Abstract

Ultra-deepwater field developments require highly insulated flowline and riser systems. Coflexip Stena Offshore are developing a reelable system which combines the advantages of both pipe-in-pipe and heat tracing technology. Heating can be used either continuously or during shut down, to extend the cool down time or to elevate the flowline temperature.

Introduction

The risk of wax or hydrate formation is of major concern for the development of deepwater and ultra-deepwater fields. The heating of pipelines emerges as an attractive method to prevent deposition by actively maintaining the temperature of the flowline above a critical limit, typically in the range 20 to 40 °C. Coflexip Stena Offshore (CSO) are developing an electrically heated pipe-in-pipe (PiP) which combines high passive and active insulating performance. The results of a successful thermal test programme confirmed the high efficiency and potentials of the system. From the conclusions of these tests, a base design of a heated PiP design is presented. Issues related to the onshore fabrication and offshore installation, using the reeling technique, are also addressed.

Principle Of The System
Insulation Requirements.

Governed by flow assurance concerns, the general trend of deepwater and ultra-deepwater field developments is the requirement for highly insulated flowlines. The insulation efficiency is often defined on the basis of worst case scenario such as the cool down of a flowline. The hydrocarbon products circulating along ultradeepwater risers may experience a significant temperature drop due to the Joule-Thomson phenomenon. In this particular configuration, passive insulation alone, even of infinite efficiency, may not prevent the flow temperature from dropping below the critical level of wax and hydrate formation (Ref. 1). Heating provides a mean of maintaining temperature in the flowline. The duration and intensity of the heat input can be monitored to tailor the insulation requirements of the line throughout the field life. In several design cases, the requirements for passive insulation would be based on the less restrictive operational flowing conditions and heating would be used in the situation of a shut down, to lengthen the cool down time or maintain the flowline temperature.

CSO Heated PiP.

The heated pipe-in-pipe (HPiP) developed by CSO is a solution to industry requirements. Materials of low thermal conductivity, such as low density polyurethane, mineral wool or microporous panels, located in the annulus of the HPiP provide an efficient passive insulation. The heat tracing system is used when the passive insulation alone cannot prevent the formation of wax or hydrates. Centralisers are clamped on the flowline to transmit loads between the flowline and carrier. Finally, the HPiP is also fitted with optical fibres, which provide data on the temperature profile of the flowline in real time. Figure 1 illustrates a heated PiP prototype.

Description Of Heating System And Temperature Monitoring
Heat Tracing System.

The heating system consists of conventional heat tracing cables, in which alternating electrical currrent circulates. The core of these cables consists of a copper alloy of low electrical resistance. The heat power is proportional to the square of the current circulating in the cable.

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