BJ Services Keeping Ireland Green

Recently, BJ Process and Pipeline Services (BJ PPS) completed a pipeline degassing operation in a remote and environmentally sensitive area of Ireland. The asset operator needed to clear a 9.5-mile (15.3-km) section of an 18-inch diameter natural gas pipeline and render it safe to tie-in a new section. In the past, crews would have depressurized the line to as low a pressure as possible before "cold venting" the residual gas to the atmosphere. New quality, health, safety and environmental (QHSE) policies, however, deem the practice of cold venting as detrimental to the environment and potentially hazardous To ensure that the pipeline operation complied with local environmental protection legislation, BJ PPS provided 'clean burn' flaring services that proved to be a successful alternative.

Consideration of the Kyoto Protocol targets and 'carbon credit' systems demonstrate why the practice of cold venting is not desirable. Clean burn flaring offers a viable alternative. Both of these environmental systems function on the basis of carbon dioxide (CO2) as a common measurement for various harmful gases. Methane has a contributory factor of twenty one (21) times that of carbon dioxide with respect to global warming, and accounts for sixteen percent (16%) of global greenhouse gas emissions when measured in terms of carbon dioxide equivalents (CO2e). Flaring the gas is advantageous as it converts methane molecules to less harmful carbon dioxide. For example, clean burn combustion of one tonne of methane produces 2.75 tonnes of carbon dioxide (CO2). This yields a potential saving of 18.25 tonnes carbon dioxide equivalents (CO2e) versus cold venting of the same quantity of methane.

Achieving a clean burn requires a number of conditions to be simultaneously controlled.

• The waste gas/air mixture must be regulated to ensure sufficient oxygen for all waste gas to be combusted
• Gas velocity at the burner tip must be high enough to cause turbulence and ensure good mixing of the waste gas and air
• Retention time in the burner must be long enough to allow complete combustion (counter-balance with turbulence requirements)

Failure to control these conditions can lead to incomplete combustion with the subsequent release of unburned waste gas, undesirable by-products such as carbon monoxide, and solid carbon deposits which can be found in the form of a sooty flame. BJ's 20-megawatt trailer-mounted, self-erecting mobile flare unit is designed to achieve these conditions in the safest possible manner.

Rig-up Made Simple
For the first phase of the Ireland project, the operator transferred product downstream to reduce pipeline pressure to7 barg (101.5 psig). The second phase involved clean burn flaring for the remaining 20,000 cubic meters (706,000 scf)of waste gas and rendering the pipeline inert with nitrogen to make it safe for mechanical work.

The BJ PPS team positioned the mobile flare unit near the pipeline and raised the flare stack from a horizontal to vertical position with the unit's integrated hydraulic power pack. Temporary transfer piping was tied into the client's pipeline while final spool connections were made on the unit and leak checks performed to ensure pipeline integrity. Due to the unit's user-friendly, self-sufficient design, rig-up and function testing were completed within just a few hours of arrival at the job site.

Optimized Combustion
Residual pressure was used to transfer natural gas from the pipeline to the flare unit for incineration. The unit's programmable logic controller (PLC) system regulated waste gas and air feed to the burner tips to ensure optimum combustion conditions. As the job progressed and pipeline pressure decayed to atmospheric levels, the flare unit's blowers drew natural gas from the pipeline to maintain flow rate. Crews also injected nitrogen gas upstream into the pipeline to push the remaining waste gas toward the flare unit and maintain the pipeline at atmospheric pressure. The nitrogen injection rate was carefully controlled to match the waste gas incineration rate of 1,200 cubic meters/hour (about 42,300 scf/hr), minimizing mixing at the interface. Nitrogen was supplied using an ambient vaporizer and liquid nitrogen storage tanks.

The operation was completed when all waste gas was displaced from the pipeline by nitrogen and combustion could no longer be sustained. Gas samples taken from the pipeline verified it was in an inert and safe condition. The line was left under a positive pressure nitrogen blanket at the client's request.

Optimum Flaring Emission Levels Achieved
BJ's on-line flue gas monitoring system allows emissions to be monitored in real time. Information includes material types, concentrations and total quantities in the flue gas emissions. The report format is suitable for presentation to local environmental agencies to illustrate compliance with legislation.

BJ PPS completed the operation in less than 24 hours, with emission levels falling well below the parameters established by the client. There was no visible flame during the job, and noise levels were maintained at a minimal level of less than 85 dBA. The operation was carried out by BJ PPS personnel from its base in Great Yarmouth, England.

How the Mobile Flare Unit Provides a Clean Burn
The 20-megawatt trailer-mounted mobile flare unit that BJ PPS used to carry out this operation provides 'clean burn' services by maintaining optimum combustion conditions at all times, even for varying composition and flow rates. Key features include:

• Programmable logic controller (PLC) system
• Variable burner tip
• Real-time monitoring of waste gas, air feed and flue gas analysis
• Waste gas flow control
• Air feed flow control

The PLC system takes real-time readings from a number of sources. Monitoring devices measure feed-gas, pressure, temperature and flow rate. Flue gas composition is also analyzed. Integrated probes monitor flame and flue gas temperatures. The PLC processes this information and automatically regulates flow rate and pressure of waste gas via a control valve.

Primary air feed to the burner is through three natural air ducts positioned on the base of the stack. Damper positions are automatically controlled to provide optimum air feed rate to the burner tips.

Variable-speed blowers on the unit are also controlled by the PLC system and can be used to either provide excess air to the burner tips for pre-mixing with waste gas, or to draw waste gas from the sources where feed pressure falls at or below atmospheric pressure.

The BJ PPS mobile flaring unit was designed with safety in mind, with particular attention to features that closely monitor gas levels, flame, temperature and sound.