Gilberto Bruno Ellwanger

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.

A coupled approach for dynamic analysis of CALM systems

This paper presents a time domain coupled numerical model, integrating the buoy and the slender structures, for the dynamic analysis of catenary anchor leg mooring (CALM) systems under random environmental loading. The model is based on the finite element discretization of the slender structures and the buoy hydrodynamics can be represented by two approaches: one based on Morisons equation and another using the diffraction/radiation theory. The moored tanker dynamic behavior is represented in the model by the hawser dynamic tension. The numerical results of a CALM system, designed to operate in 400 m water depth, are compared to model tests data available, resulting in a very good agreement between them.

VIM and Wave-Frequency Fatigue Damage Analysis for SCRs Connected to Monocolumn Platforms

The large Vortex Induced Motion (VIM) due to current acting on a circular-shaped monocolumn platform induces low-frequency stress variations on the SCRs (Steel Catenary Risers) connected to it. These stresses together with stress variations associated to wave effects must be accounted for in the fatigue analysis of these risers. Normally, the joint statistics of waves and currents show that these environmental variables may be considered as statistically independent. Therefore, the number of global riser analyses necessary for the SCRs fatigue analysis becomes extremely high in order to consider a suitable number of combinations (including intensities and directions) of waves and currents. This paper describes a methodology for computing the fatigue damage in SCRs (Steel Catenary Risers) due to wave-frequency and VIM (Vortex Induced Motion) load effects based on a combination damage formula presented in DnV-OS-F204 [1]. The wave-frequency and VIM fatigue damages are calculated separately (by a time-domain rainflow approach) and the combined damage is evaluated by means of the DnV formula. This methodology reduces considerably the number of global riser analyses and consequently the computational burden associated to the fatigue analyses of SCRs connected to monocolumn-type platforms.Copyright

On the extreme response of heave-excited flexible risers

The focus of this paper is on the development of a high effective practical approach to assess the short-term extreme response statistics of flexible risers excited by the first-order heave motion of a floating unit. The extreme response statistics is obtained by fitting a probability distribution directly to a sample of extreme response values. Each sample is obtained through the analysis of a short-time window of a heave motion realization that encompasses the instant when the extreme response takes place. The location of this window is determined with the help of a time-dependent nonlinear transfer function relating the response to the excitation. The analyses of three different flexible risers configurations illustrate the accuracy and the robustness of this approach to calculate the extreme response statistics.

Structural design of process decks for floating production, storage and offloading units

This work analyzes the influence of ship motion and deformation on the structural design of decks to support production plants in floating production, storage and offloading units (FPSOs). These decks are space frames with lengths ranging from 30 to 100 mr, with a variable number of rigid supports linked to the ship hull. The deck and ship hull are modeled together for naval and structural analysis. A global, coupled analysis is performed considering the deck and ship structures; sea loads are evaluated by means of probabilistic methods.

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 FE Model to Predict the Stress Concentration Factors in the Tensile Armor Wires of Flexible Pipes Inside End Fittings

In this work, a bidimensional finite element (FE) approach is proposed to estimate the stresses induced in the tensile armor wires inside end fittings (EF) of flexible pipes. This approach accounts for the residual stresses caused by the mounting procedure and the deformed configuration of the wire. The resin and its interaction with the wires are also addressed. A parametric study was performed aiming at investigating the influence of three parameters on the stress state along the wire, i. e., the contact conditions between the resin and the wire inside the EF, the stress levels induced during the factory acceptance test (FAT) or the offshore leak test (OLT) and the resin elastic properties. The study pointed that high stress concentration is induced in the transition between the flexible pipe’s body and the EF and the stress distribution along the wire may be significantly affected by these parameters. Moreover, the apparent axial stiffness of the wire is also modified by its anchoring conditions, which may lead to non-uniform stress distributions among the wires of the tensile armor layers.© 2013 ASME

A Frequency Domain Approach for Random Fatigue Analysis of Steel Catenary Risers at Brazil’s Deep Waters

The steel catenary riser was adopted by Petrobras as a cost-effective alternative for oil and gas export and for water injection lines on deepwater fields, where large diameter flexible risers present technical and economic limitations. The installation of the P-18 SCR was a pioneer project of a free-hanging steel catenary riser linked to a semi-submersible [1] and demonstrated the technical feasibility of the concept. Fatigue damage verification is an important issue in SCR design, demanding a high number of loading cases to be analyzed. The random time domain nonlinear analysis is considered an attractive and reliable tool for fatigue analysis as nonlinearities are properly modeled and the random behaviour of environmental loadings is considered. As time domain analysis is high computer time consuming, the frequency domain analysis has been considered as an alternative tool for the initial phases of riser design to be used mainly for fatigue damage verification. This paper presents a methodology developed to perform a linearized frequency domain analysis aiming at fatigue damage verification. Two drilling risers were analyzed with the frequency domain procedure developed. The model of a steel lazy-wave riser was analyzed both in frequency and time domain in order to compare fatigue damage results. The analyses were performed using the Petrobras’s in-house computer codes ANFLEX, ALFREQ and POSFAL developed and implemented as part of projects from CENPES/PETROBRAS with “COPPE/UFRJ -The Engineering Post-Graduating Coordination of the Federal University of Rio de Janeiro”.© 2004 ASME

Numerical Analysis of a Flexible Pipe With Damaged Tensile Armor Wires

This paper focus on the structural analysis of a 2.5″ flexible pipe with one up to five broken wires in its outer tensile armor. The pipe is supposed to be under pure tension and the effect of the number of ruptured wires on its response is discussed. A three-dimensional nonlinear finite element model devoted to analyze the local mechanical response of flexible pipes is proposed and employed in all performed analyses. This model is capable of representing each wire of the tensile armors and, therefore, localized defects, including total rupture, may be adequately represented. The obtained results pointed to high stress concentration in the wires near the damaged ones as well as a significant increase in the axial rotation of the pipe. Moreover, the stresses in the inner carcass and the pressure armor are also affected by the rupture of wires in the outer tensile armor.Copyright

A Reliability-Based Comparative Study for Mooring Lines Design Criteria

The characteristic load effect for the design of mooring systems can be defined by means of three procedures: 1) an extreme sea state with a given return period, 2) a set of sea states on a contour line associated to a return period or 3) extreme response (tension) statistics for a long-term period. This work presents the result of a reliability-based partial safety factor calibration study for a LRFD mooring line design criteria considering the three approaches mentioned above. The calibration exercise is applied to three FPSOs considering North Sea environmental conditions and different water depths: 200m, 800m and 3000m. The mooring systems investigated take into account lines made up of chains and polyester ropes. It is shown that the design procedure based on the long-term response, among all water depths investigated, is the one that presents less scattered reliability indices around the target level.Copyright