Monitoring of Beryl 'B' Platform Response to Pile Driving

ABSTRACT A monitoring program for investigation of jacket vibrations induced by plle driving on the Beryl 'B' platform in the North Sea is described. The objective of the program was to establish whether dynamic loads generated by large pile hammers could be damaging to various critical parts of the jacket or jacket appurtenances. Of particular concern were stress levels developed in pile sleeve-to-jacket connections, anode connections, and grout lines. Accelerometers were installed on pile sleeves and jacket legs prior to launch with a hardwire connection to deck level. After the jacket was mated to a subsea template, connections were completed to monitoring equipment aboard the vessel which tended the installation. Acceleration time histories from each of the measurement points were recorded during a test drive which employed the MRBS 12500 steam hammer on one- sleeve and the MHU1700 underwater hammer on the adjacent sleeve. Implications of the data in terms of current design practice are discussed. INTRODUCTION The Beryl ' ' platform is a fixed steel structure that was installed at a 120m (394 ft.) water depth location in Block 9/13 of the British sector of the North Sea during 1983. As described by Lenihan, et. al. (1984), the jacket was installed in record time, including time spent mating the jacket to a subsea template and driving the piling. Two pile driving systems were available during the installation. The first consisted of a series of steam hammers, operated above water, which drove the piles through followers. The largest available steam hammer was the Menck MRBS 12500. The second pile driving system utilized the Menck MHU 1700 hydraulic hammer. This hammer could operate underwater and drove the piles directly without the need of followers. The mechanical design and performance characteristics of these hammers have been documented by Heerema (1980). Prior to installation, there was concern that vibrations caused by the pile driving operation might reduce the life of connections between the jacket leg and pile sleeve and possibly overload certain appurtenance connections such as anode supports. This concern was supported by circumstantial evidence from two other recent North Sea installations that reportedly experienced anode support damage and grout packer failure as a result of stresses induced during pile installation. Two factors specific to the Beryl IBI installation suggested that the effects of pile driving vibrations should be seriously evaluated:Difficult pile driving with two of the world's largest hammers was expected due to heavily overconso1idated soils and the relatively large pile size of 1.83m (72 in.) diameter by 63.5mm (3 in.) wall thickness, andthe jacket would experience a large number of load cycles produced by the pile driving process since the foundation consisted of seven skirt piles on each of four jacket legs.