Assessment of Existing Structures and Repairs - Offshore Structures: Design, Construction and Maintenance - page 463

structures from at least the late 1970s, guidance on marine growth was included
in RP2A only in 1993, with the introduction of the new loading provisions in
the twentieth edition. Detailed guidance on marine growth is now given in the
fourth edition of Den/HSE notes, but only descriptive guidance was given in the
1977 edition.
Due to increased knowledge and experience, the effect of breaking waves,
wave slam and slap can now be assessed.
Techniques have improved for allowing for wave-current interaction and for
stretching the current profile to the instantaneous wave surface. The so-called
mass continuity method was widely used until the 1980s, but this assumption
can be shown to be un-conservative. Until the mid 1980s, highly sheared cur-
rent profiles were assumed for design; today, a more uniform slab profile is gen-
erally preferred for the extreme design conditions. In the twentieth edition of
API RP2A, the Doppler effect of the current on wave period is included.
Another main area of change has been the hydrodynamic coefficients used
in Morison s equation. Considerable research has been done over the years to

quantify the coefficients. A variety of tests in wave flumes, involving multiple
and inclined members, and full-scale test structures have been undertaken. The
use of these data by operators in practice cannot be easily quantified.
The following Morison

'

s coefficients are based on information from one
major North Sea operator and may be considered indicative of general practice:

'

Up to 1971 C d = 0.5 C m = 1.5

1972 - 1980 C d = 0.6 C m = 1.8

From 1981 C d = 0.7 C m = 2.0

7.5 DEVELOPMENT OF API RP2A MEMBER
RESISTANCE EQUATIONS
According to OTO report index (1999) , every edition of API RP2A-WSD has
included the following text (with minor modifications to the wording) referring
to AISC for cases not covered by RP2A:

Unless otherwise recommended the platform should be designed so that all members are
proportioned for basic allowable stresses specified by the AISC Specification for the
design. Where the structural element or type of loading is not covered by this
recommended practice or by AlSC, a rational analysis should be used to determine
the basic allowable stresses with factors of safety equal to those given by this
recommended practice or by American Institute of Steel Construction AISC.

The earliest editions of RP2A did not contain any recommendations for
allowable stresses and relied totally on the AISC or rational analysis. This reli-
ance has been eroded over time, with more and more guidance being given in
RP2A. The later editions of API RP2A-WSD warn that the AISC load and