Living with defects – Replace/repair or prove Fit-For-Service?

RBI guru and FFS assessment expert RON SELVA, Engineering Director of PP SIMTECH Solutions Ltd (UK), reveals via 3 examples how to get the best out of deteriorating plant items and optimise spend.

“PP SIMTECH has saved clients substantial sums of money using this holistic approach, incorporating FFS and RBI to successfully demonstrate the integrity of aging plant items.”

When oil, gas, petrochemical or fertilizer plant items are replaced or repaired simply because, for example, the corrosion allowance has been used up or cracking has been detected or material property changes and/or metallurgical degradation are suspected, the cost implication to companies is enormous. With the availability of established proven Fitness-For-Service (FFS) assessment technologies, rejection of these items is not necessarily automatic.

Application of proven state-of-the-art FFS technologies based on active and potential Damage Mechanisms (DMs) and their root causes, along with best-practice Risk Based Inspection (RBI) technology is changing the way in which such decisions are made to optimize spend, whilst ensuring safety and reliability of the affected equipment.

For many deteriorating or aging plant items the RBI implementation is supported by FFS assessments to derive the optimum remaining life and inspection intervals based on acceptable level of risk and consequences of failure. A comprehensive knowledge of DMs and their root causes in static equipment deterioration based on reliable operating data are crucial to correct application of FFS and RBI technologies.

By this principle, an item is considered to be fit for the intended service, if it can be demonstrated (with acceptable safety margin) that the conditions to cause failure is not reached within a predetermined time period, giving due regard to its integrity risk and the HSE and Business consequences of failure.

This holistic approach to asset integrity management guarantees delivery of the 5 strategic goals aimed for by plant sites, which are: desired operational reliability between TAs; desired optimum plant run-length time between TAs; maximum cost effective life out of aging equipment; optimum inspection interval for each item; optimum spend on CapEx; and optimum spend on RevEx.

Scope of FFS Application includes all types of pressure vessels (reactors, crackers, distillation columns, absorbers, strippers, reformers, fired heaters, storage tanks; utility plant items such as boiler drums, de-aerators, headers).

Codes & Standards used in an FFS assessment include BS-7910 and/or API-579. The assessor may also have to refer to design codes such as ASME-VIII and/or British standard BS-5500 and other Guidance documents issued by recognised Associations or Regulatory Bodies.

Depending on the reasons for the FFS assessment, the Output may include one or more of the following - tolerable corrosion/erosion damage sizes and damage rates; tolerable crack sizes and crack growth rates; remaining life; integrity driven operating limits and other risk mitigating measures; design improvements; suitable intrusive and/or non-intrusive NDT inspection methods.

The Output of an FFS assessment becomes an input to RBI implementation team study to formally determine whether - to run the item as it is and at what optimum inspection interval; to monitor the defect and at what monitoring frequency; to repair / replace item and decide when it should be carried out; to revise operating conditions; and to modify design or upgrade material. These decisions will be influenced by the RBI study output such as the Risk profiles of the applicable DMs and the respective HSE and Business Consequences of Failure.

"This holistic approach to asset integrity management
guarantees delivery of the 5 strategic goals aimed for by plant sites"

3 examples are given to illustrate the substantial benefits of FFS application in conjunction with RBI.

Project-1: FFS assessment of several redundant Distillation Columns for re-use on a new duty.

Under the previous duty, the columns had suffered various levels of localised corrosion damage (below design corrosion allowance) and cracking at some weld seams. New duty involved higher working pressures and temperatures and a low temperature pressurised start-up.

Approach: FFS assessment was supported by stress analysis, fracture mechanics, material damage studies including toughness testing, supported by specific NDT inspections to assess condition of the columns, followed by RBI team study.

Outcome:

◙ Mechanical integrity of the Columns on new duty was established proving that the previous damage, apart from 2 defects, did not require any repairs and the respective DMs confirmed inactive under new duty.
◙ RBI plan incorporating optimum inspection interval was implemented, with additional NDT to match newly identified potential DMs under new operating conditions.
◙ Several $million savings on capital expenditure.

Project-2: FFS assessment of Pressure Swing Absorbers (20 years old), which were approaching their allowable design pressure cycles. Requirement was to establish the remaining life based on actual operating cyclic pressures; future optimum inspection interval and scope of inspection; and replacement strategy for the vessels.

Approach: The FFS assessment was supported by stress analysis, initial fatigue analysis using BS 5500 method to assess remaining life (cycles) and a refined remaining life assessment based on fracture mechanics approach (BS7910) to establish crack growth rates and allowable crack sizes. The latter assessment was based on postulation of a surface crack size that can be reliably detected by established NDT inspection methods. This was followed by RBI team study.

Outcome:

◙ Mechanical integrity of the Absorbers was proven for a minimum of further 12 years.
◙ Optimum inspection interval from RBI study output set at 6 years (alternative turnarounds) incorporating two reliable Intrusive and Non-Intrusive NDT methods with extensive inspection scope covering vulnerable areas of the vessels where fatigue cracking can initiate.
◙ Deferred substantial capital expenditure for replacement absorbers/piping.

Project-3: FFS assessment of a 15,000T Ammonia Storage Tank (double wall construction) operating at 50 mbarg and at -33°C. Its last internal inspection was in 1994, with its next inspection due in 2006 (after 12 years). The MPI inspections required for the Inner tank to detect SCC (Stress Corrosion Cracking) at internal welds are prohibitively expensive, due to size of tanks and preparatory requirements. The key DM which drives internal inspection is SCC, which is caused by presence of O2 in the liquid ammonia.

There is also interest industry-wide to minimise the number of Intrusive inspections by supporting it with Non-Intrusive inspections for SCC, because the potential for damage is most likely as oxygen can get into the tank during re-commissioning. For these reasons, the project scope was to assess the possibility of deferment of internal inspection to 2012.

Approach: FFS assessment was carried out supported by our RBI technology process to assess the possibility of deferment of inspection to 2012. Extensive work was initiated following the RBI team study which involved:-

◙ Comparing the RBI outcome with the EFMA RBI process
◙ Fracture mechanics work to establish critical SCC defect sizes
◙ Setting-up a minimum sub-critical defect size having an acceptable safety margin (against critical size)
◙ Developing a new on-line ultrasonic procedure to detect this minimum size defect on a manufactured test piece
◙ Qualifying NDT technicians
◙ Development of an on-line entry regime into tank annular space & risk assessment
◙ Carrying out the on-line ultrasonic inspection
◙ Installing an oxygen monitoring system

Outcome:

◙ Inspection interval was safely extended to 2012.
◙ Non-Intrusive ultrasonic inspection was implemented to inspect the most vulnerable welds on a sample basis.
◙ Procedure was implemented to ensure no ingress of O2.
◙ The local Regulatory Body approved the work carried out.
◙ This outcome deferred inspection costs of ~£1m to 2012, whilst reducing the potential for SCC initiation.

PP SIMTECH has saved their clients substantial sums of money using this holistic approach incorporating FFS and RBI to successfully demonstrate integrity of aging plant items, which have been subjected to deterioration caused by various DMs. This article complements the Executive Interview on ‘Best Practice RBI Implementation', published in the Oct 2010 Issue of NG Oil & Gas Journal.