August 2014 Mission Statement Background

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MSc Dissertation Thesis

MSc in Sustainable Energy Systems

Self-Embedding and

Self-Removing Sand Anchor

Maja Persson


August 2014

Mission Statement

One of the challenges with wave energy devices is the attachment to seabed. This is often solved if the specific seabed has certain features for instance greater rocks that the Wave Energy Converter (WEC) can be anchored to. However this is not always the case; it may be desired to install a WEC where the seabed just consists of sand. Installing permanent hardware in the seabed is unlikely to be accepted and so a possible design of an anchoring device has been made that needs model testing to analyse its capability.

It is made out of three hollow legs that enable the anchor to float if they are filled with air and to sink when a small amount of water is pumped in. The more the legs are filled with water, the more the anchor is embedded into the seabed. A vacuum is created in the three legs once the anchor is fixed into place. It is self-removing as well, which is useful when the WEC has reached its end of life. Letting the hollow legs fill up with air again enables this. But this is rather problematic when sand grains are involved, and so the water in the anchor legs is first forced out, creating a fluidised bed around the anchor and the fluid-behaving sand and water mixture then enables the anchor to move.

  • To see if the friction created between the anchor legs and the sand grains is enough to hold the force of the waves.

  • To see if fluidising the seabed allows the anchor to both sink and float back.


This will involve testing a representative model leg of the anchor in a water butt filled with sand and water working as the seabed.

  • For the tests, a vacuum will be created through the anchor leg using difference of height instead of using a pump as with the real model.

  • Friction force of the anchor leg will be tested when it is fully embedded. A crane with a load cell will be used as well as a scale under the water butt.

  • It will be investigated if the anchor can suck the water and sand mixture up and if it can be installed and removed by fluidising the bed.

  • A scale will be placed underneath the water butt to inform how much water there is between the sand grains in the seabed.

  • The water flow and the vacuum pressure will be measured.

Interim Targets

By having output data in flow rates, holding power and positioning, the practical experiments will give an insight of the effectiveness of this possible WEC anchor design. If proven successful, then it will be very useful for future WECs since the features of the seabed will then no longer be a limitation. This will also aid in further understanding of fluidised beds and an alternative usage of it for the student.

The purpose of this project is to investigate the feasibility of a new design of an anchor for mooring a wave powered desalination device in a sandy seabed and where permission requires future removal. It consists of a tripod made out of three hollow post-tensioned concrete legs joined by a steel shell. It is buoyant for easy towing then flooded to sink. It embeds itself easily by pumping water to fluidise the seabed then pulls a vacuum to force the surrounding particles to travel towards it due to pressure difference, creating a strong grip that can resist the maximum wave. The tripod can be removed from the sand by pumping air to get buoyancy and then re-fluidising the seabed.
The project tested a model of one of the tripod’s legs. The sand grip was measured. Scaling rules show that the 40 MN force from a 100-year Atlantic wave acting on 12 metre diameter 25 metre desalination unit could be taken by a roughly 3 metres diameter anchor leg, where 12 metres length is appropriate considering water depth of 40 metres.
The suction flow rates were also measured. From these investigations, it was found that the design has the potential to be a successful mooring for the desalination project. There is some concern about high flow rates, but it is expected that the build up of plankton and fine sediment will cause these to fall.

Declaration of Originality

I declare that this thesis is my original work, except where stated otherwise. This thesis has never been submitted for any degree or examination at any other University.


A = Area (m2)

a=Acceleration (m/s2)

Cp=Discharge coefficient

d= Diameter (m)

g= Acceleration due to gravity (m/s2)

= Newton’s law proportionality factor (=1 for SI units [All14]1)

F = Force (N)

= Height (m)

L = Length of bed (m)

m=Mass (kg)

= Pressure drop N/m2

P = Pressure (Pa)

Q=Volumetric flow rate (m3/s)

q=Angle (degrees)

sp = Surface area of single particle (m2)

t= Wall thickness (m)

U=Velocity (m/s)

= Superficial velocity (m/s)

= Minimum fluidising velocity (m/s)

V=Volume (m3)

v=Velocity (m/s)

xi= Weight fraction of particle size

= Specific gravity

= Porosity/void fraction

= Absolute viscosity (Pa*s)

ρ= density (kg/m3)

=Hoop stress (Pa)

= Surface-volume ratio

anch=Anchor leg




i=Internal measurement



m=sand and water mixture

o=Outside measurement



sc=Sand crystal



V=Void space



wv=Water in voids
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