Mechanics of Heat Loss in Dry Tree Top-Tensioned Risers
Authors: Chin Y. Doreen , Wang Xuegeng 
Published: October 2008

Application of dry tree top-tensioned riser systems provides direct vertical access to reservoir via production risers and surface trees, wellbore access for artificial lift and wireline and coiled tubing. While the dry tree riser is gaining growingly attention for deepwater development, the challenge has raised for flow assurance engineering, mainly for the thermal analysis of the dry tree riser - a complex heat transfer system.

This paper focuses on the mechanisms of heat transfer in dual and single casing dry tree riser systems. The dry tree riser is a complex system and the heat transfer through the multiple wall of dry tree riser is a complicated process. Current understanding of such heat transfer mechanism is far from satisfactory, often resulting in a large deviation between the predicted and actual system thermal behavior, and temperature profiles or trends. To compensate this, the industry often applies a large safety factor for the system design.

This paper reviews dry tree riser related heat transfer study and research in the industries and academics analysis, and provides the state-of-the-art of heat transfer analysis methods for dry tree riser to offshore system engineering and flow assurance, to improve predictions of riser thermal performance at system design phase and for steady state and transient production operation.

Introduction

Dry tree top-tensioned riser (TTR) systems provide remarkable benefits for deepwater field development such as efficient drilling and major workover access, valve and choke access, wireline logging and coiled tubing access. The system also provides efficient production by allowing no commingling of well fluids, therefore can reduce production downtime and allow higher production rates.

The inner and outer annuluses of the dry tree production systems are often filled with N2, inhibited clear-brine fluid, or gel type thermal insulated packer fluid. Due to the extremely large height to gap ratio (H/d), secondary flow or vortices are induced by the buoyancy along the gap, causing an enhanced internal natural convection heat transfer coefficient, thereby an increased overall heat transfer coefficient and heat loss.

Currently, there are limited literatures in the industry directly related to the heat transfer through the TTR, either on steady state or transient [1,2], while abundant literatures may be found in the academia and other industries that can be utilized for TTR [3-12].

This paper reviews and study the heat transfer researches that are relevant to TTR. The intension is to provide the stateof- the-art of heat transfer analysis methods and the recommended practices for dry tree riser to offshore engineering and flow assurance.

Characteristics of Top-Tensioned Risers

A TTR primarily consists of pressure barrier conduit, i.e., production tubing, and environmental barrier conduit, i.e., casing or casings. Depending on workover philosophies and limiting top tension capabilities, the TTR may be single casing or dual casing configurations, as shown in Figures 1 and 2, respectively.

In deepwater producing wells the bottom-hole temperature is much higher than the production flowing temperature at the surface, while the temperature of the fluid is much higher than the surroundings.