Session: 02-12-02 Structural Analysis and Optimization 2

Paper Number: 79773

79773 - American and European Hydrostatic Tubular Beam-Column Equation Comparisons 

For offshore structures hydrostatic pressure is obviously a load condition that a structural member needs to be designed for. For jacket type offshore WTG (Wind Turbine Generator) foundations, member designs are often based on ISO 19902 or NORSOK N-004 equations. Tubular member design equations are almost identical between these two standards.

With the rapid increase in the planning and construction of offshore wind farms in US waters, API RP-2A is also becoming important. For structural design engineers, there is an apparent gap in how the hydrostatic pressure is treated in these two systems. API RP-2A has a long tradition dating back to 1960s. In this system, the beam-column equations treat the axial and bending capacities as the same when there is no hydrostatic pressure. This is physically not correct, as member utilization is a combination among hydrostatic, axial and bending actions. An increase in hydrostatic load requires the reduction of capacities that axial or bending actions can take.

In 1990, Jim Loh published an important paper on hydrostatic, axial and bending interactions. Loh’s equations were adopted by ISO 19902 in 2007, and later NORSOK N-004 adopted the same equations. Although Loh’s equations are based on API-funded researches, they did not find their way into API RP-2A WSD (Working Stress Design) or LRFD (Load and Resistance Factor Design). In the latest edition of API RP-2A LRFD (2019), the hydrostatic beam column equations remain the same as it has been for more than 40 years.

In this research, actual test data are overlain with the API and ISO capacity equations for a comparison to reality. A third set of capacity equations, provided by Chen et al. is also considered. This provides an additional check and can be used to judge the quality of API and ISO equations.

Unity check (UC) results for typical tubular members encountered in actual designs are also compared. The results from current research show that, although API equations lack the proper hydrostatic reduction in axial/bending capacities, it does not impact the final member design.

A minimum of four checks is required for a hydrostatic beam-column under compression:

1. Beam-column strength controlled

2. Beam-column stability controlled

3. Hoop buckling and axial local buckling interaction, and

4. Hoop ultimate

For the API equations, the lack of proper hydrostatic reduction in (1) and (2) are compensated by (3) and (4). In other words, when hydrostatic pressure becomes high, UC checks from (3) and (4) become controlling. Thus, the negligence of hydrostatic reduction in axial/bending capacities in (1) and (2) becomes irrelevant.

It is also observed in this study, that the ISO 19902 equations for (1) and (2) become overly conservative under high hydrostatic load.

For hydrostatic beam-column under strength control, API equations are more conservative than ISO equations. Chen and Han have provided a theoretical basis on the axial and bending interaction, which follows a cosine function. The European System has adopted this functional form. In API, linear functional form is used, and this provides noticeable conservatism when compared to the cosine function. Hence, the issue of hydrostatic difference is secondary in the presence of this functional form difference.

In summary, for engineering applications very similar member designs will be obtained by either the European or the American Systems. This is particularly true for offshore WTG supporting jackets, where the water depth is less than 50m. In this water depth range, the hydrostatic effect is small to moderate.

This research provides a sound basis to argue that there are no gaps of engineering significance between European and American design standards when hydrostatic pressure is concerned.

 

Presenting Author: Albert Ku DNV

Authors:

Albert Ku DNV
Mark Richmond DNV