[Thought the discussion on “suction anchors” should best be...

[Thought the discussion on “suction anchors” should best be transferred to the “Foundations” thread since we might want to refer to it later - and we would never find it again under boat watching.]

Megafauna’s comment and questions were:

“I can sort of understand the concept of how the plate anchor is embedded below the sea floor. From what I can tell (audio would have been helpful), the caisson (with attached plate anchor) is open at the bottom and sinks into the soft sediment. The caisson is withdrawn once it gets to the desired depth, leaving the plate anchor behind in a vertical position. After the caisson has been withdrawn, the chain attached to the plate anchor is hoisted causing the plate anchor to rotate and key into the sediment.
What I am not quite clear on is how the caisson penetrates into the sediment. Is it the weight of the caisson or is a vacuum formed inside the open bottomed cylinder causing it to be drawn into the sediment?
I would be grateful if you could help me and others to understand how this process works. I would also like to know if you think this process could be used to anchor ceto buoys?”
Thanking you
MF


My response:

A suction anchor caisson is essentially an upside down cylindrical bucket. In the situation of being suspended by the supporting chain above the seafloor, the downward water pressure exactly matches the upward pressure. When lowered into the seabed sediment the bucket edge will penetrate sufficiently, under its own weight, to form a crude seal so that the water can be pumped out via ports in the top. Thus a vacuum occurs inside the caisson while the exterior downward water pressure remains the same. There is therefore a net force pushing the caisson into the sediment.

What is the scale of this force? It is entirely due to the weight of the water column plus the air column at the surface above. The well known rule of thumb is that the pressure increases by 1 atmosphere for every 10 m you go down. For a CETO operating depth of say, 50 m, the downward pressure will be, 5 + 1 = 6 atmospheres. (It can be seen that for deep sea oil platforms in 1000 m of water the pressure increases to a very useful 100 atm.) 1 atm = 100 000 N/m2

Taking an arbitrary example of a 5 m dia caisson, the total downward force,

F = pressure x (top) area
= 11.8 M N
= 1200 tonnes weight

This force is exerted through the narrow edge of the cylinder onto the sediment - bit like pressing a knife edge into butter. The caisson sinks until level with seabed or until it hits bedrock. A good prior knowledge of the seabed geomorphology is therefore essential. CWE through its subcontractors have just completed this task (last ASX ann of 19 Aug). Just by increasing the diameter of the caisson the force can be multiplied. As these forces are high the top surface of the caisson has to be structurally engineered. At this point it becomes an exercise in optimum design for the particular situation.

Note that no jackup rig, drilling crane, or long piles and concreting is involved, but an appropriate launch vessel for the caisson is required as shown in the video clip in MF’s link.

The weight of the caisson is likely to be in the range 25 to 100 tonnes but is small compared to the weight of the water column.

Suitability for CETO anchoring

For oil platforms a ring of anchors may be employed with lines extending at angles of ~40 degrees to the seafloor. Much of any excessive force is resisted strongly by the horizontal force component in the sediment.

For a CETO device the tether is always near vertical and the anchor point has to endure innumerable cycles of high forces that tend to have a lifting effect. If the sediment is pure sand I can see some potential difficulties. In particular wet soils a vibration effect can put particles into suspension which can then flow around an obstacle or anchor point. I suspect that this is an important element in the research that COFS is intending to undertake.

For millennia slack line anchors were the in thing because there was no need for change. As civilisation advanced and marine entrepreneurs moved further offshore steep line anchoring systems became essential. It should be recognised that the development of ‘suction’ anchors has a history of about 20 years. Now that we need secure vertical fixings specialised variations will be devised. As always, necessity is the mother of invention.

The vertical plate anchor detachable from the caisson may not be suitable for the C 6 as is but I can envisage modifications that would enable the spreading of the load over a wider area at depth.

Hope this helps.

Juke