Strength of Tubular Members– Numerical Comparison of API RP2A to ISO Codes

Worldwide, currently there are three codes used for offshore jacket
platform design i.e. API RP2A WSD, API RP2A LRFD and ISO
19902. Of these, the ISO code is formulated for application all over the
world. API RP2A WSD is still being used for jacket platform design,
not only in the Gulf of Mexico, but also in most of developing
countries.API RP2A LRFD and ISO have introduced limit state design
in place of working stress design. The limit state design is followed all
over the world for most of the design codes. ISO will introduce
environmental load factors for each region, in its appendix. The
reliability of jacket platforms is maintained in API RP2A LRFD by
setting target safety factor the same as that provided in WSD designs,
which means structures designed as per LRFD code will have same
reliability as API RP2A WSD (which has already provided safe
structures and the best available practice for design). For ISO code, the
API RP2A LRFD was based as the base code of design. In this paper,
the nine stress equations of the above three codes are compared. The
knowledge of the strength equations in the codes is useful for
determining resistance factors for code calibration. The resistance
formulations are numerically compared and the similarities and
differences are determined. The member resistance formulae for local
buckling, hydrostatic pressure, interaction formulae for axial
compression, bending stress, axial tension and hoop stress introduced
in the 6th edition of WSD in 1975, have undergone major changes. In
the 11th edition (1980), equations were introduced for allowable hoop
stress, a formula for combined effects of axial compression, bending
and hydrostatic pressure. When API LRFD version was introduced in
1993, some formulae were modified. Incorporating these, the 21st
edition of WSD was published in 2000. ISO 13819 was published in
1998, modified in 2007(ISO 19902). Some provisions in ISO were
directly adopted from API RP2A LRFD while others improved and
included.