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a Hydrodynamic Analysis of a Moonpool structure with Mooring Lines Using the ANSYS AQWA


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

PRE

SENTATION 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
Dimensions External dimension: 11,25 m Internal dimension: 5,5 m Depth: 6,66 m Displacement (moorings): 1708,176 ton CG: (0, 0, -3.33) m (referece in Water Line) The unit system recomended for the geometry creation in ANSYS DesignModeler is meter.

An hexagonal floating monocolumn system

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
In AQWA, users must define its point mass (mass, inertia and position of CoG)

The geometry must be created in DM, with the fairlead points (Create > Points)

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
The anchor connection points are defined in AQWA

Catenary Sections (cable segment properties)

Definition of each cable of the mooring system (different types, e.g. catenary, linear and elastic)

Diffraction/Radiation analysis parameters

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.
AQWA Classical

AQWAWB 12.0

AQWA 13.0

Diffraction / Radiation Results (New)
3D – Plots

Force x Frequency x Direction

Radiation Damping x Frequency

Mooring System (New)
The mooring system designed is composed by 9 cables, disposed 3x3 equally and espaced by 120?.

Mooring System
? Fairlead and anchor points table:
Line 1 2 3 4 5 6 7 8 9

X(m) 0.87 0 -0.87 -9.06 -8.66 -8.19 8.19 8.66 9.06

Fairlead (*) Y(m) 9.96 10 9.96 -4.23 -5 -5.74 -5.74 -5 -4.23

Z(m) -5.66 -5.66 -5.66 -5.66 -5.66 -5.66 -5.66 -5.66 -5.66

X(m) 54.63 0 -54.63 -568.12 -542.87 -513.49 513.49 542.87 568.12

Anchor Y(m) 624.46 626.85 624.46 -264.92 -313.43 -359.55 -359.55 -313.43 -264.92

Z(m) -2200 -2200 -2200 -2200 -2200 -2200 -2200 -2200 -2200

Farilead points must be defined in Design Modeler

Anchor points must be defined as Connection Points

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.


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