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Failure and Process Stability Investigation of Gas Lift Compression System |
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Client
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Petroleum
Development of Oman LLC (PDO)
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Field/Platform
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Saih
Rawl
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Task/Objective
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Investigate
and address system's instability and improve plant availability
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Key Results
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- Stability of the plant
dramatically improved
- Availability increased to 98%
(improvement of around 20%)
- Production increased
- Manning levels significantly
reduced
- Environmental emissions
minimised
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Core
Competencies
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MSE’s
knowledge base
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Field
testing, performance monitoring and validation
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Availability
and failure analysis
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Compression,
process and turbine design
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Dynamic
and static simulation
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Overview
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The client was experiencing persistent compressor failure, frequent facility shutdowns, oil production deferment and gas flaring. MSE solved severe process control problems at the Saih Rawl gas compressor station.
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Saih Rawl produces oil and associated gas from a large number of low and high-pressure wells, with the oil and gas being separated in three separators. Oil is pumped to Qarn Alam for treatment and export. Next, the associated gas is compressed in two parallel trains.
In the early 1990s a new centrifugal compressor with variable-speed motor driver was installed. This driver suffered from operating problems from the outset. The compressor tripped during the day on motor overload and during the night due to surge, all trips causing excessive flaring.
PDO’s internal investigations highlighted process instability and compressor surging as the roots of the problem, and commissioned MSE to study the dynamics of the whole system.
Following our collection of data from the process system and observations of events, MSE created a computer model of the entire system. We then carried out a Dynamic Simulation study covering a wide range of scenarios to fully understand the plant behaviour. This revealed
that the compressor train was operating at the maximum power limit of the electric motor
and power demand exceeded available power during transients. This also explained the failure of the motor on overload or at high winding temperatures - a problem exacerbated during summer when ambient temperatures reach 45-50°C.
The study also confirmed that the process control system was inadequate to perform its task whilst being also over-complex. The master controller could not achieve process stability because it was simply trying to control too many devices, all with very different response rates and time constraints. For example, station recycle valves and flare valves are small and fast acting, whilst other devices like centrifugal compressors are massive, with large moments of inertia, making them slow to react.
MSE investigated a number of alternative process control strategies to achieve plant stability. In the long term Saih Rawl required greater installed power but this could not be justified immediately. We therefore developed an alternative and much more cost-effective solution. We tested alternative set points for separator pressure controllers and optimised the operating conditions, which minimised surging whilst staying within motor maximum power.
We recommended that the control philosophy be simplified, with the master controller being limited to fast-acting devices only, and slow-acting devices be de-coupled.
The new system was implemented and operated successfully during field trials that were supervised by MSE. The scheme has lead to much smoother operation and improved process stability, with a relatively constant compressor speed / power and separator pressure.
The simplification of the master controller allowed PDO to open the recycle valve to avoid compressor surging and maintain stability, thus avoiding excessive flaring.
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