Field Experience Installing A-Shape Piles With a Vibratory Hammer
Authors: Ligterink Aart , Martin Robert 
Published: October 2008

The paper is the result of collaboration between a piling equipment manufacturer and a subsea piling contractor and describes the development of a novel rapid deployment high holding-power piling system.

The paper describes the installation of proprietary A-Shape piles using a vibratory hammer, the ICE 1412, in two key projects in the UK North Sea during 1991. On the first project 12 piles were installed in soft clay to support three specially desigend wellhead protection structures. In the second project, 10 piles were installed in very soft clay as anchors for pipeline initiation.

Furthermore, results are presented of the post-analyses for the different locations and the dynamic multiplication factors are given.

INTRODUCTION

This paper describes the installation of the proprietary A-shape Anchortech piles using an ICE 1412 vibratory hammer in two key projects in the UK Sector of the North Sea during 1991. In total 22 piles were installed in soft clay.

Most offshore piling operations are performed from heavy-lift crane barges as part of the installation of jacket foundations. Originally driven by topside steam hammers through followers, such piles are now generally driven by large subsea hydraulic hammers.

The recent growth in the number of subsea structures being installed for satellite field developments has increased the frequency of pile driving from DSV's. Using much smaller piling equipment, and with different vessel characteristics, different handling techniques have been developed from those used on crane barges. The operations described in this paper relate to this category.

A part of the Shell UK Exploration and Production Gannet project consisted of the installation of three over-trawlable subsea protection structures and the driving of four A-shape foundation piles at each structure. Because of the over-trawlability, the structures were to be smooth and flush on the outside, pile sleeves and guides were prevented and a special latching system was designed to connect the pile to the structure. This latching system required an accurate pile position and orientation. The design of the pile and latching system was aimed at minimising the risk of fishing gear snagging on the structure.

The Anchortech Piles were installed using a removable pile stabbing frame, eliminating the requirement for pile sleeves on the structure, in itself a 20% weight saving on each structure.

The pile handling and extraction capabilities of the vibratory hammer were proved to advantage in addition to driving performance. As part of the Amerada Hess Limited's Scott Project the A-shape initiation piles were installed with the vibratory hammer. In total 10 no. of initiation piles were vibrated into very soft soil to enable the pipe-laying vessel " Stena Apache" to proceed pipe-laying. Accurately positioned anchoring points were required because of the close separation (3m) of individual flowlines, and the proximity of existing pipelines. Furthermore, the anchor points should have sufficient holding capacity in the very soft clay.

Initially, it was the intention to extract the piles without use of the vibratory hammer, but by pulling vertically with the crane. piles.