RCM Analysis 'Term of Reference GDU'

In this tutorial an example ToR document is provided aiming at an upstream plant, which can be used in an IMS RCM training. It holds relevant data for an RCM study.
An RCM ToR document is recommended to be drafted when a RCM study is to be done. With management approval of the RCM ToR, a better plan is provided to execute the RCM study and to get approval for the end result, allowing it to be operationally implemented.

This ToR document enables us to illustrate how RCM information flows from the preparation phase to the actual registration in IMS-RCM, and allows the RCM workshops to be conducted with IMS-RCM.

Terms of Reference for IMS-RCM study of a GDU unit

Introduction

After an internal benchmark study done on the Eastland Oil and Gas Production site of the Unified Petrol company last year, an RCM study was scheduled to be executed for this year. This ToR provides the plan for the RCM study of the GDU in the Eastland Oil and Gas facility to be executed in IMS and approved for execution by the Unified Petrol Reliability Manager, justifying the work to be done, resources to be used and results to be reported out.

RCM

RCM is a structured and multidisciplinary decision support process to effectively document the optimum maintenance requirement of a physical asset in its operating context. The general objective of RCM is to improve reliability and availability of Equipment by selecting appropriate PM (Preventive Maintenance) tasks and techniques without jeopardizing Health, Safety and the Environment. A RCM review targets to develop a cost-optimum proactive maintenance plan that will meet the site’s targets in integrity, reliability and HSE. The RCM process used focuses on dominant Failure Modes which are assessed in terms of its risk to the business, indicated as a Criticality, and driving the maintenance effort to reduce the risks to tolerable levels. A cost-benefit Analysis is carried out by comparing the consequences of ‘doing no Preventive Maintenance’ and the cost of ‘doing recommended Preventive Maintenance’ to justify and optimize the PM tasks recommended. This optimization can involve modification of existing tasks, adding new or even delete maintenance tasks if it appears to be not cost-effective; and is expressed in an MEI (Maintenance Efficiency Index) indicating there is economic benefit in doing the maintenance.

A multidisciplinary team coordinated by RCM facilitators carries out the RCM study where the team originates from the plant’s maintenance and operations engineers and crafts – using site specific and Equipment specific data, documentation and local experience. To this end the RCM assessments stored in the IMS-RCM tool come forward from decisions agreed in the multidisciplinary team.

For this RCM study the targets are:

• The RCM Analysis should bring the plant performance of the GDU from quartile 4 performance (90% operational availability) to quartile 2 performance (97% operational availability) according to the latest internal benchmark study.
• High priority repairs should reduce from 20% (Prio 1+2) to 5%.
• The IMS-RCM Analysis should support a Turnaround interval of 4 years, though current TA interval is 1yr and next plans is to go to 2yr TA interval.
• Equipment with a criticality of M-level or higher are analyzed in detail.
• Residual risk should be M-level or lower.
• A MEI score lower than 1 will be ‘run to failure’.
• The MEI score should preferably be larger than 2.
• The number of lower criticalities in the study (N and L) should not be greater than the amount of higher criticalities (M, MH, H and E) (so max 50%).
• The maintenance tasks recommended should be presented in a way to enable efficient execution, where tasks that can be efficiently executed together are presented as a group.
• The Analysis is to be executed in the IMS-RCM web-tool and reported out in a period of 4 months from sign-off date of this ToR.

Plant data

The Glycol Dehydration Unit (GDU) is used to remove 91% of the water and all condensate from produced gas to reach a water content of <= 77ppm (a dew point temperature of 18°F for 1,000 psia).
The normal wet gas flow is 150MMscf/d or 147t/hr (1000psi 90°F / 68.9 bar 32°C).
The financial margin = 0.1$/Sm3 so the production value with a normal gas flow of 176550Sm3/h is 17,655$/hr.

The inlet water is 867ppm (128kg/hr) and the outlet water 77 ppm (11 kg/hr), so there is 116kg/hr water removal. The dry glycol purity if 99.5% (» 0.5% water) and the wet glycol purity 96% (» 4% water).
The normal glycol recirculation flow is 5.4 t/hr (» 4.8m3/hr).

There is no holdup or tankage in the System so any downtime results directly into production loss.
In case of a plant-trip there is 4hrs extra production loss to ramp up and recover production on specification.
In case the glycol circulation stops the plant can run 1hr off-spec (too high a water content in outlet) before the unit stops.

Major plant maintenance is done yearly in the summer when there is low domestic gas demand and a plant shut down can be aligned with the main utility customer: the energy plant. This is a 10 day planned shutdown where inspection is driving the work scope.

Over the year the plant is down for additional maintenance work, often involving cleanout and repair work caused by off-spec operation and linked to glycol degradation. The unit is notorious for glycol consumption.

Unplanned downtime of 1-2 weeks duration has become normal, dropping the plant availability from the 97% target down to 90%, at a cost of 12M$/yr.

In above schematic the unit is shown with basically two Systems: A high pressure contacting System and a low pressure regeneration System.

High pressure contacting System

The function of the high pressure contacting System is to treat 150MMscf/d Natural Gas and remove 91% of the water and all condensate.

Functional Failures of the contacting System causing production loss are:

• Less than 150MMscf/d natural gas throughput of the System
• Unable to remove condensate.
• Unable to remove free water.
• Unable to remove 91% water from natural gas.
• Gas breakthrough to Glycol stream.

Low pressure regeneration System

The function of the low pressure regeneration System is to dry 5.4t/hr glycol and remove the water to create a purity of 99.5% glycol.

Functional Failures of the regeneration System causing production loss are:

• Unable to feed the contactor tower with 5.4t/h glycol throughput.
• Unable to reach 99.5% Glycol purity,
• Unable to circulate clean glycol,

The main Equipment in the GDU relevant for the RCM study are distributed of these two Systems:

Low pressure regeneration System:

 
Based on these 24 Equipment about 50 Failure Modes are estimated for IMS-RCM Analysis – taking about 15 days in Analysis time.

An IMS-RBI study for the GDU is on the year plan for next year with implementation completed on the following year. No SIFpro study is done.

Current operational rounds involve a daily operator route through the GDU, there is no operator round list.
Current maintenance activities involved are:

Level controls: The level controls involve level instruments and control valves and are found on the contactor tower and on the skimmer. Both low and high liquid level in these locations can contribute to serious operational hick-ups, involving off-spec glycol, increased corrosion and foaming.
The typical operator response is to increase the glycol circulation causing the plant heat balance to be disturbed, increasing the operational problems. There is no preventive maintenance done on the level controls. The level control failures and consequential losses contribute to 37% of the unplanned downtime.

Pumps: These belt driven plunger pumps feed glycol to the high-pressure contacting tower. It is a 1-out-of-2 redundancy set up and checked by operations as they are involved in the operator rounds to report issues. Operations can switch over the pumps in case of problems. Current performance is a MTBM of 1 year with many belt failures and valve failures. Also, often the A and B pump are reported to be defect quickly after each other (or on the same time). Glycol circulation pump failures contribute for 26% to the unplanned downtime.

Fire tube heater: Fouling of the burner causes local overheating and decomposition of the glycol. Off spec dry glycol is also found due to low outlet temperatures. Fire tube heater issues contribute to 11% of the unplanned downtime

Heat Exchangers: These are inspected in each TA and the recommendations from Inspection are followed up. Little corrosion involved, no external leakages reported. Current performance is that operations are unhappy with the heat exchangers and that there are often internal leakages and fouling involved. Inspection does not report fouling and requires cleaning just for inspection. The heat exchanger failures contribute for 10% to the downtime.

Columns: The columns are inspected in each TA and the recommendations from Inspection are followed up. Little corrosion involved, no external leakages reported. Every other TA there has been an issue with one of the columns, basically by poor maintenance from contractor workers. This typically relates to internals being ‘non pressure containing parts’ like tray’s, weirs and demisters. These failures contribute for 4% to the downtime.

Vessels: The vessels – which are actually separators - are inspected in each TA and the recommendations from Inspection are followed up. Little corrosion involved, no external leakages reported. Each TA there has been an issue with one of the vessels, basically caused by shortcomings with the internals. These failures contribute for 2% to the downtime.

RCM Methodology References

For further background and details on RCM Analysis these key documents are referred to:

1. RRM S-RCM Manual, Shell Global Sol. V2 OG 04-30260
2. RCM II book by John Moubray; ISBN 9780831131463
3. SAE JA1011, Evaluation criteria for Reliability Centered Maintenance (RCM) Process
4. SAE JA1012, A guide to the Reliability Centered Maintenance (RCM) Standard
5. ISO 60300-3-11, Application guide Reliability centred maintenance
6. ISO 14224; Collection and Exchange of reliability and maintenance data for Equipment
7. IEC 60812; Analysis techniques for System reliability – Procedure for Failure Mode and effects Analysis (FMEA)