Civil Engineering Reference
In-Depth Information
subsidence. For the GoM, explicit guideline values for deck height have been
re-introduced: for deep water
>
500 feet (152.4 m), the recommended deck ele-
vation remains 48 feet (l4.6 m) above MLLW, but for shallower waters (depths
of 100 feet or 30.5 m), it has been increased up to 53.5 feet (l6.3 m).
The Latest Editions of RP2A WSD and LRFD
In general, the change to the loading guidance between subsequent editions of
RP2A has been small. However, with the publication of the twentieth edition of
RP2A in 1993 and the simultaneous introduction of the first edition of RP2A-
LRFD, the environmental loading guidance has been extensively revised and
updated. Several factors have been introduced or modified that are intended
to produce more realistic estimates of platform hydrodynamic loading.
The changes include:
Doppler effect of current on wave period, because a current in the wave direc-
tion tends to stretch the wavelength, making the wave less steep. Guidance is
provided for the evaluation of an apparent wave period.
●
Wave kinematics reduction factor, because
waves are not two-dimen-
sional. The kinematics reduction factor is applied to the horizontal particle
velocities and accelerations from two-dimensional wave theory in order to
approximate the effects of directional spreading. For tropical storms, the
factor given in RP2A is in the range 0.85 to 0.95, and for extra-tropical
storms it is in the range 0.95 to 1.00. The commentary in API RP2A pro-
vides some further guidance on assessing the reduction factor and also dis-
cusses an irregularity factor to account for the nonsymmetric shape of real
waves.
“
real
”
●
Current blockage factor: current speed in the vicinity of the platform is
reduced from the free stream value by
●
because the structure
causes the incipient flow to diverge. The factor is in the range 0.7 to 1.0,
depending on the
“
blockage,
”
of the platform. The multiplying factor on cur-
rent velocity is evaluated from:
“
drag area
”
1/
ð
1
+ ð
hydrodynamic drag area/4
×
frontal area
ÞÞ
Conductor shielding factor: depending on the configuration and spacing of
the well conductors, the wave force may be reduced due to hydrodynamic
shielding. Guidance on the reduction factor to apply to the drag and inertia
coefficients for the conductor array is given; the factor ranges from approxi-
mately 0.5 for very closely spaced conductors to 1.0 for spacing-to-diameter
ratios greater than four.
●
Current profile stretching: because the design current profile is specified
only to the mean water level, the profile must be stretched to the local
wave surface.
●
is preferred in RP2A, although linear
stretching is considered to be a good approximation in some circumstances.
“
Nonlinear stretching
”
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