Mitigating Hydrates in Subsea Oil Flowlines: Consider Production Flow Monitoring and Control

Risk of gas hydrates occurring in subsea flowlines presents serious problems to offshore oil and gas production operations. Hydrate incidents can occur in oil and gas production systems provided that the favourable compositional mix and thermodynamic conditions exist. The principal method used to mitigate hydrates in subsea flowlines is the injection of hydrate inhibitor chemicals. Thermodynamic hydrate inhibitors (THIs) have traditionally been used but these have disadvantages, namely high OPEX with increasing chemical volume demand, space constraints, high CAPEX, and safety and toxicity concerns. Recently, low dosage hydrate inhibitors (LDHIs) such as kinetic hydrate inhibitors (KHIs) and antiagglomerants (AAs) which offer advantages of minimal volume and space requirements have received considerable attention. However, widespread application of these new classes of inhibitors has been limited by yet a number of concerns including water-cut and/or subcooling temperature range limits, lack of predictive models, toxicity, biodegradability, compatibility, and produced water quality and disposal issues. This paper presents a case study of an alternative “do-nothing” operational technique involving the comingle flow of well streams from different reservoirs at a pre-determined volume flow ratio, with temperature monitoring. Effective well-stream volume flow ratio was determined by predictive model studies of fliud systems hydrate equilibrium and pipeline operating envelops, followed by field trials. Results show that risk of hydrates in flowlines with multi reservoir fluids can be minimised by the control of the in-line fluid compositional mix by conventional flow control operation. Conceptual assessment studies suggest that this method of hydrate control could provide a cost-effective alternative to the use of chemical inhibitors, on a short-medium term basis, particularly in cases of seasonal hydrates and minimum-facility marginal oil production. Further study of effect of salinity of the produced crude on hydrate formation, given anticipated water-cut breakthrough with time, would further confirm the economics of this strategy on a field lifecycle basis.