Seismic design for vessels

Step 5 Since the seismic design for pressure vessels is based on allowable stress rather than ultimate strength, the base shear may be reduced by a factor of 1.4. 

Seismic design basis for the reactor vessel (including attachments, pressure boundary, and CRD and internals). 

A seismic stress analysis is not made as a routine procedure in the design of vessels for sites in the United Kingdom, except for nuclear installations, as the probability of an earthquake occurring of sufficient severity to cause significant damage is negligible. However, the possibility of earthquake damage may be considered if the site is a Major Hazards installation, see Chapter 9, Section 9.9. 

The MCSS is designed for high stiffness in order to maintain structural integrity during an SSE event. The seismic analysis indicates that the fundamental frequency is 32 Hz and a spectrum load of approximately 0.5 g. Comparison with ASME Boiler and Pressure Vessel Code Section III, Div. 1, Subsection NG stress criteria demonstrates that failure of the structure is not credible. 

In between these extremes there are numerous examples of floating devices that have more or less in common with ordinary shipping. One group comprises vessels specifically designed for standby, supply services, anchor handling, seismic or geological exploration, subsea work and so forth. These vessels may both look like... 

Adams, N.J.I. (1992) Seismic design rules for flat bottom cylindrical liquid storage tanks , Int. Journal Pressure Vessels and Piping, 49, pp. 61-95. 

The procedure for designing a simple cylindrical vessel to Code Case 2286 or Section VIII, Division 2 is to first establish geometry as weU as determine shell thickness values to begin with. Typically, some thickness is estab-hshed from, say, internal pressure and those thickness values are used for starting the procedure in Code Case 2286. External loads from wind and seismic design criteria must be estabhshed to complete the procedure. 

As a loading, wind differs from seismic in that it is more or less constant whereas, seismic is of relatively short duration. In addition, the wind pressure varies with die height of die vessel. A vessel must be designed for the... 

Seismic design procedures can be accomplished for most vessels by one of the two methods as follows ... 

Seismic design criteria may provide linear and nonlinear seismic response history procedures, but these are not as commonly used for vessels. 

For rigid vessels, the first few modes may represent the majority of the modal mass participation, whereas for flexible vessels, the number of modes may be 20. It is for this reason that dynamic analyses lend themselves to computerized models. Many seismic design standards indicate that the number of modes to be included must have a combined mass participation of at least 90%. 

Anchors for tall vertical vessels are frequently not required to resist shear, since the shear is resisted by friction created by the large compressive force attributable to overturning. See 3.6 for the evaluation of frictional resistance. However, there are cases where the anchor may be required to transfer the shear load, such as for shorter vertical vessels or those subject to seismic design. See 3.11 for seismic design of anchors. Following are two alternative methods for designing the anchors to resist shear ... 

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