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Moorings for Marine Energy Converters

Moorings for Marine Energy Converters. Violette Harnois v.harnois @ exeter.ac.uk 28/04/2014, SuperGen Summer class. Outline. Moorings in general Marine Energy Converter (MEC) moorings Types of moorings Materials Mooring analysis. General mooring requirements.

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Moorings for Marine Energy Converters

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  1. Moorings for Marine Energy Converters Violette Harnois v.harnois@exeter.ac.uk 28/04/2014, SuperGen Summer class

  2. Outline • Moorings in general • Marine Energy Converter (MEC) moorings • Types of moorings • Materials • Mooring analysis

  3. General mooring requirements

  4. General Requirements of a mooring • Station-keeping: • “keeping a floating structure under a restricted range of horizontal excursion (slow and large horizontal surge and sway motion), and also possibly of heading (slow yaw motion)” • This includes Survivability Oceanlinx device: May2010 – “Abnormal weather and unforeseeable damaging waves meant the MK3 prototype broke free of its moorings at Port Kembla.”

  5. General Requirements of a mooring • Durability: • Need to survive over the service life of the deployment (fatigue) • Avoid line abrasion, contact • Consider marine growth and corrosion Mussels on a rope recovered from the South West Mooring Test Facility after a few months in the water: different rope diameter, weight, drag coefficient...

  6. General Requirements of a mooring • Cost-effective: • Easy installation and maintenance • Low loads in mooring lines, anchor and the floating structure

  7. Specifications for the mooring of a Marine Energy Converter (MEC) ?

  8. Specifications for the mooring of a Marine Energy Converter (MEC) • Device specification • Wave energy • Tidal energy • Wind energy • Moorings for arrays of devices

  9. Wave energy Motion independent device (MID): power production do not depend of motion Fixed on the seabed/shore: no mooring system Oyster: pitching device Wave dragon: overtopping device Pico plant: fixed OWC Motion dependent device (MDD): should move to produce energy! Bolt 2 Lifesaver: point absorber

  10. Requirements of mooring of a MDD MEC • Low or positive interaction with power production • Low loads on power cable -> restrain excursion

  11. Tidal energy • Marine Current turbines • Sabella • BlueTec

  12. Tidal energy From Carbon Trust, 2011, Accelerating Marine Energy

  13. Requirements of mooring for tidal energy devices • Not enough floating devices yet

  14. Offshore and floating wind Require less material Can withstand greater sea loads

  15. Hywind Floating wind • Operational platforms • Blue H Technologies (Tension Leg Platform) • WindFloat (Semi-submersible) • Hywind (Spar) Blue H Technologies WindFloat

  16. Requirements of mooring of floating wind turbine • Stability: main issue

  17. Requirements of mooring of an array of MECs • Redundancy (if a line breaks, all the devices in the array are not lost!) • Clearance distances (no crash between devices) • Removal of a single device without affecting other devices (if one need maintenance) • Reducing costs (e.g. sharing anchors)

  18. Types of moorings

  19. General type of moorings • Catenary Mooring • Tension provided by “in-water” weight of the line • Compliant (=flexible) • May be single point or spread (one or several lines) • Occupy a large area • Tension leg mooring • Tension provided by the buoyancy force of the floating device • Stiff • Always spread (=several lines) • Need to stay taut (problem with large tidal range) • Vertical loads on anchors

  20. Catenary and taut moorings Catenary Taut Tension ratio TH/TV Tension ratio TH/TV Non-dimensional excursion Non-dimensional excursion (Johanning)

  21. General type of moorings • For wave energy, addition of buoy/clump weight/use of synthetic ropes (Fitzgerald)

  22. Materials

  23. General type of anchors • Clump weight anchor: dead weight • relatively easy to deploy BUT • require large weight • Drag embedment anchor • greater holding power BUT • directional • poor in rock seabed • cannot support vertical loads well

  24. General type of anchors • Vertical loaded anchor (VLA) • greater holding power, can support vertical loads well BUT • require specialised handling • directional • poor in rock seabed

  25. General type of anchors • Piled anchor • Very high holding power • Can take vertical loads But • Costly • Requires specialist drilling and installation equipment • Suction anchor • Larger diameter than piled anchor

  26. Anchor properties

  27. General mooring line material • Chain • Studless (more popular for long term moorings) or studlink (more heavy) • Good abrasion characteristics but subject to corrosion • Heavy: ideal for catenary • Wire • Light • Subject to corrosion and abrasion • Limited bending: for taut moorings

  28. General mooring line material • Fibre rope: nylon, polyester, aramid, HMPE • New materials • Very light • Elasticity (nylon, polyester) • Non-linear stiffness • Compromise strength/elasticity • Reduction in the strength of fibre ropes and change in stiffness and other properties due to water absorption, eye splice (for some rope construction) or ageing and fatigue

  29. Example of Superline Nylon Non-linear stiffness Ref: Bridon catalogue Linear stiffness

  30. General mooring line material from Baltrop (1998) and Bridon catalogue

  31. Connectors • Shackles: connection • Swivels: avoid torsion • Eye splice: end connection

  32. Mooring analysis • Introduction to the SWMTF at FabTest case study • http://fabtest.com/sites/default/files/Appendix-9-FaB-Test-site-characteristics-05.03.2012.pdf • http://exchange.dnv.com/publishing/downloadPDF.asp?url=http://exchange.dnv.com/publishing/codes/docs/2013-10/OS-E301.pdf

  33. Numerical model

  34. Mooring Analysis

  35. Mooring analysis • Converter device • Hydrodynamics parameters (weight, RAOs, QTFs, radiation damping, added mass, hydrostatic stiffness, drag forces...) • Installation location • Water depth • Tidal range • Seabed type (seabed stiffness, possible anchors) • For more precise design: map of the seabed (sediment and bathymetry)

  36. Mooring analysis • Environmental loading • Wave direction • Current direction • Max current speed • 10 year or 25 year return period Hs • Additionnally, scatter diagram

  37. Mooring Analysis

  38. Mooring analysis • Static calculation • Footprint area: what area is occupied by the buoy? (collision in an array) • Excursion: maximum surge motion (power cable, array) • Natural frequency • wave energy: near wave frequency • floatind wind: far from wave frequency

  39. Mooring Analysis

  40. Mooring analysis • Intact and damage analysis • Intact: with all mooring lines • Damage: with a mooring line broken • Fatigue analysis • Following standards and factor of safety • The strength of the weakest element of the mooring multiplied by the factor of safety > Maximum estimated load

  41. Thank you for your attention Questions? Violette Harnois v.harnois@exeter.ac.uk

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