The Asset

If total E&P U.K.’S north sea operations were an oyster , the pearl that has contributed significantly to the international conglomerate’s bottom line would be its prolific Alwyn North B platform that lies within the U.K. sector of the North Sea .

The North Alwyn B platform (popularly known as NAB Processing) is a facility that processes oil and gas produced from the Alwyn North, Nuggets, and Grant/Ellon/Dunbar fields. In essence, the NAB facility is responsible for processing about 180,000 barrels (approximately 28,600 standard cubic meters) per day of crude oil and 600 million standard cubic feet (roughly 17 million standard cubic meters) per day of gas. This translates into gross revenues of approximately $8,250,000 (based on a crude oil price of $30 per barrel and $4.75 per million standard cubic feet).

NAB Processing is responsible for feeding the Sullom Voe Oil Terminal in Shetland , Scotland and the St. Fergus Gas Terminal in Peterhead , Scotland . At NAB Processing, gas is initially processed, then compressed with the aid of multi-stage compressors, and finally transported to the St. Fergus Gas Terminal via the Frigg pipeline system.

The Damage and the Challenge

In the summer of 2002, the NAB Processing facility experienced severe problems with its compression system that jeopardized the gas supply to the St. Fergus Gas Terminal. Severe noise and vibration in the platform’s existing control valves were the first indicators of these problems. The intensity of the noise and resulting valve vibration quickly escalated into a critical situation that threatened to impair the performance of the compression system.

As a result of the rapidly deteriorating situation on the NAB Processing platform, Total E&P U.K. called on the experts within the field of controls equipment (CCI) and engineering, procurement, and construction (Kellogg Brown and Root Production Services). An expedited detailed analysis of the compression process revealed that the increased capacity requirement at the processing facility had caused severe noise and vibration and rendered the current piping and control equipment critically unstable. To prevent catastrophic failure of the gas exportation system and to minimize lost revenue, it was crucial to revamp the piping and controls equipment as quickly as possible.

Initial analysis revealed a need to increase the pipe sizes and control valve sizes to accommodate the system’s increased capacity. Due to the massive revenues at stake, secondary analysis conducted by the three parties determined the specific requirements that would ensure the compression system adhered to the highest industry standards.

The Compression System and the Recycle Valve

The gas compression system primarily consists of a compressor and a compressor recycle valve. Gas is compressed to high pressures by the compressors and then sent via pipeline to the destination point. The pressure to which gas is compressed depends on fluid mechanics as well as contractual requirements.

For a gas compression system, the control valve is by far the most critical piece of equipment. An improperly configured control valve can not only have devastating effects on the compressors, but it can also cause catastrophic failure of the entire system, resulting in lost time, production, and profits.

Main Features of a Compressor Recycle Valve

The primary function of a recycle system is to prevent surge conditions for the compressor by maintaining a certain minimum flow rate corresponding to the compressor speed. In the event of a surge condition, the control valve becomes critical – it must open rapidly (typically in less than two seconds) to relieve the surge condition. For this reason, compressor recycle valves are often referred to as anti-surge valves.

Noise is usually the next critical concern for the compressor recycle valve. In addition to being hazardous to personnel aboard a platform, extremely high levels of valve-generated noise can give rise to severe system vibration, which often ends in catastrophic system fatigue and failure.

Capacity and controllability are two critical factors that must be weighed carefully when designing a compressor recycle valve. On one hand, it is essential that the valve have adequate capacity to deal with the surge conditions. However, too much capacity will cause the valve to operate at very low lifts for normal operating conditions.

Improper actuation and controls selection combined with an “oversized” valve can lead to controllability problems. Often, limiting the stroke lengths of valves can result in over-dependence on the actuation and controls equipment, leading to instability issues such as hunting.

The Solution

To ensure reliable production for the future, officials from Total E&P U.K. , Kellogg Brown and Root Production Services, and CCI banked on designing NAB Processing’s compressor recycle valve based on design criteria recommended by the Instrumentation Society of America (ISA).

CCI constructed two 12 x 16-inch, 1500 ANSI valves of 316 stainless steel material in angle body configuration. To satisfy and exceed ISA’s trim exit velocity head criterion of 480 KPa, CCI utilized DRAG® technology to divide flow into many streams to minimize the mass and energy levels. In CCI’s DRAG® trim, each flow passage consists of a specific number of right-angle turns to form a tortuous path in which each turn reduces the pressure of the flowing medium.

For Total E&P U.K. ’s customized solution, CCI equipped each valve with 6-inch trims including 24 pressure-reducing stages (right-angle tortuous paths). The right-angle turns successfully dropped the fluid velocity to levels that provide the expected control, thereby eliminating noise and vibration. The noise levels previously recorded at 115 dBA were markedly reduced to below 80 dBA after the CCI valves were installed. Each of these trims was also constructed out of 316 stainless steel material for maximum durability.

CCI specializes in such customized solutions, and under normal circumstances the procedure for proposing, designing, and manufacturing the solution for Total E&P U.K. would have been standard. However, with millions of dollars at stake in this case, the procedure was anything but standard. In order to provide the valves by the target platform shutdown date specified by Total E&P U.K. , CCI had eight weeks to turn a list of system requirements into a customized solution and deliver it to the platform. With superb teamwork between CCI, Total E&P U.K., and Kellogg Brown and Root Production Services, the valves were completed in record time (and a day to spare!) without any safety incidents.

An Example for the Industry

CCI’s compressor recycle valves at NAB Processing were installed in October 2002 and passed initial testing with flying colors. To date, the valves have ensured the smooth supply of gas from the Alwyn North, Nuggets, and Grant/Ellon/Dunbar fields to the St. Fergus Gas Terminal in Peterhead without any hiccups. The project proved to be an excellent example of brilliant talent and superb teamwork fusing together for maximum productivity, speeding up the supply chain network to a dynamic velocity.

Published in Summer Solutions 2004