Accurate Vertical Pile Installation by Using a Hydraulic Vibratory Hammer on the Arbroath Project

Abstract In the Summer of 1989 the Arbroath platform was installed on behalf of Amoco in the UK Sector of the North Sea. A part of the project involved the installation of two docking piles, for locating the platform accurately over the template, and four under-leg piles for giving initial stability to the platform, before the skirt piles had been driven. The paper describes the use of a vibratory hammer, the ICE-14l2, during the installation of the docking and under-leg piles. It describes how the penetration by vibrator was predicted, and how during the initial stage of penetration, control was kept over the verticality of the pile. For the first time the pile inclination was measured continuously during penetration, providing the opportunity to correct the verticality as required. The paper highlights the principles and results of these measurements. The paper concludes that the combination of vibratoryhammer and conventional hydraulic underwater hammer can be very effective for underwater pile installation. The pile verticality can be effectively controlled by continuous measuring of the pile inclination during vibration. Introduction A part of the Arbroath Project was the installation of two docking piles and four under-leg piles, all with an outside diameter of 1067 mm in addition to eight foundation skirt piles. The docking piles were required for an accurate positioning of the platform over the existing template, whereas the under-leg piles were needed to support the jacket during the actual piling and to avoid uneven settlements during driving and when standing unpiled in a Summer storm. Because the verticality of some of the piles wasextremely important on this project and because of the uncertainty of the top soil layer, the piles were predriven with an ICE-14l2 vibratory hammer, and taken to final penetration by an IHC S-400 hydraulic hammer. Due to the water depth, the entire operation was performed under water. To ensure the verticality during the initial penetration of the piles, two inclinometers were mounted on the vibratory hammer housing. Hard wiring ensured that the inclinations were read out on a display on deck of the semi submersible crane vessel. Due to the smooth operation and because all piles were installed well within the tolerances, (the maximum pile out-of-verticality was only 0.75 degrees according to the instrumentation and 1.25 degrees according to a check by divers), the job was performed fully to the Client's satisfaction. The Arbroath Pr01ect The Arbroath project consisted of a 3300 tones platform, to be positioned with the aid of a crane vessel over a pre-installed sub sea wellhead template with a number of pre-drilled wells. The water depth at the locations was 93 m. The jacket was installed in 1989; the template having been installed 10 years earlier.