The Impact of Wave Drift on the Tow Resistance and Towing Stability of a Towed Vessel

The Impact of Wave Drift on the Tow Resistance and Towing Stability of a Towed Vessel: A wave drift estimation method assessment and towing stability simulation for course stable and unstable barges

Baas, Mathieu (TU Delft Mechanical, Maritime and Materials Engineering)

Huijsmans, Rene (mentor)
Schreier, Sebastian (graduation committee)

Delft University of Technology

2017-07-31

Long distance ocean towing is a commonly used transportation method in the oil & gas and renewable energy industries. Barges with cargo, vessels and various offshore structures are towed from and to location. Boskalis participates in this industry through its subsidiaries Smit and Fairmount Marine. From experience with these transports, it was found that the preliminary calculations do not match with the experiences offshore and that the impact of wave drift on these calculations is most promising for further research. To investigate this impact, the problem is split into three parts: wave drift estimation for a stationary vessel, wave drift estimation for a sailing vessel and the analysis of the towing stability.From the literature, eleven methods are identified for the wave drift estimation of a stationary vessel. These are compared for the surge, sway and yaw force components. The different methods are computed using the diffraction program Delfrac, the strip theory solver ShipMo and own implementations in Matlab. With vessel velocities included, ten wave drift estimation methods for the sailing vessel are compared. Specific attention is paid to the wave drift damping, which relates the stationary wave drift to vessel velocities. It is implemented using Aranha's formulations and the panel pressure output of Delfrac. The most applicable methods with respect to the sailing tow resistance are the far field and near field methods in combination with the wave drift damping. With respect to the total tow resistance, the wave drift contribution is found to make up roughly 25% of the total resistance, irrespective of forward velocity. The dynamic behaviour of the tow operation is examined for two barges: one course stable and one course unstable barge. Their towing stability properties are assessed without waves present and for head waves conditions. It is found that the head waves do not affect the stability of the course stable barge assuming the towing tug has sufficient available towing force. For the course unstable barge, the main stabilising effect is identified as the extra resistance due to the head waves and not the direct wave drift moment. The relative contribution of the waves with respect to the wind and current shows that the head waves have a significant stabilising contribution to the towing stability although the specific relative contributions are highly dependent on the environmental conditions. Head waves either reduce the path width of the fishtailing motion or dampen these oscillations into a stable towing position. This was verified by simulations in the time domain. Irregular waves also promote the stability: they stabilise the unstable barge in the entire range of physically relevant wave periods, irrespective of towline length.Overall, the contribution of the wave drift to the tow resistance is significant, especially while sailing. Head waves are beneficial for the towing stability of a course unstable barge, both in regular and irregular sea states. This increased insight reduces the uncertainty in the tow resistance prediction and contributes to the understanding of the towing stability properties of unstable barges.

Wave Drift
Wave Drift Forces
Tow Resistance
Towing Stability
Course stable
Course unstable
Barges
Wave drift estimation
Delfrac
ShipMo
MATLAB

http://resolver.tudelft.nl/uuid:d73c22b1-4731-4283-b5ac-9e76cb99d14a

Student theses

master thesis

© 2017 Mathieu Baas