The pipeline isolation operation was carried out on behalf of field operator Shell UK Ltd, which required the tie-in of a new 16-in gas export pipeline to the existing 20-in gas export pipeline infrastructure (see schematic). While carrying out the sub-sea tie-in, Shell needed to control the pressurized gas flow in the existing pipeline system. In addition, Shell requested full certification of the new sub-sea tie-in by carrying out a full hydro test of the new system to 375bar. The project consisted of two different types of pressure isolations. The first, put in place at the beginning of the work, provided isolation from pipeline gas, which made it possible to complete subsea mechanical tie-in works. The second isolation allowed the hydrotesting. TDW provided three complete plug trains, each of which were equipped with two isolation modules. One train created a safe and efficient double-block isolation against existing pipeline gas inventory of 30bar; the two other trains created quadruple block isolation against the pressure of 375bar in the new pipeline. TDW began working on the tie-in delivery project during the summer, 2006, with extensive engineering, design and testing. In November, 2007, the remotely-operated SmartPlug trains were pigged into position in the pipeline, upstream of the subsea tie-in point. While in the line, the tools were managed remotely from two platforms, the normally-unmanned launching and recovery platform Corvette and, as dictated by logistics and weather conditions, Shell's Leman platform. "The ability to keep the plug train in a pipeline for a long period is one of the significant advantages of the tool," explains Paul Christie, project manager for TDW. "Keeping tools in production offshore pipelines for lengthy periods is definitely not a pipeline operators' favourite task. We can, however, ensure that when this is required, our tools carry the highest safety and quality certificate standard, which is underscored by our extensive track record."