OS-C501

Offshore Standard DNV-OS-C501, November 2013
Sec.14 Calculation example: two pressure vessels – Page 185
Table 14-3 Duration of each phase and related safety class
Phase Duration Safety class Comments
Manufacturing
Fabrication/
Assembly
NR Low Not determining design dimensions- structure is not loaded.
Thermal stresses may have to be considered (not done in this
example)
Transport
Handling
Storage
NR Low Appropriate precautions are assumed to be taken, so that point
loads and other loads will not determine design dimensions.
Not considered in this example, but transport loads can often
be critical.
Testing (tank alone) Some hours Low Pressure test with water. Test set-up must be secured (safety
cage). Possible pressure shocks or vibrations may fatigue
connected pipes/valves/flanges and fittings, but not considered
to be a problem for the vessel body.
Installation NR Low No pressure during installation. Overloading during hook-up
because of over-tightening of bolts is assumed to be prevented,
and will thus not determine design dimensions.
Commissioning Some hours / Gas: High System test. Will not determine design dimensions, since it is
days Water: High not different from operating conditions.
Operation 25 years Gas: High
Water: High
Maintenance NR NR Not considered*
Repair NR NR Not considered*
Scrapping / recirculation
Not considered*
NR = Not relevant (no loads are applied or duration is so short that the actual time is irrelevant).
* We assume maintenance and repair to be carried out without pressure in the vessel.
2.5.3 This evaluation should be repeated for each part of the structure.
2.5.4 Only the two phases testing and operation will be considered in this example.
2.6 Functional requirements (ref. Sec.3 [6])
To be de-pressurised every week (1300 load cycles during the
design life). Safety class high is chosen here for both vessels
to allow a better comparison between the two designs.
2.6.1 In this example connected pipes, fittings and supports are not included.
A checklist of more functional requirements is given in App.A [A.2]. Only the relevant ones for the design of
the laminate shell with respect to testing and operation are listed in Table 14-4.
Table 14-4 Relevant functional requirements for specified phases
Phases
Functional
With liner (gas)
Without liner (water)
requirements
Testing Operation Testing Operation Comments
Pressure
X X X X
containment
Tightness
(of laminate)
X X A tight liner is sufficient
for the gas vessel
2.6.2 The other functional requirements given in App.A [A.2] were excluded for the reasons given in Table 14-5.
Table 14-5 Relevance of functional requirements
Minimum list of functional requirements Relevance for this example
Load carrying capacity
Pressure containment is the only load
Dimensional stability
No dimensional requirements
Environmental, chemical and UV resistance Used inside
Maximum vibrations
No vibrations are present
Fire Resistance
No fire requirements
Temperature insulation
Everything is at room temperature
Erosion, abrasion, wear
Not relevant here, but some designs may have sliding supports, strapping
devices etc.
Electrical Resistance or Insulation
Not relevant
Static Electricity / Grounding
Not relevant here, but is important if the fluid may be flammable
Lightning resistance
Inside storage
DET NORSKE VERITAS AS