Infotec (United Kingdom)

Abstract Subsurface Safety Valves (SSSV) hydraulically surface-controlled are fail‑safe valves that remain open under applied control pressure. Most of the SSSV mechanisms rely on an inner shroud called flow tube to push the flapper mechanics to open position. Evidence gathered in Brazilian fields suggests that some models of SSSVs were experiencing a fail-to-open failure mode, in which the flow tube could not complete its entire course and would remain in uppermost position, instead of moving down. Without the covering and support provided by the flow tube, injection can still be performed, since flapper allows for downward flow. However, the flapper would be directly exposed to downward turbulent flow, which could cause vibration (chattering effect) and can incur cumulative damage. A series of studies were conducted to evaluate risks of flapper falling downhole or of compromising well safety barrier. These studies started with a Computational Fluid Dynamics (CFD) analysis, to characterize the turbulent flow pattern surrounding the exposed flapper and calculate the fluid forces arising during injection at high flowrates. A structural numerical model was then built, using a Finite Element Analysis (FEA), incorporating both the fluid forces and structural interactions of the components. History of stress was obtained for the components deemed more susceptible to fail: flapper, hinge pin and torsional spring. Erosion analyses were also performed, to determine if the unstable behavior of the flapper body, during injection, could enhance impacting velocities to inner walls of the SSSV, which could produce higher erosion rates and a shorter productive life. Safety factors were found to be sufficient for fatigue and for erosion, considering a mission of 27 years. This result indicates that, if flow tube is unable to move, the increased vibration in flapper system should not produce significant damage, which would risk in rupture of any subcomponent or loss of sealing capability. The decision to perform a maintenance intervention must be made by operator, considering the encountered low risks of leaving the degraded state as is, as opposed to incurring various risks present in a Hard-workover operation, such as releasing of packers and pulling completion accessories out of hole through the blow-out-preventer (BOP).

Abstract The design of a Pre-Salt well with open hole completion typically involves cementing the production casing to cover the entire saline section. This approach facilitates drilling through the entire reservoir with minimal overpressure while using a Water Based Mud that is compatible with logging, completion and acid stimulation. However, due to difficult ties in geological interpretation on a Itapu well, it became necessary to drill the carbonate section with exposed salt using oil-based mud, while preserving the original completion design. This represents an unprecedented development in the Brazilian pre-salt. This article discusses the formulation of the oil-based mud and a method for removing the filter cake from the wellbore. It also presents the wellbore diameter, measured at the end of drilling, and some indicators of the well cleanliness after the completion fluid swap, and the damage ratio assessed following the stimulation operation. The oil-based mud prevented dissolution of the salt rock in the region that was drilled with water-based mud and further down. The conditioning string was pulled out of the well after the displacement operation with brine and a surfactant pill, with a remarkable condition of cleanliness and without trace of oil-based mud adhered. Laboratory tests with the surfactant pill have shown that it is possible to minimize, or even remove, the organic layer of the filter cake created by the oil-based mud on the wellbore by the drilling. After the acid stimulation the carbonatic reservoir exhibited high injectivity and high flow capacity. This drilling operation on the Itapu field represents a paradigm shift because demonstrated, in certain scenarios, that it is possible not only to drill the reservoir phase with exposed salt, but also to do so with oil-based mud while removing the filter cake from the wellbore. This technique can make possible more ambitious projects on the Pre-Salt scenario, such as the development of mature Pre Salt fields with a high degree of depletion pressure on the reservoir.

Предлагается новый метод сжатия двух точек (до $2\lceil\log_2(q)\rceil + 4$ битов) для эллиптической кривой $E_b : y^2 = x^3 + b$ с $j$-инвариантом $0$ над конечным полем $\mathbb{F}_q$ при $q\equiv 1\pmod 3$. Точнее, получены простые явные формулы преобразования координат $x_0, y_0, x_1, y_1$ двух точек $P_0, P_1 \in E_b(\mathbb{F}_q)$ в два элемента $\mathbb{F}_q$, дополненные четырьмя битами. Для восстановления (на этапе разжатия) точек $P_0, P_1$ предлагается извлекать корень шестой степени $\sqrt[6]{Z} \in \mathbb{F}_q$ из некоторого элемента $Z \in \mathbb{F}_q$. Известно, что при $q\equiv 3\pmod 4$, $q\not\equiv 1\pmod {27}$ это можно сделать с использованием только одного возведения в степень в $\mathbb{F}_q$. Таким образом, новый метод сжатия оказывается значительно быстрее классического метода для координат $x_0, x_1$, в котором разжатие использует два возведения в степень в $\mathbb{F}_q$. Показано, что новый метод можно использовать для сжатия одной $\mathbb{F}_{q^2}$-точки на кривой $E_b$ с $b \in \mathbb{F}_{q^2}^*$.

This review systematically examines the research literature published in the period 2002-8 on structured violence risk assessment instruments designed for use in mental health services or the criminal justice system. It adopted much broader inclusion criteria than previous reviews in the same area in order to capture and summarise data on the widest possible range of available instruments. To address two questions: (1) what study characteristics are associated with a risk assessment instrument score being significantly associated with a violent outcome? and (2) which risk assessment instruments have the highest level of predictive validity for a violent outcome? Nineteen bibliographic databases were searched from January 2002 to April 2008, including PsycINFO, MEDLINE, Cumulative Index to Nursing and Allied Health Literature, Allied and Complementary Medicine Database, British Nursing Index, International Bibliography of the Social Sciences, Education Resources Information Centre, The Cochrane Library and Web of Knowledge. Inclusion criteria for studies were (1) evaluation of a structured risk tool; (2) outcome measure of interpersonal violence; (3) participants aged 17 years or over; and (4) participants with a mental disorder and/or at least one offence and/or at least one indictable offence. A series of bivariate analyses using either a chi-squared test or Spearman's rank-order correlation were conducted to explore associations between study characteristics and outcomes. Data from a subset of studies reporting area under the curve (AUC) analysis were combined to provide estimates of mean validity. For the overall set of included studies (n = 959), over three-quarters (77%) were conducted in the USA, Canada or the UK. Two-thirds of all studies were conducted with offenders who had either no formal mental health diagnosis (43%) or forensic samples with a formal diagnosis (25%). The Psychopathy Checklist-Revised was tested in the largest number of studies (n = 192). Most studies (78%) reported a statistically significant (p < 0.05) relationship between the instrument score and a violent outcome. Prospective data collection (chi-squared = 4.4, p = 0.035), number of people recruited (U = 27.8, p = 0.012) and number of participants at end point (U = 26.9, p = 0.04) were significantly associated with predictive validity. For those instruments tested in five or more studies reporting AUC values, the General Statistical Information on Recidivism instrument had the highest mean AUC (0.73). Agreement between pairs of reviewers in the initial pilot exercises was good but less than perfect, so discrepancies may be present given the complexity and subjectivity of some aspects of violence research. Only five of the seven calendar years (2003-7) are completely covered, with partial coverage of 2002 and 2008. There is no weighting for sample or effect sizes when results from studies are aggregated. A very large number of studies examining the relationship between a structured instrument and a violent outcome were published in this relatively short 7-year period. The general quality of the literature is weak in places (e.g. over-reliance on cross-sectional designs) and a vast range of distinct instruments have been tested to varying degrees. However, there is evidence of some convergence around a small number of high-performing instruments and identification of the components of a high-quality evaluation approach, including AUC analysis. The upper limits (AUC ≥ 0.85) of instrument-based prediction have probably been achieved and are unlikely to be exceeded using instruments alone. The National Institute for Health Research Health Technology Assessment and Research for Patient Benefit programmes.