In iceberg prone regions, subsea substructures placed on the seabed are atrisk of impacts from free-floating and scouring iceberg keels. Here themethodology for assessing iceberg loads and two mitigation strategies aredescribed. The iceberg load model was an extension of previous work forestimating iceberg impact loads on offshore surface-piercing structures.Components of the algorithms were modified such that global design loads fromkeel contacts account for the change in contact location (i.e., longer leverarm in the vertical direction resulting in greater rotation effects). Theiceberg eccentricity model and the relationship between contact area andpenetration distance were also modified to account for iceberg keel contactswith a generic low profile structure on the seabed. One concept considered wasa single wellhead structure fitted with a special weak shear link incorporatedinto the design at the expected scour level. The shear link, or failure joint,would act as a mechanical fuse designed to fail in a combination of shear,tension and buckling during keel loading. The failure joint minimizes downholestructural response during iceberg keel loading on the production tree. Thedesigned failure mechanism would allow the well to be re-entered by protectingthe well casing from damage. Another concept considered was a steel truncatedcone structure installed over the well installation and fixed to the seabed byone of several identified foundation concepts. The protection structure absorbsenergy through crushing of the ice keel and encourages the iceberg to deflectaround and over the structure. The steel structure would be designed accordingto ultimate limit states accounting for energy absorption through elastic andplastic deformation of the structure. Design loads would correspond to anAbnormal Level Ice Event (ALIE) with an annual exceedance probability of 10-4.The size of the frame is governed by the size of the wellhead and tree system,Remotely Operated Vehicle (ROV) access requirements, and slope to encourageiceberg keel deflection. Piles may be the best option for securing a protectionstructure to the seabed, especially if a local vessel can be sourced to performthe installation. As an alternative to piles, using a drill rig to install wellcasings may be an option; however, market conditions for drilling rigs maydictate economic feasibility.
Subsea structures on the seabed may be impacted by free-floating or scouringicebergs. A drift-based Monte Carlo iceberg contact model was developed as partof the SIRAM (Subsea Ice Risk Assessment and Mitigation) program forcalculating iceberg impact risk for subsea structures on the northeast GrandBanks offshore Newfoundland and the Makkovik Bank on the Labrador Shelf. Themotivation for developing this model was to characterize the influence ofbathymetry (i.e., seabed orientation, ridges and basins) on iceberg interactionrates with subsea structures. Results were incorporated into a GIS-basedapplication to allow iceberg contact rates to be calculated for structures witha range of plan dimensions and elevations at various locations.
Ice feature interaction with subsea infrastructure or the seabed is acomplex nonlinear event, for which many analytical and advanced computationaltools have been developed with demonstrated application. Although subsea fieldshave been developed in ice gouge environments, such as the Grand Banks,consideration of alternative methods for protecting subsea infrastructure is ofgreat importance. A more in-depth understanding of ice feature mechanicalbehavior and interaction with subsea infrastructure is required.
For various iceberg shapes and loading conditions, the finite element modelspresented in this paper examine the interaction of free-floating ice featureswith protective structures located above or partially above the mudline. Apreliminary assessment of an interaction scenario involving a gouging icebergkeel with a buried protection structure is also presented. The outcome of thisstudy enhances understanding of the primary factors to be considered for thedesign of protection structures in ice environments and highlights some of thetechnical issues associated with the development and calibration of advancedsimulation tools.
The design of offshore arctic pipelines must evaluate technical engineeringchallenges, primarily related to system demand and system capacity, and addressproject execution risk, primarily associated with pipeline trenching andlogisitics. One of the significant hazards, particularly in deeper water, isthe presence of extreme ice features; such as icebergs and multi-year pressureridges, that may gouge the seabed. A comprehensive engineering framework existsto support the analysis and design of offshore pipelines in ice gougeenvironments. However, there exists some aeas of technical uncertainty withinthe current state-of-practice that are highlighted in this paper. This studyfocuses on specific technical issues associated with the simulation of contactmechanics, definition of interface parameters, and need for physical datasetsfor the validation of advanced numerical simulation tools. Study specificconclusions and recommendations that address these technology needs to resolveuncertainty associated with the simulation of ice gouging events areprovided.
In predicting the geotechnical constraint against pipeline movement usingfinite element methods, the treatment of the pipe/soil interface contactbehavior is of utmost importance, especially in the tangential direction. Thisstudy focuses on the interpretation of soil resistance to axial pipe movementin cohesive soil material for oblique loading, specifically the effect ofchanging the interface shear stress limit and friction coefficient. The mainfinding of the present study is that the incorporation of a shear stress limitin the definition of tangential shear behavior has a considerable effect on theaxial pipeline reaction forces. Without the shear stress limit, the maximumaxial forces due to oblique pipe movement are effectively doubled in comparisonto a limit equal to half of the undrained shear strength. A simple analyticalmethod is provided to estimate the maximum oblique axial soil resistance inundrained conditions. The effect of changing the assumed frictional behavior isalso discussed with respect to predicting the soil reaction forces acting on anice keel during an undrained gouging event in cohesive soil.
Some coreflood literature points to the initial wettability state undergoing change during waterflooding, usually towards water-wetness. The current study aimed to directly probe the adsorbed/deposited oil components on model silicate substrates prior to and after flooding. Bare glass and kaolinite-coated glass in the initial brine were drained with crude oil and aged, after which the oil was displaced with the flooding brine. For a matrix of initial and flood brines (comprising sodium and calcium) of varying salinity and/or pH, the oil remaining on the substrates was analyzed by high-resolution scanning electron microscopy, contact angle and spectroscopy. On glass, the oil layer contacting it in the initial (aged) state retracts and detaches during flooding, to typically leave individual oil nanodroplets separated by clean substrate. Brines less able to overcome the oil-glass adhesion displayed a higher coverage of more irregularly shaped, semiretracted drop-lets and a higher frequency of larger microscopic residues. On kaolinite-coated glass, the added porosity and roughness increased the presence of these adhering, stranded residues. On bare glass, the residual deposit after high salinity floodingis generally least at intermediate flood pH 6, while residues decrease with decreasing pH of low salinity floods. However, on kaolinite-coated substrates, residual deposit is greatest after flooding at intermediate pH 6, and also increases on reduction of flood salinity
Lee, Yun-Hee (Korea Research Institute of Standards and Science) | Kim, Yongil (Korea Research Institute of Standards and Science) | Park, Jong Seo (Korea Research Institute of Standards and Science) | Nahm, Seung Hoon (Korea Research Institute of Standards and Science) | Yoon, Ki-Bong (Chungang University)
In order to estimate the hardness and yield strength of an indented material, advanced methods have been developed for extracting closed boundaries of the contact area and the plastically deformed zone from 3D nanocontact morphologies. However, this image processing technique cannot be applied to shallow indentations as it results in weak surface pile-ups. Based on the modified volumetric approach, the new hardness and yield strength of Au film and fused quartz are compared with those from the indentation curve analysis and differential contact analysis.
Nanoindentation measuring applied load and indenter penetration depth during a contact deformation is one of the most powerful techniques for evaluating the mechanical properties of small volume materials (Oliver and Pharr, 1992). Typical nanoindentation researches have been constrained within the determination of elastic modulus and hardness. However, the research scope is now being expanded to the analysis of plastic flow curve, yield strength, residual stress, fracture toughness, interfacial adhesion and various tribological properties (Ahn and Kwon, 2001; Lee et al., 2006; Lee and Kwon, 2002; Lee and Kwon, 1999). However, since the deformation morphology under indentation loads less than mN cannot be easily observed, 2 models have been developed (Oliver and Pharr, 1992; Doerner and Nix, 1986) for characterizing Ac at the peak indentation load from the nanoindentation curve. The method commonly used for analyzing the nanoindentation load-depth curve is that proposed by Oliver and Pharr (1992), expanding on an earlier work by Doerner and Nix (1986). Below, the analyzed data based on the Oliver and Pharr method will be denoted as O&P. However, the O&P method (Oliver and Pharr, 1992) can strongly underestimate the contact area if a material pile-up is involved, as reported in the finite element simulation work of Bolshakov and Pharr (1998).
Compositional heterogeneities of H2S have been noticed in many sour gas reservoirs. Its occurrence is an important factor of economic depreciation. Thus, the knowledge of its content and distribution is a critical parameter when planning field development. The paper aims at exploring the role of an active aquifer in the creation of H2S heterogeneities in high H2S-bearing gas reservoir. Indeed, under conditions of pressure and temperature of typical reservoirs, H2S is far more soluble than hydrocarbons and other gases. A preferential leaching of H2S (e.g. versus CH4) over time is thus possible.
This mechanism is controlled by: (1) Differential solubility of gases, which change the relative amounts of each gas near the gas-water contact (GWC); (2) Contact with an active aquifer, which can export the dissolved gases thus enhancing dissolution on the long-term; (3) Diffusional transport in the gas phase, which transfers the compositional anomalies farther from the gas-water contact; (4) Geological parameters (type of aquifer, permeability heterogeneities) which can modify the transport scenario.
To illustrate and quantify this process, we show the results of numerical simulations, performed with the two-phase transport and geochemical software Hytec. First, a very schematic reservoir with a composition considered uniformly distributed within the reservoir, has been simulated to quantify the leaching of H2S. The results highlight the potential role of the active aquifer, which can leach the gases and export them outside the reservoir. In a second phase, the effect of geological parameters on the H2S heterogeneity development was studied: additional simulations were performed on geometries closer to natural cases. The amount of leached H2S depends strongly on the geometry: the larger the GWC area, the larger the amounts leached.