OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.3 Design input – Page 37
combined to produce a design effect. For this purpose, a (limited) number of possible load effect and/or
environmental condition combinations are considered. The most unfavourable combination among these shall
be found and will govern the design.
11.2.5 The most unfavourable relevant combinations shall be defined for every point in time during the design
life.
Guidance note:
In most cases the most unfavourable relevant combinations are the same over the entire design life. However, in some
cases conditions may change with time, which may in turn cause changes in the relevant combinations.
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11.2.6 The format of this standard for the combination of two or more independent random load effect
processes is based on Turkstra’s rule. The rule states that the maximum value of the sum of two independent
processes occurs when one of the processes has its maximum value.
11.2.7 The design load effect corresponding to the combination of two independent load effect processes A
and B should be determined as:
Where:
S d
γ Sd
S A k
γ A F
Ψ A
S B k
γ B F
Design load effect
Load effect model factor
Characteristic value of load effect A
Partial load effect factor for load effect A
Load effect combination factor for load effect A
Characteristic value of load effect B
Partial load effect factor for load effect B
Ψ B Load effect combination factor for load effect B.
11.2.8 The design load effect corresponding to the combination of a number of N independent load effect
processes should be determined by the maximum of the following N combinations:
Where:
S d
γ Sd
S i k
γ i F
Design load effect
Load effect model factor
Characteristic value of load effect i
Partial load effect factor for load effect i
Ψ i Combination factor for load effect i.
S
A
⎧γ
F
. S
.max⎨
A
⎩γ
F
. S
B
+ γ . S
11.2.9 The load effect combination factor Ψ = 0.7 should be used for independent load effect processes,
unless a detailed probabilistic analysis can justify a different value. For permanent load effects and permanent
environmental conditions Ψ = 1.0.
11.2.10 Some load effect processes are correlated such that the value of the one load effect process to some
degree depends on the simultaneous value of the other load effect process. The combination rule for design load
effects quoted in clause 206 for independent load effect processes can be extended to be used also for correlated
load effect processes. When applied to combination of correlated load effect processes, different (usually
higher) values of the combination factors Ψ apply, depending on the degree of correlation.
11.2.11 The load effect combination factor Ψ = 1.0 shall be used for correlated loads, unless a detailed
analysis can show that the load effects are correlated in a different way.
Guidance note:
For example:
- Water level (height) and pressure load are fully correlated processes
- Wave height and wind speed are somewhat correlated processes: waves are wind driven, so high mean wind speeds
are usually accompanied by large significant wave heights, maybe with some delay, whereas the instantaneous
wind speed and the simultaneous wave height are independent once the mean wind speed and significant wave
height are given.
. Ψ
A
B B
= k F k
d
γ
Sd
A A B
Ψ +
B
k
. γ
F
. Sk
N ⎡
⎤
j j
i i i
S
d
= γ
Sd
.max⎢γ
F
. Sk
+ ∑γ
F
. Sk
. Ψ ⎥
j=
1
⎣
i≠
j ⎦
DET NORSKE VERITAS AS