Offshore Floating Wind is finally emerging as a promising source of Renewable Energy, but the Industry still faces important challenges. Among these, the impact the floater's motions have on the product life-cycle performance still requires evaluation and validation. In this sense, the industry has struggled to define an approach that minimizes the Levelized Cost of Energy (LCOE). The reasons for this, among others, are that each floater typology induces different motions on the nacelle equipment, each metocean site conditions are different, and the wear at the nacelle depends on the turbine model and manufacturer. Hence, the industry recommendations have so far usually limited the maximum longitudinal accelerations at the nacelle to 0.3-0.4g, and the maximum roll-pitch combination is set below 10°. The induced motions add dynamic loading, wear down the mechanical equipment, produce fatigue damage on the structural members and are a source of potential downtimes, due to machine failures or simply due to unacceptable accelerations at the nacelle. Therefore, the different components of the turbine should be reinforced, increasing the CAPEX; or the maintenance and repair activities with its associated costs should increase; being both a possible combination. The purpose of this paper is to analyze the different costs items associated with more or less conservative approaches with regards to the motions at the nacelle highlighting the combination of accelerations, operational weather windows, construction and maintenance strategies that can lead to the optimum LCOE. Several response spectra of accelerations at the nacelle will be obtained for two different floaters, namely; a spar and barge preliminary sized for the DTU-10MW reference wind turbine. The effect of the turbine controller on the motions is not considered. For different acceleration cutoff values, the energy produced is estimated.

This research advances the use of microwave radar to assess geomaterial properties by the novel application of frequency modulated continuous wave (FMCW) radar to uniaxially loaded sandstones in the laboratory. Here contrasts in reflection coefficient are interpreted to represent microfracture formation and cross-linking under compressive loading prior to through-going sample failure. The Darney and Red St. Bees sandstones are characterized via optical microscope, X-ray radiography and tomography and by SEM methods and are tested for FMCW response under uniaxial compressive loads, with results showing that FMCW interrogation in the K-band is highly sensitive to the progression of bulk axial shortening and material bulk composition. The results reported here present an external, non-contact radio-frequency sensing modality using microwave radar to evaluate the samples as they are deformed. These samples are sandstones chosen for their distinct petrophysical properties and their significant differences in textural heterogeneity and anisotropy. This novel sensing application yields repeatable and consistent results, which are currently verified for four laboratory-deformed sandstone samples, where characteristic spikes in reflection coefficient, Γ, are observed. These spikes, which have not been observed in any other sensing modality, and occurring approximately 20 seconds prior to macroscopic yield, were observed in all samples tested. This research offers new insights into the evolution of deformation and failure in a porous geomaterial, here sandstone, and represents a departure from the current practices of post failure analysis. The effects of these new measurands (and how they change as a function of load) can be used to recognize more clearly the step-wise evolution of the sample progression to bulk failure and potentially be developed to provide a precursor warning of imminent sample failure. By increasing understanding of the damage development this can help refine the associated geomechanical theory. Better theory underpinning better calculations and simulations of a geomechanical reservoir issue can be critical in managing key seal and trap properties of a reservoir, enabling the operator to make improved operational decisions, increasing field productivity and production efficiency.

The characteristics of a primary well pattern in offshore oilfields are: large well control reserves, large well spacing, thin well pattern, multi-layer combined production and develop highest quality reserves. Infilling adjustment project is of great significance for field life extension, full utilization of facilities, efficient production, remaining oil tapping, well pattern improvement, water flooding optimization, reservoir pressure maintains and production stabilization for offshore This paper takes an oil field in Bohai as an example to analyze the potential and risks of the infilling project.

Multi-turbine floating offshore platforms (MUFOPs) are emerging as a viable concept for reducing levelized cost of energy in offshore wind developments. If properly designed, the cost per megawatt of electric power generated can be lower compared to single-turbine platforms. To maximize yield, minimize cost and ensure a safe design, the spacing between rotor-nacelle assemblies (RNAs), must be carefully considered. This spacing (R), is the sum of the platform column spacing (c), and tower horizontal projected length (h). Rotor diameters pose considerable challenges to the arrangement of multiple wind turbines on one platform; challenges pertaining to safe operations, feasibility of construction and transportation. Specific insights are necessary to facilitate the development of viable concepts. The parametric study presented in this paper discusses the optimization of MUFOPs using tower inclination and column spacing. Representative configurations (adjusting tower inclination and / or column spacing) are developed with a multi-turbine semi-submersible-type platform and analyzed in time domain using coupled analysis. The configurations consist of two 5 WM reference turbines of the National Renewable Energy Laboratory (NREL), U.S.A. A non-dimensional parameter (R/c), is used to characterize the configurations. Wind, wave, and current loads are applied in analysis to assess the behavior of the system holistically. Hydrodynamic, aerodynamic, elasto-dynamic, servo-dynamic and mooring-dynamic effects are captured interactively at each time-step of analysis. An operational turbine condition is simulated in analysis using full-field turbulent wind to capture spatial variations of wind loads acting on each turbine of the system. Characteristic responses of the nacelle, tower and platform are assessed to determine the optimal combination which avoids both inadequate and excessively conservative designs of multi-wind-turbine platforms. By analyzing the spectral densities of the responses, the potential impact of the observed responses on fatigue design is qualified. Optimal configurations from the scenarios considered, allow minimal or no wake interactions, tolerable towerbase loads and acceptable accelerations and motions of the nacelle and platform. Results indicate that optimal solutions exist at R/c ratios greater than 1.0. An assessment of a range of tower inclinations and column spacing for optimal design of multi-wind-turbine platforms is a study that has not been documented in literature, and deserves attention considering current industry trends for floating offshore wind turbines. This paper offers significant insights on the characteristics of optimal tower and column arrangements for such platforms and provides reliable benchmarks for future designs.

This work foucs on the safety transportation and risk evaluation of LNG accidental release and vapor cloud flame. The experimental was conducted in water tank with dimension of 1×0.5×0.5 in height × long × width and placed in a wind tunnel. The LNG released from a storage tank which connected by a flexible insulation tube. A stainless steel pipeline placed through the plexiglass tank with three orifice sizes and 0.6 m water depth. The leakage pressure and volume flow rate was measured by a pressure gauge and liquid flow meter with online monitoring. The dynamic behavior of LNG jet release was recorded by a high speed camera from a full and micro view. The flammable vapor was immediately ignited on the water surface under different crosswinds. In addition, a new correlation of the radio between flame height to width with mass loss rate and wind speed has been proposed. The flame temperature was employed by an array of K-type thermalcaplous in vertical and horizontal directions. An infrared camera was used to compare the thermal distribution for quantitative.

An introduction to the history, concepts, use cases and technology of dynamic computer-based simulation modeling, including case studies and model demonstrations. The focus of this paper is how simulation models are developed to analyze and predict the performance of a wide range of systems and processes in the energy sector.

Power generation costs must be competitive for the offshore wind industry to survive and advance consistently. It is widely believed that adopting high-capacity wind turbines (10 MW or higher) is an effective approach to reduce levelized costs of energy. Industry trends indicate that use of high-capacity turbines is imminent, and the suitability of existing floating substructure concepts is being challenged. This paper assesses characteristics of a floating substructure for supporting high-capacity turbines. A 10 MW wind turbine application with the floating structure concept in 100m water depth is investigated and verified by using aero-hydro-servo-elastic dynamic simulations. Environmental loads considered are wind, wave and current, and simulations are performed in time domain to capture interactions and non-linear responses. Wind loading on the RNA is modeled using turbulent wind fields, with turbulence intensities representative of offshore environments, whereas wind loads on the platform are captured using reliable wind load coefficients. Effects of a 10MW turbine on the nacelle, tower, platform and moorings are highlighted, and correlations between the responses are discussed. The responses are quantified and compared using power spectral densities (to delineate low, wave and high frequency effects) and extreme statistics. Comparisons discussed in this paper underscore the importance of adaptability of platform features to maintain favorable responses of floating substructures for high-capacity turbine applications. A hull steel efficiency indicator is adopted for the quick and simple measure of substructure hull efficiency. Findings of this study offer one solution to drive down the cost dramatically and provide insights for future developments.

The third Member of Dongying Formation reservoir in CFD oilfield is steep slope fan-delta deposits; it was penetrated by 6 wells, each well belongs to different fan body. 143 meters oil reservoir was found in well 1, the lithologic combination are complex pebbly sandstone and sandstone with undeveloped interlayer. The reservoir feature is low porosity and low permeability, large thickness with quick lateral variance, and strong internal heterogeneity, which made reservoir characterization challenging. Firstly, high resolution sequence stratigraphic framework of the third Member of Dongying Formation reservoir was established by using of core, log and seismic data, guided by sequence stratigraphy and sedimentology. Then, microfacies classification scheme was determined by actual data and the depositional model. The spatial distribution regularity of fan delta and its microfacies was researched by seismic sedimentology attribute slice and sedimentary numerical simulation. Combined with rock composition features, physical properties, diagenetic characteristics, etc., reservoir classification evaluation was researched. Lastly, a geological model was established for quantitative prediction of the 3d distribution and physical distribution of each reservoir type. The third Member of Dongying Formation reservoir of well 1 block was divided into Lowstand Systems Tract and Transgressive Systems Tract, subdivided into six subsequence sets and 11 subsequences. Good reservoir was mainly distributed in the upper subsequences of Transgressive Systems Tract. Through fine seismic explanation, the plane distribution of each subsequence was determined. The fan delta was divided into six microfacies, including main channel, sheetflood sand beach, braided channel, overflow sand beach, sheet sand and lacustrine mudstone. Middle-fine sandstone and well sorted pebbly coarse sandstone in braided channel and main channel are good reservoir. Based on seismic sedimentology, different strata slicing schemes were used to extract seismic attributes for spatial distribution prediction of good reservoir. Together with sedimentary numerical simulation, the planar distribution and vertical evolution of fan delta and its microfacies were researched, then fan delta sedimentary model of good reservoir developed and Transgressive Systems Tract was established. Based on the study above, the reservoir was divided into four types with different physical characteristics. Then reservoir distribution and physical property distribution of all types were quantitatively predicted by geological modeling. Lastly, a more accurate geological model was provided for oilfield development plan design. Geological model of CFD oilfield was established by comprehensive application of sequence stratigraphy, seismic sedimentology and sedimentary numerical simulation. The modeling method adequately simulated the reservoir heterogeneity and fluid flow characteristics of complex fan delta reservoir with sparse wells in oilfield development plan design stage. The method will be used to guide the fine characterization of more similar oilfields in Bohai Bay Basin.

The Archean buried hillss reservoir of Z Oilfield is the first one developed in the Bohai Bay Basin in recent years. In order to develop the offshore metamorphic buried hillss reservoir with limited wells, several solutions were present in the process of ODP (Overall Development Plan) implementation. The characterization and quantitative description of fracture: The seismic forward modeling was used to identify the relationship between seismic attributes and fracture. The results showed that pre-stack shear wave impedance inversion could predict favorable reservoir development and the different frequency properties could detect the fracture. Furthermore, the rock stress field simulation based on rock physical properties was implemented to study the direction of fracture, the detailed characterization of fractured system, the quantitative prediction of high quality reservoir space distribution including porosity of fracture. This paper studied the development mechanism of buried hills reservoir with double system-fracture and matrix by comprehensive physical simulation, numerical simulation and other data. A new injection-production pattern named PT-IB (produced wells on the top of hills and injected wells in the bottom of hills) was proposed to maximize recovery factor. A new method was adopted to avoid the injected water channeling along the fractures and to enlarge the sweep efficiency by periodic change flow field. This paper also put forward a plan about early warning and judging the water channeling based on improved water oil ratio and water oil ratio derivative curve.

Offshore oil production with the employment of FPSOs (Floating, Production, Storage and Offloading) unit faces the challenge of increasing well volumes, processing and storage capacity and ultimately oil offloading and transportation. Following the natural activity development in some oil fields, the number of spread-moored FPSOs and the employment of Dynamic Positioning Shuttle Tankers increased, however represent altogether a bottleneck for the production capacity and considerable transportation cost increment. The need to implement alternatives for the use of larger and conventional tankers is evident, and several attempts and use of technology are being tested and proposed. The work presents preliminary numerical and cost analysis of an innovative Oil Loading Terminal (OLT) for deep water. Based on field data, metocean studies and extensive practical experience, the OLT should allow direct offloading from the FPSOs onto conventional shuttle tankers including VLCCs. The OLT concept allows the transfer of oil from an FPSO to a conventional tanker moored in CALM Buoy through submerged oil offloading lines (OOLs) supported by a tethered submerged buoy. The conventional tanker will receive the oil through a floating hose string. The cornerstone of this OLT concept is the subsurface buoy application to support the OLLs and consequently de-coupling the FPSO and CALM Buoy motions simultaneously. As a result, the OOLs loads and fatigue efforts under the CALM Buoy also decrease. The paper evaluates an OLT specific conception for the Brazilian offshore pre-salt area and results related to the numerical analysis carried out are presented considering one submerged buoy connected to a FPSO and CALM Buoy via flexible offloading lines respectively. The results appoint to a technically feasible solution that can be complementarily laboratory and field tested. Further, the solution cost impact has been assessed and initial figures demonstrate that the final testing, construction and installation of one system will need investment that cost a fraction of tanker lifting costs currently requiring Ship-to-Ship oil transfers. The economies assessed with the use of this innovative solution include total avoidance of Ship-to-Ship costs; cuts transportation cost per ton in up to 50% (fifty percent), eliminates offloading bottlenecks allowing better use of FPSO storage and plant processing capacity, and ultimately decreases the number of tanker offloading operations with considerable benefit to operational safety by reducing risk exposure.

The purpose of this paper is to demonstrate the following features of the Advanced Vessel Economizer, derived from the principle of the Advanced Vessel Economizer and comparison with the Conventional Vessel Economizer. Equipment which needs a host refrigerant process. Maintains the characteristics of the host process. Improves efficiency of host process. Is able to be applied not only to marine, onshore and offshore, but also to industrial refrigerant processes and household appliances. Is able to be applied not only to the plant to be built in the future, but also to the plant that is currently being operated thanks to the simple configuration of this technology. Two case studies were conducted for this purpose: The first case study is a comparison of the newly developed process with the commercialized process. Since the Advanced Vessel Economizer is not a process but equipment, it requires a process to be a host therefore the one of commercialized SMR process [ 3 ] has been selected and the Advanced Vessel Economizer is applied for optimization. As a result of this, the newly developed process shows that it not only improves the performance of the host process [ 3 ] by about 7.4%, but also maintains the properties of the host process. The second case study demonstrates efficiency of the newly developed process by comparing the commercialized BOG reliquefaction processes and the offshore liquefaction processes (see table below) to show the possibility of commercialization. It is concluded that the BOG SMR process [ 2 ] was the most efficient among BOG reliquefaction processes and LNG SMR-II process [ 3 ] was the most efficient in offshore liquefaction processes. However, the newly developed process showed about 7% higher efficiency than these two processes. Also, thanks to the small number of equipment which is similar to LNG SMR process [ 3 ] and BOG SMR process [ 2 ], a low capital cost, a compact configuration in terms of layout and simple operating and control can be expected. That is, the newly developed refrigerant system using Advanced Vessel Economizer can be commercially attractive. Through two case studies, the features of the Advanced Vessel Economizer described above are demonstrated as well as the performance of the newly developed process using the Advanced Vessel Economizer.

Based on the characteristics of shallow buried, non-trap structure and close cover, non-diagenesis, weak cementation and easy fragmentation of natural gas hydrate deposits in China, a solid fluidization well testing and production scheme to develop these kind of natural gas hydrate is proposed. The main idea is to transfer the non-diagenetic natural gas hydrate sediments buried in shallow layer of deep-water into a closed multi-phase transporting and lifting system through mechanical crushing fluidization, during the lifting riser system from seabed to surface, hydrate will be dissociated gradually based natural rule of sea water temperature rising and the hydrostatic pressure decrease with the change of water depth during the lifting process, to turn the uncontrollable decomposition process of non-diagenetic hydrate into controllable, and to achieve safe field exploitation of natural gas hydrate in the surface and shallow layer of deep-water. After the validity was verified by the laboratory experimental study, CNOOC successfully implemented the world's first solid fluidization well testing and production of marine non-diagenetic hydrate with the water depth of 1310m and the hydrate sediments layers 117~196m depth in May 2017. It means China has found an innovation way about natural gas hydrate exploration and development with self-developed key technology and tools. The implementation relied on the Deepwater Engineering & Survey Vessel "HYSY708" at the station of Li Wan 3 in the South China Sea and it was supported by fully self-developed technology, process and facilities.

In the context of a field development in deep offshore West Africa, 1200 to 2000m water depth, we have collected three high resolution datasets (bathymetry, side scan sonar mosaics and 2D sub-bottom profiles), spatially overlapping and acquired within a 6 year period using Autonomous Underwater Vehicle (AUV) technology. Since the area is geohazard prone, with the occurrence of multiple hydrocarbon-fluid migration features (pockmarks, asphalts domes, High Impedance Reflectors (HIR), etc.) at or near seabed, the analysis of the different datasets, following a time-lapse approach, was a unique opportunity to investigate the potential evolution of these geohazards over an engineering time-scale. Our study presents the methodology deployed to compare the datasets, with a particular focus on quality control to properly estimate changes over time on the seabed and within the shallow overburden. None of the AUV datasets were designed for a time-lapse approach (e.g. baseline plus monitoring survey with accurate subsea positioning, similar sensors, etc.). Consequently, significant horizontal and vertical mismatches on fixed objects (e.g. wellheads), clearly associated with positioning and measurement uncertainties were identified. The horizontal mismatch is the most penalizing, with offsets observed randomly oriented and up to 7m. To overcome this mismatching, we based our analysis on a method developed by Gafeira et al . (2012) , which provides semi-automatically geomorphological characteristics of pockmarks. The method is applied to each bathymetry dataset and the numerical outputs are directly compared using a Geographic Information System (GIS). Any pockmark showing significant changes through time (above the vertical and horizontal uncertainties) is thoroughly analysed to detect any unreliable data, before final time-lapse comparison, including interpretation of the other AUV sensors (e.g. sub-bottom profiler). The methods detected 670 pockmarks in overlapping areas of the three datasets. The analysis of their geomorphological characteristics confirmed that 5 of these pockmarks have potentially evolved over the 6-year period between the surveys. Among these 5 pockmarks, 3 are localized on top of a salt diapir associated with hydrocarbon fluid migration. Modifications are only observed at the very bottom of the pockmarks (no widening) and consist only in a localized depression reduction. In the same area, the interpretation of 2D sub-bottom profiles reveals changes over time of HIR buried depth (upward migration). Simultaneous modifications in time at seabed and within the overburden suggest an area of active hydrocarbon migration. This study reveals unprecedented information regarding the potential evolution of hydrocarbon migration features over an engineering time-scale. It also provides quantitative information regarding positioning uncertainty effects and proposes a methodology to compare datasets not specifically designed for a time-lapse approach.

A new logging-while-drilling (LWD) ultrasonic imaging tool was designed and tested to acquire high-resolution borehole surface images in any type of drilling fluid. The LWD ultrasonic imaging tool uses ultrasonic transducers to acquire pulse-echo signals from the borehole wall and pulser/receiver and acquisition electronics to store the ultrasonic image and caliper data. The transducer, pulser/receiver electronics, and acquisition electronics have been successfully tested to meet functional targets. The design of the transducer incorporated ultrasonic modeling to achieve the desired sensitivity and bandwidth using a composite piezoelectric element, two impedance matching layers, and a high-impedance backing. Two designs variants of the transducer were built and tested. The final design was selected by evaluating the performance of each transducer design before and after environmental testing. The pulser/receiver electronics were designed to deliver wideband excitation with optimal damping to reduce transducer ringdown. The acquisition electronics necessitated a low distortion path, variable gain control, a low noise floor, and consistent channel-to-channel performance. Functional testing on the electronics system verified the analog front-end had low channel-to-channel variation with various input signals and with exposure to 165° C. A Digital Signal Processor (DSP) and a Field Programmable Gate Array (FPGA) managed the high-capacity memory to store the high-resolution ultrasonic image and caliper data with a high spatial sampling rate. The FPGA was verified with Universal Verification Methodology (UVM) and with code coverage and functional coverage metrics. UVM increased design reliability and reduced development time by facilitating the creation of many test vectors. The LWD ultrasonic imaging tool has a sufficient spatial sampling rate to fully sample the borehole in the typical ROP and RPM ranges experienced while drilling. Sample downhole images show the LWD ultrasonic imaging tool compares favorably with wireline ultrasonic and resisitivity images and correlates well to LWD resistivity images in water-based mud.

Having full knowledge of the reservoir is crucial for the development of any hydrocarbon reservoir. Beyond gathering information about skin, boundaries, effective permeability, and production rates, knowing what type of hydrocarbons are being produced is often the key to optimizing the reservoir's production (i.e., reducing costs and raising profits). This paper describes the process of using wireless acoustic technology during downhole sampling to obtain important information to aid in the development decisions for a reservoir. During the sampling process, valuable insight is gained about fluid sample collection, retention, and basic composition. This information can help the operator gain a better understanding of what is occurring during the lifecycle of the sampling event—before and after sample collection, during sample recovery to the surface, and in the sample transfer. Being able to follow each stage of the sampling process in real time provides important data that confirms whether the sample remained in single phase, as a representative fluid sample, for the entire duration of the well test. It is important to keep the sample at high pressure during the recovery process to ensure a monophasic sample. This is accomplished with a nitrogen pressure-compensation system to maintain pressure during recovery. In addition, using the pressure from the common nitrogen chamber inside the sampling system in conjunction with the new data interpretation system, it is possible to confirm, in real time, the basic composition of the samples (black oil, volatile oil, condensate, dry gas, water) that are captured inside the sampler chamber housings. Additionally, the number of samples collected and the retention of those samples can be determined. This type of knowledge about the sampling event is normally obtained once the toolstring has been pulled to surface and after the sampling cylinders have been sent off to the lab for pressure, volume, and temperature (PVT) analysis. Gaining insight into the characteristics of the reservoir in real time allows for operational decisions to be made quickly, ensuring necessary sample collection, thereby saving time and money.

Wax deposition poses severe risks to crude oil production systems. In order to remediate wax deposition, pigging operation is performed routinely to scrape wax deposits from the pipe wall. Proper determination of the pigging frequency is crucial to estimating the operating costs associated with the pigging operations as well as the risks of pipeline blockage by wax deposit. In order to predict the wax deposition rate and the deposit thickness to be pigged, existing wax deposition models simulate the hydrodynamics, heat and mass transfer of oil pipe flows based on Newtonian fluid mechanics. However, when temperature of the oil drops below the wax appearance temperature (WAT), wax molecules precipitate to form a suspension of wax crystals in oil, resulting in non-Newtonian fluid characteristics. In order to generate more reliable wax deposition predictions, the methodology to model the hydrodynamics, heat and mass transfer as well as deposit growth considering the non-Newtonian fluid characteristics needs to be developed. In this study, we present an improvement of the existing university-developed wax prediction model 1 by incorporating the non-Newtonian fluid characteristics of waxy crude oil described by the suspension of fractal aggregates (SoFA) model. This enhancement is first presented for laminar flow regime. This improved model is then applied to provide insights on 1) the impacts of non-Newtonian characteristics on the heat and mass transfer aspects of wax deposition, 2) the effect of shear on wax deposition and 3) the role of wax inhibitors on wax deposition.

Estimation of environmental and complex structural responses, such as fatigue for risers on deepwater floating production systems, is a critical and generally computationally intensive process. Long term damage estimates require the determination of host vessel motions used for riser stress calculations. In principle, riser stress could be calculated for each of a large number of directional sea states, a considerable computational burden. However, it might be possible to identify a representative subset of directional sea states for vessel motion and subsequent riser stress analysis, such that estimated fatigue characteristics (from the full set of sea states and the subset thereof) were equivalent. This would be advantageous as it would require considerably less computational effort. In this work we use non hierarchical K-MEANS cluster analysis to partition a large set of directional wave spectra for contiguous sea states at a location offshore Brazil, corresponding to a period of approximately 2 years into a number of clusters. We adopt the set comprised of cluster centroids only as representative sea states for efficient characterization of the environment and structural response. We demonstrate that the representative sea states provide an efficient basis for estimation of overall sea state bulk, wind sea and swell characteristics. We evaluate the effect of cluster size on the performance of the representative sea states using custom built visualization tools utilizing the Kolmogorov-Smirnov test statistics. The representative sea states are further used as input for a VLCC-class FPSO vessel motion analysis. For heave at the turret, roll motions, and relative vessel heading, distributions of vessel motions from analysis of representative sea states are in excellent agreement with those from analysis of all sea states. Guidelines for the application of the methodology are provided.

The development of floating offshore wind farms requires the parallel development of suitable floaters to support the wind turbines. These floaters must be economic and must also exhibit good motion characteristics to limit the accelerations and inclinations imposed on the turbines during operation. This paper reviews the required characteristics of these floaters, addressing the requirements of fabrication, turbine integration, tow to site, and offshore installation, as well as the required behavior of the floater once installed. The benefits of an integrated design approach considering all of the above is demonstrated, and consideration is given to the industrialization of the production process for large numbers of floaters for full-scale wind farms. Based upon this review of requirements, an innovative light weight structural solution incorporating tensioned mooring legs has been developed as an economic solution for the floater. The modularity of the design facilitates construction, and offshore installation can be accomplished using standard anchor handling vessel (AHV) means. The floater design exhibits low turbine inclinations and low accelerations due to a combination of its mooring arrangement and its high degree of transparency to waves, which reduces fatigue loads and maintenance issues on the turbine. The floater behavior during towing to site and in the installed condition is described, and key performance characteristics are reported based on analytical simulations and model tests results conducted at 1:40 scale. The paper seeks to clarify the key factors to be considered in developing a floater to support a wind turbine, and to propose a solution that achieves good motion characteristics whilst satisfying the economic constraints of wind farm development.

Multiphase flow is a central component in all flow assurance strategies in oil and gas production. The risk of plugging of flowlines due to hydrate formation is also one of the most prevailing flow assurance problems in subsea deepwater oil and gas operations. Hence, it is critical to develop a model to account for the inter-coupling of hydrates and multiphase flow. We present a new framework to model the effect of multiphase flow on hydrates and vice-versa. In the current work, we use a two-phase (gas/liquid) hydrodynamic slug flow model and explicitly incorporate hydrates as a third phase. The model is based on fundamental multiphase flow concepts and has the capability of predicting hydrodynamic slug formation and propagation in three-phase flow. The utility of this tool is demonstrated in the modeling of hydrate formation in gas-water and gas-oil-emulsified water systems by comparing the multiphase flow behavior in terms of flow regime, slug length distribution, number of slugs, and slug frequency. For the gas/oil system with emulsified water, we perform a systematic study on the aggregation of hydrate particles and the subsequent viscosification of the slurry, and its impact on the multiphase flow behavior of the system. The results reveal that hydrate formation has a significant effect on the slugging exhibited by the system, suggesting that one may be able to infer hydrate formation from the changes in the flow characteristics. As part of the validation of this model, we have performed an extensive comparison of the two-phase flow regime predictions of our model with available experimental data, as well as with a number of models available in literature. The results show that our model predictions are in good agreement with literature and the prediction accuracy of our model is higher than all the models compared. This establishes our model as a viable multiphase flow modeling tool to predict the flow behavior in oil and gas pipelines. Hence, in this work, we demonstrate the capabilities of the model to perform transient simulations of two-phase and three-phase flow. This is the first step in the development of the model, with the ultimate goal being completely understanding the interdependence of hydrates and multiphase flow, hence enabling flow assurance engineers to develop better hydrate management strategies.

This paper analyzes several years of observations from eight sites off northern Angola and finds strong coastal trapped waves (CTW) that are energetic and narrow-banded on the continental shelf. Seaward of the shelf break, the CTW spectral peak becomes less pronounced and more broadly banded. By 1000 m they are almost undiscernible. On the shelf, the CTWs are characterized by phase speeds of about 0.5 m/s, periods of roughly 11 days and peak unfiltered velocities of 100 cm/s; far exceeding the energy from other processes except in the upper few meters of the water column. CTW velocity profiles are sometimes barotropic (nearly constant with depth) and at other times display low-mode baroclinic characteristics. Analysis of winds in the equatorial Atlantic show substantial energy in the biweekly period range suggesting a simple source for the CTWs but a coherence analysis reveals low correlation at all frequencies. Previous work by Guiavarc'h et al. (2005) identified CTWs in this region and using a model, they traced their origin to equatorial waves driven by fluctuations in the equatorial winds. This paper also compares the global HYCOM and WANE models to the observations and finds that HYCOM shows spectral peaks in the CTW period range but they are several days longer than in the observations and too weak on the shelf. WANE underestimates the energy in the CTW frequency range on the shelf by an order of magnitude and shows no CTW frequency peak on the slope.