Impacts of Insulation Techniques on Transient Multiphase Flow Characteristics of Deepwater Catenary Risers
Authors: Chin Y. Doreen , Krishnathasan K. 
Published: October 2008

For deepwater production, the pipeline/riser system operates in a high external pressure and low seabed temperature environment, facing a high risk of hydrate formation and paraffin deposition. The leading passive strategy for flow assurance is to maintain the well stream temperature beyond the hydrate/paraffin formation zone by minimizing the system heat loss via insulation. Identification of current and new insulation methods that meet technical and commercial demands is a principle issue for deepwater field development.

This paper investigates the effect of different insulation scenarios on the transient characteristic of oil/gas/water multiphase flow in deepwater catenary risers. Numerous insulation techniques are available for flow assurance in these pipeline/riser systems. This paper presents a systematic study of risers with the pipe-in-pipe configurations and different insulation materials and with an external insulation material. This study also clarifies the impact of insulation techniques on multiphase up-flow crude in the riser and discusses the phenomena based on the fundamentals of multiphase flow and heat transfer.

Numerical transient analyses were conducted to simulate the system shut-in conditions. The effects of insulation scenarios on the temperature, pressure, and liquid holdup responses during the system cool down conditions are investigated. The results of this study will enhance the understanding of multiphase flow transient behavior in insulated deepwater pipeline/riser systems.

Introduction

Multiphase production means that the produced fluids are not subjected to any surface process at the field site. The well stream is conveyed from subsea in form of multiphase flow to remote process facilities at a fixed production platform or at floating production units. All deepwater platform concepts rely on riser systems to bring produced fluid to the topside facilities, which is one of the major challenges for deepwater field development.

The flow assurance of pipeline/riser system needs to consider steady state operation as well as transient events such as system shut-in. Understanding of the cool down process of a system shut-in is an important aspect of deepwater production system design. For an insulated flowline/riser system in which the produced fluids arrive at the process facility above the hydrate formation temperature, regular or continuous injection of inhibitor is not needed. However, precautions for preventing hydrates must be considered for thecase of unplanned shut-in, of which the flowline/riser system is shut off at both the topside facility and at the flowline inlet at wellhead. When production is shut-in, temperature and pressure decrease, and the liquid phase flows down to the low spots in the system. An evaluation of temperature and pressure contours as a function of time can provide information on duration of a pipeline/riser system after shut-in before hydrate formation becomes a concern.

Transient multiphase flow modeling has become an integral part of the design and operation of multiphase pipelines and related facilities [1]. The simulation techniques enable specific operation guidelines to be formulated by reviewing time-dependent temperature and pressure predictions along the entire length of the pipeline [2, 3].