Wind loading on drilling rigs is covered by API Specification 4F 3rd Edition, Specification for Drilling and Well Servicing Structures. The specification gives procedures for determining design wind loads on new drilling rigs. These wind loads impose significant loads on offshore facilities and are critical for the design of derrick tie-downs.
The API 4F wind methodology was developed for wind loading on square derricks. However, drilling structures exist in shapes other than square. One example is a dual derrick.
BP has a dual derrick on the Thunder Horse platform. During a reassessment of the derrick, it became apparent that current methodologies may not properly determine the wind loads on a dual derrick. To determine if the existing 4F wind methodology was suitable for a dual derrick, BP undertook a program consisting of a literature review and wind tunnel testing. The testing was performed with major components added to the structure in stages to determine the contribution of various major components on the overall wind load.
This paper discusses the results of the wind testing on the dual derrick, provides comparisons of the resulting wind loads with the current API 4F methodology, methodology based on a literature search, a modified methodology viewed as minimizing risk of underestimating loads, and provides suggestions for future wind tunnel testing.
Effenberger, M., Ross, G., Martinez, J., Beck, R.D., Botros, T., Jesudasen, S., Jennings, T., “Wind Testing of a Dual Derrick”, Paper Number 24262, Offshore Technology Conference, May 6-9, 2013, Houston, Texas.