NEMOH is a commercially feasible and tailored subsea water treatment package. (Image courtesy of CETCO Oilfield Services)

For more than 10 years CETCO Oilfield Services Co., a wholly owned subsidiary of AMCOL International Corp., has been developing innovative techniques for the treatment and management of wastewater fluids.

Specializing in the removal of hydrocarbons, solids, toxic materials, and other contaminants from oilfield wastewater streams, CETCO Oilfield Services has now developed NEMOH, a Nomadic Environmental Media Host vessel for processing and managing polluted fluids in a subsea environment.

The company’s first foray into the subsea sector, NEMOH can treat chemically contaminated water without the need for a topside facility. Referred to as an equalized underwater media vessel, NEMOH is capable of operating at depths of up to 6,000 ft (1,830 m).

Subsea depressurization

In 2000 CETCO Oilfield Services was brought in on a project by a major deepwater operator in the Gulf of Mexico (GoM) to determine a method of treating water inhibited with chemicals as part of a hydrotest depressurization of a riser.

Typically fluid is pressure-tested and depressurized to a topside facility. However, due to project delays, the topside facility was not on location in time to take displacements. With strict permit regulations enforced and no topside equipment available to capture the small volume of chemically treated fluids, CETCO Oilfield Services was contracted to find a way to pump and treat the fluids subsea, negating the need for running lines subsea and pumping the fluids back to a support vessel.

With the tie-in points all in deep water, the operator required a solution that could act as a temporary and portable installation for operating at varying sea levels.

Over the last eight years CETCO Oilfield Services Engineering and R&D divisions have further developed the technology to create a commercially feasible and tailored subsea water treatment package. Fast-forward since that first job in the GoM, and NEMOH is now a permanent fixture on the same operator’s subsea dive boat.

Strict discharge regulations in the GoM compel operators commissioning new field developments to consider the effects of pressure-testing pipelines.

During the installation of new infrastructure offshore, the integrity of equipment needs to be verified. Connections, seals, valves, and pipelines all need to be checked for leaks. This is routinely done by pressure testing the lines. To do this, the lines must first be filled with water, which is often chemically treated to protect the lines and equipment from corrosion. These chemicals, which typically include biocide, corrosion inhibitor, and oxygen scavengers, can be inherently toxic to the environment.

After pipelines have been pumped with chemically treated water to maintain integrity, additional water is pumped into the equipment until a desired pressure is achieved. The pressure is held and charted for variances. Once this is complete, the pipeline is depressurized. The excess fluid that results from the depressurization then has to be captured and treated accordingly before being discharged into the sea. NEMOH has been developed to work onsite to capture, treat, and release these resulting fluids in a safe and environmentally compliant manner.

Though hydrostatic testing goes beyond just that of pipelines, the same procedure is often adopted for testing the integrity of, for example, risers and manifolds. These tests are typically done during new installation or maintenance procedures on a section where new components need to be evaluated. In cases where the equipment has been in use for some time, there may be additional contaminants such as oil and grease, entrained gas, production chemicals, and other forms of maintenance chemicals that need to be treated.

The vessel is shaped like a vertical tank and is fitted with a protective cage and lifting frame. Initially NEMOH is operated by topside technicians; following its immersion in the water, it is controlled by remotely operated vehicles (ROVs).

Operational advantages

There are a number of advantages to deploying this system as an enhanced subsea water treatment technology. Treating potentially hazardous chemicals, including corrosion inhibitors and biocides, at a subsea level can be more time-efficient than using traditional topside methods.

It is flexible and portable and is proving very effective as a temporary installation able to operate at varying sea levels and in all types of marine environments including fresh, salt, or brackish water.

Deployment involves safely landing the vessel on the bottom of the ocean and allowing the ROV to make inlet and outlet connections.

This development enables CETCO Oilfield Services to treat various constituents depending on whether the materials need to be oxidized, coalesced, or neutralized, since it can be operated with different media types.

When the treatment process is complete, NEMOH can either be recovered in a reverse process, disconnected, or be guided by an ROV along the seabed to another location.

The industry’s need to complete cost-effective subsea projects that are focused on environmentally harnessed results have provided the bedrock for the development and deployment of NEMOH as a viable technological waste treatment solution.With the ability to be operated either as a standalone vessel or in conjunction with a series of vessels, the system allows operators to begin treating fluids before permanent facilities are available, debottlenecking subsequent operations.

Saving time and costs

Having the ability to work ahead of time means operators can speed up commissioning of new facilities. Additionally, this flexibility means the technology can be applied to various applications, including over-flushing and de-oiling operations.

When equipment is being replaced, repaired, or modified, the presence of preservation fluids often means over-flushing is required. When subsea infrastructure is compromised—be that planned or inadvertently, due, for example, to hurricane damage—seawater may enter the line or system. When the line or system is then pumped out, the resulting seawater that was exposed is often displaced with preservation fluids or oil.

The fluids pumped out then have to be collected and/or processed. NEMOH can be deployed to treat those fluids onsite, ensuring that the subsea equipment is protected while also ensuring oil and other discharge materials do not enter the marine environment.

Similarly, when repairs or additional tie-ins to subsea architecture are required, the subsea infrastructure must be de-oiled prior to opening the system up to the environment. Typical flow loops are flushed using pigs. However, sections that are dead legs and/or cannot be pigged require an alternative method. To de-oil a line so new subsea architecture can be installed often means a new well will need to be added, meaning operators have to cut into lines to ensure no oil is present.

NEMOH can act as a solution for reducing flushing time. Proven to act as a flushing mechanism that is up to 20 times faster than traditional flushing techniques, it can be utilized at the close of such an application and treat additional chemicals such as Methanol/glycolate, which is often used for hydrating lines and additional cleaning.

If an entire line has to be flushed repeatedly for a deadleg, this can take up to eight hours, which obviously cuts into precious production time. NEMOH can flush the same volume of fluids within 15 minutes.

The real value is the reduced downtime that can be achieved. The system can perform operations such as flushing faster and more efficiently than traditional topside techniques, often helping to reduce downtime and accelerate production.

At the same time, NEMOH can reduce an operator’s liability because it reduces the amount of waste fluids that would previously have to be returned to shore. It is often the case that where treatment of waste fluids is taking place, vessels that require pumps and heavy equipment cannot directly access the site. With NEMOH, because water is treated within a contained space at a subsea level, there is no need for a fully equipped offshore support vessel to intervene.

The flexibility derived from the fact the system can be deployed weeks ahead of time and recovered post-treatment means operators can access a section of line easily without having to compromise the entire flowline. Essentially, NEMOH can turn what could potentially be a liability into an asset.