Offshore Structure Platform Design - Offshore Structures: Design, Construction and Maintenance - page 161

may be subject to variations of stress due to wave, wind and other environmental
loads or operational loads. Operational loads include loads associated with
machine vibration, crane usage and filling and emptying of tanks. Where varia-
tions of stress are applied to conventional weld details, identified in ANSI/
AWS D1.1- 2002, the associated S-N curves provided in AWS Figure 2.11 should
be used, dependent on degree of redundancy. Where such variations of stress are
applied to tubular nominal stress situations identified in ANSI/AWS D1.1-2002,
the associated S-N curves provided in AWS Figure 2.13 should be used. Stress
categories DT, ET, FT, Kl and K2 refer to tubular connections where the SCF
is not known.
The hot-spot stress concentration factor can be determined, taking into con-
sideration the exposure of the joint to random variable loads, seawater corro-
sion, or submerged service with effective cathodic protection system.
The referenced S-N curves in ANSI/AWS D1.1.-2002 Figure 2.11 are class
curves. For such curves, the nominal stress range in the vicinity of the detail
should be used. Due to load attraction, shell bending, and other, not present in
the class-type test specimens, the appropriate stress may be larger than the nom-
inal stress in the gross member. Note that the geometrical stress concentration
and notch effects associated with the detail itself are included in the curves.
ET is a simple T-, Y- and K-connection with groove welds or fillet welds as
well as complex tubular connections in which the punching shear capacity of
the main member cannot carry the entire load. In addition to that, load transfer
is accomplished by overlap (negative eccentricity), gusset plates and ring stif-
feners. The main stresses are due to tension and compression failure.
FT is a simple T-, Y- or K-connection loaded in tension or bending, having
fillet or PJP groove welds and main stress due to shear in the weld.
DT is a connection designed as a simple T-, Y- or K-connection with CJP
groove welds, including overlapping connections in which the main member at
each intersection meets punching shear requirements. The main stresses are due
to tension, compression and bending with reversal action.
K2 is a simple T-, Y- or K-connection in which the gamma ratio R/tc of the
main member does not exceed 24 and the main stresses are due to punching.
Reference may alternatively be made to the S-N criteria similar to the other
joint (OJ) curves contained within ISO DIS.
The ISO 19902:2004 code proposal uses a weld detail classification system
whereby the OJ curves include an allowance for notch stress and a modest geo-
metrical stress concentration.

S-N Curves for Tubular Connections
Design S-N curves are given below for welded tubular and cast joints. The basic
design S-N curve is of the form:

Log 10 ð N Þ = Log 10 ð k 1 Þ − mLog 10 ð S Þ (3.109)