Offshore Applications Research Articles

A low-fidelity model for the dynamic analysis of full-lattice wind support structures

This work aims to develop a low-fidelity model for a lattice support structure for offshore wind applications. The proposed low-fidelity model consists of a sequence of regular Timoshenko beams, each of them characterized by homogenized mechanical and mass properties representative of the single bays of the reference space-frame structure. The homogenized elastic coefficients of the sequence of beams are then computed by means of two alternative procedures: case (a), via analytical expressions available in the literature and accounting for a partially isotropic behaviour; case (b) by means of an optimization procedure, with ad hoc calibration factors. The suggested methods to derive the homogenized elastic coefficients are then tested for both straight and tapered lattice structures. The prediction performance is evaluated in terms of estimation of the first five natural frequencies and mode shapes, response to dynamic loads, and ability to predict rotor-structure interaction phenomena. A parametric study is then performed to evaluate the potential and limitations of the proposed models. To bypass the optimization procedure (b), a data-driven approach is also proposed for the case of straight lattice structures.Overall, the developed low-fidelity model leads to a computational speed-up factor of at least 60. The prediction reliability of the low-fidelity model is discussed for a tapered and regular straight lattice structure. However, for the latter one, a more detailed comparative study between the various modelling assumptions is performed and discussed. With reference to the straight lattice tower, whenever an optimization procedure is used (case (b)), and with reference to a typical subset of the investigated geometrical parameter space, the mean prediction error of the first five natural frequencies is lower than 1%. On the other hand, for case (a) and for the same investigated subset, the mean prediction errors for the first two bending modes and the torsional mode are, 5.2%, 13.3% and 18.8%, respectively. These results are improved in case a data-driven regression model is used to predict the calibration factors, leading to mean prediction errors below 5% for the entire investigated parameter space.

On the development of a generalized atmospheric boundary layer velocity profile for offshore engineering applications considering wind–wave interaction

Within industrial standards, effects due to wind–wave interaction on the marine atmospheric boundary layer (ABL) may be included via the Charnock sea-surface roughness parameter for open-sea and near-coastal waters. This roughness parameter does not accommodate the wide variety of wave states possible, nor does it modify the ABL to include speed-up effects resulting from an undulating wave profile. In an attempt to provide an updated definition of the general ABL profile for offshore wind engineering applications, an experimental and numerical study is performed to assess the interaction of the ABL on a fixed wave geometry. By extracting the mean velocity field during the wind–wave interaction, bespoke values of the velocity profile power exponent and wind risk factor can be obtained. A range of wave heights and wave lengths are considered from which a Kriging surrogate model is trained to supply the relevant wind profile parameters depending on the wave state. These newly garnered parameters enable the practitioner to define a reference wind velocity, and then adjust the velocity profile characteristics to contain the influence of the wave. This approach provides a new inlet velocity condition that can be used within computational wind engineering investigations without the need to explicitly model the wave surface, as well as flexibility in specifying the underlying wave conditions. Application of the re-calibrated velocity profile shows that wind forces are significantly greater throughout an offshore wind turbine (OWT) swept blade area when large (H= 15 m) and small (H= 1.5 m) wave heights are compared.

Review of Offshore Chemical Flooding Field Applications and Key Lessons Learned

Summary This paper presents an overview of both current advancements and field applications of offshore chemical flooding technologies. Along with offshore oilfield development strategies that require the maximization of oil production in a short development cycle, chemical flooding can become a potential avenue to accelerate oil production in secondary oil recovery mode. This makes it different from onshore chemical flooding processes that mostly focus on enhanced oil recovery in mature or maturing reservoirs. The advancements in offshore chemical flooding field applications are reviewed and analyzed. By summarizing offshore application cases, the presented analysis also assesses the chemical formulations applied or studied and injection/production facilities required in offshore environments. The main technical challenges are also discussed for scaling up the applications on offshore platforms or floating production storage and offloading (FPSO) systems. The chemical flooding technologies reviewed include polymer flooding, surfactant-polymer (SP) flooding, and alkaline-surfactant-polymer (ASP) flooding. By assessing the technology readiness level of these technologies, this study presents their perspectives and practical relevance for offshore chemical flooding applications. It has been long realized that chemical flooding, especially polymer flooding, can improve oil recovery in offshore oil fields. The applications in Bohai Bay (China), Dalia (Angola), and Captain (North Sea) provide the know-how workflows for offshore polymer flooding from laboratory to full-field applications. It is feasible to implement offshore polymer injection either on a platform or in an FPSO system. It is recommended to implement polymer flooding at an early stage of reservoir development to maximize the investment in offshore facilities. By tuning the chemistry of polymer products, they can present very good compatibility with seawaters. Therefore, choosing a proper polymer is no longer a big issue for offshore polymer flooding. There are also some interesting findings reported on the development of novel surfactant chemistries for offshore applications. The outcome from a number of small-scale trials, including the single-well chemical tracer tests on surfactant, alkaline-surfactant (AS), and SP in offshore Malaysia, Abu Dhabi, Qatar, and South China Sea, provided valuable insights for the feasibility of chemical flooding in offshore environments. However, the technology readiness levels of surfactant-based chemical flooding processes are still low, partially due to their complex interactions with subsurface fluids and the lack of interest in producing residual oil from matured offshore reservoirs. Based on the lessons learned from offshore applications, it can be concluded that several major challenges still need to be overcome in terms of large well spacing, reservoir voidage, produced fluid treatment, and high operational expense to successfully scale up surfactant-based chemical flooding processes for offshore applications. Key Terms and Phrases offshore chemical flooding; polymer flooding; surfactant-based chemical flooding; offshore application cases; lessons learned

Marine Applications of Fiber-Reinforced Polymer Matrix Composites

Composite materials are formed by bringing together two or more materials that are insolublein each other at a macro level. Composites consist of two main elements such as reinforcement (thecarrier) and matrix (the binder). These materials are generally classified according to matrix andreinforcement elements. In the classification made according to the matrix element, polymer, metal, andceramic matrix composites are examined. In this classification, polymer matrix composites are widelyused in practice. A polymer matrix from the thermoset or thermoplastic group is reinforced with varioustypes of continuous or short fibers. Composites belonging to this group are widely used in basic sectorssuch as automotive, aviation, and marine. Especially in the marine sector, polymer matrix composites areused in the construction of marine vehicles (ships, boats, yachts, etc.) and equipment. Marine systems andstructures include the hull and shipbuilding industries (ship and submarine masts, propellers, and interiorparts), the offshore applications industry (gas pipelines, tendons, and support structures), and therenewable energy sector (turbine devices and rotor blades). The importance of lightweight design isincreasing day by day in vehicles used in land, air, and sea transportation. Today, the increase in the valueof both safety and energy savings causes research on composite materials to intensify in the marinesector. It is advantageous to use composite materials in many parts so that negative environmental effectssuch as corrosion, biological pollution, seawater aging, and hydrostatic pressure cause minimal damage tomarine structures. With the developments in composite science, the level of use of these materials isincreasing in the marine sector, as in every other field. This review presents an overview of the use ofpolymer matrix composites in the marine industry.

Overall Design of Multi-module Offshore Floating Bridge and a Method for Random Analysis of Anchor Tension

This paper proposes a multi-module floating bridge design for offshore applications and establishes a coupled motion response analysis method for the anchoring system of the floating bridge. Time-domain coupled analysis is performed under the combined effects of wind, wave, and current loads, resulting in the maximum tension of the anchor chain in random waves. Using the typical island and reef sea area of Zhoushan, China as an example, the randomness of waves is considered, and a random sampling method is adopted. The main configuration, connection structure, and mooring system design schemes are presented, and the maximum tension of the anchoring system is numerically calculated and analyzed. The random distribution of the maximum anchor chain tension is obtained, the goodness-of-fit test is conducted for the fitting results, and the main degree-of-freedom motion response is analyzed under different exceedance probabilities. The results show that the multi-module offshore floating bridge meets the design requirements, and the statistical characteristics of the maximum tension of the anchor chain under random waves conform to the Gumbel distribution. The mode of the random distribution of the maximum anchor chain tension increases by 7.65% when fully loaded compared to the unloaded condition. In different exceedance probability scenarios, there are significant differences in the maximum tension of the anchor chain and the motion response of the main degrees of freedom of the floating bridge.

Wave-by-wave prediction for spread seas using a machine learning model with physical understanding

Accurate surface wave predictions have the potential to greatly enhance the safety and efficiency of many offshore applications, such as active control of wave energy converters and floating wind turbines. However, real-time wave prediction becomes increasingly challenging when large directional spreading is considered. To address this challenge, the present study introduces a machine learning model that utilizes an Artificial Neural Network (ANN) for predicting moderate directional spreading waves. Linear, short-crested wave time histories are synthesized numerically to assess the capability of our machine learning model. The ANN model demonstrates better prediction capability than a recently developed theoretical scheme (Hlophe et al., 2022), extending the prediction horizon by approximately one peak period into the future. Further, a quantile loss function is introduced to quantify the uncertainty, enhancing the practical value of the developed model in decision-making processes and engineering applications, such as the active control of offshore renewable energy systems.

Investigation of Natural Mucilage for Enhanced Oil Recovery: the Potential of Corchorus Olitorius Hydrocolloid.

The need for an effective offshore enhanced oil recovery (EOR) solution led to the focus on natural hydrocolloids. Polysaccharide hydrocolloid research is constantly expanding in a variety of petroleum applications such as drilling, flow assurance, and EOR. Corchorus olitorius is being examined in the present study as a potential natural polymer for chemical flooding. This study investigated the rheology and fluid flow characteristics in porous media, focusing on the effects of the concentration, temperature, and salinity of the fluid. Furthermore, core flooding was carried out to evaluate the potential recovery was characterized and found to contain a significant amount of polysaccharides and cellulose. The rheological behavior demonstrated an increase in viscosity with concentration. The relationship between viscosity and temperature is inversely proportional. Additionally, the mucilage viscosity significantly increased in the presence of 35,000 ppm NaCl, varying from 39 to 48 cp. The improvement of oil recovery by a unit PV injection is around 10 and 20% at 0 and 35,000 ppm of NaCl, respectively. In sandstone with a moderate porosity and permeability, the overall oil recovery ranges between 59 and 70%. C. olitorius has complex polysaccharide/cellulose derivatives that improved rheology and produced results that are promising for future offshore applications.

A study on the influence of the numerical scheme on the accuracy of blade-resolved simulations employed to evaluate the performance of the NREL 5 MW wind turbine rotor in full scale

We present a numerical investigation on the influence of the numerical arrangement in the accuracy of the NREL 5 MW wind turbine rotor performance evaluation in full scale, using a Computational Fluid Dynamics (CFD) methodology which employs the Finite Volume Method (FVM) implemented in the OpenFOAM software. The nonlinearity of the Navier–Stokes equation, which requires special treatment and still represents a subject under intense debate in the scientific community, is tackled by using two different discretization schemes for computing the convective term. In addition, since the advection scheme numerically interacts with the turbulence closure model employed in the numerical simulations, we considered the URANS k-ω SST and DES k-ω SST models to perform the blade-resolved simulations. A hybrid central/upwind scheme, namely the LUST scheme, was considered for discretizing the convective term for both URANS and DES simulations, while the second order accurate upwind given by the LUD scheme was considered only in the URANS simulation. Considering the LUST scheme, an investigation on the temporal and spatial discretization was performed for both turbulence closure models. The performance of the NREL 5 MW rotor for offshore application in full scale was assessed in terms of power production, generated thrust, and forces distribution along the blade span, for the operation condition of optimal wind-power conversion efficiency. We provide detailed information regarding the flow features and computational cost, and verified the results from each case by comparing the CFD results against values obtained using the blade element momentum theory implemented in OpenFAST. For the spatial discretization considered in one of the meshes, the LUD scheme showed low accuracy in the results, being more susceptible to the grid influence for the URANS simulations compared to LUST, while the URANS-LUST and DES-LUST approaches were both suitable options for all the meshes investigated. The DES-LUST approach was less affected by the variation in the size of the time step employed. Additionally, finer flow structures were captured with the DES-LUST simulations at an affordable computational cost.

Factors influencing powders’ flowability and favorable phases like crystalline (Mullite and quartz) and amorphous phases of plasma-sprayed fly ash coatings suitable for marine and offshore applications

Due to its rich mineralogy, fly ash (FA), an industrial waste, has been used to combat erosive, corrosive environments. Powder flowability dictates coating properties. In this investigation, raw FA powder was obtained from a thermal power plant and sieved in various sizes to assess their flowability. Powder's physical characteristics, such as specific surface area, Blaine's fineness number, and bulk density, were determined, and their influence on powder flowability was analyzed. Of these properties, bulk density affects more. Rietveld refinement was performed on the powder to quantify the phases. The powders had 45.08 ± 11.38 amorphous and 11.00 ± 2.76 % of mullite phases. Later, alumina was added between 10 and 50 wt% to FA, and samples were subjected to high-temperature X-ray diffraction at 1150 °C. A ∼32.27% rise in Mullite content was observed for 50 wt% alumina, with ∼119% decrease in the amorphous phase. Finally, one set of FA without additives coating was plasma sprayed onto a marine-grade steel substrate. The coating showed ∼17.31 ± 0.6% of mullite and ∼69.43 ± 0.6 % of the amorphous phase, with decent Mechanical properties. Therefore, 50 wt% alumina in FA powder has improved the mullite phase, bulk density (43%), and flowability by decreasing the amorphous phase content.

Effect of epoxy resin addition on properties and corrosion behavior of sintered joints in power modules serviced offshore

Power modules applied in offshore applications are facing risks of corrosion failures on die-attach materials due to high humidity and H2S exposure. To investigate such corrosion behavior for sintered die-attach materials, we conducted a study with four groups of samples fabricated using copper and silver metal particles under different solvent systems. Such samples were firstly subjected to high-humidity-H2S conditions for 168 h to simulate the harsh offshore environment. After undergoing corrosion, the primary compounds formed were CuO/Cu2O and Ag2S through SEM, XRD, and XPS analysis. Notably, the incorporation of epoxy resin into sintered copper joints resulted in a remarkable reduction in corrosion and a substantial improvement in electrical conductivity after the reaction. In contrast, while the addition of epoxy did not evidently reduce corrosion in silver joints, it did lead to a significant increase in shear strength. Furthermore, to gain further insights into the effect of epoxy resin on corrosion behavior, electrochemical analysis, and molecular dynamics simulations were conducted. Finally, the mechanical reliability of the corroded copper and silver joints was evaluated through thermal shock tests. In summary, sintered copper joints exhibited better anti-corrosion behaviors than sintered silver under high humidity and H2S exposure, especially with the addition of epoxy resin. However, the corrosion products of sintered copper suffered from a sharp decrease in shear strength after thermal shock tests than sintered silver, which is probably due to the coefficient of thermal expansion mismatch.

Coordinated Predictive Control of Offshore DC Collection Grid and Wind Turbines for Frequency Response: A Scheme Without Secondary Frequency Drop

From both technical and economical perspectives, all-dc offshore wind farm (OWF) is a trend for future offshore wind energy applications. Furthermore, in an all-dc OWF, both the capacitors in dc collection grid and offshore wind turbines have the potential for onshore grid frequency response. In light of this, this paper presents a model predictive control scheme, which coordinates the operation of offshore dc collection grid capacitors (dc-grid capacitors) and offshore wind turbines for the onshore frequency support. With this scheme, offshore dc-grid capacitors provide fast inertia support right after the frequency event, while offshore wind turbines provide primary frequency response for a longer period. A key feature of the proposed scheme lies in the ability to suppress the secondary frequency drop issue, which is caused by sudden power reduction of offshore dc-grid capacitors during the frequency recovery phase. An analytical framework is derived to validate this ability. Finally, simulation results verify the effectiveness of the proposed scheme.

Electromagnetic design and performance analysis of large-scale offshore wind generators with modular fractional-slot concentrated winding

Future offshore wind engineering raises demands on higher reliability, better power generating performance, as well as lower manufacturing and transportation cost. This paper presents a direct-driven permanent magnet generator (DDPMG) with modular fractional-slot concentrated winding (MFSCW), to meet the large-scale, low-cost and high-reliability requirements of offshore wind power applications. The corresponding pole–slot combination method and electromagnetic characteristics analysis for this novel generator are proposed and performed in detail, including the winding coefficient analysis, magnetomotive force (MMF) harmonic analysis, cogging torque analysis, finite element simulations and generator parameter optimization. After finite element simulations, a 3.3 MW prototype MFSCW-DDPMG is developed to validate the proposed scheme. The results provide a complete and feasible scheme of pole–slot​ combination and electromagnetic design for future MFSCW-DDPMG engineering applications.

Modelling and Thermophysical Properties of TiO2-SiO2-CaO and SiO2-CaO-MgO Based Electrode Coatings for Offshore Applications

Modelling and Thermophysical Properties of TiO2-SiO2-CaO and SiO2-CaO-MgO Based Electrode Coatings for Offshore Applications

Torsional installation and vertical tensile capacity of helical piles in clay

Helical piles are being increasingly considered for offshore applications as they avoid the acoustic emissions associated with pile driving, and they provide additional capacity relative to a driven pile. In this paper, the installation and tensile capacity of helical piles is considered through a combination of centrifuge experiments and large-deformation finite-element analyses within a coupled Eulerian–Lagrangian framework. The experiments provide a basis for validating the numerical simulations, but also quantify the expected installation torque and undrained tensile capacity, including the variation with time after installation. The numerical simulations extend the parameter space investigated experimentally, considering the number of helices, their spacing and pitch, in addition to the ratio of pile shaft to helix diameter and the profile of undrained shear strength. Mechanisms revealed through the numerical simulations are reflected in a new analytical model for calculating undrained tensile capacity, which is seen to agree reasonably well with the numerically and experimentally determined capacities.

Robust damping control for VSC-MTDC system in offshore applications considering disturbance uncertainty

Multi-terminal direct current transmission based on voltage source converter (VSC-MTDC) has become an ideal solution for long distance offshore wind farm access systems. However, the integration of VSC-MTDC with offshore wind farms is vulnerable to various disturbances, which can lead to instability. To address this issue, this paper presents a robust damping control for VSC-MTDC in offshore applications that accounts for disturbance uncertainty. First, a small-signal VSC-MTDC model is developed considering the coupling characteristics of the cable, and the DC cable model is suitable for both unipolar and bipolar structures. Secondly, the key parameters affecting the oscillation mode are analyzed. On this basis, the paper proposes a robust damping strategy that adapts to the changing operating conditions of the system without impairing steady-state performance, while enhancing transient performance. Finally, the paper builds a VSC-MTDC with offshore wind farms on MATLAB/Simulink to validate the correctness and effectiveness of the proposed scheme. The simulation results demonstrate that the proposed control strategy exhibits better robustness and suppression ability under multiple types of disturbances compared to traditional additional damping control.

Microstructure and Mechanical properties of Dissimilar Welds of Duplex and API Steel for Offshore Applications

Microstructure and Mechanical properties of Dissimilar Welds of Duplex and API Steel for Offshore Applications

Testing and validation of travelling wave based fault detection and location method for offshore wind farm applications

This work investigates the efficiency of Travelling Wave (TW) based fault detection and locationing for multi-sectional cross-bonded export cable for offshore wind farm applications. The cables, instrument transformers and the offshore wind farm are modelled according to the TW method and practical conditions. In order to correctly reflect the offshore collection grid dynamics, an aggregation method that preserves the frequency response of the wind farm impedance is implemented. The result of the EMT simulation of the developed offshore system model is played back on an actual TW relay to test the reliability of the current-based TW FDL method. A simulation of TW FDL using wavelet transform is performed to verify the relay response. Through cross-validation with the relay testing result, the paper provides another aspect of the applicability of TW relay for offshore applications.

Seawater Effects on Thermally Aged Ambient Cured Carbon/Epoxy Composites: Moisture Kinetics and Uptake Characteristics.

Carbon fiber-reinforced epoxy matrix composites using ambient- and moderate-temperature curing non-autoclave processes have broad applicability in marine, offshore, and naval applications. This research focuses on the characterization of moisture kinetics of ambient cured carbon/epoxy composites subject to immersion in seawater for up to 72 weeks after prior periods of extended thermal aging. A two-stage model is shown to best describe the overall kinetics and response. The level of maximum moisture uptake shows an increasing trend with the temperature and time of prior thermal aging, reaching asymptotic levels at the highest levels. The transition point is seen to represent a shift between the diffusion and relaxation-/deterioration-based dominant regimes, and the ratio of uptake at the transition point to the maximum uptake can be correlated to the relaxation coefficient. Diffusivity, as expected, generally increases with the temperature of prior aging and shows changes based on the level of post-curing and network changes with time. Moisture uptake kinetics and characteristics developed through the sequence of exposures provide a better understanding of phenomena towards the development of a future comprehensive model capable of long-term prediction based on the sequential prior history of exposure to elevated temperatures and immersion in seawater.

A novel smart coating with hexacyanoferrate intercalated layered double hydroxides nanoadditive for early detection of carbon steel corrosion

The detection of corrosion at early stages could increase the service life of metal-based infrastructures in a cost-effective manner. Despite the recent progress in “smart” self-reporting corrosion sensing coatings, the development of environmentally friendly systems appropriate for steel substrate used in offshore applications remains a relevant challenge. In this study, a novel smart corrosion sensing coating, based on hexacyanoferrate intercalated Mg-Al LDH nanoadditive, was developed, aiming at the detection of early-stage corrosion of carbon steel. The detection mechanism is based on the ability of hexacyanoferrate ions to react with iron cations generated during the corrosion process, giving rise to a colorimetric signal, while LDH carriers provide a controlled release of active ions under corrosion conditions. The sensing nanoadditive was embedded into a commercial pigment-free water-based acrylic polyurethane coating. The nanomaterial was characterized structurally (XRD) and morphologically (STEM). The compatibility of the additive with the polymer formulation and its influence on the resulting coating performance was investigated in terms of rheological behavior, structure (FTIR), morphology (SEM/EDS), thermal (TGA, DSC) and mechanical (adhesion, hardness) properties. The corrosion protection ability of the coating was evaluated via EIS, while the sensing functionality was analyzed by visual analysis of the surface. The developed coating successfully detects early-stage corrosion of steel substrate at a lab scale, in conditions relevant to the use of metallic structures in offshore applications, demonstrating a correlation between the level of material degradation and the spectroscopic signal associated with the presence of the LDH functional nanoadditive. Furthermore, the observed decrease in coating barrier properties, caused by the presence of LDH, was overcome by the subsequent development of a multilayer coating system. Two different topcoats (epoxy- and polyurethane-based) were surveyed for this purpose, showing an improvement in the coating barrier properties without influencing the corrosion detection functionality of the sensing layer. The results were successfully validated by standard salt spray tests. The multilayer approach opens up the possibility to model coatings with different characteristics for various operating conditions.

Fatigue Life and Crack Initiation in Monopile Foundation by Fatigue FE Analysis

The construction of new renewable energy infrastructures and the development of new ocean resources continues to proceed apace. In this regard, the increasing size and capacity of offshore wind turbines demands that the size of their accompanying supporting marine structures likewise increase. The types of marine structures utilized for these offshore applications include gravity base, monopile, jacket, and tripod structures. Of these four types, monopile structures are widely used, given that they are comparatively easy to construct and more economical than other structures. However, constant exposure to harsh cyclic environmental loads can cause material deterioration or the initiation of fatigue cracks, which can then lead to catastrophic failures. In this paper, a 3D fatigue finite element analysis was performed to predict both the fatigue life and the crack initiation of a welded monopile substructure. The whole analysis was undertaken in three steps. First, a 3D non-steady heat conduction analysis was used to calculate the thermal history. Second, a thermal load was induced, as an input in 3D elastoplastic analysis, in order to determine welding residual stresses and welding deformation. Finally, the plastic strain and residual stress were used as inputs in a 3D fatigue FE analysis in order to calculate fatigue crack initiation and fatigue life. The 3D fatigue finite element analysis was based on continuum damage mechanics (CDM) and elastoplastic constitutive equations. The results obtained from the 3D fatigue finite element analysis were compared with hot spot stresses and Det Norske Veritas (DNV-GL) standards.