City Research Online

Abstract

It is well understood that the spilled oil from damaged oil tankers poses a severe threat to the marine environment. Although great efforts have been devoted to studying the oil spilling from damaged oil tankers, especially double hull tanks (DHTs), the majority is subjected to an ideal condition (e.g., fixed tanks in still water; simple damage conditions) and adopts hydrostatic theories or quasi-steady models with over-simplified assumptions on data analysis or analytical prediction. These conditions or assumptions may not stand in the complex dynamic spilling process in the real spilling accident. This study brings a step further on the knowledge of oil spilling from a damaged tank by combining experimental and numerical investigations, with a focus on the dynamic spilling process from damaged oil tankers which is either fixed or subjected to motion, which have not been systematically investigated.

In the experimental investigation, the submerged oil spilling from DHTs under different accidental scenarios including grounding and collision is studied. Two new sets of laboratory tests are carried out, where the damaged tank is fixed in still water. In the first set, the axial offset between the internal and the external holes on two hulls of the grounded DHT is considered to widen the scope of damage conditions which the tanker may suffer from during grounding accidents. Although all cases in this set are subjected to the same hydrostatic conditions, completely different dynamic spilling processes are observed. In the second set, the initial water thickness inside the ballast tank of the collided DHT is considered. This aims to represent the real scenarios that the external hull is generally damaged prior to the internal hull and, therefore the ballast space is partially filled by the water flowing from the surrounding environment before the internal hull is damaged. These experiments do not only advance the state of the art of the experimental study in this field, but also provide a reference for validating the numerical models developed in this study. Based on the experimental data, the correlation analysis for the discharge through the internal hole by using quasi-steady Bernoulli’s equation is presented, contributing to the development of an improved analytical model for predicting the oil spilling from damaged oil tankers.

The numerical study is carried out using a numerical model developed in OpenFOAM framework, where the VOF is applied to deal with the air-oil-water multiphase flow. This model enables the users: (1) to consider air, oil and water three phases of fluid and their interaction with solid tanker hull using dynamic mesh technologies;
(2) to model turbulence associated with the oil spilling process using various available turbulent models; and (3) to investigate the effects of the compressibility of the fluid. The oil spilling from damaged DHTs is simulated and validated by the experimental data. Intensive investigations are carried out to clarify uncertainties in existing numerical modelling of the oil spilling from damaged DHTs. These include (1) the associated turbulence behaviours and selecting an appropriate approach to turbulence modelling; (2) the role of fluid compressibility during the oil spilling; and (3) the effect of tank motion on the oil spilling process. For the turbulence modelling, various approaches to model the turbulence, including the large eddy simulation (LES), direct numerical simulation (DNS) and the Reynolds average Navier-Stokes equation (RANS) with different turbulence models are attempted. It is concluded that the oil spilling from DHTs is more sensitive to the turbulence modelling than that from SHTs. For DHT cases, the effective Reynolds number (Re) considering both oil outflow and water inflow is suggested to classify the significance of the turbulence and to correspondingly select the appropriate turbulence model. The investigation on the role of the air compressibility in the oil spilling from damaged DHTs reveals that the air compressibility may be considerable in a small temporal-spatial scale (e.g., jet-jet and jet-structure impact pressure), but plays an insignificant role in the macroscopic process of the oil spilling (e.g., spilling discharge and volume). In order to approach the spilling phenomena in the more realistic environment, a systematic numerical study is carried out to investigate the effect of the periodic ship motion on the oil spilling from the damaged tank. Different tank designs (i.e., SHTs and DHTs), accidental scenarios (i.e., grounding and collision) and tank motion parameters (i.e., types, frequencies and amplitude) are considered. The result indicates that the tank motion does not only cause a periodic oscillation of the oil/water flow through the broken hole, but also induces a second long-duration stage of spilling after a quasi-hydrostatic-equilibrium condition occurs, resulting in the more significant amount of spilled oil.

By using both the experimental data and numerical results produced in this research, an improved prediction model for oil spilling from damaged DHTs in still is formulated. This model considers the case-dependent hydrodynamic interaction between the oil and water jet flows inside the ballast tank and its effect on the spilling process. The result using the improved model is compared with the numerical result indicating its superiority over the existing model.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TC Hydraulic engineering. Ocean engineering
Departments:	School of Science & Technology > Engineering Doctoral Theses School of Science & Technology > School of Science & Technology Doctoral Theses

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