Ten years after the installation of the Beryl platform, severe corrosion to several sea water lift pump caissons necessitated structural repairs. The repairs to the caissons were effected underwater using structural clamps to provide a structural load path and to seal large holes in the caissons.
The repairs were carried out entirely by remote control from the surface. There was no diver involvement. They are believed to be the first diverless undawater structural clamp repairs to an offshore structure. The reasons for adopting a derived solution were safety and costeffectiveness.
The design of the clamps used state-of-the-art technology on the load transfer mechanisms for clamp systems. The installation methodology was derived from a variety of onshore techniques, ROV technologies and new developments established within the project.
This paper discusses the design and installation of the repair clamps, including the various innovative features introduced to facilitate diverless installation. The implementation of structural strengthening/repair clamp systems using the diverless technology described herein has direct application in improving the cost-effectiveness and safety of repair and maintenance operations for existing offshore installations. At water depths beyond diving limits, diverless techniques represent the only available options for strengthening and repair. The technology described herein represents a major milestone in this field..
Mobil's Beryl Bravo platform was installed in the northern North Sea in 1983 and has a ‘traditional’ barge launched steel jacket with a number of caissons, including 10 used for seawater lift. The platform is operated by Mobil North Sea Limited on behalf of co-venturers Amerada Hess Limited, BG North Sea Holdings, Enterprise Oil plc, and OMV (U.K.) Limited.
During a routine annual general visual examination by ROV in 1990, two dead fish were found pinned by hydrostatic pressure to the wall of a seawater lift caisson. Following shutdown of the pump within the caisson, two 10mm diameter holes were observed in the caisson wall and, as the external condition and the CP potential were good, the cause was deduced to be galvanic corrosion from the inside following upgrading of the pump strainer located inside the caisson at the level of the holes.
A detailed survey was conducted the following summer, which identified severe corrosion of six seawater lift caissons, with lesser corrosion to a further four caissons. The depth range of the corrosion corresponded exactly to the locations of the pump strainers, confirming the hypothesised cause.
In 1992, Mobil appointed MSL Engineering as Consulting Engineer for the Beryl Bravo diverless repair project. Mobil also awarded a long term contract in December 1992 for the routine underwater inspection of the Beryl area facilities and associated Scottish Area Gas Evacuation (SAGE) gas export pipeline to SubSea Offshore Limited (SSOL). The contract allowed for the inclusion of this repair. Initially, a conventional diver-intervention repair was designed for implementation in 1992. Subsequent analyses of the structural integrity of the caissons allowed the repairs to be postponed to 1993. The analyses also revealed that at least the two worst affected caissons would require repair before the 93/94 winter. Following further design work in 1992, the feasibtilty of a diverless repair was demonstrated, recognizing the significant inherent dangers of diving operations on jackets.
