Aqwa Programs Tutorial-2

Document Title: AQWA PROGRAM TUTORIAL 2

FREQUENCY DOMAIN ANALYSIS OF COUPLED SYSTEMS

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Other Information: This report contains a simple example in which potential tools of the AQWA FER program are utilized. The RAOs of coupled systems are reported and also the dynamic modes of vibration are studied within the module AQWA LIBRIUM/FER.

Created: 17-09-2010

Signature legend:

WGU - Wilson Guachamin - Naval Architect

AQWA PROGRAM TUTORIAL 2

Frequency domain analysis Rev.1 Page 2 of 17

EXAMPLE 2

Frequency domain analysis



TABLE OF CONTENTS

1 INTRODUCTION ............................................................................................................ 4 1.1 General ....................................................................................................................... 4 1.2 Objective .................................................................................................................... 4 1.3 Scope ......................................................................................................................... 4

2 MODELING CONSIDERATIONS AND ASSUMPTIONS ................................................ 5 2.1 General ....................................................................................................................... 5 2.2 Description of the models ....................................................................................... 5 2.2.1 Main particulars of the HLV in pre-lift condition ........................................................ 5 2.2.2 Main particulars of the topside modules ................................................................... 6 2.2.3 Main particulars of the rigging .................................................................................. 6 2.3 Selection of the environmental conditions .............................................................. 6 2.4 Approach ................................................................................................................... 6

3 ANALYSIS ..................................................................................................................... 8 3.1 Hydrodynamic parameters ....................................................................................... 8 3.2 Free floating RAOs ................................................................................................... 9 3.3 Combining the structures ......................................................................................... 9 3.3.1 Data file preparation in AQWA LIBRIUM ................................................................ 10 3.3.2 Data file preparation in AQWA FER ....................................................................... 13

4 RESULTS ..................................................................................................................... 14 4.1 General ..................................................................................................................... 14 4.2 Coupled RAOs ........................................................................................................ 14 4.3 Dynamic stability modes ........................................................................................ 15 4.4 Significant motions ................................................................................................. 15



1 INTRODUCTION

1.1 General

The AQWA FER program is a module that can be employed to analyze the response of free floating and coupled bodies. Typical examples of coupled systems are: a vessel with catenary mooring lines, a HLV lifting or lowering a topside module from or onto a floating structure, multiple vessels moored by means of fenders and elastic lines, etc. In previous examples, it is considered that the response of the system is linear with the wave excitation since no nonlinear parameters are included. However, nonlinear systems could also be analyzed in the frequency domain if the nonlinear parameters are properly linearized (i.e. drag forces); a typical example could be the lowering of subsea structures.

The frequency domain analysis of coupled systems in AQWA FER has many advantages and therefore is preferred whenever possible:

- Saving in time analysis - Easy to identify all modes of vibration from the RAOs - Nodal RAOs respect to the fixed reference axis and relative between floating bodies - Direct calculation of significant responses - Possibility of including wave spreading and several sea states simultaneously. - Include wave and drift frequency motions. - Basis or preliminary study for further time domain simulations, among others to list a few of

them

Most of the times it is not necessary to spend time carrying out complete time domain simulations of linear systems. In fact, the problem of all coupled systems in which the lifted object is above the mean water line should be solved in the frequency domain.

1.2 Objective

The main objective of this report is to explain some important features when using the AQWA FER program to solve problems in the frequency domain.

1.3 Scope

This tutorial is limited to the frequency domain analysis of coupled systems. If it were necessary some aspects of the other program modules will be briefly explained.



2 MODELING CONSIDERATIONS AND ASSUMPTIONS

2.1 General

This section contains the considerations and required information to complete the modeling and analysis of the coupled system and its procedure.

2.2 Description of the models

The model that is selected for this analysis corresponds to the HLV Oleg Strashnov installing topside modules onto an offshore platform. Only the models of the HLV, the main hook, rigging arrangement and topside modules are included in the analysis.

The model is composed of the following elements:

Diffracting model of the Oleg Strashnov Simplified model of the topside modules Hoisting system including cables, slings and hook

Main particulars of the structures are listed below:

2.2.1 Main particulars of the HLV in pre-lift condition

Description Notation Units Value Water depth d [m] 1000 Length between perpendiculars LPP [m] 171.6 Breadth B [m] 47 Depth D [m] 18.2 Mean draught Tmean [m] 11.926 Displacement [Ton] 63232 Longitudinal center of gravity LCG [m] 72.825 Center of gravity above keel KG [m] 13.90 Transverse metacentric height GMT [m] 11.35 Mass moment of inertia about x Ixx [kgm2] 2.3553E10 Mass moment of inertia about y Iyy [kgm2] 1.3173E11 Mass moment of inertia about z Izz [kgm2] 1.4247E11 Linearized roll viscous damping B(1) [KNms/rad] 5.0103E8 Natural period for roll T [s] 12.4

Table 2.2.1.1: Main particulars of the HLV Oleg Strashnov [1]

Figure 2.2.1.1: Diffracting model of the HLV Stanislav Yudin (Side View)



2.2.2 Main particulars of the topside modules

Description Notation Units Value Overall length along the X axis L [m] 50 Width along the Y axis B [m] 40 Depth H [m] 18 Weight on air Wair [KN] 40000 Mass moment of inertia about x Ixx [kgm2] 1.3724E09 Mass moment of inertia about y Iyy [kgm2] 1.1124E09 Mass moment of inertia about z Izz [kgm2] 1.3724E09

Table 2.2.2.1: Main particulars of the topside modules Note: The geometry could be a simple box!

2.2.3 Main particulars of the rigging

Description Notation Units Value Cables

Length of cables L [m] 36.1 Stiffness C [KN/m] 4.7E5 Slings

Length of slings L [m] 26.2 Stiffness C [KN/m] 5.9E4 Hooks

mass m [Ton] 210 Table2.2.3.1: Main particulars of the rigging

2.3 Selection of the environmental conditions

The environment is characterized by the following parameters:

Wave spectrum

Peak period Peak enhancement

factor

Significant wave height

Wave direction

Tp[s] [-] Hs[m] [] JONSWAP 3.5 2.4 2.0 180 JONSWAP 4.5 2.4 2.0 180 JONSWAP 5.5 2.4 2.0 180 JONSWAP 6.5 2.4 2.0 180 JONSWAP 7.5 2.4 2.0 180 JONSWAP 8.5 2.4 2.0 180 JONSWAP 9.5 2.4 2.0 180 JONSWAP 10.5 2.4 2.0 180 JONSWAP 11.5 2.4 2.0 180 JONSWAP 12.5 2.4 2.0 180 JONSWAP 13.5 2.4 2.0 180 JONSWAP 14.5 2.4 2.0 180 JONSWAP 15.5 2.4 2.0 180 JONSWAP 16.5 2.4 2.0 180 JONSWAP 17.5 2.4 2.0 180

Table 2.3.1: Environmental conditions

2.4 Approach

The following procedure in the present example:

First, a diffraction analysis of the HLV in AQWA LINE, the main goal is to obtain the hydrodynamic parameters (damping, added mass coefficients) and free floating RAOs. (Note: The modeling of the vessel could be done using the *LIN file available at SHL.

Second, the HLV is joined to the topside modules by means of linear cables and the equilibrium position is found; for this purpose the AQWA LIBRIUM module is used.



Third, the frequency domain analysis is carried out in AQWA FER. In this part of the exercise, some RAOs of the coupled structures are compared with the free floating ones.

Fourth, results from comparisons of RAOs and significant responses are reported. Finally discussion of results from this exercise when necessary.



3 ANALYSIS

3.1 Hydrodynamic parameters

The hydrodynamic parameters such as added mass and radiation damping of the structures are automatically calculated from the diffraction analysis carried out in AQWA LINE and the hydrodynamic data base is stored in the *.hyd file. To verify previously calculated results before going on with the analyses, the following results for the VSS template could be cross checked.

It is understood that the user is able to model the structures and carry out the diffraction analysis according to specifications given in section 2 of this report. Anyway, some modeling issues of the structures are included in this tutorial.

>>Open AGS (AQWA Graphical Supervisor) >>Graphs>>File>>Open>>*.PLT >>Radiation damping heave>>heave>>confirm selection>>OK

Figure 3.1.1: Added mass heave-heave of the HLV Oleg Strashnov

Figure 3.1.2: Radiation damping heave-heave of the HLV Oleg Strashnov

All hydrodynamic coefficients are stored in the*.hyd files and will be automatically used when called for in the frequency domain analyses.



3.2 Free floating RAOs Using previous calculated hydrodynamic parameters, the RAOs of the structures are internally calculated in the program and are also stored in the *.hyd file. Similarly it can be displayed as follows.

>>Open AGS (AQWA Graphical Supervisor) >>Graphs>>File>>Open>>*.PLT >>Free floating raos>>roll>>dirn (90)>>confirm selection>>OK

Figure 3.2.1: Roll free floating RAO for the HLV Oleg Strashnov, =90 Notice that the natural frequency for roll is indeed 0.51rad/sec (12.3sec) and the maximum response is about 11.5/m.

If the results from the diffraction analysis match the ones in figures 3.1.1, 3.1.2 and 3.2.1 then the structures could now be coupled together as shown below.

3.3 Combining the structures Both structures HLV and topsides have to be combined into one file and they have to be connected by means of the suspension lines. If the complete model is described in the file, then it can be executed and the equilibrium position for the specified sea state can be found; this equilibrium position will provide the correct coupling terms in the stiffness, added mass and damping matrices of each structure. It is done in AQWA LIBRIUM and some important considerations are briefly explained in this section.



Figure 3.3.1: Set up of the combined structure

3.3.1 Data file preparation in AQWA LIBRIUM The DAT file contains simply the information of each vessel and has to be prepared in the same way as it is done for a multi-body analysis.

>>Options and restart stages

Because new points have to be defined within the structures, it is recommended to restart the model from stage 1 and use the diffraction analysis results only to call fro the hydrodynamic coefficients.

>>Nodal coordinates of the HLV

The nodes of each panel are defined in this section (it is automatically generated if the lines plan utility is used) but it could be necessary to add the NOD5 card to allow more than 9999 nodes.

>>Define additional nodes

Extra nodes for mooring line connection and relative motion analysis are required to be defined



>>Define nodes for the hook and the topsides

The structures could be modeled with aid of any program (The main hook is just a single point STRC2 and STRC3 is the topside). The format is similar to the one of the HLV.

Note: The topsides are modeled using the internal modeling utilities of the program. Reference is made to the AQWA reference manual.

>>Define extra nodes for the connection of the slings in the topsides

>>Paneling of the structures

The meshing of the HLV would be done automatically by the program but the one of the topsides could be input manually. Remember that the normal of the panels must be pointing outwards with a counter clockwise numbering.

The HLV panelling format should look like the one shown below.

In this case the panelling of the topsides follows:



>>The mass of the structures is input in deck 3

>>Mass moment of inertia of the structures and global parameters

>>The hydrodynamic parameters could be copied from a previous AQWA LINE *.hyd file

>> Environmental conditions Wind, current and waves could be defined in deck 13. In this case only waves are considered as per table 2.3.1. Note than the number of spectral lines (NSPL) is set to 200 to obtain the best accuracy as possible.

>>The mooring lines are defined in deck 14



>>The initial position of the structures before iteration towards the equilibrium position may be specified as follows

>>Run the *.DAT file The structures should now converge and the equilibrium position card as well as the geometry defined in this data file will be used in the preparation of the drift file as shown below.

3.3.2 Data file preparation in AQWA FER >>Restart the model from stage 4 by calling the previous AQWA LIBRIUM *.RES (ABOS1) file

>>Additional linear damping for drift frequency motions The following damping coefficients could be considered applicable to the structures.

>>Decks 13 and 14 are the same as the one of the AQWA LIBRIUM file

>>Specify points from which significant (absolute/ relative) motions are required

The RAOs of the specified points (or any other defined in the model) could be plotted through the graphical supervisor and is shown in the following section.

>>Run the AQWA FER *.DAT file



4 RESULTS

4.1 General In this chapter, some important comparison between free floating and coupled bodies RAOs are presented. Different RAO peaks due to different coupled modes of motion are also identified graphically. Then, results of significant motions are also analyzed.

4.2 Coupled RAOs Because of the coupled body interactions, some new peaks will be observed in the RAOs. For the same degree of freedom the phase and anti-phase motions of other connected bodies may substantially change the response of the structures.

>>Open AGS (AQWA Graphical Supervisor) >>Graphs>>File>>Open>>Drift *.PLT file (of the present AQWA FER run) >>Select structure1 (at the mid left hand side of the window) >>Response amplitude operators (Depending on what is needed, you could also plot the response spectrum, transfer function and force spectral density) >>Mooring configuration #1 - Spectrum #1 (by default) >>About y axis (to plot the pitch RAO for 180 as defined in the *.DAT file) >>Confirm selection

Since it is going to be compared with the free floating condition, the correspondent pitch RAO for head waves should also be plotted:

>>Graphs>>File>>Open>>Drift *.PLT file (of the previous AQWA LINE run) >>Free floating raos >>pitch(y) >>Dir # (180) >>Confirm selection >>OK

>>Select both plots by clicking on them >>Merge

Figure 4.2.1: Pitch RAO for free floating and including coupling effects

In previous picture, new peaks are observed and some existing ones are slightly different; it could easily be explained looking at the different modes of motion.



4.3 Dynamic stability modes

>>Within the AGS in the main menu select: Run>>AQWA LIBRIUM

Figure 4.3.1: AQWA LIBRIUM program menu

>>In the new window that pops out: File>>Open>>*.RES (AQWA FER file) >>In the same new window menu select: Display>>Dynamic Stability modes

Figure 4.3.2: Dynamic stability modes

Here is displayed all information about all modes of motion. If you click on the PLOT tab as shown in the previous picture, an animation of the structure in that mode of vibration is displayed. For instance the mode # 6 has a natural frequency of 0.385rad/sec and corresponds to the anti-phase motion of the topsides when the HLV is pitching and it is in agreement with the peak shown in figure 4.2.1.

Other modes and fully coupled RAOs could be displayed in the same manner.

>>Select: Display>>Equilibrium Positions

A table containing all equilibrium positions for all defined wave spectra and mooring configurations are displayed in a table (see figure 4.3.2). Note that this information is used in AQWA FER when reading the *.RES file from the previous AQWA LIBRIUM run. Similarly and in the same menu some useful options could be found.

4.4 Significant motions Unfortunately the significant motions for previously defined sea states could not be plotted trough the AGS and therefore the output has to be copied from the *.LIS file created when executing the AQWA FER data file.

>>Open *.LIS file >>By the end of the document find the significant motions and forces for the defined wave spectra



Figure 4.2.2: Significant motions of the COG of the HLV Oleg Strashnov for various sea states

Similar tables are available in the *.lis file and can be used to easily plot the significant responses for several Hs/Tp combinations from a scatter diagram and quickly give figures about workability analyses of coupled systems.



REFERENCES

1. Load condition 3. 4000 MT @ 41.89m 180. Shipshape output. Doc. GPGS.pdf. 2. WGU. Spectral density sensitivity to NSPL and SEED values in the AQWA time domain

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