Development of an improved shielding model for vessels and offshore structures

Development of an improved shielding model for vessels and offshore structures

El Marini, S.

Metrikine, A.V. (mentor)

Mechanical, Maritime and Materials Engineering

Maritime and Transport Technology

Offshore & Dredging Engineering

2015-10-13

MARIN has developed the computer program WINDOS for the calculation of wind loads on vessels and various offshore structures. To account forwind shielding effects,semi-empirical relations based on Schlichting’stheory for free turbulent flow are used within the program.An extensive WINDOS investigation is carried out in comparison with the wind tunnel experiments that were carried out for the Offshore Operability JIP’s with an emphasis on the wind shielding effects during tandem offloading operations. Models of a shuttle tanker and an FPSO in 1/250 scale were used during the study. During the wind tunnel measurements the shuttle tanker was rotated over the full range of headings in ballast condition at 300 meters and 450 meters full scale downstream the FPSO in fully loaded condition with a heading of 30° relative to the undisturbed wind. Thewind load coefficients over the full range of shuttle tanker headings were calculated for both positions. The same situation is modelled within WINDOS and the shuttle tanker coefficients for both positions are gathered. There is a significant error between the wind tunnel and WINDOS results. The disturbed velocity distribution behind an asymmetrical object relative to the flow direction is asymmetrical. Schlichting’s relation in its original form only calculates the variation in longitudinal wake velocity for growing distances and is only able to calculate the disturbed velocity distribution at a certain distance behind a symmetrical objectcorrectly as the output is always a symmetrical wake velocity distribution. In the wind tunnel the velocities at a distance of 300 meters downstream the FPSO in ballast condition with a heading of 0° and 30° relative to the undisturbed wind were also measured. A transformed form of Schlichting’s relationfor free turbulent flow is presented that attempts to calculate the asymmetrical wake distribution behind a rotated object. From the investigation follows that the semi-empirical relation shows acceptable results for the FPSO with a heading of 0° (symmetrical in flow direction), but still gives, despite the transformation, unacceptable results for the FPSO with a heading of 30° (asymmetrical in flow direction). A prediction method is proposed based on Schlichting’s relation in its original form with fundamental assumptions and additions to account for asymmetry. Thelongitudinal force coefficients show a better agreement for both positions, especially when compared to the coefficients obtained with WINDOS. The transversal force and yaw moment coefficients still lack in accuracy but show potential when the relative distance is increased from 300 meters to 450 meters full scale. Overall the obtained results are closer to the measured values.

Shielding

http://resolver.tudelft.nl/uuid:7c254958-1a06-476a-a400-0ddb5aac5785

2020-10-13

Student theses

master thesis

(c) 2015 El Marini, S.