Wind Testing of a Dual Derrick

Abstract

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. Introduction On the BP Thunder Horse semi-submersible is a dual derrick. A dual derrick is a relatively new drilling structure used for deep water drilling operations. Efficiencies exist from allowing dual hook load operations that make this type of structure advantageous. Figure 1 presents a photograph of the Thunder Horse derrick from the V-door face (long front). The derrick has a width to depth ratio of 2 to 1 versus the 1 to 1 ratio of a more traditional square derrick. The Thunder Horse Dual Derrick was designed using the methodology in the 2nd edition of API 4F. API has issued a new edition of API 4F, the third (3rd) edition. API 4F 3rd Edition had extensive revisions to the wind methodology. In addition, environmental design conditions for the Gulf of Mexico have also been revised. The new metocean design criteria contain updated/higher wind speeds for the location in the Gulf of Mexico where the Thunder Horse semi-submersible is located. BP wanted to investigate the dual derrick using the new API wind methodology and the new metocean criteria. Evaluations were made with the wind methodologies from API 4F 3rd Edition, ASCE 7, and other wind documents for loading on the dual derrick long and short faces. Inconsistencies were found among the methods. As a result, BP was faced with a decision to use the most conservative wind load values or perform wind tunnel testing to better understand the loads on the structure. BP elected to perform wind tunnel testing.

Unconstrained Dynamic Simulation on Offshore Dual Derrick

“…Simulation methods used to obtain characteristics such as natural frequencies and mode shapes, design assessment and safety, and vibration response are becoming more popular and useful for performing studies on the derrick [1][2][3][4][5][6]. Due to the complex conditions of marine environments [7][8][9], extensive research is needed to understand the nature of loads acting on structures operating in the marine environment [10,11]. The derrick is a lattice structure, hence, various approaches such as mathematical approaches [12], experimental approaches [13], and simulation approaches are used to evaluate stress and loads acting on its structure.…”