Civil Engineering Reference
In-Depth Information
“
”
The term
has always been unclear. In the sixteenth edition (and
subsequent editions) the following sentence was added without amplification:
Where there is sufficient knowledge of wave current joint probability, it may
be used to advantage. In the draft edition of RP2A-LRFD that was published
in 1989, the definition for combined extreme wind, wave and current is dis-
cussed. The following three definitions are used:
associated
1. 100-year wave + associated current and wind
2. 100-year platform load
3. 100-year wave + 100-year current + 100-year wind.
Offshore structures are affected by wave loading, and the definition pre-
ferred by API is to use the l00-year return period wave height with the statis-
tically expected value of current and wind. It is noted that for structures whose
extreme fluid loading is not dominated by waves, any
combina-
tion of parameters leading to the l00-year return period load, such as base
shear or overturning moment, may be used.
Using the l00-year return period wave height combined with the 100-year
return period current speed and the l00-year return period wind speed is recog-
nized as conservative, and for some structures, for example in areas of the GoM,
too conservative.
“
reasonable
”
Deck Clearance or Air Gap
The air gap is a very simple and very important factor that affects structure
safety. Where waves strike a platform
'
s deck, a large force affects the deck;
however, it is only in the latest edition that general (nonregional) guidance is
offered in RP2A.
The air gap was first discussed in RP2A in the seventh edition (1976), when
regional guidance was introduced for the GoM. Then it was recommended that
use of reference level wave heights for water depths greater than 200 feet (61 m)
in the GoM should result in a deck clearance elevation of at least 48 feet (14.6 m)
above mean low water (MLLW). This allowed for storm and astronomic tides,
but it did not include an explicit safety margin.
When guidance for other regions in U.S. waters was introduced in the
eleventh edition (1980), tabulated values of corresponding reference deck ele-
vations were also included. The values included an appropriate safety margin,
and were unchanged until 1986 when the sixteenth edition was introduced.
Then, instead of providing values for minimum deck elevation, detailed
guidance was provided for its evaluation; this included an explicit safety margin
or air gap of at least 5 feet (l.5 m).
These recommendations still apply in the twentieth edition and the LRFD
version; however, they are now considered part of the general provisions and
not region-specific. In the latest editions it is also recommended that an addi-
tional air gap should be provided for any known or predicted long-term seafloor
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