Underwater Modifications to the Brent Field Gas Export Facilities

1. ABSTRACT Underwater modifications to gas export facilities in the Brent Field included removal of subsea valves and replacement of gas risers on two production platforms. The work which required the assistance of four diving spreads was one of the most extensive diving construction programmes carried out in the North Sea. This paper summarises these diving operations and highlights the engineering design and installation methods used to accomplish the work over three years. 2. INTRODUCTION Standing in 460 feet of water Shell/Esso's Brent Field is located in North Sea Block 211/29 some 116 miles Northeast of Lerwick, Shetland Islands. The original gas export system included subsea Emergency Shutdown (ESD) valves on two of the field's four platforms. Concern about the safety and reliability of using these subsea valves in extended underwater service led to a decision in early 1979 to remove them. Instead, above water ESD valves would be incorporated in the upper risers on each platform where they could be maintained throughout the operating life of the gas export system (see Figure 1). The planned scope of work included the following activities:On Brent Alpha (a steel jacket) - removal of one 24-inch valve on the gas export riser which also entailed replacement of the upper 300 feet of riser pipe (see Figure 2-A). Restoration would be accomplished using two flanged spool pieces and a vertical hyperbaric welded tie-in.On Brent Bravo (a concrete gravity platform) removal of one 36-inch valve on the gas export riser and one 30-inch valve on the gas import riser, both located 270 feet down on the cell tops (see Figure 3-A). The redundant 30-inch crossover valve and pipework would also be removed. Restoration of each riser would be accomplished using multiple hyperbaric welded pup pieces. One year later the work scope was extended to include replacement of both gas risers on Brent Bravo (see Figure 4). This would require onshore fabrication of new riser spool pieces for offshore installation and connection using a combination of flanged and hyperbaric welded tie-ins. Replacement of the 36-inch riser would be accomplished using two spool pieces, the upper section measuring some 350 feet long and weighing 90 tonnes. Replacement of the 30-inch riser would be similar; however, a third spool piece would be required to cross over the cell tops. To facilitate riser spool installation 51 original "fixed" riser guides would also be replaced by "adjustable" guides which could be easily aligned by the divers. This paper presents an overview of these underwater diving construction operations briefly covering the work on Brent Alpha followed by a more detailed discussion of the various activities carried out on Brent Bravo. 3. BRENT ALPHA-SUBSEA MODIFICATIONS The removal of the 24-inch valve and replacement of the upper 300 feet of the riser on Brent Alpha was carried out by two diving spreads; Brown & Root's semi-submersible laybarge "SEMAC 1", and Comex Houlder Diving Ltd's (CHDL) dynamically positioned semi-submersible "UNCLE JOHN". The work entailed two unique operations, the replacement of riser guides and a vertical hyperbaric weld.