Living with defects: replace/repair or prove fit-for-service?

By Ron Selva

When oil, gas, petrochemical or fertilizer plant items are replaced or repaired simply because the corrosion allowance has been used up, cracking has been detected or material property changes and/or metallurgical degradation are suspected, for example, the cost implication to companies is enormous. However, 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 optimise 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 is 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 (within acceptable safety margins) that the condition 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 five 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.

The scope of FFS application includes all types of pressure vessels (reactors, crackers, distillation columns, absorbers, strippers, reformers, fired heaters, storage tanks and 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; and suitable intrusive and/or non-intrusive NDT inspection methods.

The output of an FFS assessment becomes an input to the RBI implementation team's 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 the 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.

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

Project 1

Scope: This project involved the 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. The new duty involved higher working pressures and temperatures and a low temperature pressurised start-up.

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

Outcome: The mechanical integrity of the columns on new duty was established, proving that the previous damage, apart from two defects, did not require any repairs and that the respective DMs were confirmed inactive under new duty. An RBI plan incorporating optimum inspection intervals was implemented, with additional NDT to match newly identified potential DMs under the new operating conditions. Several million dollars were realised in savings on capital expenditure.

Project 2

Scope: This involved FFS assessment of pressure swing absorbers (20 years old) that were approaching their allowable design pressure cycles. The requirement was to establish the remaining life based on actual operating cyclic pressures; future optimum inspection intervals and scope of inspection; and a 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 a 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: The mechanical integrity of the absorbers was proven for a minimum of a further 12 years. An optimum inspection interval based on RBI study output was set at six 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. The project helped defer substantial capital expenditure for replacement absorbers/piping.

Project 3

Scope: This involved FFS assessment of a 15,000T ammonia storage tank (double wall construction) operating at 50mbarg 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 were to detect stress corrosion cracking (SCC) as internal welds are prohibitively expensive, due to the size of tanks and preparatory requirements. The key DM that drives internal inspection is SCC, which is caused by the presence of O2 in the liquid ammonia. There is also interest industry-wide to minimise the number of intrusive inspections by supporting these with non-intrusive inspections for SCC. The potential for damage is more likely during the former 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 2010. 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), and developing a new on-line ultrasonic procedure to detect this minimum size defect on a manufactured test piece using qualifying NDT technicians; development of an on-line entry regime into tank annular space and risk assessment; carrying out the on-line ultrasonic inspection; and installing an oxygen monitoring system.

Outcome: The inspection interval was safely extended to 2012, while non-intrusive ultrasonic inspection was implemented to inspect the most vulnerable welds on a sample basis. This procedure was implemented to ensure no ingress of O2, and the local regulatory body approved the work carried out. This outcome deferred inspection costs of around UK£1 million to 2012, whilst reducing the potential for SCC initiation.

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 that have been subjected to deterioration caused by various DMs.

This article complements the executive interview on 'Best Practice RBI Implementation', published in the October 2010 Issue of NG Oil & Gas.

About

Ron Selva has over 35 years of industry recognised experience relating to static equipment integrity, with the last 20 years specialising in the development and application of best practice RBI and fitness-for-service assessment technologies. He has published many papers on these subjects. He is also a member of several relevant British Standards Technical Committees.