Assessment of Existing Structures and Repairs - Offshore Structures: Design, Construction and Maintenance - page 455

In 1993, the twentieth edition presented a modification to the peak hot-spot
stress curves to account for the new wave force recipe. This edition also intro-
duced a correction term to the S-N curves to account for the scale effect due to
plate thickness.

7.2.6 Pile Foundation Design
The pile is very critical in assessing the existing platform because the structural
analysis should include the geotechnical data. In some cases, geotechnical data
for the nearest area are used as a guideline for performing the in-place analysis
or the nonlinear analysis. Note that, for older platforms constructed from 1970
onward, the design depends on the practical wisdom and capability of the instal-
lation contractor. Pile foundation design guidance was initially limited in API
RP2A. In the first edition (1969), safety factors were 1.5 and 2.0 for the extreme
and operating conditions, respectively, and these have remained unchanged
until the last edition.
Design guidance for the axial capacity of tubular piles in clay was initially
based on engineering practice that had previously been followed for about 30
years in the GoM. This guidance was unchanged until the sixth edition (1975),
when it was replaced by the so-called API method 2. This was a substantial
change that led to a significant increase in design pile penetration. As stated in
the OTO report, due to concerns raised by the projects, the previous method
was reinstated in the seventh edition (1976) for highly plastic clays, such as those
found in the GoM. API method 2 was categorized for use with other types of clay.
The design guidance for clays remained almost unchanged until the seventeenth
edition (1987), when it was completely changed and a new method was intro-
duced. In the first two editions of RP2A, guideline bearing-capacity factors
and soil friction angles for sands and silts were recommended for a limited
range of soil types, along with limiting values. In the third edition, the limiting
values were removed, and this guidance remained almost unchanged until the fif-
teenth edition was introduced in 1984. Following an extensive review of all the
available test data, the guidance was changed extensively.
It is very important to highlight that one of the most significant changes was
for piles under tension, where the earth pressure coefficient was increased from
0.5 to 1.0 for full displacement for plugged or closed-ended piles. Other
changes were to expand the range of soil types covered by guideline parameters
and to reintroduce limiting values on end-bearing and skin friction piles. The
last edition of API RP2A (2007) presents more methodology for pile design
using known CPT data, as discussed in detail in Chapter 4 .

7.3 DEN/HSE GUIDANCE NOTES FOR FIXED OFFSHORE DESIGN
In 1974, the Den/HSE Guidance Notes were originally produced by the United
Kingdom s Department of Energy (Den); they have since been re-issued by the

Health and Safety Executive (HSE). The main purpose of the notes is to explain

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