Refinery embraces risk-based inspection plan

A risk-based inspection (RBI) plan allowed the Conoco Inc. Commerce City, Colo., refinery to markedly cut its inspection costs and evaluate crudes, according to David Long, reliability leader at the refinery.

From May through November of 1998, the Process North American division of Det Norske Veritas (DNV) conducted an RBI of the fixed equipment (process towers, drums, exchangers, and process piping) in the crude unit at Conoco's Commerce City, Colo., refinery.

DNV's Process North America division provides risk and reliability services in process safety and risk, mechanical integrity services, and RBIs. The division also provides field services in materials engineering to the refining and petrochemical industries.

The project followed the American Petroleum Institute's standard RBI methodology. The RBI technology was developed by both DNV and API. The API process ranks processing equipment based on risk and develops an inspection plan for each equipment item in accordance with the safety and financial risk it poses and the damage mechanism driving the risk.

Although Conoco began its quest for an RBI as a way to optimize its overall pressure vessel and piping inspection program, the study uncovered numerous risk management application benefits and fiscal windfalls.

The study enabled the Conoco facility to reduce its future inspection budget on the equipment studied by a factor of six, without any increase in risk.

Before implementing RBI, Conoco's estimated inspection and maintenance-related expenditure for the 10-year plan period was a little over $1 million. Using RBI, the cost of the planned inspections (for the 10-year plan period) fell to just under $200,000.

The study also examined the effect of processing higher sulfur and naphthenic acid content crudes and the intrinsic risks associated with those feeds. Although these crudes potentially yield improved margins, they may also increase plant-degradation rates.

What does RBI involve?

As the first step toward an integrated risk management program, RBI is more than an on-site risk assessment of safety-related issues. It is a program for other types of risk, such as production-loss risks, risk of environmental impact, and off-site risk to the community.

The RBI approach is flexible and allows any combination of these and other risk types to be factored into decisions concerning when, where, and how to inspect process equipment at a plant.

RBI is also not just confined to refineries. It can be used throughout the general processing industry, in all types of processing plants, including chemical and ammonia production plants, to realize significant short-term and long-term savings in inspection and maintenance costs.

Fig. 1 [81,146 bytes] shows the contribution of equipment to risk in a typical plant. After completing a risk assessment, a processing plant usually finds that about 20% of its equipment contributes to 80% of the risk. The way to optimize then is to focus attention on the 20% areas and de-emphasize the 80% areas.

"In the long run, RBI reduces inspection costs and down time. It puts money towards the problem areas. Now, it may take some money from one area and move it to another, but your overall risk is reduced. You're looking at the whole picture, not just one part of it," said Long.

Conoco RBI elements

In the Conoco/DNV study, information from the risk analysis and prioritization was used to identify several types of information:

Priority for inspection, based on risk
Equipment, which required immediate inspection attention, based on the likelihood of failure
Equipment requiring mitigation based on consequence of failure
Vessels which did and did not require entry or downtime to provide adequate inspection
Equipment items which could safely operate with less-frequent inspections
Specific areas and equipment of concern when more- corrosive feed is charged, in this case sulfur and naphthenic acid.

Scope of project

Prior to beginning the project, a cross-functional team of equipment-management professionals was created, consisting of inspection personnel, chemical process engineers, metallurgists, corrosion specialists, fixed-equipment engineers, and refinery management members.

Conoco's initial decision to hire DNV as its RBI inspector was based on a number of factors. Conoco considered DNV's expertise, its broad-based industrial experience, software applications, development skills, mechanical integrity abilities, and the company's pre-existing familiarity with the company's core business operations.

"We used DNV because it had already assisted us in developing our mechanical integrity system, so it was familiar with our processes. DNV's expertise is broad-based. It has a lot of experience and abilities in other industries, and risk-based inspection is new to petrochemicals," Jim Slovacek, chief inspector of Conoco's Commerce City refinery, said.

Deliverables

DNV's project deliverables included:

Models of the refinery unit in the RBI software for three crude feeds
Identification of the likelihood and consequence drivers
Recommended action plans to reduce risks.

Recommended plans were based on alternative cases of 0.1% and 0.15 TAN (total acid number), 0.5% and 0.28 TAN, and 0.8% and sulfur 0.67 TAN crude. TAN is an indication of the presence of organic acids.

DNV prepared a cost benefit analysis comparing Conoco's traditional approach with inspection and recommendations developed from the RBI analysis. In addition, DNV prepared a business case demonstrating the specific advantages of applying RBI in the crude unit and other processing units.

Analysis of the RBI study

Risk for each individual equipment item was categorized according to the standard risk matrix (Fig. 2 [64,867 bytes]). The relative likelihood of a scenario is plotted against the magnitude of the consequence to categorize risk.

Table 1 [21,140 bytes] shows the impact of the increasing corrosivity of the modeled crude slate and the percent of increased risk as a result of processing the higher-sulfur crude. This table does not take into account any inspection that could be done to reduce risk.

"The effect of processing different quality feeds was incorporated in the unit risk-based analysis," said Tony Poulassichidis, a DNV project manager. "Therefore, Conoco was able to check the impact of operational changes to the unit's risk and evaluate the cost-benefit of processing various feeds. A similar approach may be used in refinery units for other operational variables such as, flow rate, pressure, etc."

The study showed that Conoco's crude unit operation presented a low overall risk (both consequence of failure and likelihood of failure) when processing the low-sulfur crudes, as demonstrated by the low percentage of equipment requiring inspection. This relatively low-risk operation contributes to the factor of six reduction in inspection and maintenance cost opportunity.

Processing the medium-sulfur crude, which contains significantly higher levels of H2S and naphthenic acid, increased, however, the potential for thinning corrosion and cracking in the unit in specific equipment. Continuous operation of this more-corrosive crude would result in the need for three times the budget to maintain the same risk level. Conversely, keeping the inspection effort the same would result in a two-fold risk increase (for both safety and financial risk).

Installing a more-resistant metal, however, can lower the risk. DNV and Conoco found that they could lower risk by upgrading the piping and vessel metallurgy through which the crude was processed.

"RBI analysis revealed that the equipment items more prone to risk increase in number when feed is changed," Poulassichidis said.

Fig. 3 [49,371 bytes] shows the risk profile. According to DNV, the RBI focus should be to look at the risk profile and to distinguish the high from the medium and the low. The high risk areas are the weak link and are the most likely to fail.

"Based on a cost-benefit analysis, Conoco, in order to reduce the risk associated with the operational change (feed quality in this study), can choose to upgrade the construction material for these items. So RBI identified the weak link in the chain which may need reinforcement before general replacement occurs."

At the conclusion of DNV's RBI study, Conoco realized that the application of quantitative RBI to its refinery resulted in significant expenditure savings on inspections. Plus, compared with traditional approaches, the RBI plan also reduced the risk associated with the plant.

"In the end, with DNV's risk-based inspection, we were able to better define our equipment risk, better manage our risk, compile our high-risk documentation in a single area (the software program), and utilize RBI as a tool within our mechanical integrity system," Conoco's Slovacek concluded.