Subsea Installations Using Vibratory Piling Hammers

ABSTRACT During the summer of 1987 ICE hydraulic vibratory hammers were used 3 times successfully for subsea installations of anchorpiles and template foundation piles. The hammers proved to be, for the soil conditions encountered, a suitable alternative to conventionally used impact hammers. The present paper will briefly describe the results of the driveability studies on the basis of which the hammers were accepted and give full details of the three projects. The installations also proved that for the combination of small diameter piles and overconsolidated clays, pile plugging may occur and has to be taken into account in pile driveablity calculations. The second part consists of the description of the extension of the stresswave programme TNOWAVE for vibratory hammers and in which case this programme can be of great help to judge the reliability of the vibratory pile driveability studies. INTRODUCTION Hydraulic Vibratory hammers are ideal tools to drive, as well as to extract light-weight foundation piles. The piles presently designed as foundation piles for most subsea structures such as SPM-piles, (temporary) anchor piles, Plem piles, docking piles, template piles etc. can all be driven with hydraulic vibratory hammers. Specific advantages thereby are:–total handling weight of pile and hammerspread low compared to impact hammers (no pile-sleeve follower required).–vibratory hammer and pile are clamped together and can be handled as one unit.–the hammers are easily adjustable without costs to a wide range of pile diameters (up to 3.5m)–hammer can be activated before the pile tip touches the seabed avoiding the need for a temporary guiding structure at the seabed.–if soil-conditions are suitable for vibratory driving the installation speed is generally a number of times greater than with impact hammers. The ease in handling and (pile) preparation time furthermore reduces the overall installation time. Apart from being used as the only hammer for installations, the vibratory hammers should also be considered to a greater extent as complementary tool to impact hammers, i.e. for the initial driving of skirt and mainpiles, TLP-piles, Tripod structures etc. A specific feature of the hydraulic vibratory hammers worth mentioning separately is their capability of extracting piles. This feature enlarges the range of offshore foundation applications such as the use of temporary anchorpiles, temporary casings, auxiliary piles for temporary use, reaction piles for pre-tensioning anchors etc. Further applications in the extracting mode are to release stuck pipelines, stuck drillpipes, pulling a part of the conductor pile assembly for re-use of the conductor guides (directionally drilling) etc. To offer a more complete picture, the principle of operation of vibratory hammers will be explained first, followed by a brief description of the driveability analyses for these types of hammers, after which the actual installation will be dealt with in full detail.