Why Good Mooring Systems Go Bad - Maine Marine Composites
TODAYS MOWII WEBINAR:Why Good Mooring Systems Go BadFatigue Factors in Mooring Systems for Floating Offshore Wind TurbinesRichard H. Akers, PEChief Technology OfficerMaine Marine Composites, LLCJuly 16th, 10:00 AM ETThe presentation will begin at 10:03to allow all attendees time to loginPlease Join us for the Next MOWII Webinar: Subject and Time to be announcedFor more information on this and other Ocean & Wind Energy events visit us on the web at:www.mainewindindustry.com
Why Good Mooring Systems GoBadFatigue Factors in Mooring Systems for FloatingOffshore Wind TurbinesRichard H. Akers, PEChief Technology OfficerMaine Marine Composites, LLCPortland, ME USAhttp://www.mainemarinecomposites.com2
Agenda Floating Offshore Wind Turbines:Background Mooring Components: Background Anchor Failures Mooring Failures Fatigue Mechanisms Rules & Regulations Modeling, Simulation, andPrediction Inspections3
Floating Offshore Wind Turbines Performance of oil & gas platforms well understood Key differences between oil & gas platforms and FOWTs Additional uncertainties Any accidents will receive significant publicity Interaction between turbine, control system, floating platform, mooring system not well understood (yet) Performance & dynamics FOWT likely to be located in shallower waterHave lower massWind loads have greater influence on global performance & loadsTurbine control systems, angle of attack, nacelle yaw Introduce additional complexity Can complicate relative severity of “operational” and “survival” conditions UnmannedThe New York Times, “Offshore Wind Farm Approvedin New 3/offshore-wind-farm-approved-in-newjersey/? php true& type blogs& r 0, July 15, 2014 Economics: StatoilHydro Hywind Spar: 62.5 million Perdido Spar (world’s deepest oil & gas spar): 3 billion Design margins Current practice is for less redundancy onmooring systems of FOWTs Less risk of environmental pollution4
Floating Offshore Wind TurbinesPao, Lucy Y., and Kathryn E. Johnson. "A tutorial on the dynamics and control of windturbines and wind farms." American Control Conference, 2009. ACC'09. IEEE, 2009.Hurst, T., “Plans for Floating Offshore Wind MakingWaves in Mass,” Crisp Green, October 10 ingoffshore-wind-farm-making-waves-in-mass/.5
Mooring ComponentsSound & Sea Technology,"Advanced Anchoring andMooring Study,"November 30, 20096
Existing Floating Offshore Wind TurbinesStatoil Hydro HywindSpar, 2.3 MW3 catenary mooringchains in 200-220mwater depth withsingle clump weight“Floating Wind Turbine,”Wikipedia, July 9 2014,http://en.wikipedia.org/wiki/Floating wind turbinePrinciple PowerWindFloat, 2 MWSemi-taut moorings,drag anchorsFukushimaWind Farm, 2MWKabashima IslandSpar, 100 kW (2MW full scale)3 catenary 56mm anchor chainsSurvived typhoonPrinciple Power,“WindFloat TheOffshore WindSolution,” IBC DeepWater Wind FarmsSeminar, London, 2013.The Japan Times,“Floating windfarm debuts offFukushima,”November 11,2013.Utsunomiya et al.2014. Dynamicresponse of a spartype floating windturbine at powergeneration. OMAE.VolturnUS, 12 kW3 catenarymooringsExperienced scaled50-year and 500year eventsCianbro,http://www.cianbro.com/7
Mooring Materials: ChainDuggal, A.S andFontenot, W.L. 2010.Anchor Leg SystemIntegrity – FromDesign throughService Life, OffshoreTechnology Conf.,Houston, TX, pp. 1-5.BreakChain Wire 7690102114127R3KN1.32E 032.23E 033.55E 034.88E 036.65E 038.32E 031.01E 041.22E 04ProofR3StudKN8.75E 021.48E 032.36E 033.24E 034.41E 035.52E 036.71E 038.08E 03WeightStudAverage EAKgs/mN3.20E 01 1.379E 085.50E 01 2.388E 089.00E 01 3.912E 081.26E 02 5.516E 081.77E 02 7.736E 082.28E 02 9.936E 082.85E 02 1.241E 093.53E 02 1.540E 09Source: API RP 2SK8
Mooring Materials: Wire RopeFontaine et al. Semi-empiricalmodeling for seawater corrosionof wire rope. ISOPE, Osaka,Japan, June 21-26, 2009.Source: API RP 2SK9
Anchor TypesSound & Sea Technology, "Advanced Anchoringand Mooring Study," November 30, 2009Toal et al. Gryphon Alpha FPSO –Experience gained duringmoorings replacement and hookup. OTC-25322, Houston, Texas,May 5-8, 2014.10
Drag/Embedment Anchors Lots of drag anchor choices Vryhof Stevpris, Stevmanta shown Uplift in anchor/mooring design?Vryhof ed July 15, 2014 Depends on how deep the anchor is imbedded Inverse catenary of mooring line (E) allows for uplift up to 20degrees before anchor loads change Proof load testrequired 50% of breakingload of chainAnchor Manual2010, “TheGuide toAnchoring,”Vryhof Anchors.11
Suction Pile AnchorSource: API RP 2SKSound & Sea Technology, "Advanced Anchoring andMooring Study," November 30, 200912
Suction Pile Failures Scouring Tilting, loss of friction drag Normalized scour depth reducesquickly with increase of pilediameter Actual scour depth depends oncaisson diameter and “stick-up”height Prevention Geotechnical Analysis (depends onbottom type, other factors) Add skirts, artificial fronds at baseof caissonOpen Course, "Offshore Windfarm Design,Foundations" OE 5662, Delft University WindEnergy Research InstituteLi, Y., et. al., “Is Scour Important for PileFoundation Design in Deepwater?,”OTC-19906, 2009 Offshore TechnologyConference, Houston, TXR Riemers. “Self Installing Wind Turbine (SIWT),” SPTOffshore, Network Event Paris, November, 2011.13zlow
Suction Pile FailuresBhattacharjee et al. 2014. Serpentina FPSO mooringintegrity issues and system replacement: unique fast trackapproach. OTC-25449, Houston, Texas, 2014.14
Abrasion Failure: Synthetic RopeBanfield et al. Durability of polyesterdeepwater mooring rope. OTC-17510,Houston, Texas, 2005.Ayers et al. Effects of fiber rope – seabed contacton subsequent rope integrity. OTC-25136,Houston, Texas, May 5-8, 2014.15
Mooring Failure MechanismsExcessive Loads Load exceeds breaking strength ofmooring components Cause/physics Extreme storm events High pretension causes higher tension fromwave motions Decrease in breaking strength due tofatigue Line goes slack and snaps How to avoid it (case studies, examples) Recommended practice API, DNV, ABS mooring design guidelines Software analysis Accurate model and appropriate choice ofenvironmental conditions Model testsJean et al. Failure of chains by bending on deepwater mooringsystems. OTC-17238, Houston, Texas, May 2-5, 2005.16
Mooring Failure MechanismsUmbilical Failure Cause/physics Extreme weather Low pre-tension leads toexcessive offset Platform excursion so largethat umbilical snapsLi, S., Nguyen, C. 2010.Dynamic Response ofDeepwater Lazy-WaveCatenary Riser. Deep OffshoreTechnology International,Amsterdam, Netherlands. How to avoid it Umbilical bend restrictorsand other accessories Umbilical designed tosurvive bending, offset,and tensions"Ship’s anchors and trawlers can causedamage and failure of undersea Survivability.html, downloaded 07/2014MarineTechnologyReporter, vol57 (5) June2014, pg. 42.17
Mooring Failure MechanismsCascading damage Cause/physics Line snaps, FOWT moves to newposition Secondary umbilical or line failure Loss of stationkeeping Worse for FOWTs than oil and gasplatforms (less redundancy) How to avoid it Mooring design should account for Stationkeeping with one/two failed lines Transient effects of a line breaking eventSource: API RP 2SK18
Fatigue DamageBrown et al. Phase 2 Mooringintegrity JIP – summary offindings. OTC-20613,Houston, Texas, May 3-6,2010.Fontaine et al. 2012. Investigationof severe corrosion of mooringchain in west African waters.Proceedings of the Twenty-secondInternational Offshore and PolarEngineering Conference, Rhodes,Greece, pp. 389-394.19
Fatigue DamageCorrosion of Chain Cause/Physics Water temperature Water velocity (can disrupt rust buildup & marine growth) Dissolved oxygen Abrasion (can disrupt rust build-upmarine growth) Microbiologically InfluencedCorrosion Other factors that have less effect Alloy composition of steel Water pHDuggal, A.S and Fontenot,W.L. 2010. Anchor LegSystem Integrity – FromDesign through Service Life,Offshore Technology Conf.,Houston, TX, pp. 1-5.Fontaine et al. SCORTHJIP – Feedback on MICand pitting corrosionfrom field recoveredmooring chain links.OTC-25234, Houston,Texas, May 5-8, 2014.20
Fatigue DamageCorrosion of Chain How to Avoid it Design practice: over-design chainto account for material loss Empirical models exist for predictionof corrosion rate Current standards suggestcorrosion/wear allowances based ononly a few factors Several case studies show corrosioncan exceed allowances in standards Routine inspectionMelchers et al.Corrosion of workingchains continuouslyimmersed in seawater.J. Mar. Sci. Technol.12:102-110, 2007.Melchers, R.E.2005. The effectof corrosion onthe structuralreliability of steeloffshorestructures.CorrosionScience 47, pp.212391-2410.
Fatigue DamageCorrosion of Wire Rope Cause/Physics Driven by environmental factors Water temperatureWater velocityDissolved oxygenEffectiveness of lubricantRate of zinc dissolution How to Avoid it Protective measures Protective zinc coating Empirical models have been proposedto predict Corrosion rates Rate of deterioration of protectiveelementsFontaine et al. Semiempirical modeling forseawater corrosion ofwire rope. ISOPE, Osaka,Japan, June 21-26, 2009.22
Case StudyMMC Investigation of Chain Corrosion Investigation of corrosion of US Coast Guard aid-tonavigation (ATON) buoys Significant loss of chain link material in touchdownregion Believed to be caused by corrosion & abrasion Prevention of rust build-up by abrasion Increased contact roughness by sand/shell on seafloor Current investigation efforts by MMC Field measurements to quantifymaterial loss Examination of logs to assess trends Computer models of ATON chaindynamics in chafe zone23
Case StudySevere Pitting Corrosion Investigation of FPU off tropical WestAfrica Pitting corrosion discovered in mooringchain 35% decrease in cross-section after 7 years Significantly higher loss than recommendedallowances in existing codes Breaking load between 80-90% of original Attributed to Microbiologically InfluencedCorrosion (MIC)Reported by Fontaine et al, 2012 (ISOPE) &Fontaine et al, 2014 (OTC) as part of SeawaterCorrosion of Rope and Chain (SCORCH) JIPFontaine et al. SCORTH JIP –Feedback on MIC andpitting corrosion from fieldrecovered mooring chainlinks. OTC-25234, Houston,Texas, May 5-8, 2014.24
Fatigue DamageMaterial Abrasion Cause/Physics Contact between surfaces Consecutive chain links Mooring & seafloor Mooring & fairlead Function of Contact force Material hardness Relative motion How to Avoid it: Design so rope never contacts seafloor Predict abrasion on chain links based onChain geometry, mooring line dynamics,steel hardnessBrown et al.Phase 2 Mooringintegrity JIP –summary offindings. OTC20613, Houston,Texas, May 3-6,2010.25
Fatigue DamageTypes of Abrasion Adhesive wear: weldsform between wearingsurfaces and are shearedoff Abrasive wear: hardmaterial abrades softerone Fretting: small oscillationsbetween surfaces causeoxidizationwww.machinerylubrication.com26
Case StudyWear on Buoy chain Installed in 1982 with asymmetric chain mooring layout Failure during typhoon 2 months after installation 40-70 knot winds & 30 ft. waves over 3 day period Failure caused by material wearShoup & Mueller. Failureanalysis of a Calm buoy anchorchain system. OTC-4764,Houston, Texas, May 7-9, 1984.
Fatigue DamageOut of Plane Bending Relatively new source of fatigue Cause/Physics Chain bending in chainhawseHigh pretension in mooring lineDeformation of link due to proof loadingHigh loads cause links to behave like solidbeam members How to Avoid it Prediction Empirical modelsAnalytical beam modelsFinite element modelsHot-spot S-N analysisJean et al. Failure of chains bybending on deepwatermooring systems. OTC-17238,Houston, Texas, May 2-5, 2005.28
Case StudyGirassol Offloading Buoy Offloading buoy designed in accordance withAPI RP2SK with design fatigue life 60 years Several chains broke within 1 year due tofatigue failureJean et al. Failure of chains by bending ondeepwater mooring systems. OTC-17238,Houston, Texas, May 2-5, 2005.29
Fatigue DamageSnap Loads Cause/Physics Slack line followed byspike in tension as linegoes taut Can lead to large increasein tension close to orabove breaking strength How to Avoid it Further research needed Determine how snap loadsaffect fatigue Is Miner’s rule violated?30
Fatigue etraining/pre04/05pre04.htm Cause/Physics Abrupt tension changes and smallbend radii in touchdown region Torsion & trenching Changes in line behavior due tocorrosive losses How to Avoid it Avoid rope contact with seafloorDuggal, A.S and Fontenot, W.L. 2010. AnchorLeg System Integrity – From Design throughService Life, Offshore Technology Conf.,Houston, TX, pp. 1-5.31
Case StudyHaewene Brim FPSO Installed with chain/unsheathedwire rope mooring system 1998 Birdcaging discovered onnumerous occasions Reported by Leeuwenburgh &Brinkhuis, 2014 (OTC-25232)Leeuwenburgh & Brinkhuis. Lifetimeextension North Sea FPSO, mooring systemreplacement; integrity and designchallenges. OTC-25232, Houston, Texas,May 5-8, 2014.32
Rules and RegulationsStandard Based Design to AvoidFailure For good analysis Key Standards for Mooring & FOWT Good metocean model needed Good environmental model needed Pick relevant design and survival loadcases Cross between reasonable and worst case Use accepted engineering practices to ensuresurvival Environmental cases Flaws in statistical methods Climate change, growing history of weatherevents mean changing long term statistics Upper limits to wave conditions are neglectedin long term statistics When hurricane size increases to a pointwaves start to get smallerDesign: American Petroleum Institute RP 2SK Design and Analysis of StationkeepingSystems for Floating Structures American Bureau of Shipping Guide for Building and Classing FloatingOffshore Wind Turbines Guidance Notes on the Application of FiberRope Mooring Guide for the Certification of OffshoreMooring Chain Bureau Veritas 493NI Classification of Mooring Systems forPermanent Offshore Units Det Norske Veritas OS-E301 Position MooringOS-E302 Offshore Mooring ChainOS-E303 Offshore Fibre RopesOS-E304 Offshore Mooring Steel Wire Ropes33
Rules and RegulationsStandard Based Design to Avoid Failure“ABS Guide for Building andClassing Floating OffshoreWind Turbine Installations.”American Bureau of Shipping,2013.34
Rules and RegulationsStandard Based Design to Avoid Fatigue S-N curves available in standards for mooringcomponents/materials Many design standardsrecommend sizecorrosion/abrasion allowances for chain Growing number of case studiesshow allowances are insufficientOffshore Standard DNV-OS-E301. “PositionMooring,” October, 2010.35
Modeling, Simulation, and PredictionStationkeeping Analysis: MMC Tools CAD: Development of platform/hull model ANSYS-Aqwa Radiation/Diffraction analysis in frequencydomain Determine wave loads, Response AmplitudeOperators (RAOs) of platform/vessel NREL FAST Analysis of turbine performance & loads intime domain Quasi-static mooring line model Orcina OrcaFlex Nonlinear finite element mooring model intime domain Coupled with FAST for best analysis of FOWThydrodynamics including platform, turbine,moorings36
Inspection Goals: detect problems, evaluate remaining life (potential life extension) Maintenance Retrieve & inspect critical components regularly Rotate/replace chain linksAllan et al. Mooring system life extension usingsubsea inspection technologies. OTC-24184,Houston, Texas, May 6-9, 2013. Inspection Visual Inspection: high level inspection forsignificant & obvious damage, clean, identifyareas of potential risk Measurement: quantify corrosion, abrasion,other observed damage 3D Modeling: assess remaining strength ofcomponents Monitoring equipment Many floating systems: can’t tell if mooring is intact Some mooring failures detected months after failure Measurement options Line tension measurement using load cells Angle measurement using inclinometers Position & heading measurement using Differential GPS37
Conclusions Mooring systems are underappreciated Design standards lack details Corrosion/abrasion allowances Affect of snap loads Selecting environmental conditions Inspection needed to prevent failures Significant additional research needed Cause of corrosion/abrasion Selection of design load & survival conditions Effect of snap loads on mooring integrity38
References1. “ABS Guide for Building and Classing Floating Offshore Wind TurbineInstallations.” American Bureau of Shipping, 2013.2. “Floating Wind Turbine,” Wikipedia, July 9 2014,http://en.wikipedia.org/wiki/Floating wind turbine3. Allan et al. Mooring system life extension using subsea inspectiontechnologies. OTC-24184, Houston, Texas, May 6-9, 2013.4. American Petroleum Institute (API). Design and Analysis ofStationkeeping Systems for Floating Structures. API RecommendedPractice 2SK Third Edition, Washington DC, 2005.5. Anchor Manual 2010, “The Guide to Anchoring,” Vryhof Anchors.6. Ayers et al. Effects of fiber rope – seabed contact on subsequentrope integrity. OTC-25136, Houston, Texas, May 5-8, 2014.7. Banfield et al. Durability of polyester deepwater mooring rope. OTC17510, Houston, Texas, 2005.8. Bhattacharjee et al. 2014. Serpentina FPSO mooring integrity issuesand system replacement: unique fast track approach. OTC-25449,Houston, Texas, 2014.9. Brown et al. Phase 2 Mooring integrity JIP – summary of findings.OTC-20613, Houston, Texas, May 3-6, 2010.10.Cianbro, http://www.cianbro.com/11.Duggal, A.S and Fontenot, W.L. 2010. Anchor Leg System Integrity –From Design through Service Life, Offshore Technology Conf.,Houston, TX, pp. 1-5.12.Fontaine et al. 2012. Investigation of severe corrosion of mooringchain in west African waters. Proceedings of the Twenty-secondInternational Offshore and Polar Engineering Conference, Rhodes,Greece, pp. 389-394.13.Fontaine et al. SCORTH JIP – Feedback on MIC and pitting corrosionfrom field recovered mooring chain links. OTC-25234, Houston,Texas, May 5-8, 2014.14.Fontaine et al. Semi-empirical modeling for seawater corrosion ofwire rope. ISOPE, Osaka, Japan, June 21-26, 2009.15.Hurst, T., “Plans for Floating Offshore Wind Making Waves in Mass,”Crisp Green, October 10 n et al. Failure of chains by bending on deepwater mooringsystems. OTC-17238, Houston, Texas, May 2-5, 2005.17.Leeuwenburgh & Brinkhuis. Lifetime extension North Sea FPSO,mooring system replacement; integrity and design challenges. OTC25232, Houston, Texas, May 5-8, 2014.18.Li, S., Nguyen, C. 2010. Dynamic Response of Deepwater Lazy-WaveCatenary Riser. Deep Offshore Technology International, Amsterdam,Netherlands.39
References cont.19. Li, Y., et. al., “Is Scour Important for Pile Foundation Design in29.Deepwater?,” OTC-19906, 2009 Offshore Technology Conference,Houston, TX30.20. Marine Technology Reporter, vol 57 (5) June 2014, pg. 42.21. Melchers et al. Corrosion of working chains continuously immersed 31.in seawater. J. Mar. Sci. Technol. 12:102-110, 2007.22. Melchers, R.E. 2005. The effect of corrosion on the structural32.reliability of steel offshore structures. Corrosion Science 47, pp.2391-2410.23. Offshore Standard DNV-OS-E301. “Position Mooring,” October,2010.33.24. Open Course, "Offshore Windfarm Design, Foundations" OE 5662,De
Vryhof Stevpris, Stevmanta shown Uplift in anchor/mooring design? Depends on how deep the anchor is imbedded Inverse catenary of mooring line (E) allows for uplift up to 20 degrees before anchor loads change Proof load test required 50% of breaking load of chain Anchor Manual î ì í ì, “The Guide to Anchoring,”
API-RP-2SK recommends equal or higher design strength than mooring line, [3]. The IEC support structure design is consistent with other mooring codes. Anchors or foundations design is of primary importance for a good mooring system design. Compar-ison of anchor holding capacity design safet
by time domain potential flow theory, and the safety of the ship is analyzed and verified. The dock mooring design is carried out to calculate the mooring force of each cable based on the wave load and compare it with the specification value to verify the reliability of the mooring method. The main dimensions of the LNG ship are shown in Table 1.
mooring ropes and single point mooring hawsers at Viana do Castello, on the coast about 80km north of Porto, Portugal. The factory covers some 3,600m² production facilities together with 2,400m² storage area. In addition to state-of-the-art rope production machinery, the factory includes a large
anchor. Instead of anchoring, boat users tie off to the mooring and this lessens damage. Mooring buoys can also be used as an ongoing aid to coral reef conservation. They may be used to zone an area for a particular activity and help avoid conflicts between, for example, fishermen and divers. If an area is being overused, moorings can
4. The graphic display of the fleet arrangement should indicate the location of all mooring devices, cells, clusters, deadmen, etc., and the normal and high water arrangement of mooring lines and barge configuration. 5. At what river stage you can no longer access your barge mooring facilities?
(Chapter 13-257, Hawaii Administrative Rules). Anchoring elsewhere in a day-use mooring zone is permitted in areas of sand, rock, or rubble bottom types where no live corals exist. Any use of a state day-use mooring shall be at the sole risk of the owner or operator of the vessel using the mooring (Chapter 13-257, Hawaii Administrative Rules).
together by conveyor-belt loops. Wave attenuation and mooring-force features were established based on data from 402 separate runs in which incident and transmitted wave heights were recorded, along with the tension in the seaward mooring line. Test results are dompared with those of earlier experiment
Graphic Symbols N Up 17R Stair direction symbol Indication arrows drawn with straight lines (not curved); must touch object Note Note Note North point to be placed on each floor plan, generally in lower right hand corner of drawings 111/ 2 T The symbols shown are those that seem to be the most common and acceptable, judged by the frequency of use by the architectural offices surveyed. This .
