Celestino Valle-Molina

Reliability Analyses of Clay-Embedded Offshore Pipelines Under Vertical Buckling Considering Lower-Bound Pipe-Soil Capacity

This paper presents the reliability formulation and analyses for studying and quantifying probabilistically the impact of the main parameters involved in the upheaval buckling of offshore buried pipes due to high pressure high temperature conditions (HPHT) on the reliability of the pipeline. Pipelines are considered installed in a clayey trench and naturally covered. The limit state function is established in terms of the vertical pipe-soil capacity and vertical loading. A lower-bound capacity of the pipe-clayey soil system is included in the reliability analysis in order to represent more realistic conditions with regards to the uncertainty in the capacity. The lower-bound capacity is the smallest possible physical limit of the pipe-soil capacity. Reliability assessments using the main parameters that control the vertical buckling in terms of loading and capacity were performed. Consequently, the variations of the reliability index (β) with the vertical imperfection (δ) and the ratio of the cover height (depth of pipe embedment) to the pipe diameter (H/D) were quantified. The reliability index was evaluated by means of Monte Carlo simulations. The inclusion of the lower-bound capacity by means of a left-censored lognormal distribution was found to increase in some cases the values of β.Copyright

Analytic Simulation of the Elastic Waves Propagation in the Neighborhood of Fluid Filled Wells with Monopole Sources

An analytic formulation to understand the scattering, diffraction and attenuation of elastic waves at the neighborhood of fluid filled wells is presented. An important, and not widely exploited, technique to carefully investigate the wave propagation in exploration wells is the logging of sonic waveforms. Fundamental decisions and production planning in petroleum reservoirs are made by interpretation of such recordings. Nowadays, geophysicists and engineers face problems related to the acquisition and interpretation under complex conditions associated with conducting open-hole measurements. A crucial problem that directly affects the response of sonic logs is the eccentricity of the measuring tool with respect to the center of the borehole. Even with the employment of centralizers, this simple variation, dramatically changes the physical conditions on the wave propagation around the well. Recent works in the numerical field reported advanced studies in modeling and simulation of acoustic wave propagation around wells, including complex heterogeneities and anisotropy. However, no analytical efforts have been made to formally understand the wireline sonic logging measurements acquired with borehole-eccentered tools. In this paper, the Graf’s addition theorem was used to describe monopole sources in terms of solutions of the wave equation. The formulation was developed from the three-dimensional discrete wave-number method in the frequency domain. The cylindrical Bessel functions of the third kind and order zero were re-derived to obtain a simplified set of equations projected into a bi-dimensional plane-space for displacements and stresses. This new and condensed analytic formulation allows the straightforward calculation of all converted modes and their visualization in the time domain via Fourier synthesis. The main aim was to obtain spectral surfaces of transfer functions and synthetic seismograms that might be useful to understand the wave motion produced by the eccentricity of the source and explain in detail the new arising borehole propagation modes. Finally, time histories and amplitude spectra for relevant examples are presented and the validation of time traces using the spectral element method is reported.

Calibration of partial safety factors for suction caissons in connection to catenary and taut-leg mooring systems

A risk and reliability based calibration of partial safety factors for the ultimate limit state of suction caissons subjected to inclined tension loads from floating production systems is presented. The formulation is for the case of normally consolidated clay deposits with undrained loading conditions.The load capacity analysis is carried out using the plastic limit model proposed by Aubeny et al. [1–3]. A procedure to calibrate the plastic limit model based on finite element numerical results is described. Line tensions from two mooring systems of an FPSO designed for two sites in deep water at the Bay of Campeche, Gulf of Mexico, are used.A procedure to characterize probabilistically the load capacity of the caisson at mudline is presented. The physical lower limit of the load capacity and soil-chain interaction are taking into account. The mean and dynamic tensions of mooring lines are modeled through response surfaces in terms of uncertain metocean variables describing extreme sea-states. Reliability analyses are performed using FORM and considering both mooring line tensions and load capacity of the soil-caisson system at the mudline, rather than at the padeye. Partial safety factors for the design of caissons in connection to catenary and taut-leg mooring systems are calibrated separately. This is considered to be most appropriate taking into account their differences in terms of relative contributions of the mean and dynamic tension components, failure mechanisms controlling the loading capacity, and lower bound capacity. Calibration of safety factors is performed for a target reliability index equal to 4.2.Copyright

Pore-pressure prediction and wellbore stability in the deep Mexican Gulf of Mexico

Safe and economical exploration drilling largely relies on appropriate analyses and estimation of the overpressures in the subsurface. In contrast, incorrect predictions of these abnormal pressures can cause delays and increase costs due to failures of rock formations, lack of drilling mud circulation, collapse of the casing, and closures for differential pressures.