Operability comparison of three ultra-deepwater and harsh environment drilling vessels

Abstract

This report deals with how the design principles of different drilling vessels affect the motion characteristics and operability. Furthermore, the vertical heave limitation’s influence on the operability has been analyzed. Drill strings and risers permit only minimal vertical relative motions between the vessel and the seabed. Seakeeping and wave load analyses are therefore very important in operability studies. The motion behavior of the three analyzed vessel concepts can, due to the hull design, be described by very different characteristics. This manifests itself both in terms of natural periods, deck load capacities, waterline areas and dynamic amplification magnitudes. In addition, transit speed, price and build complexity are greatly affected by the design philosophy. The operability of the vessels has been calculated for the “Southern Green Canyon” field in the Gulf of Mexico, the “Ormen Lange” field in the North Sea and for conditions typical for the west coast of Africa. The analyses show that all three vessels achieve a high operability in the Gulf of Mexico and west coast of Africa. In North Sea conditions, the SEVAN unit shows an unsatisfactory vertical response in the winter season, with a low operability as consequence. The operability in the mentioned areas can be seen in Table 1.

Table 1: Operability in three potential areas of operation. The operability can however be somewhat deceptive regarding indication of general motion behavior. In the most frequently encountered sea states the SEVAN unit performs better than the West Navigator in spite of lower operability. Furthermore, the West Navigator has in average twice the heave amplitudes compared to the Aker H6 under normal operational conditions.