ABSTRACT

As the step out distance for subsea wells increases, the ability to maintain the temperature of the production fluids to prevent the formation of wax and hydrates becomes even more critical. Deepwater flowlines are particularly vulnerable to hydrate formation since the surrounding water near the seabed is constantly cold. The conventional method of maintaining flowline temperatures is by using a pipe-in-pipe system to provide the insulation. However, pipe-in-pipe is relatively expensive and more difficult and time consuming to install. This paper presents an alternative method of insulation utilizing the thermal properties of the seabed soils obtained by burial of the flowline in combination with a conventional insulation coating. This approach provides significant thermal benefits, which can rival those of pipe-in-pipe, and results in considerable cost savings to the project. The paper will discuss the thermal properties of seabed soils with specific reference to the deepwater conditions and their influence on the overall thermal behavior of the subsea flowlines. In addition, an economic evaluation will be presented comparing buried and coated flowlines with more conventional solutions.

Introduction

Flowline burial has been used in recent years as an economical and effective method to achieve thermal insulation. The benefits of buried and coated (BC) flowlines as compared to pipe-in-pipe (PIP) systems include:

  1. potential cost savings,

  2. an increase in allowable transient shutdown times, and

  3. longer duration between wax appearance and hydrate formation.

The primary purpose of this paper is to describe the most pertinent soil parameters and burial methods to verify the feasibility of using burial as an alternative for providing thermal insulation for deepwater flowlines. The information includes a description of preferred burial methods and potential improvements, input data for modeling the thermal and structural aspects of flowline design, and comparative cost analyses.

Flowline Burial Methods

Flowline burial is common in shallow water, mainly for protection against external loads such as fishing gear. There are a wide variety of trenching and backfill techniques available for submarine flowlines. Traditionally, a surface towed plow, a jet sled, or a tracked vehicle has been used to dig a trench in a variety of soil conditions and subsequently backfill soil over the rigid pipe.

In deeper water, the use of surface-towed equipment is much more difficult due to the control of the catenary of the heavy tow wire and control umbilical. Thus, the use of an ROV based jetting system as described by Bruton 1 has proven more feasible and economical for deeper water applications (>1000m). In general, deepwater sites are commonly associated with soft clays which are ideally suited to the use of jetting-trench systems.

The "Talon" ®?burial system (Figure 1) is a good example of a proven jetting system that is designed to trench and subsequently bury pipelines up to 24-in (600 mm) diameter2. Other jetting trenchers include the tracked type shown in Figure 2.

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