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Over the last few years, the oil and gas industry has been challenged by prolonged volatile hydrocarbon prices and market instability. As a direct consequence of these challenging conditions, the industry has experienced a shift of focus to the development and application of more advanced and efficient technologies.
One of the most revolutionary subsea technological developments implemented in the latest years has been the subsea compression system installed at the Åsgard field, located offshore Norway. It has successfully been in continuous operation since September 2015. This has proven the feasibility of installing a compressor on the seabed, and at the same time verified a large number of power and control system technologies required for the control and operation of the subsea compressor. This includes large subsea transformers, power cables and terminations and high voltage wet-mate connectors, all of which are essential technologies for subsea substations for offshore wind application.
Today, the rapidly expanding offshore wind power industry could benefit from utilizing some of these high voltage power techonlogies recently qualified for subsea oil and gas processing applications. Many of the subsea power technological developments, skills and expertise gained in the oil and gas industry can be adapted for offshore wind application.
This paper presents the concept of a subsea high voltage substation applied to floating offshore wind parks. Both the system characteristics and the individual power technology features that are required to make the substation compact, efficient and reliable are emphasized. In addition, the differences and benefits of a subsea substation versus a floating substation will be highlighted.
Floating offshore wind turbine technology, much of it developed domestically, is rapidly advancing and is in the early implementation phase, while floating substation technology is still at an early development stage. This study presents novel floating wind power substation platform designs for deepwater wind farm applications. Two types of floating substations configurations are considered to compare technical and cost performance: a semi-type "X-WindStation" and a TLP-type "TX-WindStation". The floating substation platforms are considered for a 200 MW wind farm located in 100 m (328 ft) water depth off the Northeast coast of the United States. The floating substation supports a two-deck electrical power facility that provides sufficient electrical power equipment layout area and includes temporary quarters. Both floating substation platforms are evaluated for global performance and mooring systems (catenary for semi-type and tendon for TLP-type) with the site design metocean conditions for the extreme and survival storm seas. The results are assessed in accordance with industry standards ABS and API, and offshore engineering practices. Capital expenditure (CAPEX) of both substation platforms for a 200 MW farm is estimated by including the electrical substation, platform hull, mooring lines, anchors, integration, installation and commissioning costs. Installed CAPEX costs of the platforms show that the semi-type substation platform cost is lower than the TLP-type cost for the case where each tendon has a dedicated anchor, whereas the cost for the TLPtype with two tendons sharing an anchor is highly comparable to, if not less than, the semi-type platform.
New innovative services and goods based on artificial intelligence, next-generation robotics, autonomy, digitization, and electrification are taking root in the ocean economy. More typical competitive forces, such as value enhancement and safety improvement, are also present, as suppliers constantly strive to improve their competitive positioning. Concurrently, the Oil and Gas sector is experiencing mounting pressure to decrease its operational carbon emissions, forcing efforts to reduce or eliminate liquid fuel consumption offshore. Companies in the ocean economy are having to react to this combination of macro-and micro-level drivers and innovate at an ever more rapid pace. However, before many of these new capabilities can be implemented, new, clean, reliable power sources are needed, especially as liquid fuel grows increasingly out of favor. For many of these loads--both old and new and ranging from watts to megawatts--wave energy systems provide the best solution for remote power generation. As such, research, development and demonstration activities are underway to prove wave energy's ability to provide reliable, consistent energy supplies for certain offshore Oil and Gas activities, paving the way for near term commercialization. This paper outlines these drivers, competitive reaction to them, and why and how wave energy can be a preferable choice for power at sea.
James, Scott C. (Baylor University) | Olson, Sterling S. (Sandia National Laboratories) | McWilliams, Sam (Integral Consulting, Inc.) | Jones, Craig A. (Integral Consulting, Inc.) | Roberts, Jesse D. (Sandia National Laboratories)
The effect a marine hydrokinetic device, or array of devices, has on the environment is a key component of design, permitting, and viability of a project. To accurately understand physical processes and their potential relationship to environmental stressors at a current-energy converter (CEC) site, a numerical model was developed using SNL-Delft3D-CEC-FM to facilitate an understanding of the potential changes to the system as shown in Conceptual flowchart of the modeling methodology.
Conceptual flowchart of the modeling methodology.
Using turbine and river data provided by the Alaska Hydrokinetic Energy Research Center, a steady-state flow model was constructed for varying river discharge levels. The wakes of University of Alaska at Fairbanks New Energy Systems vertical-axis, 5-kW turbine as well as commensurate changes to the steady-state flow field were simulated. Finally, an example optimization study was undertaken for turbines arranged in various arrays to demonstrate the effects of lateral and downstream interference, wake recovery, and overall momentum removal on the flow conditions and power generation.
Considering a distribution of flow conditions over a 10-year period for the Tanana River at Nenana, variations in number of CEC devices and array layout provided insights into power production and environmental effects. Specific quantities of interest included changes in velocity and bed shear stress, which each showed changes in proportion to the number of devices in the array. Using SNL-Delft3D- CEC-FM with calibrated turbulence constants allowed for a design that maximized CEC array power while remaining within constraints of minimally altered environmental conditions. That is, array layouts could be selected to optimize power generation while minimizing flow-field changes.
This work highlights the importance of investigating array performance at each site under consideration. These findings are helpful in optimizing turbine arrangements in the Tanana River at Nenana that maximize power production and minimizes undesirable/unintended changes to the river's natural flow (flow depth, velocity, and bottom shear).
This research applied a Genetic Algorithm model to identify suitable JONSWAP spectra parameters for specific locations, solving restrictions such as the influence of bathymetry and met-ocean events that could limit the coefficient selection of the spectra. Also, this study assessed available parametrized equations of the JONSWAP spectra in order to verify their applications for easing the application of the GA model. The modeled results demonstrated the capability of the GA model to find the best alpha and gamma values of the spectra for normal and extreme sea state conditions. In addition, the assessed spectra parameterized equations evidenced limitations to represent the wave energy distribution at the study area, then, this study performed a successfully modification of one of these parameterized equations to model the proper JONSWAP spectra. According to the results was evidenced the necessity to apply numerical approaches such as Genetic Algorithms to solve wave spectra equations for offshore structure designing, and the assessing of support parameterized equations before their application because the wave loads could be wrongly estimated.
Service firms are diversifying their portfolios, in part driven by large-scale budget cuts among operators since the industrywide downturn. Subsea advancements in the works include longer tiebacks, an underwater drone that lives on the seafloor, and a robotic manifold capable of actuating dozens of valves. Do these new capabilities, born of necessity, signal a sea change in industrywide technology development? The biennial SPE Offshore Europe conference will explore a diverse set of topics, including the application of digital technologies and preparing for a low-carbon energy future and ongoing work around standardization and decommissioning. Hurricane Energy is still on pace for first oil in 2019 for the Lancaster field, which may lead to more significant development in the UK North Sea.
This work concerns the quantification of numerical accuracy for focused wave interactions with floating structures, using results obtained via a commonly used computational fluid dynamics (CFD) and linear wave superposition approach. It represents an individual contribution to the CCP-WSI Blind Test Series 3, where numerical predictions for a structure’s motion are submitted for comparison with physical data, without prior access to this data. An estimation of accuracy based on the reproduction of physical empty tank data, and previous experience, is compared with the observed error in the structure’s motion. The results imply that the error in peak values of heave and empty tank surface elevation are comparable, but the peak surge and pitch are substantially larger. This is likely due to a combination of numerical modelling errors, although numerical uncertainty must also be reduced in order to fully assess the problem.
Two key issues that are limiting the routine use of computational fluid dynamics (CFD) are the uncertainty in its accuracy and the time required to obtain numerical results. The time taken to run a simulation is notoriously long, but this can be decreased through use of a larger computational resource. However, an often overlooked factor is the man-hours required to set up a case through processes such as mesh design; this has the potential to be considerably more time-consuming than the simulation time. For industry to benefit from the strengths of CFD models, the efficiency of the setup process needs to be increased, and this could be achieved through increased confidence in prediction by parametrically understanding numerical accuracy and providing standardised, “best-practice” implementations. An ever expanding use of CFD simulations for wave–structure interaction (WSI) applications (Windt, Davidson, and Ringwood, 2018; Palm et al., 2016; Devolder et al., 2018) has led to preliminary studies seeking to set the foundations for standardisation, such as the expansion of mesh convergence schemes to estimate uncertainty (Eskilsson et al., 2017; Wang et al., 2018), assessment of available wave generation methods (Windt et al., 2019a), the influence of mesh deformation scheme (Windt, Davidson, Akram, and Ringwood, 2018), and turbulence modelling under breaking waves (Brown et al., 2016). However, in general, there are very few established guidelines for design of WSI CFD simulations. Bearing in mind the enormous number of techniques and settings available to a user, it is therefore neither uncommon nor unexpected to see a wide range of solutions for a single problem where the desired solution is not known a priori, even when applying the same base CFD code (Ransley et al., 2019, 2020).
In this work, our computational fluid dynamics (CFD) solver naoe-FOAM-SJTU is adopted to simulate the interaction between focused waves and a moored hemispherical-bottomed buoy. This solver adopts a two-phase Navier–Stokes model and a spring mooring system. Three crest-focused wave groups, based on NewWave theory, are generated and validated against the experimental measurements from the Collaborative Computational Project in Wave–Structure Interaction (CCP-WSI) working group. Numerical results for the buoy’s heave and surge displacement, pitch angle, and mooring load are compared against corresponding physical data. The effects of wave steepness on the behavior and mooring loads are discussed.
Under extreme wave conditions, strong nonlinear impact phenomena such as severe wave runup, relative motion, and green water may occur, which will cause a large local impact load on wave energy converters (WECs). Exploring the interaction between extreme waves and WECs has great importance for the design and protection of these kinds of structures. As an extreme wave is highly nonlinear and can arise as a highly transient event within a multifrequency sea state, a focused wave group is typically adopted to model an extreme wave in physical or numerical modeling. The focused wave group where many wave components in a spectrum focus simultaneously at a position in space can represent an extreme wave profile with a specified wave energy spectrum. Thus, a focused wave can play the role for extreme wave conditions. The accurate prediction of the motion of a WEC under extreme wave conditions can be viewed as that under the focusing wave.
The paper presents the contribution to the CCP-WSI Blind Test, in which the responses of wave energy converters subjected to extreme waves are considered, by a hybrid model, qaleFOAM, coupling a two-phase Navier–Stokes (NS) model and the fully nonlinear potential theory (FNPT) using the spatially hierarchical approach. The former governs a limited computational domain (NS domain) around the structures and is solved by the OpenFOAM/InterDyMFoam. The latter covers the rest of the domain (FNPT domain) and is solved by using the quasi Lagrangian-Eulerian finite element method. Two numerical techniques have been developed to tackle the challenges and maximizing the computational efficiency of the qaleFOAM, including a modified solver for the six-degree-of-freedom motions of rigid bodies in the NS model and an improved passive wave absorber imposed at the outlet of the NS domain. With these developments, the accuracy and the computational efficiency of the qaleFOAM are analyzed for the cases considered in the blind test.
A reliable prediction of the responses of the offshore structures in a realistic extreme sea plays a fundamental role in the safe and cost-effective design of such structures. Numerous numerical models and software have been developed based on wide ranges of theoretical models, including the Navier–Stokes (NS) models and the fully nonlinear potential theory (FNPT), which assumes that the flow is incompressible, inviscid, and irrotational.
This paper presents a three-dimensional hybrid model that couples the weakly compressible smoothed particle hydrodynamics (SPH) and the quasi-arbitrary Lagrangian-Eulerian finite element method (QALE-FEM) for modelling the wave-structure interaction (WSI) in extreme sea states. The former is a fully Lagrangian mesh-free approach that solves the one-phase incompressible Navier–Stokes model and has shown satisfactory performance in simulating the WSI. The latter is an arbitrary Lagrangian-Eulerian approach based on the fully nonlinear potential theory and can accurately simulate highly nonlinear nonbreaking water waves in a large scale with high computational efficiency. These two models are coupled using a one-way zonal approach, in which the SPH uses a small computational domain near the structure, whereas the QALE-FEM covers the rest of the computational domain and provides wave conditions at the inlet of the SPH domain. The present hybrid model is validated against the experimental results and applied to the CCP-WSI blind test on modelling wave energy converters subjected to extreme focusing waves. The accuracy and convergence of the presented model are discussed.
The wave-structure interaction (WSI) in extreme sea status has been receiving extensive attention in the design and operation of the offshore and marine structures for their safety and survivability. The wave in such a condition is typically highly nonlinear and may exhibit local wave breaking, typically exceeding the range of applications of the linear, second-order, or Stokes wave theories. For a full development/formation of the extreme wave, the associated wave fetch length or propagation distance must be sufficiently large (Wang, Ma, and Yan, 2017, 2018). On the other hand, the structures subjected to such waves normally undergo significant motions and/or deformations. Consequently, small-scale physics, such as the viscous/turbulent effects, breaking wave impact, and aeration, may be important. This calls for numerical models with a capacity of dealing with both the large-scale wave propagation and the small-scale near-field physics simultaneously to deliver a reliable prediction on WSI in extreme sea conditions. Conventionally, two types of numerical models have been developed and applied in the engineering practices.