Morison Equation Offshore Wave Force Calculator (Piles & Jackets)

Formulated by Morison, O’Brien, Johnson, and Schaaf in 1950, the Morison equation predicts the total inline hydrodynamic wave force on slender cylindrical members.

A structure is classified as "slender" when its diameter is much smaller than the wavelength (D/L < 0.2), meaning wave diffraction can be neglected.

The drag force Fd arises from flow separation and wake vortex shedding, scaling with the square of water particle velocity (u|u|).

The inertia force Fi arises from the pressure gradient of the undisturbed wave plus the force required to accelerate the added mass of fluid around the cylinder (du/dt).

Because velocity and acceleration are 90° out of phase in linear waves, peak drag and peak inertia occur at different moments in the wave cycle.

Large diameter piles (e.g. 8 m offshore wind monopiles) are inertia-dominated.

Small diameter jacket braces (e.g. 0.5 m) are drag-dominated.

• Marine growth: Barnacles and kelp increase effective diameter and double surface roughness, dramatically boosting drag loads.
• Wave-current interaction: Collinear tidal currents add vectorially to wave velocity, compounding quadratic drag.

• Structural design of oil and gas jacket platforms, offshore wind turbine monopiles, and marine jetties.
• API RP 2A, DNV-RP-C205, and ISO 19902 design code checks.

• Valid for slender cylinders where D/L < 0.2; large volume gravity structures require diffraction theory.