Development of an instrumented mooring system for VFFS model testing

Abstract

Floating structures have developed significantly in recent years. As land is becoming scarce, the use the water surface will contribute to ease this scarcity. Therefore in recent years, floating structures covering large areas have been developed, also called flexible floating structures.
In this project the focus is set on the mooring system very flexible floating structures (VFFS). At the TU Delft, two towing tanks can be used to investigate the mooring system of VFFS, however first a reliable measuring system is required that is able to examine a specific part of the mooring system. Conventional setups that measure the mooring forces consist of large instruments, as these instruments only have a small effect on their investigated structure (vessels). The response of VFFS is dominated by elastic deformations and differs from conventional rigid structures. For VFFS, these type of instruments will have a large effect on the structure motions and thus these conventional setups cannot be used. Therefore, a new measuring system is required to conduct small scale experiments with VFFS, and the following objective is formulated: Develop an instrumented mooring system for VFFS at model scale for the towing tank at the TU Delft and determine its accuracy.
A new concept is developed in this project. This concept resulted from an extensive concept development where all functions of the system were analyzed. With the use of a Morphological Chart and a Multi Criteria Analysis the best concept was selected. For this concept, it was determined that the focus should be on the sensor configuration and calibration procedure.
First, the optimal sensor configuration of the concept was specified by analysing the working principle of the concept. Second, the calibration procedure was further analyzed. From this analysis, three calibration procedures were developed: the single sensor calibration matrix, the full fixed calibration matrix and the full rotated calibration matrix. From literature and theory, it was not possible to determine in advance what calibration procedure should be selected, and therefore the performance of the procedures were verified with experiments. All calibration procedures were executed, whereafter the performance of the different procedures were compared. The two main considerations for the comparison were the accuracy and the usability of the procedures. After performing the comparison, the main conclusion was that the full upright calibration procedure is the optimal procedure.
To verify the concept under realistic conditions, an example application was performed in the towing tank No.1 at the TU Delft. By doing this, the concept has proven to be suitable to measure the mooring force and transform them into usable data.
In this project a new concept was developed into a working system. This system forms an excellent base for extensive research into the mooring system of VFFS, and is a good addition to the measurement instruments for the towing tank at the TU Delft. It is concluded that the system is able to measure the mooring forces and the direction. The accuracy of the system still has to be improved, and with additional research the working concept can be further developed.