Assessment of Existing Structures and Repairs - Offshore Structures: Design, Construction and Maintenance - page 452

Together, these loading provisions provide a package that is intended to
model platform hydrodynamic loading more realistically.

7.2.2 Regional Environmental Design Parameters
Some of the design parameters are the responsibility of the platform owner, as
stated in API RP2A, and are usually requested by the engineering companies.
RP2A did not contain any recommendations for regional design parameters
until the seventh edition (1976), when guideline wave heights for the GoM
were introduced. These values were referred to as the reference level, where
it is really used as a guide only, along with guideline values for Morison coeffi-
cients, wave steepness (1/12) and other specific guidance (current was not men-
tioned). By the eleventh edition (1980), guideline wave parameters were
provided for 10 areas in U.S. waters, covering offshore Alaska, California and
the Atlantic coast. This has gradually been increased, and the twentieth edition
covers 20 areas.
There are specialized companies and government authorities in most coun-
tries that can provide environmental data. The environmental data are obtained
from mathematical hindcasting and predictive forecasting based on huge data-
bases of environmental data (such as wind and wave measurements) and are
supplemented with hurricane data taken from aircraft and free-floating buoys.
It is understood that the peaks-over-threshold method is used to estimate signif-
icant wave height from 3-hour sea-state data. According to Forristall (1978) , the
short-term distribution of wave height within a sea state is assumed to be based
on a 2-parameter Weibull distribution.
As illustrated in Table 7.2 for omnidirectional wave heights in the GoM, the
guidelines were very uncertain to begin with, and, with time, the range of values
has been narrowed.
API recognized that, based on specialist oceanographic and hydrodynamic
advice, alternative metocean design parameters can be used; however, RP2A
does not provide detail about a specific methodology to follow.

TABLE 7.2 Reference Wave Heights for the Gulf of Mexico

Reference Wave
Height (m)

Range of
Values (m) RP2A Edition (Year)

First - sixth (1969 - 1975) — —

Seventh - fifteenth (1976 - 1984) 21.2 17.4 - 25.3

Sixteenth - nineteenth (1984 - 1991) 21.64 19.5 - 23.8

Twentieth (1993 - ) 20.73 Directional factors