Hydrodynamic behaviour of perforated mudmat foundations

Abstract

Throughout the offshore industry large structures installed on the seabed use mudmats as a foundation to provide in sufficient bearing capacity and stability. Due to their size, the mudmats have a significant influence on the hydrodynamic behaviour and installation requirements. It is proposed to change the design of the mudmats from a near solid design to a perforated structure in order to reduce the total loads.
When describing the hydrodynamic behaviour of a structure in a body of water, two terms are of importance: the added mass and added damping. For solid flat plates there is much data available to compute the coefficients for models. However the coefficients are influenced by the degree of perforation and the presence of boundaries. Since there is little information available where the presence of a boundary is combined with a perforated structure, it is proposed to conduct experiments to elaborate on the hydrodynamic behaviour of a perforated structure close to an impermeable boundary.
A total of six scale models were constructed with a perforation ranging from 0 up to 75%, these models were oscillated in the water with different amplitudes, from 10 mm up to 160 mm. These oscillations were performed at different frequencies, from 0.2 Hz to 2 Hz. These tests were performed in still water and subjected to a uniform current of 5 mm/s and a current of 20 mm/s. All data obtained with these experiments was collected and analysed.
The first conclusion drawn from the analysis of the experimental data is that both the added mass as the damping decrease with an increasing perforation. The added damping is found to be larger for high excitation frequencies, however the difference decreases for increasing perforation ratios. The added mass is smaller for high frequencies, but this relative difference does not decrease for larger perforation ratios. Where the damping is not significantly affected by the presence of a current, it is observed that the added mass decreases when a current is present and this effect is larger for lower perforation. The added damping shows linear behaviour when plotted against the amplitude of oscillation, except for experiments where the smallest amplitudes are tested in combination with a low excitation frequency. For the latter experiments a more quadratic behaviour of the damping is observed.
The majority of the energy is found at the excitation frequency. For the added damping it is noticed that similar trends are distinguished for the first order and third order, the energy at three times the excitation frequency. At two times the excitation frequency there is only little energy found for the added damping. For the added mass there is no significant trend observed when comparing the first, second and third order energy at the load signal against the perforation. It is however found that there is a similar trend in the dependency of the added mass on the excitation frequency in both the second and third order, although the energy is decreasing for higher orders.