Efficient Welded Joint Sealing System for Pipe with Thermoplastic Liners

Abstract

Thermoplastic pull-through liners are an effective method or preventing corrosion of carbon steel pipe. Offshore pipelines using thermoplastic liners require a means of welding pipe joints without damaging the heat-sensitive liner. Methods used to date include the use of corrosion- resistant alloy (CRA) connectors that place the plastic liner far enough away from the weld zone to prevent heat from damaging the liner, and thermoplastic connectors that utilize insulation and/or an annular air gap to prevent heat from damaging the plastic liner at the weld zone. In the case of the latter the thermoplastic connectors require electrofusion welding of the connector to the liner in the pipe, which is time consuming. This paper reviews the design and shop tests of a welded joint sealing system that permits the use of insulating thermoplastic connectors that seal to the liner in the steel pipe by means of a mechanical seal. This welded joint sealing system permits far faster joint assembly in the field than other methods, by avoiding the time-consuming processes of CRA welding or electrofusion of the thermoplastic connector to the thermoplastic liner in the steel pipe. The joint assembly process on a lay barge takes seconds per joint and is orders of magnitude faster than other methods, making it economically feasible for use in J-Lay and S-Lay vessels. Hydrostatic pressure tests have proven the welded joint sealing system to consistently prevent liquids from reaching the weld zone at pressures exceeding 500 bar. The key to achieving the mechanical seal is the novel means of locking the thermoplastic liner to the steel pipe wall. By preventing the liner from moving inside the steel pipe, it is possible to provide a fixed seat for the thermoplastic connector to seal against. Weld tests have proven that weld quality is unaffected by the connector and that heat exposure to the connector is below the degradation threshold of its thermoplastic material. The welded joint sealing system provides a cost-efficient, highly repeatable means for protecting the weld zone of thermoplastic lined steel pipe from corrosion. It makes it economically feasible to use thermoplastic liners for the transport of corrosive and hazardous liquids, including the use of specialty thermoplastic liners in case of high temperature sour oil and gas.