Monitoring the Three-Dimensional Position and Three-Axis Rotation of Submerged Structures
Authors: Cook Peter L. 
Published: December 2007

Monitoring the Three-Dimensional Position and Three-Axis Rotation Position and Three-Axis Rotation of Submerged Structures

During many subsea structure installations, the knowledge of x, y, and z coordinates is not sufficient. There is an additional need to appreciate spatial attitude-i.e., pitch, roll, and yaw. This paper describes equipment and methods for continually measuring each of these parameters underwater and presenting them in an ergonomic format to a presenting them in an ergonomic format to a controller at the surface.

Introduction

A submerged object generally is subjected to a system of nonconcurrent spatial forces (water currents, weight, buoyancy, etc). From the laws of mechanics, these forces can be expressed in terms of a single resultant force and a single resultant couple. The consequent motion induced is, therefore, a combination of translation and rotation.Most existing navigation and survey equipment measure translation - i.e., a relative component of motion vs. a fixed reference system. Strictly speaking, the fixed system in Newtonian physics is the primary inertial system, which is assumed to have no motion in space. For engineering applications, however a set of axes attached to the surface of the earth usually is taken. These axes translate as a local frame to which velocity or acceleration can be referred.A position fix underwater can be related directly to a static surface or subsea datum. Alternatively, it can be related indirectly by means of a moving system which itself is positioned with respect to such a datum. Much commercial equipment is available to provide underwater position vs. either reference, but provide underwater position vs. either reference, but very few can determine rotation of the local frame.A survey package to monitor translation and rotation of a submerged structure can be configured in a variety of ways -with substantial differences in measurement technique, accuracy, redundancy, and complexity. The approach described in this paper has been proved on several projects in the North Sea and offshore Brazil.

Requirement

Recent years have seen increasing numbers of subsea installations despite difficulties experienced with the new technology involved. By far the majority of such installations have been associated with hydrocarbon exploitation, particularly in the marginal and deepwater offshore fields.Such structures as satellite wells eliminate the problems of directional drilling, and subsea problems of directional drilling, and subsea templates can facilitate earlier production than the conventional drilling platform approach. Cost effectiveness and performance of units now operating establishes the subsea completion as a viable proposition. proposition. In general, as the development of an offshore field progresses, the positioning requirement becomes progresses, the positioning requirement becomes more demanding. In deeper water, the increased risk and level of capital investment by operators calls for more precise control. On the other hand, the sensors (as risk items) should be relatively inexpensive.Broadly, the sensor requirement during installations is to provide survey data to enable movement control of the submerged structure. Typical sensor output specifications are shown in Fig. 1. Positioning demands vary between + 10 and0.5 m, with rotation normally within 0.5 degrees. What often is left unqualified, however, is the reference to which such measurements are to be made.An example of current field development projects requiring this type of monitoring is the positioning of a steel platform jacket of 18,000 DWT over a drilling template in 150 m of water.

JPT

P. 1557