Suction anchor or suction caisson is an offshore foundation which is quite popular for offshore installation. It utilizes a negative pressure concept to drive the suction anchors down. This article will demonstrate some simple calculations used for the suction anchor.
Example:
A suction anchor, 40” OD x36” ID, is deployed as part of a mooring anchor for a floating production platform in 900 ft of water. The initial penetration due to its weight is 5 feet into the sea. Soil resistance is 450 lb/ft2 by average. Based on the given information, determine values from these two questions below;
- What is the volume evacuated for each foot of penetration?
- How long of a suction anchor will be needed so that the top of the suction anchor is 5 ft above the seabed at the end of the operation?
Figure 2 – Suction Anchor Submerse by Its Weight
Important Information
The sea water density = 64.0 lb/cu-ft (8.6 ppg)
Steel specific gravity = 785
What is the volume evacuated for each foot of penetration?
Assumption: Impermeable formation
Volume (ft3) = 7.07
How long of a suction anchor will be needed so that the top of the suction anchor is 5 ft above the seabed at the end of the operation?
Figure 3 – Suction Anchor Diagram (Before and After)
The initial penetration support weight of the suction anchor therefore is only the frictional force between soil at the seabed and the hydrostatic pressure from the water column will be taken into account.
L = the length of the suction anchor resisting the force from hydrostatic pressure applied at the top of the suction anchor.
This is assumed that the friction generated by initial penetration continues to oppose and equal the buoyed weight of the suction anchor.
Based on the assumption, the force from soil resistance is equal to force from the hydrostatic pressure acting against the top of the suction anchor.
450 × surface area = Hydrostatic pressure × area of top of the suction anchor
L = 56.2 ft
Total length of the suction anchor = 5+ 56.2 + 5 = 66.2 ft
Figure 4 demonstrate the final condition compared to the initial condition.
Final Diagram
References
James G. Speight, 2014. Handbook of Offshore Oil and Gas Operations. 1 Edition. Gulf Professional Publishing.
Trond Bendiksen, 2015. Commissioning of Offshore Oil and Gas Projects: The manager’s handbook. Edition. AuthorHouse.
Joseph A. Pratt, 1997. Offshore Pioneers: Brown & Root and the History of Offshore Oil and Gas. Edition. Gulf Professional Publishing.
SEMAR AS, (2013), Shelley Field [ONLINE]. Available at: http://www.semar.no/semar/bilder/Shelley-field.jpg [Accessed 29 July 2016].
