Description
ABSTRACT
In this research study, the pitch motions analysis of the Agbami FPSO vessel moored in the deep sea and subjected to head sea, is carried out. The study includes the linear wave analysis of the motion of the vessel on the basis of Airy’s wave model and the strip theory, to determine the excitation forces and moments, hydrodynamic coefficients, and the response amplitude operators. The result of this was used to carry out a spectral analysis with the modified Pierson-Moskowitz wave energy density spectrum on the basis of the 100-year period storm data of the West Africa offshore, to account for the irregularities associated with a real sea state, while predicting the responses of the vessel. Furthermore, a computer programme (HaPMotA), which made use of the formulae derived from the regular wave and spectral analyses, was developed using MATLAB script for accurate and expeditious predictions and analyses. The result indicates that the pitch motions occur at opposite phases. Moreover, the most probable maximum pitch motion responses of the vessel as predicted for offshore West Africa were found to be within the allowable motion limits for the smooth functioning of oil separators.
Cover page
Title page I
Abstract ii
Declaration iii
Certification iv
Acknowledgement v
Dedication vi
Table of Contents vii
List of Tables
List of Figures
List of Plates
Nomenclature
Chapter 1
Introduction
Objectives of the study
Chapter 2
Literature Review
Chapter 3
Materials and Methods
Chapter 4
Results and Discussion
Chapter 5
Conclusion and Recommendations
References
Nomenclature
k Wave number
Φ Velocity potential ξ̂ Amplitude of wave elevation
t Any time instant
g Acceleration due to gravity
ρ Density of sea water
ε Phase displacement
η̂3 Amplitude of heave response
Sr(ω) Vessel response Sw(ω) Wave spectral density
FPSOs Floating, Production, Storage and Offloading units
CFD Computational fluid dynamics
Table of Contents
List of Tables
Table1:MainParticularsoftheVessel
List of Figures
Figure1:Magnification Factor
Figure2:PitchMomentswith Wavelength
Figure3:PitchRAOwith Wavelength
CHAPTER ONE
1.0 Introduction
1.1 Background of the study
Energy is the major need for every human being. Human need for energy has been on steady increase during the past three decades now, resulting in a worldwide demand for Floating, Production, Storage and Offloading units (FPSOs). Because of the economic advantage the FPSO vessels have over oil platforms in adapting to different water depths, its flexibility and mobility among other benefits, their use in the Nigerian oil fields have soared (Wang et al., 2018). Thus, about fourteen (14) FPSOs are currently working in the offshore oil fields in Nigeria.
Agbami is a producing conventional oil field located in deepwater in Nigeria and is operated by Star Deep Water Petroleum. According to GlobalData, who tracks more than 34,000 active and developing oil and gas fields worldwide, the field is located in block OML 127 (OPL 216) and OML 128 (OPL 217), with water depth of 4,528 feet.
The Agbami conventional oil field recovered 88.79% of its total recoverable reserves, with peak production in 2011. Based on economic assumptions, production will continue until the field reaches its economic limit in 2031. The field currently accounts for approximately 4% of the country’s daily output(Wang et al., 2018).
According to Xu (2017), ship motion study is the initial point of ship’s sea keeping research, and is usually investigated in four ways namely: theoretical analysis, model experiment, empirical formulae, and trials analysis. The theoretical analysis of ship motion involves the methods of strip theory, spectral theory, and computational fluid dynamics (CFD) technique. Depending on what approach adopted to solve motion problem, the theoretical analysis of ship motion can be done in the frequency domain or in the time domain. However, Chakrabati (2017) reported that analysis done in the frequency domain results in less computation time compared to time domain, and gives satisfactory results as the effect of the nonlinear drag force is less. The analyses in this research were all done in the frequency domain.
Ship motion can be analyzed independently or coupled as can be seen in (Heurtieret al., 2011). Although the uncoupled approach is uncomplicated and simple, it does give fairly accurate predictions and used extensively in the preliminary design stage of a vessel and its mooring system.
The vertical motion modes which are introduced by hydrostatic restoring forces significantly affect the response of a ship or floater subjected to wave loads in deep waters. Although the West African offshore region have been a lucrative site for oil exploration with FPSOs, not much work have been reported on the uncoupled motion of FPSO there with the metocean data of this benign region.
This study modeled the Agbami FPSO as a rectangular shaped floating body spread moored in the deep sea and subjected to head sea waves. The mooring lines are considered to be of negligible mass; as such the hydrodynamic loads on the mooring lines will be neglected. The research thus first determined the motion behaviour of the vessel in regular waves and then carried out a spectral analysis to predict the responses of the vessel in a realistic sea. The forces and moments, and the hydrodynamic coefficients were determined by the strip theory. On the basis of the formulae and principles so evaluated, a computer programme that can evaluate and predict the motion of an FPSO is developed.
Objectives of the study
- To investigate the pitch motion of the Agbami FPSO in regular wave
Ii. To analyse the pitch responses of the Agbami FPSO in irregular waves
iii. To develop a computer software to analyse the pitch motion of Agbami FPSO
