Development of a Design Method and Implementation of Installation Procedures for Long Vertical Piles: MOSSGAS FA Platform, South Africa
Authors: Press A.J.C. , Zietsman J.F.W. , Rossouw Jan 
Published: October 2008

This paper describes the design method developed for the initial full stick up condition, during installation, of exceptionally long vertical piles, used on the MOSSGAS FA Platform in South Africa, An approach was developed in which the risks of exposure to resonance were reduced to acceptable limits. The analysis, and formulation and execution of the installation procedure is also described, The specially developed tools and the environmental monitoring systems, used for the operation, are outlined.

INTRODUCTION

The most sensitive operation during the installation of the first offshore platform, in 105m of water off the coast of South Africa, was the installation of the vertical piles. This activity warranted special consideration because of a number of factors which combined to make it particularly critical.

Firstly, the low capacity of the upper 67m of soil at the offshore site, resulted in the need for an arrangement of four corner clusters of six exceptional Iy long 84inch diameter piles. Sixteen of these required to be 145.5m long and needed to be driven to 122m penetration, with the remaining eight being 118.5m long and required a penetration of 95m. The use of vertical piles over battered piles was preferred as considerable cost savings were evident in both fabrication and installation.

Of all the previous projects researched, it was concluded that the 145.5m piles for the MOSSGAS platform would be the longest single piece vertical piles yet to be installed. A review of the industry codes and rules, governing offshore design practices, confirmed that no specific guidelines existed for the structural checkimz of vertical piles.

These factors were further aggravated by difficulties encountered in retrieving continuous core samples of a hard limestone crust at the seabed, that overlays the substrata. This led to uncertainty concerning the exact geotechnical properties of this layer, which together with a lack of any previous offshore piling experience for the region, resulted in the need to assume, for development of the piles installation procedure, that no initial self weight penetration of the piles would occur,

For this initial full stick up condition of 145.5m, it was established that the piles natural period of 12 seconds, would fall within the mnge of predominantly occurring seastate energy peak periods (10 to 13.5 seconds). A potential resonance problem was consequently anticipated, or alternatively, control of the operation by avoidance of such seastates, it was concluded, would result in high downtime. Furthermore, studies of environmental data enabled us to conclude that the seastates offshore South Africa would be persistently higher than those in the North Sea.

As a result of these observations a delailed programme of studies was instigated to establish the bounds of this envisaged problem with the objective of formulating a suitable design checking method and establishing an installation procedure.

ANALYSIS OF ENVISAGED PROBLEM

Resonance in Irregular Seas

The perceived resonance problem was based on a line up of the natural period of the free standing piles with that of the peak energy period (Tpeak) of the seastates.