OS-C501

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Offshore Standard DNV-OS-C501, November 2013 Sec.14 Calculation example: two pressure vessels – Page 184 2.3.2 The laminate has the following interwoven winding sequence: ±15, ±85, ±85, ±85, ±85, ±15 Each sequence has the same thickness of 1mm. 2.4 Division of the product into components (ref. Sec.3 [3]) 2.4.1 Both pressure vessels can be divided into the following parts: — main cylindrical body — two end-caps with: — inlet nozzle for water or gas pipe — nozzle for venting — nozzle for pressure gauge — lifting rings — support structure to store vessel on the ground — liner (gas vessel only) — liner - laminate interface (gas vessel only). 2.4.2 Only the design of the laminate shell (cylindrical part) is considered here, to keep the example simple. 2.5 Phases and safety class definitions (ref. Sec.3 [4] and [5]) 2.5.1 The vessels will be built, transported, installed and operated. The minimum phases to consider are construction and operation. A more detailed division into phases is shown in Table 14-2. Further comments are given in App.A. Table 14-2 Minimum of phases to consider Manufacturing Fabrication / Assembly Transport Handling Construction Storage Installation Testing Commissioning Operation Maintenance Operation Repair Retrieval / recirculation Post-operation. 2.5.2 Consider the duration of each phase and the corresponding safety class, as summarized in Table 14-3. DET NORSKE VERITAS AS
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
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