Roll stabilization by anti-roll passive tanks

doi:10.1016/S0029-8018(00)00015-9

Ocean Engineering

Volume 28, Issue 5, May 2001, Pages 457-469

https://doi.org/10.1016/S0029-8018(00)00015-9 Get rights and content

Abstract

Since the most severe roll motion occurs at resonance (known as synchronous rolling), the best way of reducing it is to increase the damping. The most common means of doing so is by the installation of bilge keels. If more control is required, both anti-roll tanks and fins are used. Tanks have the advantage of being able to function when the ship is not underway. The use of tanks with liquid free surfaces for reducing roll motion of ships is an old idea. Many researchers have studied the design of anti-roll tanks. However, most of the past effort has concentrated on studying the performance of anti-roll tanks in damping the roll motion of the ship. Little attention has been paid to the fluid motion inside the tank itself. Another important issue is the tank tuning. Proper tuning of the anti-roll tank, to match the ship's natural frequency, is very important in reducing the roll motion. This paper concentrates on the most familiar type, which is the U-tube passive tank as a mechanical absorber of roll motion. A detailed study, covering tank damping, mass, location relative to the ship CG, and tuning, is presented. New suggestions and observations are stated concerning tank damping and tuning.

Introduction

The reasons for attempting to control and reduce the motions of a ship are as varied as the types of ships. Excessive motions interfere with the recreational activities and comfort of passengers on a cruise ship. Often more than half of the load of a containership is stowed above deck where it is subjected to large accelerations due to rolling. In some situations this may cause some internal damage to the contents of the containers; in more severe situations the lashings can fail and containers may be lost overboard. Offshore platforms, pipe-laying ships, and drill ships require only very small motions to perform many of the individual operations. In these types of engineering tasks, the amplitude of the motions may be the most important feature. Anti-roll tanks are commonly used as mechanical absorbers to reduce roll motions. Generally, they can be divided into passive, controlled–passive, and active tanks.

The anti-roll tank system dates back to Froude. In 1874 Froude installed water chambers in the upper part of a ship for the purpose of achieving stabilization against roll. Nothing further was done with anti-roll tanks until about 1910 when Frahm began to use the U-tube tank. Before World War II, Frahm's passive tanks were installed in over 1,000,000 tons of German shipping, including the passenger liners Bremen and Europa. Many researchers studied anti-roll tanks in the 1960s and 1970s. Vasta et al. (1961) made a review of the Navy's development and installation of passive tanks. They derived the equations of motion, discussed model techniques, and proposed a tank design. Stigter (1966) studied the roll stabilization by a passive U-tube tank. He derived the equations of motion of the fluid in the tank and obtained the coupling terms between the ship and the tank. Today many researchers consider his equations of motion to be a classical basis for studies of U-tube tanks.

Bell and Walker (1966) investigated two types of controlled–passive tanks. First, control is effected by valves in the water channel, and second, control is effected by valves in the air channel. They also proposed an activated tank system with a propeller continuously driven in one direction to save power. Webster (1967) gave a detailed study of the control of pump-activated U-tube tanks. He explained that tank control can best be selected on the basis of minimizing the response of the ship to an impulsive roll velocity. Vugts (1969) designed and compared four passive tanks for the same ship.

A comparative study between U-tube and free-surface passive tanks in regular beam seas was carried out experimentally by Field and Martin (1976). Lewison (1975) proposed a mathematical model to optimize the design of free-surface passive tanks. Barr and Ankudinov (1977) provided a critical review of a number of predictive methods for roll motion and its reduction using anti-roll tanks. Webster et al. (1988) gave a detailed study of free-flooding anti-roll tanks during the major upgrade of the USS Midway in 1986. Recently, Lee and Vassalos (1996) investigated the effect of flow obstructions inside the tank. A challenging problem is to estimate the roll-motion coefficients (especially roll damping). Details of these coefficients were found by Dalzell (1978), Troesch (1981a, b) and Mathisen and Price (1984), Troesch, 1981a, Troesch, 1981b and Mathisen and Price (1984). A substantial amount of effort has gone into studying nonlinear rolling motion (i.e. motion that can only be described by nonlinear equations) in different sea conditions, such as the work by Nayfeh and Khdeir (1986a) for symmetric ships in regular beam seas, Nayfeh and Khdeir (1986b) for biased ships in regular beam seas, Nayfeh (1988) for laterally symmetric ships in head or following regular seas, Gawthrop et al. (1988), Nayfeh and Sanchez (1990) and Sanchez and Nayfeh (1990) for longitudinal seas (i.e. the waves crests are oriented at right angles to the vessel's centerline), Nayfeh and Oh (1995), and Falzarano et al. (1995).

In all of the above investigations, researchers concentrated on either theoretical manipulation of the equations of motion or model testing and analyzing existing tanks. Less effort was made to link the theoretical investigations with the demands of actual operating conditions. This study aims to extend the theoretical investigations to optimize the performance of passive U-tube tanks over a wide range of excitation frequencies. Passive U-tanks have the following advantages: (a) they have no moving parts; (b) they require little maintenance; and (c) they avoid the small resistance penalty produced by fins and bilge keels. On the other hand, they occupy a considerable volume of the ship's hull, which creates accessibility problems. The free surface of the tank reduces the metacentric height of the ship, which may create stability problems. New methods for tank tuning are proposed. This tuning has a unique importance in real operating conditions where the load of the ship and consequently its natural frequency may change frequently.

Section snippets

Equation of tank fluid motion

A configuration of a simple U-tube passive tank is shown in Fig. 1. The tank consists of two side reservoirs and a connecting duct of constant rectangular cross section. The coordinate system shown is used to determine the motion of the liquid in the tank caused by the motion of the ship. The origin 0 is at the midpoint of the connecting duct and the axis y runs along the duct and up the reservoirs of the U-tube. The fluid velocity along the positive y direction (up the port reservoir) is v.

Equation of motion for a ship with a passive tank

For a purely roll motion, Lloyd (1989) derived the following equation: $A_{44} φ ̇̇ +b_{44} φ ̇ +c_{44} φ+ a_{4τ} τ ̇̇ +c_{4τ} τ =F sin ω_{e} t$ where A₄₄ is the ship roll inertia, b₄₄ is the ship roll damping coefficient, c₄₄ is the ship roll stiffness, a_4τ is equal to a_τ4, c_4τ is equal to c_τ4, F is the amplitude of the excitation roll moment, and ω_e is the wave encounter frequency. The expression inside the square brackets is the roll stabilizing moment from the liquid inside the tank. It follows from , that the tank natural

Effect of tank damping

The damping inside the tank depends on the machining accuracy smoothness of the tank walls and the design and structure requirements. Fig. 2 shows the response amplitude operator (RAO) or the roll angle normalized by the wave slope amplitude at ω_t/ω_s≈1.0. In this case, the worst motions of the ship (motions near the resonant frequency) couple easily into motions of the stabilizer. In practice, it is typical for this ratio to be slightly larger than unity to account for the difference in damping

Effect of tank mass

The proper choice of the stabilizer mass is vital. A light stabilizer would not be able to affect the ship motion. A very heavy stabilizer takes a considerable amount of the hull space and dangerously lowers the metacentric height of the ship and reduces its stability. Fig. 4 shows the effect of the tank length x_t, which represents the tank mass, on the roll angle, tank angle (left axis), and loss in metacentric height LMH (right axis). The tank damping is chosen to be the optimal value of

Effect of tank location

The effect of location of the duct (bottom part of the tank) with respect to the ship center of gravity (CG) is shown in Fig. 5. Negative values of r_d/w mean that the duct is above the CG. The maximum roll response remains small and almost constant for all values less than or equal to zero for both the 2.0 and 4.0% mass ratios. The roll response increases dramatically when it is placed below the CG. This increase is delayed for higher tank masses. The tank angle decreases with increasing r_d/w.

Tank tuning

During regular cruising, a ship is subjected to changes in its natural frequency (see Fig. 6). This is more obvious for tankers and cargo ships, where the load is changing from one port to another. Also, warships are subjected to a change in their load due to firing missiles, launching air-crafts, and burning fuel. Since the optimal effect of the stabilizer is obtained when ω_t≈ω_s, it is vital to change ω_t to be as close as possible to ω_s. It follows from Eq. (6) that ω_t is a function of the

Conclusions and recommendations

This study gives a link between the theoretical investigations and actual operating needs of anti-roll tanks. Special attention is paid to tank damping and tuning. From the present study, the following points can be stated.

1.
There is an optimal value for the tank damping. Each tank should be equipped with damping devices or vortex generators to control the fluid motion.
2.
A compromise should be made between the space occupied by the tank and the maximum tank angle to avoid tank stall. A mass ratio

Acknowledgements

This work was supported by the Office of Naval Research under Grant No. N00014-96-1-1123.

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Roll stabilization by anti-roll passive tanks

Abstract

Introduction

Section snippets

Equation of tank fluid motion

Equation of motion for a ship with a passive tank

Effect of tank damping

Effect of tank mass

Effect of tank location

Tank tuning

Conclusions and recommendations

Acknowledgements

Ship rolling, its prediction and reduction using roll stabilization

Marine Tech.

Activated and passive controlled fluid tank system for ship stabilization

SNAME Trans.

A note on the form of ship roll damping

J. Ship Res.

A combined steady-state and transient approach to study large amplitude ship rolling motion and capsizing

J. Ship Res.

Comparative effects of U-tube and free surface type passive roll stabilization systems

Royal Inst. of Naval Arch.

Parametric identification of nonlinear ship roll motion from forced roll data

J. Ship Res.

An investigation into the stability effects of anti-roll tanks with flow obstructions

Int. Shipbuild. Prog.

Optimum Design of Passive Roll Stabilizer Tanks

Seakeeping-Ship Behaviour in Rough Weather

Estimation of ship roll damping coefficients

Royal Inst. of Naval Arch.

On the undesirable roll characteristics of ships in regular seas

J. Ship Res.

Nonlinear rolling of ships in regular beam seas

Int. Shipbuild. Prog.