OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.6 Failure mechanisms and design criteria – Page 122
10 Long term static loads
10.1 General
10.1.1 The sustained load conditions, as defined in Sec.3 [9.5], shall be used as the applied load when checking
for the effects of long term static loads. The observation period is defined as the duration during which stress
corrosion is likely to take place. An approach similar to Sec.3 [9.5] should be used, indicating the applied stress
or strain level(s) during the observation period divided into one or several time intervals. The load conditions
shall be based on a conservative estimate.
10.1.2 The observation period is defined as the period at the end of which effects of long term static loads shall
be calculated.
10.1.3 Analysis with sustained load conditions may be performed on a ply (lamina) level or laminate level.
However, long term data shall always be measured on laminates with representative lay-ups to ensure that the
data represent the interactions between plies.
10.1.4 If a component is exposed to static and cyclic long term loads the combined effect shall be taken into
account. As a conservative choice the effects may be taken to be additive. Other combinations may be used if
experimental evidence can be provided. If fatigue is analysed or tested with a mean load that corresponds to
the permanent static load, effects of static and cyclic fatigue may be considered separately.
10.2 Creep
10.2.1 The effect of creep is a reduction of the Young's modulus. The reduction of the Young’s modulus is
denoted as the creep modulus. How the modulus changes with time is described in Sec.4 and Sec.5. Usually
experimental confirmation of creep behaviour is required.
10.2.2 The result of creep can be a redistribution of stresses in a larger structure or exceeding of a maximum
displacement requirement. If the redistribution of stresses is of concern a stress analysis with the changed
elastic constants shall be performed. If displacement requirements shall be observed the displacement criterion
(see under [8]) shall be checked by using the relevant creep moduli in the analysis.
10.3 Stress relaxation
10.3.1 The effect of stress relaxation is a reduction of the Young's modulus that causes a reduction of stresses
under constant deformation. How the modulus changes with time is described in Sec.4 and Sec.5. Usually
experimental confirmation of the behaviour is required.
10.3.2 The result of stress relaxation can be a redistribution of stresses in a larger structure or the loss of a
certain contact pressure. If the redistribution of stresses is of concern a stress analysis with the changed elastic
constants shall be performed. If a certain contact pressure is needed the structure shall be checked for the
reduced moduli.
Guidance note:
A specified contact pressure is often needed for bolted connections, or if a component is kept in place by friction.
Stress relaxation will be less pronounced when:
- the glass content in the laminate is increased
- more of the fibres are orientated in the load direction
- the temperature is lowered.
10.4 Stress rupture - stress corrosion
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
10.4.1 Materials may fail due to the permanent application of loads: this process is called stress rupture. If the
permanent loads act in combination with an aggressive environment the process is called stress corrosion. The
analysis is generally the same for both processes, but the material curve describing the reduction of strength
with time depends on the surrounding environment. In the following parts only the notion of stress rupture will
be used.
10.4.2 All significant failure mechanisms shall be checked for stress rupture, i.e. all failure mechanisms that
are linked to a critical failure mode and limit state. The approach is basically the same for all failure
mechanisms, but different stress rupture curves and residual strength values shall be considered. These are
described in Sec.4 and Sec.5.
10.4.3 A stress rupture analysis shall provide the answers to two questions:
— can the structure survive the expected load sequence?
— is the structure strong enough that it can survive all relevant extreme load cases on the last day of its service
life?
DET NORSKE VERITAS AS