Offshore Platform Displacement with an Inertial Reference System
Authors: Schmidt T.R. 
Published: October 2008

Platform displacement measurements are useful in evaluating analytical techniques, basic assumptions, and conditions related to platform dynamics. Light portable equipment using an inertial reference to measure platform motion is described. Amplitudes in the range of a few thousandths of an inch to a foot or more over a frequency band of one twentieth of a Hertz to several Hertz can be measured. Examples of calibration runs and actual platform tests are given. Methods of presenting data are illustrated including strip chart recording, horizontal plane platform motion plots, displacement spectrum plots and extended time multiple spectra plots.

INTRODUCTION

Fixed offshore platforms are in constant motion. During relatively calm conditions this motion may be no more than a few hundredths of an inch and undetectable without instrumentation. Heavy weather or ice can cause motions of an inch or more and be an annoyance to platform personnel. More important, at least to the design engineer, is the amount of information contained in platform motions whatever their amplitude.

Platform motion, seemingly random and disorganized at first observation, actually represents the cumulative response of the structure, with its own unique characteristics, to the various excitations it is being subjected to. Waves, ice, wind, currents, earthquakes, platform equipment, boat mooring, and helicopters can each contribute to this motion and be a part of the excitation forces. Platform design, loads, bottom conditions, and structural integrity are included in the overall response characteristic of the structure to this excitation. Platform displacement is the result of the interaction of these many factors.

Records of these displacements are full scale experimental data which can be used to evaluate and refine design criteria, to provide information on modes of behavior that are not well understood theoretically, and to evaluate structural integrity. Measured time histories of displacement can be analyzed to provide information relating to wave force magnitudes, platform frequencies and mode shapes, and percent critical damping. If data describing the ambient sea is available, studies can be made to compare measured and predicted motion.

THE INERTIAL REFKRENCE APPROACH TO DISPLACE MENT DETERMINATION

Motion is relative and is measured with respect to a fixed reference. A fundamental problem with an offshore structure is obtaining a reference point for displacement determination.

The ocean floor is the nearest fixed reference point and is used to position vessels through the use of anchors, sonar transponders, or taut wire systems. These approaches were considered but generally lacked the desired resolution and required a substantial installation effort. Because of these limitations, inertial reference techniques were explored and have proven to be not only workable, but convenient to use.

Inertial reference techniques are based on Newton's laws of motion. A given body at rest will remain at rest unless acted on by an unbalance force and if subjected to an unbalance force, will accelerate in proportion to this force. Inertial devices consist of a seismic mass and some form of support.