Ocean Engineering

Journal: Ocean Engineering

Abbreviation

Ocean eng.

Publisher

Elsevier

ISSN

0029-8018

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Publications 1 - 10 of 11

Effect of pipeline-seabed gaps on the vertical forces of a pipeline induced by submarine slide impact
Fan, Ning; Sahdi, Fauzan; Zhang, Wangcheng; et al. (2021)
Ocean Engineering
In recent years, the interaction between submarine slide runout and offshore pipelines has received extensive attention, based on the need to protect pipelines crossing regions susceptible to submarine slides. The interaction force between the slide material and a pipeline may be resolved into horizontal and vertical components, but with most attention to date focusing on the former while the latter has not been studied thoroughly. This paper reports the results from a series of multiphase flow (slide material and the ambient seawater) numerical simulations using a computational fluid dynamics (CFD) approach aimed at investigating the vertical forces during submarine slide flow around a pipeline at different gap ratios (pipeline-seabed gap normalized by pipeline diameter) from 0.08 to 10.0 and at Reynolds numbers ranging from 0.38 to 267. The oscillatory characteristics and magnitude of the vertical forces for different gap ratios are discussed. The effects of the gap ratios on the geotechnical and inertial components of the slide-pipeline vertical forces are analyzed systematically using a hybrid geotechnical-fluid mechanics framework. From the results of the CFD simulations, a modified hybrid geotechnical-fluid mechanics method to estimate the slide-pipeline vertical forces is developed considering the effects of pipeline-seabed gaps. © 2020 Elsevier
Evaluation of full scale shear performance of tension anchor foundations: Load displacement curves and failure criteria
Bergamo, Paolo; Donohue, Shane; Callan, Damian; et al. (2017)
Ocean Engineering
Higher-order gap resonance and heave response of two side-by-side barges under Stokes and cnoidal waves
Ding, Yunfeng; Walther, Jens Honoré; Shao, Yanlin (2022)
Ocean Engineering
Coupled piston-mode fluid response and the heave motion of two identical barges in side-by-side configuration is studied under finite-depth and shallow-water waves using a two-dimensional fully nonlinear numerical wave tank. To understand possible critical responses of the gap flow and the floating barge, regular-wave conditions which are able to excite up to 5th-order nonlinear gap resonance and also the resonant heave motion of the barge are considered. In shallow-water waves, high-frequency oscillations, featured by secondary peaks in the time histories, are observed for both wave elevation in the gap and the heave motion of the barge. The shallow-water wave-induced 4th- or 5th-order gap resonance can be equally crucial as the 1st- and 2nd-order resonances due to finite-depth waves. At higher-order gap resonance, the higher-harmonic heave motion of the barge is negligibly small, in contrast to the gap-flow response. Compared with fixed barges, the free-heave motion of an upstream barge tends to increase the wave elevation in the gap in most of the resonant conditions, except at 1st-order gap resonance where the gap response is greatly reduced. When the resonant heave motion of a floating barge, either located upstream or downstream, is excited, significant barge motion is observed. However, the relative motion between the gap flow and the floating barge is seen to be very small, ascribed by small phase difference between the two. The present study suggests that the effects of heave motion and water-depth should be carefully considered in the design of side-by-side marine operations, and hiding the small bunkering ships behind the large receiving ships is regarded as a preferred arrangement during the bunkering operations in offshore and coastal environments.
Impact of initial slide on downslope seafloor with a weak layer
Han, Yunrui; Zhang, Wangcheng; Yu, Long; et al. (2024)
Ocean Engineering
Submarine landslides are a major offshore geohazard posing increasing threat to offshore and coastal developments. A submarine landslide may usually involve multistage failure triggered by the first failure. During submarine landslide evolution, the emergent sliding mass may act as an impact load on the downslope seafloor potentially leading to surficial erosion of sediments or deeper ploughing failure with the presence of a weak layer. The study presents a comprehensive analysis of seafloor instability and post-failure behaviors subjected to the first slide mass impact, using a large deformation finite element (LDFE) method. Different failure patterns are investigated and the corresponding critical impact loads for triggering the seafloor instability are analyzed. The parameters controlling the strain softening behaviors of soils and the undrained shear strength in the weak layer have significant effects on the critical condition for the seafloor failure. The dynamic inertia of the slide mass, however, has little effect on the results. An empirical equation for assessing the seafloor instability subjected to the first slide impact, with respect to the impact load from the initial slide and the gravity stress relative to the shear strength in the weak layer, is proposed based on a parametric study.
Offshore renewable energies: A review towards Floating Modular Energy Islands—Monitoring, Loads, Modelling and Control
Marino, Enzo; Gkantou, Michaela; Malekjafarian, Abdollah; et al. (2024)
Ocean Engineering
Floating Modular Energy Islands (FMEIs) are modularized, interconnected floating structures that function together to produce, store, possibly convert and transport renewable energy. Recent technological advancements in the offshore energy sector indicate that the concept of floating offshore energy islands has the potential to become more cost-effective and more widespread than previously anticipated. This review is specifically meant as a basis for the development of new approaches to the sustainable exploitation of multi-energy sources in the offshore environment leveraging the know-how of existing technologies and, at the same time, exploring new solutions for the specific challenges of FMEIs. The paper critically analyzes the current state of data-driven approaches and structural health monitoring techniques in the offshore energy sector. It also covers topics such as met-ocean data, load estimation, platform dynamics, coupling actions, nonlinear dynamics of mooring lines, modelling considerations, and control of electrical subsystems. It is believed that this systematic and multidisciplinary review will facilitate synergies and further enhance research and development of offshore renewable energies.
Three-dimensional modelling of cavitation bubble collapse using non-orthogonal multiple-relaxation-time lattice Boltzmann method
Peng, Haonan; Fei, Linlin; He, Xiaolong; et al. (2024)
Ocean Engineering
In this study, we employ a three-dimensional (3D) non-orthogonal multiple relaxation time (MRT) pseudo-potential lattice Boltzmann (LB) model to simulate the dynamics of cavitation bubble evolution. We benchmark the model against the Laplace law and the Rayleigh–Plesset (R–P) equation, confirming its efficacy in accurately capturing cavitation phenomena. We then apply the model to examine the collapse dynamics of a singular bubble located near a plane wall boundary and right-angled wall corner. Additionally, the dynamic interactions among five cross-shaped bubbles revealed the dimensionless jet volume Vj*, which is the ratio of the jet volume to the maximum bubble volume, exhibits a power relationship with the bubble distance δ. The simulation results demonstrate the accuracy of the model in discerning the effect of the wall boundary and the protective mechanisms inherent to multi-bubble interactions. These results further validate the aptness of the model for cavitation bubble dynamics simulations. Moreover, the tested case studies provide a foundational basis for future research into more complex cavitation behaviours. In summary, the developed 3D non-orthogonal MRT pseudo-potential LB model is capable of reproducing fluid flows, capturing pressure waves, and measuring wall pressures. Our work provides a deep insight into cavitation bubble dynamics and a solid basis for both applied and fundamental research.
High-order shallow water expansions in free surface flows: Application to steady overflow processes
Castro-Orgaz, Oscar; Cantero-Chinchilla, Francisco N.; Hager, Willi H. (2022)
Ocean Engineering
Free surface flow modeling in the natural environment is frequently conducted within a depth-integrated framework using the non-linear shallow water equations or Saint-Venant equations. However, these equations underestimate the discharge in overflow processes over obstacles, such as during the flooding of coastal levees. Alternative models are constructed by expansions in terms of a shallowness parameter σ, with the 2nd-order results being the so-called Serre-Green-Naghdi theory, widely used in ocean research. Higher-order expansions are required in some instances to obtain more accurate results or to set the validity limit of the leading order term of these perturbation results. However, most of the high-order results available apply to water wave problems over horizontal beds, yet not to the overflow over obstacles of generally uneven bathymetry. High-order shallow water expansions for open channel flows over uneven beds are developed in this work by an iterative procedure to generate the corresponding asymptotic expansions up to the high-order O(σ6), resulting in high-order ODEs when the Bernoulli equation at the free surface streamline is invoked to include gravity effects. These new equations are applied to the overflow process, as occurs during the coastal flooding in a levee-protected area. A perturbation solution to this problem is presented up to O(σ6) for the main overflow variables, which are successfully and systematically compared with a new set of experiments conducted in a large scale obstacle model. The novel theoretical solutions presented are further demonstrated to be better than former theories available in the literature.
Multi-objective optimization of a negative stiffness vibration control system for offshore wind turbines
Kapasakalis, Konstantinos A.; Gkikakis, Antonios Emmanouil; Sapountzakis, Evangelos J.; et al. (2024)
Ocean Engineering
A systematic multi-objective optimization approach is presented for designing vibration control systems for monopile Offshore Wind Turbines (OWT) under the combined actions of wind and wave. An extended configuration of the KDamper (EKD) is employed, with second-order tower phenomena and soil–structure interaction effects taken into account. A holistic approach is employed to design an optimal EKD by considering competing objectives and limitations, such as engineering requirements, manufacturing specifications, available budget, and uncertainties in the design parameters. A global sensitivity analysis is conducted to identify the influence of each parameter of the OWT-EKD system on performance. Subsequently, a global multi-objective optimization is employed to explore the trade-off between conflicting objectives (Pareto front) and demonstrate that the EKD effectively operates even without specific components (e.g., dampers). In accounting for the uncertainties involved, such as manufacturing tolerances and environmental conditions, a robust analysis on a bi-objective Pareto front is performed, leading to the selection of EKD designs that exhibit superior average performance. The numerical results demonstrate the improvement in the peak tower dynamic response and the effective damping compared to a conventional Tuned Mass Damper comprising 20 times higher mass.
Offshore foundations in low-plasticity cohesive soils: Cyclic degradation experimental evidence and simplified numerical analysis
Antoniou, Maria; Gelagoti, Fani; Herzog, Ralf; et al. (2024)
Ocean Engineering
The paper studies the effect of soil strength and stiffness degradation on the undrained cyclic performance of offshore foundations in low-plasticity cohesive soil using 3D finite element modelling. Cyclic triaxial tests on reconstituted kaolin are conducted at the ETH Zurich laboratory, providing insights into key parameters affecting the degradation process. A simplified soil constitutive model accounting for cyclic degradation is developed and encoded in Abaqus via a user subroutine. The model is calibrated against experimental results and validated with published centrifuge model tests of monopiles under cyclic lateral loading. It is subsequently used to evaluate the performance of suction caisson foundations with different aspect ratios (L/D = 0.5 and 2) under short-term cyclic and seismic loading. Due to its ductile resistance mechanism, the L/D = 0.5 caisson exhibits superior performance under vertical cyclic loading in fast-degrading soil. Under inclined cyclic loading, the slower degradation rate of the L/D = 2 caisson governs response, reversing the trend. Under seismic shaking, the degradation-induced resistance imbalance amplifies the irrecoverable settlements produced by kinematic shearing at the caisson sidewalls. For the fast-degrading soil examined, degradation is shown to increase settlements by up to 50%.
Convex modelling for ship speed optimisation
van Dooren, Stijn; Duhr, Pol; Onder, Christopher H. (2023)
Ocean Engineering
Optimising the ship speed depending on the forecast oceanic and atmospheric conditions is an effective operational measure to reduce both fuel consumption and carbon dioxide emissions. In the literature, this optimal control problem (OCP) has been formulated using different types of mathematical models and solved using various optimisation methods. In this paper, we use convex functions to reformulate the models and cast the OCP as a convex optimisation problem. This type of problem can be solved very efficiently and yields the globally optimal solution. As a first step, we assume that the environmental conditions do not vary with time and only depend on the distance along the route. For this case, we show that the convex reformulation of the OCP is accurate and the computation time to solve it is low. Second, we introduce an iterative method to solve the problem under time-varying conditions, drawing inspiration from the literature on hybrid electric vehicles. Whilst this method converges quickly, it does not converge to the global optimum, which was calculated using dynamic programming. We present an artificial scenario to explain why convergence to the global optimum is not guaranteed. Finally, we discuss the implications of our findings for potential future research on this topic.

Publications 1 - 10 of 11

Journal: Ocean Engineering

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