Moodu Praveen Naik

11NA30010

Under the guidance of Dr. Joydip Bhattacharjee 

Department of Ocean Engineering and Naval Architecture

IIT Kharagpur2014

Mooring Systems for Oscillating Water Column

CONTENTSIntroductionWave Energy ConvertersOWC’s and its typesMooring LinesProject OverviewResultsConclusionFutureReferences

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IntroductionCurrently, wind is biggest renewable energy source.

But, all ocean energy resources, including wave power, would combine to create energy 800 times more powerful than wind.

While solar energy might yield 150 watts per square meter on a sunny midday and wind power could produce 300 watts during a similar time period, wave energy has the potential to create 30,000 watts per meter wave front.

Ocean Energy has the potential to supply 10% of the world’s energy.

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Wave Energy Converters

Oscillating Water Columns

Overtopping Devices

Wave Activated Bodies

There are three different kinds of Wave Energy Converters:

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Wave Energy Converters(WEC)Oscillating Water Columns (OWC)Waves cause the water column to rise and fall, which alternately

compresses and depressurize an air column. The energy is extracted from the resulting oscillating air flow by using a

Wells turbine.OWC’s constructed at shore line, near shore(bottom standing) and

offshore(Floating).Concrete and steel are used in there construction.Shoreline devices have the advantage of

- easier installation and maintenance

- do not require deep-water moorings

- do not require long underwater electrical cables

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OWC’s and its typesOWC’s

Fixed Structure

Isolated

Pico, Limpet

Break Water Sakata, Mutriku

Floating Structure

Mighty Whale, Ocean Energy,

Oceanlinx

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OWC - Fixed StructureThe oscillating water column (OWC) device

comprises a partlysubmerged concrete or steel structure, open below the watersurface, inside which air is trapped above the water free surface

In general these devices stand on the sea bottom or are fixed to a rocky cliff.

The oscillating motion of the internal free surface produced by the incident waves makes the air to flow through a turbine that drives an electrical generator.

The cross-sectional area of these OWCs lies in the range 80–250 m2. Their installed power capacity is in the range 60-500 kW.

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OWC - Floating StructureThis class of device exploits the more powerful

wave regimes available in deep water (>40m depth).

More recent designs for offshore devices concentrate on small, modular devices, yielding high power output when deployed in arrays.

There is a huge importance to Mooring lines in order to decrease the oscillatory motions of the device and to increase the efficiency of the power output.

Well’s turbine is used in both Fixed and Floating structure’s of OWC’s.

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Mooring LinesThe two major requirements for a WEC mooring are to

withstand the environmental and other loadings involved in keeping the device on station, and to be sufficiently cost effective so that the overall cost of the device remain viable.

These are designed to increase the power extraction efficiency and thus to the income stream.

The excursion of the device must not permit tension loads in the electrical transmission cable and should allow for suitable specified clearance distances between devices in multiple installations.

The mooring system should allow the removal of single devices without affecting the mooring of adjacent devices.

Contact between mooring lines must be avoided.

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Mooring systems

Mooring Systems

Single Point mooring

Turret mooring

Catenary Anchor Leg mooring

Single Anchor Leg mooring

Articulated Loading Column

Dynamic Positioning

Spread mooring

Catenary mooring

Multi-Catenary mooring

Taut Spread mooring

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Mooring Components

Mooring Component

s

Mooring Line

Chain

Wire Rope

Synthetic Rope

Mooring Anchor

Driven Pile/Suction Anchor

Vertical Load Anchor

Drilled Anchor

Gravity Anchor

Drag-Embedment Anchor

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Project Overview Considering OWC as a rectangular barge analysis done to obtain

mooring forces and hydrostatic forces.o Length - 20mo Breadth - 10mo Height - 15mo Draught - 10m

Wave input : An irregular wave is considered with 3 meter significant wave height with 11 sec wave period and 0.34 Hz peak wave frequency

Analysis done by connecting three different spread mooring to the barge.o Catenary mooringo Taut spread mooringo Linear catenary mooring

Catenary properties: Diameter – 0.25 m, Max. tension – 600 kN, Stiffness- 323968.3 kN, Bending stiffness – 8441 N.m^2

Four cables are connected to the barge in its corner positions.

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Catenary mooring Max. cable tension at structure 460572.906 N Max. cable tension at sea bed 131267.453 N

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Taut spread mooring14

Max. cable tension at structure 474958.531 NMax. cable tension at seabed 139663.25 N

Linear catenary mooring

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Max. cable tension at seabed 495232.12 NMax. cable tension at structure 142623.35 N

ConclusionPrimary objective of mooring lines is station keeping of the

OWC device.Mooring systems are designed to decrease the cost of the

setup. In Floating OWC’s any type of the mooring lines can be

used since energy is not converted from oscillatory motion of the system.

However, mooring loads will play an important role in the efficiency of the device as the relative motion of the rigid chamber and the water column may interfere either positively or negatively.

Spread moorings using catenary lines are common for semi submersible platforms.

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Future

Design analysis of Oscillating Water Columns.Different Mooring analysis on OWC’s to

generate maximum energy and power output.Material properties for mooring lines.

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References“Wave Energy: the Department of Energy's R&D

Programmed 1974-1983”, P. G. Davies (ed), ETSU Report Number R-26, March 1985.

CEC, (1993). “Wave Energy Converters - Generic Technical Evaluation Study”, a report for the Commission of the European Communities, published by Danish Wave Power aps, August 1993.

McCormick ME. Ocean wave energy conversion. New York: Wiley; 1981.

DET NORSKE VERITAS – DNV, 2001, Offshore Standard NV_OS_E301, Position Mooring, Norway

AMERICAN RETROLEUM INSTITUTE – API, 1996, Recommended Practice for Design and Analysis of Station keeping Systems for Floating Structures, API RECOMMENDED PRACTICE 2SK, 2nd Edition

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THANK YOU

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