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An experimental study that allows for thermal-structural analysis of steel structures subject to non-spreading cryogenic spills is described. A local, non-spreading cryogenic spill was replicated by building a liquid nitrogen pool in the middle of six EH36 steel plates with a central crack of different length. The results of the temperature measurement and the thermal analysis are reported in the first and second part of this three-part companion paper. The computational study reported herein comprises stress analysis based on temperature-dependent von Mises plasticity combined with the SED failure criterion. Comparison of experimental and numerical results for stresses, fracture initiation and crack path shows satisfactory agreement. The numerical study indicates that the SED failure criterion is a useful concept for practical design of ship and offshore structures subjected to sub-zero temperatures and thus exposed to the risk of brittle failure when exposed to cryogenic spills. [ABSTRACT FROM AUTHOR]

In Part I of this paper, six liquid nitrogen (LN2) pool boiling tests were carried out to locally cool down six different EH36 steel plates to cryogenic temperatures. Leindenfrost and Critical Heat Flux (CHF) points of LN2 boiling curve were estimated. These estimations together with the recorded temperature histories are used in this part for the development of a heat flux curve for EH36 steel-LN2 pool boiling through Finite Element Thermal Analysis (FETA) of the problem investigated experimentally in Part I. Thermal conductivity and heat capacity of EH36 are defined by two unprecedented temperature functions based upon experimental studies in cryogenic temperatures. A user subroutine written in FORTRAN defines the air convection coefficient as a function of surface-air temperature difference, which changes with time and location. Results of this study contribute to the Accidental Limit State (ALS) design of marine and offshore structures for liquified gas spill scenarios. [ABSTRACT FROM AUTHOR]

The input for fatigue analyses of offshore wind turbines is typically chosen based on design values provided by design standards. While this provides a straightforward design methodology, the contribution of different input parameters to the uncertainty in the fatigue damage estimates is usually unknown. This knowledge is important to have when improving current designs and methodologies, and the parameters governing the uncertainty is typically found through a sensitivity analysis. Several sensitivity studies have been performed for monopile‐based offshore wind turbines, typically focusing on specific turbines and engineering disciplines. This paper performs a sensitivity study for three monopile‐based offshore wind turbines (5 MW, 10 MW, 15 MW) using parameters from several engineering disciplines. The results show that the fatigue utilization is primarily governed by the uncertainty in the SN curves and fatigue capacity. Following this, the uncertainty in the environmental conditions is the dominating uncertainty, with wind loads becoming increasingly important as turbine size increases. Additionally, the effect of modelling uncertainties is investigated. The wind‐related model uncertainties dominate in the tower top, while uncertainties in the wave and soil models dominate in the tower base and monopile. Designers wanting to reduce the uncertainty in a design are recommended to focus on the environmental conditions, and using as accurate models as possible. All modelling uncertainties are significant, but research should particularly be focused on wave directionality and soil models. [ABSTRACT FROM AUTHOR]

Liquefied Natural Gas (LNG) spills may cause steel to embrittle, leading to the brittle fracture of marine structures due to the cryogenic temperatures. For the integrity assessment of marine steel structures in the event of an LNG spill, first and foremost, an accurate heat transfer analysis is required. This study presents an experimental set-up that allows for thermal-structural analysis of steel structures subject to non-spreading cryogenic spills. A local, non-spreading cryogenic spill was replicated by building a liquid nitrogen (LN2) pool at the central zone of an EH36 steel plate under tensile loading. Six pool boiling tests were conducted on six different specimens. Temperature histories recorded by nine thermocouples are presented and discussed as well as the effect of surface roughness on cooling rates and boiling regimes. Numerical simulations of the experiments, discussed in Part II of this study, resulted in a heat flux curve for EH36 steel-LN2 pool boiling. [ABSTRACT FROM AUTHOR]

The cost of offshore wind energy can be reduced by incorporating control strategies to reduce the support structures' load effects into the structural design process. While effective in reducing the cost of support structures, load‐reducing controls produce potentially costly side effects in other wind turbine components and subsystems. This paper proposes a methodology to mitigate these side effects at the wind farm level. The interaction between the foundation and the surrounding soil is a major source of uncertainty in estimating the safety margins of support structures. The safety margins are generally closely correlated with the modal properties (natural frequencies, damping ratios). This admits the possibility of using modal identification techniques to reassess the structural safety after installing and commissioning the wind farm. Since design standards require conservative design margins, the post‐installation safety assessment is likely to reveal better than expected structural safety performance. Thus, if load‐reducing controls have been adopted in the structural design process, it is likely permissible to reduce the use of these during actual operation. Here, the probabilistic outcome of such a two‐stage controls adaptation is analyzed. The analysis considers the structural design of a 10 MW monopile offshore wind turbine under uncertainty in the site‐specific soil conditions. Two control strategies are considered in separate analyses: (a) tower feedback control to increase the support structure's fatigue life and (b) peak shaving to increase the support structure's serviceability capacity. The results show that a post‐installation adaptation can reduce the farm‐level side‐effects of load‐reducing controls by up to an order of magnitude. [ABSTRACT FROM AUTHOR]

Bridges across busy navigation channels are under the threat of accidental ship collisions. Many research works have been conducted to investigate the structural responses during ship bow-bridge pier collisions. However, the collisions between the ship deckhouse and the bridge girder have not been well studied. In this paper, a container ship deckhouse impact with a floating bridge girder is investigated. The impact demand and the structural deformation during the deckhouse-girder collision are numerically obtained. Based on the deformation mechanism observed from the simulation, a simplified analytical method is proposed for the fast prediction of the impact force and energy dissipation. The impact resistance and energy dissipation calculated by the simplified analytical method are validated against the numerical results for various collision scenarios. Results show the proposed method can accurately and efficiently estimate the impact demand of deckhouse-girder collisions. [ABSTRACT FROM AUTHOR]

In the past decades, a prevailing way for the assessment of responses in ship collisions and groundings has been to decouple the problem into two parts: external dynamics and internal mechanics. The external dynamics deals with global motions of the two interacting bodies prior to, during and after the collision. The main outcome of an external dynamics assessment is the energy loss during the collision, which will be dissipated by structural deformations in the assessment of internal mechanics. The decoupled method works well for right-angle collisions in general, but the accuracy can be much less in many scenarios, e.g. skew collisions with small collision angles and collisions with long durations. This is mainly because the assumptions in the decoupled method are violated in the studied cases. This paper reviews the assumptions and simplifications behind the external dynamic models and discusses validity of the assumptions by comparing with coupled simulation results. Decoupled and coupled models in both planar 3DOF and full 6DOF are addressed. Various collision scenarios are studied, including colliding with oblique plates, grounding on a sloping sea floor, crushing into rigid plates with normal vectors misaligned with coordinate axes, and collision with a submersible platform. In some scenarios, cases with different attack angles and impact velocities are simulated. The outcome will help understand potential limitations of the decoupled method, which should be used with care. [ABSTRACT FROM AUTHOR]

The dimensions of offshore wind turbine (OWT) support structures are governed by fatigue considerations. For 6‐ to 10‐MW OWTs, wave loads are often dominating in terms of fatigue utilization. The present work proposes a control scheme to reduce the wave‐induced fatigue loads in OWT support structures. The control scheme applies collective pitch control to increase both the damping and stiffness of the fore‐aft vibration modes. With conventional active tower damping, efficient wave disturbance rejection is restricted to a narrow frequency range around the first fore‐aft modal frequency. The proposed control scheme achieves efficient wave disturbance rejection across a broader frequency range. Here, tower feedback control is implemented via an auxiliary control loop. Based on a low‐fidelity model, the effect of the tower feedback loop on the stability margins of the basic controller is analysed. The results show that, within certain boundaries, the stability margins are improved by the stiffness term in the tower feedback loop. Consequently, the need to reduce the bandwidth of the basic controller to accommodate tower feedback control is relaxed. Based on time‐domain simulations carried out in an aero‐hydro‐servo‐elastic simulation tool, the lifetime effects of the proposed control scheme are analysed. Compared with conventional active tower damping, a more favourable trade‐off between adverse side effects and the support structure's fatigue damage is achieved with the proposed control scheme. [ABSTRACT FROM AUTHOR]

The fatigue damage sensitivity to wave directionality on large-diameter monopile foundations for use in the offshore wind turbine industry is investigated. A frequency-domain approach with the Dirlik method is used to estimate fatigue damage and to evaluate the effect of wave spreading and swell separation compared to a total sea representation. To evaluate the long-term impact, 30 years of hindcast data from the Dogger Bank area are used. Furthermore, a computationally efficient time-domain model of a 10-MW offshore wind turbine is used to compare with the frequency-domain results. Results show that benefits in terms of structural fatigue can be obtained with directional considerations. [ABSTRACT FROM AUTHOR]

Floating bridges across wide and deep fjords are subjected to the environmental wind and wave loadings. In this study, the dynamic response of floating bridges under such loadings is investigated. A floating bridge concept, which consists of two cable-stayed spans and 19 continuous spans, is selected. An eigenvalue analysis is first conducted and it is found that the period of the first mode is typically in the order of one minute or more. This implies that the amplified response effect should also be evaluated for the second-order wave load in addition to the first-order wave load. By performing a nonlinear time domain dynamic analysis, the bridge dynamic responses from wind and wave loadings are obtained. The effects of the wind load, first-order and second-order wave loads are studied considering different load combinations. Structural responses including girder displacements, accelerations and moments are investigated for each load combination. [ABSTRACT FROM AUTHOR]

Ship collisions with floating ice in which the ship sustains damage are in the shared-energy regime – both the ice and the ship dissipate energy through inelastic deformations. The physics of these events are rarely studied. Experience on the conditions of shared-energy collision tests is limited. The aim of this paper is to present an experimental study on a scenario where the impacted structure undergoes permanent deformations together with ice failure. The paper describes a series of laboratory-scale impact tests of freshwater granular ice blocks against stiffened steel panels, and presents the analysis of the main test results and lessons-learned. [ABSTRACT FROM AUTHOR]

The complete stress–strain relationship is needed for nonlinear finite element simulation of ship collisions and other events with plastic deformations, whereas only the material grade is known in the design phase. The analyst has to rely on minimum requirements to material strength found in class rules and statistical distributions of the material parameters. This paper investigates the effect of various features of the complete stress–strain curve on the predicted outcome of a collision simulation. This is discussed on the basis of published literature, and the effect of the assumed stress–strain curve is determined through nonlinear finite element analysis (NLFEA) simulations of a full-scale impact scenario. The influence of strain-rate effects is investigated. The findings reveal that the slope of the stress–strain curve determines how strains localise, and thereby when and where fracture propagates. The slope is strongly dependent on the yield ratio, yield plateau and the elongation to fracture, parameters that have significant statistical variation within a material grade. Some challenges related to simulation of strain-rate effects are also discussed. [ABSTRACT FROM AUTHOR]

This study introduces a generalised model for the shape of the sea floor, which is relevant for ships during stranding. The sea floor is parameterised by a mathematical expression as a quadratic surface, namely, an elliptical paraboloid. Two scenarios were considered in this study using the LS-DYNA code, implemented with the user-defined Rice-Tracey-Cockcroft-Latham (RTCL) material routine. A simple procedure to estimate the global collapse load of the grillage of the bottom structure based on plastic analysis is presented. The solution obtained by this simplified procedure also illustrates a promising scheme that can be applied for rapid assessment of the ship hull in stranding analysis. The ultimate goal of the study is to understand the behaviour of the ship bottom structure and to incorporate the developed procedure into a decision support tool, which is intended to facilitate assessment of critical situations. [ABSTRACT FROM AUTHOR]

A simple method for the rapid assessment of ship bottom structures subjected to grounding over seabed obstructions with large contact surfaces is proposed in this paper. It has been recognised that the shape and size of the seabed obstruction is of crucial importance in relation to the characteristics of bottom damage. Most studies on ship grounding are concerned with ‘rock-type’ sharp obstructions, where plate tearing is the dominating failure mode. However, very few studies are found related to grounding over blunt obstructions with large contact surfaces such as ‘shoals’. Denting rather than tearing is more likely for the bottom plating as observed from actual grounding incidents. The sharp obstruction may cause earlier penetration and result in very unfavourable consequences such as compartment flooding. In contrast, the bottom plating may not fracture when moving over a blunt-type sea floor. But it may threaten the global hull girder resistance and give rise to even worse consequences such as hull collapse. The proposed simple method is established on the basis of a series of closed-form solutions for individual structural members developed from the plastic mechanism analysis. The primary deformation modes for the major bottom structural members are sliding deformation of longitudinal girders, denting and crushing of transverse members and indentation of bottom plating. The effect of friction is considered and estimated in a simple manner. The vertical resistance that governs the vertical ship motion is derived. It is found that the vertical resistance is free of friction. The proposed simple method for bottom strength is verified against large-scale non-linear finite element analyses, where a good correlation is obtained. [ABSTRACT FROM PUBLISHER]

The objective of this paper is to study oil spills from damaged tanks during ship collision. The work comprises development of analytic models for estimation of oil spill and computational fluid dynamics (CFD) simulation with the Fluent software for verification of the simple models. In previous papers, oil leakage from a damaged tank with an opening in the bottom was investigated. The purpose of the present study is to extend the scope to cover oil spills for the case of collisions where the openings occur in the sides. In such cases, the gravitational force is very important as for the grounding scenario. However, for collision scenarios, there is a local imbalance due to different densities of the fluids even when the internal pressure is equal to the external pressure. A combination of water inflow and oil outflow through the opening may occur. Analytical calculations and time-domain simulations are applied to calculate the volume of oil outflow and the outflow rate versus time. Good agreement is obtained between the analytic model and the CFD analysis of the oil spill. [ABSTRACT FROM PUBLISHER]

A simple method for the rapid assessment of ship bottom structures subjected to grounding over seabed obstructions with large contact surfaces is proposed in this paper. It has been recognised that the shape and size of the seabed obstruction is of crucial importance in relation to the characteristics of bottom damage. Most studies on ship grounding are concerned with ‘rock-type’ sharp obstructions, where plate tearing is the dominating failure mode. However, very few studies are found related to grounding over blunt obstructions with large contact surfaces such as ‘shoals’. Denting rather than tearing is more likely for the bottom plating as observed from actual grounding incidents. The sharp obstruction may cause earlier penetration and result in very unfavourable consequences such as compartment flooding. In contrast, the bottom plating may not fracture when moving over a blunt-type sea floor. But it may threaten the global hull girder resistance and give rise to even worse consequences such as hull collapse. The proposed simple method is established on the basis of a series of closed-form solutions for individual structural members developed from the plastic mechanism analysis. The primary deformation modes for the major bottom structural members are sliding deformation of longitudinal girders, denting and crushing of transverse members and indentation of bottom plating. The effect of friction is considered and estimated in a simple manner. The vertical resistance that governs the vertical ship motion is derived. It is found that the vertical resistance is free of friction. The proposed simple method for bottom strength is verified against large-scale non-linear finite-element analyses, where a good correlation is obtained. [ABSTRACT FROM PUBLISHER]