50 API RECOMMENDED PRACTICE 2T
c) In irregular waves, the potential theory and WFE predict a steady and a slowly varying horizontal force
called the wave-drift force. In potential theory this occurs at the difference frequencies of the wave
energy. A current adds to the drift force through an interaction with the viscous forces resulting from wave
kinematics.
d) In irregular waves, both potential theory and WFE predict sum frequency forces, which, when they occur
at pitch/roll/heave resonant frequencies, excite springing responses (Naess 1994
[195]
). In regular waves,
potential theory predicts double frequency forces.
6.5.4.1.2 Subharmonic Wave Forces normally have periods greater than 30 seconds and are produced by
the difference of wave components in wave frequencies (0.033 < f < 0.2 Hz, or 5 < T < 30 s). Subharmonic or
slow drift or difference frequency wave forces can cause large motions in surge, sway and yaw, if their natural
frequency falls in the range of the exciting forces. In TLP designs, these low-frequency forces cannot be
ignored. Due to the sensitivity to spectral peak periods, a range of periods should be checked.
6.5.4.1.3 Details of sub-harmonic and super-harmonic forces are described in Section 7.
6.5.4.2 Damping
6.5.4.2.1 Damping in resonant modes is important in the calculation of responses.
6.5.4.2.2 Verley and Moe (1980)
[240]
have published work investigating the drag for very small oscillations
of large cylinders. Model tests can be used to determine the damping, but the viscous effects are Reynold’s
number dependent. Care should be exercised when estimating full-scale damping values from model test
results.
6.5.4.2.3 Special attention should be given to the high Reynolds number, low Keulegan-Carpenter number
dependence of damping for vertical mode resonance.
6.5.4.2.4 The presence of current, waves, or both generally increases damping. (Wickers and Huijsmans,
1984
[245]
; Simiu and Leigh, 1983
[224]
). For severe condition responses, the damping should be estimated
using techniques such as model tests or time domain calculations.
6.5.4.2.5 The damping in the vertical modes (heave, roll and pitch) includes contributions from structural
and soil damping, as well as from hydrodynamics. These should be estimated with consideration for the mode
shapes and the strains in the various parts of the system. This is especially important in the calculation of the
springing response which contributes significantly to the tendon fatigue damage.
6.6 Ice Loads
Superstructure icing can affect tendon tension and increase local wind loads due to increased frontal area.
Wave induced motions of floating ice can impose local impact forces which should be considered in the
design of the structure.
6.7 Wave Impact Forces
Wave slap and wave slamming forces should be evaluated for local effect on structural or flotation members
and, if warranted, be included in the overall solution of the equation of motion. Wave slap forces on the
columns are a potential source of tendon “ringing” responses, and should be evaluated for design of column
structure. For hull external appurtenance design, wave slamming forces due to wave particle velocities and
wave run-up jets should be considered for locations subject to free surface encounter (possibly including top
of column).
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