Design and Assessment of Offshore Structures for Extreme and Abnormal Accidental Loading

A simplified method for reliability- and integrity-based design of engineering systems and its application to offshore mooring systems

Maintaining the integrity of an aging offshore facility often requires the repair of corrosion. With advances in adhesive technology, epoxy adhesives (cold bond repairs) are increasingly being adopted to repair corrosion damage on offshore structures. Structural elements, protection barriers, and occupied buildings, for example a platform living quarters, designated as safety critical often must comply with project or facility performance standards that specify minimum design accidental loading (e.g., from fire, explosions, or impact). In addition to satisfying conventional structural design criteria, a corrosion repaired structure must also perform within acceptable limits when subjected to accidental loads. The present study outlines approaches to assess the performance of cold bond repairs subjected to accidental explosion loads. The living quarters considered in this study featured approximately 50 individual cold bond repairs; each arbitrarily located over the building’s external surfaces. Under accidental blast conditions, failure of a cold bond repair was deemed unacceptable due to the threat of pressure leakage into the building’s interior. As such, the study demanded an analysis capable of considering both the global response of the living quarters as well as the local response of cold bond repairs. There have been several studies conducted by researchers around the globe to evaluate the response characteristics of adhesive bonds. However, these studies are largely focused on experimental, local, and micromechanical analysis of adhesively bonded materials. Numerical analysis of adhesive bonds, for purposes of global structural system response assessments, especially in the case of accidental loading, appear to be nonexistent in available literature. Inspired by this gap, we present a case study involving an assessment of global structural performance of a living quarters building repaired with cold bond adhesives subject to loading due to accidental and rare events. In this study, the local behavior of cold bond adhesives was calibrated using numerical simulations of the ASTM tests specified by the adhesive vendor. The calibrated properties were implemented in finite element analyses used to validate cold bond seal plate repairs at various locations on the living quarters building. The study includes a discussion about the methods and approaches used to model cold bond repairs in a practical and efficient manner. The approach outlined herein provides a useful framework that can be adapted to similar assessments by a practicing engineer responsible for blast analysis of marine structures.

Purpose Accidental loadings such as fire constitute a great majority of potential and actual fatalities in both onshore and offshore installations. In order to prevent human loss and for a safe design of an asset, the risk of fire loading needs to be quantified, in terms of both probability/frequency and consequence aspects. In this paper the authors propose a novel risk-based approach for the assessment against accidental fire loading. Design/methodology/approach In a conventional passive fire protection (PFP) analysis using ductility level analysis (DLA), fire loads are deterministically applied to a structure whose response is then analyzed. The initial PFP scheme is developed based on the analysis and then optimized. This approach is sometimes misinterpreted as a “risk-based” approach; however, it does not take into account the frequency aspect of the risk assessment. In a risk-based PFP analysis using DLA, fire scenarios are developed in a particular target zone. Then DLA is performed to determine the structural consequence. If personnel safety is of interest, the consequence of the structure is then linked to individual risk (IR) to determine fatalities. The amount of PFP to be applied on the structure is fully based on the risk that is produced by the fire scenarios in target zones. Findings A new perspective on safe design of onshore/offshore structures for accidental loadings is outlined to estimate the associated risk to potential targets such as personnel as well as asset. The proposed assessment methodology will contribute toward identifying the mitigation measures and safety-critical procedures and equipment and toward a safer design. Originality/value This paper presents a new perspective in a safer design of onshore and offshore structures for a fire accidental loading based on risk calculation. Risk is defined as a combination of the frequency and consequence. An event frequency analysis is carried out to determine how often one should expect the event to occur. A consequence analysis is carried out to determine the severity levels of the event. In a risk-based consequence analysis, the severity levels are fully determined based on the risk associated with the event. The proposed novel risk-based assessment methodology against accidental fire loading contributes toward fully understanding the risk from an impact to personnel and to asset perspectives and leads toward safer and optimal design.

Road accidents are often caused by short abnormal events, including traffic violations, abrupt change in vehicular motion, driver fatigue, etc. Observing an accident event from the right camera perspective plays a crucial role while detecting accidents. However, it may not be possible to capture such abnormal events from a limited camera perspective. We present a deep learning framework to analyze the accident events recorded from multiple perspectives. First, we estimate feature similarity in videos recorded from multiple perspectives. We then divided the video samples into high and low feature similarity groups. Next, we extract spatio-temporal features from each group using two-branch DCNNs and fuse them using a rank-based weighted average pooling strategy followed by classification. We present a new road accident video dataset (MP-RAD), where each accident event is synthetically generated and captured from five independent camera perspectives using a computer gaming platform. Most of the existing road accident datasets use egocentric views or they are captured in fixed camera setups. However, our dataset is large and multi-perspective that can be used to validate ITS-related tasks such as accident detection, accident localization, traffic monitoring, etc. The dataset contains 400 accident events with a total of 2000 videos. We provide temporal annotations of all videos. The proposed framework and the dataset have been cross-validated with latest accident detection baselines trained on real-world road accident videos and vice-versa. The sub-optimal detection accuracy obtained using the baselines indicates that the proposed framework and the dataset can be useful for ITS related research. Code and dataset is available at: https://github.com/draxler1/MP-RAD-Dataset-ITS-

Reliable evaluations of the radiological consequences of hypothetical severe accidents in nuclear power plants (NPPs) play a key role in supporting the emergency and preparedness plans during such abnormal events. Also triggered by the growing requests of the national regulators, one of the milestones of the Nuclear Safety Research program (NUSAFE) of the Karlsruhe Institute of Technology (KIT) is the improvement of the performance of the codes employed for such evaluations in the current and innovative NPPs, which are in operation and going to be built in EU and worldwide. Having this in mind, a calculation platform of state-of-art codes has been assessed at KIT to evaluate the accident progression, the radiological source term, and the behavior of the fission product dispersion to the environment by also taking into account the corresponding uncertainties. In particular, the CASMO5 lattice physics code, developed by Studsvik Scandpower, is used for a proper evaluation of the fuel inventory loaded in the reactor core. The European reference Accident Source Term Evaluation Code (ASTEC), developed by IRSN (France), is employed to evaluate the accident progression from the initiation to the fission produced release to the environment. The use of the KArlsruhe Tool for Uncertainty and Sensitivity Analysis (KATUSA), developed by KIT, aims at performing the uncertainty quantification of the source term. Finally, the Java Real-time On-line DecisiOn Support (JRODOS) is employed for analyzing the radiological consequences of the accident. The KIT platform has been employing since some years in the frame of the KIT participation to different European projects, i.e., MUSA, SASPAM-SA on SMRs, SANE. In the paper, an overall view of the KIT strategy on severe accident analyses will be provided, and examples of the application of the above-mentioned platform to generic NPPs will be provided, i.e., PWR Konvoi.

Within the oil and gas industry major inconsistencies have been found in the correlation of predicted pile response using design code prescribed methods and the measured response from installed on-line monitoring systems. This paper presents the first phase of a study conducted to determine whether a finite element continuum model which uses an elasto-plastic constitutive soil model can better determine the measured/observed response of piled offshore platforms. The first phase of the study entailed a comparison of the measured natural period of an existing platform to the natural period determined numerically using a dynamic eigenvalue analysis with nonlinear soil springs. Sensitivity studies were also conducted to identify how modelled parameters such as topsides mass, conductor mass and marine growth affected the calculated natural period of an existing platform. Single pile models were also developed for detailed finite element (FE) analyses using the nonlinear springs (API soil springs) and a continuum soil model with the soil properties defined using the Mohr-Coulomb plasticity constitutive soil model. The results of the analyses have confirmed industry observations that, even accounting for the various parameters considered in the sensitivity studies, the design code derived soil springs show poor correlation to the measured response of the jacket. The dominant reason for this is the lateral stiffness (p-y) of the API derived soil springs which required an amplification factor in the range of 8-10 to obtain a suitable correlation to the measured jacket response. The continuum model demonstrated a significantly improved correlation between the numerically derived pile response and the measured data. The numerical representation of the soil-pile interaction described in this paper provides a more realistic determination of the structural response of the modelled platform. The results obtained have major implications for both the design and assessment of offshore structures. An increased lateral soil stiffness will largely eliminate the foundation failures predicted by conventional ultimate strength assessments and improve correlation to observed post hurricane structural performance. In the case of platform risk assessments, this improved representation of the soil-pile interaction will reduce the industry observed excessive conservatism in the prediction of failure probabilities of existing offshore facilities. In extreme cases, platforms have had to be strengthened or even removed based on predictions of foundation failures that were inconsistent with measured (OLM systems) and observed performance (hurricane survivals). Based on the results obtained from the first phase of this study, further work is planned to validate the FE continuum approach single pile model and finally provide a numerical solution for pile-soil interaction which can be easily adopted by practicing engineers in the oil and gas industry.

Risk‐targeted seismic design maps for Mexico are presented. Risk‐targeted approach establishes an acceptable risk level, measured with the annual collapse probability, to determine seismic design coefficients. Seismic design maps are calculated for an annual probability of collapse of 1 × 10 −4 and 2 × 10 −4 . These maps are then compared with the seismic design maps for fixed return periods of 475 and 2475 years. An additional idea is proposed for setting reasonable values for the probability of collapse associated with the design intensity, usually denoted as X in the literature, incorporating over‐strength ( OS ) as part of the input parameters. The results indicate that the risk‐targeted methodology, based on these annual collapse probabilities, yields higher design coefficients across most of the country, compared to the 475‐year return period maps, whereas leading to lower design coefficients in large areas when compared to a 2475‐year return period. Additionally, we note that the use of generic capacity/fragility curves may introduce significant variations in seismic design coefficients relative to those obtained from site‐specific analyses. This underscores the importance of evaluating capacity/fragility curves for different structural configurations to enhance accuracy. Furthermore, ensuring a uniform level of reliability across all regions could result in high construction costs in certain areas, emphasizing the need to integrate cost considerations into seismic design decisions.

Understanding how regional climate, and the extremes associated with it, will change over the coming decades is a vital step towards effective local adaptation. understanding of the vulnerability and risk from severe winds is derived from a number of factors, including the frequency and intensity of the hazard, community exposure and the relationship between gust wind speed and impact/loss associated with residential structures. Australian Government, through the Australian Building Codes Board (ABCB), has responsibility for maintenance of the Building Code of Australia (BCA), which establishes performance requirements that buildings must satisfy when subjected to the actions of, among other hazards, severe winds. Australian /New Zealand Wind Loadings Standard (AS/NZS 1170.2, 2011) specifies the gust wind speeds used in calculating loads on structures in different of the country. Design wind speeds are similar to wind hazard (both described in terms of return period gust wind speeds), and the levels are set in an effort to equalise the risk between the low and high hazard across the country. Areas around the northern coastline are subject to the impact of tropical cyclones, and as such are deemed to experience the highest wind hazard. This results in Australia being divided into four regions, within which the wind speed is broadly similar. Regions C and D are those where the hazard is dominated by tropical cyclones. A recent report of climate change on design wind speeds in cyclonic regions (Holmes, 2008) prepared for the Australian Building Codes Board recommended that: • wind Region C should be extended south on the Queensland coast to 27 o S. This is justified based on the recent simulation studies on the effects of climate change, and restores the boundary to that in the Standard between 1975 and 1989. • The existing uncertainty factor for Region C, FC, should be increased to 1.10 (+5%), primarily in response to predictions of increased Category 3-5 cyclones by climate model simulations. An area where the impact of such regulatory changes would be significant is the Sunshine Coast region in Queensland. Changing the AS/NZS 1170.2 region from B to C would render most of the existing homes non- compliant, hence increasing the risk by virtue of increased hazard without reduction in vulnerability via adaptation. This study seeks to develop the underpinning science that could be used to evaluate the need for adaptation (re-classification and retrofit of existing housing to comply with revised classification). Cyclonic wind hazard for the Sunshine Coast region has been estimated using Geoscience Australia's Tropical Cyclone Risk Model, utilising synthetic tropical cyclone event sets derived from observations (1981-2006) as well as IPCC AR4 climate models. regional wind hazard levels have been modified for terrain, topographic and shielding effects to reflect localised variations in wind hazard. resulting local wind hazard maps depicting annual exceedence probabilities for severe wind gusts have been integrated with the National Exposure Information System (NEXIS) to provide an analysis of impact, in terms of residential buildings affected and the cost of replacement. Return periods (annual exceedence probabilities) associated with severe wind gust hazard were evaluated for the Sunshine Coast and Southeast Queensland for both current climate and future climate scenarios. This wind gust hazard was subsequently used to evaluate annualised loss, which represents the average annual cost to the region of exposure to wind hazard if viewed through a very wide window of time. costs and benefits of potential changes to wind loading for the Sunshine Coast area have been analysed in a Regulation Impact Statement (RIS) prepared by the Australian Building Codes Board (ABCB, 2010). RIS focuses on changes to standards for new buildings and houses; it does not address the impacts on existing buildings. This study presents analyses of risk for both existing and new buildings considering the variation in structural vulnerability within the building stock.

This work focuses on examining the dynamic behavior of large floating offshore wind turbine (FOWT) exposed to extreme (wind and wave with 50-year return period) and accidental (mooring line breakage) loadings. The FOWT considered is 15 MW reference turbine supported on semi-submersible platform stationed using catenary moorings. As the mooring configuration greatly affects the response of FOWT, two different mooring configurations namely non-redundant (three-line) and redundant (six-line) systems were studied and compared. The coupled multi-body dynamic system was solved using Openfast. When simulating the mooring line failure, both the operating and extreme loading conditions were considered. Failure of one mooring was considered at a time. The response of the coupled system due to breakage of the mooring indicates high displacements in surge and sway directions in comparison to the intact system especially for the non-redundant mooring system. Furthermore, change in platform yaw angle and increased tension in the other intact mooring lines were observed. The findings from this study will be helpful in accidental limit state design and preventing failure of similar large FOWT systems. In addition, insights into using non-redundant and redundant mooring configurations for such large structures with respect to extreme and accidental loadings were also discussed.

<div class="section abstract"><div class="htmlview paragraph">The hydrogen internal combustion engine technology, with its potential for almost full carbon emissions reduction and adaptability to a wide range of fossil fuel-based internal combustion engine (ICE) platforms, offers a promising future. However, as with any innovative technology, it also presents challenges, such as abnormal combustion phenomena. These challenges, including intake backfire, which is more common when using port fuel injection (PFI), and pre-ignition in the combustion chamber, which can be experienced with PFI or direct injection (DI), require detailed investigation to understand and optimize the engine’s performance and efficiencies.</div><div class="htmlview paragraph">This study comprehensively investigates the main abnormal combustion events that could happen in a spark ignition (SI) hydrogen engine. It examines both direct and port fuel injection systems and uses high-resolution in-cylinder, intake, and exhaust pressure measurements alongside a suite of fast-response gas analyzers. The study provides a direct comparison between abnormal and normal combustion events, sampled over 200 consecutive cycles, and uses ultra-fast NOx, HC, and CO2 emissions analyzers to help analyze pre-ignition combustion, backfire, and partial burn events.</div><div class="htmlview paragraph">Seemingly for the first time, the study has demonstrated the direct link between the in-cylinder combustion events and exhaust gas emissions from a spark ignition hydrogen engine. Pre-ignition caused by lubrication oil is realised with controlled oil injection. Such pre-ignition on the in-cylinder combustion process and its impact on the instantaneous production of HC and CO2 from lubrication oil are quantified. The advanced data acquisition (DAQ) system enabled accidental pre-ignition combustion events to be detected and captured. The backfire cycles are investigated using simultaneous recordings of instantaneous intake and in-cylinder pressures alongside the fast gas analyzers to show the risk of backfire on both the intake system and the in-cylinder combustion process. Finally, the study shows how adopting highly boosted air for ultra-lean burn combustion can lead to instability and misfire under high-load operating conditions.</div></div>

This paper presents a probabilistic study of various Gulf of Mexico (GoM) MODU mooring systems that was performed as part of the MODU Mooring JIP (Joint Industry Project). The intent was to determine the relative changes in reliability between different types of MODUs, with different mooring systems, in different water depths, designed for different hurricane conditions and located at different regions in the Gulf. In addition, the effects of assuming reduced mooring line strength were investigated. Twenty cases were selected which cover four different classes of MODUs, four different types of mooring systems, three different numbers of mooring lines, as well as three different water depths. These base cases were chosen so that trends or differences between different hull shapes and mooring systems could be identified. These twenty base cases are further expanded into sixty cases according to location in the Gulf as well as the return period used to design the mooring system. The results indicate that on average the " notional?? (i.e., relative) annual failure probability of mooring line failure decreases by one order of magnitude when increasing the MODU mooring system design return period from 10-yr to 50-yr. Two orders of magnitude decrease are observed if the design return period is increased from 10-yr to 100-yr. In addition to the above findings, reliability sensitivity to four different GoM Metocean regions is also provided. Redundancy of the mooring system for 8, 12 and 16-lines is also examined in terms of the additional reliability gained by the extra lines. Introduction The probabilistic reliability approach was performed using an updated procedure that was originally developed in prior work by DeepStar [1]. The results can be used by operators, drilling contractors, API work groups and regulators alike to understand, on a comparative basis, the performance of various MODUs and mooring systems. A base set of 20 separate MODU and associated mooring cases were evaluated as shown in the first twenty cases of the matrix in Table 1. These selected twenty base cases cover four MODUs (or MODU classes), four types of mooring systems, three numbers of mooring lines and three water depths. The footnotes of Table 1 provide definitions of these cases. These base cases were chosen so that trends or differences between different hull shapes and mooring systems can be identified. This base set of cases was then further defined into a larger set of 60 according to location in the Gulf as well as the return period used to design the mooring system. The Gulf locations were defined as the Central, West Central and West according to MODU JIP metocean study [5] and the latest API guideline [6], and the design return periods were defined as 10, 25, 50 and 100 year. The remainder of Table 1 shows the resulting matrix of cases studied. The full set of 60 cases was initially evaluated for reliability using 100% CBS (Catalogue Break Strength) for the mooring line. The entire set of 60 was then re-evaluated using 80% CBS to determine the sensitivity of the results to lower than expected mooring line strength. For the 8-10 mooring line cases, the first and second line failures were determined. The second line failure was used to represent the mooring system failure. For the 12 and 16 line cases, the third line failure was also determined, with the third line failure representing the mooring system failure. This allowed a comparison of the effect of mooring line redundancy in terms of the first line failure, versus the mooring system failure.

The tension leg platform (TLP) is a moored floating offshore structure whose buoyancy is more than its weight. The mooring system, known as tethers, is vulnerable to failure due to extreme (maximum and minimum) tensions. In the present study the reliability of these tethers under maximum and minimum tension (ultimate limit state) has been studied. Von-Mises failure criteria has been adopted to define the failure of a tether against maximum tension. The minimum tension failure criteria has been assumed to meet when the tethers slack due to loss of tension. First Order Reliability method (FORM) has been adopted for reliability assessment. The reliability, in terms of reliability index, and probability of failure has been obtained for twelve sea states. The probabilities of failure so obtained for different sea states have been adopted for the calculation of annual and life time probabilities of failure.

Characteristic loads for design are defined in terms of their annual exceedance probability, q. For ultimate limit state (ULS) q = 10−2, while q = 10−4 for accidental limit state (ALS). In principle a full long term analysis is required in order to obtain consistent estimates. This is straight forward for linear response problems, while it is a challenge for non-linear problems in particular if they additionally are of an on-off nature. The latter will typically be the case for loads due to breaking wave impacts. The Contour line approach is an alternative convenient method to estimate the long term extreme response, based on short term statistics from an appropriate sea state. The consequence of very large short term variability (large coefficient of variation for 3-hour extreme value) on the application of the contour method will be discussed. The long term integral is carried out over all sea state combinations. The lowest sea states will of course not affect the extremes. However, for the impact problem the short term variability is much larger than for most response cases. The coefficient of variation of the 3-hour maximum impact pressure is often between 0.5 and 1, while for a typical response process it is between 0.1 and 0.2. Due to the large variability, lower sea states than normal will contribute to the long term response. In this paper the irregularity of the response surface, and the uncertainties related to the number of seeds used in each sea state is looked into. The focus is on slamming loads from breaking waves, and some results from a model test are presented. The uncertainties in long term response from slamming loads are compared to a more common response process. The effect on the long term response when integrating over a reduced area of sea states in the scatter diagram is discussed.

Rainfall date obtained for thirty-two meteorological stations distributed in Iraq to find the rainfall returns period, which contributes in the improvement of water management plans all over Iraq, especially during dry seasons. Mean annual summation of rainfall has a symmetrical increasing pattern from southern west towards northern east, according to the increasing ratio of rainfall in the northern region of Iraq. The northern east region of Iraq has characterized by very abnormal and abnormal events regarding rainfall (P) in term of return periods, while the northern west, middle and southern regions characterize by a normal distribution of rainfall. , Iraq has only two types of weather conditions, according to rainfall returns periods: the humid weather condition located in the northern east part and dry weather condition in the other parts of it.

<p>Compound flooding hazard in estuarine delta is increasing due to mean sea-level rise (SLR) as the impact of climate change. Decision-makers need future hazard analysis to mitigate the event and design adaptation strategies. However, to date, no future hazard analysis has been made for the Kapuas River delta, a low-lying area on the west coast of the island of Borneo, Indonesia. Therefore, this study aims to assess future compound flooding hazards under SLR over the delta, particularly in Pontianak (the densest urban area over the region). Here we consider three SLR scenarios due to climate change, i.e., low emission scenario (RCP2.6), medium emission scenario (RCP4.5), and high emission scenario (RCP8.5). We implement a machine-learning technique, i.e., the multiple linear regression (MLR) algorithm, to model the river water level dynamics within the city. We then predict future extreme river water levels due to interactions of river discharges, rainfalls, winds, and tides. Furthermore, we create flood maps with a likelihood of areas to be flooded in 100 years return period (1% annual exceedance probability) due to the expected sea-level rise. We find that the extreme 1% return water level for the study area in 2100 is increased from about 2.80 m (current flood frequency state) to 3.03 m (under the RCP2.6), to 3.13 m (under the RCP4.5), and 3.38 m (under the RCP8.5).</p>