A Simple Way To Drive Freestanding Subsea Anchor Piles
Authors: Jansz Joost W. , Brockhoff Henk S.T. 
Published: December 2007

Puppet systems have been developed to drive unsupported piles in deep water Puppet systems have been developed to drive unsupported piles in deep water without the assistance of any structure on the seafloor to provide lateral stability. These systems are ideally suitable for subsea anchor piles. Integral to the systems are modern underwater pile-driving hammers.

Introduction

All offshore pile-driving projects, as far as we know, have one aspect in common; there is always a structure of some kind sitting on the seafloor prior to driving. The structure generally has sleeves to receive the pile. Once stabbed, the pile derives its stability from the structure. The driving of piles for Shell Oil Co.'s Cognac platform in the Gulf of Mexico in 1977, among other projects, is such an example.But is it possible to drive a freestanding 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 pile-toe touchdown pile upright at the moment of pile-toe touchdown and during the first hammer blows.After 2 years of research, the answer has been found. It has been named the "puppet system." There are two methods, and both are described in this paper; not only the underlying theory but also the first practical application in the North Sea are reported.One indispensable part of the development of the puppet systems is the availability of underwater puppet systems is the availability of underwater pile-driving hammers. Today these hammers are pile-driving hammers. Today these hammers are reliable tools that fulfill the requirements of offshore pile driving.

Puppet Systems Puppet Systems When a pile hangs freely, supported only from its top, it is in positive balance. This is the case in air as well as underwater where outside forces such as wind or currents may affect this balance; nevertheless, we can call it a stable balance because it is a self-stabilizing system. This is correct as long as the pile toe is not supportede.g., by the seafloor. In that case, the pile system becomes unstable, especially when there is a hard soil, and the pile will topple.To solve the problem, it soon became clear that a stabilizing force must be introduced, preferably one that acts automatically and comes into force at the right moment and in the right sense. The stabilizing system also should be as simple as possible, preferably using only known and proved techniques. preferably using only known and proved techniques. The system should not complicate the total operation unnecessarily, it should be extendable to greater depths, e.g., 5,000 to 6,000 ft (1500 to 2000 m).During the 2-year research period, a large variety of possible methods was studied, of which two finally were selected for application in practice (Fig. 1).

Puppet System 1. This a method where only lateral Puppet System 1. This a method where only lateral soil resistance is used to stabilize the pile and hammer.

Puppet System 2. A solid mass, the puppet weight, Puppet System 2. A solid mass, the puppet weight, provides self-stabilization to both pile and hammer; provides self-stabilization to both pile and hammer; this method works independently of soil properties.

JPT

P. 2223