Determining Pile Capacity And Pile Driveability In Hard, Overconsolidated North Sea Clay
Authors: Durning Paul J. , Rennie Iain A. 
Published: December 2007

Heather Field Drilling and Production Platform foundation piles were driven into hard, overconsolidated clay. The required pile penetration was computed by several methods to take advantage of all field and laboratory tests, experience and different prediction methods. A single prediction method was not relied upon because of the high shear strength soil at the Heather site, the large pile diameters and the high pile loadings that are all outside the range of field load test records. An attempt was made to estimate the soil stress condition in the ground.

Methods for predicting the soil resistance during pile driving are discussed. The soil resistance at the time of driving is needed for pile driveability studies when using the wave equation prediction method. Predicted soil resistance at the time of driving is compared with estimated soil resistances determined from driving records.

Introduction

The Heather Field is located on Block 2/5 the UK Sector of the North Sea and lies approximately 100 km (64 mi) east of the north end of the Shetland Islands. The platform sits some 650 m (2130 ft) to the west of the original platform site in 143 m (470 ft) of water. A site investigation and foundation design were made for the original location. However, in early 1976 the platform site was moved in order to reduce pile penetration requirement and to improve the likelihood of driving the piles to the required penetration. The jacket and piles were designed and partially fabricated for the original site. Therefore, the present location had to be carefully selected to insure that the jacket cold be used without requiring a major redesign.

The general stratification of the soil consists of a deposit of stiff to very hard, sandy, silty clay to the maximum depth investigated. Between the depth of 40 m (130 ft) and 60 m (200 ft), successive layers of fine, very dense, silty sand were found. They varied in thickness from a few centimeters (inches) to 2 m (6 ft).

Pile Makeup

The base of the jacket is approximately a 61-m (200-ft) square at the seabed with the 24 skirt piles battered one on 10.8. The six 1.52-m (60-in.) diameter skirt piles at each corner of the jacket are spaced around the jacket leg on a 7.4 m (24 ft) diameter circle. Figure 1 shows a typical pile group arrangement. The piles were fabricated from BS 4360 grade 50D steel tubes with a continuous 64 mm (2.5 in.) wall thickness and were fitted with an open-ended, internal driving shoe. The driving shoe was 0.482 m (1.58 ft) long with an internal diameter of 1.35 m (53 in.). The internal upset was tapered with a 1 in. 3 chamfer and the outside corner of the shoe at the pile end was the chamfered at 45 ° leaving a 25 mm (1 in.) wide ring at the end of the pile.

Additional details on pile design, pile makeup and the pile driving sequence can be found in EUR 46.1

Site Investigation

In mid-1976 the new site investigation was conducted from the vessel Surveyor equipped for offshore soils investigations. Five boreholes were drilled extending westward from the edge of the buried valley that was encountered at the original site. Later in 1976, a program of four seabed cone penetrometer tests were made from the semi-submersible Choctow II. The five boreholes ranged in depth from 19.6 m (64 ft) at 81 to 89 m (292 ft) at B2. The seabed cone penetrometer tests penetrated the upper foundation soils to a maximum depth of 12.8 m (42 ft). From 296.6 m (973 ft) of drilled borehole, 170 samples were taken and 55 wireline cone penetrometer tests were made.

The borehole and seabed cone penetration test locations are shown on Figure 2 along with the position of the jacket. The position of the jacket on Figure 2 shows jacket leg A1, B1, B5 closest to borehole B4 and jacket legs A5 nearest to borehole B5. Soil profiles as determined from borehole soil samples are shown on Figure 3. The borehole profiles are presented as a section that traverses the platform site (shown as line XY on Figure 2).