Reliability Analysis of Piles in Multilayer Soil in Mooring Dolphin Structures

Reliability of complicated mooring dolphin structures (MDS) is estimated using few deterministic evaluations and an improved response surface method denoted as IRSM-second-order reliability method (SORM). It is a hybrid approach consisting of an IRSM, SORM, and several advanced factorial schemes. For this type of sophisticated analysis, simulation-based algorithm is impractical to implement. The concept is applied to estimate the risk of an existing MDS at the shore of Nile Delta. It is a large diameter steel-pile embedded in the sea bed. The Pile-Soil-System is represented by a nonlinear finite element model (NLFEM). In NLFEM, the steel pile is assumed to behave linearly under the considered working loads, but the soil is considered to behave nonlinearly. Moreover, the contact nonlinearity between the pile and the soil is taken into account. It is demonstrated that the reliability information on MDS can be extracted using tens of deterministic evaluations. It has been found that incorporation of the contact nonlinearity into analysis has no effect on the pile behavior. In the probabilistic analysis, the uncertainties in loading, material properties, and geometric details are taken into account. Both operational and structural limit states are considered. For the MDS considered in this study, it has been observed that the strength limit state (flexural) is more critical than the operational limit state (drift). The most important variables are the mooring loads, the radius and thickness of the pile, and the modulus of elasticity of steel.