Strength of an anchor design
Anchors should be designed to withstand the loads applied on them in the different loading situations. Typical loading situations and areas of special attention for anchors are:
•During the proof loading of the anchors in the factory, after construction has been completed. On basis of the proof load results, the classification societies issue the approval certificate.
While embedded in the seabed
•Depending on the soil conditions, different loading situations can occur on the anchor. In sands and clays, the load tends to be spread equally over the anchor, which generally presents no problems. Retrieval is also very simple, without excessive loads
placed on the anchor.
32 • In very hard soils, the anchor has to be able to withstand the load with only one or two of the fluke tips buried in the soil, as penetration in very hard soil conditions is generally shallow.
•In very soft clays (mud) penetration of the anchor is uncomplicated. However, recovery of the anchor can cause high loads, sometimes exceeding the load that was used to install the anchor.
•Sidewards forces on the top of (shallow) buried anchors can be so extreme that no anchor is capable of resisting them.
During anchor handling
•Care should be taken during the handling of the anchors, as the loads exerted by the winches, vessels and chain can sometimes exceed the structural strength of the anchor and cause damage. Anchor designers attempt to design the anchors for these high loads, however this is not always possible due to variations in the magnitude of the loads during handling operations.
•Large forces can be exerted on the anchor when high winch power is used, the anchor is caught on the anchor rack or caught behind the stern roller of the AHV.
Strength of an anchor design
•The use of an improper anchor/chaser combination. When a chaser is used that is either too small or too large, the chaser could jam on the shank of the anchor and cause damage.
The strength of the Stevpris anchor is now more closely examined in the light of the remarks made before.
Strength of the shank
The prismatic shape of the Stevpris anchor not only ensures optimal penetration of the soil but also guarantees maximum strength. Although the Stevpris design also has limitations, it is one of the better designs to withstand sideward forces on the shank, a frequent occurrence in practice. When using an
anchor in very soft clay (mud), the bending moment 33 on the shank is low during the installation and when
the anchor is in the soil. However, during the breaking out of the anchor, high bending moments could be introduced in the shank due to the high retrieval forces required in very soft clay. In extremely sticky soils, the breaking out force of the anchor can rise to 80% or 90% of applied anchor load; in certain instances, it can even exceed 100%. To reduce these forces the breaking out procedure is undertaken at low speed to allow time for the anchor to break out.
Strength of the fluke
The strength of the fluke and especially the fluke points of an anchor are very important when working in extremely hard soils such as coral, limestone and other rock types. It is possible in such instances that the total holding capacity of the anchor will have to be sustained by the fluke points alone. This means the structure must be strong enough to withstand extreme bending forces. Loading in normal soil conditions is not a problem due to the fact that the load is equally spread over the fluke.
