Assessment in action
With the background condition, operating parameters, damage mechanisms and established level of FFS confirmed, we can demonstrate the true value of FFS in action.
During a routine inspection localised corrosion under insulation (CUI) was detected above a horizontal stiffener ring in a large vertical vessel. Local metal loss spanned approximately 200mm upwards from the stiffener ring covering locally the entire circumference of the shell. Accurate thickness measurements were not immediately possible due to surface condition, but estimates suggested only 7mm remaining of the original 16mm wall in the worst affected area.
Oceaneering was contacted for advice on how to accurately assess the safety and ongoing operability of the equipment. The production facility had capacity for a short-term, partial shutdown of 5 days for surface preparation and inspection of the damaged vessel. Beyond this, if a longer repair time was necessary, the facility would require a complete shutdown, resulting in substantial financial loss. Primary concern centred around the immediate safety of personnel and equipment, followed by mitigation of any required down time. FFS would provide valuable information on whether continued operation was safe and achievable, while a suitable repair strategy was investigated, designed and implemented.
The vessel dimensions did not satisfy the requirements for Type A component classification, as additional loading conditions had to be considered. The location of the defect, immediately adjacent to a stiffener also did not satisfy Level 2 applicability requirements, indicating finite element stress analysis would be required to evaluate the local stress and strain distribution. Advanced FFS (Level 3) was therefore the only suitable assessment.
Oceaneering quickly provided a preliminary (Level 2) assessment based on the initial information available, while the operator proceeded with short-term shutdown for surface preparation and detailed inspection. Although not suitable to certify the integrity, the indicative assessment provided a preliminary indication of the potential failure risk and the likelihood of a successful Level 3 assessment outcome, thereby enabling the operator to focus immediate efforts on recommissioning or repair. Oceaneering also initiated geometric modelling for FEA to expedite the assessment process.
The original vessel design included consideration of both internal pressure and vacuum conditions, with normal operation under partial vacuum. Oceaneering’s indicative assessment showed that the original design was governed by the vacuum loads, not internal pressure. It anticipated that the vessel would withstand internal pressures well in excess of the maximum design pressure, as well as full vacuum conditions, at the initially reported thickness levels. However, subsequent detailed inspection revealed the metal loss was substantially greater than originally stipulated, with remaining thickness of only 2.5 mm in the worst affected area. This increased the urgency for a Level 3 assessment, and reduced confidence in a successful outcome.
To expedite results, a phased approach to assessment load cases was followed. Firstly, combined vacuum, weight and thermal loads were assessed to demonstrate safety for continued normal operation. The limit load and buckling assessment indicated overall structural stability and adequate resistance to buckling under full vacuum, with no change in design buckling behaviour in the presence of the defect, and no excessive plastic strains. The vessel was therefore deemed fit for continued service for normal operation (partial vacuum) and preparations for recommissioning could commence, with appropriate protections to avoid upset conditions.
Secondly assessment of internal pressure, weight and thermal loads during upset conditions were assessed. The assessment indicated potential structural instability and excessive plastic strains at pressures exceeding 70% of design maximum pressure, requiring derating of the vessel for potential upset conditions. Finally, assessment of wind loads indicated that vessel integrity would not be compromised at design wind speeds.
Oceaneering concluded that due to the low minimum remaining thickness and the required derating for internal pressure, the vessel would not be able to sustain any significant further metal loss. It was deemed fit for short-term continued service, provided it was derated to 70% of its original design maximum pressure, that further degradation was inhibited by temporary corrosion protection, and a suitable repair is designed and implemented in a reasonable timeframe.
Proven benefits of applying FFS
Due to the criticality and urgency associated with FFS, regular progress feedback and preliminary results are critical to enable accurate decision making without the need to wait for a final formal report. For the above example, Oceaneering provided the following value-adding benefits to its customer:
Rapid and responsive indicative results – providing the right level of info for the operator to make an informed decision within three days
Detailed assurance on the condition of the equipment – meaning the vessel could be safely recommissioned under certain conditions in tandem with seeking repair solutions
A cost-effective service that provided valuable insight into the ongoing safety, compliance and remaining life of the equipment - enabling the customer to accurately manage risk to mitigate future safety or operational issues
In most cases, when FFS is conducted by competent industry advocates, the cost of the assessment is greatly outweighed by the benefits of gaining a more detailed understanding of the damage, potential risk, safe operating boundaries and likelihood of repair. FFS provides valuable insight into the risks associated with component and defect combinations and it supports effective future integrity management.
Including FFS technology and assessment capabilities as part of asset management strategy can create substantial operational efficiencies, reduce the likelihood of unplanned and costly repairs. Ensuring that operators are fully aware of what FFS is and why it exists as part of a maintenance programme budget can prevent extended shutdowns, enhance recovery and keep assets safe for longer.
This is part two of a two-part article. Part one is available to read here: https://www.oilfieldtechnology.com/special-reports/18032020/fit-for-service-fit-for-life-part-one/
Read the article online at: https://www.oilfieldtechnology.com/special-reports/19032020/fit-for-service-fit-for-life-part-two/
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