Assessment of Time Effects on Compressive Bearing Capacity of Steel Pipe Piles Driven in Clay Deposits of Persian Gulf

Offshore oil and gas extraction structures at shallow waters are conventionally supported by long driven steel pipe piles. In recent years, the direct CPT- or CPTu-based pile design methods have broadly been used to predict the bearing capacity of offshore piles in a more reliable manner. On the other hand, previous investigations have shown that the pile capacity is time-dependent (set-up and relaxation phenomena). However, time effects are missing in most CPT- or CPTu-based prediction methods. The main objective of this paper is to estimate the axial compressive bearing capacity of the offshore steel pipe piles driven in the marine clay deposits of the Persian Gulf based on some popular CPT/CPTu as well as static -based prediction methods. The estimated results are compared with the measured capacities obtained from the Pile Dynamic Analyzer (PDA) and the Case Pile Wave Analysis Program (CAPWAP). The measured values have been recorded at End-Of-Drive (EOD) and Beginning-Of-Restrike (BOR) conditions periodically up to nine months after pile installation. Then, the most reliable bearing capacity prediction methods are determined based on the shaft, base, and ultimate capacity values in short, medium, and long-term conditions. Here, five open-ended long steel pipe piles driven into very soft to hard marine clays of the Persian Gulf, Iran are considered to verify and evaluate the prediction quality of each method. It is shown that the ratio of predicted to measured ultimate bearing capacities obtained from the static analysis methods averagely have around ۶۴% more scattering than the corresponding values obtained from the CPT and CPTu-based methods. The results of the current investigation can be employed in offshore piling projects of the Persian Gulf in which the time constraints of installation do not allow running dynamic load tests at different time intervals.

Keywords:

Offshore pile , Bearing capacity , CPTu , PDA test , Time function , Set-up