The Prediction And Subsequent Measurement Of Pile Driving Behavior At The Hondo Platform In Santa Barbara

Abstract

It is important to be able to accurately predict the driving behavior of the piles to be installed predict the driving behavior of the piles to be installed with an offshore platform because of high offshore construction costs. This paper describes the procedures that were followed to investigate the accuracy procedures that were followed to investigate the accuracy of the wave equation pile driving behavior predictions for the Hondo platform. Also described is the manner in which pile head instrumentation driving data were used for comparison to the wave equation predictions. Introduction In the summer of 1976, Exxon Company, U.S.A. installed an offshore platform in 850 feet (259 meters) of water. This platform, known as the Hondo platform, is located 5 miles (8 kilometers) offshore and 23 miles (37 kilometers) west of Santa Barbara, California. The Hondo jacket is 865 feet (263.6 meters) high (Figure 1) and weighs 24,400 kips (107 MN) (Ref. 1). A total of 20 steel piles was required to anchor the jacket to the seabed. Of these 20 piles, 8 were main piles which extended from the top of the jacket piles which extended from the top of the jacket through the jacket leg penetrating into the seafloor. The remaining 12 piles were skirt piles that extended through the skirt pile sleeves, which were located approximately 110 feet (33.5 meters) above the seafloor, and into the ocean bottom. From a detailed analysis the required pile design penetrations were determined to be 340 to 375 feet penetrations were determined to be 340 to 375 feet (104 to 114 meters) for the main piles, and 250 feet (76 meters) for the skirt piles. The main piles had a 48 inch (122 cm) O.D. and the skirt piles had a 54 inch (137 cm) O.D. Because of the different penetrations and a sloping seafloor, the main pile lengths penetrations and a sloping seafloor, the main pile lengths varied from 1215 to 1255 feet (370 to 382 meters). Again because of the sloping seafloor, the skirt pile lengths, including the follower, varied from 1125 to 1145 feet (343 to 349 meters). All piles were fabricated from 50 ksi (345 MPa) steel. The skirt pile follower was fabricated from 36 ksi (248 MFa) steel. The main pile string was made-up of 13 add-ons which were welded together as the pile installation progressed. However, the skirt pile follower add-ons progressed. However, the skirt pile follower add-ons were not welded together but were fastened together using Rockwell Breech Block connectors. See Ref. 2 for details of the various pile makeup. From a boring at the Hondo platform site, the soils were determined to be fine-grained normally consolidated cohesive soils. Laboratory tests indicated that the soil sensitivity (undisturbed shear strength divided by the remolded shear strength), as well as the in situ shear strength, approximately increased linearly with depth, Figure 2 is a plot of measured undisturbed and remolded shear strengths for the soils at the Hondo site. PILE INSTALLATION STUDIES PILE INSTALLATION STUDIES Efforts were initiated to develop reasonable estimates of pile drivability to assist in hammer selection and pile installation procedure development because: There was no prior pile driving experience available in Exxon's area of interest in the Santa Barbara Channel. The Hondo piles were the longest piles (up to 1255feet, 382 meters) ever to be driven. There had been historic inability to accurately anticipate pile driving requirements in new areas. The "wave equation" analysis technique (Ref. 3) for predicting pile drivability was selected as the basis for a study of the pile installation. The study included an effort to determine methodology for establishing soil and hammer input parameters for the Hondo site. Since there were no available data concerning pile installations in soil in the Santa Barbara area and since the soil at Santa Barbara appeared to be made up of normally consolidated, fine grained materials, it was decided to develop the methodology for determining soil parameters for this installation using hindcasts of pile installations in various Gulf of Mexico locations. P. 95