New Thermal Insulations For CDTM Bundles:Foamed Polyurethane And Silica Sphere Slurries
Authors: Nelson D.O. , Wozniak Tom , Colquhoun Robin 
Published: October 2008

To help prevent wax dropout in oils and prevent hydrate formation in gases being transferred through submerged offshore pipelines, producers have sought to insulate lines by surrounding them with various thermal-resistant compounds, including solid and foamed polymers.

A method recently introduced into North Sea operations provides a more economical alternative by which a pumpable, gelling but non-setting slurry of hollow, high-strength silica spheres and seawater is pumped into the main annulus of a controlled depth tow method (CDTM) bundle. Minor percentages of cement and bentonite are included to impart desired colloidal stability to the slurry. The slurry is pumped into the main annulus of the bundle after it has been installed on the sea floor. By this method, the design is thus similar to that of one in which no insulation is required; this simplifies and economizes onshore fabrication and the CDTM operation.

This paper describes development research and the practical application of the improved methods under field conditions, with emphasis on silica sphere slurries and foamed polyurethane. Thermal properties are also presented.

Introduction

The technique of using a slurry of high-strength silica microsphere (HSM)l can be applied to flowline, gathering line, and transmission line CDTM bundles in which one or more of the lines requires thermal insulation. The technique increases the range of applications in which CDTM bundles offer a viable alternative to flexible flowlines.

Tests at Heriot Watt University have established slurry mix designs that combine low initial viscosity with gelling speed sufficient to prevent separation of the HSM from the seawater. In practical applications, use of the silica sphere method has shown several advantages. When using HSM grout, application of insulation to CDTM bundles can be deferred until late stages in the productive life of the field when throughput is decreasing and flow heat conservation is more important. This saves front-end capital investment as well as economizing on bottom-line cost. The HSM mixture also provides a contingency to back up or supplement other forms of thermal insulation.

Brief Recapitulation of the CDTM Bundle Concept

Groups of parallel flowlines can be made up under factory conditions on shore, bundled together, inserted into an outer steel carrier pipe and sealed in, pressure tested, then launched into the sea and towed to the offshore location between a leading and trailing tug, lowered into position on the seabed, and tied in to the wellhead manifolds, platforms, or floaters at each end.

The bundle must be designed to have the appropriate buoyancy at the appropriate time. The annulus and flowlines are typically filled with pressurized nitrogen before launch; the bundle is designed to have slight positive buoyancy in this condition, Ballast chains are attached to the carrier pipe soffit at regular intervals along the whole length and the amount of ballast chain is calculated to provide a slight overall negative buoyancy when the bundle is allowed to float freely