Penetration initiation
Up Anchor geometry Penetration initiation Failure wedge Anchor settling

Situation 1: anchor is resting on the seafloor

At the end of the shank of the anchor there is a horizontal pulling force of the anchor-handling vessel, Ft. The anchor is about to rotate because the normal force, Fn is already concentrated at the end of the fluke of the anchor. Fn Would be straight under the gravity force Fz when Ft=0.

When the anchor starts to rotate situation 2 follows. 

Situation 2: anchor is no balancing on his fluke

All the normal force of the seafloor is concentrated in a very small region of the soil. Here we make the assumption that Fz (= Fn) is that large and the region where the fluke rests on the seafloor is that small that the soil will yield.

When the soil yields situation 3 follows out of situation 2. 

Situation 3: flukes are penetrating the seafloor

 A lot of extra forces appear now. We make the assumption that the anchor is penetrating the seabed. The anchor will meet 5 forces. The gravity force, Fz ; The pulling force of the anchor-handling vessel, Ft; A resulting normal force of the soil which can be pointed downwards or upwards, but in with the assumption that the anchor penetrates further it will point downwards, Fn; A resistance force of the soil sliding along the both fluke sides, Fw; Because of the assumption that the anchor will penetrate further is the soil, there is also a force for cutting the soil, Fs.

All these forces can be decomposed in a force parallel to the x-axis and a force parallel to the y-axis. When the anchor is penetrating the soil, then follow the next equations:

In x-direction:

                                                   

In the y-direction:

                                                  

When we make the assumption that the anchor is slowly penetrating the soil than:

                                                  

so we can continue our deduction. The aim is to get a relation between the weight of the anchor, angle β and the pulling force of the anchor-handling vessel:

                                                  

When we replace Fn in the equation of the x-direction, the following is the result:

                              

In this equation it becomes clear that the weight of the anchor is always a positive factor for the penetrating behaviour. Furthermore is becomes clear that a low value of (Fw + Fs) is positive for the penetrating behaviour. Streamlining the anchor and sharpening the flukes is very positive, which matches with our first thoughts.

(Fw + Fs)-Fz  will be larger than 0 because the anchor is designed to hold a much larger load than its own weight. So the anchor will not penetrate the soil when tan β << 1. This is the case when β comes close to 90 degrees. The exact “non-penetrating-angle” is dependent of the exact value of (Fw + Fs)-Fz, which is soil dependent; the weight of the anchor; And the pulling force of the anchor-handling vessel.

An anchor should be designed in such a way that the angle α and the length of the shank prevent a to large angle β. A designer knows the coefficients of the soil and the cutting resistance of the anchor, so he must be able to do so.

 

Copyright © 2005 Project group 1
Last modified: 10/27/05