The thermodynamics of gas flow in offshore pipelines differs from that onshore. Offshore pipelines have higher density fluids with larger heat capacities and Joule-Thompson coefficients flowing at higher pressures and higher mass flow rates. Thus the environment has much less effect on the flowing temperature of the gas offshore than in the onshore case. The operational consequence is that compressor stations operating without aftercooling will likely deliver gas to the next station at higher-than-environment temperatures, significantly degrading compression efficiency. In one realistic case the required compression was almost 40% higher without aftercooling. The time required to establish a new thermal state following a change in inlet temperature is studied, and a formula for computing the thermal relaxation time is given. The relaxation time is useful in estimating the time to settle to a new thermal state after changing the inlet temperature.
The purpose of this work is to examine differences between the pipeline transport of gas in offshore and onshore environments. It is further to give quantitative insight into the causes of the differences which may be valuable in planning for design or operation of a pipeline. The conclusions are:
The suction temperature at a compressor fed by an offshore pipeline is likely to be much moredependent upon the discharge temperature of the upstream station than that in an onshoreenvironment. This results because mass flow rates and fluid heat capacities are significantly higher offshore.
The value of aftercoolers at the discharge of upstream stations is substantially greater in the offshore than in the onshore environment. In one example using coated pipe offshore a station downstream of a station operating without aftercooling would use 39 % more compression power than if aftercooling were practiced. The savings from the aftercooler comes in large part because of lowered suction temperature. Even when an offshore pipe was shallow-buried without a mastic wrap the cost of foregoing aftercooling is 24% in a quite realistic example.
For an offshore pipeline at steady state prior to a sudden drop in inlet temperature, about three changes of cold gas are required to produce a lowering of outlet temperature. In a typical 36- inch 85mile pipeline flowing 1.45 BCFD, the drop in outlet temperature occurred about 14 hours after a 40 "F lowering in inlet temperature. The first cold gas traversed the pipe in 4.7 hours.
Paradoxically in the foregoing example, the initial response to the 40" lowering of inlet temperature is a 1 degree & in outlet temperature over the first 13 hours.
Analysis of transient radial heat flow in a pipe-wrap-ground system following a change in pipe wall temperature leads to a formula for thermal relaxation time. This is the approximate time required to settle by a factor of e (- 2.7) toward a new steady thermal state.