TDW Offshore Achieves Record-High Pipeline Pressure in N. Sea

TDW North Sea Tie-back
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TDW Offshore Services AS announced that two new gas production platforms on Shamrock and Caravel fields in the North Sea were tied in to an existing pipeline system on Corvette field last month. TDW Offshore Services provided SmartPlug services enabling hydro test activities to be completed at a pressure of 375 bar in a 20-inch pipeline system. The pressure is the equivalent of an axial load of more than 680 tons. This hydro test pressure is the highest pipeline pressure ever isolated by TDW Offshore Services, following more than 120 pressure isolation projects it has completed worldwide.

The pipeline isolation operation was carried out on behalf of Shell U.K. Limited, which required the tie-in of a new 16-inch gas export pipeline to the existing 20-inch gas export pipeline infrastructure. 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 375 bar. The Caravel, Shamrock and Corvette fields are operated by Shell U.K. Limited.

The project consisted of two different types of pressure isolations. The first isolation put in place at the beginning of the works provided isolation from pipeline gas inventory, which made it possible to complete sub-sea mechanical tie-in works. The second isolation allowed hydro test activities to be carried out. 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 30 bar. The two other trains created quadruple block isolation against a pressure of 375 bar in the new pipeline.

Typically, a high pressure isolation project ranges from pressures of 50 to 350 bar. TDW achieved a new world record by sustaining a full pressure hydro test of the completed new pipeline installation at 375 bar.

TDW began working on the tie-in delivery project during summer 2006, with extensive engineering, design and testing. In November 2007, the remotely-operated SmartPlug trains were pigged into position in the pipeline. The tools were pigged upstream of the sub-sea 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 SmartPlug train in a pipeline for a long period of time 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, assure that when this is required, our tools carry the highest safety and quality certificate standard, which is underscored by our extensive track record."

By using technology such as the SmartPlug tool during maintenance and up-grade works, Shell was not required to vent or flood the pipeline. Flooding the pipeline with water or another fluid is an alternative to the SmartPlug approach. However, this technique requires that pipeline gas and/or oil inventory be evacuated, and has a substantial impact on the length of the operation and associated costs.

TDW Offshore Services has carried out more than 120 successful pressure isolation projects around the world. The recent pipeline isolation operation for Shell in the North Sea illustrates how SmartPlug technology has once again proven its value as an effective pipeline intervention method, making it possible to carry out necessary maintenance and up-grade works, safely and efficiently.


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