This paper presents improved numerical methods to predict slamming forces/pressures on a wedge with different tilted angles and on a ship section with various drop heights. The water-entry problem was solved by using a constrained interpolation profile (CIP) method on a fixed Cartesian grid. Both incompressible and compressible solvers and three interface capturing schemes were used to examine their effects on the solutions. Effects of prescribed and free fall motions on the solution were investigated. Convergence studies were carried out using various domain sizes, grid sizes, and time steps. The grid convergence index was employed to estimate the uncertainties due to spatial discretization. Validation studies were performed by comparing numerical solutions with experimental data.
The water-entry or slamming problem is highly nonlinear since it involves breaking water surfaces and air bubbles. The water-entry problem of wedges has been extensively studied by many researchers. The theoretical analysis of the similarity flow induced by the wedge entry was pioneered by Von Karman (1929) and Wagner (1932). A review of earlier research on the theoretical analysis of water-entry problems was given by Korobkin and Pukhnachov (1988).
Various numerical methods have also been developed to address water-entry problems. The potential flow theory has been applied to solve water-entry problems of wedges, for example, by Vinje and Brevig (1981), Greenhow (1987), Zhao and Faltinsen (1993), and Zhao et al. (1996). Chuang et al. (2006) developed a boundary element method (BEM) based on the desingularized Cauchy formula and removed the corner singularity at the intersection point of body and water surface. Xu et al. (2008) simulated oblique water entry of an asymmetrical wedge based on a BEM with an analytical solution for the jet. Xu and Wu (2015) used a BEM with vortex shedding to simulate the oblique water entry of a wedge. The pressure jump was addressed by imposing the Kutta condition at the wedge apex. Bao et al. (2016, 2017) studied the oblique water entry of a 2-D wedge with prescribed and free fall motions. Wang and Faltinsen (2017) improved the numerical method proposed by Zhao and Faltinsen (1993) and presented the results for wedges with small dead-rise angles. Although the water-entry problem can be solved successfully in various degrees by the potential flow theory, there are difficulties in treating distorted free surfaces. Special treatments are usually needed, for example, for detached flow jets. Most of the studies based on the potential flow theory are limited to short-time simulations and simple 2-D geometries.
Measured post-storm beach recovery and swash velocities were simulated using the process-based, depth-averaged 1-D cross-shore numerical model CSHORE over 30 tide cycles. The field data were collected during a three-week experiment along the Delaware coast of the United States following a nor’easter storm. A pronounced ridge formed immediately after the storm and rapidly grew in height and shoreward extent. The model was able to capture the ridge accretion process (crest location, elevation, and ridge slope) and time-averaged swash flows accurately once sediment transport related parameters were calibrated. In addition to sensitivity analyses of the most important numerical parameters, profile evolution and swash velocity results are presented.
Intertidal sand bars are a common morphological feature often observed along sandy coasts with micro- and mesotidal beaches. The formation and dynamic behavior of bars, or bar and trough systems, located in the surf and swash zones of the beach are controlled by a variety of morphological and hydrodynamic characteristics of the surrounding environment. The dynamic evolution of these features is thought to play a critical role in beach recovery mechanisms after erosive storm events. In particular, the ridge and runnel (RR) type of swash bar is a single bar and trough system, asymmetric in shape and generally found in the upper intertidal zone on sandy beaches subject to relatively small tidal ranges (<3 m; Kroon and Masselink, 2002). Wave runup coinciding with the rising storm tide carries a large amount of sediment landward that is left in the inner swash zone, building a pronounced berm face, or a ridge, followed by the landward slip-faced dipping, or runnel (King and Williams, 1949). The formation and onshore migration of RR-type bars is linked to beach recovery processes since the onshore migration of a (partially) submerged nearshore bar during low-energy wave conditions following a storm can carry large amounts of sediment back up on the subaerial portion of the beach.
The ratio of soil volume driven into a suction pile to total soil displaced by the pile (soil heave ratio) has a direct impact on the external skin resistance and pile length. Recent observations have indicated that the common assumptions of this ratio for self-weight and under-suction installations (50% and 100%, respectively) may overestimate the soil heave inside the pile. In this paper, an equivalent finite element model was used to simulate the fine-grained, normally consolidated sediment behavior adjacent to the pile tip. Afterward a method was proposed to estimate the soil heave ratio considering tip geometry effects. Finally, the method was validated by a field case and other studies.
A suction pile (also called “suction anchor” or “suction caisson”), which is among the most frequently used offshore foundation systems, is an open-ended and closed-top pipe pile. It is partially installed by its own weight followed by insertion to the final depth by applying suction inside the pile. The use of internal and external stiffeners allows the application of relatively high ratios of diameter to wall thickness (d=t) compared with ordinary driven pipe piles (Randolph, 2003).
During installation, a certain amount of soil is displaced by the pile wall. Despite the high diameter-to-wall-thickness ratio (d=t) of suction piles, the amount of displaced soil can be considerable because of the large dimensions of these piles. Hereafter, the ratio of soil volume driven into the pile to the total volume of the displaced soil is called the SHR (soil heave ratio). Andersen and Jostad (2002) indicate that the amount of soil pushed outward from a pipe pile would increase the external skin friction. This alone shows the importance of a correct SHR estimation, as the external wall capacity is approximately 40%–50% of the total transient capacity for a pile with an aspect ratio of approximately 5, and this ratio is even higher when the reversed end-bearing mechanism is not activated and the pullout capacity rules the failure load (Clukey et al., 2004). In addition, as a portion of the displaced soil is pushed inside the pile, the final lengths of the suction piles are always longer than the desired embedment depth imposed by pullout capacity requirements. Therefore, any probable source that can lead to a higher SHR than predicted should be addressed carefully to avoid pile installation failure at the site.
With the advent of higher steel grades for offshore pipelines and the reliance of the UOE forming process on trial and error, knowing the final yield strength of the pipe beforehand would be beneficial in terms of time and cost. However, predicting the yield strength of the UOE pipe constitutes a difficult task because of the alteration of the material properties throughout the forming process. Moreover, the yield strength is measured by tensile test executed on specimens obtained by flattening samples cut from the formed pipe, but this flattening process also alters the properties of the material. Accordingly, this study presents a 2-D finite element method (FEM) program considering both forming and flattening processes to predict the yield strength of the UOE pipe measured by tensile test. The results show that the simulation predicts the yield strength with good accuracy.
UOE forming is a popular method used for the production of longitudinally welded thick-wall pipes. The longitudinal edges of the steel plate are first crimped, and the beveled plates are then formed into a U shape using a U-press followed by O-ing using an O-press. After welding of the edges, the formed pipe is expanded by applying internal pressure to improve the cross-sectional ovality and to relieve the residual stress on the pipe wall (Yi et al., 2017).
Throughout these forming stages, the steel plate undergoes a series of plastic deformations that alter its material properties and endow each fiber located along the circumferential direction with a unique strain hysteresis (Kyriakides et al., 1991; Kyriakides and Corona, 2007). Among the material properties, the yield strength is a key design parameter that can be measured by tensile test on flattened samples cut from the formed pipe. This yield strength is also an indicator of the pipe quality specification in the API Standard (American Petroleum Institute, 2012).
The discrete-finite element coupling method is an effective approach to simulate the complex interactions between sea ice and offshore structures and ice-induced vibrations (IIVs) of structures. However, the small time step in the discrete element method, as the time step of the coupled method, is time-consuming. Adoption of a time multiscale strategy can solve this problem. This paper proposes a coupled discrete-finite element method based on a domain decomposition method to analyze the interactions between sea ice and a conical jacket platform. Moreover, IIVs of the platform were analyzed. The computational domain is split into several subdomains based on whether sea ice interacts with the platform. The subdomains directly impacted by sea ice use small time steps of the discrete element method. The numerical results show that the proposed time-efficient method is reliable and stable for the simulations of ice-platform interactions.
In cold regions, the vibrations of offshore platforms induced by sea ice can be harmful for not only the routine production but also the serviceability and safety of platforms. Conical jacket platforms have been used considerably in the Bohai Sea of China. The forces induced by sea ice are the dominant environment loads acting on the platforms. Ice-induced vibrations (IIVs) of platforms have also been reported by Yue et al. (2009).
To overcome IIVs of platforms, some beneficial work including field measurements, model tests, and numerical simulations has been conducted on the interactions between sea ice and offshore platforms (Huang et al., 2013; Nord et al., 2015). Because field and scale tests are difficult and expensive, numerical simulations are usually adopted for investigating the dynamic behaviors of offshore platforms under ice loads (Hopkins, 1997; Paavilainen and Tuhkuri, 2013). Kärnä and Turunen (1989) calculated the IIVs of a narrow structure by assuming ice load to be a function of the relative displacement and relative velocity between ice and the structure. The finite element method (FEM) has also been utilized in ice load analyses in which the sea ice is approximated using the material’s nonlinear model (Sand and Fransson, 2006). However, the continuum-based FEM is limited by the inherently discrete nature of sea ice, especially in the case of floe ice.
An experiment is performed to demonstrate water-wave cloaking of a surface-piercing cylinder by an array of eight surrounding cylinders. The objective is to confirm cloaking for one wave number (5 rad m.1) and steepness (305%) by measuring the second-order mean drift force on the inner cylinder and the far-field surface elevation. The influences of the tank walls and viscous forces are explored, and uncertainties in the measured quantities are evaluated. A geometry is selected using a linear potential-flow solver coupled with an optimizer, and an apparatus is built for tank testing. For the configuration tested, tank-wall effects are important, but viscous forces and load-cell cross talk effects are small. Elimination of the measured second-order mean drift force is observed with the addition of the outer cylinders. Experimental wave amplitudes are in agreement with numerical predictions at almost all measurement points.
Invisibility has long captured the public imagination. Over the last decade it has also drawn the interest of the scientific community, with researchers in various fields pursuing methods of concealing objects from distant observers. Pendry et al. (2006) proposed a method of deflecting electromagnetic waves around a hidden cloaked volume by manipulating the properties of a surrounding cloaking region. Cloaking has since been theoretically investigated and experimentally demonstrated for wave phenomena including microwaves (Schurig et al., 2006), and elastic waves in solids (Milton et al., 2006).
When applied to surface water waves, cloaking describes a reduction in the far-field scattering that occurs when a wave interacts with a fixed structure. Perfect cloaking occurs when scattering is eliminated and the structure’s presence is not revealed by diffracted waves. In the general uncloaked case, the amplitude of scattered waves can be related by momentum conservation to the time-averaged, second-order mean drift force exerted on the body. This relationship dictates that the mean drift force is zero if there are no scattered waves. Such elimination of the second-order mean drift force may be of practical use in reducing the size of moorings for large offshore structures.
Aggarwal, Ankit (Norwegian University of Science and Technology) | Chella, Mayilvahanan Alagan (Norwegian University of Science and Technology) | Bihs, Hans (Norwegian University of Science and Technology) | Pákzodi, Csaba (SINTEF Ocean) | Berthelsen, Petter Andreas (SINTEF Ocean) | Arntsen, Øivind A. (Norwegian University of Science and Technology)
Offshore structures are exposed to irregular sea states consisting of breaking and nonbreaking waves. They perpetually experience extreme wave loads after installation in the open ocean. Thus, the study of steep waves is an important factor in the design of offshore structures. In the present study, a numerical investigation is performed to study steep irregular waves in deep water. The irregular waves are generated using the Torsethaugen spectrum, which is a double-peaked spectrum defined for a locally fully developed sea and which takes both the sea and swell waves into account. Thus, the generated waves can be very steep. The numerical investigation of such steep waves is quite challenging because of their high wave steepness and wave–wave interaction. The present investigation is performed using the open-source computational fluid dynamics (CFD) model. The wave generation and propagation of steep irregular waves in the numerical model are validated by comparing the numerical wave spectrum with the experimental input wave spectrum. The numerical results are in good agreement with experimental results. The changes in the spectral wave density during the wave propagation are studied. Further, the double-hinged flap wavemaker is also tested and validated by comparing the numerical and experimental free-surface elevations over time. The time and the frequency domain analysis is also performed to investigate the changes in the free-surface horizontal velocity. Complex flow features during the wave propagation are well captured by the CFD model.
Offshore wind turbines are exposed to extreme irregular sea states. Extreme waves exert extreme hydrodynamic loads on substructures. Thus, the study of such irregular waves is very important in the design of offshore wind turbines. Several experimental and field investigations have been performed in the past to study extreme waves. Such spectra exhibit two peaks, because of the presence of swell and wind waves. Ochi and Hubble (1976) carried out a statistical analysis of 800 measured wave spectra in the North Atlantic Ocean. They derived a six-parameter double-peaked spectrum that is composed of two parts: the first primarily includes the low-frequency wave components and the second contains the high-frequency wave components. Each part of the wave spectrum is represented by three parameters. The six-parameter spectrum represents almost all stages of the sea conditions associated with a storm. Guedes Soares and Nolasco (1992) analyzed wave data from the North Atlantic and the North Sea and proposed a four-parameter double-peaked spectrum. This double-peaked spectrum was formulated by superimposing individual spectral components of the JONSWAP-type single-peaked spectrum.
The objective of the present study is to investigate the ultimate hull girder strength of an asymmetrically damaged ship under a sagging condition. Two kinds of ships are taken as the object of analysis. The cross section of the ship is considered and assumed to have remained plane. The simply supported boundary condition is applied to both sides of the cross-section. The ultimate hull girder strength is attained when a plate and/or stiffened plate element at the specified location called “critical element” reach its ultimate strength. To investigate the ultimate hull girder strength, a simplified approach is proposed. The result obtained by the simplified approach is compared with the progressive collapse analysis to determine effectiveness and for validation.
The ability to predict accurately the ultimate strength of ship hull girder when subjected to longitudinal bending is one of the most important aspects of ship structural design. Collision and grounding damage may take place on the ship’s hull, which may threaten safety of ships and surrounding environment. In this regard, to enhance the safety of ship’s structure and minimize the risks, the International Maritime Organization (IMO, 2009) has required in Goal Based Standard for New Ship Construction (GBS) to consider the residual strength of the hull girder in specified damage conditions as one of the functional requirements for the structural rules for bulk carriers and tankers.
Many studies have been conducted on the analyses of the residual hull girder strength as a result of collision and grounding damages. Pedersen (1994) presented a mathematical model to estimate the contact pressure between the grounded ship and the sea bottom. The grounding contact force was compared with the force that would crush the forward bottom of the ship. The sectional bending moment due to grounding was determined and compared with the ultimate hull girder strengths. The model experiments and full-scale controlled grounding experiments were also performed to validate the mathematical model. Paik et al. (1998) developed a rapid procedure to identify the possibility of hull girder failure after collision and grounding damages based on the closed-form formulae of the ultimate hull girder strength and section modulus after the damages. Guedes Soares et al. (2008) evaluated the ability of simplified structural analysis methods based on Smith’s formulation to predict the ultimate strength of damaged ship’s hull. Muis Alie et al. (2016) investigated the influence of the superstructure on the ultimate strength of Ro-Ro ship under a vertical bending moment. The cross section was considered to be analyzed. The results obtained by beam theory with and without superstructure were compared with one another.
Ship-mounted cranes are used widely in the transportation and installation of heavy loads at sea. To minimize the sway motion induced by the harsh environment, the cranes are equipped with antisway-compensation equipment. To effectively test the feasibility of the antisway algorithms at the early design stage, the hardware-in-the-loop simulation (HILS) technique can efficiently be used for the proposed technique of this study. In this study, it is applied to the example of the antisway control of a crane on an offshore support vessel during the installation operation of subsea equipment using the HILS.
The cranes on an offshore support vessel (OSV) are used for offshore transportation and the installation of subsea equipment at sea (Hong et al., 2016). An OSV-mounted knuckle boom crane is shown in Fig. 1. The knuckle boom crane can perform various tasks, as it is characterized by the design of a folded knuckle that is attached to an extension rod.
During the installation operation, the sway motion of the suspended load (e.g., subsea equipment) is inevitable. In addition, the wind at sea can intensify the sway motion. Meanwhile, the waves induce the OSV motion, and the OSV motion induces motions of the suspended load directly or indirectly. In this situation, the installation operation by the crane sometimes pauses under harsh environmental conditions to avoid above-deck collisions, those between the suspended load and the ship structure, or those between the suspended load and the crane boom (Jeong et al., 2016). Therefore, a controller with a suitable antisway control algorithm is necessary to significantly reduce the residual sway motion of the suspended load.
In most cases, the reduction of the sway motion is achieved by the crane control. Abdel-Rahman et al. (2003) provided a well-classified review of the crane control. Gjelstenli (2012) used the cascade control method to solve the antisway problem for the offshore crane. More recently, Ramli et al. (2017) also conducted a comprehensive review of the control strategies for different crane types. Most of the researchers concentrated on the overhead crane, tower crane, and boom crane. Abe et al. (2011) used radial basis function networks for the trajectory planning of overhead cranes and reduced the payload sway motion. In addition, an experiment was conducted to verify the proposed controller.
Large deformation finite element analyses were performed to study the undrained vertical bearing capacity of subsea pipelines installed on a surficial high-shear-strength crust overlying a soft clay deposit. A detailed parametric study was carried out for different combinations of shear strength ratio of crust to underlying soil, crust thickness, and strength heterogeneity of the underlying soil. Results show that the presence of a crustal layer significantly influences the vertical penetration response of pipelines. Simple relationships are developed to predict the maximum penetration resistance and the displacement required to mobilize the maximum resistance for the assessment of punch-through failure.
As the offshore industry is exploring hydrocarbon reserves in remote geographic locations, a thin layer of crust exhibiting an abnormally high shear strength overlying a normally consolidated soft clay deposit is often encountered. The high shear strength of the crust can be attributed to the biochemical activities of microorganisms living in the vicinity of the seabed (Kuo and Bolton, 2013). Deepwater subsea pipelines carrying hydrocarbons at very high temperatures and pressures are often directly installed on this crustal sediment, without any trenching or additional protection. Self-weight and dynamic oscillations of laying vessels (Randolph and White, 2008a) lead to partial embedment of these pipelines. The vertical penetration of the pipe during the installation process is generally an undrained phenomenon involving the formation of soil heave around the pipe. Sudden reduction in the bearing capacity of the soft soil underneath the crust could lead to the possibility of punch-through failure during installation. Thus, it is essential to accurately assess the penetration response of pipelines installed on a surficial crust for the design of on-bottom submarine pipelines.
In the recent past, researchers have conducted several studies on the vertical bearing capacity of subsea pipelines using either classical plasticity theory (Randolph and White, 2008b; Martin and White, 2012) or small strain finite element analysis (Aubeny et al., 2005; Merifield et al., 2008, 2009). Wang et al. (2010) and Chatterjee (2012) performed finite element analyses adopting the large deformation approach to simulate the change in seabed geometry and formation of soil heave around the pipe during penetration. Experimental studies in geotechnical centrifuge (Dingle et al., 2008; White and Dingle, 2011) were also carried out to understand the pipe–soil interaction, although they are limited to specific cases. Very few studies available in the literature have considered the effect of a high-strength crustal layer present near the seabed on the penetration response of offshore pipelines.