The Effect of Deepwater On The Performance Of Slender Underwater Hammers

ABSTRACT Measurements of the impact energy of slender hydraulic underwater hammers show that the performance decreases as the hammer drives deeper under water. Energy is lost into the air circulating around the ram as it moves up and down, and probably also into the adhered water under the anvil. These energy losses should be taken into account in the design of piled foundations in deep water. 1. INTRODUCTION Slender hydraulic underwater hammers were introduced in 1980. Their slim shape introduced substantial improvements over the first generation of large diameter underwater hammers. Slender hammers are much more versatile. When given an extension pipe (Figure 1a), they can follow a pile through the guides, replacing followers without requiring any structural modifications to the jacket. When equipped with a pile sleeve (Figure 1b), they can drive freely riding on free-standing piles, and when the pile diameter is wide enough, even on a shoulder inside the pile (Figure 1c). The author's employer owns two types of slender hammer: the small MHU 195 with a specified net impact energy of 195 kNm, and the huge MHU 1700 with a specified net impact energy of 1700 kNm. Both hammers belong to the same series manufactured by Menck. These hammers have been in use since 1980. On two piling jobs in 1981 the pile driving with the MHU 195 appeared unexpectedly difficult. Several piles could not achieve their target penetration. At that time, the hammers were not equipped with an adequate monitoring system, so the source of the problems remained unclear. Since 1983, it has been possible to measure the impact energy of the MHU 1700s. The first time the measuring system was used in the field, during the pile installation of the Iwaki platform, offshore Japan, the hammer performed substantially less well than it had performed on test piles. Improvements to the hydraulic control of the hammer could not raise the energy to the target level. The cause could not be in the energy supply, as an underwater power pack with short hydraulic hoses was used, fed by an electric umbilical that had minimal energy losses. Suspicion was directed to the compressed air circulating around the ram. After the completion of the Iwaki installation, the vent ports in the inner wall of Heerema's MHU hammers (Figure 2) were enlarged. Exxon later reported similar problems with their MHU 220s during the installation of the Lena Guyed Tower in 1983 (1). Extensive instrumentation of hammer and pile disclosed that the source was indeed the circulation of the compressed air around the ram. The air could not escape quickly enough, and caused cushioning of the ram just before the blow. External piping around the hammer eased the air flow and enhanced the efficiency from below 50% to between 60 and 80%. Several instrumentations of the MHU 1700 were carried out in 1984 and 1985.