Offshore Conditions Research Articles

The Role of Beach Morphology and Mid-Century Climate Change Effects on Wave Runup and Storm Impact on the Northern Yucatan Coast

Wave runup is a relevant parameter to determine the storm impact on barrier islands. Here, the role of the beach morphology on wave runup and storm impact was investigated at four coastal communities located on the northern Yucatan coast. Current wave conditions based on regional wind simulations, topo-bathymetric transects measured at each location, and a nonlinear wave transformation model were employed to reconstruct multi-year runup time series. Dune morphology features and extreme water levels (excluding storm surge contributions) were further employed to determine the storm impact at each site for different return periods. Despite the similar offshore conditions along the coast, extreme water levels (i.e., runup and setup) showed intersite differences that were mainly ascribed to subaerial and submerged morphological features. Numerical results showed that the average surf zone beach slope, sandbars, berm, and dune elevation played an important role in controlling extreme water levels and storm impact at the study sites under the present climate. Moreover, in order to assess the potential effect of climate change on coastal flooding, we analyzed wave runup and storm impact in the best-preserved site by considering wave conditions and sea level rise (SLR) projections under the RCP 8.5 scenario. Modelling results suggest no significant increase in the storm impact regime between the present and future conditions in the study area unless SLR is considered. It was found that to accurately estimate SLR contribution, it should be incorporated into mean sea level prior to performing numerical wave runup simulations, rather than simply adding it to the resulting wave-induced water levels.

Evaluation of natural frequency, Campbell diagram and forced torsional vibration of deepwell pumps for the marine, oil and gas industry

Deepwell marine and offshore pumps need to design in a way to present high performance and reliable lifetime. Deepwell pumps should be designed in a way to operate under offshore conditions. In order to investigate the resonance and find critical rotational speeds of the pump, it is needed to find the rotational natural frequency for undamped model. In order to avoid any resonance, the intersection points between natural frequency and excitation (1X and 2X) should not be in the interval of 90%-110% of operating speed, based on API standard (API 610). Deepwell pump structure is combination of motor, motor-shaft coupling and pumping configuration. Based on the size of tank and type of ship the length of whole deepwell pump can be more than 40 meters. Elastic rotating shafts connect with appropriate shaft-couplings with needed numbers of impellers and inducer. Small and linear angle of rotation theory are used to extract natural frequencies of deepwell pumps. Different damping scenarios are illustrated in detail. Holzer method is implemented to analyse torsional forced vibration. Free-free boundary conditions considered. Combination of discrete model, lumped mass model and finite element method are employed to present the Campbell diagram. Parametric strategy is used for forced torsional vibration analysis. It is found that how to model and consider the effective length of intermediate shaft in pumping configuration has effect on second natural frequency. It is seen that the first natural frequency of the whole system is highly depended on modelling of motor-shaft coupling. The results are compared when the whole pump is model by lamped mass model and finite element model. Results of this paper suggest that combination of three mentioned methods is acceptable and accurate. It is shown that Holzer method is suitable for torsional forced vibration analysis. The developed codes are verified with findings in the literature.

Feature recognition and strength estimation of chain links by 3D inspections

In the past mooring chain failure probabilities have been much higher than the design failure probabilities. The high failure rates are attributed to prolonged exposure to various degradation mechanisms in harsh offshore conditions. In addition to understanding failure mechanisms, researchers have stressed on reducing the risk of failures through regular and reliable inspections. Current inspection techniques involve both in-air and in-water inspections. However, current in-air inspections are time consuming and expensive, while in-water inspection techniques are not yet reliable. The present paper proposes a condition monitoring and strength estimation methodology based on three-dimensional (3D) scans of links. The study first identifies and estimates changes in simple geometrical features of chain links using finite element (FE) modeling for condition monitoring. The methodology has two stages: 3D general imaging (3D-GI) to identify degraded chain links using feature monitoring, followed by 3D detailed imaging (3D-DI) of the identified weak links. The weak links 3D point clouds could be further converted to solid FE models for residual strength estimation by employing a suitable material degradation model. The geometrical feature monitoring methodology is verified by 3D scans of an undeformed and a deformed studded link.

Improvement of offshore adaptability of main cryogenic heat exchanger in FLNG dual mixed refrigerant liquefaction process

The dual mixed refrigerant liquefaction process and its key equipment applied in FLNG (floating liquefied natural gas) suffer performance deterioration due to the offshore conditions. In order to improve the offshore adaptability of FLNG dual mixed refrigerant liquefaction process, the experimental device of FLNG dual mixed refrigerant liquefaction process, the numerical simulation model and the visualization experimental device of falling film flow are developed. Multi-scale marine adaptability is studied from the aspects of the liquefaction process and its key equipment (meter scale) and liquid film thickness distribution (micron to millimeter scale). The results show that the effect of offshore conditions is more obvious on the spiral wound heat exchanger (SWHE) than the plate-fin heat exchanger (PFHE) of precooling circuit and the low temperature throttle valve of natural gas. It is recommended to use the corrugated tube with the diameter difference of 2 mm in the FLNG spiral wound heat exchanger to improve the offshore adaptability of dual mixed refrigerant liquefaction process.

Potential of a repair system for grouted connections in offshore structures: Development and experimental verification

Grouted connections are intensively used in offshore rigs, platforms as well as jacket and monopile offshore wind turbine structures. Being located in remote offshore conditions, these connections can experience considerable adverse loading during their lifetimes. Degradation was reported inside similar connections, which were installed in the last three decades. Grouting in the offshore sites may often be proven difficult, which eventually leads to reduced load-bearing capacity of connections in the long run. Thus, repair and rehabilitation of such connections should be planned ahead to minimize operational delays and costs. In this study, scaled grouted connections were manufactured using a novel mould, whose integrity were monitored using digital image correlation (DIC). The connections were loaded under static load to visualize the main failure pattern using distributed fibre optic sensors and acoustic emission (AE) analysis. Grouted connections were then repaired using a cementitious injectable grout. The effectiveness of the grout injection was monitored using dye penetration technique. Finally, specimens are reloaded to identify the potential of such repair for grouted connections.

Two-phase flow boiling characteristics in plate-fin channels at offshore conditions

In order to optimize the plate-fin heat exchangers for floating liquefied natural gas and avoid the degradation of performance at offshore conditions, the flow boiling characteristics in offset strip fins channels of plate-fin heat exchangers under offshore heaving conditions were investigated experimentally, covering the mass fluxes in the range 64–128 kg/(m2·s) and vapor qualities (vapor mass fractions) in the range 0.1–0.8. The results show that the influence of heaving is stronger for vapor quality equal to 0.6, leading to the increment of the heat transfer coefficients by 8.4%. The heat transfer coefficients under heaving conditions vary with the sloshing motion in a sinusoidal-like way. The enhancement of time-average heat transfer coefficients varies in the range of 98.4–108.4% for different heaving conditions. As the vapor quality increases, the relative fluctuation amplitude Ah and sloshing time-average factor Fh of heat transfer coefficients initially increase and then decrease, showing a maximum at a vapor quality of 0.6; for increasing mass flux, Ah decreases and Fh increases; for decreasing sloshing period and increasing sloshing amplitude, both Ah and Fh decrease. A novel correlation model for the heat transfer coefficients under heaving conditions for flow boiling in offset strip fin channels is proposed, and the predicted deviations within ±10%.

Analytical method for loads determination on floating solar farms in three typical environments

Floating solar energy is an industry with great potential. Farms can be installed on lakes, reservoirs or offshore. Numerous projects are under study or have been realised. This study presents three reference cases for floating solar farms, with adapted environmental data: a small lake (2.5 MWp – Mega Watt Peak - island), a large lake (7.5 MWp island) and offshore (2.5 MWp island). Mooring is one of the biggest challenges of floating solar photovoltaics (PV), compared to ground-mounted solar PV. To date, most mooring designs of industrial projects have been performed by computing environmental loads using an analytical analysis. This study aims to present one of the methodologies currently used in industry to perform a mooring design and to highlight the areas where further research is necessary before continuing developing industrial projects. Wind and current loads are computed with drag forces, considering drag coefficients from norms, wind tunnel tests, computational fluid dynamics modelling and the literature. Waves drift loads are computed with the Maruo formula. Results show that wind loads dominate for all cases, except for the offshore conditions, where waves have a significant contribution to the total load (around 50%). Even though the large lake island is 3 times larger than the offshore island, the total load applied on the offshore island is higher. The main challenges in the mooring design for each case are also underlined.

Laboratory methods for assessing the effectiveness of dispersants used in various countries for oil spill response in offshore conditions

Laboratory methods for assessing the effectiveness of dispersants used in various countries for oil spill response in offshore conditions

CASE STUDY OF NEARSHORE CURRENTS HAZARD ANALYSIS FOR RECREATIONAL BEACH DEVELOPMENT

Nearshore current generation at two coastlines contemplated for beach resort development is studied with the use of a numerical model for coexisting waves and currents. A nested-mesh technique was applied to consolidate the 2 domains of coarse and fine bathymetric data and to translate deep water wave conditions at the nearshore mesh boundary. The hydrodynamic model is validated using tide data at the nearest tide stations, while offshore wave conditions, determined from a wave hindcasting method, are inputted as quasi-stationary forcing. Simulations results of wave-current co-existing fields indicate local areas of rip currents within the project coastlines. In order to evaluate the safe swimming zones, an analysis of threshold currents under idealized conditions of human characteristics was carried out, that indicated a threshold of 0.16 mps for pure currents. With a safety margin to account for co-existing waves, rip current zones not exceeding 0.1 mps are considered safe and are used to designate the safe swimming areas for the 2 locations.

Evaluation of a roughness length parametrization accounting for wind–wave alignment in a coupled atmosphere–wave model

AbstractThe importance of wind energy as an alternative energy source has increased over the latest years with more focus on offshore winds. A good estimation of the offshore winds is thus of major importance for this industry. Up to now the effect of the wind–wave (mis)alignment has not yet been taken into account in coupled atmosphere–wave models to study the vertical wind profile and power production estimations of offshore wind farms. In this study the roughness length parametrization of Drennan et al. in 2003, and its extension addressing the wind–wave (mis)alignment proposed by Porchetta et al. in 2019, are investigated in the Coupled Ocean–Atmosphere–Wave–Sediment Transport (COAWST) model. This study shows that the yearly mean wind estimation at hub height (100 m) is improved by the roughness length parametrization of Porchetta et al. compared to Drennan. This is mainly due to the increased roughness of the former parametrization compare to the latter, even in aligned wind–wave conditions. This difference in roughness is caused by the dataset used to obtain the constants, deep‐water conditions versus mixed offshore conditions. Moreover, the roughness length parametrization of Porchetta et al. performs better in two of three alignment categories. Furthermore, similar model performances are obtained if we exclude the wind directions from the wind shadow zone of the measurement mast or the wind directions from the recently built Alpha Ventus wind farm, which is in close vicinity of the measurement mast. Investigating different wind conditions shows that the new roughness length parametrization of Porchetta et al. performs best for both offshore and onshore winds. Additionally, we show that the coupled model estimations of the vertical wind are only slightly affected by significant wave height estimations. Similar model performances for different accuracies of significant wave height estimations are presented. One exception is the perpendicular alignment category where the new roughness length of Porchetta et al. outperforms the roughness length of Drennan when investigating the wind estimations related to significant wave heights with a higher accuracy. The roughness length parametrization of Porchetta et al. reduced the power production overestimation of the coupled model from 5.7 to 2.8%. We also show that the standalone atmospheric model including the roughness length of Charnock in 1955 has a degraded performance compared to the coupled model including the roughness length parametrization of Porchetta et al. for yearly average wind profiles.

Failure analysis of an offshore drilling casing under harsh working conditions

An N80 casing in a certain oil well was broken. After the casing was pulled out, many cracks, sand holes, and corrosion and rust marks were observed in the casing fracture. To test the cause of the fracture, failure simulation using ANSYS software was carried out. Metallographic analysis test, chemical composition analysis test, and conventional mechanical performance test of the casing body were designed, and finally the failure analysis of the casing fracture was completed. The results revealed several large-scale oxide metallurgical inclusions in the casing near the upper end of the wellhead, which reduced the strength of the casing material and caused the casing to crack at the maximum stress position. In addition to harsh service environment, the casing was subjected to strong stress, which exceeded the strength limit of the casing and eventually caused the casing to break. The casing material met the standard requirements at all parts except the fracture areas. A calculation method is proposed for the ocean current and wind force; the method has high accuracy and can be used for analyzing similar offshore conditions. To prevent accidents, it is recommended to strictly control the heat treatment process of the casing material, optimize its microstructure, test its performance before deployment, and establish a real-time analysis system. Timely adjustments should be made to the operating schedule when operating in harsh environments.

Optimised design of screw anchors in tension in sand for renewable energy applications

The offshore deployment of floating offshore structures such as wind turbines or wave energy converters is expected to strongly increase during the next decade, to face the appetite for green energy sources. The growing size of these structures’ dimensions, inducing very large mooring forces, makes the anchoring solution adopted a critical issue for the commercial success of floating marine energy farms.The upscaling of the screw anchor technology from onshore to the offshore environment has been recently proposed as an efficient way of providing a large tension capacity while their installation generates far less noise and vibrations than impact pile driving. Most of recent studies on screw anchors have focused on separated geotechnical problems such as their uplift capacity or installation requirements. This paper incorporates within a single procedure geotechnical and structural constraints to calculate the optimal anchor geometry able to maximise the uplift capacity available.Performance envelopes for screw anchors have been derived in a parametric study, covering a broad range of soil conditions as well as in a case study, representative of offshore conditions. Results show that single screw anchors are more efficient (e.g. shorter and lighter) than driven piles to sustain tension loading. The results presented in this study support the applicability of screw anchors to be used as part of the mooring system for wave energy converters. However, tension requirements for tension-leg platform wind turbines would probably require the use of group of anchors.

Complex thermophysical modeling of processes in the foundation soil of oil pipelines in the Arctic and offshore conditions

The paper presents the results describing the mathematical model in operation and testing methodology aimed at heat engineering calculations needed to do for multilayer oil pipelines. A section of the pipeline running under difficult geological conditions is under simulation. The methodology aimed at calculating the thermal processes and its results are described. The simulation was carried out by using some insulating materials with a constant internal diameter of the pipeline and a pumping mode. The simulation was carried out with the universal software system of end-element modeling – ANSYS and with a module developed by the authors for TPS thermotechnical calculations.

Flux-gradient relation and atmospheric wind profiles — an exploration using WRF and lidars

A common closure for the planetary boundary layer in numerical weather models assumes a direct relation between turbulent fluxes and the mean wind vertical gradient, i.e., the flux-gradient relation or K-theory. This assumption implies that the angle β between the momentum stress vector and the mean gradient of the velocity vector are aligned, i.e., β = 0°. This is not what we observe from measurements. We quantify the misalignment of β in offshore conditions using measurements from a long-range Doppler profiling lidar and numerical simulations from the New European Wind Atlas mesoscale model output. We compare vertical profiles of wind speed, wind direction, momentum fluxes, and β up to 500 m, hence covering the rotor areas of modern offshore wind turbines and beyond. The results show that β ≈ −18° on average, with a lower, but still non-zero, value under stable stability conditions, ≈ −7°. We illustrate that the simulations describe well the mean wind speed and momentum fluxes within the observed levels, but the characterization of wind turning effects could be improved.

Monthly variation of growth, biochemical composition, fatty and amino acids patterns of gilthead sea bream (Sparus aurata) in offshore cage systems under brackish water conditions in the Black Sea

The aim of this study was to investigate seasonal variations of growth performance and biochemical composition, fatty and amino acids profiles of cage-cultured gilthead sea bream (Sparus aurata) in the Black Sea. The research was carried out on a yearly basis in the offshore cage systems of a commercial fish farm located on the coast of Samsun province, Turkey. Fish with an initial average weight of 2.44 ± 0.03 g reached a harvest weight of 474.60 ± 8.64 g with a survival rate of 94.26 ± 0.60% in 14 months. The average specific growth rate and feed conversion rate were 0.92 ± 0.30% and 1.45 ± 0.05 respectively. Seasonal variation of total essential and total non-essential amino acids and EAA/NEAA ratios ranged between 7.80 ± 0.00–11.22 ± 0.02 g/100 g, 8.19 ± 0.00–9.94 ± 0.01 g/100 g and 1.01 ± 0.00–1.18 ± 0.01 respectively (p < 0.05). The average of total ɷ-3, ɷ-6 and ɷ-9 values were 11.63 ± 0.68%, 24.29 ± 0.84% and 28.78 ± 0.17% respectively. It was determined that gilthead sea bream showed a remarkable growth performance over the 14 months period under offshore marine conditions in the Black Sea. Growth performance, biochemical composition, fatty and amino acids profiles in fish were influenced by seasonal changes of water temperature and nutritional value of the used diets. Findings of the present study provide remarkable information in terms of the year-round growth performance and meat quality of cage-cultured gilthead seabream based on a long-term evaluation throughout the full-cycle of 14-month production period of a commercial cage farm operating in offshore conditions in the Black Sea area.

Adsorption and desorption of water-soluble naphthenic acid in simulated offshore oilfield produced water

The removal of water-soluble organics compounds in oilfield produced water (OPW) is currently one of the biggest challenges to meet the environmental legislation, as conventional remediation focuses mainly on removing solids and the dispersed oil fraction. Naphthenic acids (NAs) are present in significant amounts in water-soluble organics compounds; therefore, this work milestone is to properly understand the adsorption process by using an acutely recalcitrant O2-NA as WSO model. For this, a comprehensive screening of commercial adsorbents and waste was tested for NAs remediation, assessing also the influence of adsorbent properties and functionalities. Afterwards, there were selected the resins MN 202 and L 493 as adsorbents to further evaluate the regeneration and other variables such as temperature, pH and salt content, focusing on on-site offshore conditions. The initial screening and the equilibrium data suggest that characteristics such as surface area, pore diameter, and ionic form have a great influence on the adsorption process. Furthermore, the adsorption mechanisms involve anion exchange and hydrophobic interactions, showing a transition between multilayer to monolayer adsorption with the increase of the adsorbent dosage and the kinetics, while increasing NA encourages pore diffusion, resulting in irreversible adsorption. The use of eluents, while avoiding irreversible adsorption, allows recovering the adsorbent, improving the process feasibility at the targeted conditions.

Marine floating solar plants: an overview of potential, challenges and feasibility

The offshore environment represents a vast source of renewable energy, and marine renewable energy plants have the potential to contribute to the future energy mix significantly. Floating solar technology emerged nearly a decade ago, driven mainly by the lack of available land, loss of efficiency at high operating cell temperature, energy security and decarbonisation targets. The vast majority of the available technology and projects in operation are located in inland freshwater bodies. Currently there is momentum in the sector to develop floating solar systems to be deployed in marine environments. Experience from inland floating solar projects could open up possibilities to scale up and move to nearshore or even offshore conditions. Hence, it is relevant to explore the technical feasibility and challenges of designing floating photovoltaics (FPV) in marine environments, with a natural transition of FPV applications first to nearshore locations and then further exposed offshore environments. This research study provides a literature review of the potential of marine applications of floating solar plants, exploring the current available technologies, the technical challenges and the risks in designing and building these projects in the marine environment.

Carbon Nanotube Reinforced High Density Polyethylene Materials for Offshore Sheathing Applications.

Multiwall carbon nanotube (CNT)-filled high density polyethylene (HDPE) nanocomposites were prepared by extrusion and considered for their suitability in the offshore sheathing applications. Transmission electron microscopy was conducted to analyse dispersion after bulk extrusion. Monolithic and nanocomposite samples were subjected to accelerated weathering and photodegradation (carbonyl and vinyl indices) characterisations, which consisted of heat, moisture (seawater) and UV light, intended to imitate the offshore conditions. The effects of accelerated weathering on mechanical properties (tensile strength and elastic modulus) of the nanocomposites were analysed. CNT addition in HDPE produced environmentally resilient nanocomposites with improved mechanical properties. The energy utilised to extrude nanocomposites was also less than the energy used to extrude monolithic HDPE samples. The results support the mass substitution of CNT-filled HDPE nanocomposites in high-end offshore applications.

Exploring role of Indian workplace spirituality in stress management: a study of oil and gas industry

PurposeThe primary objective of the present study is to explore the relationship between workplace spirituality and work stress among offshore and onshore employees of the Indian oil and gas industry. The present study also tends to study the difference in the stress level of offshore and onshore employees of the Oil and Gas Industry.Design/methodology/approachThe size of the sample for the present study was 202 respondents. It includes 128 onshore employees and 74 offshore employees of oil and gas companies. Respondents were mainly managers and supervisors working in various departments of Oil and Natural Gas Corporation (ONGC), Cairn India, Reliance India Ltd (RIL), Bharat Petroleum Corporation Ltd (BPCL) and Indian Oil and Gas Ltd (IOCL). Since the different level of stress is experienced by employees at different stages of the organizational structure, thus study selected population comprising of managers and supervisors since they are believed to face similar work stressors. A variety of statistical tools like mean, t-test, correlation and multi-regression is used for the analysis of collected data.FindingsResults show that all six dimensions of workplace spirituality are significantly negatively correlated with stress for onshore employees. However, the sense of community and gratitude are found insignificantly associated with stress for offshore employees. Stressful offshore conditions and excessive specialization might not allow offshore employees to cherish the community at the workplace and also the virtue of gratefulness. The offshore employees might have a certain level of gratitude and community system, but it is not sufficient for the employees to perceive a lower level of work relates to stress. The result gives the impression that the normal working conditions (onshore workplace) provide adequate opportunity to workplace spirituality to transcend its impact on work stress.Originality/valueThis is one of the pioneer studies that examined the role of workplace spirituality and stress in stress management of offshore and onshore employees of Indian Oil and gas companies.

Infragravity Waves in a Complex Macro-tidal Environment: High Frequency Hydrodynamic Measurements and Modelling

Pedreros, R.; Idier, D.; Le Roy, S.; David, A.; Schaeffer, D.; Durand, J., and Desmazes, F., 2020. Infragravity waves in a complex macro-tidal environment: High frequency hydrodynamic measurements and modelling. In: Malvarez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 1235–1239. Coconut Creek (Florida), ISSN 0749-0208.Nearshore high frequency (> 1 Hz) hydrodynamic measurements over month to year scale face many issues, especially in inter-tidal areas. We present measurements done in the intertidal area of a macro-tidal site (Gâvres, France), and on a time span of 2 years. The measurements are done with a WiSens NKE pressure sensor (burst mode of 17 minutes). Infragravity (IG) waves are present in the signal with IG wave heights reaching 0.5 m. The hydro-meteorological conditions (tide, surge, offshore short wave, wind) are characterized for each burst, showing for instance a linear increase of IG wave height with the offshore wave height (Hs) as long as Hs remains smaller than 4 m. To further investigate the temporal and spatial variability of IG waves, the analysis is completed using an unsteady phase resolving model (SWASH). We show how the IG wave height depend on the offshore wave conditions, the still water level, local wind conditions, and the local configuration, and provide hypothesis on the IG wave mechanisms generation on Gâvres.