mooring of ships

8/13/2019 Mooring of Ships

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Automated design of a ship mooringsystem

The challenge: To investigate a mechanism to control and automate a mooringsystem between two ships at sea

Maplesoft, a division of Waterloo Maple Inc., 2009

Editor's Notes

Introduction

Problem Statement

1. Physical Model

Representation2. Modeling WaveDynamics

3. Open-Loop Response

4. Closed-Loop Response

Results

** This application was developed using Maple and MapleSim


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Editor's NotesThe mooring of ships to harbors, terminals, and offshore structures is a common and essential procedure inmost seafaring operations. Inadequate mooring can result in significant structural damage to the berthingvessel and moorings. To this day, most mooring operations are still performed in the same manner as theywere decades ago; they are dependent on heuristics, or in other words, the captain or mooring mastersexperience. Unfortunately, the effects of global warming and climate change are altering the hydrodynamics of the sea, making manual mooring operations a very risky venture.

The challenge: To design a mechanism to control and automate a mooring system between two ships at sea.

MapleSim and Maple are used to:

Create a physical model of the two ships and mooring cablesDevelop a realistic model of the external environmental forces and use the results to simulate conditions atseaDesign and tune an appropriate controller to stabilize the system

Developing a realistic model of the external forces was critical to the design of the controller. This allowed theengineer to tune the controller, eliminate vibrations, and stabilize the tension in the line. With the initial controlsystem complete, further improvements can be made to the disturbance model by including irregular wavesfrom multiple directions, wind gusts, wave drift forces, and the interaction effects from passing ships.

Introduction

The mooring of ships to harbors, marine terminals, floating terminals, and offshore structures is an essentialpart of most seafaring operations. Inadequate mooring can severely jeopardize the success of most ship-to-

ship operations resulting in large operational downtimes. In addition, it can also result in significant structuraldamage to the berthing vessel and/or the moorings. Consequently, optimizing and stabilizing the mooringsystem is a high priority for most seafaring operations.

Mooring operations today are still performed in the same manner as they were decades ago. That is, theydepend on heuristics, that is, the captain or mooring masters experience. Unfortunately, the effects of globalwarming and climate change are altering the hydrodynamics of the sea, making un-mechanized mooringoperations a very risky venture.

This application investigates a preliminary design of a control system to automate the mooring processbetween two large vessels, such as an oil tanker, in the open sea. The hydrodynamic interactions of thewaves and the presence of wind are modeled by a second-order linear partial differential wave equation. As

expected, the open-loop response of the system results in severe vibrations on the tension in the mooring line. The incorporation of a PID controller to the open-loop system eliminates the vibrations on the line, andstabilizes the tension at 70 N/m.

Problem Statement

To investigate the hydrodynamic interaction of a ship mooring system and develop a controller to ensure thatthe mooring system can securely withstand external forces in the form of wind and waves.


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As can be seen in the diagram, the physical model is made up of a controller and plant model. The controllermodel adjusts the motor voltage driving the Mother Ship so that the tension in the mooring system remains at72 N / m. The tension in the mooring line is fed back to the control system through a force sensor that isconnected between the Mother Ship and the Mooring Cable.

The Mother Ship submodel was modeled by a DC Motor and Gear submodel, where the DC Motorwas modeled as an equivalent electric circuit and an ideal gear. The mass of the Mother Ship wasassumed to be significantly larger than that of the Daughter Ship which is why it was modeled as afixed, non-moving surface.

The Mooring System which normally entails affixing the Daughter Ship to the Mother Ship with atleast 6 cables, was modeled by a single stiff spring.

The Daughter Ship submodel was modeled with a sliding mass component of 100 tons. For thismodel, it was assumed that the engine driving the Daughter Ship was turned off.

The data used to model the Wave Dynamics submodel was derived in Maple. The details can be

found in the next section.

Each of these submodels, and the components that were used to model them can be seen in the .msim file.


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The wave dynamics felt by the Daughter Ship when it is 0.5 m away from the Mother Ship can be seen in thefollowing plot.

WaveEquationSoln :- plot x = 0.5, t = 0.05 .. 20

t2 4 6 8 10 12 14 16 18 20

4

3

2

1

0

1

2

3

According to research, the effects of wind on the wave dynamics defined above can be modeled by Gaussianwhite noise.

WindRandomVariable d Statistics RandomVariable Normal 1, 0.152

:

WindFunction d

t /

Statistics Sample WindRandomVariable , 1 1 :WindSignal d seq WindFunction t , t = 0.05 ..20, 0.05 : plot seq t , t = 0.05 ..20, 0.05 , WindSignal , title = "Wind Signal"


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2 4 6 8 10 12 14 16 18 20

0.3

0.4

0.5

0.6

ind ignal

The total wave dynamics, which take into account the effects of wind, can be seen below.

WaveDataProc d WaveEquationSoln :-value x = 0.5, t = 0.05 ..20 :

WaveData d seq rhs WaveDataProc i 3 , i = 0.05 ..20, 0.058

:

TimeData d seq i , i = 0 .05 ..20, 0.05 :TotalWave d zip x, y / x$ y C 0.01 , WaveData , WindSignal :

plot TimeData , TotalWave , title = "Wave Input (wave C wind)"


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(3)(3)

2 4 6 8 10 12 14 16 18 20

0.2

0.1

0

0.1

0.2

ave Input (wave + wind)

WaveDynamics d convert TimeData , Vector column convert TotalWave , Vector column

400 x 2 Matrix

Data Type: anything

Storage: rectangular

Order: Fortran_order


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3. Open-Loop Response

As expected, the open-loop response to the system resulted in severe vibrations far surpassing the desiredline tension value.

# Without Controller

WaveWithoutController

Time5 10 15 20

A m p

l i u

d

0.2

0.1

0

0.1

0.2

ave Input

OuptutWithoutController

Time

5 10 15 20

T n s i o n

N / m

40000

30000

20000

10000

0

10000

2000030000

40000

50000

Cable Tension Output

Note: These plots were obtained by simulating the model with the parameter set defined in "OpenLoop. params" file.


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4. Closed-Loop Response

By adding a PID controller, the tension of the line was stabilized at around 70 N/m. The PID gain values wereobtained by tuning the model with a step pulse of 1m.

# With Controller

WaveWithController

Time5 10 15 20

A m p l i u

d e

0.2

0.1

0

0.1

0.2

ave Input

OuptutWithController

Time5 10 15 20

T n s i o n

N / m

20

10

0

10

20

30

40

50

60

70

Cable Tension Output

Note: These plots were obtained by simulating the model with the parameter set defined in "ClosedLoop. params" file.


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Results

This application illustrates the preliminary design of a controller to stabilize the mooring system of a ship onthe open sea. As mentioned previously, inadequate mooring could severely hamper ship-to-ship transfers of

cargo and other goods. Moreover, in the worst case, it could result in significant, and costly damage, to boththe mother and daughter ships.

Developing a realistic model of the external forces in the form of wind and waves is critical to the design of thecontroller. That said, the next step towards improving the control system design is to make improvements onthe disturbance model by including irregular waves from multiple directions, wind gusts, wave drift forces, andthe interaction effects from passing ships. In this application, the wave dynamics are only applied to thedaughter vessel; however, in the future, the wave dynamics should also be applied to the mother vessel.

Legal Notice: Maplesoft, a division of Waterloo Maple Inc. 2009. Maplesoft and Maple are trademarks ofWaterloo Maple Inc. This application may contain errors and Maplesoft is not liable for any damages resultingfrom the use of this material. This application is intended for non-commercial, non-profit use only. ContactMaplesoft for permission if you wish to use this application in for-profit activities.

mooring of ships

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