Anchor Penetration  

One method used to determine anchor penetration was first formulated by True (1975), Ref. [1], for analyzing soil failure around an advancing penetrometer to determine seabed undrained shear strength. Penetrometers used for tests carried out by the British Research Establishment (BRE 1998), Ref. [2], were essentially slender torpedo shaped vehicles weighing up to 3200kg and impacting the ocean floor at velocities over 65 m/sec. Penetration depths of over 60m were realized.

Analysis Model

The equilibrium of forces on an advancing anchor may be formulated as follows:                                                                             

                      M a = W - B - FBE - FAD - FD  - Fchain

                
where                   M         =  anchor mass
                            a          =  acceleration
                            W         =  weight of anchor in air
                            B          =  anchor buoyancy
                            F_BE       =  bearing component force
                            F_AD      =  side adhesion force
                            F_D        =  fluid drag force from water and soil phases
                            F_chain   =  anchor chain drag *
                                            *  (not included in True’s formulation)

The terms involving soil strength are:

                        FBE = Se (su Nc Af)

where                   S_e         =  soil strength strain rate factor
                             s_u         =  undrained shear strength
                             N_c        =  bearing capacity factor
                             A_f         =  anchor frontal area

 and

                        FAD  = Se (su As d / St)

where                     As    = anchor wall area
                              S_t     =  soil sensitivity
                               d        =  wall adhesion factor **
                                      ** (d is less than or equal to one)

Soil sensitivity, S_t, is the ratio of the undrained shear strength of intact clay to the remolded state i.e. S_t = s_u,intact/s_u,remolded. Although the latter equation includes S_t and gives an indication of reduced wall friction, it does not differentiate between contractant and dilatant soils. S_t is measured using a falling cone test on a soil sample where the pore pressure is essentially zero at the surface and increases by some function into the sample. For contractant soils the pore pressure will have its greatest value at the anchor wall and decrease with distance from the wall. One would therefore expect less wall friction resistance than what the soil sensitivity suggests for contractant soils.

The wall adhesion factor d is to account for reduced side resistance from separation or reduced contact pressure between the anchor and soil during anchor descent through the seabed sediments and is difficult to accurately assess. This effect will be greatest just behind the anchor frontal cone and decrease towards the anchor top end. The same effect will be realized around the front edge of each fluke. Although not included in True’s expression, d should be a function of anchor velocity (decreasing with velocity) and soil characteristics. As is its an average factor for given soil conditions.

Typical variation in anchor penetration as a function of different values of d (soil separation parameter)  for velocity 25m/sec.