Safer flare design with CFD
Stephen Ferguson, CD-adapco

The flaring of natural gas plays a critical role in the global oil and gas industry. According to the World Bank over 150 billion cubic meters (or 5.3 trillion cubic feet) of natural gas are flared and vented annually, mostly as part of the oil and gas production process).

Flaring systems were originally developed to dispose of the waste gas produced as a side effect of the oil production process, although continuous flaring has been effectively outlawed by strict legislation and economic and environmental concerns. Today, flaring systems are principally deployed as safety systems, protecting the production system from over-pressurisation during the extraction process. During surges in production, gas, and occasionally liquids, are routed by a pressure-relief valve (or by an emergency safety valve) up through the flare-header towards the flare tip.

An obvious consequence of combusting large amounts of natural gas is the considerable amount of radiation emitted by an operational flare. Modern flare systems are specifically designed to reduce the radiation, pollution and acoustic impact of a flare, by using the energy associated with the high-pressure gas to entrain large amounts of air (typically using Coanda effects, or sonic or super-sonic nozzles). These aerated flames are small and have a relatively low radiation signature – however the amount of radiation that they generate is still enough to cause significant damage to personnel and equipment on the installation.

Although the American Petroleum Institute API 521 effectively limits the amount of incident radiation on production surfaces to 1390 Wm-2 during normal operation -- a level at which “continuous exposure is allowed without causing permanent injury” –- the consequences of exceeding those levels can be serious. At 1580 Wm-2, exposure of more than a minute will cause symptoms similar to mild sunburn. At 1890 Wm-2, bare skin will begin to feel pain after 50 seconds of exposure; by 2840 Wm-2 this time is reduced to 30 seconds; at 4730 Wm-2 bare skin will begin to feel pain after 18 seconds, and personnel without protective clothing have just 23 seconds to escape to a safe area.

In practice, safety conscious operators try to limit radiation to well below the minimum API standard. This is partly due to economic necessity as, during unexpected surges in gas production, should the radiation levels rise to unacceptable levels, operators have no choice but to reduce production or to stop it all together, effectively limiting the overall profitability of the installation.

Traditionally, the radiation signature of a flare on a specific installation has been calculated using a combination of empirical calculations and ad-hoc post-installation modification Increasingly manufacturers and operators are turning towards Computational Fluid Dynamics as a way of predicting how flares will perform under realistic operating conditions, before even the first prototype is built.

Computational Fluid Dynamics (or CFD) is a powerful technique that simulates fluid flow phenomena using computer technology. Although its origin is in the aerospace and automotive industries, CFD is increasingly finding application in many areas of the oil and gas industry.

Unlike testing of physical prototypes, CFD simulations are typically carried out at full scale (the computer model has the same dimensions as the actual production platform rather than those of a smaller experimental model). This has the considerable advantage that results can be interpreted directly and do not have to undergo scaling, a process that can introduce a significant uncertainty, especially for problems involving combustion and radiation.

One of the biggest advantages of CFD is that its rapid turn-around time helps to break the dependence of design on pre-existing design codes. Although design conditions are a useful starting condition for offshore design analysis, CFD simulation allows designers to more easily pursue multiple “what if?” scenarios.

CD-adapco has recently performed a number of radiation studies deployed on both fixed and floating units, including a project undertaken on behalf of DPS, a leading engineering design company with a vast amount of experience in the design, supply and support of process equipment for the Oil and Gas industry, which involved the analysis of a flare deployed on a Floating Production Unit (FPU).

Unlike fixed installations, FPUs are limited by stability considerations in the length of boom that they can deploy in order to reduce the incident radiation on the deck. During the DPS study the impact of various mitigation scenarios was considered, including variations in boom angle, installation of physical shielding and the deployment of a protective water curtain. The simulation results allowed the design team to accurately assess which areas of the deck would be exposed to high levels of radiation and to adjust their protection strategy accordingly.

“Using CFD analysis we were able to predict the performance of the flare installation with confidence, which allowed us to carefully select a level of protection that would ensure the safety of personnel and equipment aboard the FPU” said Jasbir Landa, Project Manager for the FPU project at DPS.

The CFD model was also used to examine the influence of wind speed and direction on the flame combustion and the shape of flame that it generates, something that is generally not possible with less sophisticated flare modelling packages. “In this case flame shape had a significant impact on the radiation footprint of the flare, which was something we had to address carefully when choosing a mitigation strategy.”