< Previous | Contents | Next >

Section 5 Calculations

501. General

1. Pressure hulls, hatches, windows, suspensions etc. are to be calculated in accordance with the relevant Rules of the Society or other code of engineering practice. Pressure hulls and pressure vessels sub- jected to external overpressure may be calculated in accordance with Guidances separately provided.

2. The calculations on which the design is based are to be submitted to the Society. Where the calcu- lations are to be performed with the aid of computers, proof of the suitability of the programs is to be furnished to the Society.

3. Allowance is to be made for the loads due to nominal diving pressure, test diving pressure, col- lapse pressure, internal overpressure and any dynamic loads, reaction forces and additional local stresses caused by fastening attachments and supports. The calculations are also to take account of the environmental conditions specified in Ch 4, Sec 2.

4. The load factors for dynamic loads are to be agreed with the Society. Account is to be taken of the fatigue strength of the pressure hulls are to be designed for at least 5000 operating cycles.

5. The weld factor of welds is in accordance with 402.

6. The allowance for corrosion and wear is normally to be 1 mm. The allowance may be dispensed with in the case of plates 30 mm thick, stainless steels or other corrosion resistant materials and where special corrosion protection is applied.

7. The wall thickness of the shells and ends of seamless or welded pressure hulls shall generally not be less than 6 mm.

502. Design criteria

The following design criteria are to be applied to the calculation of components subjected to ex- ternal over pressure:

(1) Tensile, compressive and bending stresses at nominal and test diving pressures shall not exceed the permissible values stated in 503.

(2) Components critical to stability is to be designed with a sufficient margin to withstand buckling,

bulging and lateral buckling at nominal and test diving pressures in conformity with the safety factors stated in 504. For cylindrical, shells, proof is to be provided of resistance to both asym- metrical and symmetrical buckling.

(3) The collapse pressure/nominal diving pressure ratio indicated in Table 1.5.3 are not to be reduced. The possibilities of failure critical to stability and of plastic failure are to be analyzed. Allowance is to be made for the reduction in the modulus of elasticity between the limit of proportionality and the yield point or 0.2 % proof stress. Generally, the material shall be as- sumed to behave elastically and plastically without strain hardening. Where the compressive load/deformation(P-d) curve for the material has been determined in the presence of the Society's representative, this curve may be used as the basis for calculations.

Table 1.5.3 Test pressure and collapse pressure in relation to nominal diving pressure

503. Permissible stresses

The smaller of the following two values is applicable:

ĄŶËŊU or

JA

ĄÁĂ ŹU

JA

where

ĄŶËŊU = guaranteed minimum tensile strength [NĤmm Ë] at room temperature (may be disregarded in the case of established finegrained steels with ĄÁĂ ≤ ĖÈŊ NĤmmË or where external material. overpressure exerts a compressive load)

ĄÁĂ ŹU = guaranteed yield point or minimum value of 0.2 % proof stress at design temperature

The safety factors A and B are shown in Table 1.4.5.

Table 1.5.4 Safety factor

504. Safety factors against buckling and tripping

′

1. Cylindrical shells are to be designed to withstand elastic buckling and stiffening rings to resist trip- ping at nominal diving pressure by the application of a safety factor ÅÝ of at least 3.0 (under test ÅÝ = 2.2).

2. Dished ends are to be designed with at least the safety factor ÅÝ

Ý

or Å′

to be calculated according

to the following formulae taking into account the wall thickness Å and the reference radius Ą of

the connected cylindrical shell:

Ą ÅÝ GĖĞŊĦŊŊËJÅ

ËĦË

ÅÝ′ G JĖ ÅÝ

505. Allowance for manufacturing tolerances

If the manufacturing tolerances on which calculations have been based are exceeded, the deviations

observed are to be

pressure.

used to carry out a mathematical verification of the maximum permissible

< Previous | Contents | Next >