Elisabeth de Campos Porto

Undrained Load Capacity of Torpedo Anchors Embedded in Cohesive Soils

This paper presents a numerical based study on the undrained load capacity of a typical torpedo anchor embedded in a purely cohesive isotropic soil using a three-dimensional nonlinear finite element model. In this model, the soil is simulated with solid elements capable of representing its nonlinear physical behavior and the large deformations involved. The torpedo anchor is also modeled with solid elements, and its geometry is represented in detail. Moreover, the anchor-soil interaction is addressed with contact finite elements that allow relative sliding with friction between the surfaces in contact. A number of analyses are conducted in order to understand the response of this type of anchor when different soil undrained shear strengths, load directions, and number and width of flukes are considered. The results obtained indicate two different failure mechanisms: The first one involves significant plastic deformation before collapse and, consequently, mobilizes a great amount of soil; the second is associated with the development of a limited shear zone near the edge of the anchor and mobilizes a small amount of soil. The total contact area of the anchor seems to be an important parameter in the determination of its load capacity, and, consequently, the increase in the undrained shear strength and the number of flukes and/or their width significantly increases the load capacity of the anchor.

Undrained Load Capacity of Torpedo Anchors in Cohesive Soils

This paper presents a numerical based study on the undrained load capacity of a typical torpedo anchor embedded in a purely cohesive isotropic soil using a three-dimensional nonlinear finite element (FE) model. In this model, the soil is simulated with solid elements capable of representing its nonlinear physical behavior as well as the large deformations involved. The torpedo anchor is also modeled with solid elements and its complex geometry is represented. Moreover, the anchor-soil interaction is addressed with contact finite elements that allow relative sliding with friction between the surfaces in contact. Various analyses are conducted in order to understand the response of this type of anchor when different soil undrained shear strengths, load directions as well as number and width of flukes are considered. The obtained results point to two different failure mechanisms: one that mobilizes a great amount of soil and is directly related to its lateral resistance; and a second one that mobilizes a small amount of soil and is related to the vertical resistance of the soil. Besides, the total contact area of the anchor seems to be an important parameter in the determination of its load capacity and, consequently, the increase of the undrained shear strength and the number of flukes and/or their width significantly increases the load capacity of the anchor.Copyright

A Study on the Holding Capacity Safety Factors for Torpedo Anchors

The use of powerful numerical tools based on the finite-element method has been improving the prediction of the holding capacity of fixed anchors employed by the offshore oil industry. One of the main achievements of these tools is the reduction of the uncertainty related to the holding capacity calculation of these anchors. Therefore, it is also possible to reduce the values of the associated design safety factors, which have been calibrated relying on models with higher uncertainty, without impairing the original level of structural safety. This paper presents a study on the calibration of reliability-based safety factors for the design of torpedo anchors considering the statistical model uncertainty evaluated using results from experimental tests and their correspondent finite-element-based numerical predictions. Both working stress design (WSD) and load and resistance factors design (LRFD) design methodologies are investigated. Considering the WSD design methodology, the single safety is considerably lower than the value typically employed in the design of torpedo anchors. Moreover, a LRFD design code format for torpedo anchors is more appropriate since it leads to designs having less-scattered safety levels around the target value.

Reliability-Based Design of Torpedo Anchors

The use of powerful numerical tools based on the finite element method has been improving the prediction of the ultimate bearing capacity of fixed anchors applied in the offshore oil industry. One of the main achievements of these numerical tools is the reduction of the uncertainty related to the bearing capacity prediction of these anchors. Therefore, it is possible to reduce the design safety factors values that have been calibrated based on prediction models with higher uncertainty, without impairing the original level of the structural safety. This paper presents a reliability-based safety factors calibration study for the design of torpedo anchors considering the statistical model uncertainty evaluated using the results from some experimental tests performed by PETROBRAS and their correspondent finite-element based numerical estimates.Copyright

Numerical Simulation of Installed Torpedo Anchors Embedded in Cohesive Soil

Torpedo anchors have proven to be one of the most important alternatives for mooring systems, especially in Brazilian offshore fields. Their installation involves many hydrodynamic and soil-structure interaction aspects and, therefore, their vertical stability cannot be always assured. This often results, after their installation, in an inclination between the anchor and a vertical axis called tilt angle. The traditional approach to assess the holding capacity of torpedo anchors relies on the hypothesis that the relative angle between the load direction and the axis of the anchor may be used in conjunction with a finite element model in which a perfectly vertical anchor is considered. In this work, this assumption is discussed. A parametric study in which four different tilt angles are combined to various load inclinations was conducted. The study relied on a previously proposed FE model. The results obtained indicate that the use of the traditional approach is safe for relative angles higher than 40° and lower than 140° For other angles, the approach leads to values slightly higher than those predicted with the direct consideration of the tilt angle.Copyright

The Development of the Torpedo-Piezocone

The paper describes the development of a soil investigation equipment, consisted of a piezocone installed at the tip of a torpedo-pile. The new equipment, named torpedo-piezocone, is able to measure cone resistance (qc ), sleeve friction (fs ), pore-pressure at the cone face (u1 ) and cone shoulder (u2 ) as well as cone temperature during free-fall and some time after final stop. Velocity, as well as displacement (depth) are obtained from accelerometer data, as in the case of the torpedo-pile. The various steps to develop the equipment are presented, from the requirements of the transducers until the calibration procedures in the laboratory. The first tests performed onshore are also presented. In general, very good results have been obtained.Copyright

A Simple Approach for Determining the Holding Capacity of Torpedo Anchors Embedded in Cohesive Soils

Torpedo anchors have been used in various offshore applications especially due to its low cost installation and the ability to withstand high inclined loads. This anchor consists of a shaft in which flukes are welded in order to increase the soil-anchor contact region and, consequently, its holding capacity. Since this anchor presents a singular geometry, different from a regular cylindrical anchor/pile, the computation of the holding capacity of a torpedo anchor is not straightforward. In a previously presented work, the holding capacities of typical torpedo anchors were assessed with a finite element (FE) model in which both the anchor and the surrounding soil are represented with three-dimensional finite elements. However, this FE model demands a significant computational effort and, consequently, simpler approaches would be desirable in order to design these anchors. Relying on the FE model and a parametric study, this paper presents simple formulae to predict the holding capacities of torpedo anchors embedded into cohesive soils. These formulae are employed to predict the holding capacities of two different torpedo anchors, which are compared to those estimated with the FE model. Results agreed very well indicating that this simpler approach may be employed to quickly evaluate the holding capacities of these anchors.Copyright