Several codes are currently available for design and analysis of pressure vessels. Two of the main contributors are the American Society of Mechanical Engineers providing the ASME VIII code, Ref /4/ and the Technical Committee for standardization in Brussels providing the European Standard, Ref /2/.
Methods written in bold letters will be considered in the discussion presented in this paper.
The ASME VIII code, Ref /4/, contains three divisions covering different pressure ranges:
Division 1: up to 200 bar (3000 psi)
Division 2: in general
Division 3: for pressure above 690 bar (10000 psi)
In this paper the ASME division 2, Part 5, “design by analysis” will be considered. This part is also referred to in the DNV-OS-F101, Ref /3/, for offshore pressure containing components.
Here different analysis methods are described, such as:
Elastic Stress Analysis
Limit Load Analysis
Elastic Plastic Analysis
The Elastic Stress Analysis method with stress categorization has been introduced to the industry for many years and has been widely used in design of pressure vessels. However, in the latest issue (2007/2010) of ASME VIII div. 2, this method is not recommended for heavy wall constructions as it might generate non-conservative analysis results.
Heavy wall constructions are defined by: (R/t ≤ 4) with dimensions as illustrated in Figure 1.
In the case of heavy wall constructions the Limit Load Analysis or the Elastic-plastic method shall be used. In this paper focus will be on the Elastic-plastic method while the Limit Load Analysis will not be considered.
Experience from recent projects at IKM Ocean Design indicates that the industry has not been fully aware of the new analysis philosophy mentioned in the 2007 issue of ASME VIII div.2. The Elastic Stress Analysis method is still (2012) being used for heavy wall constructions.
The NS-EN 13445-3; 2009, Ref /2/, provides two different methodologies for design by analysis:
Method based on stress categories.
The method based on stress categories is similar to the Elastic Stress Analysis method from ASME VIII div. 2 and it will therefore not be considered in this paper.