Motion Response Project pdf

Motion response of an intact and damaged

vessel in head waves

Sainath Atul Nashikkar

B.Tech Naval Architecture and Ocean Engineering

Indian Maritime University

INTRODUCTION

• The design for marine structures requires a lot of understanding

about the behaviour and characteristics of the ship at sea .

• Once in the water the ship has to meet its operational

requirements , structural strength requirements and also its

environmental and safety of the passengers and crew on board

.

• The uncertain behaviour of the sea adds to the efficient design

of ship for various sea conditions .A wide and reliable study of

the characteristics of the ship help the designers in designing an

efficient ship .

• Towing tanks ,wind tunnels ,cavitation tunnels and wave basins

have been considered as reliable inputs of the knowledge of

ship characteristics at sea. But they are expensive so only ship

owners with big budgets opt for it.

The Use Of CFD in Marine

• Résistance and wave prediction .

• Manoeuvring and seakeeping analysis

• Propulsion system flow analysis

• Ship motion in waves

• Flow over control devices in a ship

• Movements of fluid within internal tanks of a ship

• Using CFD results for structural analysis .

Ship Motions

Number Degree of freedomForce and

moments

Linear and angular

velocity

Position/

Euler angle

1 Motions in the x- direction (Surge) X u x

2 Motions in the y-direction (Sway) Y v y

3 Motions in the z- direction (Heave) Z w z

4 Rotation about x-axis (Roll) K p ϕ

5 Rotation about y-axis (Pitch) M q θ

6 Rotation about the z-axis (Yaw) N r Ψ

Translatory motions

ROTATORY MOTIONS

INTACT SHIP

In this condition the ship does not have any leakages or any ingress of water

into the hull.

DAMAGED SHIP

In this condition the ship has an opening in the hull because of collision with another ship, grounding or explosion .

HEAD WAVES

When the waves approach the ship opposite to the direction of the heading of the ship .

Terms used

IMPORTANCE OF THE MOTIONS

Heave Motion : The heave motion of the ship leads to increase and

decrease in the hydrostatic pressure around the vessel . The cases of

hogging and sagging occur because of this .Heaving plays an

important role in drill ships and also for helicopters landing on vessels .

.

Pitch motion : The pitch motion of the ship is responsible for slamming

or pounding and an idea of the pitching motion helps in identifying

the forces acting on the bow

Roll Motion : The roll motion in a ship is particularly important

because excessive rolling may lead to capsizing of the ship . In

passenger ships rolling leads to a very uncomfortable ride and

causes sea sickness.

Ship damages

IMPORTANCE OF THE STUDY

The study of ship motion is very important as it helps the designers

and vessel operators.

• To understand the behaviour of the ship in various sea states .

• Designing of roll damping devices with adequate strength and

efficiency .• Calculating the forces that would be generated by these motions

helping in proper strengthening of ship.

• Damaged Ship responses are particularly important to passenger

vessels which have to comply with the International Maritime

Organization (IMO) Safe-Return-to-Port (STRP) requirement .

• Damaged ship responses also help naval vessels to understand the ship motions after damage.

Paper and Geometry

• Experimental Ship Motion and Load Measurements in Head and

Beam Seas

Authors :

E.Begovic –Department of Naval Architecture and Marine

Engineering , University of Naples Federico II , Napoli , Italy

A.H.Day , A.Incecik –Department of Naval Architecture and Marine Engineering ,

University of Strathclyde , Glasgow ,UK

• The geometry used is an unappended hull of DTMB 5415 ,a benchmark model of ITTC . Full scale dimensions of the ship are

taken into the computation.

GEOMETRY DETAILS

SETTING THE CASE

• The case was setup in EHP

(Estimate Hull Performance) and

modifications to the setup were

done as required .

• EHP sets up the case for half

model but the half model was

then replaced with a complete

one as a complete model is

required for roll response .

• The case was first run with Flat

waves as for testing the setup .

MESHING

• The cell count was one

million cells.

• Thickness of Near Wall

Prism Layer is Changed for

each wave to keep the

Wall Y+ value within the range of 100-150.

VOLUME MESH

REPRESENTATION

PHYSICS SETUP

• The EHP sets up a flat wave . It

was later changed to a Fifth order wave .

• Rest all models were the same .

Fifth Order Wave specifications

• Four different wavelengths were chosen randomly from a set of twenty one waves .

• The wavelength was calculated from the Wavelength to Ship

length ratio .

• The wave height was calculated according to the steepness

ratio given .

• The sea wave period of the waves were given from which the

wave speed was calculated .

• The water depth was taken as the depth of the domain.

DFBI Solver

• The hull mass for the full ship was

inserted manually as EHP inserts

the mass of half ship.

• The Centre of Mass was also

calculated by EHP which also

matched with the original data.

• EHP only calculates Moment of

Inertia in the Y-axis ,the Z and X

axis Moment of Inertia was

calculated manually using

empirical formulas .

CALCULATION OF MOMENT

OF INERTIA

I = m*r^2

Rx = 0.33B

Ry =0.26Lpp

Rz =Ry

Where m = mass

B = width

Lpp = length between perpendiculars

SOLVING CONDITIONS

• The implicit unsteady solver time step was kept at 0.1

seconds

• The maximum inner iterations were kept to 15.

• All Cases were run for a maximum physical time of 350

seconds

• Computing time was nearly 3 days for each wavelength

case using 4 processors .

WAVE DATA

1st Case

Wavelength = 44.5086m

Wave height = 0.890172m

Wave period = 5.336 s

Velocity of wave = 8.3411 m/s

2nd Case

Wavelength = 146.466m

Wave Height = 2.929 m



3rd Case

Wavelength = 62.1414 m

Wave height = 1.243 m



4th Case

Wavelength = 72 m

Wave height = 1.44 m



Intact Heave RAO

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.2 0.4 0.6 0.8 1 1.2

HEa

ve

RA

O

λ/L

RAO – Response amplitude operator ( Heave amplitude / Wave

amplitude )

HEAVING

Intact Pitch RAO

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.2 0.4 0.6 0.8 1 1.2

Pitc

h R

AO

λ/L

PITCHING

DAMAGED CONDITION

In this case the damaged condition has been simulated by simulating the final condition of the ship after damage .

DAMAGED HEAVING

DAMAGED PITCHING

FUTURE WORK

• Modelling water intrusion into the ship and simulating flooding

into the compartment . Accurate modelling of the internal of

the ship would be an added advantage.

• The present work is performed on an unappended hull but in

reality the ship has many appendages such as the bilge keels ,

fin stabilizers ,rudder , propeller ,etc. Superstructure of the ship is

also not considered .

• Higher sea state conditions can be simulated to know the

survivability of the ship .

• The current work simulates regular waves but in reality the sea

waves are mostly irregular .So simulations with irregular waves

can be a major breakthrough.

Motion Response Project pdf

Documents

Transcript of Motion Response Project pdf