Putting hydrates under high-pressure

While methanol/glycol cleaning could be considered the primary removal method, its efficiency in open water is limited. BOURBON has started using high-pressure water jet cleaning as an alternative.

The BOURBON Trieste lifting the Schilling heavy duty work class ROV.  Images from BOURBON.

Removing potentially consolidated hydrates from subsea infrastructure is a concern for the oil and gas industry.

Hydrates are concretions, similar to “ice” in particular pressure and temperature conditions, which can form on subsea infrastructure in deepwater. Hydrates can affect the operation of equipment by restricting movement of parts, so they need to be removed periodically.

Cleaning at these depths is carried out using heavy duty or ultra-heavy duty underwater remotely operated vehicles (ROVs), which most commonly apply methanol to dissolve the hydrates. While this can be highly effective, methanol treatment has its disadvantages.

This primary method for cleaning involves pumping the methanol from a floating production system, and diverting the subsea injection point by re-routing piping and installation hoses to “attack” the subsea equipment.

An entire cleaning operation might require more than 10-20,000 liters of methanol, because the effectiveness of using methanol when cleaning the exterior of equipment is reduced by the diluting effect of the seawater.

Due to the volume required, the injection of methanol directly from an ROV-borne skid, typically containing 15 gal bladders, was disregarded.

BOURBON has developed a method using a high-pressure water jet, using local sea water, to remove the hydrates, avoiding using chemicals. The company performed its first water jet cleaning operation using the inspection, maintenance and repair vessel BOURBON Trieste, working with its ultra-heavy duty ROV. The job was done on a subsea Xmas tree in 1400m water depth offshore West Africa in 2013/14.

The project involved nearly two year’s preparation between the parties. BOURBON first tested three types of nozzles to find the most effective design for the job – a pointed nozzle proved better at cleaning than one with a flatter end or a rotating nozzle. The nozzles were tested and demonstrated to the client before BOURBON personnel embarked on the operation.

Operations underway in the deep.

The first part of the operation involved using a low-pressure jet (10-20 bar) created by a Zipjet pump, to remove the loosest layers of concretions. The remaining hydrates were then removed using a high-pressure jet (300-800 bar) generated by a Dynaset pump, in several stages.

In both cases, the ROV positioned the jet 1.5-2m from the subsea equipment, and then gradually approached it to a distance of 50cm. The pressure from the jet could be adjusted by the ROV operator during cleaning up to a maximum level, which was set before the ROV was sent down. The highest pressure setting used was 500 bar, which is able to deal with nearly all hydrate deposits.

The nozzle was located at the end of a “wand,” a rod attached to one of the ROV’s robotic arms. At the same time the ROV hung on to the subsea infrastructure with is grabber arm. Precise indications on structure architecture were given by the client, this meant the wand could be controlled very precisely to clean even the most difficult-to-access parts of the equipment, accurately, safely and with no risk that the jet would damage it.

During this entire process, the client was also able to follow the operation from the FPSO in real time by viewing the images from the video cameras on the ROV. That meant the client was able to provide instructions, verify that the operation was progressing smoothly and call a halt if they had concerns – or when they were satisfied that the cleaning process had met their requirements.

The operation removed all of the hydrates, while protecting the lines and cables on the structure. Since then, BOURBON has successfully carried out this cleaning process several more times for the same client. The frequency of such operation depends on the density of shallow gas in the seabed.

While the project was a success, it is unlikely that the technique will completely replace low pressure methanol cleaning across the oil and gas industry in the near term, despite the lower cost, environmental risks and time savings, due to operators being more familiar with methanol cleaning, which is perceived as being less risky for the subsea assets.

A growing track record of success, improved knowledge and technological advances are likely to promote wider use of the water jetting technique in the future.

Field operators could also adopt a hybrid approach to hydrates cleaning if they remain concerned about the risks, using high-pressure jets to complete the bulk of the cleaning process and then turning to methanol cleaning for the most sensitive areas of the subsea infrastructure.

Whatever technique is used, removing hydrates is a delicate business. But, as confidence in the ability of ROV operators to use high-pressure jets grows, the established trend towards greater use of this method will only become stronger.