The API flange design is a well-known commonly used solution. The flange concept was developed in late 1920s and 1930s by Waters and Taylor. The design methodology of the flange was published in 1937[1], well known as the “Taylor Forge method”. This is still the basis of the present ASME flange calculation. The design is based on the simple elastic principles and linear stress analysis/calculations. The conventional flange type dimensions are described in API 6A [2] and analyzed in API 6AF [3] and 6AF2 [4]. On the other hand, the Compact Flange concept was presented first by Webjørn in 1989 VCF joint [5]. It is based on plastic theory equations and plastic collapse capacity. In 1989 the initial concept was adopted by the Steel Product Offshore (SPO) company for oil industry by equipping flange with HX seal ring for raiser and subsea use. After that a topside budget version (with simpler IX seal ring) was prepared by SPO and presented on PVP 2002 conference [6][7][8]. The Compact Standardized and simplified flange design with IX seal ring is defined and described in ISO-27509 [9]. As for today, along ASME B.16.5 [10] pressure classes range, SPO CF 5K, 10K, 15K and 20K rating flange classes were designed and are in use. The main advantages for CF design are reliability, low weight/compact dimensions and static behavior compared to the conventional design. The design is already well known and commonly uses for European region (mostly Norway). Despite its benefits, CF is still rare outside Europe region. A comparison between those two different concepts will be presented in this paper followed by the examples and Finite Element Analysis (FEA). In case of FEA the Compact Flange design is more suited to the plastic collapse analysis than to elastic stress evaluation as it is for API, therefore comparison between different FEA approaches will be studied in addition.

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