Evaluation of Large-bore composite Cryogenic Hoses effects during Offshore LNG unloading – CFD Turbulence Modelling, Boil-Off Gas and Annual Energy Costs Sensibility Analysis

LNG industry has been moving towards E&P of offshore gas reservoirs. While in mild conditions the LNG transfer system comprises rigid arms coupled with swivels, in harsh environments flexible cryogenic hoses have the ability to withstand tension loads and twist. Their helically corrugated profile increases the friction losses and subsequently the BOG. At typical LNG unloading rates of 10,000 m3/hr, two 16” ID hoses are required to obtain reasonable pressure drop. Nevertheless, development of large-bore cryogenic hoses is seemed as a potential cost reduction.

Traditionally, offshore LNG unloading has been approached considering each component of the regasification terminal separately. The innovative software proposed here is able to combine CFD and gas processing simulations in HYSYS® to produce a statistical treatment of BOG and annual shipping costs at different operational conditions and design variables. Sensitivity analysis showed that pressure drop in cryogenic hoses may account up to 70% of the total pressure drop. Although the friction factor in these hoses is significantly important for the LNG pumps, 10% reduction of its value will affect BOG only 2%. When the transfer system is reduced to a single 20” ID hose, higher pressure drop increases the pump’s energy consumption by 17%. However, using a 24” hose, not only the pressure losses decrease, but also heat transfer reduces significantly and the total pump’s consumption decreases by 35%.

A CFD study of 3D fully developed turbulent flow at Re≈1.0x107 was carried out to validate three empirical correlations. The fluid domain consisted on a 16” ID hose, length of 1D and a mesh of 8,973,417 elements. Simulations showed an independent trend between friction factor and Re with discrepancies of 10%. Finally, the friction factor is mainly composed by two effects: the counterforce at the reattachment point and the skin friction at the top of the corrugation.

 

The Author

Jose Fuentes Casella
Salford University – UK