A Simple Way to Drive Free-Standing Subsea Anchor Piles

ABSTRACT The paper describes the "Puppet System" that have made it possible to drive unsupported piles in deep water without the assistance of any structure on the sea floor to provide lateral stability. Theory as well as recent North Sea experience is presented. The systems are ideally suitable for subsea anchor piles. A "Puppet System" makes use of an underwater Hydroblok hammer. The system allows also the largest Hydroblok hammer to be used in combination with a very large pile. The hammer carries the pile during its downward travel from the surface vessel. It is a temporary connection that keeps pile and hammer safely together until the driving is started. Correct positioning, and giving the pile a certain orientation prior to driving, as well as bringing the pile into the required verticality or batter and maintaining it in that position have been exercised; monitoring the operation showed that all functions could be kept under full control using existing remotely controlled devices. Mathematically the problem has been fully assessed and a computer program has been made, that takes into account all possible influences. "Puppet Systems" in principle can be used in any waterdepth; they do not require diver assistance. Anchorpiles in deeper water may serve TLP foundation purposes as well as OTEC's. The paper shows an example that has been worked out for a waterdepth of 1500 m. Utilization of the Hydroblok hammer in the "Puppet System" yields real soil information, because of the hammer's ability to derive such dynamic soil properties while actually driving. Late 1978 the system found practical application for the first time, when a number of 60-in O.D. piles were driven in 480 ft waterdepth in the North Sea, without any lateral support other than the interaction between the soil and the pile. The driven piles serve as anchor piles, their verticality is well within ±1 degree. This operation involved two extra complications. A pre-attached heavy anchor chain connected approximately halfway the pile had to go down simultaneously with the hammer and the pile. The top of the pile had to be driven to a certain distance below the sea floor. SUBSEA ANCHOR PILES DRIVEN UNSUPPORTEDLY All offshore piledriving projects, as far as our knowledge goes, have one aspect in common; there is always a structure of some kind sitting on the sea floor prior to driving. The structure generally has sleeves to receive the pile. Once stabbed the pile derives its stability from the structure. Driving piles into Shell Oil's Cognac platform (1977) in the Gulf of Mexico, amongst other projects, was such an example. But, is it possible at all to drive a free standing pile underwater without a structure? It has been a desire for many years to be able to drive heavy anchor piles into the seabed without the help of a template to keep the pile upright at the moment of touch-down of the pile-toe and during the first hammer blows.