Internal Corrosion Monitoring of Subsea Production Flowlines - Probe Design, Testing, and Operational Results
Authors: Joosten M.W. , Kolts J. , Humble P.G. , Keilty D.M. , Blakset T.J. , Sirnes G. 
Published: October 2008

Abstract
The design, testing, and operational results from a subsea, electric-resistance probe system are discussed. Subsea monitoring provides the advantage of measuring the corrosion inhibitor efficacy at the point of injection, rather than inferring performance from platform measurements. The internalcondition of pipelines can be monitored in a variety of ways. Electric resistance probes are one technique for acquiring subsea corrosion rate data. The optimum monitoring technique will change with pipeline age, location, accessibility, and operating conditions. More importantly, the applicable methods may change based on the type of information required. For evaluation of corrosion inhibitor performance a high-sensitivity corrosion monitor is required. A prototype dual-element, electric-resistance probe has been evaluated for pressure and temperature stability under subsea operating conditions simulating the Britannia field. The probe functioned well under all test conditions. As expected, temperature had the greatest impact on the stability of the corrosion measurements. Comparison of the relative response of the dual probes to the variety of test conditions is useful in evaluating the validity of field data and the functionality of the probe.
The value of the field data from the subsea probes was apparent from an operational perspective. The development of a correlation between the topsides and subsea corrosion ratesis a useful prediction tool. The probes were successfully installed, demonstrated the sensitivity to detect design corrosion rates, enabled evaluation of corrosion inhibitor performance, and confirmed proper location of the inhibitor injection point. However, the overall performance of the probes has not been as favorable as experienced during the evaluation program. Unfortunately, one of the two probes has failed for unknown reasons during the first year of operation. Plans for replacement are being developed.

Intoduction
The Britannia gas-condensate field is situated 210 kilometers northeast of Aberdeen, Scotland, covers approximately 112 square kilometers of the UK North Sea. Recoverable reserves are approximately 3.1 trillion cubic feet of gas and 145 million barrels of gas condensate. Britannia's reserves are being developed through a single drilling, production, and accommodation platform at the east-end of the reservoir and a subsea well center with 14 slots. The duplex stainless steel subsea center is located 15 km west of the platform. It is commected with the platform via a carrier pipe containing the flowlines, methanol line, and water return line. This bundle is described elsewhere.1 The platform has 36 well slots and is supported on an eight-leg steel jacket in 140 meters of water. Britannia provides approximately 8% of the UK gas supply. Details of the development are presented elsewhere.2,3
The Britannia production fluids are corrosive. Corrosion inhibitor is injected into the subsea test and production headers.4 Corrosion monitoring subsea and on the platform is required to monitor inhibitor performance. Uninhibited corrosion rates for the Britannia subsea system are 10 mm/y. For reasons of economics, the design is based on steel flowlines (one 14-inch production line and one 8-inch test line) protected by corrosion inhibitor. The flowlines are designed for a 25-year life with a 25-mm wall thickness including a 2-mm corrosion allowance. The "allowable" corrosion rate for the flowlines is 0.08 mm/y. The three factors that influence the detection limits for electrical resistance probes are the corrosivity at the monitoring location, time, and scatter caused by physical environment changes such as temperature fluctuations. Accoun