Abstract

The Laminaria FPSO installed in 1999 represented the first use of suction piles in the calcareous soils offshore Australia. This paper will describe aspects of the site investigation performed, it will discuss the a-prioi predictions for the suction pile installation characteristics and will present a detailed back analysis of the actual measured data obtained during the suction pile installation. In light of these findings, centrifuge model tests reported previously at OTC will also be reinterpreted. The implications of the installation characteristics on the in-place performance will also be summarised.

Introduction

The Laminaria/ Corallina field is located in the Timor Sea in the Zone of Cooperation between East Timor and Australia. In October 1998, nine suction anchor piles were installed at this location to provide the mooring for the Northern Endeavour FPSO (installed in 1999) on behalf of Woodside Energy Ltd (WEL) and its partners. This is believed to represent the first use of suction anchor piles in a predominately carbonate material. The anchor piles were designed by a Kvaerner - Single Buoy Moorings consortium (KSC) and installed by Coflexip Stena JP Kenny (CSK). Advanced Geomechanics (AG) was retained by WEL to organise the site investigation and to provide the geotechnical interpretive report, to verify the design of the suction piles and to provide an interpretation of the measured installation data.

Historically, carbonate soils have proven to be problematic in offshore foundation engineering2. However, most of the previous problems have been concerned with uncemented carbonate sands and calcarenites, where very low skin friction may be mobilised for driven piles. At the Laminaria location the soil comprised carbonate mud, which superficially appeared very similar to normal highly plastic clays. For a predominately laterally loaded anchor pile, there was little concern as to the expected lateral resistance behaviour, and this was further documented through centrifuge model tests1. However, even for this type of anchor, an axial load component is applied due to the inclination of the chain at the buried padeye and also due to installation tolerances.

The Laminaria soil appeared to exhibit several "clay like" properties and consequently there was some expectation that traditional pile design rules such as those in API3 could have been applicable in this case. However, being acutely aware of the extensive problems experienced with Australian carbonate soils in the past and with conflicting data available, it was believed that a less optimistic approach was appropriate.

At one stage KSC proposed using a temporary lid for the suction piles but, after discussion, agreed to adopt fixed permanent lids. It transpired that this conservative approach was well rewarded.

This paper presents the story of the site investigation (which also included the first commercial application of the T-bar), presents key aspects of the suction anchor pile design and presents the installation data and an interpretation of the findings. A re-evaluation of the original centrifuge tests1 is also provided.

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