A Methodology for a Hydrodynamic Analysis

of a Moonpool structure with Mooring Lines

Using the ANSYS AQWA Product

Maira Vargas, M.Sc - ESSS

Lucas Froes Cavalcanti - COPPE

PRESENTATION TOPICS

• Introduction

• Problem Description;

• Methodology;

• Conclusion and next steps.

Introduction

• This paper describes two examples of hydrodynamic

analysis methodology of a particular floating system with

(and also without) considering the effects of its mooring

lines and of a typical ship hull using ANSYS AQWA

programs

• AQWA allows to develop different types of analysis,

working independently, in the time and frequency domain.

CASE 1 – HYDRODYNAMIC EVALUATION OF A FLOATING

STRUCTURE WITH THE EFFEC OF MOORING LINES

Problem Description

• Geometry to be reproduced in DM

An hexagonal floating monocolumn system

DimensionsExternal dimension: 11,25 m Internal dimension: 5,5 m Depth: 6,66 mDisplacement (moorings): 1708,176 tonCG: (0, 0, -3.33) m (referece in Water Line)

The unit system recomended for thegeometry creation in ANSYS DesignModeler is meter.

Worbench Platform with AQWA 13.0 (New)

The time response analysis takes the

geometry, model and /or results obtained in

frequency domain (Hydrodynamic

Diffraction).

Geometry and Meshing

The geometry must be

created in DM, with the

fairlead points (Create

> Points)

In AQWA, users

must define its point

mass (mass, inertia

and position of CoG)

Meshing (allows

user to control

many important

parameters)

Geometry and Meshing (New)

• It’s important that the points created be a component of the platform.

It’s done by creating a line connecting the point and the structure. In

AQWA, user must supress the line, so it doesn’t interfer in the results.

Line connecting point and platform

Fairlead points (New)

• To define the fairlead points in Design Modeler, it’s possible to create

a .txt file, containing the points coordinates and import it with Create >

Points > From Coordinates File

ANSYS AQWA Outline

Definition of each cable of the mooring system

(different types, e.g. catenary, linear and

elastic)

The anchor connection points are defined in

AQWA

Diffraction/Radiation analysis parameters

Catenary Sections (cable segment properties)

Time Response analysis parameters

Wave Frequencies

• It’s important to define the range of waves frequencies or periods we

want to analyze. In our case, it was set the initial frequency (0,02 Hz)

and also the end frequency (0,2 Hz), for a number of intermediate

values equal to 8. This last value automatically calculates the interval

frequency.

Diffraction / Radiation Results

In AQWAWB it’s possible to visualize the wave contours in the frequency domain. Note that it changes by considering different frequencies / wave heights.

Diffraction / Radiation Results (New)

• AQWA 13.0 comes with even more graphic results, allowing users to

plot all these parameters below:

Diffraction / Radiation Results

• The graphic comparison shows that the results are the same with the

upgrades.

AQWAWB 12.0

AQWA 13.0

AQWA Classical

Diffraction / Radiation Results (New)

Radiation Damping x Frequency Force x Frequency x Direction

3D – Plots

Mooring System (New)

The mooring system

designed is composed

by 9 cables, disposed

3x3 equally and

espaced by 120º.

Mooring System

• Fairlead and anchor points table:

Line Fairlead (*) AnchorX(m) Y(m) Z(m) X(m) Y(m) Z(m)

1 0.87 9.96 -5.66 54.63 624.46 -22002 0 10 -5.66 0 626.85 -22003 -0.87 9.96 -5.66 -54.63 624.46 -22004 -9.06 -4.23 -5.66 -568.12 -264.92 -22005 -8.66 -5 -5.66 -542.87 -313.43 -22006 -8.19 -5.74 -5.66 -513.49 -359.55 -22007 8.19 -5.74 -5.66 513.49 -359.55 -22008 8.66 -5 -5.66 542.87 -313.43 -22009 9.06 -4.23 -5.66 568.12 -264.92 -2200

Anchor pointsmust be definedas Connection

Points

Farilead points mustbe defined in Design

Modeler

Cable sections (New)

For each section, user must define the following main properties:

Cable summary (New)

Each cable (1-9) is composed by the sections (1-6 already

described) shown below:

Time Domain Analysis (New)

• In time domain, we can compute the cable influence in the system

response (including or not its dynamics), considering many different

sea and environment conditions.

Time Domain Analysis Results (New)

• In Time Response analysis, it’s possible to visualize the results of all

these parameters below:

Parametric Analysis (New)

• It’s possible to create a parametric analysis, to study the influence of

the systematic variations in geometry and hydrodynamic (frequency

and tme domain) parameters.

Parametric Analysis (New)

• Here are some examples of parametrization with time domain and

cable system. Note that every item with the blank box can be

parametrized.

Parametric Analysis

• In WB, it’s possible to define the values of some parameters, in a way

to define sucessive analysis. In this particular case, the wave forward

speed will be changed, to obtain the maximum RAO along the x

rotational axis.

PRESENTATION TOPICS

• Company Overview (2-3 minutes);

• Problem Description;

• Methodology;

• Goals;

• Conclusion and next steps.