Thermal Analysis and Design of Hot Water Heated Production Flowline Bundles
Authors: Zhang Jeff J. , Chen M. Jack , Wang Xuegeng , Chacko Jacob 
Published: October 2008

The high pressure and low temperature environment in deep water creates tremendous flow assurance challenges for subsea tiebacks, which are frequently used in offshore oil and gas development. During extended production shutdown, gas hydrates can form and block the flowline unless necessary time consuming and/or complex prevention measures are taken. For passively insulated flowlines, these measures include hydrate inhibitor injection and dead oil displacement of produced fluids. In an emergency shutdown, the flowline is only protected by the cooldown time of the insulation since the prevention measures usually are not possible in the emergency shutdown situation. For production start-up or restart, hydrate inhibitor injection and/or other procedures such as hot oil circulation are necessary.

Heated flowline systems can significantly reduce the complexity of shutdown and start-up operations, and provide better overall flow assurance. Heating methods developed by the industry include hot water/liquid heating, electrical trace heating, and direct electrical heating.

This paper discusses the issues in the design of hot water heated production flowline bundles using two typical offshore West Africa developments as case studies. Two design options, direct and indirect heating, are analyzed and compared. Factors affecting the warm-up time, such as water flowrate and temperature, pressure limitations, and topsides heating capabilities are analyzed and compared.

Determining the warm-up time of the heated bundle with good accuracy is one of the most challenging tasks for the design of water/liquid heated flowline bundles. A thermal analysis method was developed and implemented in an inhouse computer program. The paper describes the method and discusses its validation by comparing the results with those from three-dimensional transient finite element analyses.

Introduction

Advancement of technology and the discoveries of abundant petroleum reserves have produced a rapid growth of deepwater offshore development activities in recent years. This trend is expected to continue in the foreseeable future in areas such as the Gulf of Mexico, West Africa, and offshore Brazil. One of the frequently used development concepts is subsea tieback, either for producing satellite fields to a production platform, or for total field production to a floating production, storage and offloading vessel (FPSO).

The term "deepwater" evolves quickly and at present generally refers to water depth over 1000 m. In this environment, the pressure is generally high and the ambient temperature is low (about 4°C), a combination that creates tremendous flow assurance challenges for the production system. The most challenging problem is gas hydrate formation. At typical deepwater flowline pressures, gas hydrates can form at 10-20°C above the ambient water temperature at seabed. During extended production shutdown, gas hydrates can form and block the flowline unless necessary prevention measures are taken. These measures typically are very time consuming. Some are also quite complex.