Water Hammer Pile Driver

Abstract

ABSTRACT The water hammer phenomenon appears to be naturally suited to the underwater driving of large offshore piles. Attributes include: no pile extensions to the surface; no practical depth limitations, and driving ability increases with depth; good physical compatibility and impedance-match to the pile; driving can be near the mud line and in any direction; diesel-electric high-voltage primary power is utilized more efficiently due to the 1ow blow rate; high energy/weight ratio and greatly expanded rough-weather operational window. The foregoing advantages should permit substantial economies in platform and pipeline design and installation. Physically, a long, cylindrical cavity is created underwater by pumping out a water hammer tube fixed to the pile; then sea water is fast-valved into the resultant vacuum to act as the battering ram. The energy expended in evacuation is analogous to the potential energy of raising the conventional drop-weight. The basic hydromechanical principles are introduced by a numerical example presenting idealized operating parameters associated with a water hammer tube 3 ft dia × 250 ft long operating between 1-1000 ft deep: driving energy=3,000,000 - 100,000,000 ft-lbs/blow; the rectangular 100 millisecond force-time characteristic = 3,000 - 15,000 kips; calculated transformation efficiencies are 70% for force and influx velocity and 50% for energy. A 4000 hp submersible motor pump would permit 30 - 1 blows/minute. A comparison with conventional hammers is made on a power input basis. Some test results from a small scale, land-based, model are presented along with those from wave equation analyses. INTRODUCTION SAFETY. As the offshore structures get larger and are emplaced in deeper waters, their safety from storms, collisions, earthquakes, and settling demands more secure foundations. The time-proven method of insuring permanent foundations is by nailing to the sea-bottom using cylindrical piling of suitable size driven to an adequate depth. The pile's load capacity depends principally on the pile-soil friction. This determines the diameter and wall thickness necessary to drive the pile and then to resist the vertical bearing and pull-out forces. An additional wall thickness usually is provided at the mudline to accomodate the side-loading. Thus, requirements exist for driving piles several feet in diameter, hundreds of feet long, and weighing hundreds of tons. INSTALLATION. When the driving means is inadequate, palliative techniques such as pre-drilling, under-reaming and grouting, jetting, etc. are used. These incur multiple costs both in additional time, equipment, and personnel plus reduced load capacity. Similarly expensive are the pile extensions to above the water surface to accomodate the air/steam drop-weight hammers, especially in water depths exceeding a few hundred feet. IMPULSE. Unrelated to the operational "weather window" for offshore pile driving, there is a "force gate" requirement on the driving hammer - it must be force-matched to the driven load. If the force is too small to advance the pile, the pile and soil are simply compressed and the hammer energy stored in this elastic compression is returned in the form of hammer rebound. If the force is too great then the pile is overstressed, and its top becomes damaged.