Orcina August 2009

7/29/2019 Orcina August 2009

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TEXT DATA FILES

The advent o Text Data Files or OrcaFlex is a signifcant development....

OrcaFlex has, until v9.3, relied on a proprietary binary ormat or data entry.New or v9.3, and alongside the existing binary ormat, we haveintroduced the option to use text data les as a meanso generating human readable input data.

We do not intend that the text le ormat will replacethe binary ormat. Rather the two will work together,each to be used to achieve slightly dierent aims. Inconsidering this new capability, several points areworth noting:

a) The binary data ormat has excellent versioncompatibility (old programs read new datalesand new programs read old datales). Achievingthe same level o compatibility is hard to do withtext les.

b) You need OrcaFlex to understand the binary ormat, which means having a paid up licence(although the OrcaFlex Demo version can help).In principle, the text le ormat allows manualcreation o, or amendments to, an input le.

c) With the binary ormat, printing, checking and archiving input data is not as easy as it might be. A ull licence is needed (not just the Demo version) to exportin the current binary ormat. A well structured text le ormat makes all o these activities much easier.

d) The text le is not a ‘Keyword’ type le (typied by ‘# Keyword’ ollowed by a sequence o numbers representing the input data). These les have the advantageo brevity but lack QA rigour as each number is then not uniquely reerenced. The OrcaFlex text le has each input number uniquely reerenced to thevariable’s batch script name. This makes the le more verbose, but also ar more readable hence producing much stronger QA.

Mindul o these thoughts, here’s how we see the two ormats complementing each other: we think that most users will want to use the GUI or datalepreparation because, or all but the most trivial models, it will be ar more productive to use the GUI. However, the text le is much better or printing andchecking input data, comparing dierent datales (File | Compare Data acility already in OrcaFlex) and certain mass modications and automation tasks.

Though, depending on your requirements, the last two tasks can oten be easily accomplished using the existing acilities in OrcaFlex.One point to note: comments added to the text datale will not ‘round trip’, ie, i a text datale with comments is imported into OrcaFlex and the text lesubsequently re-saved, comments rom the original will be lost.

For those who like their input data as text data les and or those who want improved QA, we hope that thiseature will be a big advantage.

With the release o OrcaFlex v9.3 in Agst 2009 it’s time or another newsletter setting orthsome o the great new eatures it contains. Although we have embarked on the next developmentround leading to v9.4, a release provides a good opportunity to see what has been achieved.

We’re pleased to note that despite recent turmoil in world markets and the oil price more thanhalving, our business continues to hold up. As a sotware house (with most o our revenue romthis source) it is particularly gratiying or us to see that sotware sales and leases continue apace,though it’s also interesting to note that our consultancy team is busier than ever! Predictions arealways hard to make, however most client sentiment about the uture remains positive, althoughperhaps the orward order book is looking slimmer than it did 12 months ago.

So, over to OrcaFlex: released in v9.3 are several major eatures, which include the advent o text data fles and extreme response statistics. We now have the ability to save static results,to add coatings and linings to a line type and to have non-linear material or bend stieners.

The longstanding interace to Shear7 has seen a major overhaul, the API RP 2RD stress code checks have been added and there are some improvements to the random wave discretisation process. Our major article ocuses on the various options or determining vessel motion response,in particular to draw attention to the ully coupled vessel/line capability.

The recently included OrcaFlex Applications page eatures again, as does the new Agents page. Asever we hope that we achieve a useul balance in these newsletters. We always welcome suggestionsor improvements / uture content so please do drop us a line.

In This Month’s NewsletterPage

Editorial and Contents........................................... 1

Short Articles:

• Textdatales .............................................. 1 • SavingStatics .............................................. 2 • Extremeresponsestatistics........................ 2 • ImprovedSHEAR7interface ..................... 2 • Bettercontrolonrandom

wave discretisation ...................................... 3 • MoreLINEimprovements......................... 3

Main Articles:

• Fullycoupledvessel/lineanalysis............. 4 • Applications(Trawlgearimpact&WECs).... 6 • Agentsnews(Brazil&SEAsia) ................. 7 The Rest:

• NewsinShort ............................................. 8 • DidYouKnow ............................................ 8 • Cominginthenextissue ........................... 8 • Orcinaoutandabout................................. 8

Newsletter www. orc ina.co m

OrcaFlex 9.3 (Aug ‘09)......another feature-rich release!

rofessional software for

ngineering professionals



SAVING STATICS

This new eature removes a long standing ‘niggle’....

Short Articles

EXTREME RESPONSE STATISTICSTo the extremes o analysis...We are oten in the position o wanting to dene a characteristic design valueto use, based on an irregular time history o that result. DNV (OS-F201) callsthis the expected or most probable extreme or a storm o given duration(eg. 3 – 6 hours) – based on the assumption o a stationary random process.Typically this translates to wanting to predict the maximum line tension,bend moment, etc, in a 100-yr storm simulation. (Note that this requirementis distinct rom the extreme values o an ensemble o ‘stationary’ events;computations such as this are typically done by the metocean community toyield (eg. the 100-yr return period wave height.)

As an example thescreenshot shows atension time trace

or a 3-hr randomwave simulation. Thequestion is then whatsingle value o tensiondo we use in our design?We could repeat thisrun (eg. 10 times) eachwith a dierent randomwave seed and then (say)take the average o the

maxima. Alternatively one very long simulation could be created and taketheaverageofthemaxima.Eitheroftheseoptionsareacceptable,butrequirelong simulation times.

To reduce the simulation time and introduce some ‘justication’ or

identiying design values, an alternative is to identiy and t the peaks o a single shorter irregular simulation using standard statistical methods or‘extremes’. Such methods then allow prediction o extreme values or thereturn period o interest. This is what we now provide in OrcaFlex as shownin the screenshot.

There are three statistical methods available or this extrapolation – Rayleigh,Weibull and Generalised Pareto (GPD). Rayleigh is oten used or a time history which is stationary and Gaussian. Where these assumptions break down, theWeibull and GPD are used with the latter being preerred by the statisticscommunity. Use o the Weibull and GPD methods also allows estimates o the condence interval or the given return period. Diagnostic graphs areprovided which allow the assessment o goodness o t, appropriateness o the selected method, etc.

This approach would not typically be used with simulations incorporating

low-requency foater motions, or to estimate (eg) 100-year return levels onresponse parameters. Although this can be done in principle, the length o simulation required to capture enough data or good tting and extrapolationis typically quite high.

We know that some users have been taking OrcaFlex results to other packagesto obtain the extremes, so we hope that or them, and all other users, this

somewhat overdue eature is o some considerable use.

IMPROVED SHEAR7 INTERFACE Making lie or SHEAR7 users even easier....OnecapabilityofferedinOrcaFlexistheinterfacetoSHEAR7fortheanalysisofvortex induced vibration (VIV). Previous versions o this interace generated thenecessarySHEAR7structuralinputdataleforagivenstaticcongurationandtheSHEAR7modeshape(.mds)inputle(ortheusercouldrelyonSHEAR7togeneratethemodes).SHEAR7wasthenmanuallyrunwiththeselesfortheVIVanalysis.ThenewSHEAR7VIVcouplinginvolves:

1) OrcaFlex line statics with Cd as initially specied in the line type data.

2) SHEAR7isthencalledtodetermineanydragenhancementduetoVIV.

3) OrcaFlex then recalculates line statics using these new Cd values.

Steps 2 and 3 are then repeated until the static position has converged. Perormingthis‘coupling’bydroppingouttoSHEAR7eachiterationwastimeconsumingand

errorprone.ThebigadvantageofthenewinterfaceisthatSHEAR7canberundirectlyfromOrcaFlexwithSHEAR7resultsautomaticallyreturnedtoOrcaFlex.Inthis respect it is very similar to the long-standing interace with the VIVA sotwarealsousedforVIVpredictions.AswellasthemanualoptiontorunSHEAR7,thethree new options are (see screenshot):

a) Full (coupling): EachiterationproducesanewmodeshapeleforusewithSHEAR7.

b) Partial + auto .mds fle: The OrcaFlex generated mode shapes rom the initialstatic conguration are used and kept constant or each subsequent VIVcoupling iteration.

c) Partial + user .mds fle: The user-specied (on the data orm) mode shape leis used and kept constant or all subsequent VIV coupling iterations.

Inormal experience suggests that there are not many cases where the ull couplingoption makes a signicant dierence compared to partial coupling, however, it isnoticeably more time consuming to run. So, whilst the sensitivity o the modelto ull coupling should be understood, or most cases partial coupling will be thebetter option.

It should also be noted that there are specic OrcaFlex batch script commands toproducetheSHEAR7.outand.pltles–indeed,ifOrcaFlexisrunfromthebatchorm, these les are produced automatically.Consequently, the main advantages o this new eature are (i) to allow ull couplingto be easily checked, and (ii) considerably more straightorward le handling andautomationbyallowingSHEAR7toberunfromwithinOrcaFlex.

In previous versions o OrcaFlex there was no way o saving the results at the end o a Statics calculation. Forsimple models and ones which converge quickly, this was not a particular problem. However, with complex models and / or where the solve took a long time, this limitation could be rustrating. There were twoworkarounds: (i) run a very short dynamic simulation with no excitation, and (ii) use the OrcaFlex post-processingspreadsheet,specifyingthe.datratherthan.simle.Bothapproacheswerealittlecumbersome.

Now, however, it is possible to directly save the results at the end o the Statics calculation. The Static results

aresavedasasimulationle.File|Save(CTRL+S)attheendofStaticswillnowsavelename.simwhichholds all the static results.

So a simulation le potentially now has two ‘sets o results’: (i) as beore, the results or a dynamic simulation,(but which now, because o these developments, also include the static state results) and (ii) it can now alsohold the results just rom the static solve. Not only is this eature useul or those only interested in staticsresults, but a ‘Static’ .sim le can be loaded into OrcaFlex and the dynamic simulation started rom thatstate.

Note that we deliberately decided not to create yet another le name extension to add to those already in use(.dat, .sim, .sct, .tg, .xls). Instead we expanded the idea that a .sim le holds any results rom OrcaFlex.

We think that that allowing static results to be saved will be a major advantage, probably in ways we haven’t yet anticipated.

age 2 v9.3 released August 2009



MORE LINE IMPROVEMENTS

The OrcaFlex Line object gets yet more unctionality (neatly seen on the screenshot)...

NON-LINEAR STRESS-STRAIN RELATIONSHIP (1)

In v9.2 we introduced the Homogeneous Pipe category o line type, retainingthe previous line data entry approach as the General category. This distinctionwas created to ease the modelling o line sections with diameter proles(eg, tapered stress joints and bendstieners). O course, this eature alsohad the benet o making it simpler tomodel steel pipes.

In v9.2 this implementation only supported linear material propertiesie, a constant value o Young’s Modulus(E). For elastomeric bend stiffeners,in particular, it can be importantto include the non-linear materialproperties and v9.3 now allows that.

COATINGS AND LININGS (2)

For many ‘homogenous pipes’, it iscommon to dene various coatingsand linings in addition to the strengthsection. These are typically used withsteel pipes to model the additional

mass and displacement o concretecoatings, plastic linings, etc. In v9.2 andbeore, these eects could be includedbut only ater an o-line calculation to generate ‘equivalent’ line properties.

For the Homogeneous Pipe line type category, v9.3 removes the need or o-line preparation o equivalent properties by allowing the denition o

any number o coatings and linings. OrcaFlex then does the sums and usesthe equivalent set o properties, though remember that coatings and liningsdo not contribute strength, nor are they assumed to be load bearing.

This new capability makes data specication easier and clearer or coatingsand linings without ‘losing’ the source data. This has very obvious QAbenets, but also allows much clearer parametric variation o the coating/

lining data.

API RP 2RD STRESS CHECK (3)

The API RP 2RD code includesa von-Mises type strength check.

Implementation is not completely trivial, so we’ve built it into OrcaFlex.(Note: 2RD contains errors, mosto which were corrected in a recenterrata).

As seen in the screenshot,implementing this code check required a new View Mode anddata page. This page contains datanecessary or the code check but notrequired by OrcaFlex or a dynamicsimulation. Two new results (APIRP 2RD Stress and API RP 2RD

Utilisation) are now available. We hope that this proves a signicant

convenience to all users o 2RD. At the moment this is the only code check supported in OrcaFlex, but we intend to add other widely used codes, soplease let us know i you have any candidates.

Short Articles

BETTER CONTROL ON RANDOM WAVE DISCRETISATION

We have added more user control or wave spectrum discretisation...

A wave spectrumis traditionally discretised in one

o two ways: (i) by dividing it into anumber o equalenergy intervals, or(ii) to divide it intoequal requency intervals. Both areshown schematically in the chart (thoughwith coarse intervalsor clarity).

Although there are pros and cons to both methods (there is little publishedwork on this, though see, or example, DNV OS F201, Appendix D, SectionC), OrcaFlex only uses the equal energy approach or the ollowing reasons:

a) The equal energy component requencies are not related to each other ina multiplicative way, which means that the repeat period o the resultingwave train is eectively innite. To avoid a short repeat period with theequal requency interval approach many more components are required.

b) The equal energy approach automatically gives ner discretisation aroundthe spectral peak, a generally desirable eature. The equal interval approachcan achieve the same, but because the same interval is used throughout therest o the spectrum, more requency components are required.

Since simulation runtimes increase with more components, the equal energy approach is generally more ecient. The exception is i you pre-computethe wave kinematics when using the equal requency spacing approach.

As described in the DNV reerence this can be more ecient, but thisapproach then requires justication o the spatial interpolation scheme thatis subsequently required.

However, the main disadvantage o the equal energy approach is that the

intervals in the tails o the spectrum can be quite large. Potentially, a systemresonance might be missed, or excited, in a non-realistic way because therequency component is assigned to the middle o quite a large interval.Consequently we have added user control to set the maximum componentrequency interval that the discretisation will use – see screenshot.

Thismeans themaximumsize of the energyinterval can becontrolled,ensuring that that the requency discretisation is ne enough in the tails. Notethat i this upper limit is chosen realistically it will not usually aect the levelo discretisation around the peak as this is normally at much ner resolution.In most cases the deault values suggested will be perectly adequate.

We have also introduced user control over the minimum and maximumrequencies considered during the discretisation. Previously these werehardcoded at 0.5m and 10m respectively so these values are set as the deaulton the dataorm.These changes mean that the Waves page now has a new option called‘SpectrumDiscretisationMethod’.Thisiseither‘Legacy’(ie,discretisationasper v9.2 and earlier) or ‘New’ (as described here). We recommend that the‘New’optionisusedforallnewanalyses,butthe‘Legacy’optionisretainedto allow the same wave realisations as earlier versions o the programto be reproduced.

.3 released August 2009



DEFINITIONS

System: the combined oater and connected Lines(risers, moorings, etc).

Motion: the time varying position o the foater.

Oset: the time invariant position o the foater.

RAOs: Response Amplitude Operators are derivedrom model tests and / or diraction programs anddescribe vessel response to waves in the wave requency regime. These are generated in both the displacementand load orm.

QTF: Quadratic Transer Function loads are computedrom diraction programs. They describe vesselresponse to waves in the low requency regime, and only arise in an irregular seastate. The QTF leading diagonalis derived rom 1st order wave diraction analysis. O-diagonal terms are either computed by the diractionprogram or derived with Newman’s approximation.

Wave Loads: are the orces and moments on the foaterrom wave action. They induce foater motions which,depending on the analysis approach, may be used asmotions or osets. Wave loading is traditionally thoughto in three loading ‘regimes’ – mean, wave requency and low requency. The real system has no knowledgeo these articial distinctions, but this industry-wideapproach makes analysis more understandable andtractable.

Mean Wave Loads are sometimes called steady drit,meanwavedriftor(rarely)zerothorderwaveloads.They are time invariant loads which only arise in anirregular seastate. They cause a moored foater to take

up an oset known as the mean (or steady or wavedrit) oset.

Wave Frequency (WF) Wave Loads are sometimescalled 1st order wave loads, and are characterised by RAOs. They arise in both regular and irregular wavesand induce WF (or 1st order) foater motions andosets. These are time varying eects, with periodstypically in the range o 3s to 30s.

Low Frequency (LF) Wave Loads are also called 2ndorder wave, or wave drit, loads and are characterisedby the QTF. They arise only in an irregular seastate andcause low requency (or 2nd order) foater motionsand osets. These loads are much smaller than the WF

wave loads and are only signicant when they excite asystem resonance period. This might be o the order o 60s+dependingonthesystem.

In De-coupled analysis the foater motion is solvedin the time domain, with the moorings and risersincluded quasi-statically. This means that all othercoupling eects (eg, current loads on lines) need to beseparately assessed and included.

Coupled analysis: Here the complete equations o motion or the system are solved in the time domain.This means that all coupling eects are automatically included in the analysis, including the dynamic loadsrom the lines.

FULLY COUPLED VESSEL/

LINE ANALYSIS

TO COuPLE OR NOT TO COuPLE- THAT IS THE quESTION

It’s obvious really, but such systems respond to environmental loads (wind, wave,current, VIM) in complex ways and all (excepting VIM) act both on the foater andthe line. In what ollows we concentrate on the dierent contributions to total vesselmotion rom wave loads.

Despite the complexity, various approaches to the analysis o such systems exist, eachwith dierent levels o approximation. The precise approach(es) used depend on thesystem type, and the results o interest (eg, foater displacements, mooring tensions,risers stresses, etc).

The ollowing gives a high level description o the most common approaches tosystem analysis. Several variants on the ‘naming’ o dierent approaches exist andthere is nothing sacrosanct about the names used below.

Option 1: Quasi-Static (QS) Analysis: here foater motions due to wave loads are

‘translated’intorepresentativeoateroffsetsforeachofthemean,WFandLFregimes.These osets are then combined to give a total foater oset considered representativeo the total foater motion in waves. Firstly the system’s static equilibrium osetunder mean (wind, wave & current) load eects is ound. A urther oset is addedrepresentingtheWFandLFmotionoffsetcomponents.Linestaticsaresolvedforat this total oset, yielding the quasi-static solution. Typically this approach is usedin the early stages o mooring design, though some simple systems can be designedentirely by this method (noting the requirement to use higher saety actors).However, most systems have an element o dynamic analysis; or highly non-linearsystems the dynamic analysis typically orms the biggest part o the analysis.

Option 2a:ImposedWFoatermotions+imposedmeanandLFoffsets:aspertheQSapproach,themeanoffsetissolvedfor,andtheLFdynamicoatermotionsare imposed as a urther foater oset in addition to the mean oset. At this oset

the wave requency dynamics are then represented using the displacement RAOs.Consequently the vessel acts as a boundary condition which ‘drives’ the line motion.

Option 2b:ImposedWFoatermotions+calculatedmeanandLFmotions:theLFand mean foater motions are computed directly in the time domain by solving thesystemequationofmotionincludingLFwaveloads.Thisgeneratesasteady(mean)+slowvaryingoatermotion(thelatterassumingsomesystemresonanceisexcited),onto which the WF foater dynamics are superimposed using the displacementRAOs.

Option 3:De-coupledsolution(fulloatermotionsolution+nolinedynamics):here the foater equation o motion is solved directly in the time domain, includingmean,WF andLF orderwave loads. Lineload effects areincluded from a look-up table o quasi-static line loads or with non-linear springs. This approach istraditionallyusedfortheglobalanalysisofoatermotions.Lackofcouplingwith

lines is a signicant limitation, though estimates o the missing damping eectscan be made and included. This approach is more ecient than coupled analysis,allowing much longer / more simulations.

Option 4:Coupledsolution(fulloatermotionsolution+linedynamics):theoatermotion is calculated as per the de-coupled approach, but now also includes thedynamic loads rom any attached lines. It is considered the most accurate approach,but also the most time consuming (depending on the level o detail included orthe lines). Thereore, it is common to work in two stages: (i) Determination o thefoater global response - lines are initially o secondary importance, but are includedwith sucient detail to give acceptable foater motions rom any coupling eects.(ii) Then re-use the foater motion time history rom (i) as an imposed boundary condition, together with a much more detailed model o the line(s) o interest.

OK, so how can OrcaFlex be used to implement the above? The Vessel data orm

screenshot (shown on the next page) shows the available options or determiningVessel motions. The options, and approach to setting them might seem a littlebewildering at rst, but hang in there...




Basicallytherearethreeoptionsfordeterminingthemotiono the Vessel: (i) to impose Vessel motions, (ii) to calculateVesselmotions,and(iii)amixof(i)and(ii).By‘impose’vessel motions we mean that the foater equation o motionis not solved to give foater displacement - the foater motioncomes instead from some pre-determined method. By

‘calculate’ vessel motions we mean that the foater equationo motion is solved to yield a time history o the foatermotion.

The table below identies which options on the orm imposeVessel motions and which ask OrcaFlex to calculate the vesselmotions.

The Included Effects section (see screenshot) turns onvarious loadings to be applied when the vessel motion isto be calculated, ie, they only apply when the vessel’s staticposition is to be determined, and / or when either o theCalculated options or Primary Motion are selected.

OK, having seen what OrcaFlex has, how do these map ontothe common approaches to the determination o foater

motions outlined earlier? The table on the right attempts toclariy this.

So with the correct selection o vessel controls, OrcaFlex canperorm any o the commonly accepted analysis approaches.Even the less‘routine’coupledanalysishasbeen availablein OrcaFlex since v8.5 (Jun-04), though it was not untilv9.0 (Sept-06), when requency dependent added massand damping were included, that comprehensive coupledanalysis was ully realised.

Though coupled analysis is seen as the most accurate option,it suers rom being computationally the most demanding.There are several analysis strategies to reduce computationtimes, all involving various approximations to the inclusion

o line loads. However, sotware advances can also make a bigdierence in reducing computation time and OrcaFlex helpsin this regard by:

• Implementingaveryefcientsinglesolveforthecoupledsolution

• Beingoneofthequickestsolversinitspeergroup

• Havingmulti-threadedbatchcapability(asstandard,ie.at no extra cost!)

• Offering Distributed OrcaFlex (as standard, allowingnetworked, OrcaFlex-licensed computers, to transparently run OrcaFlex jobs using spare processor time)

The collective eect is to massively enhance the speed

at which large numbers o coupled solutions can beperormed. As an illustration a three-hour irregular-wave(100 components) global foater motion simulation withvery simplied models or 12 mooring chains and threerigid risers took approximately 30mins on a single thread onamodernworkstation.Basedonthis,100loadcasesrunningon a quad core machine becomes tractable or an overnight

Calc Options: To ‘impose’ Vesselmotions, select:

To ‘calclate’ Vesselmotions, select:

Vessel initialposition

specied n/a

Included in StaticAnalysis

None 3 DoF or 6 DoF

Primary Motion None or Prescribed

or Time History

Calculated 3 DoF or

Calculated 6 DoF

SuperimposedMotions

None or Disp. RAOs+HarmonicMotion

or Time History

n/a

Initial position Inclded in Statics(& Inclded Eect)

Primary Motion(& Inclded Eect)

SperimposedMotion

1 qasi-static analysis (done in 2 stages. 1st solve or static equilibrium with mean loads. Ofine, add to this a pre computedoffsetrepresentativeofWFandLFmotions.2ndre-dostaticlinesolvewithvesselinitialposition=totaloffset.)

Nominal1 Yes (Hydro Damping, WindDamping, Wave Drit)

n/a n/a

2a Imposed WF foater motions + imposed mean and LF osets

mean+LFoffset None None Displacement RAOs

2b Imposed WF foater motions + calclated mean and LF motions (note that or technical reasons this option can only be run using the explicit integration scheme)

Nominal1 Yes(Hydro Damping, WindDamping, Wave Drit)

Calculated(2nd order wave load)

Displacement RAOs

3 De-copled analysis (OrcaFlex solves or foater motions, but ‘lines’ only included with non-linear springs – no acility or use o pre-prepared look-up tables)


Calculated(1st and 2nd

order wave load)

None

4 Copled analysis (Floater motions solved as in the previous option, but the dynamic loads rom the lines are alsodirectly included in the solution)


Calculated(1st and 2nd

order wave load)

None

1 ‘nominal’ means that the initial position should probably be set as something sensible, but where specied above it is only used as a starting point or themean calculation

run! Clearly very good news, but o course there is usually then a second round using themotion time histories rom the above with more detail in the line models. However, this doesshow that coupled analyses are very much more tractable than they have ever been.

So, great as this all is, there are nonetheless some eatures missing rom OrcaFlex which we’dlike to add in the uture. These include: use o the ull QTF, 2nd order high requency terms,hydrodynamic coupling between vessels, wave drit damping and requency domain. In theusual way, please let us know how important these terms are relative to other competingeature requirements.

References‘F201’: DNV-OS-F201, Dynamics Risers, Jan-01 (amended Oct-03),‘302’: DNV-OSS-302, Offshore Riser Systems, Oct-03 (amended Apr-09).‘F205’: DNV-RP-F205, Global Performance Analysis of Deepwater Floating Structures,Oct-04 (amended Apr-09).‘E301’: DNV-OS-E301, Position Mooring, Oct-08 (amended Apr-09). 2SK’: API RP 2SK, Design and Analysis of Stationkeeping Systems for Floating Structures, 3rd Ed., Oct-05.

3 released August 2009



Applications

DESIGN OF STRuCTuRES FOR FISHING INTERACTIONuSING ORCAFLEX

An application through thinking out-o-the-box by Jonathan Jury, Senior Consultant, RiserTec Ltd.

It appears that in the available literature there is no attempt to use analysistools or evaluating the dynamic loads caused by shing gear on structures.The main reerence (Wellhead Protection at Gannet B, D Heal and L Naughton

Underwater Technology Vol. 17 No 4,1991) uses small-scale model testing romsome time ago; other load data is based on similar scale testing or calculationsbased on energy methods and warp breaking loads. Consequently RiserTecundertook in-house research to assess the possibility o developing a tool toassist in the layout and design optimisation o subsea structures.

We’ve used OrcaFlex routinely or riser-to-riser and riser-to-vessel impactassessment during operation and installation. It thereore seemed possible, i a challenge, to use the sotware to consider snagging and impacts rom shinggear.TheanalysesshownhereconsiderimpactsfromBeamTrawlsbutOtterTrawl systems could be similarly modelled.

Typicalvesseldataforbeamtrawlerswasused(assumeddisplacementof750Tonnes and trawl speed o 5 knots). The trawl beam was 9m long, weighingaround 5T. The rigging arrangement was typical or this vessel type, producinga warp angle o ≈25˚ when towing at constant speed. The arrangement isshown in the screenshot with a net model included. The structure shape wastaken rom a Central North Sea Protection Structure designed by Genesis Oiland Gas with 55˚ rake on the braces. The model could be more detailed but itis a reasonable representation o the problem.

The key issues with shing interaction are the impact loads on the sidemembers, structure pullover loads and snag loads i the trawl gear becomesentangled. The results predict a side brace impact orce o 65T with an impactenergyof 20KJ,apulloverforceof 17Tandamaximumwarpsnagloadof80T at 30 ̊ tothe horizontal. They comparewell against standarddesign

values where or example maximum side impact energy is commonly takenas 15KJ, the extreme impact as 30KJ, the pull over orce as 15T and peak netcaptured snag load as 100T.

Sensitivities: reducing the side angle rom 55˚ to 45˚ reduces the clashorce and energy by ≈45%. Introducing a 1m high skirt angled at 45˚ (eg,by a concrete shaped block) provided a similar substantial reduction in sideimpact energy.

Conclsions: it appears possible to model shing interaction with a 3Danalysis package such as OrcaFlex. This allows the designer to assess in arational manner the eciency o alternative structure design & layout, waterdepths,deectorsandprospectivechangesinshinggearandvesselsize.

Acknowledgements: thanks to Duncan Warwick at Genesis Oil and GasConsultants or providing assistance on typical structure layouts and designloads, and or challenging me!

A WAVE ENERGY CONVERTER WITH ORCAFLEX

An application or a new but growing market area or OrcaFlex by Bil Stewart 1 , Stewart Technology Associates (STA)...

STA, who are one o Orcina’s joint US Agents, has been involved withnumerous marine renewable energy projects. One o the most interestingWEC concepts involves an eccentric rotating “pendulum” mass turningaround a central vertical shat inside a surace foating buoy (see screenshot).OrcaFlex can be used to model the buoy motions and the pendulum rotationin any sea state. The buoy motions cause the pendulum to rotate and this

rotation strongly infuences the buoy motions. The direction o pendulumrotation may change and the pendulum may stall, depending upon the powertake-o load. The system is non-linear and sensitive, demanding time domainsolutions.

In prototype systems the rotating pendulum may drive pure electricalgeneratorsviadirectdrives,orviahydraulicsystems.Electromagneticpowergeneration is also possible. In OrcaFlex riction can be used to simulate powertake-o.

Upperandlowerhorizontalarmsattachthependulummasstothecentralshat in the buoy. OrcaFlex single segment line elements are used to modelthe arms and pendulum mass. Two 6D OrcaFlex buoys (green cubes in thewirerame drawing) connect the upper and lower shats to the pendulum.

The line element simulating the pendulum mass can be seen inside the

riction cylinder, pushing onto the inside ace o the cylinder. The stiness o the cylinder and the pendulum line element are arranged to give a sensiblecontact orce. The riction coecient between the cylinder and the pendulumline element is adjusted to simulate varying power take-o scenarios.

The graph shows rotation o one o the arms around the central shat. Powertake o is ound by integrating the distance travelled by the pendulum around

the inside o the riction cylinderand multiplying by the rictionorce. Account must be taken o the reversals in direction and inthe mechanical and electricallosses in eciency this will cause.

Many mechanical variablesmay be investigated or any

given buoy geometry and massdistribution, including pendulummass,horizontalarmlength,andriction coecients.

he intention o this new section is to give readers an understanding o some o the more unusual models and applications we’veen with OrcaFlex.….

1Bil Stewart is Chairman of the ASCE COPRI Marine renewable Energy Committee.




Whilst our agents are mostly geographically remoterom us, we have very close working relationships withthem, oten with daily contact. With most we havelong standing relationships going back many years.This engenders strong technical communicationwhich is very important in seeking to serve our clients

in the best way possible.We hope that this page will keep you abreast o thelatest developments with our agents so as to betterunderstand what they do and hence get the most romyour use o our sotware.

Our list o our agents and their contact details areshown in the table to the right:

....AND FROM S.E. ASIA

ZeeEngineeringConsultants

Pte, Ltd (ZEE) is a multi-disciplinary consultingengineering company specializing in the offshoreoil and gas industry. ZEEwas established in 1986(becoming an agent orOrcina shortly thereater)and to date has successully completed over 200 projectsintheMiddle and Far Eastregion.DuringthistimeZEEhasbeenverysuccessfulinbuildingaversatileteamofengineersforoffshoreinstallationespeciallyinSubmarinePipelines.In2003ZEEenhanced its capabilities by orming a joint venture with PT Indonesian Service

Bureau(ISB)forprojectsinIndonesia.Besides submarine pipeline engineering ZEE has also been engaged in design &simulation o oshore foating structures, such as SPM’s, moorings, FSO’s, FPSO’setc.InalltheseprojectsZEEhasusedOrcaFlexasthemaindesigntool.

Over the years, during which we evolved rom punch card operated programs tomodern fullgraphicmodellingcomputer simulations,ZEE hasutilizeda numberofindustryapprovedproprietarysoftwarepackages.DuringthelastfewyearsZEEhas carried out a number o studies comparing OrcaFlex to other packages and havearrived at the ollowing conclusions:

a) OrcaFlex gives realistic results especially or large diameter pipe in very shallow waterforpipelayusingaminimumfacilityLayBarge.Inaparticularinstancetheinstallation contractor was able to reduce the minimum tensioner requirements by approximately 20%.

b) In OrcaFlex the start-up (including ‘bow line’), lay-down, and davit lit can besimulated as a continuous operation, whilst in other packages this needs to bebroken down to a number o stages.

c) The behaviour o the lay barge can be simulated in OrcaFlex by giving precisebarge characteristics such as RAOs, QTFs, and damping at required orientation.

d) In OrcaFlex, support rollers can be modelled very easily and the reactionsare calculated based on line clashing orces which will provide accurate rollerreactions.

e) OrcaFlex has the capability to model heave compensators. This will result in thelowering o the system stresses.

) OrcaFlex has superior graphics or modelling and reviewing results. This makes iteasy to model and to understand the integral behaviour o the barge and the line.

ZEEhasswitchedovertoOrcaFlexforallpipelineinstallationwork.Zeeengineershave been given the encouragement and the resources to research and to implementall the eatures in OrcaFlex, resulting in a much better value added solution. Our in-house experience is passed on to other OrcaFlex users in the region through normaltechnical support, seminars, presentations and workshops. The greater awareness o OrcaFlex in the region, and the presentation o results to the major oil companiesparticularly in Malaysia and in Indonesia, has meant in some cases OrcaFlex beingspecied as the preerred sotware in tender documents.

Agents News

NEWS FROM SOuTH AMERICA.........

Our South American agent

is SUPORTE Consultoria eProjetosLtda.,whohavebeenworking with Orcina since1998. Our main contact thereis the company presidentNelsonSzilardGalgoul,whoholds a Dr.-Ing. Degree romGermany and who is also a ullproessor at the FluminenseFederal University and an

associate proessor at the Federal University o Rio de Janeiro.

The Brazilianmarket has been very active since the mid-80s inshallow waters, and in deep waters since the mid-90s. In this marketOrcaFlex is the most successul sotware or fexible lines, thanks to

its application in several dierent analysis types.

SUPORTEisageneraloffshoredesigncompanywithmorethan100employees. They have been very successul using OrcaFlex or rigidsubsea pipeline design, or structural installation analyses (dynamicimpact simulations) as well as or analysis o fexible lines.

OntheengineeringsideSUPORTEstartedoffin1986performingonly consultancy work related to structural and naval architecture.This encompassed platorm structures including construction andinstallation analyses plus subsea pipelines. Ater a while it was only natural forSUPORTE tograduallyundertake other disciplines aswell, so the company now develops multi-discipline projects ortheoffshoreindustry,notonlyinBrazil,butalsoinLatinAmerica(Mexico, Peru, Ecuador and Argentina) and in the Middle East

(EmiratesandIran).The company has ully designed over 30 xed platorms in waterdepths up to 200m and has re-analyzed over 70 other existingstructures because o problems such as lack o strength, insucientfoundationsorfatiguefailure.SUPORTEhasalsoparticipatedinover20 foating platorm projects, including FPSOs, semi-submersiblesand pipe-lay barges.

Rigid subsea pipeline courses, in which OrcaFlex is the mainlecturing tool, have become part o the curriculum o both graduateand postgraduate level courses, which are being taught not only by Nelson, but also by 4 other proessors o these same universities(bothinRiodeJaneiro)andwhoworkatSUPORTE.ThishasgreatlyenhancedtheknowledgeofthesoftwareintheBrazilianoffshoremarket. In addition to these courses SUPORTEprovides normaltraining courses to the local market and technical support to localusers.

On a personal note, in addition to Nelson’s academic activities(which have led to over 100 published papers) and his proessionalroleasSUPORTE´smainstructuralconsultant,hehasalsopublishedseveral books in the religious eld.

USA, CANADA&MEXICO

Bil Stewart, President, Stewart Technology [email protected],www.stewart-usa.com,+1(713)7898341

Paul Jacob,JTEC,[email protected],Cell+1(713)3989595

SOUTH

AMERICA

Nelson Galgoul,President,SUPORTEConsultoriaeProjetosLtda

[email protected],www.suporte-cp.com.br,+552121131717

KOREA J.T. Jang , President, SACs Korea Inc [email protected],www.sacs.co.kr,+8224218018

MALAYSIA,INDONESIA&SINGAPORE

Herman Perera,ManagingDirector,[email protected],www.zee-eng.com,+60(03)78778001

INDIA&MIDDLEEAST

Tarun Rewari,ManagingDirector,[email protected],www.aryatech.net,+911146018102

v9.3 released August 2009



ORCAFLEX uSER GROuP MEETINGS, 2009

The most recent round o OrcaFlex User Group Meetings was held between September andDecember 2009. We were delighted to see over 250 colleagues registering, especially given the currentclimate. In addition to the standard ‘What’s New in OrcaFlex’, the 2009 topics (see www.orcina.com/Support/UserGroup or available presentation materials) included:

Hydrodynamics: an introduction to the hydrodynamics o relatively simple geometries as

exemplied by piles and surace piercing buoys. With some o the principles understood, derivinghydrodynamic properties or more complicated geometries should be more straightorward or theuser.

Fatige: we look at the atigue lie o an SCR with and without the non-linear seabed. For thenon-linear soils model we examine the eect o varying mudline shear strength and suction ratioindependently, and then including both eects. We also show how the atigue lie o an unbondedfexible is aected by the choice o the bending stiness model employed (linear, nonlinear-elasticand hysteretic).

Advanced Atomation: beore dealing with the advanced topics, the existing automation acilitieswere outlined. Then the new Python interace was introduced, showing how this can be used toeciently automate both the pre- and post-processing. We have also just developed a new Matlabinterace and a brie demonstration o this was given.

Our gest speakers were again all well appreciated. They included:o Aberdeen:DavidFielding&NeilBotterill,ProspectFlow:VIV of Drill Casing with

OrcaFlex and CFD

o Perth: Pro. Dave White and Zack Westgate, UWA: Non-linear Seabeds

o KL:CecepHendra,ZEEEng:Lay Analysis

o Houston:JamieMcClellan&ChrisMungall,KBR: Application of Automation in a Project

o Stavanger: Anders Rødstøl, JP Kenny: Using OrcaFlex for tie-in and in-place behaviour at amanifold hub

o Rio:JoséPedrosa,Subsea7:Installation Analysis of Short Dual Flexible Jumper at Campos Basin

o Paris:CecileMelis,SBM:Generating Models suitable for Purpose

We’d like to say a big thank you to all those

who kindly contributed as guest speakers tothe UGM events. Their eorts are very muchappreciated by Orcina and other attendeesalike, and without their continued supportthis slot would simply not happen.

We also exhibited at OTC’09 in May and Offshore Europe’09 in Aberdeen inSeptember. These ran pretty much to ormwith most o the usual crowd present – asar as we could tell, the mood was one o quiet optimism about the uture. It wasgreat to catch up with old riends and tomake a ew new ones (though at Oshore

Europewedidnearlyfalloutwiththeaudiosystem on one o the nearby stands ;-)).

In November our Houston Agents BilStewart and Paul Jacob presented apaper at the PECOM 2009 conference inVillahermosa, Mexico. The paper was titled‘The Role of Dynamics in Subsea SystemsInstallation Analysis’ . The ocus was onillustrating the benets that accrue by undertaking dynamic studies prior to goingtoeld.Examplesincludedvesselmotions,fow line installation, package resonanceand fying lead operations.

IN THE NEXT NEWSLETTER

Although OrcaFlex version 9.3 went out in August2009, what with holidays and the preparationor, and delivery o, the User Group Meetingswe’ve not completely nalised our priorities or9.4. However, eatures which are high on our listinclude:

• Pipe-in-pipe,piggybacks• Slug ow, contents density variation along

arclength, ree-fooding lines• PyandTzcurvesforverticalrisers• Multi-threading for the OrcaFlex post-

processing spreadsheet• Restarts• Fatigue:T-Ncurves,meanstress• Built-inhinge/articulationmodellingelement• Bettersplashzonemodelling,slamming• FullQTF,sumfreq.QTF• Wholesystemmodalanalysis• Line payout, especially for modelling inertia

and drag on winches• Pipelaycodechecks

Clearly not all o these will appear in the nextrelease (v9.4), but is likely that most o these willat some stage! As ever, i you have any eedback

on existing eatures, or planned new ones, thenwe’d be delighted to hear rom you.

a) ...that a Bend Stiffener Attachment does notappear as a separate object in the ModelBrowser,butitdoesappearasaseparateobjectin the Results orm. As the bend stiener andproduct line can have dierent properties thisallows OrcaFlex the big advantage o being ableto extract separate results or each!

b) ...that(forsome timenow) 6DBuoyscanbeconnected to other 6D Buoys. This allowsmultiple 6D Buoys to be treated as a singlerigid body - particularly useul or modellingmore complex shapes than the stacks o co-axialcylindersthatSparBuoysandTowedFishallow!

c) ...that the OrcaFlex spreadsheet can acceptstatic-state simulation les and thereoreextract static results much aster than beore. Asalways, we strongly recommend using the latestversion o the OrcaFlex spreadsheet, suppliedwitheachOrcaFlexrelease.See“AboutOrcaFlex

Spreadsheet”onthespreadsheetOrcinamenuor your spreadsheet version.

d) ...that OrcaFlex batch script can place theresulting output les into existing directories.For example: SaveData “pathname\Case01.dat”,wherepathnamecanbeabsolute(eg,N:\Projects\Analyses\P101\OrcaFlex\Results\”, orpathname can be relative to the directory romwhichthescriptlewasloaded(eg,Results\).The latter is usually more convenient.

e) ...that the contact vertices or spar buoys’interaction with shapes and the seabed arebased on square cylinders, even i the ‘Draw

circular cylinders’ option is used. The contactvertices can be seen by selecting ‘Draw squarecylinders’ rom the drawing page o the sparbuoydataform.LumpedBuyswithnoverticeshave no interaction eects.

) ...as the author o this Newsletter discoveredwhile preparing this edition, the onts used orx- and y-axes graph labels and ticks can be setby the user! The preerred setting can also be setas the deault. Particularly useul when copyinggraphs into documents as the text on the axescan sometimes be hard to see :-)

Exhibitions, User Group Meetings and Training Courses:

The normal round o OrcaFlex User Group Meetings andassociated Open Training courses now regularly occurs duringthe September – December period each year. The most up-to-date ino on these can be ound at: www.orcina.com/UpcomingEvents. Inadditiontotheseannualeventswe haverm plans to attend:

• OTC2010: 3-6 May, 2010 at the Reliant Park, Houston. Seewww.otcnet.org or more details.

• Oceanology International 2010: 9-11 March, 2010 at theExcelCentre,London.Seewww.oceanologyinternational.com or more details.

We are also looking at attending some other events during2010, particularly in emerging market areas.

Daltongate Ulverston Cumbria LA12 7AJ United Kingdom

Telephone: +44 (0)1229 584 742 Fax: +44 (0)1229 587 191

Web: www orcina com Email: orcina@orcina com

News in Short….

Did yo Know….

Orcina - ot and abot

Orcina August 2009

Documents

Transcript of Orcina August 2009