GoM deepwater producers demand certification of subsea chemicals

May 1, 2006
Chemical delivery via umbilicals to treat subsea wells has been occurring for over a decade.

Ensuring umbilical-ready chemicals

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L.M. Cenegy, S.J. Allenson - Nalco, Energy Services Division

Chemical delivery via umbilicals to treat subsea wells has been occurring for over a decade. While in the beginning attempts were made to pump more or less traditional chemistries down umbilicals, it soon became apparent that the extreme conditions of subsea treatment would require more technically advanced products to meet the needs of the industry. During this period of industry development, umbilical lengths increased dramatically to accommodate subsea wells located great distances from the host platforms in increasingly deeper water.

Today in the Gulf of Mexico, it is not uncommon for chemicals to be pumped from a host platform to deepwater subsea wells more than 40 mi away. Corrosion inhibitors, scale inhibitors, paraffin inhibitors, and many other products are often applied in this manner. While the performance of the product is always of paramount importance, an increased research effort has been placed on the factors influencing the deliverability of the chemical to the subsea injection point. As a result of this research, Nalco has developed a special line ofSurflo Certified chemicals for subsea umbilical delivery.

Umbilical chemical requirements

In deepwater, there is no margin for error in umbilical chemical treatment programs. The designated chemical products must reach the subsea wellheads and manifolds to treat the production continuously in order to provide flow assurance and asset integrity to the facility. While an interruption of chemical treatment would be an inconvenience in a land-based operation, the same problem can become a very expensive event at a subsea field located a mile or more deep in the water and many miles from the host platform.

Umbilical treatment from a floating platform to subsea wells (typical GoM scheme).

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A failure in an umbilical could be extremely costly; in fact, umbilical replacement cost could run up to $1 million per mile. The longest umbilicals have reached over 50 mi and there are plans to lay umbilicals that are over 60 mi long. If a plugged umbilical delivers a chemical that could immediately impact production, such as a hydrate inhibitor, the cost to the operator will rise significantly in the event of lost or shut-in production.

For this reason, a variety of factors should be considered in the design of a subsea umbilical treatment chemical. These include:

  • stability of the product at low and high temperatures
  • viscosity at subsea pressures and temperatures
  • level of impurities or dust in the product
  • compatibility of the product with the materials of construction used in the chemical delivery system.

In consideration of the above factors, a comprehensive testing and evaluation program is required to certify that the products developed will meet the rigorous conditions of application. Such an evaluation and testing program is discussed later.

Product stability

Due to the increasing lengths of umbilicals and the very low dosage rates of some chemicals, a day or more may elapse from the time the chemical is injected at the platform until it reaches the subsea well. Chemicals used under these conditions must not only be effective at controlling wax, inhibiting corrosion, preventing scale, or performing whatever other function that they were designed for at low concentrations, but they must also be stable at ocean temperatures that are near freezing (40°F). If downhole injection is required, an additional requirement is that the product be stable at downhole temperatures, which can reach 250°F or higher.

Viscosity

When it comes to chemicals that are applied down an umbilical, the viscosity of that chemical is very important. The higher the viscosity of a chemical, the more pressure is needed to pump it down an umbilical. Since umbilicals are now exceeding 50 mi in length, the pressure needed to pump even a low-viscosity product can be significant. Of course, the pressure needed to pump a chemical down an umbilical is dependent on several factors including: umbilical length, wellhead pressure, chemical viscosity, umbilical diameter, pump design, and pump rate.

High pressure viscosity at operating temperature


Effect of temperature and pressure on the viscosity of a typical corrosion inhibitor
Pressure
(PSIA)
Viscosity (cP)
at 40°F
Viscosity (cP)
at 120°F
15,00051.18.8
10,00034.26.5
5,00022.94.5
50015.93.2

Umbilicals do not operate under atmospheric pressure, so measuring viscosity at only atmospheric pressure is not effective. In order to get the correct viscosity of a chemical for umbilical use, the viscosity must be measured at the operational pressure of the umbilical, which could be as high as 15,000 psi. Viscosity measurements taken at high pressure can be greatly different from viscosity measurements at atmospheric pressures. Also, varying the temperature from a subsea temperature of 40°F to a downhole condition of 250°F or more will greatly affect the viscosity, as the table shows.

Impurities in the product

It is critical that the chemical products be free of particulate contamination such as dust that may plug the narrow orifices in the chemical delivery system. These issues present substantial challenges in the formulation and manufacture of chemicals for umbilical use, challenges that must be addressed prior to approval of the product for use.

Borrowing from the aerospace industry, a test and measuring system was developed for subsea chemicals that measures and ranks the level of contamination in products. The system involves the use of a laser particle counter and the NAS (National Aerospace Standards) rating system.

Studies indicate that it is not the small diameter of the umbilical tube itself that causes plugging problems, but rather the subsea fittings and valves, some with tolerances as small as 0.03 in. (750 microns), which are most likely to plug. Experience has indicated that particles greater than 70 microns are most likely to cause plugging problems, and are of special interest.Surflo Certified products typically meet a NAS Class 9 rating or better for particles greater than 70 microns (similar to SAE AS4059 bin 9F). This means that in a distribution of particle sizes in a sample, there cannot be more than 128 that are 70 microns or greater in size.

Material compatibility

One of the most important aspects that must be understood before a chemical is applied down an umbilical is material compatibility. If the chemical in use is not compatible with any of the metals and elastomers that are used in the chemical delivery system, then system failure is likely. No chemical will be compatible with all metals and elastomers, therefore it is necessary to determine what materials are compatible with the chemical before the system is built.

Material tests typically involve exposing the elastomers and metals to the neat chemical at an elevated temperature for a time period of one week to one month. The elevated temperature is used to accelerate the aging process. After the test period the elastomer is examined to determine if any changes occurred. For elastomers, changes such as increased brittleness, change of mass, changes in pliability, and others would constitute a failure. For metals, any substantial corrosion of the metal or pitting would indicate a failure. Typically, a suite of 16 elastomers and plastics, including nitriles, polyketones, and fluoroelastomers, and seven metals, including carbon steel, stainless steel and other alloys, are evaluated. For special projects, additional materials may also be tested.

Operational considerations

Once a chemical is deemed to be umbilical worthy, it is still necessary to make sure the chemical is applied in a manner in which the umbilical is protected. A chemical can be manufactured that passes all the requirements and specifications that would allow the chemical to be successfully applied down an umbilical. However, the oilfield is not a pristine palace. Just because a chemical leaves a manufacturing facility ready to be applied down an umbilical doesn’t mean it will be so when actual umbilical application takes place. All along the supply chain route the chemical can be contaminated with foreign material.

A typical chemical can be transferred two or more times on its way to an offshore facility. Dirty hoses and pumps may have been used to make these transfers. The chemical may arrive at the facility ready to be applied down an umbilical, but may not stay that way for very long. A few precautions should be taken when handling an umbilical-ready chemical at an offshore facility:

  • All storage tanks must be sealed so that rain, seawater, dust, and other material cannot enter the tank.
  • All lines and pumps that are used to transfer chemicals must be thoroughly cleaned after each use.
  • Extreme caution must be taken as not to accidentally mix chemicals.

Common sense should be used when handling the chemical to make sure it is not contaminated.

Laboratory testing to determine product fitness

In order to evaluate the fitness of product for subsea umbilical usage, a modern laboratory equipped with application-specific equipment is required. The testing required is extensive and provides comprehensive information about the performance of the product under subsea umbilical conditions. Among other tests, the umbilical test apparatus (see above) provides particularly valuable information about product performance in low temperature, high pressure environments.

Umbilical test apparatus

The umbilical test apparatus subjects the chemical to simulated subsea umbilical conditions. It is essentially a working umbilical that can be equipped with actual pumps, filters, meters, valves, and other equipment for topside and subsea use that would be used for an actual umbilical.

The apparatus simulates actual umbilical conditions of low temperatures (40°F), high pressure (5,000 psi), and long resident time (seven days).

Umbilical test apparatus and 3/8-in., 1,000-ft tube

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A full-size umbilical apparatus such as this gives significant advantage over capillary-designed simulated umbilicals. Since the full-size umbilical can be equipped with the actual valves, meters and other equipment that may be used in a production system both topside and subsea, it more completely measures a chemical’s suitability to be applied via an umbilical.

Conclusion

A decade ago, when umbilical delivery of production chemicals first came into operation, chemical-related failures were common. Today, with a better understanding of what criteria are needed to deploy chemicals down an umbilical, chemical-related failures are very uncommon. By understanding the processes outlined above, it is possible to consistently be successful at applying chemicals subsea via an umbilical.