Heba Wael Leheta

Reliability-based method for optimal structural design of stiffened panels

Abstract A methodology for structural optimization of stiffened panels based on reliability is developed. The stiffened panels are typical of those found in the deck or bottom of longitudinally stiffened ships. They are assumed to be under stillwater and wave induced loads, resulting in predominantly compressive loads. Both serviceability and collapse limit states are considered. The design problem is formulated as a nonlinear programming problem that aims at minimizing weight with behaviour constraints on reliability and physical constraints on the dimensions. The safety index is calculated using a first order reliability method (FORM). The PNET method is then employed to determine the representative failure modes and the system safety index. Finally, a numerical example is carried out to demonstrate the applicability of the design methodology and the results are presented and discussed.

Fate of drilling waste discharges and ecological risk assessment in the Egyptian Red Sea: an aquivalence-based fuzzy analysis

Egypt is located in the Northeast of Africa where oil and gas (O&G) are produced offshore from the Gulf of Suez and the Southeast part of the Mediterranean. The O&G production in Egypt is distributed as follows: 70% from Gulf of Suez, 16% from Western desert, 8% from the Sinai Peninsula and 6% from Eastern desert. Past O&G activities, refining and transport have resulted in chronic pollution in Egyptian offshore and numerous environmental programs have been initiated to protect new development areas from the environmental impacts. The offshore drilling process uses drilling fluids (mud) and generates waste fluids and cuttings, which could be the largest discharges going into the receiving water bodies. There are several options to manage offshore drilling wastes: offshore discharge, offshore down-hole injection and onshore disposal. Water-based drilling fluids (WBF) are commonly employed for drilling in Egyptian offshore because of their expected environmental benign behavior. The main objective of this paper is to develop a methodology to determine the fate of heavy metals associated with WBF drilling waste in the marine environment and estimate the associated ecological risks. Proposed contaminant fate model is based on an aquivalence approach, which has been integrated with fuzzy-based analysis to study the uncertainties. This research concluded that the impacts of heavy metals associated with the drilling waste discharges in the receiving waters are minimal.

Multi-attribute risk assessment of LNG carriers during loading/offloading at terminals

A methodology and software tool for the risk assessment of LNG gas carriers during loading/offloading at the ship/terminal interface is developed in this work. The approach is based on multi-attribute risk assessment and utility theory. The use of a semi-quantitative risk assessment approach improves upon existing qualitative methods and allows the ranking of risk alternatives based on a unified utility measure. The developed method also allows modelling of the decision makers attitude towards risk, i.e., risk aversion/neutral and/or risk taker. This methodology is also applicable to other ship-operating modes such as transit in open sea and/or entering/leaving port. The developed software tool allows ranking of risk alternatives and sensitivity analyses to be carried out to assess the sensitivities of the risk models recommendations to various modelling variables. A case study is presented to demonstrate the developed program capability.

Reliability of subsea pipelines against lateral instability

An approach for the reliability assessment of subsea pipelines against on-bottom lateral instability is presented in this work. Combined stresses/lateral displacement acting on offshore pipelines due to combined hydrodynamic loads including wave/current effects are computed using a non-linear finite element model. The model takes into account the effect of pipe–soil interaction. The resulting combined pipe stresses/lateral displacement are used as input for reliability assessment. Probabilities of pipeline overstress/lateral movement due to hydrodynamic loading are computed using a finite element reliability code. First-/second-order reliability methods are used for computation of safety indices. These values are then compared with target safety levels as set in pipeline design codes. Three case studies for actual pipelines off the shore of Egypt are analysed using the proposed approach. Results show that computed safety levels are within the target values. The proposed approach can be a valuable tool for the pipeline designer/operator for assessment of pipeline safety and reliability with respect to on-bottom stability.

Fire safety design of Nile-floating hotels

The importance of Nile-floating hotels as a national income source in Egypt is highlighted in this work. Statistical analysis of collected data of previous accidents that occurred during the past 12 years is presented. The results showed that fire is ranked third among the four common types of recorded accidents. Records also showed that fire is one of the causes of killing or injuring passengers and crew members aboard Nile-floating hotels. An integrated fire and evacuation simulation programme, “FDS+EVAC” is used to analyse the consequences of a worst-case cabin fire scenario that could occur in an accommodation deck aboard a Nile-floating hotel. A series of simulations of the fire scenario under consideration involving an existing design of an accommodation deck as well as seven suggested alternative designs/arrangements of the same deck which are assumed to have equivalent or better level of fire safety have been carried out. The results are then analysed and discussed.

Fuzzy inference system for fire and explosion risk assessment of floating storage and offloading vessels

A case study for the fire and explosion risk assessment of a floating storage and offloading (FSO) vessel is presented in this paper. The fire and explosion hazards affecting the storage tanker for different operating modes were first identified. Two approaches for risk assessment were then adopted. The first is a qualitative risk assessment approach using a qualitative risk matrix. The second is a fuzzy risk assessment approach using a fuzzy inference system (FIS). The use of a fuzzy representation for probabilities, consequences, and risk values takes into account the imprecision associated with the assignment of their values. A comparison is provided between the results obtained using the two approaches for an FSO vessel which is located about 120 km offshore from Libya. Results show that the use of an FIS approach provides a more robust framework for fire and explosion risk assessment and more output information than the qualitative matrix approach. The results also show that the computed risk values using a fuzzy risk index measure are consistent with those obtained using a qualitative risk matrix approach.

Reliability analysis of novel stiffened panels using Monte Carlo simulation

ABSTRACT The present study aims to perform reliability analyses using Monte Carlo simulation to compute and compare the time-invariant reliability indices of stiffened panels with either conventional T-stiffeners or novel Y (Hat + Tee/Angle) stiffeners profiles in a double hull oil tankers bottom and deck panels under axial compressive loads. The ultimate strength and the applied axial compressive stress formulations used in the limit state functions are obtained based on the International Association of Classification Societies-Common Structural Rules for oil tankers considering the following failure modes: unstiffened plate buckling, stiffener beam-column buckling, and stiffener torsional/flexural buckling (tripping). With the Y-stiffeners profile, a considerable improvement in the panels’ reliability index throughout the vessel lifetime is achieved.

Analysis of Fatigue Crack Growth in Ship Structural Details

Abstract Fatigue failure avoidance is a goal that can be achieved only if the fatigue design is an integral part of the original design program. The purpose of fatigue design is to ensure that the structure has adequate fatigue life. Calculated fatigue life can form the basis for meaningful and efficient inspection programs during fabrication and throughout the life of the ship. The main objective of this paper is to develop an add-on program for the analysis of fatigue crack growth in ship structural details. The developed program will be an add-on script in a pre-existing package. A crack propagation in a tanker side connection is analyzed by using the developed program based on linear elastic fracture mechanics (LEFM) and finite element method (FEM). The basic idea of the developed application is that a finite element model of this side connection will be first analyzed by using ABAQUS and from the results of this analysis the location of the highest stresses will be revealed. At this location, an initial crack will be introduced to the finite element model and from the results of the new crack model the direction of the crack propagation and the values of the stress intensity factors, will be known. By using the calculated direction of propagation a new segment will be added to the crack and then the model is analyzed again. The last step will be repeated until the calculated stress intensity factors reach the critical value.

Assessment of Safety Performance Indicators: A Case Study for the Bouri Field

An assessment of the safety performance indicators on board two fixed offshore structures handling petroleum is presented in this paper. Results are based on an analysis of a database of safety activities covering a period of 3 years from 2006 to 2008. The database is compiled by the operating company of the platforms, which are located about 120 km offshore from Libya in the Bouri field. The key safety performance indicators assessed are the potential loss of life, fatal accident rate, average individual risk, lost-time injury frequency, and accident severity index (gravity index). The assessment provides important information on personnel safety performance and rates of injury occurrences on board fixed offshore platforms. The computed values for the safety indicators are then compared with acceptable standard values and with other platforms and regions to assess the current safety performance level on board the platforms. The results could be a valuable tool for improving personnel safety on board the platforms, for identifying changes over time, and for focusing efforts where improvements are needed.