INTRODUCTION
Thermoplastic lined carbon steel pipelines offer a cost-effective solution to the prevention of corrosion in flowlines transporting hydrocarbon fluids. Field experience and an earlier Thermoplastics Linings JIP demonstrated that liner collapse is the major technical hurdle preventing more widespread adoption of this technology - particularly for multi-phase hydrocarbon applications. Liner collapse results when permeated gases, through the thermoplastic liner, collect in the annular space between the liner and outer steel pipe. During depressurization this accumulation could cause the liner to collapse. Venting these permeated gases is the key to successful application of thermoplastic lined steel pipelines for multiphase hydrocarbon transport. Phase 1 of COREL JIP (April 1998-December 1998) identified 3 design concepts with the potential to provide a means to effectively vent the annulus. The main objective of COREL JIP Phase 2 project (March 1999-August 2000) is to verify the potential of these 3 design options through proof of concept testing and analysis. If possible a field trial of the most promising technology will be undertaken towards the end of the project. Proof of concept testing within Phase 2 will be aimed at answering the following question as a priority: Can gas that has built up in the interface between the steel pipe and the liner vent at a sufficient rate to avoid liner collapse on depressurisation - or can the need for venting be eliminated?
The three most promising concepts have been identified as a perforated liner, externally longitudinally grooved liner and a liner incorporating an impermeable barrier (alurninum). Theoretical and experimental work has been undertaken to date and learnings have been derived from earlier, liPs and participants' in-house research. This paper summarizes findings to date.
Polymer liners have been used successfully as corrosion resistant barriers for new-lay subsea water injection pipelines over recent years in the North Sea and also used extensively for low pressure hydrocarbon service in onshore lines in North America and The Middle East. Liners enable the use of carbon steel pipe with a reduced corrosion allowance, as opposed to applying corrosion resistant alloys, clad pipe, or employing high doses of corrosion inhibitor. The purpose of the liner is to act as a barrier between the aggressive pipeline fluids and the carbon steel outer pipe. The reduction in both capital and operational costs can be significant. It has been estimated in some conceptual pipeline studies, either for onshore and offshore applications, that the cost savings of lined carbon steel pipelines can be between 25-50% of alternative material or operational solutions t. Corrosion is most prevalent in multiphase pipelines in which oil, water and gas phases are present, often with significant levels of H2S and CO2. Once the fluid has been processed the corrosive nature of the fluid is reduced. Hence, the opportunity for lined pipelines lies largely in infield flowlines, both onshore and offshore and tiebacks from satellite developments. Therefore, the application envelope for lined pipelines is likely to be in the diameter range 4-24" nominal diameter and pressures up to 350 bar (average pressure approximately 150 bar) and temperatures (at entry point) up to 160°C. This is borne out by a survey of operators which concluded that 68% of candidate kilometerage was at 50-200 bar pressure and 57% had an inlet temperature<100°C. However a significant proportion (21%) were in the range 100-200°C. An earlier thermoplastic liner JIP, for which BG Technology was the technical contractor, demonstrated that thermoplastics provided an effective corrosion re
