The Ocean Cleanup Barrier

The Ocean Cleanup Barrier: A hydro-elastic model for fatigue lifetime assessment of The Ocean Cleanup barrier

Tanis, A.H.

Kaminski, M.L. (mentor)

Mechanical, Maritime and Materials Engineering

Marine and Transport Technology

Offshore & Dredging Engineering

2016-10-03

The Ocean Cleanup proposes to place a huge barrier in the ocean in order to clean it from plastic waste. Plastic will be caught as it flows towards the barrier on the ocean’s currents. As such a structure has no precedent, the concept has to be engineered from the bottom up and tested extensively. The barrier has a relatively shallow draft and moves in the splash zone, making it difficult to model the barrier’s behaviour. First, this study focusses on the development of a numerical two dimensional (2D) hydro-elastic model. This model has to be able to simulate the motions of such a barrier as observed in two dimensional tests and allows for a better understanding of the barrier’s hydrodynamic behaviour. The wave forces on the barrier are calculated with the Morison equation, which is often used in offshore hydrodynamics to calculate the wave forces on slender members. Including all details necessary for a correct simulation, the model is simplified wherever possible. This reduces runtime so that the model can be used for simulations for a fatigue assessment in the second part of this thesis. These assumptions are validated using the results from 2D tests carried out by Deltares. In addition, another student will develop a CFD model to validate these simplifications by an alternative way. The results of the validation using the 2D tests suggest that the motions can be simulated very well using this approach. The validation also shows that the horizontal position of the barrier seems to be mostly affected by the forces on the screen, while the vertical position is dominated by the buoyancy force of the floating boom. The second part of this study consists of a first qualitative assessment of the fatigue life of the barrier. Earlier tests have shown that the loads on the barrier are relatively low. This is why fatigue is expected to play an important role, given the fact that the barrier will be deployed for about 5 to 10 years. Fatigue life of the rubber component at the point where the floating boom is connected to the plastic capturing screen is investigated. To carry out this assessment, the prototype currently deployed in the North Sea is used as a starting point. The hydro-elastic model from the first part of this thesis is used to simulate the barrier’s motions. Using a structural model which simulates the rubber’s behaviour, these motions are superimposed from the global to the local structure. For this structural model a hyperelastic material that resembles the rubber component is assumed. After superimposing the barrier’s motions, fatigue life of the barrier is calculated using a damage model suitable for rubber. The results show that fatigue life seems to be dominated by seastates with steep waves or with large velocity and acceleration amplitudes. Also, the fatigue life in the Pacific Ocean will be about two and a half to three times longer than for one deployed in the North Sea.

http://resolver.tudelft.nl/uuid:6a62d2a0-cf7a-4c66-9edf-2d7e1bef26ac

Student theses

master thesis

(c) 2016 Tanis, A.H.