Underwater Pile Driving Test Offshore Louisiana

ABSTRACT This report presents results of an underwater pile driving test conducted at South Marsh Island Block 130, offshore Louisiana. A 24inch diameter tubular pile was driven to 310 feet penetration with a submerged hydraulic hammer. The hammer drove the pile top from a position 33 feet above water to a point 175 feet below water. The test was performed as a part of technology development for constructing an offshore drilling structure in 1,050 feet water depth. PURPOSE AND SCOPE The pile foundation for platforms on the Louisiana Continental Shelf are currently installed with steam pile driving hammers, operated above the water. The purpose of this test was to demonstrate the feasibility of pile driving with the hammer underwater. Such a capability would make a significant impact on the cost of installing piling in very deep water because of the expense involved with extending the pile from the mud line to above water. Thus, the test was performed as a part of the design effort for the 1,050-foot water depth Cognac platform, since its concept included features which would allow the piling for the platform to be installed with (1) an underwater pile driver or (2) drilling and grouting methods. An early evaluation of these alternates indicated that the underwater pile driving would be several million dollars more attractive than cementing the piling in a drilled hole. HAMMER SELECTION After reviewing the status of several hammers with the potential capability of underwater pile driving, a hydraulic pile driver was selected for the test. It was the only hammer in field operational use which could be readily adapted for underwater use. Other available hammer designs were either in the conceptual or development stage. Also, the manufacturer had two similar but larger hammers (600,000 ft-lb energy rating) under construction at the time for use in the North Sea. Such a large hammer was considered adequate to drive the Cognac platform piling. Much development engineering work had been completed for adapting this hammer design for underwater use. Thus, this hydraulic hammer was chosen for the test because it presented the best alternate to test the underwater pile driving concept and provide a sufficiently large underwater pile driver in time for the platform installation. HAMMER MODIFICATIONS FOR THE UNDERWATER TEST Since the basic test objective was determining the ability of the hammer to drive at its rated energy while submerged in seawater, no attempt was made to solve the problems of positioning the hammer underwater. The hammer would be placed on the pile above water and pile driving would proceed until the mud line or refusal is reached. Divers would be used to reposition the hammer if it becomes necessary to remove it. Thus, the hammer was only equipped with devices for:holding it on the pile top,offsetting submerged buoyancy, andmaintaining a gas environment inside the hammer casing and a gas cushion around the pile top.