High-pressure water jet offers superior alternative to methanol for cleaning subsea hydrates

Bourbon used its service vessel, Bourbon Trieste, and an ultra-heavy duty ROV to clean subsea hydrates, with a high-pressure water jet, as a cost-effective alternative to methanol spraying.

One of the challenges for the offshore oil and gas industry is the removal of potentially damaging gas hydrates from the exteriors of subsea infrastructure. The most commonly used method is cleaning with a methanol spray, which has limited efficiency in open water. Bourbon has developed high-pressure water jet cleaning as a viable alternative, providing time-savings at a lower cost, as well as reduced risk to both people and the environment.

Similar to “ice,” hydrates are concretions of water and natural gas that form on subsea structures under certain pressure and temperature conditions. Hydrates can affect the operation of equipment by restricting movement of parts; hence, the need for periodic cleaning.

METHANOL DISADVANTAGES

Typically, ultra-heavy-duty, remotely operated vehicles (ROVs) are employed to spray methanol across subsea structures for dissolving hydrates. While this method can be highly effective, there are certain disadvantages:

• The cleaning involves pumping methanol from a floating production system, and diverting the subsea injection point by re-routing piping and installation hoses to “attack” the subsea equipment.
• The removal operation might require around 30,000 l of methanol, as its effectiveness is reduced by the diluting effect of the seawater. When cleaning the inside of subsea infrastructure, methanol injection is highly effective, as it is confined and not diluted by seawater.
• Each multi-stage transfer requires careful supervision to safeguard against methanol spillage, which is dangerous to both people and the environment.
• Due to the volume required, the injection of methanol directly from an ROV-borne skid, typically containing 15-gal bladders, was disregarded as being impractical.

HIGH-PRESSURE WATER JET

Bourbon proposed the avoidance of chemicals to remove the hydrates by using a high-pressure water jet. By employing local seawater, there would be genuine cost and time reductions. One concern, however, was that poor application could damage delicate subsea structures. A misdirected water jet could damage electrical flying leads, hydraulic lines, or the coatings on steel pipes, with potentially huge financial and environmental consequences for an FPSO operator.

Bourbon conducted this type of water jet cleaning, using the Bourbon Trieste, a vessel specializing in inspection, maintenance and repair (IMR), working with an ultra-heavy duty ROV. Recently, the vessel performed the water jet cleaning operation on a subsea X-mas tree at a 1,400-m water depth. Bourbon first tested three types of nozzles to find that a pointed nozzle was the most effective design for the job, rather than one with a flatter end or a rotating nozzle. Once the process was approved by the client, Bourbon launched the operation with all necessary caution.

The first part of the operation involved a low-pressure jet, created by a Zipjet pump, to remove the loosest layers of concretions. The remaining hydrates were then removed via a high-pressure jet, generated by a Dynaset pump, in several stages. The ROV started by positioning the jet 1.5–2 m from the subsea equipment, and then gradually approached it to a distance of 50 cm. The pressure from the jet could be adjusted during cleaning, up to a maximum level, set before the intervention. The highest pressure setting used was 500 bar, sufficient to remove almost all types of hydrate deposits.

The nozzle was located at the end of a “wand,” a rod attached to one of the ROV’s robotic arms. The ROV was attached to the subsea infrastructure with its grabber arm, meaning 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 wand would damage it. The jet cleaning removed all of the hydrates, while protecting the lines and cables on the structure. Since this first successful operation, Bourbon has conducted this cleaning process several times, reflecting the client’s confidence in the water jet cleaning process.