The purpose of a pipe-in-pipe flowline is to provide a highly insulated system to minimise the transfer of heat between the transported fluids and the surrounding ambient environment. The dry environment between the concentric pipes allows high performance dry insulation to be used, ultimately achieving a thermal performance greater than that which is possible with a simpler wet insulated flowline. For the exploitation of High Pressure / High Temperature (HP/HT) reservoirs, a pipe in pipe system can provide the necessary thermal insulation and the integrity for transporting hydrocarbons at high temperatures above 120°C – the current limit of conventional wet thermal insulation.
Compared to a wet insulated pipeline, the mechanical configuration of a pipe-in-pipe is inherently more complex and consists of several components (insulation, centralisers, water stops, field joints) which all have differing heat transfer properties. The magnitude of each component’s effect on the overall thermal performance must be understood during the design to avoid either under or over insulating the system potentially leading to operational problems and/or wasted expense. However, due to the relatively complex heat transfer processes at work, very accurate quantification of the effects is often cumbersome to calculate with little or no added meaning due to greater uncertainties in other parts of the design.
However, there are cases when the design comes together in a way such that a small change in insulation thickness may tip the design into requiring a larger sleeve pipe diameter to accommodate the extra thickness. As pipes are most economically purchased in standard diameters (8, 10, 12 inch, etc.) the jump from a 14 inch to a 16 inch sleeve pipe for example not only adds significant material cost, but may have huge implications to the installability of the pipeline.
Hence before making the jump to a larger pipe diameter, it makes sense to first evaluate the basis for the required thermal performance (e.g. the criticality, operational and flow assurance impacts of a marginally reduced thermal performance), and also the calculation methodology – often conservative simplifications are included early on in the design when the fabrication and installation implications are not fully evaluated or understood.
This article aims to provide an overview of the current thermal design approaches for pipe-in-pipe systems, and to highlight the choices available to the engineering team and the areas with scope for optimisation. The relative magnitude of the various resistances to heat transfer will be quantified, with focus on the most relevant aspects of the heat transfer with respect to pipe-in-pipes.
While this paper does not address active heating, the improvements to the passive insulation discussed would directly affect the power requirements of actively heated systems.
