OrcaFlex User Group 2003 Slide 1 of 14 OrcaFlex - Intro to Statics Convergence Presentation Outline Introduction Why do, and what is, Static

OrcaFlex User Group 2003 www.orcina.com Slide 1 of 14

OrcaFlex - Intro to Statics Convergence

Presentation Outline

• Introduction• Why do, and what is, Static Analysis?• Numerical Static Convergence Parameters• ‘Use Calculated Positions’ Button

• Improve Line (Inner) Convergence Loop• Improve Outer (System) Convergence Loop• Conclusion

• Statics progress window• Line Statics Methods



Introduction

• OrcaFlex analysis comes in 2 distinct parts:– Static Analysis (F = Kx)– Dynamic Analysis (F = Ma + Cv + Kx)

• Occasionally static analysis can cause problems - this presentation outlines what statics is, and how to tackle ‘problem cases’

• Both are normally quick and straightforward



Why do Static Analysis?

• We sometimes want to know the results of a static analysis (esp. true at early design)

• More often in OrcaFlex it is to provide a good starting point for the dynamic analysis:

– ‘True’ static config. Fewer dynamic transients

– Arbitrary static config.

Dynamic transients & possible

failure of dynamics

Dave:Dave:



What is Static Analysis?

• One where static forces & moments are balanced

• Discretised modelling means linked iterative calcs.

Dave:Dave:

• Two iterative loops are potentially required:

– Inner (line) statics loop(line statics with ‘fixed’ ends)

– Outer (system) statics loop(line + buoy DoF)



Convergence Parameters(1/3)

• Inner & outer loops are numerical iterative schemes

• Parameters influence convergence behaviour

Dave:Dave:

• Inner(line) parameters on the Line Data form • Outer(system) parameters on the General Data form




• Max. Iterations: – Static Progress Window is useful here– Increasing this may help, but if you need more than 500, then statics are

probably not going to converge!

Dave:Dave:

• Min. Damping:– Reduces size of iteration step

– Increasing this may help (typically to 1.2 - 1.5, sometimes 10!)

– But high values can cause slow convergence at end

– Changes should be progressive

The most common are:




• Tolerance:– Controls solution accuracy– Increasing this may help (may also give ‘bad’ static solution)– Increasing too much may also prevent convergence

Dave:Dave:

The most common are:

• Mag. Of Std. Change:– Limits size of step taken

– Reducing this can sometimes prevent taking too large a step between iterations

– But convergence is still rapid towards the end



‘Calculated Positions’ Button

• Used once a static solution has been found

• Set initial positions of free bodies and (optionally) lines to the values calculated in the static solution

Dave:Dave:

• Useful for ‘long’ static convergences or iteratively for difficult problems

• Found on the Statics Tab of the General Data form

• Can also use during dynamics to set initial positions to the dynamic values



Statics Progress Window

Dave:Dave:

• Found under Window menu• For each Outer (System) iteration it shows:

– Out-of-balance forces and moments

– Calculated position at that iteration

• Useful check on ‘difficult’ statics



Line Statics Methods

• Step 1: Find an equilibrium solution without line bending stiffness or interaction with shapes

• Step2: Find the ‘full’ equilibrium solution including bend stiffness and interaction with shapes

Dave:Dave:

• Step 2 uses the solution from Step 1 as the starting point

• The Inner (Line) statics loop consists of 2 steps:

• Step 1 method = ‘Catenary’ is nearly always sufficient



Improve Inner (Line) Convergence

• Arbitrary change to initial position

• Check line end orientations are correctly set

Dave:Dave:

• User specified Step 1

• Try altering the convergence parameters

• Application of current



Improve Outer (System) Convergence (1/2)

• 6D buoys can be optionally included (6DoF or 3DoF), or excluded from the static analysis

• 3D buoys can be optionally included or excluded from the static analysis

Dave:Dave:

• ‘Use Calculated Positions’ button iteratively to progressively include more DoF for the free bodies

• Vessels can be optionally included (surge, sway, yaw) or excluded from the static analysis



Improve Outer (System) Convergence (2/2)

• Avoid line connections at CoG of 6D buoys

• Obtain a quasi-steady dynamic solution

Dave:Dave:

• Arbitrarily change initial conditions

• Outer loop convergence parameters can be altered



Conclusion

• Some are tricky, but simple ‘tweeks’ sucess

• Really stuck? Email us the file!

Dave:Dave:

• Vast majority of static analyses are quick easy and robust

Documents

OrcaFlex User Group 2003 Slide 1 of 14 OrcaFlex - Intro to Statics Convergence Presentation Outline Introduction Why do, and what is, Static