ASSIGNMENT 1 Department of Civil & Environmental Engineering National University of Singapore 1 DATE 7 FEBRUARY 2022 REF NO. 5206-22-A1-0 PAGES 3 MODULE OT5206 OFFSHORE FOUNDATIONS INSTRUCTOR CHOW YEAN KHOW ACADEMIC YEAR 2021/2022 SEMESTER 2 SUBJECT PILE DRIVEABILITY ANALYSIS SUBMISSION DUE DATE: 21 FEBRUARY 2022 The foundations of a four-legged offshore steel platform at 60 m water depth in the Gulf of Thailand consist of open-ended steel pipe piles 36 in (0.9144 m) in diameter with two wall thicknesses of 1.5 in (38.1 mm) and 1.75 in (44.5 mm). The yield stress of the steel used to fabricate the piles is 345 MPa. Based on estimated long term static capacity of the piles and design axial load, a 110 m pile penetration depth (from the seabed level) was targeted. The total pile length is 188 m. Table 1. Pile Section Pile Section Section Length(m) Thickness (mm) Outer Diameter (m) S5 (top) 24.0 44.5 0.9144 S4 23.0 38.1 0.9144 S3 23.0 38.1 0.9144 S2 46.0 44.5 0.9144 S1 (bottom) 72.0 38.1 0.9144 The seabed consists of soft clay to stiff clay soils of increasing shear strength with depth. The design shear strength profile is shown in Table 2 and Figure 1. An average clay sensitivity of 3.0 was estimated from remoulded shear strength data. Table 2. Design shear strength profile Depth (m) Soil Type Undrained Shear Strength Su (kPa) Effective Unit Weight γ’ (kN/m3) 0.0 – 13.0 Soft CLAY 20 7.0 13.0 – 19.0 Firm CLAY 70 9.0 19.0 – 32.0 Stiff CLAY 125 10.0 32.0 – 55.0 Stiff CLAY 80 9.0 55.0 – 73.0 Very stiff CLAY 150 10.0 73.0 – 90.0 Stiff CLAY 90 9.5 90.0 – 110.0 Stiff CLAY 100 10.0 110.0 – 150.0 Stiff CLAY 125 10.0 OT5206 Assignment 1 2 During the installation, the piles were driven by a Menck MHU-500T hammer. No cushion was used during driving. The hammer input energy was varied by the hammer operator as the piles were driven. The recorded “hammer efficiency” defined as the ratio of actual input energy to hammer rated energy including the temp
orary pile length is given Table 3. The recorded hammer blow count per 0.25m is shown in Figure 1. Table 3. Actual Hammer Efficiency and Temporary Pile Length Pile Penetration Depth (m) Temporary Pile Length (m) Hammer Type “Hammer Efficiency” 0.0 – 15.0 118 – Self-weight penetration 15.0 – 25.0 118 Menck MHU-500T 10% 25.0 – 35.0 118 Menck MHU-500T 20% 35.0 – 50.0 141 Menck MHU-500T 35% 50.0 – 70.0 164 Menck MHU-500T 50% 70.0 – 80.0 164 Menck MHU-500T 60% 80.0 – 110.0 188 Menck MHU-500T 70% Figure 1. Soil Shear Strength Profile and Recorded Hammer Blow Count per 0.25m 0 10 20 30 40 50 60 70 80 90 100 110 120 0 25 50 75 100 125 150 Su (kPa) Su Design Su 0 10 20 30 40 50 60 70 80 90 100 110 120 0 10 20 30 40 50 Blow Count per 0.25m (CPT) P1 (Actual) P2 (Actual) P3 (Actual) P4 (Actual) De pt h (m ) OT5206 Assignment 1 3 As an offshore geotechnical engineer, you are asked to perform a back analysis and a calibration of the pile driveability parameters based on the recorded pile driving data for subsequent pile installations at the next platform locations. Perform the following tasks: a) Present in your report a “print screen” of your GRLWEAP input for the pile section properties, and the unit shaft resistance and unit toe resistance for the driveability analysis. State your assumption for the percentage of internal shaft friction. b) Report the soil resistance at the time of driving (SRD), blow count, and driving stresses versus pile penetration depth as per graphical output and tabulation from GRLWEAP. c) Compare your pile driveability analysis against the actual blow count data for all the four platform legs and comment on potential sources of discrepancy between the prediction and the actual blow count. Are the driving stresses in the pile within acceptable limits d) Re-do the pile driveability analysis (if applicable) to get a closer match to the actual blow count data by adjusting relevant parameters, e.g. modifying the sensitivity of the clay.