WAVE AND CURRENT LOADS
Wave and current loads generated forces applied to submerged structural members in platforms and floating hulls are analyzed through linear and nonlinear kinematics in accordance with the API RP 2A specifications.
The PSE software computes wave and current forces applied on the structural members. The wave kinematics can be established using either Airy’s linear theory or Fenton’s nonlinear theory.
NONLINEAR KINEMATIC THEORY
The linear kinematic theory is valid where the wave height is small compared to the water depth. On the other hand, the nonlinear kinematic theory, proposed by J.D. Fenton, solves the motion equations by representing the velocity potential and surface elevation with a Fourier series. The later method minimizes the error of each parameter governing the wave motion equations and is valid over the entire spectrum.
MEMBER WAVE LOADS
The input for the member wave loads consists of the following six parameters:
– Current profile
– Wave profile
– Marine growth profile
– Drag coefficient
– Inertia coefficient
– Shielding factor
EFFECT OF MARINE GROWTH ON MEMBERS
Marine growth increases the cross section diameter and surface roughness of the members, and it is defined by a set of elevation-thickness pairs.
The PSE software accounts for the following wave profiles and kinematic parameters:
– Wave period
– Incidence angle
– Elevation of the sea bed
– Elevation of the still water line (SWL)
– Kinematic reduction factor
– Crest position criterion
A preview of the wave surface profiles, velocities and accelerations at any point is readily available.
With the PSE Petroleum Structural Engineering® software, the current profile is described with respect to the sea bed. The current speed is defined by a set of elevation-velocity-angle triplets and the reduction of the current speed in the vicinity of the structure or the blockage factor is accounted for.
In order to combine the current with the wave profile, the current needs to be stretched, or compressed, to the local wave surface. Two stretching methods are available:
– The linear stretching method, also known as the Wheeler stretching
– The nonlinear method or hyperbolic stretching
According to commentary C.3.2.1 of the design code API RP-2A- 2003, the Doppler effect is accounted for by calculating an apparent period defined as the wave period as seen by an observer moving with the current.
The member forces, calculated using Morison equation, vary according to the position of the waves with respect to the structure. In order to obtain the maximum forces in the members, the critical position of the wave crest is determined by the program.