Physical and numerical modeling of offshore anchor piles under mooring forces

Floating Production Storage Offloading vessels (FPSOs) are widely used in offshore oil and gas industry in harsh environments reach at the Grand Banks. Vessels working at an offshore site must be held in position despite the effects of wind, waves, and current. Many FPSOs are keeping position using seafloor anchors which are commonly secured using anchor piles. Anchor piles are very effective in many soils. The pile can either be drilled in and grouted, using an offshore mobile drilling rig, or driven in with an underwater hammer or a follower. The anchor pile resists pullout by a combination of bending plus passive resistance and skin friction shear.;Correctly designed pile anchors should transfer the environmental loads on the floating platforms to the seabed safely. In-service, these anchor piles are subjected to a wide range of monotonic and cyclic lateral to oblique pull forces. The large cyclic forces applied during extreme storm will tend to govern the design.;The presented work aims at identifying the behavior and capacity of anchor piles used for anchoring offshore floating structures in dense sand. As full-scale experimental verification is not always possible, this raises up the need to design a physical model which can simulate the behavior of the full-scale case. To simulate the important gravitational component, the physical model tests were conducted using the geotechnical centrifuge to investigate the anchor piles response to mooring forces in saturated dense sand. Two centrifuge tests setup were carried out. In each test setup four model pipe piles were jacked in flight in homogeneous saturated sand and subjected to monotonic and cyclic pull-out forces with inclination angle 0° (pure lateral loading), 16°, 30°, and 90° (pure tension loading) with the horizontal. The soil pile interaction behavior was monitored through the strain gauges attached on the pile. While the undisturbed soil stiffness distribution with depth will be measured using a shear wave measurement system of bender elements which can provide soil shear modulus distribution with depth.;To study many factors that are affecting the model, a 3-D finite element model (FEM) was validated from the experimental centrifuge results. The validated FEM was used to do a parametric study to get design procedures and provide better understanding of the response of anchor piles to a variety of loading conditions. The parameters that will affect the pile behavior as suggested from the previous researchers and will be studied here are pile diameter, pile flexibility, load inclination angle, and padeye depth on the pile.;From the present study, it was found that there is a significant interaction between lateral and tension loading. A design method was proposed to predict the ultimate capacity of offshore anchor pile depending on pile flexibility, loading angle and padeye depth. Also, a design method was proposed to predict the maximum bending moment.