1. 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

2. OrcaFlex User Group 2003 www.orcina.com Slide 2 of 14 OrcaFlex - Intro to Statics Convergence 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

3. OrcaFlex User Group 2003 www.orcina.com Slide 3 of 14 OrcaFlex - Intro to Statics Convergence 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:

4. OrcaFlex User Group 2003 www.orcina.com Slide 4 of 14 OrcaFlex - Intro to Statics Convergence What is Static Analysis? One where static forces & moments are balanced Discretised modelling means linked iterative calcs. Dave: Two iterative loops are potentially required: –Inner (line) statics loop (line statics with ‘fixed’ ends) –Outer (system) statics loop (line + buoy DoF)

5. OrcaFlex User Group 2003 www.orcina.com Slide 5 of 14 OrcaFlex - Intro to Statics Convergence Convergence Parameters(1/3) Inner & outer loops are numerical iterative schemes Parameters influence convergence behaviour Dave: Inner(line) parameters on the Line Data form Outer(system) parameters on the General Data form

6. OrcaFlex User Group 2003 www.orcina.com Slide 6 of 14 OrcaFlex - Intro to Statics Convergence Convergence Parameters(2/3) 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: 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:

7. OrcaFlex User Group 2003 www.orcina.com Slide 7 of 14 OrcaFlex - Intro to Statics Convergence Convergence Parameters(3/3) Tolerance: –Controls solution accuracy –Increasing this may help (may also give ‘bad’ static solution) –Increasing too much may also prevent convergence 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

8. OrcaFlex User Group 2003 www.orcina.com Slide 8 of 14 OrcaFlex - Intro to Statics Convergence ‘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: 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

9. OrcaFlex User Group 2003 www.orcina.com Slide 9 of 14 OrcaFlex - Intro to Statics Convergence Statics Progress Window 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

10. OrcaFlex User Group 2003 www.orcina.com Slide 10 of 14 OrcaFlex - Intro to Statics Convergence 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: 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

11. OrcaFlex User Group 2003 www.orcina.com Slide 11 of 14 OrcaFlex - Intro to Statics Convergence Improve Inner (Line) Convergence Arbitrary change to initial position Check line end orientations are correctly set Dave: User specified Step 1 Try altering the convergence parameters Application of current

12. OrcaFlex User Group 2003 www.orcina.com Slide 12 of 14 OrcaFlex - Intro to Statics Convergence 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: ‘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

13. OrcaFlex User Group 2003 www.orcina.com Slide 13 of 14 OrcaFlex - Intro to Statics Convergence Improve Outer (System) Convergence (2/2) Avoid line connections at CoG of 6D buoys Obtain a quasi-steady dynamic solution Dave: Arbitrarily change initial conditions Outer loop convergence parameters can be altered

14. OrcaFlex User Group 2003 www.orcina.com Slide 14 of 14 OrcaFlex - Intro to Statics Convergence Conclusion Some are tricky, but simple ‘tweeks’  sucess Really stuck? Email us the file! Dave: Vast majority of static analyses are quick easy and robust