Transporting High Temperature Fluids: The Elgin/Franklin Insulated Bundle
Authors: Stirling J.R. , Summers J. 
Published: October 2008

Abstract
The paper will describe the design constraints and solutions for the transportation of gas condensate between Franklin and Elgin Fields. The initial wellhead conditions at Franklin are 860 Barg wellhead pressure, and 190°C topsides temperature. Franklin Wellhead Platform (WHP) will have no processing capacity, but will be remotely controlled from the Elgin Production/Utilities/Quarters platform (PUQ), 5.4km away. Applications are as follows:Maximum pipeline design temperature 165°C, maximum pressure 150 barg.Start up temperature 80°C, peaking at 165°C and then falling at end of field life.Minimum pipeline temperature of 38°C required to avoid wax/hydrate formation.Design PUQ temperature not to exceed 145°C, due to expansion.
A temperature regulator within the pipeline system was required. This was successfully achieved by installing a caisson surrounding the Franklin risers, which could accommodate nitrogen, circulate water/glycol, air, or insulation according to the temperature requirement. Having provided sufficient flexibility in the riser design to meet the varying temperatures over life of field, it was important to design the pipelines on the seabed to be highly insulated. This was achieved by installing a bundle containing the 2 flowlines, pipe-in-pipe, fully insulated with injected Polyurethane thermal insulation. The foam also providing a bond to the flowline wall and sleeve, thus sharing expansion stresses. The pipeline material tests proved the long term integrity of the mechanically bonded corrosion resistant alloys on the X65 steel for the onerous conditions. The automatic hyperbaric welding of 625 incoloy carried out in August 1999 is a first for the Industry, and will be significant for future projects. The controlled mid-depth tow, carried out successfully last year was for the largest diameter bundle installed in the North Sea.

Introduction
A significant current trend in the North Sea is oil and gas production from progressively deeper reserves. As a result the product is hotter and reaches the seabed at a higher pressure than in shallower fields, and this has led to the development of a growing number of high pressure/high temperature (HT/HP) pipeline systems. These pipelines are now commonly required to carry products with temperatures exceeding 140°C at pressures of several hundred atmospheres.
The high temperature and pressure involved present the designer and fabricator with challenges well beyond those typically encountered on more conventional pipeline projects. Specialist corrosion resistant materials are often required to protect the pipe from a sour product and a high performance insulation system is usually needed to prevent wax formation.
The design must not only account for the effects of elastic stress, but also plastic strain, which becomes far more prevalent in HP/HT systems because of the increased axial thermal loading and start-up/shutdown pressure variations. This calls for a far more comprehensive approach to the analysis of the pipe to determine acceptable levels of plastic strain, in order to keep the wall thickness to within an economically viable limit.