A Vessel Motion Instrumentation System

Abstract

Abstract This paper describes an instrumentation system for a drilling barge in the open ocean; the system measures and records barge motion, waves, anchor line forces, wind and current in a form suitable for time series analysis. The instrumentation system was built by Chevron Research Co. and installed on the drilling barge Western Offshore III. It recorded data during drilling operations by Standard Oil Co. of California, offshore California, Oct., 1964 to Jan., 1965, and offshore Oregon, May to Oct., 1965. Data from this system are suitable for an accurate determination of the dynamic characteristics of the drilling barge and its anchoring system in waves. The system, with the exception of the wave height sensors, proved to be highly reliable and easy to operate. The system, in general, is suitable for obtaining measurements on any anchored drilling barge or semi-submersible. Introduction The use of floating vessels to drill exploratory and producing oil wells is increasing as the search for oil is carried into deeper water all over the globe. An important factor to be considered when evaluating a drilling vessel for a particular drilling program is its motion characteristics in waves; in some areas of the world, these characteristics can be a determining factor in ability to operate profitably, or at all. Motion characteristics of a particular drilling vessel may be determined in varying degrees of certainty by calculations, model tests or full-scale measurements. This paper describes an instrumentation system for making full-scale measurements to determine the characteristics of a drilling vessel and to evaluate other methods. The instrumentation system measured vessel motions, waves, anchor line tensions, wind and current, and recorded the measurements in a form suitable for time-series analysis. Approximately 180 hours of recorded data were obtained that include several storms and periods of marginal operating conditions. This paper describes the system built for the barge Western Offshore III, and discusses criteria for selecting the components, characteristics of the measured data and experiences with operation of the system. The paper serves to illustrate what can be done and what factors should be considered in developing a practical system. Description of System GeneralThe system consists of functional groups of transducers, a central signal conditioning unit and a recorder. The location of each of these system parts on the Western Offshore Ill is shown in Fig. 1. Each of the following sections contains a description of components, accuracies of recorded data, discussions of the characteristics and limitations of the resulting data, and a summary of experiences from handling and operating the equipment. Accuracy and resolution figures quoted for each measurement are based on the following: accuracy of most measurements was determined by system calibrations performed before installation of the system on the barge and at the completion of the project. A system calibration includes a measurement of uncertainties in the recording system as well as in the transducers. In some instances where calibration standards were not available, manufacturer's specifications were used in combination with recorder accuracy. Resolution figures are based on either minimum transducer resolution, quoted by the manufacturer, or the signal required to make a 0.01-in. deflection on a record, whichever is greater. The 0.01-in. deflection on a record is the resolution of the machine available to digitize records. Basic criteria for selecting components in the system were to make accurate measurements with the simplest, available equipment adequate to the task. Commercially available transducers were used throughout the system; no attempt was made to develop special measuring devices. In addition, an effort was made to avoid interference with drilling operations or dependence on operating equipment whenever possible. The analog recording system was used because the components and a data digitizing system were available. Under other circumstances, a digital recording system would have been seriously considered. Our experience with the system bears out these original criteria. System components performed reliably with few exceptions during the total period of operation; the system proved simple to operate and maintain. JPT P. 1041ˆ