Bespoke solution for Qatar offshore platform

Edited by the Engineeringtalk editorial team Jun 2, 2009

Furmanite has recently completed a project to replace the upper sections of three risers and emergency shutdown (ESD) valves on an offshore platform off Qatar.

Furmanite carried out the project for Occidental Petroleum of Qatar, replacing the top sections of six, eight and 10in risers and ESD valves on the PS1D platform in the Persian Gulf, to enable the risers to be pigged for cleaning purposes.

The wall thickness (or schedule) of the existing topside risers was too thick to allow the cleaning pigs to pass through.

Initially the project appeared to involve a standard hot tap and line stop operation (involving drilling into the riser, using a valve to contain the pressure and inserting a plug), to isolate each riser and allow the top section of each to be replaced.

However, the isolation was required at a location on the riser below the ESD valve, some three metres above sea level in the splash zone, where a standard hot tap and line-stop completion plug and blind flange arrangement could be considered a potential leak path and could not be left in place.

The challenge, therefore, was to achieve and maintain the isolation at the required location, while cutting and replacing the line above this, which included the point of entry into the riser some two metres and a 5D bend away from the point of isolation.

Furmanite was contracted by Occidental to undertake a detailed review of the requirements and produce a full engineering solution for the project.

Furmanite's solution involved the supply and use of mechanical bolt-on hot tapping tees and temporary flange (an alternative used where welding is not permitted or is not suitable), coupled with a custom-developed plugging tool with a special launching arrangement.

This involved a set of umbilical hoses run through a steel tube, which was sealed into a housing using a glanded-seal arrangement.

Furmanite also performed the pipe-cutting operations to enable the old riser sections to be removed.

The procedure involved an initial standard hot tap and line-stop procedure to create an isolation and allow a pipe cut to take place above this, where the temporary mechanical flange was fitted.

The purpose-designed plugging tool was bolted on to the mechanical flange.

The initial line stop was then retracted and the bespoke plugging tool was launched to achieve an isolation at the required point (below the initial line stop).

This required deployment along a two-metre section of the riser and round the 5D bend.

With the plugging head in place and isolation achieved, the mechanical flange and plug-launching tool were removed and a second pipe-cutting operation was carried out at the lower point required to remove the section of riser needing to be replaced.

A new section of riser was then welded in place and the mechanical flange re-fitted to this new section.

The line plug could then be moved up the riser, and the process repeated until the point at which the new ESD valve was fitted, when the plug was finally retrieved through the valve.

The work was carried out in three mobilisations of five 24-hour days each, to work on each riser separately in December 2008 and January and March 2009.

In each case, the riser was shut down and vented but contained residual hydrocarbon gas and oil.

The project was successfully completed with all work in each phase carried out on time, safely and to the customer's satisfaction.