Performance Prediction of Floating Drilling Vessels for Various Operating Areas

Abstract

This paper describes a computer program, composed of weather and drilling models, that was developed to help select floating drilling vessels for specific operating areas and start-up times. With these models, expected performance of different vessels can be simulated and their costs performance of different vessels can be simulated and their costs compared. Introduction Weather conditions vary considerably from area to area and even during the course of a year in a single area. Consequently, drilling vessels that have been used successfully in one region or at one time might not be suitable for different circumstances. Since the day rate for different vessels also may vary, selection of a proper vessel involves both engineering and economic factors. If expected adverse weather or sea ice place time restraints on operations, reaching a desired place time restraints on operations, reaching a desired well depth by a certain date becomes an important consideration in vessel selection. Criteria necessary to establish estimated performance of a floating drilling vessel are moron characteristics, operating limits with respect to sea conditions or vessel motion, well design and expected drillability of the formation with no vessel motion, and weather data for the geographical area. This information is incorporated in drilling and weather models that then are combined to calculate predicted performance of the vessel. Vessel-Motion Characteristics Heave, roll, and pitch data for a specific vessel are determined from model tests in wave basins or from theoretical calculations. These data usually occur as response-amplitude operators (RAO) for regular waves (Figs. 1 through 3). Since sea conditions are not regular, vessel-motion characteristics in irregular seas must be determined. As described by Michel and Minkenberg and Gie, vessel motion in irregular seas (heave, pitch, and roll spectra) can be determined by multiplying wave spectral density by the square of the vessel response for a range of frequencies. The wave spectral density, Sw(w), for a frequency w is given by the Bretschneider spectrum: 4,200 H 2 - 1,050s Ts 4 w 4 Sw(w) = -------------- e, ..............(1)T 4 w5s where Sw(w) = wave spectral density, sq ft-sec forfrequency wHs = significant wave height, ftTs = significant wave period, secondsw = wave frequency, sec-1. With this definition, the area under the spectral curve equals the square of the significant wave height. Fig. 4 illustrates this spectrum for a significant wave height of 17 ft and a significant wave period of 10 seconds. The heave spectrum in Fig. 5 was obtained by multiplying the wave spectral density by the square of the RAO for the corresponding frequency. Z(w) = Sw(w) . RAO 2 (w),...................(2) where Z (w)= heave-spectral density, sq ft-sec forfrequency w. Significant heave, Zs, is obtained by Zs (Hs, Ts) = Z(w) dw,...................(3) JPT P. 1688