Skirt Pile Foundations in Offshore Platforms: Group Effect and Behavior in Shallow Waters

Abstract This study investigates the behavior and group effect of skirt pile foundations in offshore platforms located in shallow waters with a water depth of 178 m through 3D numerical models representing the soil–pile group interaction. The foundation includes four skirt pile groups located in the corners of the fixed offshore platform. Each skirt pile group includes 4 open-ended tubular skirt piles with a distance (S) between them of 6.17 m, each with a diameter (D) of 2.743 m (108 in), extending 120.5 m below seafloor (BSF). The study site is in the Bay of Campeche, Gulf of Mexico. The geotechnical parameters for the pile analyses were derived from a site-specific geotechnical investigation, which included exploration up to 125 m BSF. The site features calcareous clay deposits with shear strengths ranging from 5 kPa at the seafloor to 190 kPa at the maximum exploration depth. A numerical model using the Mohr-Coulomb constitutive model was developed to simulate load tests and analyze the load-settlement behavior of the piles. The model includes a 3D finite difference mesh, incorporating both the skirt piles and their connecting casing to the substructure and considers the stiffness of these elements in the transmission of loads, as well as the annular space that is filled with grout. The results indicate the group effect reduces the axial load capacity of the piles with S/D = 2.25 by approximately 33 %, which must be considered in the foundation design. Additional analyses were conducted for two other sites with piles extending to 150 m BSF, confirming similar load capacities and group effects reducing the axial load capacity of the piles by approximately 33 % and 35 %, reinforcing the study's findings. Ignoring this reduction in pile capacity induced by pile-group effect can result in an overestimation of the foundation's capacity, potentially leading to unsafe designs and structural failures. Therefore, incorporating the pile-group effect ensures a more accurate and reliable design, enhancing the safety and performance of the foundation. This study is part of ongoing research addressing the challenge of designing very tall offshore platforms in shallow waters in the Bay of Campeche and underscores the importance of validating the study with field or laboratory measurements. To achieve this, scaled load tests on skirt piles in marine soils are being conducted to refine and verify the design hypotheses for these foundations.