| Summary: | The demand for drilling operations in offshore Arctic regions with harsh weather conditions is steadily growing due to the significant increase in the need for oil and gas. In order to still have an acceptable working environment and protection of the drilling and pipe handling equipment it is necessary to make improvements of upcoming drilling rigs. One of the items needed to be developed is the derrick/drill tower, which nowadays is a framework structure with only a limited protection against the severe weather conditions. A low weight of the derrick is desired in order to achieve improved stability conditions of the rig and hence be able to obtain a higher deck load capacity. The objectives with the current work were to propose a weight-optimized basic design of a stressed skin derrick and to compare it to a covered conventional truss derrick with respect to weight and strength. Aker MH has provided data about a conventional truss derrick which was used as a reference derrick for input of loads, dimensions and weights. In order to ensure enough structural strength, structural analyses have been carried out using FEM. The optimization software, LSOPT, has been used to weight-optimize the outer plating of the structure against yielding. The buckling strength of the plates and stiffeners was checked analytically and modifications were done to fulfill the buckling criteria. A limited buckling check of some stringers was also carried out to study the influence of the stringer weights. After the optimization against yielding, a structural weight of 619 mT was achieved and with the adjustments made for the buckling stability of plates and stiffeners a weight was obtained at 641 mT. The initially chosen stringers were shown to be significantly heavier than necessary and with the limited check and modifications of the stringers a final weight of 550 mT was achieved. The weight of a covered conventional truss derrick is estimated to 590 mT. The obtained weight of the stressed skin derrick is less than the covered ...
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