Thermal Insulation of Flowlines with Polyurethane Foam
Authors: Palle S. , Ror Logstor 
Published: October 2008

This paper will present the state of development concerning the use of Polyurethane Foam (PDF) as part of the sandwich construction of bonded pipe in pipe systems with steel jacket pipe. The concept of bonded pipe elements is that the thermal expansion of the hot inner pipe is constrained by the outer pipe. The range of thermal insulation properties and the strength properties will be given taking into account temperature and expected service life. The paper introduces considerations concerning design, production and installation.

Introduction

There is an increasing demand for thermally insulated flowlines for deepwater oil and gas fields. The bonded Pipe in Pipe system with the PDF is a cost effective method of achieving both excellent insulation and long reliable service life.

Polyurethane Foam

PUF is a material with excellent insulating properties. It has a long history of application in building and aerospace industry and in particular in preinsulated pipelines for district heating hot water distribution. The PDF is produced by mixing polyol and isocyanate together with a foaming agent. Today cyclopentane is used as the foaming agent because it has no impact on the ozone layer. contrary to freons which were used years ago. However, cyclo-pentane has the same stability and insulating properties as the earlier used freons.

Figure 1 Pipe production.(available in full paper)

With the traditional method the carrier pipe is centered in the jacket pipe with optional spacers and closed at both ends to build a confined space. The two pipes are positioned at an appropriate angle. The mixture of polyol, isocyanate and cyclo-pentane is injected into the annulus, where it reacts and fills the space between the pipes completely. Using the correct recipe and controlling the temperature of the pipe surfaces, the cleaning of the surface and the amount of injection, you can obtain a complete filling with no voids and with proper adhesion between the PDF and the pipes.

PUF insulation properties

The insulating properties of PUF are not a fixed value - they depend on a number of aspects such as: foam density, temperature and cell gas composition. To illustrate and assess the interaction between these factors, the thermal conductivity lambda value of the foam is divided into a number of components:(mathematical equation)(available in full paper)

The solid matter component contributes approx. 40%, the cell gas component approx. 50% and the radiation component approx. 10% to the total lambda value of PUF, while the convection is minimal. It is assumed that the contribution of the solid matter component is independent of the temperature, and that the contribution from gas and radiation can be described by one lambda value, as a function of temperature. Thus, the lambda value of PUF can be described as a function of both temperature and foam density within the temperature and density interval in question. On this basis, the following simplified model for the lambda value of PUF can be constructed:(mathematical equation)(available in full paper)

From a number of measurements of the lambda value of PUF with different densities, the functions Sand G have been determined.