COGNAC Positioning System
Daniel Peterson · Offshore Technology Conference · 1979
ABSTRACT An acoustic position-measurement system was designed for aiding the installation of the Cognac platform. This system was configured specifically for each phase of the installation requiring position information. Before the details of the system are described, a brief review of some basic principles involved is given. This review includes a discussion of the "range-range" solution, which provides a high degree of accuracy over a wide operating area; tilt-angle measurement by pulse-position modulation; and the range-angle solution which provides backup in the event of either shadowing of the acoustic path or transponder degradation. The system usage during each phase is described, and results are given to illustrate the system performance in estimating the platform position and in aiding pile installation. INTRODUCTION Imagine the task of assembling a steel structure as tall as the Empire State building, with a base approaching the size of the Astrodome, all done under water from the sea surface. The installation of Shell Oil Company's Cognac platform in the Gulf of Mexico, off the mouth of the Mississippi River, was a project of this sort. The complete platform, consisting of three main sections and a top deck, was successfully installed, as shown in Figure 1. The installation plan devised by Shell Oil involved shipping the sections from the construction yard, launching them into the water between two derrick barges, and lowering them into place in a building-block style. This plan called for stringent requirements in position measurement and control. These requirements lead to the development and use of a sophisticated position-monitoring system which was tailored to each phase of the platform installation. The system used underwater acoustic instrumentation and signal processing for obtaining the measurements. Extensive data processing and display equipment were used to present the information in a form that could be easily understood by the installation crew. Base Section Installation Phase The base section had to be lowered in a controlled manner to a predetermined landing site. Its final location had to be within a 50-foot radius of the site; its final heading, within ±2.5 degrees of the target heading; and the verticality error needed to be less than 1 degree. The 14,000-ton base section, measuring 380 by 400 by 175 feet high, was lowered to the 1,025 foot-deep sea floor with lines from two anchored derrick barges. The positioning system continuously indicated barge position and attitude relative to a fixed reference frame, during launch preparations and lowering. It also provided the position and attitude of the base section in the same reference frame after its launch and during its lowering. Repositioning and heading changes of the base section were accomplished by changing line lengths in the six anchor lines from each barge. Final position for setdown, within a 50-foot radius of a target point, was established leveling angles were measured, using acoustics to back up the primary angle measurements, which used water-loop sensors on the base section. Heading was accurately measured with acoustics to confirm that satisfactory final platform emplacement had been achieved.