Commercial Deployment of Low Dosage Hydrate Inhibitors in a Southern North Sea 69 Kilometer Wet-Gas Subsea Pipeline

Abstract

Abstract Threshold Hydrate Inhibitor (THI) has been successfully commercially deployed at a low dose rate in a wet gas pipeline connecting the Hyde-West Sole gas fields to the onshore Easington gas terminal. These gas fields are BP operated assets and are located in the UK sector of the southern North Sea, approximately 70 kilometers off-shore of the Easington gas terminal. Three gas platforms Hyde, West Sole Charlie and West Sole Bravo are connected to a common 24" diameter flow line to shore and typical gas production lies in the range 50-180 mmscf/d (million standard cubic feet per day). The three platforms were sequentially converted from glycol to the new THI reagent. During the changeover period to THI reagent the pipeline operated a mixed system containing both THI reagent and glycol. As of 31st October 1996, the THI reagent has been deployed for fourteen weeks. In the last three week period glycol content of the produced water arriving at the on-shore terminal has been at a low level, typically <0.5 wt %. Introduction Natural Gas Hydrates are icelike crystalline solids composed of cages of hydrogen bonded water molecules surrounding 'guest' hydrocarbon gas molecules. Natural Gas Hydrate formation has been a major concern in Oil and Gas production systems ever since Hammerschmidt identified pipeline hydrates in the 1930s. The present technical solutions available to prevent hydrate formation in wet gas applications include methanol or monoethylene glycol solvents as inhibitors, triethylene glycol to dry gas and pipeline heating/insulation to keep the wet gas system outside the hydrate forming region. In recent years low dose Hydrate inhibitors have been receiving considerable industry attention. Replacement of the traditional thermodynamic inhibitors methanol and glycol is highly desirable from both commercial and Health & Safety considerations. The operating costs for these solvent based inhibitor treatments are expensive, and the off-shore facilities for these treatments can be complex and logistically intensive. From a safety perspective, it is becoming increasingly unacceptable to store large inventories of solvent on off-shore platforms. BP Exploration has been working for several years to develop a robust and cost effective low dose inhibitor technology which can be commercially deployed in its Oil and Gas production operations. From 1994 new Inhibitor technology has been developed in collaboration with Shell Research BV. THI technology is being commercialised by TR Oil Services (TROS) of Aberdeen. The present THI chemistry is effective up to 10 C of subcooling inside the hydrate forming region. The THI Project programme has progressed through three distinct phases initial laboratory screening, field trials and finally routine commercial deployment. This paper reports on progress so far with commercial deployment in the Hyde-West Sole gas field. Extended Cleeton-Ravenspurn Field Trial carried out in May 1995. Prior to commercial deployment of THI in the Hyde-West Sole system a further field trial was carried out in the Ravenspurn to Cleeton line in May 1995. This was the sixth field trial conducted on THI. This trial lasted for a total of thirty days which comprised 17 days of production and 13 days of pipeline shut-in (5 separate periods). There were two main aims of this final trial the first was to confirm that THI could function successfully over an extended period, and the second was to test the robustness of THI to shutdown and re-start conditions. P. 449^