Studies on Dynamics of Suction Piles during Their Lowering Operations

Suction piles are used for anchoring the mooring lines at the seafloor. One of the challenges of their installing is the occurrence of the heave resonance of the pile-cable system and possibly the heave induced pitch resonance during the lowering process. When the heave and/or pitch frequency of the vessel which operates the lowering of the pile matches the heave natural frequency of the pile-cable system, the heave resonance may occur, resulting in large heave oscillations of the pile and thus significantly increasing loads on the lowering cable and lowering devices. Furthermore, the large heave may resonantly induce the pitch of a pile. To predict and possibly mitigate the heave/pitch resonance of the pile-cable system during the lowering process, it is crucial to under the mechanism of heave induced pitch resonance and estimate the added-mass and damping coefficients of the pile-cable system accurately. The model tests of the forced heave excitation of pile models were first conducted to investigate the added-mass coefficient for a pile model with different opening area ratios at its top cap at the Haynes Coastal Engineering Laboratory of Texas AandM University. In the model tests, it was observed that the resonant heave may occur if the heave excitation frequency matches the related heave natural frequency and the pitch resonance may be induced by the heave resonance. The results of the following theoretical analysis and numerical simulation of the heave excitation of the pile-cable system are found to be consistent with the related measurements, which is helpful to further understand the physics of lowering a pile-cable system. The results of this study may be used to determine the magnitudes of total heave added-mass and damping coefficient of a pile and the heave natural frequency of the pile-cable system based upon its main characteristics. The heave induced resonant pitch is found to occur when 1) the pitch natural frequency is roughly equal to one half of the heave natural frequency and 2) the heave excitation frequency is approximately equal to the heave natural frequency. If only one of the two conditions is satisfied, no significant pitch resonance will occur. These results may have important implications to the operation of lowering offshore equipment to the seafloor in deep water.