Why do suction piles buckle? The Milkshake Problem.
When installing a suction pile, underpressure must be applied by an ROV’s pump to advance the pile from its self-weight penetration to its target embedment depth. Based on soil properties, engineers develop a required suction pressure curve to guide the pile design. For the suction pile to perform properly, the skirt wall must be thick enough, to be stiff enough, to maintain its shape during installation to resist buckling.
What is buckling?
Imagine drinking a very thick milkshake: when you draw too hard on the straw, it collapses on itself and you are denied ice creamy goodness. The straw’s skirt wall just buckled! The thickness was not adequate to maintain circularity during underpressure. At a malt shop, this is a simple matter of getting a new straw and drinking more carefully. Offshore, it means a ruined, expensive, heavy suction pile that needs to be extracted and replaced.
It would be easy to design a suction pile with a very thick wall that would never buckle, but it would be unnecessarily expensive, heavy, and cumbersome to handle. A little math (well, a lot, but the computer does it) goes a long way to make a suction pile’s design an ideal foundation solution. How do engineers prevent buckling? Non-linear finite element analysis is applied to a mathematical model of the suction pile. The model is distorted to a 1% out-of-roundness to reflect industrial fabrication tolerances and allow buckling to initiate. Linear springs are applied radially to the pile elements to replicate soil stiffness acting on the pile.
Once the primary buckling mode is established in an initial run, the model is perturbed to 10% of that deformed shape. Now the pressure is ramped up and the maximum displacement is recorded. When the rate of displacement accelerates, it represents the inelastic deformation of the pile. Sometimes a predetermined deflection limit will be used to indicate this, if optimization is not required. The pressure is recorded at this instance at the particular depth. This process is performed at different depths between self-weight penetration and the target embedment, until a structural allowable pressure curve is developed. This curve is reduced by a factor of 1.5 for safety. Combined with the geotechnical allowable pressure curve, the installation crew can confidently apply the prescribed underpressure to successfully embed the pile.