Edison Castro Prates de Lima

The application of the Lanczos Mode Superposition Method in dynamic analysis of offshore structures

Abstract In this work the application of the Lanczos Mode Superposition Method in dynamic analysis of offshore structures is studied. It is shown, in two typical examples, that this approach reduces remarkably the computational effort spent on dynamic response computations without loss of accuracy. Furthermore, by an adequate choice of the Lanczos algorithm starting vector, it is also verified that the localized effects, which depend on the higher modes, are well represented.

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.

Nonlinear dynamic analysis using the pseudo-force method and the Lanczos algorithm

Abstract This work presents a technique for nonlinear dynamic analysis based upon the combination of the Pseudo-Force method and the Lanczos algorithm. The equations of motion of the structural system, discretized by finite elements, are reduced through the Lanczos algorithm and the nonlinear time integration is performed by the purely iterative Pseudo-Force method. The accuracy and computer effectiveness of such methodology is shown in the analysis of a rectangular elastic plate resting entirely on nonlinear supports.

On the application of an element-by-element lanczos solver to large offshore structural engineering problems

Abstract Iterative methods for solving large sets of linear equations have been used as an alternative to direct methods of solution since the early beginning of numerical analysis. The conjugate gradient method (CCM), one of the most widely used, seeks a solution that minimizes the potential energy of the finite element assemblage. Recently, the use of Lanczos algorithm for the solution of large sets of linear equations has been re-examined. Lanczos biorthogonalization procedure is an oblique projection method that provides a solution approximation whose residual is orthogonal to a Krylov subspace. It has been shown that Lanczos and CGM share several properties but the former has the advantage of not being necessary to compute the approximated solution at each iteration. Jacobi preconditioning can also be employed in order to accelerate convergence. The Lanczos procedure was implemented using an element-by-element (EBE) scheme. The applications spread over typical offshore engineering problems encompassing regular and irregular meshes. These problems are normally ill-conditioned when compared with continuum problems. For all the analyses addressed the element-by-element Lanczos procedures presented outstanding computational efficiency.

Comparing Measured and Calculated Forces of a Manifold Deployment in 940 Meters Water Depth

In August 2001 the MRL-5 production manifold was installed by PETROBRAS in 940 meters water depth at the Marlim field offshore Brazil. The semi-submersible Amethyst, using an 18-5/8” marine riser, deployed it into the location. During the manifold deploying, several in-site measurements of the hook forces (force at the drill line dead end) and the semi-submersible accelerations were done. Both time series for the vertical accelerations and forces were obtained for two positions of the manifold along the water column. The main objective of this paper is to compare the results from the column riser system numerical analysis with the riser axial forces measurements obtained by the monitoring system.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 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