Improvements to Rich Gas Decompression Predictions for Offshore Pipelines

This paper describes how rich gas decompression predictions have been performed for use in the Battelle-Two-Curve ductile fracture model for application to offshore pipelines. The decompression behavior of rich gases in offshore pipelines is more complicated than for onshore pipelines for two major reasons, (1) the operating pressures typically are greater offshore than onshore, and (2) the effects of higher molecular weight components in rich gases are not well described by earlier methods. Although for lean gases the AGA8 method is very accurate for single phase gas and the GASDECOM method is reasonably accurate for gases which enter the two phase region, neither of these existing methods is sufficiently accurate for two phase conditions for gases containing more than about one mole percent hexanes and heavier hydrocarbons at pressures greater than about 15 MPa. This paper discusses the requirements for a method to achieve sufficiently accurate high pressure rich gas decompression predictions for use in the Battelle-Two-Curve ductile fracture model and provides examples of the use of an interim method which has suggested the path to an ultimate solution.