A Procedure To Evaluate Pile Drivability in the Difficult Soil Conditions of the Southern Part of the Gulf of Guinea

Abstract

ABSTRACT This paper presents a method for evaluating the soil resistance to driving in the hard clays, very dense sands and rock strata encountered offshore Angola. The method is calibrated on typical case histories where 48" O.D. platform piles and 30" .D. conductor pipes were instrumented during driving. Results are applicable on most of the oil fields located in the southern part of the continental shelf of the Gulf of Guinea. INTRODUCTION The Gulf of Guinea is a zone of intense oil field developments. In the southern part of the Gulf, i.e. roughly south of the equator line, the continental shelf consists of a narrow band (50 to 100km wide) extending from Cap Lopez (Gabon) to Moss Medes (Angola) (Fig. 1). The water depth increases gently from 20 to 200m from east to west then dips suddenly to 1000m. The zone is characterized by a common geological history. The submarine canyon of the Congo river is the only outstanding topographic accident. In the past few years, several oil companies had to face serious platform installation problems due to major difficulties encountered during pile driving operations. Three types of problems have been identified:development of strong set-up effects in hard clays during add-on welding,premature refusal of piles in very dense sands,collapse of pile toes on limestone layers. The paper addresses these different topics and presents a procedure to predict the soil resistance to driving in the specific soil conditions of the region. The method is based on the experience gained during the first phase of development of oil fields located offshore Angola and operated by ELF AQUITAINE. Its application may be extended to the southern part of the continental shelf of the Gulf of Guinea, wherever quite similar subsurface soil conditions prevail (South Gabon, offshore Congo, offshore Cabinda). EVALUATION OF DRIVING HAZARDS The development of Block 3, offshore Angola, started in 1985. Four jacket-type platforms have been installed so far in water depths ranging from 65 to 90m. They were founded on 48" O.D. steel pipe piles driven by hammers with rated energies up to 700 kJ (500 000 ft.lbs). The wall thickness of the piles was comprised between 1" and 2". The 30" O.D. conductor pipes were also installed by driving. Premature refusals of piles and/or conductor pipes were observed during the installation of the first two platforms. Obviously, these difficulties were due to local specificities in the soil conditions: high degree of heterogeneity of the soil formations, presence of limestone layers with a random distribution, unusual response of some materials to dynamic sollicitations