Skirt

Test Your Knowledge

Quiz: Skirts in Oil & Gas

Instructions: Choose the best answer for each question.

1. What is the primary function of a skirt in a vertical vessel?

a) To enhance the vessel's aesthetics b) To provide structural support and stability c) To improve the vessel's heat conductivity d) To increase the vessel's storage capacity

2. Skirts are typically constructed from:

a) Concrete b) Aluminum c) Steel plates d) Plastic

3. How are skirts connected to the vertical vessel?

a) By bolts and nuts b) By welding to the base plate and bottom head c) By adhesive bonding d) By magnetic attraction

4. Which of these is NOT a benefit provided by skirts?

a) Load distribution b) Protection against corrosion c) Increased vessel capacity d) Ease of access for maintenance

5. What type of skirt is used to accommodate changes in diameter or provide additional strength?

a) Straight skirt b) Tapered skirt c) Flanged skirt d) Curved skirt

Exercise: Skirt Design

Scenario: You are designing a storage tank for a liquefied natural gas (LNG) facility. The tank will be 30 meters tall and 20 meters in diameter. You need to choose the appropriate type of skirt for this application.

Task:

1. Consider the factors that influence the choice of skirt design.
2. Based on your analysis, choose the most suitable type of skirt (straight, tapered, or flanged) for this LNG storage tank.
3. Justify your choice with specific reasons related to the tank's dimensions, environment, and functional requirements.

Techniques

Chapter 1: Techniques for Skirt Design and Fabrication

This chapter delves into the technical aspects of skirt design and fabrication, exploring the various methods and considerations that engineers employ to ensure the structural integrity and longevity of these essential components.

1.1 Design Principles:

• Load Analysis: Engineers first conduct thorough load analysis to determine the forces that the skirt will need to withstand. This includes the weight of the vessel itself, the weight of its contents, wind loads, seismic forces, and other relevant environmental factors.
• Material Selection: The choice of material for the skirt is crucial. Steel is the most common choice, with different grades selected based on the specific operating conditions and required strength. Factors like corrosion resistance, weldability, and cost are considered.
• Geometry and Configuration: The skirt's shape and dimensions are determined based on the vessel's size, weight, and the loads it will experience. Design options include straight, tapered, and flanged skirts, each with its own advantages and suitability for different applications.
• Stress Analysis: Finite element analysis (FEA) is widely used to simulate the behavior of the skirt under various loads and to ensure that the design can safely handle the stresses it will encounter.

1.2 Fabrication Techniques:

• Plate Rolling: Steel plates are rolled into cylindrical shapes to form the skirt. This process requires specialized equipment and skilled operators to achieve the desired accuracy and tolerances.
• Welding: Skirts are typically fabricated using welding techniques, which involve joining the rolled plates together. Different welding processes, like shielded metal arc welding (SMAW) and gas metal arc welding (GMAW), are employed based on the project requirements and material properties.
• Quality Control: Throughout the fabrication process, stringent quality control measures are implemented to ensure that the skirt meets the specified dimensions, tolerances, and material requirements. This involves inspections, non-destructive testing (NDT), and documentation.

1.3 Challenges and Innovations:

• Fatigue Analysis: Skirts are subjected to cyclical loads, particularly in harsh environments, which can lead to fatigue failure. Advanced fatigue analysis techniques are used to assess the skirt's long-term durability and to design for optimal resistance to fatigue.
• Corrosion Protection: Skirts require robust corrosion protection due to their exposure to potentially corrosive environments. Coatings, galvanization, and other protective measures are implemented to extend the lifespan of the skirt.
• Emerging Technologies: Innovations like 3D printing and advanced materials are being explored to create lighter, stronger, and more cost-effective skirts, pushing the boundaries of skirt design and fabrication.

Chapter 2: Models and Standards for Skirt Design

This chapter focuses on the various models and standards that govern skirt design, ensuring consistency, safety, and reliability in the oil and gas industry.

2.1 Design Codes and Standards:

• ASME (American Society of Mechanical Engineers): ASME codes, such as ASME VIII (Pressure Vessels), provide comprehensive guidance for skirt design, including stress calculations, material selection, and fabrication requirements.
• API (American Petroleum Institute): API standards, like API 650 (Storage Tanks), offer specific guidelines for skirt design related to storage tanks and other oil and gas infrastructure.
• EN (European Norm): EN standards, such as EN 13445 (Pressure Vessels), provide equivalent regulations for skirt design in Europe.
• Other Regional Standards: Various countries have their own national standards for skirt design, often based on the international codes mentioned above.

2.2 Modeling and Simulation:

• Finite Element Analysis (FEA): This powerful tool is used to create digital models of skirts, allowing engineers to simulate the stresses and strains that the skirt will experience under different loading conditions. FEA helps optimize the skirt's design for safety, efficiency, and cost-effectiveness.
• Computational Fluid Dynamics (CFD): In specific cases, CFD simulations can be used to analyze the flow of fluids around the skirt, particularly in offshore applications where hydrodynamic forces play a significant role.

2.3 Design Considerations:

• Foundation Interaction: Skirt design must take into account the characteristics of the foundation, such as its stiffness and bearing capacity. This ensures that the foundation can adequately support the loads transferred from the skirt and vessel.
• Environmental Conditions: Skirt design needs to factor in environmental factors such as temperature extremes, wind loads, and seismic activity, which can significantly affect the stresses and strains experienced by the skirt.
• Maintenance and Inspection: Skirts are designed to be easily accessible for maintenance and inspection. This ensures that potential issues can be identified and addressed early, preventing downtime and ensuring the long-term integrity of the vessel.

Chapter 3: Software for Skirt Design and Analysis

This chapter provides an overview of the software tools that are essential for skirt design, analysis, and fabrication.

3.1 CAD Software:

• Autodesk Inventor: This widely used CAD software allows engineers to create detailed 3D models of skirts, including their geometry, material properties, and any required connections to the vessel and foundation.
• SolidWorks: Another popular CAD software package, SolidWorks provides a comprehensive suite of tools for designing and modeling skirts, incorporating features like parametric design, surface modeling, and assembly capabilities.

3.2 FEA Software:

• ANSYS: This software is a leading FEA package used for simulating the behavior of skirts under various loads. It offers a range of advanced features for stress analysis, fatigue analysis, and optimization.
• Abaqus: Abaqus is another sophisticated FEA software that is well-suited for modeling and analyzing skirts, especially for complex structures and nonlinear material behavior.

3.3 Specialized Skirt Design Software:

• Skirt Design Software: Some specialized software packages are specifically developed for skirt design, providing streamlined workflows and tailored features for this particular application. These packages often integrate with CAD and FEA tools to facilitate a seamless design and analysis process.
• Foundation Design Software: Software for foundation design is also relevant to skirt design, as it allows engineers to assess the soil conditions, determine the foundation's bearing capacity, and ensure that the foundation can adequately support the loads from the skirt and vessel.

3.4 Benefits of Using Software:

• Increased Accuracy: Software tools enable engineers to create precise models and simulations, improving the accuracy of skirt designs and minimizing the risk of errors.
• Enhanced Efficiency: Automated calculations and design optimization features within software save time and resources, streamlining the design process.
• Improved Collaboration: Software allows engineers to share designs and data with other team members, promoting collaboration and ensuring consistency throughout the project.

Chapter 4: Best Practices for Skirt Design and Installation

This chapter outlines key best practices to ensure the safe and reliable design and installation of skirts for vertical vessels in the oil and gas industry.

4.1 Design Considerations:

• Redundancy: Incorporate redundancy into the design to minimize the risk of failure in critical situations. This may involve designing the skirt with a safety factor greater than the minimum required by codes or using multiple support points.
• Maintainability: Design the skirt to be easily accessible for maintenance and inspection. This includes providing adequate clearance around the skirt, using readily available components, and ensuring that critical areas are easily reachable.
• Environmental Considerations: Take into account environmental factors such as temperature, humidity, and corrosive substances that could impact the skirt's longevity and performance.
• Code Compliance: Ensure that the skirt design adheres to all relevant codes and standards, including ASME, API, and EN regulations.

4.2 Installation and Fabrication:

• Foundation Preparation: Ensure that the foundation is properly prepared to support the skirt and vessel. This includes proper compaction, leveling, and reinforcement if necessary.
• Skirt Alignment: Precisely align the skirt with the vessel and the foundation during installation to minimize stresses and ensure proper load distribution.
• Welding Quality: Use qualified welders and follow stringent welding procedures to ensure the integrity of the welds connecting the skirt to the vessel and the foundation.
• Inspection and Testing: Conduct thorough inspections and non-destructive testing (NDT) to verify the quality of the fabricated skirt and its installation.

4.3 Maintenance and Monitoring:

• Regular Inspections: Implement a regular inspection program to monitor the condition of the skirt and identify any potential issues early on. This includes visual inspections, NDT assessments, and monitoring of environmental conditions.
• Corrosion Control: Implement measures to control corrosion and prevent degradation of the skirt. This may involve coatings, cathodic protection, or other corrosion mitigation techniques.
• Record Keeping: Maintain detailed records of inspections, maintenance activities, and any repairs performed on the skirt. This information is crucial for tracking the skirt's condition and planning future maintenance activities.

Chapter 5: Case Studies of Skirt Design and Installation

This chapter presents real-world examples of successful skirt design and installation projects in the oil and gas industry, highlighting the key challenges, solutions, and lessons learned.

5.1 Case Study 1: Offshore Storage Tank Skirt:

• Challenge: Design a skirt for an offshore storage tank that could withstand extreme weather conditions, including high winds, waves, and seismic activity.
• Solution: The skirt was designed using a tapered configuration for increased strength and stability. Special anti-corrosion coatings and cathodic protection were applied to address the harsh marine environment.
• Outcome: The skirt successfully supported the offshore storage tank throughout its operating life, demonstrating the importance of considering environmental factors in skirt design.

5.2 Case Study 2: Pressure Vessel Skirt with Complex Foundation:

• Challenge: Design a skirt for a large pressure vessel that needed to be supported on a complex foundation with uneven soil conditions.
• Solution: FEA simulations were used to analyze the stresses and strains on the skirt, taking into account the foundation's irregular geometry and varying soil properties. Adjustments were made to the skirt's design to ensure stability and prevent uneven settling.
• Outcome: The skirt successfully supported the pressure vessel, demonstrating the effectiveness of FEA simulations for optimizing skirt designs in challenging conditions.

5.3 Case Study 3: Skirt Replacement Project:

• Challenge: Replace the skirt on an aging pressure vessel, minimizing downtime and ensuring minimal disruption to operations.
• Solution: The replacement skirt was fabricated off-site and pre-assembled to streamline the installation process. Carefully planned rigging and lifting operations ensured that the new skirt was installed efficiently and without incident.
• Outcome: The skirt replacement project was completed successfully, demonstrating the importance of planning and coordination in minimizing downtime and maintaining operational efficiency.

Conclusion:

Skirts are essential components in the oil and gas industry, providing structural support, load distribution, and corrosion protection for vertical vessels. By adhering to sound design principles, utilizing advanced software tools, and following best practices, engineers can ensure that skirts remain safe, reliable, and efficient components throughout their service life. Case studies highlight the challenges and successes associated with skirt design and installation, providing valuable lessons for future projects in the oil and gas industry.