bluuebunny/crossref_metadata_2025_split huggingface dataset:
Numerical Simulation of Complex Fracture Network Development by Hydraulic Fracturing in Naturally Fractured Ultratight Formations
Authors: Hofmann Hannes , Babadagli Tayfun , Zimmermann Günter 
Published: November 2013

The creation of large complex fracture networks by hydraulic fracturing is imperative for enhanced oil recovery from tight sand or shale reservoirs, tight gas extraction, and Hot-Dry-Rock (HDR) geothermal systems to improve the contact area to the rock matrix. Although conventional fracturing treatments may result in bi-wing fractures, there is evidence by microseismic mapping that fracture networks can develop in many unconventional reservoirs, especially when natural fracture systems are present and the differences between the principle stresses are low. However, not much insight is gained about fracture development as well as fluid and proppant transport in naturally fractured tight formations. In order to clarify the relationship between rock and treatment parameters, and resulting fracture properties, numerical simulations were performed using a commercial Discrete Fracture Network (DFN) simulator.
A comprehensive sensitivity analysis is presented to identify typical fracture network patterns resulting from massive water fracturing treatments in different geological conditions. It is shown how the treatment parameters influence the fracture development and what type of fracture patterns may result from different treatment designs. The focus of this study is on complex fracture network development in different natural fracture systems. Additionally, the applicability of the DFN simulator for modeling shale gas stimulation and HDR stimulation is critically discussed.
The approach stated above gives an insight into the relationships between rock properties (specifically matrix properties and characteristics of natural fracture systems) and the properties of developed fracture networks. Various simulated scenarios show typical conditions under which different complex fracture patterns can develop and prescribe efficient treatment designs to generate these fracture systems.
Hydraulic stimulation is essential for the production of oil, gas, or heat from ultratight formations like shales and basement rocks (mainly granite). If natural fracture systems are present, the fracturing process becomes more complex to simulate. Our simulation results reveal valuable information about main parameters influencing fracture network properties, major factors leading to complex fracture network development, and differences between HDR and shale gas/oil shale stimulations.
Tethered Catenary Riser: A New Deepwater Concept
Authors: Legras Jean-Luc , Pillet Baptiste 
Published: November 2013

A new riser concept is proposed by Subsea 7 for field development in deep and ultradeep waters: the Tethered Catenary Riser (TCR)-patent pending. The concept consists of a number of steel catenary risers (SCRs) supported by a subsurface buoy which is tethered down to sea-bed by means of a single pipe tendon and anchored by means of a suction pile; flexible jumpers are used to make the connection between the Floating production Unit (FPU) and the buoy. Umbilicals run without interruption from the FPU to their subsea end while being supported by the buoy.
The system has all the advantages of de-coupled riser arrangements: flexible jumpers effectively absorb platform motions, thereby the rigid risers and tendon have very small dynamic excitation. The system can be installed before FPU arrival on site, which improves the time before first oil. Analyses have shown that, with adequate geometry of the buoy, the latter is sufficient stable to induce acceptable tilt and twist when different arrangements of SCRs and flexible jumpers are installed, and under accidental scenarios during the in-place life.
The riser system is best designed for a number of risers between 4 and 8, in addition to a number of umbilicals, thus convenient for one or two drilling centers.
Results of the basic engineering work on the TCR clearly indicate that it is possible to have a robust design using presently qualified materials and technology. The components used in the TCR are all field proven as they are commonly used in existing riser systems.
As a result of installation studies, a method very similar to the one commonly used by Subea7 for Single Hybrid Risers (SHRs) has been selected for the buoy and tether system. Placement of rigid risers, jumpers and umbilicals is as done by Subsea 7 for the Buoy Supporting Risers (BSRs). This method is well adapted for installation by the new Subsea 7 flagship vessel Seven Borealis which is able to perform heavy lift and pipe laying.
The Tether Catenary Riser is a credible option for use in deep water developments all over the world. Since all the components, design methods and installation procedures are fully qualified and familiar to Subsea 7, the concept is cost effective and ready for project application.
Risk Based Inspection for Aged Offshore Pipelines
Authors: Stadie-Frohbös Gundula , Lampe Jörg 
Published: November 2013

Today risk based inspection (RBI) has become very popular as it allows the inspection budget to be used more efficiently compared to conventional inspection procedures. The key parts of a risk based inspection programme for aged pipelines is the determination of all relevant risk driving effects for each specific pipeline, the definition of acceptance criteria and the assessment of the residual lifetime for the pipeline. These tasks require a high level of co-operation with the pipeline operator and the RBI-experts.
This paper describes some of the major aspects of the methodology developed to estimate the risk to offshore pipelines. Based on the assessment of the risk an optimised inspection strategy can be developed.
The complete RBI-process typically starts by gathering all relevant data used in design phase (including construction phase) and operation phase. In most cases a variety of risk assessment methods will be used, depending on the quality of existing pipeline data. When insufficient data is available a simple qualitative assessment with an index procedure can be used. With more detailed data available the method becomes a semi-quantitative approach.
This paper presents the results of a general risk assessment of 22 aged offshore pipelines. Although only basic data has been available this was sufficient to work out reliable results within a semi-quantitative approach. Depending on varying wall thickness, diameter, etc. a pipeline was divided into several different segments. The assessment reveals different governing threats and failure modes for pipelines and risers, like ageing, free span or corrosion. In a final step the results of the risk assessment are used for the optimisation of the inspection intervals within the proposed risk based inspection framework.
Non Linear Wave Loading of the 3D FE Structural Model of Floating Bodies
Authors: de Lauzon Jérôme , Sireta François-Xavier , Malenica Sime 
Published: November 2013

Different numerical methods exist to treat the non-linear wave body interaction problem. These methods go from very complex CFD simulations (VoF, SPH…) to simpler potential flow based methods. Within the potential flow methods, different types of numerical approaches also exist, going from very complex fully nonlinear time domain methods to simpler hybrid frequency-time domain methods. In this paper, only the hybrid frequency-time domain potential flow methods are of concern.
The basic principle of these methods is to use the linear frequency domain data and transfer them to the time domain (Cummings-1962, Ogilvie-1964) using the inverse Fourier transforms. Once in the time domain, all kind of nonlinearities can be added on top of the linear solution. This method showed to be very computationally efficient for global behavior of the floating bodies. However, in the context of the body structural response, it is not trivial to make the load transfer from the 3D hydrodynamic panel model to 3DFE model of the structure. One of the main problems concerns the radiation part of the potential which comes into the play through the well-known convolution integral of the impulse response functions. Within the hydro-structure interaction problem which is of concern here, the straightforward application of the Cummings principles leads to the convolution integral for every pressure point (either on hydrodynamic or structural mesh) which can lead to very expensive numerical simulations.
Previous study by Tuitman, Sireta, Malenica and Bosman (2009) proposed the use of the fast Fourier transform (FFT) to compute the radiated wave loads at each point of the structural model, as a post-processing of the seakeeping calculations.
In this way the CPU time is significantly reduced but the slight unbalancing of the 3DFE model remains. In this paper, we propose the direct use of the local radiated pressure impulse response functions and we show that a good numerical implementation of such method can result in acceptable CPU time with perfect balance of the 3DFE model.
Assessment of Loading on the Carcass and Pressure Armour due to Environment Swelling of Polymer
Authors: Fernando Upul S. , Davidson Michelle , Harries Iwan , Thompson Andrew , Sheldrake Terry 
Published: November 2013

Flexible pipes consist of multi-layer structures comprising polymer extrusions, tapes and insulation that are trapped between layers of helically-wrapped steel wires. Under certain pressure and temperature service conditions, and when the polymers are in contact with certain chemicals (such as supercritical CO2), these polymer layers may experience significant volumetric changes caused by permeation, solubility and absorption of chemicals into the polymer structure. The swelling of the polymer barrier can result in a significant increase in load on the carcass and pressure armour that may compromise the integrity of the pipe. This paper investigates the effect of the volumetric changes in the barrier and how that can influence the extra loading exerted on carcass and pressure armour under service conditions.
A simple analytical model has been proposed to predict the extra loading acting on the carcass and the pressure armour. The proposed procedure has been validated using FE results. Experiments have been performed using a three-layer simulated setup and full-scale pipe to investigate the load acting on the carcass and pressure armour. A brief description of the test program and the results are presented. Tests clearly show that there is only a limited increase in hoop stress in the carcass or the pressure armour in pipes immersed in acetone, although the barrier swelled considerably under unconstrained conditions. An empirical procedure has been proposed to account for this swelling retardation behavior. The effect of the PVDF barrier swelling in pressurized supercritical CO2 environments is discussed.
Influence of Ship Behaviour in a Seaway on CO2 Emissions
Authors: Prpić-Oršić Jasna , Faltinsen Odd Magnus , Mrakovčić Tomislav 
Published: November 2013

A reliable prediction of attainable ship speed at actual seas is essential from economical and environmental aspects. At this paper a methodology for estimating the attainable speed and related fuel consumption and carbon dioxide (CO2) emissions in moderate and severe sea is proposed. The irregular sea is handled as a series of regular waves with different amplitudes and frequencies. The added resistance in regular waves is obtained by either a direct pressure integration method or an asymptotic small wavelength formula. The in-and-out-of-water-effect and ventilation of a propeller in severe seas is accounted for by a quasi-steady averaging of experimental data for different propeller submergences. The propulsion results for regular waves are used in simulating results in irregular waves. It is shown that for higher sea states this effect has much more influence on the speed loss than the added resistance in waves. The speed loss is calculated by taking into account the engine and propeller performance in actual seas as well as the mass inertia of the ship.
The numerical model used for main propulsion engine modeling is based on a zero-dimensional model of an internal combustion engine. The main propulsion engine is represented by number of control volumes interconnected with links for mass and energy transfer between them. This model provides excellent prediction of engine dynamic response during transients with rather short computational time. Also, engine fuel consumption can be precisely determined which represents the basic presumption for estimation of carbon-dioxide emission. Furthermore, use of such model can be extended to determination of the lowest fuel oil consumption strategy for given sea condition and ship speed with resulting lowest possible CO2 emissions. The attainable ship speed is obtained as time series. Correlation of speed loss with sea states allows predictions of propulsive performance in actual seas.
Optimisation of Subsea Tie-In Spools Using Evolutionary Algorithms
Authors: Madeley Chris 
Published: November 2013

Tie-in spools must be designed to resist a large number of onerous load combinations. These loads include gravitational, temperature, pressure and environmental loads along with various imposed displacements. Additionally, there are several design constraints that must be satisfied. Due to the three-dimensional geometric freedom of the spool there are many possible design scenarios that could be evaluated in the search for the optimum solution.
It is the responsibility of the pipeline design engineer to use their own judgment and experience to find the best possible solution within the design period. Traditionally a trial and error design approach is used in an iterative manner. This method is typically slow and labor intensive and can be too focused on one design concept at the expense of others that are potentially superior. On similar engineering problems with many design parameters automated non-linear optimization routines have been shown to be very effective. Specifically, applying evolutionary algorithms is a robust, time-effective and adaptable approach. Such a tool assists the engineer in finding superior design solutions and assists in searching the entire design space.
To test this design method, a multi-objective evolutionary algorithm has been applied to two semi-constrained spool design problems. The spool design has been modeled using finite element analysis. First, the algorithm was applied to the optimization of spool geometry for multiple design objectives. Within 24-hours of runtime the algorithm was able to find superior solutions to those found using a traditional iterative approach. Also, the trade-off between conflicting design objectives could be quantified and visualized to enable the designer to select the most appropriate candidate. The second problem evaluated was the placement of supports to mitigate the onset of vortex induced vibration (VIV). The algorithm was again able to quickly find a better solution and quantify the tradeoff between conflicting design objectives. The paper presents the results of this new design process as applied by subsea pipeline engineers to find optimum spool designs.
An Experimental Investigation of Turbulent Drag Reduction in Concentric Annulus Using Particle Image Velocimetry Technique
Authors: Corredor Fabio Ernesto Rodriguez , Bizhani Majid , Kuru Ergun 
Published: November 2013

Polymer drag reduction is investigated using the Particle Image Velocimetry (PIV) technique in fully developed turbulent flow through a horizontal flow loop with concentric annular geometry (inner to outer pipe radius ratio = 0.4). The polymer used was a commercially available partially hydrolyzed polyacrylamide (PHPA). The polymer concentration was varied from 0.07 to 0.12% V/V. The drag reduction is enhanced by increasing polymer concentration until the concentration reaches an optimum value. After that, the drag reduction is decreased with the increasing polymer concentration. Optimum concentration value of PHPA was found to be around 0.1% V/V. Experiments were conducted at solvent Reynolds numbers of 38700, 46700 and 56400. The percent drag reduction was found to be increasing with the increasing Reynolds number.
The study was also focused on analyzing the mean flow and turbulence statistics for fully-turbulent flow using the velocity measurements acquired by PIV. Axial mean velocity profile was found to be following the universal wall law close to the wall (i.e., y+ <10), but it deviated from log law results with an increased slope in the logarithmic zone (i.e., y+ >30). In all cases of polymer application, the viscous sublayer (i.e., y+ <10) thickness was found to be higher than that of the water flow. Reynolds shear stress in the core flow region was found to be decreasing with the increase in polymer concentration.
Assessing the Importance of Nonlinearities in the Development of a Substructure Model for the Wind Turbine CAE Tool FAST
Authors: Damiani Rick R. , Song Huimin , Robertson Amy N. , Jonkman Jason M. 
Published: November 2013

The design and analysis of wind turbines are performed using aero-servo-elastic tools that account for the nonlinear coupling between aerodynamics, controls, and structural response. The NREL-developed computer-aided engineering (CAE) tool FAST also resolves the hydrodynamics of fixed-bottom structures and floating platforms for offshore wind applications.
Primarily due to the required modal characteristics, monopiles become progressively less economical and more difficult (or impossible) to fabricate for multimegawatt turbines and water depths of more than 25–30 m. Derived from the oil and gas industry experience, light and stiff space-frame alternatives have been proposed to alleviate this problem. Lattice structures (e.g., jackets) are more complex to analyze and design than cantilevered monopiles, especially in terms of the structural dynamics of the coupled turbine-support structure system.
This paper outlines the implementation of a structural-dynamics module (SubDyn) for offshore wind turbines with space-frame substructures into the current FAST framework, and in particular focuses on the initial assessment of the importance of structural nonlinearities. Nonlinear effects include: large displacements, axial shortening due to bending, cross-sectional transverse shear effects, etc. A nonlinear computational analysis is resource-intensive, thus it is important to assess the applicability of a linear approach to maintain high-fidelity results while still allowing for fast and efficient design simulations. Space-frame structural behavior can be controlled by a number of design parameters (e.g., member cross-sectional properties, number of legs, batter angles). Additionally, nonlinearities may manifest only at certain load levels. Several finite-element analyses were carried out via commercial and open-source codes that can capture nonlinear effects in the structural behavior of turbine substructures under different load cases. Results were compared to the output of the new linear module SubDyn. The configurations considered in this study included 5-MW, 7-MW, and 10-MW platforms: OC3 monopile, OC3 tripod, OC4 jacket, and a full-lattice tower, all supporting a 5-MW turbine; also two jackets for a 7-MW and a 10-MW turbine, respectively, were investigated. These models differed in base geometry, load paths, size, supported towers, and turbine masses. Results showed that nonlinearities (quantified in terms of the maximum differences in displacement and stresses with respect to a linear calculation) amounted to about 4% (3%) at tower top (at tower base), or about 10 cm (1 cm). This means that the absolute effects of nonlinearities are mostly associated with the tower. The linear approach used by the multimember structural module introduced in this paper was therefore deemed suitable to be utilized within FAST to analyze multimember substructures for offshore wind applications.
A Novel Concept for Self Installing Offshore Wind Turbines
Authors: Wåsjø Kasper , Bermúdez Rico Jorge Vicente , Bjerkås Morten , Søreide Tore 
Published: November 2013

The present paper describes a novel concept of a self-installing offshore wind turbine.
A concept for combined installation of the substructure and turbine in one single operation without the need of expensive installation vessels is described.
The stability of the concept during transport and installation is obtained by two structurally connected standard barges with dimension 92 × 32 m.
The concept proves to be stable with weather window equal HS = 4 m for transport and Hs = 1.5 m for installation in the waiting of more accurate analyses.
A cost saving potential in this early phase of 17% is identified compared to the more common steel jacket solution. The cost saving is related to the installation process.
A Simplified Methodology for Comparing Fatigue Loading on Subsea Wellheads
Authors: Holden Harald , Bjønnes Pål , Russo Massimiliano 
Published: November 2013

This paper presents a simplified way of comparing the fatigue load on different subsea wells. The simplest comparison is done by accumulating the number of days BOP has stayed connected to the wellhead. The wellhead fatigue load is however heavily dependent on the vessels used, water depth and weather while connected to the well. An equation for deriving a benchmark load factor for each operation phase for a subsea well is proposed. This benchmark load factor takes into account the water depth, metocean season of the operation, BOP height and weight, and the stiffness of the marine riser lower flex joint. This benchmark load factor will represent a standard number of days with a BOP connected, correcting for some known effects. The goal has been to define a measure of ‘BOP days’ that accounts for the water depth, operational season, and BOP particulars. A base case (one MODU, 100 m water depth, and all year operation), equating to one standard BOP day, has been chosen as the reference for all cases discussed.
The validity of the benchmark load equation will be shown through a comparison with 31 different global riser analyses intended for wellhead fatigue. For each of the 31 data sets, time domain load analysis is done for all sea states in the wave scatter diagram. The different analyses covers different rigs, water depths and two operational phases (with or without subsea XT installed). To enable a large scale comparison of the bench mark factor, an approach where the fatigue load is summarized using the bending moment standard deviation on the wellhead datum is presented. This methodology is then compared to four full fatigue calculations using a typical subsea wellhead fatigue capacity. Then the simplified fatigue calculation is performed for all 31 global riser analyses. The calculated damage is then compared with the corresponding bench mark formula in each case.
Finally it is shown how this benchmark load formula has been implemented into the Statoil WellSpot database as a fatigue load criticality screening tool for the different Statoil subsea wells. It is further shown how this can be used as a tool during planning of future operations, and how to prioritize wells where a detailed fatigue analysis is recommended.