Oscillation and Motion of Piles in Water Flow

Oscillation and motion of piles in water flow can have significant effects on the stability and structural integrity of pile structures. Understanding the factors that contribute to these oscillations is crucial in designing and constructing pile foundations that can withstand the forces exerted by water flow. One important factor to consider is the effective mass of the pile. The effective mass is the sum of the mass of the material of the pile and the mass of the water displaced by the pile. In the case of hollow tubular piles filled with water, the mass of the enclosed water must be added to the mass of the material. For steel tubular piles with relatively thin walls, the effective mass is approximately equal to the mass of steel plus twice the mass of the displaced water. Oscillation can occur in line with the direction of flow when the shedding of single vortices coincides with the natural frequency of the pile. Cross-flow motion, on the other hand, can be expected when the velocity-to-diameter ratio (V/ND) is 5. In-line motion, on the other hand, can be expected when the V/ND ratio is 2.5. The most severe conditions occur before the piles are braced together or tied into the deck structure. A notable example of severe oscillations during pile construction is the Immingham Oil Terminal in the Humber Estuary. Piles were driven through water with a mean depth of 23 m (75 ft) and ebb currents reaching a mean velocity of 2.6 m/s (5 knots). The piles, which were helically-welded steel tubes with outside diameters of 610 mm (24 in) and 762 mm (30 in) and a wall thickness of 12.7 mm (0.5 in), developed a cross-flow motion before they could be braced together. This cross-flow motion had an amplitude of ±1.2 m (3.94 ft) and resulted in many piles breaking off at or above the sea bed. Extensive research has been conducted to understand and mitigate pile oscillations. Design criteria have been proposed to determine the maximum current velocity that can be withstood by an unbraced pile structure and the maximum velocity before breakage of a cantilevered pile occurs due to cross-flow vibration. These criteria take into account factors such as the elastic modulus, moment of inertia, pile diameter, pile length, and effective mass. In cases where the observed current velocity exceeds the maximum values determined by the design criteria, measures must be taken to increase the natural frequency of the piles or to brace them together. Additionally, moored ships can transmit forces onto the piles supporting the mooring bollards due to current drag. The current drag on the ship can be calculated using appropriate equations. In conclusion, understanding the oscillation and motion of piles in water flow is essential in designing and constructing stable and resilient pile foundations. By considering factors such as effective mass, natural frequency, and design criteria, engineers can ensure the structural integrity of pile structures in various water flow conditions.