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FP5

Improved Dynamic Positioning for Large Vessels to Increase Safety and Effectivity in Offshore Operations and Exploration and Exploitation of Marine Resources, Publishable Synthesis Report

Project ID: ENK6-CT-2000-00104
Funded under: FP5-EESD

Abstract

The Joint Industry Project carries out an investigation to develop and make available against economically justifyable cost a method for "Improved Dynamic Positioning for Large Vessels to Increase Safety and Effectivity in Offshore Operations and Exploration and Exploitation of Marine Resources".
The project results will be useful in DP applications for deep water drilling, floating production, offshore offloading of oil and/or gas (LNG), dredging, cable laying and pipe laying.
In the project, industry and MARIN have been working together to develop and test a method for improved dynamic positioning.The improvement method is based on the notion that positioning of a large ship in open sea by use of thrusters will become more effective if the environmental forces are known in "real time". In feed back control such real time information is called "Feed Forward".
An example of feed forward which is common practice in DP systems is measurement of wind speed and direction, and the use of wind force coefficients to estimate the wind force on the ship. For large ships the so-called "wave drift forces", which are second order effects in a seaway, are dominant forces.
The estimation of wave drift forces, which cannot be directly measured, has to make use of physical relations between observables and these second order forces. It has been found that the relative water motion at the bow, stern and sides of the ship is a good observable: it is a measurement with a good signal to noise ratio and it contains the physical information to estimate the wave drift forces in various ways.
Two different methods were used to estimate the wave drift forces from the relative motions. Only a limited number of relative motions sensors (i.e. 6) was needed to get an estimate with sufficient accuracy, and even have some redundancy left. One method is the "waterline integral method", and the other method is the "modulator method".

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