PMech Consultant Pvt. Ltd.

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Double Wall Tank Engineering | Dome Roof Air Rising Methodology | LPG Storage Tanks

PMech delivered complete engineering design, various specialized engineering analysis, and detailed engineering solutions for double wall storage tanks with fixed dome roofs designed for propane and butane storage (diameter ~75 m and 65 m), as part of a major Oil & Gas expansion project in the Middle East. This scope required high precision, multidisciplinary coordination, and adherence to international standards.

The project required advanced structural modelling, load analysis, and engineering validation, ensuring compliance with API 620, API 625, and AISC 360 standards along with client specifications.

A key highlight of this project was the implementation of the dome roof air rising methodology, supported by simulation-driven engineering using STAAD.Pro and ANSYS, enabling safe, efficient, and execution-ready solutions for large-scale LPG storage tank engineering. This dome roof air rising methodology is also the focus for this case study.

Engineering Scope & Deliverables :

PMech delivered comprehensive multidisciplinary engineering services for storage tank systems, covering detailed design, advanced analysis, and execution support:

Design & Engineering Deliverables :

• Tank Design & Detailing

• General Arrangement (GA) Drawings

• Dome Roof Design & Drawings

• Air Rising Methodology Engineering

• Nozzle & Appurtenance Layouts

• Welding & Joint Details

• Structural Steel Detailing

• Foundation Loading Data

• Load Analysis Reports

• Fabrication & Erection Drawings

• Quality Assurance Documentation

Advanced Engineering & Analysis Capabilities :

• FEA Analysis (Finite Element Analysis) for structural validation

• Blast Load Analysis using time-history simulation

• Thermal Analysis for temperature impact assessment

• Radiation Analysis for operational safety evaluation

• BOG (Boil-Off Gas) Calculation for process optimization

• In-Tank Pump Selection and VDR (Vendor Data Review)

• Fire Fighting System Design and Detailing for storage tanks

• Construction and Commissioning Support for site execution

• As-built support

Engineering Approach for Double Wall LPG Storage Tanks :

In large-scale Oil & Gas storage infrastructure, the safe and efficient storage of liquefied gases such as propane and butane is critical. The double wall LPG storage tanks in this project were engineered using advanced finite element modelling (FEM) with ANSYS software and proprietary PMech tools, ensuring high structural integrity and long-term operational reliability.

For this project, PMech carried out a comprehensive analysis covering:

• Blast load analysis using time-history simulations in STAAD.Pro Advanced and ANSYS software

• Radiation impact analysis using ANSYS software

• Evaluation of insulation systems such as perlite blocks and wool glass systems

• Local structural analysis of tank shell, roof plate, and bottom plate for nozzle, pump, and platform loads

• Firefighting system design for storage tanks

• Design of temporary structures required during erection phases

Dome Roof Structure – Load Complexity :

The dome roof structure, widely adopted in industrial storage tank engineering, offers efficient load distribution and structural strength. However, in this project, the dome roof was required to support a significantly large operating platform of approximately 35 m × 17.5 m × 18 m height.

This platform supported pump well systems, large piping and large valves, overhead monorail for lowering the pumps. In addition to Main operating platform, PSV Platform & Walkways are designed on top of Dome roof.

These requirements introduced complex and huge amounts of load transfer challenges on dome roof rafters, necessitating detailed structural analysis and precise engineering validation.

Why Air Rising — and How It Works :

To address the limitations of conventional erection methods - like safety, quality, time and cost impact challenges - the air rising method for dome roof erection was adopted. This method enables the dome roof to be assembled and welded at ground level and then lifted into position using controlled internal air pressure.

Key Engineering Considerations :

• Blower sizing and selection for controlled air pressurization

• Air-tight sealing of tank shell openings

• Structural integrity under internal air pressure during lifting

• Simulation of real execution scenarios using STAAD.Pro structural analysis software

Key Advantages :

• Significant reduction in work at height → improved safety

• Enhanced welding and inspection quality

• Parallel construction activities → reduced project duration

• Improved alignment and construction precision

Key Engineering Challenges & Solutions :

Despite the advantages, the air-rising method introduced three specific challenges that needed to be resolved through analysis before construction began.

1. Centre of Gravity (COG) was Off-Centre :

Due to uneven structural rafter configurations supporting heavy platforms, the centre of gravity (COG) of the dome roof was not aligned with the geometric centre. This created a risk of uneven lifting and structural instability during the air rising process.

Solution :

• Strategic addition of extra roof plates to redistribute mass

• Validation using finite element modelling and STAAD.Pro analysis

• Alignment of COG with the geometric centre

2. Controlling Radial Lateral Deflection of Dome Roof:

During air rising, radial forces caused by internal air pressure and structural flexibility resulted in lateral deflection of the dome roof, potentially affecting geometry, load distribution, and can prevent proper seating on the tank compression ring region. If uncontrolled, this can lead to permanent deformation or difficulties in welding and sealing the roof to the compression ring.

Solution :

• Installation of high-tensile pre-tensioned ropes across the diameter

• The ropes are designed with sufficient tensile capacity to withstand the expected loads, ensuring the dome roof maintains its intended geometry.

• Continuous monitoring of rope tension and roof deflection

3. Stability After Removal of Temporary Supports:

Removal of temporary supports resulted in displacement exceeding 1000 mm against an available clearance of only 95 mm, posing a significant risk of obstruction.

Solution :

• Detailed STAAD.Pro structural simulation of actual behaviour of Dome roof after removing the fixed support & keeping vertically rested.

• Additionally, the roof plate welded to rafter & Friction resistance developed between Steel rafter to Steel plate is useful to resist the compression load developed under its self-weight. These modifications keep dome roof stable against its oven radial displacement, even after removing of fixed support.

STAAD.Pro Structural Analysis & Validation :

The structural analysis and validation process was carried out using STAAD.Pro, including:

1. Model Creation – Define dome roof geometry (diameter, curvature, thickness, stiffeners).

2. Material Assignment – Apply steel properties per API 620/625 & AISC 360.

3. Load Definition – Apply dead load, air pressure.

4. Boundary Conditions – Fixed but condition laterally due to ropes used to balance the dome against overturning & vertically released as dome is free to move in vertical direction with internal air pressure.

5. Analysis Setup – Run static/nonlinear analysis; apply load combinations per codes.

6. COG Verification – Check reactions; adjust plates until COG aligns with geometric centre.

7. Deflection Control – Monitor node displacements; validate rope effectiveness within limits.

8. High tensile ropes introduced to control displacement behaviour of roof.

9. Result Interpretation – Review stresses and deflections; confirm compliance with tolerances.

10. Validation & Reporting – Cross-check with manual/FEM; prepare report on COG balance and deflection control.

11. Site Execution – Implement STAAD.Pro solutions (extra plates, ropes) during roof air rising.

Dome Roof Air Rising Procedure :

The execution of the dome roof air rising methodology followed a structured and controlled process:

• Structural preparation and inspection

• COG balancing and validation

• Air-tight sealing and pressure system setup

• Rope anchoring and tensioning

• Controlled air pressurization and lifting

• Final alignment and welding to compression ring

• Inspection, quality assurance, and documentation

Conclusion :

PMech successfully delivered engineering design, structural analysis, and execution support for double wall LPG storage tanks with dome roof structures, overcoming complex challenges associated with the air rising methodology.

The project achieved full compliance with API 620 and API 625 standards, ensuring safety, quality, and structural reliability.

Through advanced structural analysis, simulation-driven engineering, and execution-focused design, PMech demonstrated its expertise in delivering high-performance solutions for Oil & Gas storage infrastructure and industrial tank engineering projects.