NUMERICAL ANALYSIS OF THE HOLDING CAPACITY OF TORPEDO ANCHORS CONSIDERING SETUP EFFECTS

Resumo

Torpedo anchors have proven to be an outstanding alternative of fixed anchor point for taut
leg mooring systems on Brazilian offshore fields. This type of anchor has low construction and
fabrication costs, which are not dependent on water depth and withstands high vertical loads. Torpedo
anchor has a “rocket” shape and its installation is given by free fall using heavy weights as the driving
kinetic energy. Its driving process induces an excess pore water pressure generation and causes
significant shearing and disturbance, which affects the stress, strain and strength characteristics of the
soil surrounding the anchor. Immediately after installation, the holding capacity of the torpedo anchor
is significantly reduced. Although, after the anchor is driven, holding capacity is observed to increase
with time. This phenomenon is referred as setup. This paper presents a numerical based study of a finless
torpedo anchor embedded in a purely cohesive isotropic soil using an axisymmetric nonlinear finite
element model. The plasticity Cap model was chosen to describe the mechanical behavior of the soil.
Anchor-soil interaction is simulated using surface-to-surface contact pairs with a penalty type contact
property to represent friction behavior between the surfaces in contact. A number of analyses are
conducted using Abaqus/CAE® in order to understand the response of this structure when considering
setup effects. Additionally, a parametric study is also performed. The results indicated that soil
permeability plays an important role into setup process. Furthermore, plasticity parameters are also
investigated and the results followed a pattern of behavior on structure response.