The paper describes the planning and installation of an offshore platform piled foundation in hard, overconsolidated clay, located north of the sixtieth parallel in the British Sector of the North Sea. The successful installation of the piles for the steel jacket platform in the Heather Field is used as an illustrative example throughout the paper. Engineering considerations necessary for a successful installation are outlined starting from site selection and carried through the final comparison between pile capacity, pile driveability and pile driving experience.
The Heather field is located on Block 2/5 of the UK Sector of the North Sea as shown on Figure 1. The Heather Platform is located at approximately 0 ° 57' 20" East longitude and 60 ° 57' 10" North latitude, lying approximately 100 km (64 mi) east of the north end of the Shetland Islands in 143 m (470 ft) of water. The oil field was discovered in December 1973 and a preliminary soil investigation was started in early 1974 with soil borings being taken during the drilling of the oil field delineation wells drilled from the semi-submersible West Venture. During the spring of 1974, a platform site was selected and five soil boreholes were made with the deepest borehole drilled more than 150 m (500 ft). After considering several platform proposals and the foundation conditions at the site, the steel pile-supported platform was selected for the field. The original plan was to drive a short, 1.5-m (60-in.) diameter pile, drill a 1.32-m (52-in.) diameter hole through the pile, then grout 1.22-m (48-in.) diameter insert piles. After carefully evaluating the merits of driving the piles made possible by the availability of larger pile driving hammers, the decision was made in the fall of 1975 to drive 1.52-m (60-in.) diameter piles.
A detailed geophysical study was made mapping the soil near the seabed. This study showed the platform location to be over a buried valley and the first 60 m (197 ft) of soil to be more disturbed than the soil column to the west. With the plan of reducing the required pile penetration, a new platform location was selected about 650 m (2130 ft) to the west of the first location. A series of five boreholes were drilled to verify the location of the buried valley scarp and to determine how far west the platform should be located. Based on the results of these boreholes, the final platform site was selected and the pile design finalized.
The pile design must satisfy three sets of criteria - structural, geotechnical and installation.
The structural criteria require that:
The number, size and spacing of piles must be such that the axial and lateral loads can be transferred from the jacket to the piles with acceptable stresses and deflections in the jacket and in piles.
A field-installed connection must be provided between the jacket and the piles.
The pile diameter and stiffness must be compatible with the soil stiffness and strength.
From the geotechnical point of view, pile design must satisfy three criteria as stated b Toolan and Fox1:
Each pile or group of piles must have sufficient axial and lateral capacity to carry the design loads with an adequate safety factor.
The deflection characteristics of the pile or pile group must be acceptable under long-term, transient loads.
The piles must be able to be installed to the required penetration with the driving hammers available.