Technical Challenges of a High Pressure Heavy Wall Hot Tap Connection on the Bord Gáis Éireann Brighouse Bay Project

The UK onshore high pressure gas export terminal at Brighouse Bay in south west Scotland is a key strategic facility for Bord Gáis Éireann currently providing the predominant source of gas supply to Ireland via two interconnector pipelines that cross the Irish sea. A high design pressure of 150 barg combined with a low minimum design temperature (−30°C) has led to the use of heavy wall thickness station pipework, i.e. 508 mm outside diameter × 38.1 mm wall thickness ASTM A333 grade 6 (240MPa yield strength) seamless pipe. A requirement for a new hot tap connection at Brighouse Bay to improve security of supply identified several issues that needed to be addressed. Firstly, the normal UK requirement for 2 × carrier pipe thickness for the shell of the full encirclement split tee for the main branch connection could not be achieved due to the impracticality of rolling 76.2 mm thickness material to an internal diameter of only 508 mm to match the carrier pipe. Consequently there was concern that the area replacement ratio achieved by use of a thinner fitting may not be adequate for any additional site specific system loading despite meeting the ASME B31.3 code. Furthermore, the pressurised circumferential fillet welds made between the split tee and the carrier pipe may not be of sufficient size in view of the restricted leg length and hence resultant reduced fillet weld throat thickness. The parameters for the Brighouse Bay pipework in term of pipe material specification, pipe wall thickness and design pressure were also outside the range for which the existing UK hot tap welding procedure had been qualified. Hence a hot tap simulation assembly would need to be fabricated to qualify the welding procedure. In addition, the 38.1mm thickness Brighouse Bay pipework required PWHT in accordance with the ASME B 31.3 design code, but PWHT was not feasible for the hot tap connection. Hence there would be a need to demonstrate adequate toughness and fitness for purpose in the as welded condition. The paper describes the detailed approach taken to address these concerns which included preliminary on-site material sampling and NDE, evaluation and assessment of the project pipe and fitting materials requirements, pipework stress analysis, finite element analysis and engineering critical assessment of the split tee connection, and hot tap weld procedure qualification. The paper concludes by describing the successful hot tap installation phase of the project.