QUANTIFYING THE FATE OF HYDRATE INHIBITOR IN AN HPHT GAS CONDENSATE SUBSEA TIEBACK

This case history details the passage of hydrate inhibitor after being injected at a subsea well head in an HPHT gas‑condensate tieback. Produced gas and fluids were routed through in‑field flow lines, gathering at a subsea manifold 44 km from the platform before passing through a pipeline to the platform. Industrial methylated spirit (IMS) was the selected hydrate inhibitor and was used to protect the pipeline on a cold start after depressurization. There was very little data on the phase‑partitioning of IMS, and quantities could end up at the oil terminal where they would have a serious effect on processing and product quality. This paper summarizes the management of using large quantities of IMS (30 m3). Results from a series of injection trials at the oil terminal have been summarized and show how partitioning of IMS occurred through the gas, condensate and oil process. The paper then goes on to explain how this proved crucial in risk assessing the impact and use of IMS on the platform. The data has been used to improve the model (CPA equation of state and HYSYS) data sets and details of this have also been included. Extensive sampling exercises performed offshore when injecting and processing IMS (necessary in order to meet export requirements) have also been included and lead to details of phase‑partitioning behavior through the platform process plant. Such considerations are rarely made, nor lengths taken to understand the exact partitioning and fate of a chemical when injected offshore, as well as on arrival at the terminal. The implications in this instance were significant and therefore the exercise was necessary. The results show that exact quantification is very elusive and that relying on models and standard principles can be very misleading and potentially damaging.