Modelling Rheological Behavior of Fresh Cement-Sand Mixtures

In this paper, the effect of the mix composition on rheological behavior of fresh cement-sand mixture is studied by considering a mixture as a two-phase model that is decomposed into a granular and a paste phase. The paste itself is subdivided into void paste and excess paste. Void paste fills the void space between the grains when they are in a fully compacted state while excess paste will use the remaining paste to form a paste layer around each individual grain particle, with equal thickness. By considering each grain particle covered with the excess paste layer as a single element, the rheological behavior of cement-sand mixtures can be related to their inter-element interactions for all sets of particle combinations.

Density and Drag Reduction With Hollow Glass Additives

This study concentrates on the use of materials known as hollow glass spheres, also known as glass bubbles, to reduce the drilling fluid density below the base fluid density without introducing a compressible phase to the wellbore. Four types of lightweight glass spheres with different physical properties were tested for their impact on rheological behavior, density reduction effect, survival ratio at elevated pressures and hydraulic drag reduction effect when mixed with water based fluids. A Fann75 HPHT viscometer and a flow loop were used for the experiments. Results show that glass spheres successfully reduce the density of the base drilling fluid while maintaining an average of 0.93 survival ratio, the rheological behavior of the tested fluids at elevated concentrations of glass bubbles is similar to the rheological behavior of conventional drilling fluids and hydraulic drag reduction is present up to certain concentrations. All results were integrated into hydraulics calculations for a wellbore scenario that accounts for the effect of temperature and pressure on rheological properties, as well as the effect of glass bubble concentration on mud temperature distribution along the wellbore. The effect of drag reduction was also considered in the calculations.

Erratum: “The Effect of Nickel on Fracture Toughness at Low Temperature for Hydrogen Pre-Charged Steel Samples” [ASME 2014 33rd International Conference on Ocean, Offshore and Arctic Engineering, Volume 5: Materials Technology; Petroleum Technology, San Francisco, California, USA, June 8–13, 2014, Conference Sponsors: Ocean, Offshore and Arctic Engineering Division, ISBN: 978-0-7918-4545-5, Copyright © 2014 by ASME. Paper No. OMAE2014-23274, pp. V005T03A007; 7 pages; doi:10.1115/OMAE2014-23274]

This erratum corrects errors that appeared in the paper “The Effect of Nickel on Fracture Toughness at Low Temperature for Hydrogen Pre-Charged Steel Samples” which was published in Proceedings of the ASME 2014 33rd International Conference on Ocean, Offshore and Arctic Engineering, Volume 5: Materials Technology; Petroleum Technology, V005T03A007, June 2014, OMAE2014-23274, doi: 10.1115/OMAE2014-23274.

Numerical Calculation of Residual Stress Corrected J-Integral Using ABAQUS

Residual stresses exist in welded structures due to thermal stresses. Without temperature change, large plastic deformation can result in “cold” residual stresses in a wrinkle or dent in a metallic pipe. For a crack in residual stress field, residual stresses might have strong effect on fracture parameter, the J-integral. In order to ensure its path-independence, different correction methods have been developed in consideration of residual stress effect. Recently, the finite element commercial software ABAQUS adopted one of the correction methods, and is able to calculate the residual stress corrected J-integral. A brief review is first given to the J-integral definition, the conditions of path-independence or path-dependence, and the modifications to consider the residual stress effect. A modified single edge-notched bend (SENB) specimen is then used, and a numerical procedure is developed for ABAQUS to evaluate the path-independence of the residual stress corrected J-integral. Detailed elastic-plastic finite element analyses are performed for the SENB specimen in three-point bending. The residual stress field, crack-tip stress field, and J-integral with and without consideration of residual stresses are discussed.

Variables Affecting Fracture Toughness of Welds in T-K-Y Connections

Fracture toughness of tubular welded joints is one of the critical factors affecting the structural integrity and reliability of offshore structures, such as platforms and subsea pipelines. The factors affecting the design fracture toughness of these structures are related to, both, the welding process as well as the chemical composition of the weld metal. The welding process in this application typically comprises of depositing weld metal in the tubular joints of varying thicknesses through series of weld passes. The number of weld passes required for welding these joints subjects the weld metal to repetitive cycles of heating and cooling. The effect of the thermal cycling introduces significant heterogeneity in the microstructure. This is further exacerbated by the presence of micro-alloying elements such as Niobium (Nb) and Vanadium (V) that form complex carbides, nitrides and carbo-nitrides during post weld heat treatment (PWHT). The focus of this work is to evaluate the effect of micro-alloying elements on the ductile to brittle transition temperature and the mode of fracture at temperatures relevant to offshore applications. A threshold Nb and V level has been determined for achieving acceptable weld metal toughness. The improvement in the fracture toughness using this approach has been quantified by Charpy V-Notch (CVN) and Crack Tip Opening Displacement (CTOD) measurements. The Ductile to Brittle Transition Temperature (DBTT) has been shown to be shifted to lower temperatures by 25 °C after post weld heat treatment in the welds where the total amount of Nb and V are controlled to less than 40 ppm. A wet precipitate extraction technique was used to extract precipitates from the welds to establish the presence of fine Nb rich precipitates in the welds with the higher DBTT. The weld deposited with controlled levels of Nb and V was further tested in different joint configurations and base plate thickness. The fracture toughness was evaluated by CTOD testing of the weld in two different thicknesses (50 mm and 70 mm). Increased specimen thickness resulted in lower CTOD values.

Real-Time Software to Estimate Friction Coefficient and Downhole Weight on Bit During Drilling of Horizontal Wells

The increasing complexity and higher drilling cost of horizontal wells demand extensive research on software development for the analysis of drilling data in real-time. In extended reach drilling, the downhole weight on bit (WOB) differs from the surface seen WOB (obtained from on an off bottom hookload difference reading) due to the friction caused by drill string movement and rotation in the wellbore. The torque and drag analysis module of a user-friendly real-time software, Intelligent Drilling Advisory system (IDAs) can estimate friction coefficient and the effective downhole WOB while drilling. IDAs uses a 3-dimensional wellbore friction model for the analysis. Based on this model the forces applied on a drill string element are buoyed weight, axial tension, friction force and normal force perpendicular to the contact surface of the wellbore. The industry standard protocol, WITSML (Wellsite Information Transfer Standard Markup Language) is used to conduct transfer of drilling data between IDAs and the onsite or remote WITSML drilling data server. IDAs retrieves real-time drilling data such as surface hookload, pump pressure, rotary RPM and surface WOB from the data servers. The survey data measurement for azimuth and inclination versus depth along with the retrieved drilling data, are used to do the analysis in different drilling modes, such as lowering or tripping in and drilling. For extensive analysis the software can investigate the sensitivity of friction coefficient and downhole WOB on user-defined drill string element lengths. The torque and drag analysis module, as well as the real-time software, IDAs has been successfully tested and verified with field data from horizontal wells drilled in Western Canada. In the lowering mode of drilling process, the software estimates the overall friction coefficient when the drill bit is off bottom. The downhole WOB estimated by the software is less than the surface measurement that the drillers used during drilling. The study revealed verification of the software by comparing the estimated downhole WOB with the downhole WOB recorded using a downhole measuring tool.

Finite Element Analysis Applied in Structural Integrity Management

During the periodic inspection of the Alvheim subsea system 2013 a number of cracks were observed at the Mid Water Arch (MWA) tether anchoring arrangement. The MWA and associated anchor block are critical design elements. Detailed investigations were initiated in order to determine future development of the cracks and their severity. The application of advanced non-linear finite element analysis as part of the inspection and maintenance strategy resulted in significant cost savings compared to a solution based on immediate mitigation action. This paper describes the background for occurrence of these cracks and the analyses used to determine their development: • The cracks are located in non-loadbearing locking brackets. The function of the brackets is primarily to secure the pins connecting the top part of the tether hinge to the anchor block. • During construction the locking brackets were welded to the pin and to the tether hinge. This way the non-structural element became part of the load bearing system resulting in very high stresses in the bracket and subsequent crack development. It could not immediately be excluded that the cracks observed could initiate further cracking into main bearing parts of the hinge. • FE modeling using Abaqus [1] was used to analyze the criticality of the situation. Non-linear material properties and removal of elements were applied in order to simulate crack initiation and crack growth. The system was analyzed by modelling the load paths from initial assembly on land, installation loads and finally the loads during operation. Removal of elements was introduced to replicate the crack growth pattern observed on ROV still photos from periodic surveys 2012 and 2013. The analysis documented the principle mechanism behind the crack development and further demonstrated that the risk of failure of any of the load bearing elements was negligible. The results of the analysis provided the necessary documentation for the appropriate precautions and at the same time plan for execution of mitigation measures which would have minimal economic impact.

Offshore Structural Steel Plates for Extreme Low Temperature Service With Excellent HAZ Toughness

The strength and the toughness required for steel plates used for offshore structures became higher as the installation areas move into arctic areas. The main property of offshore structure steel is the crack tip opening displacement (CTOD) property of weld joint, and CTOD testing is performed at the minimum design temperature of the structure. Thus, the demand for satisfying −40°C of CTOD test temperature specification has increased. For the improvement of HAZ toughness, coarse austenite grain is suppressed by TiN, and low-C, Ceq, Si, P, Nb design is adopted to decrease the formation of M-A constituents. Furthermore, by using Ca inclusion, which works as a pinning particle and a bainite nucleation site, very fine bainite microstructure are formed in HAZ and excellent low temperature toughness are achieved. The YP420 class plate with excellent low temperature toughness has been developed using these technology.

Cement Sheath Integrity Implications of Scaled-Down Wellbore Casing-Cement-Formation Sections

Many offshore petroleum wells have serious cement sheath integrity issues that imply costly repairs, limitations with respect to short- and long-term use of planned or converted production and injection wells, and safety issues. The annular cement sheath is exposed to a wide variation of thermal and pressure loads that potentiality may result in zonal isolation failure throughout its lifecycle. While several experimental and numerical work manage to map highly loaded scenarios that leads to radial cracks and micro-annulus generation, little effort has been taken to investigate and quantify the impact of moderate pressure and thermal loads on cement sheath stresses. This paper presents a 2D finite element assessment of impacts of numerous material properties, geometric parameters, pressure and thermal loading variables contributing to cement sheath stresses. A centralized wellbore section of casing-cement-formation is considered, representing a production casing string. This work is conducted as a preliminary effort in order to develop a down-scaled laboratory set-up, that represents realistically the impact of thermal and pressure loads on cement sheath stresses. This paper introduces a discussion about the capabilities of a down-scaled well section to characterize the stress distribution around the annular cement sheath of a conventional 9 5/8″ production casing. Results indicate that a down-scaled configuration exposed to comparable internal pressure and temperature profile resembles the stress distribution associated with completion and production operations of wells. It is shown that a proper characterization of the cement stress regimes requires the combined effect of pressure and thermal variations. Sensitivity studies conducted on cement sheath stresses, for both wellbore and down-scaled configurations, have assessed the relative influence of mechanical and thermal properties as drivers of cement stresses. For well operations with substantial wellbore temperature variations such as production and frac job, particularly uncertain parameters such as cement-formation-Young’s modulus and thermal expansion coefficient of cement have shown significant impact on annular cement sheath stresses. In terms of combined pressure and thermal loading, cement stresses have proved to be more sensitive to temperature differential than pressure differential variations.

Influence of Short Polypropylene Fibers on the Toughness of High Performance Cement Slurries

In oil wells, one of the goals of the cement sheath is the hydraulic seal. Generally, cement pastes, which are adopted in cementing operations, exhibit brittle fracture when subjected to tensile stresses. This behavior can compromise the hydraulic seal promoted through the sheath. One way to mitigate this problem is the use of slurries with more deformability. In this context, this work aims the determination of the toughness of high performance cement slurries reinforced by different volume fractions (0.50% and 0.75%) of short polypropylene fibers (6mm long). The influence of fiber addition in the rheological behavior, free fluid, density, stability and unconfined compression of the slurries was also determined. The obtained results indicated an increase in the yield strength and a reduction in the spreading of the reference mix with the increase of the fiber volume fraction. The free fluid, density and stability behavior of the reinforced slurries were similar to that of the control mix. An expressive change in the fracture behavior of the brittle matrix was observed in both mechanical tests carried out. Under uniaxial compression loads, although the ascending branch of the stress-strain curve did not show expressive differences with the fiber reinforcement (only minor modifications in the peak load), the descending branches were significantly modified with the reinforced mixes presenting a smooth post-cracking behavior. The greatest benefit provided by the fibers was observed, however, in the bending tests. Both, the maximum post-cracking strength and bending toughness, were significantly augmented with the increase of the fiber volume fraction.