Flowline Burial: An Economic Alternative to Pipe-in-Pipe
Authors: Loch Kenneth 
Published: October 2008

Abstract
Until recently, single skin flowlines were practically limited to short and medium length flowlines. To achieve longer flowline lengths with acceptable fluid outlet temperatures, it was common to rely on pipe-in-pipe (PIP) technology. However, PIP is comparatively difficult to manufacture, difficult to install, difficult to repair and all around more expensive than single skin flowlines. Recent advancements in flowline burial have demonstrated that buried and coated (B&C) flowlines have thermal properties that make single-skin pipe systems feasible for many projects thought possible only with PIP. The key parameter for such beneficial thermal properties of B&C systems is soil thermal conductivity. Recent insitu and laboratory investigations have discovered that the thermal conductivity of deepwater soils is far better than previously anticipated. B&C systems can provide the required steady state outlet temperatures for many long flowline systems at a fraction of the cost of equivalent PIP systems. In addition, due to their heat effected zone, the cool-down rates for B&C systems are far greater relative to PIP systems. This paper presents some of the new found thermal properties of B&C systems and the steady state and transient performance benefits such properties provide to oil/gas flowlines.

Flowline Heat Transfer Mechanisms
Heat flow through soil for buried, cylindrical, oil/gas flowlines can be primarily attributed to conductive and convective heat losses (Winterkorn and Fang, 1975). Conduction is a kinetic phenomenon where particles transmit their energy to nearby particles through a grossly static energy transfer mechanism whose dynamics are limited to molecular vibration, i.e., no water or soil particles permanently translate or rotate. Convection requires the movement of gross particles, generally being driven from high to low heat sources, with other particles replacing the original particles to maintain mass continuity. As will be discussed more later, deepwater soils appear to have low tendency for convective heat loss. If the fluid inside a flowline contains a non-ideal gas and there are significant pressure drops in the flowline route, the flowing fluid can experience the Joule-Thompson effect, whereby gas expands, causing a related temperature reduction. This paper focuses on the thermal benefits of flowline burial relative to PIP. Joule-Thompson cooling is relatively independent of flowline insulation options and does not change the relative thermal benefits of different flowline insulation systems. Thus, the Joule-Thompson effect will not be explicitly discussed herein except to note that its effects are implicitly included in some of the numerical results presented.

Nomenclature
OHTC (U) = overall heat transfer coefficient (W/m2-°K) Q = heat flux (W) A = reference area-circumference time length ?T = delta temperature (°K) Cp = specific heat (J/kg-°K) kn = thermal conductivity of layer n (W/m-°K) ks = thermal conductivity of soil (W/m-°K) rr = reference radius (taken as ro herein) (m) rni = internal radius of coating layer n (m) rno = external radius of coating layer n (m) H = burial cover from mudline to mid-pipe (m) D = total outer diameter of insulated pipe (m) PPF = polypropylene foam SPP = solid polypropylene PIP = pipe-in-pipe B&C = burial and coating