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Articles

Vol. LVII, No. 2

Quality Control of Offshore Positioning Surveys

Submitted
August 6, 2015
Published
2015-07-13

Abstract

This paper discusses the need for a quality control method for offshore positioning surveys and then its actual use. It limits the discussion to radio locationing systems only, but this restriction does not imply that Satellite Navigation or Underwater Acoustic Positioning Systems do not require such quality control. The subject is divided into two parts. Firstly the calibration methods necessary to determine the zero/delay settings for the survey equipment and to establish the propagation velocity required in the survey. Secondly, the positioning of the survey vessel with the necessary, but often missing, “ simultaneous quality control” of this positioning. The methods described in this paper are compared with field results, from which can be seen the often limited value of the most commonly used, and sometimes time-consuming and expensive, calibration methods. It will be shown here that the generally accepted principle of the location offshore of vessels and structures using only two position lines not only lacks reliability but can often result in costly resurveys. The lack of data redundancy in offshore surveys stands in sharp contrast to the practice in land surveying where data redundancy in observation and control surveys is common practice. With the increasing practice of having extra positioning equipment on stand-by— especially in remote areas— and with the increase in cost of offshore surveys, Shell’s Topographic Survey Department has developed a method based on redundancy of data. This method provides a continuous quality control of the offshore positioning while surveying, and avoids unpleasant surprises after the survey vessel has left the area.