow with multiple-component mixture. The quantity of liquid at a given point along the pipeline is determined not only by the local temperature and pressure, but also by the gas superficial velocity, slippage, andthe pipe geometry/inclination, since interfacial shear between the phases is the main mechanism by which the liquid is transported. When a pipeline shut-in occurs, there are relatively large changes in pressure and temperature, with correspondingly large amounts of phase change. The phases re-allocate themselves due to the combined effects of gravitational force, buoyancy force, interfacial shear force, surface tension force, and the energy re-distribution internally and externally.

One of the main problems with subsea pipelines, especially in deepwater, is the risk of blockage due to hydrate formation. For steady state operation, hydrate blockage can be typically avoided by maintaining the fluid temperature above the hydrate temperature using insulation and/or by suppre sing the hydrate formation temperature with chemical inhibitors. During a planned pipeline system shutdown, inhibitors are injected beforehand to protect the pipeline, however, for an unplanned shutdown, there may be insufficient inhibitor in the system to provide inhibition. In this case, there is a certain amount of time required before hydrate formation conditions are reached, during which the pipeline must be restarted or certain concentration of hydrate inhibitor must be reached by additional injection. It is critical to accurately predict the cooldown temperature response along the pipeline.