ABSTRACT
Failure of offshore oil and gas pipelines occurs under certain
conditions due to some applied mechanical forces. These conditions
constitute a potential threat to the integrity of in-service life span
of the pipelines which can lead to loss of resources and environmental
pollution. Several studies have shown that pipelines fail as a result of
Welding, Fatigue Crack Growth, Corrosion Fatigue, Stress Corrosion
Cracking, and Erosion due to fluid flow.
This paper presents a model by using fracture mechanics to analyze
the allowable applied stresses an in service pipeline needs to withstand
in minimizing crack growth. Furthermore, the crack size, crack shape
and hole radius with pipe thickness will be modeled. The modeling
results will be validated using experimental data. The implications of
the results will be discussed for the design or development of a robust
oil and gas pipelines.
CHAPTER ONE: Background and Introduction
1.1: Research Background
Oil and Gas Pipelines are used as a medium through which petroleum
products are transported from the wells to the tanks. When it is under
operation, it fails rarely; meanwhile, it causes extremely serious
problems like loss of resources and lives if failure does occur. Over
half of all in-service pipelines fail as a result of some externally
applied mechanical forces which must be properly analyzed to prevent
reoccurrence. Fractographic examination is to determine the causes of
failures by studying the characteristics of a fracture surface.
Griffith proposed that cracks that already exist will propagate when
the released elastic Strain Energy is at least equal to the energy that
is required to create the new crack surface. Life prediction for Fatigue
Crack by Paris has showed that range of Stress Intensity Factor, k,
might characterize Sub-Critical Crack Growth under fatigue loading. He
examined that Crack Growth Rate of Stress Intensity Factor gave straight
line.
Also, Rice’s J-integral is a commonly used Elastic Plastic Fracture
parameter for the description of the local field in the neighborhood of
the Stress Concentration and for the study of crack initiation and
propagation. His theory is also interpreted as the potential difference
in energy between two specimens that are loaded identically having
slightly different crack length. Meanwhile, Irwin proposed the Stress
Intensity Factor as crack primary driving force.
Neumann and Raju estimated the Stress Intensity Factor for hollow
Cylinder for specific Crack Aspect Ratios, Crack Depth to Thickness and
Hole Radius to Thickness. Alexander Aynbinder evaluated the thickness of
High Temperature (HT) and High Pressure (HP) pipe walls and combined
the inelastic behavior of pipe steels by using iterative computational
modeling algorithm. Idriss Malik used Lame’s solution to estimate
stresses and Neumann and Raju solutions to calculate Stress Intensity
Factor for the combined modeling of the wall thinning and crack
propagation in pipelines.
Oil and Gas Pipeline reliability is affected by welding defects,
corrosion and stresses that cause cracking. Therefore, the applied
stresses, Stress Intensity Factor, Composition, and Temperature etc are
highly considered for the prediction of the integrity of offshore
pipelines. Additionally, Stresses that are resulted to Sub-Critical
Crack Growth is a major challenge to the pipe, while the loss of
materials can result from the interaction between erosive fluids flow
and corroding pipeline.