Underwater Pile Driving Hammer At The "Maul Field"

ABSTRACT The RU-300 Underwater Hammer was designed to enable crews to continue pile driving operations on offshore structures during periods of bad weather and high seas and when no derrick ship can anchor along or near the structure. Compared with conventional pile driving hammers running on steam, the underwater pile driving hammer operates on high pressure air supplied from an air compressor package located on the structure itself. With a 150 ton revolving crane, temporarily mounted on the work deck, this set-up makes the actual pile driving operation completely self sufficient and independent from large derrick ships employed during conventional pile driving. INTRODUCTION Following the discovery of natural gas and condensate in March 1969 by the Shell British Petroleum and Todd Consortium, a partnership was formed with the New Zealand Government owned Offshore Mining Co. Ltd which is referred to hereafter as SBPT. Offshore Mining Co. purchased one-half interest in all rights in the discovery in return for meeting half the cost of the development of the field, a half share in the entire output of the field and appropriate voting rights in the joint venture as a whole. Production from the Maui field required the construction of offshore platforms. The location selected for the first platform, Maui A, was at a point 22 miles off the southwest coast of Taranaki Province in 354 feet water depth in an area subject to high winds, strong sea currents and rough seas and the possibility of earthquakes. To provide complete stability the design called for the installation of a pin pile driven in the center of each of the four tower legs, followed by six skirt piles per leg. All 28 steel piles were 48 inches outside diameter and of varying wall thickness and had to be driven to approximately 240 feet beneath the seabed. The task of driving the piles was scheduled to be done by conventional methods, but our company was retained to employ their newly developed Underwater Hammer side by side with the conventional hammers for direct comparison and possible construction time and money saving. The development of the Underwater Hammer began after recognizing the need for a pile driving method eliminating the use and handling of pile followers, steam line connections between hammer and ship boilers and the tie-up of a costly derrick ship during driving. The most important consideration was to develop a method to work In all but extreme weather conditions. During pile driving by conventional methods, an appreciable portion of the blow energy is absorbed by the follower system a fact non-existing with the RU-300 with its ram striking surface only 15 ft. above the pile, instead of 200 ft. or longer with followers. The underwater hammer first developed to produce 200,000 ft-lbs of energy (RU-200) was later redesigned to develop 300 000 ft-lbs to meet the requirements of the Maui project. Two hammers were tested in a specially constructed test well 120 ft.