Versatile DP flow metering technology solves redesign challenges on offshore platform project
When it became clear that the crude oil price decline starting in mid-2014 was no brief blip, oil producers began to brace for a long, difficult period. Many producers put capital projects on hold, as profit margins thinned or turned negative. For a company such as Wood serving as the engineering design contractor on many projects, this can be problematic, to say the least. Putting a large-scale project on hold, and then taking it up again potentially some years later, depends on scrupulous documentation, so that anyone working on the restart can understand what the previous team was doing.
In May 2017, Wood received a call from a client wanting to restart work on a major offshore wellhead platform project that had been suspended in 2014. Some of the engineers, who had carried out the original work, were still available, but mostly the project team was new and had to restart from the original notes, specifications and drawings. Much of the piping and instrumentation layout work had been completed, but a request from the client meant it had to be re-opened.
In the intervening years, the client’s engineering staff had decided to standardize on a very conventional and traditional approach of using differential pressure (DP) flow metering (Fig. 1) as its preferred technology. While this is a very practical approach for wellheads, platforms and refineries, its use was not anticipated when the original piping layouts were being configured, based on a different technology that required short meter runs. Going back this late in the game, and retrofitting a different flow metering technology with potentially longer meter run requirements, presented a challenging but ultimately manageable set of problems to solve.
This article will examine one general and two specific situations, to show how Wood approached the process of implementing the DP flow metering.
GENERAL DESIGN PARAMETERS OF DP FLOW
DP flow meter technologies remain the most commonly deployed flow device, based on a variety of advantages, including:
DP flow meters can be made in virtually any size, handle just about any type of fluid, and deliver whatever degree of precision is required. They need an effective DP transmitter to measure the pressure loss, as the flow passes through the primary element, which is designed to create a controlled flow obstruction. When combined with a temperature reading and the fluid density, a DP flow meter can provide a compensated mass flow reading, in addition to a volumetric flow reading.
One challenge associated with DP flow meters is the loss of system pressure. Additionally, achieving best precision depends on stabilized flow, achieved through relatively long lengths of smooth, straight piping, upstream and downstream of the primary element. The International Standardization Organization’s ISO 5167 standard specifies 44 diameters of straight pipe upstream from the primary element, and an additional seven diameters downstream. With something as small as 1-in. (DN 25) pipe, this equates to approximately 4 ft (1.2 m) of required upstream piping. Not all manufacturers require such extensive lengths, so it is important to understand what is required for each application and technology type to achieve the highest degree of accuracy.
Piping designers, who know that DP flow meters are going to be used, typically make allowances for the piping requirements. However, when they have to be retrofitted into existing piping, there can be problems, as was the case with this platform project. Offshore platforms must economize on the weight and bulk of equipment, so finding space for long, straight pipe runs is a challenge. In this case, flow meters have to be installed on pipes up to 14 in. (DN 350) in diameter, so total straight lengths of 50+ diameters at that pipe size are simply not practical.
FINDING A FEASIBLE SOLUTION
Wood engineers consulted with Laurentide Controls, a local Emerson Impact Partner serving the client, to find a practical alternative to conventional DP flow meters. The solution had to fit into the existing piping layouts, while still delivering accurate and reliable measurement performance. Conventional single-hole orifice plates, as the primary element, could be mounted between standard flanges to minimize the overall size, but the meter run requirement would be too long. Laurentide suggested replacing the conventional plates with conditioning orifice plates (Fig. 2) to reduce the straight run piping requirements.
A conditioning orifice plate uses four smaller orifice holes rather than one. The four holes, together, equal the same cross-sectional area and provide the same differential pressure drop. Multiple smaller orifices help flatten the flow profile without the need for long pipe lengths. In fact, using a conditioning orifice reduces the straight pipe requirement to only two diameters upstream and two diameters downstream in most applications. A conditioning orifice plate can be installed in any application, where a conventional plate has been specified without the need to make any piping modifications.
The result was that no piping changes would be required. The only design modification would be a specification change on the drawings and bills of material. In some cases, piping designers needed to make slight adjustments to the position of the flanges and impulse lines within the straight section, but this proved to be more an exception than a rule. Moving forward, the platform will be using conditioning orifice plate technology on pipe sizes from 8 to 14 in. (DN 250 to 350).
EXTENDING FLOW TURN-DOWN RANGE
In most process manufacturing plant applications, a flow meter tends to operate within a relatively narrow range of flows, generally between 50% and 80% of the instrument’s scale. The situation can be different on an offshore platform, due to the importance of minimizing the amount of hardware installed within the structure. In some situations, one instrument has to perform multiple functions and cover an exceptionally wide range.
As a case in point, when installed, this platform will serve a variety of gas wells with a wide range of production volumes. Each well’s output needs to be measured and evaluated regularly. This involves sending the output to the platform’s single test separator, so the product mix can be characterized. The platform’s operators anticipate that the flow through the test separator will be as low as 2,000 sm3/hr to a maximum of 54,000 sm3/hr. That represents more than a 25:1 flow turndown range, more than is practical for any metering technology under the accuracy requirements of natural gas production regulations.
DP flow meters can be very accurate across a wide range, given the accuracy and stability of today’s sophisticated DP transmitters. However, getting into a 25:1 turndown range requires a significant pressure drop, which is impractical when working with the higher-producing wells. Again, Laurentide Controls engineers offered a solution to create effectively two flow meters in a single device.
This simple solution requires insertion of a different primary element. When measurement of a low flow range is necessary, a worker must insert a disk with a smaller orifice to create a higher pressure drop that can be measured more easily. But this requires a shutdown, which is not practical on an offshore platform, particularly given the criticality of resealing the joint.
However, there is a mechanism to change the orifice plate without having to shut down the line or disassemble anything. A dual-chamber orifice fitting (Fig. 3) can be inserted in the pipe to hold the primary element. This device allows it to be slid out of position into a second chamber, where it can be replaced with a different orifice plate and placed back into operating position. This can be done in a matter of minutes with no pressure loss. The only interruption is to the flow reading during the change-over.
The test separator DP flow meter will now have two orifice plates available to facilitate measuring all the flowrates. The low range will cover 1,800 to 10,000 sm3/hr and the high range will cover 8,000 to 54,000 sm3/hr. Any flow, from lowest to highest, will be measured with the accuracy demanded by regulations. Both orifice plates will use conditioning orifices to minimize piping length complications.
AGGRESSIVE TWO-PHASE FLOW APPLICATIONS
Some of the wells feeding this platform use gas-lift technology to assist with extraction, resulting in crude oil entrained with dehydrated natural gas. Measuring this flow is especially challenging, and the operator’s engineers were concerned that a conventional DP flow meter would not be suitable. Laurentide Controls agreed and suggested an alternative to Wood, a wedge primary element, Fig. 4. Rather than a conventional or conditioning orifice, a wedge inserts a triangular piece through the side of a pipe to create the restriction while leaving much of the pipe profile unimpeded.
This minimizes any potential for accumulating debris, while at the same time providing a self-cleaning passage, thanks to a sweeping action of the fluid. This requires more straight-pipe length than a conditioning orifice, but still a modest 10 diameters upstream and 5 diameters downstream. The new platform will use wedge flow meters in 2-in. and 8-in. (DN 50 and 200) line sizes. These will be purchased as fully assembled flow meter units, including the transmitter and impulse lines, ready to install. Each flow meter will have a single part number.
THE COMMON ELEMENT
All these applications have one thing in common: the DP transmitter. Wood and the client have standardized on a single design for these flow metering applications, along with a variety of other applications, where DP readings are necessary, including line pressures, level measurements and others. Emerson’s Rosemount 3051S Pressure Transmitter (Fig. 5) can handle all of these applications with a high degree of accuracy, repeatability and stability. With just a few different ranges, this transmitter can fulfill all the operational needs of the company with minimum inventory. This is particularly important on offshore platforms, where weight and storage space are at a premium.
The ability to use a technology as scalable and versatile as DP flow metering, while concentrating on one family of transmitters and primary elements, provides a high degree of versatility, while minimizing the number and variety of products necessary as spares. Such considerations are paramount when working on an offshore platform, but they also reduce costs and simplify operations. As construction of the new wellhead platform continues, this will save engineering time and ensure greater profitability.
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