Improvement of the workability of monohull heavy lift vessels during lifting operations by reducing roll motion

Improvement of the workability of monohull heavy lift vessels during lifting operations by reducing roll motion

Van Heijst, J.F.

Van der Heiden, K.T. (mentor)
Keijdener, C. (mentor)
Metrikine, A.V. (mentor)

Civil Engineering and Geosciences

Offshore and Dredging Engineering

2015-11-25

In the past Jumbo Maritime has focused mostly on port-to-port transport of heavy loads. During the last decade their focus shifted slightly towards the offshore market: two of the J-type vessels have been outfitted with DP2 systems and deep water winches. The motion behaviour of these monohull vessels in offshore lifting operations, during which only small roll angles are allowed, is not considered to be optimal as the rolling motion often limits the workability. A survey of existing roll reduction systems (RRS) was conducted. Taking into account the given preconditions, two active anti-roll tank systems were selected for further investigation: U-tanks and N-tanks. Both systems create a stabilizing moment by shifting the centre of gravity of the tank by displacing water inside the hull. The displacement is controlled by adapting the air pressure at the top of the tank. The U-tank is a closed system where a horizontal duct connects water tanks located on each side of the vessel while the N-tank system uses two separated tanks with an open port to the ocean for the inlet of water. To model these RRS a 3 DOF (sway, heave, roll) time domain model in Matlab/Simulink was created. The hydrodynamic coefficients and force response amplitude operators (RAO) were extracted from output files of the radiation/diffraction panel program WAMIT. In order to calculate the motions of the vessel in the time domain, the Cummins equation was solved. The time consuming convolution integral was replaced by a 30th order state space system; a fairly new technique in ship motion modelling. Wave forces were modelled using the RAO’s. Validation of the ship model was done using Orcaflex. The U-tank and the N-tank systems were modelled using Bernoulli’s principle. The missing viscosity is captured using a discharge coefficient and empirical pipe friction. An initial design for both tanks based on literature and the infrastructure present in the vessel is created, with equal dimensions for both systems for best comparison. The two systems have been compared in multiple sea states, both regular and irregular, with various different wave periods, significant wave heights and headings. Sensitivity analyses were carried out for the main system parameters. In addition, workability analyses were done using scatter data from a West-Australian area. The two main conclusions drawn from these results are: 1. The U-tank is a more efficient system. 2. The initial design is not optimal. The U-tank is more efficient because the effective pressure (the pressure forcing the water in or out a tank) is calculated as the pressure difference over the port and starboard tank, while for the N-tanks it is relative to the atmospheric pressure. Because of this, the effective pressure is higher for U-tank resulting in an inflow velocity that is about 40% higher than for the N-tank. An improved design for the U-tank has been made based on the knowledge obtained in the sensitivity analysis. A final size of the system cannot be given as this is based on economic consideration which fall outside the scope of this report. Workability in beam waves for the selected area was improved from 43% for the base case to 94% with the proposed RRS. Lastly, the system was excited with an extreme load to analyse the effects of the RRS on the stability during an incident wave, but no unexpected behaviour was found.

anti-roll tanks
u-tank
n-tank

http://resolver.tudelft.nl/uuid:34b4b2d0-5a19-4384-ae39-ddbbd62f3a27

2020-11-15

Student theses

master thesis

(c) 2015 Van Heijst, J.F.