Heat Transfer Optimized In Offshore Pipeline Bundles
CFD-based simulations are being used successfully in many less sophisticated industries to great effect, while the aerospace industry, driven by cost and safety considerations, had relied on it for at least 30 years.
Aerodynamists know precisely when to apply it and when to rely on wind tunnel or other simulations. As a result, the industry has improved lead times, enhanced product quality and developed a keen understanding of where costs can be saved.
Precisely the same benefits await offshore operators. The adoption of CFD would enable them to reach better decisions faster and more cost-effectively. In many cases, quick, low-cost CFD simulations that could be used to ensure 'experimental' simulations achieved a high probability of success. Typical applications might include helideck modeling and ventilation and turbine exhaust studies.
Applied Computing and Engineering Ltd. in the United Kingdom recently used its CFD modeling techniques to optimize the thermal performance of offshore pipeline bundles for Statoil contractors Brown & Root Energy Services A/S and engineering partner Rockwater. The North Sea bundles will comprise a vital part of the complex subsea systems being constructed for the Asgard and Gullfaks off and gas fields off Norway. They will run from templates on the seabed to the bottom of production risers.
Rockwater's goal was to determine the most cost-effective design capable of providing the highest levels of thermal efficiency for trouble-free shutdown and start up. The selected option, which had to be proved and optimized, was a bundle containing separate lines for wellstreams, injection gas and heating, in a single insulated sleeve.
A key design parameter for the operation of such subsea pipelines is normally the overall heat transfer coefficient, or U value. In cases where flowlines are insulated in a common sleeve, the steady state flow condition U value is not a characteristic parameter. It varies, based on individual flowline temperatures and corresponding heat fluxes along the length of the bundle. Consequently, it is difficult, if not impossible, to specify an overall U value coefficient, either for the bundle as a whole or each individual flowline. However, given specified flowline temperatures, it is possible using CFD models to evaluate the heat fluxes and effective U values for each pipeline over a short bundle segment.
As can be seen above, the bundle cross section comprises two production flowlines, a gas injection line and three heat-up lines, two warm-up and one return. The product and heat-up lines are contained in a common sleeve which is insulated and packed with nitrogen at seabed pressure.
The high pressure nitrogen facilitates increased heat transfer in free convection. After a long-term shutdown, hot water is circulated in the heat-up lines to elevate the temperature of the production fluids above hydrate formation temperature. To facilitate this process, the heat-up lines are strategically positioned to provide high view factors of the product lines which maximizes radiative heat transfer. In addition, the insulation is lined with reflective material to give higher emissivity values.
As shown, the sleeve pipe is contained within the bundle carrier pipe, which is flooded with inhibited seawater after installation to provide protection and bottom stability.
The ACEL CFD analysis modeled heat transfer rates between the flowlines and heat loss to the sea. This encompassed the complex effects of turbulence and radiative heat transfer. The analysis confirmed the effective flowline U values to be significantly lower than individually insulated flowlines. As a result, overall thermal efficiency could be seen to be considerably higher, proving the design's cost-effectiveness.