Subsea waterjet cutting goes ultra-deep and ultra-high pressure

Waterjet cutting went deepwater during the BP Macondo crisis. Now it is ready to go even deeper. By Elaine Maslin

Ultra-high pressure (UHP) waterjet technology has been used since the mid- 1980s in the marine environment. Its use is well established for surface preparation and corrosion removal, as well as for cutting, both in manufacturing processes and on infrastructure. UHP waterjet has also been used underwater, but until recently, this has only been to a depth of about 365m, due to the inability to deliver abrasive to greater depths, as well as pressure loss inside the hose or high pressure tubing.

In 2010, Chukar Waterjet developed a waterjet system used at 1430m (5000ft) on the Macondo well in the US Gulf of Mexico. It needed to remove accumulating hydrates that were making it difficult to cap the well. Other methods of removing thehydrates, including using methanol dispersant, had been unsuccessful.

Chukar used seawater pressurized by a hydraulic waterjet intensifier pump, in conjunction with a methanol dispersing system, to remove the hydrates from the bottom and the inside diameter of the spool as it was being lowered into place. The sealing surface was then able to go over the flange and mate with the lower marine riser (with the spool acting as the connector), which the valve system could be mounted onto.

The system operated for extended periods while at 1430m deep and 3° Celsius (37° Fahrenheit), with ambient pressures of 150 bar (2100 psi) and exposure to salt water and methanol.

Chukar has now extended the depth at which its UHP waterjet intensifier technology can work to more than 3000m, for both cutting and hydrate removal.

In April, the latest system performed a successful test cut in in Thunder Bay, Lake Superior, Ontario. An UHP waterjet pipe cutter was used to cut a 14in.- (356 mm) diameter, ½in.(12 mm) thick steel pipe, in under 20 minutes.

Warren Christopherson, operations manager at Chukar Waterjet, says that saws designed for pipe cutting, including diamond-wire saws, can cut off pipes or other offshore structures, but they can bind, because they must enter into the saw cut gap (kerf) to complete the cut.

“Attempts to cut off the riser pipe at the Macondo oil well in 2010, with the use of a diamond- wire saw, were unsuccessful because the saw became stuck in the full flow pipe,” he says.

“After the saw was dislodged, a shear was then used to cut off the riser, leaving a jagged cut.”

Underwater welding and flame-cutting techniques are well established, but safety risks have placed increased pressure on dive operators to minimize reliance on both, he says.

The system developed by Chukar is able to carry enough abrasive on the waterjet skid in order to provide cutting of 50m (160ft) of 12mmthick steel, independent of depth, in a single deployment.

The process of abrasive waterjet cutting typically involves a UHP (3900 bar) stream of water with an abrasive, such as garnet, added. Without abrasive, water-only waterjets are typically used for cleaning or for cutting soft materials.

Using waterjet tooling, either hand-held or using an ROV, the UHP stream is directed to a surface or cutting area, for cutting holes or linear cuts, and cutting pipes from the inside or outside.

The system, controlled from the surface, is deployed with an umbilical cable, providing power and communication, using either a diver or ROV to aid skid and tooling placement, depending on water depth.

For coating removal or hydrate remediation, seawater or methanol is pressurized to 3900 bar by an intensifier-style pump, to create a uniform application to remove coatings like epoxy or concrete without damaging the substrate.

For cutting applications, seawater is pressurized to ultra-high pressure by the intensifierstyle pump, and abrasive is added to create a cold-cutting tool, effective at cutting most material, including up to 250mm steel.

Chukar’s system is powered by a 150hp electric motor, running at 3000 volts and is aimed at cutting and cleaning, decommissioning, maintenance, and salvage, as well as emergency response use. Sea water, plain water or methanol can be used, along with abrasives added for cutting applications. OE Review