Pile Load Tests In Calcareous Soils Conducted In 400 Feet of Water From a Semi-Submersible Exploration Rig

Abstract

ABSTRACT An offshore pile load test programme was conducted in calcareous silts and sands off North West Australia. The tests were carried out in 408 feet of water from the semi-submersible exploration drilling vessel the "Ocean Digger". These tests form part of the foundation investigation programme for a proposed gas production platform at the North Rankin "A" location. Both driven and grouted pipe piles were tested, the deepest test being 400 feet below the sea bed. The results disclosed that ultimate skin friction capacity was in excess of 2000 psf for grouted piles, and 300–400 psf for driven piles. Details and results of the testing programme are given, including soil profile data, together with an outline of special equipment combinations and work techniques. The programmes successful conclusion confirmed the feasibility of utilizing a floating vessel and conventional drilling equipment to perform pile load tests to obtain reliable foundation design criteria in deep water. INTRODUCTION The North Rankin "A" Site, (Fig. 1) is located on the North West Shelf of Western Australia, 85 miles offshore from Dampier. It is the location of the recent discovery of a significant gas reservoir. There are no existing offshore production facilities in this isolated area, and there are only very limited local support facilities. Reconnaissance foundation test borings identified the foundation materials to be calcareous silts and sands with varying degrees of cementation. These initial test bores showed the in-situ material to have very high voidratios and high moisture contents, with corresponding low bulk densities. Experience at Bass Strait (Ref. 1) has shown that calcareous sands and silts require special care in evaluating their engineering behaviour, and that it is not possible to apply normal engineering techniques to evaluate pile capacities. The tower type structure proposed for the North Rankin location will require high capacity piles to provide adequate support. Consideration of pile performance in the calcareous soils indicated long grouted piles involving special installation techniques would be required to develop the necessary support capacity. The combination of the location, soil conditions and the type of piles under consideration, dictated the need for a detailed site investigation, together with pile load testing, in order to arrive at reliable design recommendations necessary for planning of the proposed platform. There was no record of a previous significant on-site testing programme carried out from a floating exploration drilling vessel, and this necessitated the development of a new approach to the test programme. The use of a semi-submersible exploration rig, imposed a series of capacity and time restraints which had to be accommodated in defining an acceptable test programme. Taking cognizance of the engineering information required at the site, and the limitations imposed by the use of the floating rig, a programme of investigation and testing was defined to permit evaluation of the soil engineering properties and their extrapolation to the design situation.