INTRODUCTION

This chapter is concerned with the suitability of long and wide piles for different subsea applications A brief overview of the development of large hammers for driving long jacket piles is presented, and the suitability of wide piles for other applications is discussed Methods of interconnecting wide piles are described, and finally there is an account of the development and testing of a launching system for piling off small vessels.

Note In the context of this chapter, a "long" pile has an aspect ratio (length breadth) in excess of 20 1, whereas a "wide" pile has an aspect ratio of nearer 5 1

APPLICATIONS FOR OFFSHORE PILES

Applications for offshore piles may be divided into three broad categories

  • Jacket piles to support production platforms. Pile loads are principally axial compressive, supporting the structural weight, with some lateral and tensile loading due to the overturning effect of storm conditions on the platform

  • Subsea structures such as templates and protective structures Pile loads are much smaller than for jacket piles, principal loads being horizontal or vertical, compressive or tensile, caused by snagging or deflection of fishing gear Self-weight compressive loads are generally small by comparison.

  • Anchor piles for permanent moorings of floating production systems, loading buoys and circumstances where sea-bed flowlines do not permit conventional moorings. Pile loads are principally horizontal, with some vertical tensile in extreme conditions

Since engineering solutions are predicated by available techniques and the mainstream of offshore hammer developments has been towards large hammers for jacket piles, a brief overview is appropriate

OVERVIEW
Mainstream piling techniques

The two principal techniques for installing piles offshore are (a) Impact driving and (b) drilling and grouting Other techniques such as jetting, vibration or jacking may be advantageous or of assistance in certain circumstances, but are of application to a more limited number of soil types, and are unreliable in others Offshore construction projects tend to be of much shorter duration than their onshore equivalents, employing more-expensive equipment The penalties of downtime are therefore more severe, so that driven or drilled piles have been most frequently used.

The principal application for offshore piling has been to support the weight of steel jackets and production platforms on sedimentary sods Bearing capacities up to 2000 tonnes/pile may be required, with typical pile diameters of 1–2 m to transmit the load.

Driven piles are better suited to such large-scale applications, because the reaction to the energy input is provided by the hammer itself, and because final pile sets provide a recognized measure of the capacity of the installed pile

By contrast, a drilling technique necessitates satisfactory torque reaction, and this presents problems at large scale In addition, the installed grout envelope around the pile is often irregular and unpredictable in shape, so that the pile must be proof-tested in-situ unless it is of very conservative design

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