Moorings for Marine Energy Converters

1. Moorings for Marine Energy Converters Violette Harnois v.harnois@exeter.ac.uk 23/01/2014, Hydropower module

2. Who am I? • MSc in Ocean Engineering • PhD student, final year • Analysis of highly dynamic mooring systems: occurrence of extreme mooring loads in realistic sea conditions • Research Associate • Design of Fred Olsen mooring for FabTest trials • Tank tests of the SWMTF buoy • Study for Scottish National Heritage about entanglement of marine animals in moorings • 1.5year experience with a wave energy developer, AWS Ocean Energy

3. Outline • Moorings in general • Marine Energy Converter (MEC) moorings • Types of moorings • Materials • Mooring analysis • Sea and tank tests

4. Moorings

5. Examples

6. General Requirements of a mooring • What is the main role of a mooring system? • How do we design it?

7. General Requirements of a mooring Oceanlinx device: 2010 May – “Abnormal weather and unforeseeable damaging waves meant the MK3 prototype broke free of its moorings at Port Kembla.” • Station-keeping: • restrain the motion of a floating structure – • may be required to be RIGID or COMPLIANT • Resist the environmental forces • Over the service life of the deployment -> FATIGUE LIFE, DURABILITY • Low loads in mooring lines, anchor and the floating structure -> COMPLIANCE, STRENGTH

8. General Requirements of a mooring • Avoid line abrasion,contact-> DURABILITY • Avoid marine growth and corrosion -> DURABILITY • Easy installation and maintenance -> COSTS 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...

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

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

11. Device specification: Brainstorming • Which wave energy device do you know? • How to classify them? (in term of mooring!)

12. Wave energy Motion independent device (MID): power production do not depend of motion Fixed on the seabed: no mooring system Oyster: pitching device Pico plant: fixed OWC Wave dragon: overtopping device Motion dependent device (MDD): should move to produce energy! Pelamis:yawing/heaving device

13. Two kind of moorings • For motion-dependent devices MDD • For motion-independent devices MID • More device developers here: http://www.emec.org.uk/marine-energy/wave-developers/

14. Requirements of mooring of a MDD MEC • Interaction between moorings and motion of floating structure? • Power cable? • How long should the mooring last?

15. Requirements of mooring of a MDD MEC • Low or positive interaction with power production • Low loads on power cable -> restrain excursion • Should last >30 years. Possible change of components >5years.

16. Mooring for tidal energy • Add tidal devices to the classification http://www.emec.org.uk/marine-energy/tidal-developers/

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

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

19. Mooring for Wind energy • Add offshore wind to the classification

20. Offshore wind • Monopiles: cheap, easy to install but only in shallow water <25m • Gravity based structures: easy to manufacture, use for >30m • Space frame structures

21. Floating wind • Experimental stage: not cost effective yet, stability problem

22. Requirements of mooring of an array of MECs • Redundancy (if a line breaks, we don’t lose all the devices in the array) • 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) • Optimising power production

23. Types of moorings

24. General types of mooring • Gravity base • Steel reinforced concrete • Turret mooring • Lines connected to the turret which via bearings allows the vessel to rotate around the anchor legs. • Dynamic positioning • Propellers or thrusters move floating structure • OR moored with controlled winches • All expensive solutions/difficult to install!

25. 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 • Line kept in tension by the buoyancy force of the floating device • Stiff • Always spread (=several lines) • Need to stay taut • Both widely used!

26. Catenary and taut moorings Catenary Taut Tension ratio TH/TV Position top end

27. General type of moorings • Tethered surface buoy moorings: single point or spread • Stiffness can be “tuned”

28. Materials

29. Materials • What are the different elements of a mooring system?

30. General type of anchors • Clump weight anchor: dead weight • Advantages • Relatively easy to deploy • Disadvantages • Requires large weight and subject to dragging • Drag embedment anchor • Advantages • Greater holding power • Disadvantages • Can require specialised handling • Directional • Poor in rock seabed • Cant support vertical loads well

31. General type of anchors • Vertical loaded anchor (VLA) • Advantages • Greater holding power • Can support vertical loads well • Disadvantages • Require specialised handling • Directional • Poor in rock seabed

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

33. Anchor properties

34. General mooring line material • Chain • Studless (more popular for long term moorings) or studlink (more heavy) • Heavy -> provide line stiffness (restoring forces) • Choice of size: maximum load and water depth. • Good abrasion characteristics but subject to corrosion • Wire • Light • Subject to fretting and corrosion. • Internal elasticity

35. General mooring line material • Fibre rope: Nylon, polyester... • Very light • Good elasticity • Subject to fatigue -> need to change it every 3-5 years • Change of stiffness after deployment • Non linear stiffness

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

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

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

39. Mooring analysis

40. Catenary Mooring equations w= mass per unit length

41. Catenary mooring equations

42. A Catenary Mooring – Horizontal Distance h=z

43. Variation in line tension with surge

44. Numerical and physical simulation

45. Dynamic response - Tension • Tension at large surge become non-linear Catenary equations assuming quasi-static situations cannot be applied if dynamic loading becomes dominant

46. Effects on dynamic response • Need to use a more sophisticated numerical model (Orcaflex, Deeplines…)

47. Mooring Analysis

48. Sea and tank tests

49. Facilities

50. The SWMTF facility