Design Of Offshore Structures Research Articles

Advances in Marine Mechanical and Structural Engineering

In the design of modern ship and offshore structures, one of the key issues is the accurate prediction of strength with regard to various new materials and structures used in the structural design stage and under extreme sea environment and accidental states [...]

Breaking Wave Statistics In Short-Crested Seas

The accurate prediction of loads to allow for a safe yet efficient design of coastal and offshore structures requires a thorough understanding of the distribution of crest heights. Although deep water waves are relatively well understood and accurately modelled by existing wave and crest height models, the same cannot be said for waves in intermediate and shallow waters (Tayfun & Alkhalidi, 2020). This work addresses this gap by analysing long, random records of laboratory-generated short-crested seas. The analysis focuses on the application of a novel method to identify waves that undergo significant nonlinear amplification and breaking. Following their identification, the associated energy transfers and dissipation are calculated per wave and probabilistically approximated. A modelling suite is proposed to describe the probability of wave breaking and associated dissipation in short-crested seas. This is then converted into a mixture model to recover crest height statistics. The success of the proposed approach is demonstrated through comparisons between model predictions and measurements.

Calibration of horizontal seismic design spectra of offshore earthquake ground motions by applying improved differential evolution method

Design Response Spectrum (DRS) serves as a crucial cornerstone for seismic design, encapsulating the characteristic parameters of the design spectrum. To investigate the offshore design spectrum required for seismic design of offshore structures, improved differential evolution (DE) methods are first used to determine the characteristic parameters of the horizontal design spectra of offshore ground motions. In assessing the stability and precision of the chosen improved DE methods, we scrutinize the applicability of various renowned DE methodologies in the calibration process. Our findings indicate that differential evolution with global and local neighborhoods (GLDE) exhibits superior stability and accuracy compared to other DE methods, rendering it suitable for calibrating the design spectrum to delineate optimal characteristic parameters of the DRS. Subsequently, the properties of the offshore horizontal design spectra are investigated by applying the recommended GLDE to 6756 seismic data collected from three offshore regions. The site conditions of water depth and sedimentary thickness of the ocean-bottom stations were investigated by the finely divided seismic data with classified earthquake types. The ocean-bottom stations are classified as A-D by the water depth and sedimentary thickness, and the offshore DRS for four site classes with different earthquake types are presented in this study. According to the observed offshore response spectra with different water depth groups, it is found that the water depth should be considered in the offshore design spectra mainly due to the differences between buried and unburied seafloor stations, and the spectral values with deep water layer are considerably larger than those with shallow water. Offshore DRSs are compared with onshore DRSs of seismic codes and different peak ground acceleration (PGA) groups are presented in this study.

Current-distorted wave diffraction patterns in the near-field of a monopile in coastal seas

The simultaneous impact of a wave field in the diffraction regime and a perpendicular intersecting current on a cylinder reflects a common application case for wave-current-structure interaction with offshore wind turbines in the German North Sea. Although existing literature has thoroughly investigated structural impacts from both waves and currents individually, the complex interplay between both forcings in the near-field of an offshore monopile has not been explored as thoroughly in the past. Only little is understood concerning the resulting nonlinear 3D interaction. This laboratory study evaluates the physical insights of a broad range of wavelength-dependent pile diameters, i.e., 0.14<diameterD/wavelengthL<0.25. The study focuses on surface elevation characteristics, documented either qualitatively using video recordings or quantitatively by surface elevation measurements in close vicinity of the pile. Within the experimental boundaries of this initial study, a current-induced, D/L-dependent asymmetry in crest amplitudes is documented. These distorted diffraction patterns are explained to originate from non-linear wave–current–structure interactions, which are not represented by existing design approaches. The findings further suggest that in this context, secondary load cycle dynamics are of practical relevance to the design and operation of offshore structures.

A new protocol to evaluate the behaviour and durability of marine structures

Sustained mechanical and environmental loadings on offshore structures result in creep phenomena. Creep and relaxation significantly affects the development of strains and stresses, thus they influence the durability of structures. Creep originates from the behaviour of the phases within the cement matrix. Seawater modifies the phases composition of cement matrix because of the variety of ions and their possible interactions with cement hydration products. This, in turn, affects the mechanical behaviour of structures, including creep. For durability and safety purposes, the design of offshore structures should consider the possible coupling between cement hydration, seawater exposure, and creep. Therefore, there was a need to develop a new protocol to evaluate the behaviour and durability of marine structures. As part of this new protocol, a new rig has been developed to evaluate the creep of cement-based materials immersed in seawater. The obtained results show that the short-term creep of structures in seawater is faster than in tap water. In the long term, even though multiple phases are microscopically formed due to seawater attack, the positive and negative effects of these phases formation and dissolution lead to almost similar creep development in seawater. This protocol can be used to assess the chemo-mechanical durability of multiple structural materials in order to develop low carbon materials. Additionally, it could improve the design of offshores structures by estimating the maximum permissible load based on the rigidity of structures and their chemical degradation level.

Wave attenuation and transformation across a highly turbid muddy tidal flat-salt marsh system

Understanding wave processes within the coastal system is crucial for the appropriate design of coastal prevention engineering and offshore structures. To address the limitations of field observations and traditional methods, an array of pressure sensors and optical instruments were deployed across a shoreward intertidal flat consisting of the fluid muddy seabed, bare flat, and salt marsh in a highly turbid coastal area. We found that the Spartina alterniflora salt marsh has the strongest wave dissipation capacity. Although bare flats have the weakest wave dissipation capacity, they are the most extensive within the intertidal zone, resulting in substantial total wave energy dissipation. A noteworthy positive feedback between fluid mud and wave dissipation is observed. Additionally, the incident waves undergo complex transitions during propagation, which are strongly modulated by tidal influences. In particular, when the wave propagation direction is opposite to the tidal current, there is an amplification of wave energy within the tidal flat. This amplification is attributed to the "jacking effect" of the tidal current, hindering the transformation of the wind waves into the low-energy capillary waves. It is important to note that the high-energy infragravity waves are gradually released and steadily increase as the incident waves propagate shoreward. This implies their potentially significant role in nearshore processes, especially during extreme dynamic events or along extensive coastlines.

Determination of load-bearing capacity of piles used in stationary offshore platforms

Fastening of hydrotechnical oil-gas mining facilities to seabed soils in Caspian Sea aquatoriums is usually carried out by pile foundations. Sustainability of strength and stability during the design and construction of hydraulic structures requires to solve a number of theoretical and practical problems. Numerous static and dynamic tests (experiments) were carried out in the Caspian Sea aquatorium and in laboratory conditions to solve these issues. The widespread use of pile foundations in the development of offshore oil and gas fields revealed the inconsistency of the domestic scientific methodological and regulatory framework for calculating their load-bearing capacity over the soil. It is established that the wave strikes, acting on pile foundations, interact with the surface design of offshore structures and offshore ground bases. Sea wave banging pile foundations creates vibrations in the pile foundation - topsides offshore structures and moving piles in subgrade. Displacement piles depend on the strength of the reaction between the structure and subgrade, the intensity of the shock wave in the time that passed through piles for offshore soil. At the same time, it takes into account the rheological properties of composite models of shelf soil. Keywords: shelf; stress; ground; pile foundations; wave; marine structures; soil resistance.

A critical review of challenges and opportunities for the design and operation of offshore structures supporting renewable hydrogen production, storage, and transport

Abstract. The climate emergency has prompted rapid and intensive research into sustainable, reliable, and affordable energy alternatives. Offshore wind has developed and exceeded all expectations over the last 2 decades and is now a central pillar of the UK and other international strategies to decarbonise energy systems. As the dependence on variable renewable energy resources increases, so does the importance of the necessity to develop energy storage and nonelectric energy vectors to ensure a resilient whole-energy system, also enabling difficult-to-decarbonise applications, e.g. heavy industry, heat, and certain areas of transport. Offshore wind and marine renewables have enormous potential that can never be completely utilised by the electricity system, and so green hydrogen has become a topic of increasing interest. Although numerous offshore and marine technologies are possible, the most appropriate combinations of power generation, materials and supporting structures, electrolysers, and support infrastructure and equipment depend on a wide range of factors, including the potential to maximise the use of local resources. This paper presents a critical review of contemporary offshore engineering tools and methodologies developed over many years for upstream oil and gas (O&amp;amp;G), maritime, and more recently offshore wind and renewable energy applications and examines how these along with recent developments in modelling and digitalisation might provide a platform to optimise green hydrogen offshore infrastructure. The key drivers and characteristics of future offshore green hydrogen systems are considered, and a SWOT (strength, weakness, opportunity, and threat) analysis is provided to aid the discussion of the challenges and opportunities for the offshore green hydrogen production sector.

A Joint Probability Distribution for Multivariate Wind-Wave Conditions and Discussions on Uncertainties

Abstract This article presents a joint statistical model, which is needed in probabilistic design and structural risk assessment, that has been fitted to data of wind and wave conditions for an offshore location off South Brittany. The data are from a numerical model and contain hourly values for several wind and wave variables over a period of 32 years. The joint distribution presented in this article considers the variables wind direction, mean wind speed, significant wave height, wave direction, and peak period. A conditional model for turbulence given wind speed is introduced to yield an additional variable for the joint model. The joint model is constructed as a product of marginal and conditional models for the various variables. Additionally, the fitted models will be used to construct environmental contours for some of the variables. For significant wave height, various models are used to obtain different extreme value estimates, illustrating the uncertainties involved in extrapolating statistical models beyond the support of the data, and a discussion on the use of nonparametric copulas for the joint distribution is presented. Moreover, bootstrap has been performed to estimate the uncertainty in estimated model parameters from sampling variability. The effect of changing which variable to model as the marginal in a conditional model is illustrated by switching from wind speed to significant wave height. Such joint distribution models are important inputs for design of offshore structures, and in particular for offshore wind turbines, and the influence of the joint model in design is illustrated by a simple case study. This article is an extension of the conference paper by Vanem et al. (2023, “A Joint Probability Distribution Model for Multivariate Wind and Wave Conditions,” 42nd International Conference on Ocean, Offshore and Arctic Engineering).

Exponentially Reduced Reflection (ERR) versus Linear Wave Theory (Airy-Laplace)

The linear wave theory according to Airy/Laplace (1842) is widely used by engineers however violates the law of conservation of mass and considers local ground inclination α = 0⁰ only.Both shortcomings can be avoided(a) by extending Schulejkin's mirror method to represent orbital kinematics over flat ground to inclinations 0⁰ ≤ α ≤ 90⁰ and(b) by referring to the phase shift Δϕ between incident and reflected waves according to the author´s definition of the complex reflection coefficient CRC.This contribution includes Exponentially Reduced Reflection (ERR) for the first time as a wave theory applicable to complex boundary conditions, considering a priori an important cause of wave deformation over inclined seafloor.For instance, the change of the total orbital kinematics (orbital paths, velocities and accelerations) related to the water surface is shown for decreasing water depths on a slope 1 : n = 1 : 2.In contrast to the Airy/Laplace theory, lower orbital velocities are obtained above the still water level, which is important with respect to the design of offshore structures.

A novel post-weld treatment using nanostructured metallic multilayer for superior fatigue strength

Welded joints exhibit fatigue failure potential from weld geometry and characteristics of the heat affected zone. In order to counteract fatigue, structures and components require larger thicknesses resulting in heavier designs exhausting the finite natural resources. We hereby introduce a novel post-weld treatment, which postpones or even prevents fatigue failure of the welded connection. A Cu/Ni nanostructured metallic multilayer (NMM) is applied via electrodeposition and a 300–600% increase in usable lifetime compared to the untreated weld is observed. A FAT class 190 with a slope of k = 6 is proposed for the design of NMM treated butt welds. Material mechanisms responsible for the fatigue strength increase are introduced herein. A case study shows that the design of offshore wind turbine support structures applying NMM post-weld treatment enables a lifetime extension as well as a 28% weight reduction compared to the structure without post-weld treatment.

What a Wave Buoy Actually Measures in 3D: Analysis of a Mild Sea State

Abstract Measurements of surface waves from oceanographic buoys have been regarded as the “ground truth” for validation of sea-state prediction models, providing the basis for input to the design of offshore structures. The engineering practice is to produce wave statistics of vertical surface displacements over periods of years. However, a wave buoy can provide simultaneous time histories of its motion, one vertically and the other two horizontally, giving the complete vector displacement field in time. We investigate the measured time histories of a wave buoy in three orthogonal directions and explore the relationship between them, for a relatively benign, so typical, sea state. We adopt a NewWave-type analysis to investigate the average shape of the large events across the measured time histories. In combination with a conditioning analysis, we give a reciprocity relationship between the vertical displacement of the wave buoy and those in the horizontal plane. The relationship is of value, as it allows for the prediction of wave kinematics in the horizontal plane based on the vertical measurement only. We observe significant second-order components in the measured data in the horizontal directions and smaller contributions vertically. This data-driven analysis paves the way for wave-by-wave prediction and the active control of wave energy converters and personnel transfers offshore.

Effects of material strain rate, impulse profile on damage evolution of steel plates subjected to repeated pressures

This study deals with a numerical estimation of the damage evolution of steel plates used in ships and offshore structures due to repeated slamming-induced pressures. The numerical analysis model was developed using finite element ABAQUS software. Subsequently, the validated model was extended to investigate the effects of material strain rate and impulse profile on the damage evolution of plates having actual scantlings. Different strain rate models were used, and several types of impulse profiles that can be used to simplify the actual slamming pressure were adopted. In the simulation, the material strain hardening was defined for the plate model, and the impulsive pressures were applied uniformly over the plate while it was fully clamped at four edges. The results showed that a large effect of the material strain rate on the damage evolution of the plate was noticed for only several first impacts, while the effect of the impulse profile is remarkable for whole impact events. The occurrence of the pseudo-shakedown phenomenon of steel plates was examined accordingly. The results can be useful for the design of ships and offshore structures against slamming.

Dynamic response and liquefaction potential of porous seabed induced by partial standing ocean waves

The analysis of ocean wave-induced dynamic response of a porous seabed is particularly important for coastal and geotechnical engineers when designing and constructing maritime structures. In this study, an analytical solution is presented to analyze the dynamic response and liquefaction potential of a poro-elastic seabed induced by partial standing waves with arbitrary reflectivity. The porous seabed is modeled using Biot’s theory describing the propagation of elastic waves, and coupled deformation and water flow of porous media, whereas the ocean waves are described using linear ocean wave theory. Based on the mixed boundary-value conditions, explicit expressions of displacements, effective stresses and excess pore water pressure of seabed are derived with consideration of the effects of inertial forces, compressibility of solid and fluid, and arbitrary reflectivity of standing waves. The results of degenerated analytical solutions are compared with the existing ones to verify the correctness of the proposed method. The effects of several pertinent parameters of ocean wave-seabed system, including reflection coefficient, phase lag and period of standing waves, depth of water, permeability, degree of saturation, and shear modulus of seabed deposits, etc., on the dynamic response of seabed and liquefaction potential, are examined and discussed. It is found that the reflection of standing wave has a significant effect on the dynamic response and liquefaction potential of porous elastic seabed. Compared with that of no wave reflection, the liquefaction depth of seabed induced by fully-reflected standing waves increases 82.49% under certain conditions of wave-seabed system. In addition, phase lag, wave period, water depth and mechanical and physical properties of seabed soil such as saturation, permeability and shear modulus have different effects on the dynamic response and liquefaction potential of porous elastic seabed. The investigation of the dynamic response and liquefaction of the porous elastic seabed under partial standing ocean waves will help to better predict the influence of standing waves on breakwaters and seabed soil, and can provide some guidance for the design of offshore structures.

Influence of Rate Effects, Temperature, and Moisture Content on the Geomechanical Behavior of Marine Clays

Abstract The study of the geomechanical behavior of marine clays is a basic requirement for project optimization of the oil and natural gas industry. As part of a large-scale project, this study explored the effects of moisture content, temperature, and rate effects using laboratory vane shear tests. A series of results has helped in the identification of the effects of ground freezing on the undrained shear strength. For temperatures below freezing (−2.0 °C) the undrained shear strength increases with increasing water content and decreases with increasing shear rates for applied angular velocities in the range of 0.0025– 0.015 rpm. Apparently, with the freezing of pore fluids, the shear strength is partially governed by the strength of the ice-soil particle bonds. The increase in shear rate appears to facilitate the breakage of the ice bonds and afterwards the ice crystals reducing the viscous effects on the mobilized shear strength. Conversely, samples tested at temperatures above freezing show an increase in undrained shear strength with the increase in the imposed angular velocity and decreases with increasing soil moisture. Based on these studies, it is concluded that rate effects should be coupled to the influence of temperature and moisture content in the design of offshore structures.

Understanding the behaviour of a new robotic device for next-generation site investigation: A 3D DEM Exploration

Accelerating advancements in robotic engineering provide an opportunity for developing economical and efficient site characterisation devices, particularly for critical offshore applications. A new intelligent robotic device, ROBOCONE (Figure 1), is being developed for site characterisation and design of offshore structures, which would be capable of mimicking the load histories of the soil inside the ground at a desired depth [1]. The obtained soil constitutive properties would support an efficient design of offshore structures, considering “whole life” stress-strain histories. ROBOCONE could apply lateral, shear, and torsional modes (Figure 1(a)) of loading on the surrounding soil, operated remotely from the ground. In the pursuit of developing such a device, a prior understanding of its behaviour is essential. The lateral loading static and cyclic responses of offshore foundations are critical aspects of their design. The present work highlights the lateral module (P-Y Module) behaviour of the ROBOCONE device (Figure 1(a)) under various stresses, using 3D Discrete Element Modelling (DEM). The P-Y module moves horizontally at a desired depth in the ground and this lateral motion is numerically simulated using commercial DEM software, Particle Flow Code (PFC3D) [2].&#x0D; The proposed dimensions of the P-Y module are 200mm long and 54mm diameter (DRC). Present DEM model uses rigid walls as P-Y module placed in a rigid monodisperse 3D granular assembly with grain diameter of around 1/3rd DRC [3]. The Hertz contact model micro-parameters for grain-grain (E50 value of 27.4±2.2MPa) and grain-wall contacts are shown in Table 1. A 10 times higher shear modulus of grain-wall contact than the grain-grain contacts is sufficient to have stiffer response. The servo mechanism in PFC enables the model to reach desired average stresses. The primary objective of this work is to understand the ROBOCONE P-Y module behaviour and thus the lateral soil reaction curves without the influence of boundary. For the current testing mode, where the module moves laterally in the soil, the ratio of the moving object to soil bed dimensions for no boundary effects is unprecedented. A simple and innovative method of using periodic boundaries is proposed in the present study. Conventionally, periodic boundaries replicate the representative volume element infinitely at the boundaries. Due to the presence of ROBOCONE in the centre of the model, the required extent reduces to half in X and Y directions, as schematically shown in Figure 1b. This paper discusses the model side to ROBOCONE diameter, L/DRC, (e.g., 9, 18, 30) influence on the P-Y curves (at a given stress level). The effects of random particle generations for different models in PFC software can be minimized using same central core zone of contacts with ROBOCONE for each simulation, while the extent of model side is increased. ROBOCONE wished-in-place rather than driving/jacking in the model reduces the DEM computation costs. Limited amount of lateral displacement of the ROBOCONE (around 2.5mm) is sufficient to establish the effectiveness of the proposed modelling. Optimum loading rate (the lateral velocity of P-Y module) of around 0.8mm/s, which can maintain the inertial number lower than the threshold value of 7.9x10-5[4] further helps the computational efficiency.&#x0D; Once the optimum L/DRC value is obtained, the quantitative outcomes from these simulations are the lateral soil reaction curves at different stress levels and also the bending moments on the ROBOCONE P-Y Module. Though the ROBOCONE model in these simulations is a rigid wall and doesn’t mobilize in bending, the moments calculated from the magnitude and locations of each contact force on the P-Y module provide an estimate of bending stresses generated on the module. These results provide a basis for developing an understanding of the physical capacity of the robotic device in sustaining the lateral forces. Figure 1(c) shows preliminary results of soil reaction curve from the P-Y module at 500kPa stress level for L/DRC value of 9. The changes in the contact forces during the P-Y module motion can be qualitatively assessed through the changes in the force chain network (Figure 1(d)). Figure 1(e) shows a representative plot of bending moments calculated at different lateral movement stages of the P-Y module, and the maximum value at a desired displacement of the P-Y module suggests the physical design requirements of the ROBOCONE. The results from these DEM simulations would directly inform the design of the robotic device for ground investigation and hence improve the economical offshore foundation designs.

A reliability-based design and optimization strategy using a novel MPP searching method for maritime engineering structures

PurposeIn order to solve the problems faced by First Order Reliability Method (FORM) and First Order Saddlepoint Approximation (FOSA) in structural reliability optimization, this paper aims to propose a new Reliability-based Design Optimization (RBDO) strategy for offshore engineering structures based on Original Probabilistic Model (OPM) decoupling strategy. The application of this innovative technique to other maritime structures has the potential to substantially improve their design process by optimizing cost and enhancing structural reliability.Design/methodology/approachIn the strategy proposed by this paper, sequential optimization and reliability assessment method and surrogate model are used to improve the efficiency for solving RBDO. The strategy is applied to the analysis of two marine engineering structure cases of ship cargo hold structure and frame ring of underwater skirt pile gripper. The effectiveness of the method is proved by comparing the original design and the optimized results.FindingsIn this paper, the proposed new RBDO strategy is used to optimize the design of the ship cargo hold structure and the frame ring of the underwater skirt pile gripper. According to the results obtained, compared with the original design, the structure of optimization design has better reliability and stability, and reduces the risk of failure. This optimization can also better balance the relationship between performance and cost. Therefore, it is recommended for related RBDO problems in the field of marine engineering.Originality/valueIn view of the limitations of FORM and FOSA that may produce multiple MPPs for a single performance function, the new RBDO strategy proposed in this study provides valuable insights and robust methods for the optimization design of offshore engineering structures. It emphasizes the importance of combining advanced MPP search technology and integrating SORA and surrogate models to achieve more economical and reliable design.

Reducing the uncertainty of ULS load estimates in offshore structural design

The design process of any offshore structure, including Wave Energy Converters (WECs), aims to assess a series of critical conditions, often encapsulated in the concept of ‘limit states’, that define the ‘design boarders’ across which a structure is unlikely to respond satisfactorily, potentially leading to functional failure(s). The Load and Resistance Factor Design (LRFD) method aims to quantify such ‘design boarders’ with adequate safety margins on both the loading and the resistance sides of the design inequality. An inherent risk of such approach is that the definition of ‘adequate’ may be clouded by multiple sources of uncertainty, making it challenging to assess if a system is likely to be under- or over-designed from inception.&#x0D; Although overall guidance for the design process of a WEC can be sought from related industries – see e.g. [1], and while noting that generic guidance for WEC design is available in e.g. [2], there is at present limited (practical) guidance on how to derive estimates of long-term return period loads. In recent work, comparative assessments of different load estimation methods have been made, illustrating the dependence of the outputs on the underlying method – see e.g. [3]. Additionally, and in connection with the assessment of extreme loads acting on WECs, the load post-processing methodology was identified in [4] as the major contributor to the uncertainty in Ultimate Limit State (ULS) load estimates.&#x0D; This paper presents a novel post-processing methodology to assess the uncertainty when estimating ULS loads acting on an offshore structure. To assess which statistical distributions are best suited to estimate long-term extreme loads, goodness-of-fit tests were performed using a series of input load time-series. The methodology was applied to evaluate the 50-year return load acting on the foundation of a generic Submerged Pressure Differential (SPD) WEC, based on the generic configuration originally defined in [5]. Results suggest that current conventional practices based on visual inspection(s) may lead to the selection of non-representative fitting functions which, in turn, likely lead to inaccurate extreme load estimations.&#x0D; Ultimately, the methodology described in this paper aims to contribute to a probabilistic approach to the definition of suitable safety factors, which in turn is expected to reduce the uncertainty in key design metrics and the risk of either under- or over-design.&#x0D; This study was conducted as part of the H2020 VALID and H2020 IMPACT projects.&#x0D; References&#x0D; [1] DNVGL-ST-0437. (2016). Loads and site conditions for wind turbines.&#x0D; [2] IEC/TS 62600-2. (2019). Marine energy — Wave, tidal and other water current converters Part 2: Design requirements for marine energy systems.&#x0D; [3] Michelen, C., Coe, R. Comparison of Methods for Estimating Short-Term Extreme Response of Wave Energy Converters, Sandia National Laboratories, SAND2015-6890C.&#x0D; [4] Atcheson, M., Cruz, J., Martins, T., Johannesson, P., Svensson, T. (2019). Quantification of load uncertainties in the design process of a WEC, Proc. 13th European Wave and Tidal Energy Conference (EWTEC 2019), Naples, Italy.&#x0D; [5] Babarit, A., Hals, J., Muliawan, M.J. et al. (2012). Numerical benchmarking study of a selection of wave energy converters. Renewable Energy 41, 44-63.

Long-term observations (2014–2020) of level ice draft, keel depth and ridge frequency in the Barents Sea

Results from consistent long-term observations of sea ice draft by upward looking sonars at five sites in the Barents Sea during the period 2014–2020 are presented. The recordings were initiated by the Barents Sea Metocean and Ice Network (BaSMIN) with the objective to provide data for design of offshore structures in Norwegian oil and gas license areas. Sea ice imported from the Arctic Ocean, ocean heat of Atlantic origin as well as dynamic deformation contribute to spatial and temporal variability in drafts. The average draft of locally grown level ice at the end of winter is approximately 0.6 m. Reflecting a decreasing trend in ocean temperatures at the observation site, the timeseries of locally grown level ice draft exhibits an increasing trend during the observation period of approximately 5 cm per year. During events of sea ice drifting in from the north, the level ice draft reaches up to 1.5–1.8 m. Keels deeper than 2.5 m have an average keel depth of 3.8 m. The 99th percentile keel depth is 9.4 m, while the deepest keels protrude down to nearly 20 m. The average ridge frequency is 3.6 ridges/km, with a corresponding 99th percentile of 25 ridges/km. Sea ice on Spitsbergenbanken has deeper keels and larger frequency of ridges than sea ice on Storbanken at the northern end of the Hopen Trench. We hypothesize that more vigorous currents with convergence and shear on the shallower Spitsbergenbanken contribute to this geographical difference.

Effects of constitutive soil models on the seismic response of an offshore jacket platform in clay by considering pile-soil-structure interaction

Offshore jacket platforms are widely used for wind energy generation and gas and oil field development. The South Pars gas field, located in the Persian Gulf, stands as one of the largest gas fields in the world. This gas field is situated in a seismically active region predominantly characterized by clay soil. Considering the significant impact of clay soil on the seismic response of offshore structures and their vulnerability to damage, it is essential to conduct comprehensive studies on the seismic behavior of offshore structures in this particular area while considering the nonlinear behavior of soil and pile-soil-structure interaction (PSSI). However, due to the laboratory limitations, it is difficult to experimentally investigate the seismic behavior of these structures on a real scale by considering the actual soil environment. This paper aims to adopt a nonlinear kinematic hardening model for clay in Abaqus software and calibrate this model parameters for the South Pars gas field region to carefully investigate the effects of nonlinear behavior of soil and PSSI on the seismic response of an offshore jacket platform. In addition, the perfectly elasto-plastic Mohr-Coulomb model is employed as a simple and common constitutive soil model to compare the results. To achieve the study goals, firstly, the numerical modeling techniques are validated. Subsequently, the main parameters of the nonlinear kinematic hardening model are calibrated for the South Pars gas field region, which has not been done for this region in previous works. Detailed numerical models, including the fixed-base model and the pile-soil-structure (PSS) model, are then developed and analyzed using different selected ground motion records. The results indicate that considering the effect of PSSI has a significant effect on the seismic responses of the structure. Moreover, PSSI effect has a great dependence on the constitutive soil model. The results of this study are expected to provide useful insights into the seismic design of offshore structures in similar geological conditions.