The cutter dredge Al Sadr is National Marine Dredging Company’s (NMDC) most powerful dredge at 27500 hp. The dredge has a 900 mm (35.5-inch) suction and 850 mm (33.5-inch) discharge.
The Al Sadr was built in 1999 by IHC using Thordon composite bearings in the cutter shaft which since then has moved millions of cubic meters of material. When engineers replaced the bearing in July 2009 it showed only two millimeters (.07 inch) of wear after 10 years.
The dredge is equipped with three cutters: an Esco six-arm right-hand rotation 52-inch-diameter Spherilok HD for rock; a 48 DS Spherilock LD cutter for rock; and a 48 DS Helilok for sand and clay. The cutter motor is 2200 kW.
The Thordon distributor Rafid Qureshi of Ocean Power International LLC advised that the bearing could remain in operation but NMDC management decided it had had a good run and replaced it while the vessel was drydocked.
“It probably could have gone longer” says Qureshi. “I’ve seen these bearings last 12 to 15 years.”
Throughout its life the end shaft of a cutter is repeatedly thrust into the floors of lakes and oceans. Besides performing its heavy industrial function the bearing must withstand abrasive water that inevitably seeps through.
This is the first bearing behind the cutter says Jurjen Visser marine superintendent at NMDC. “It is always underwater and absorbs most of the shaft’s movements.”
After 10 years in contrast to the nearly-intact composite bearing the metal retaining ring that holds it in place is completely worn indicating that a metal bearing would have required several replacements under such harsh conditions. During last summer’s retrofit at Qureshi’s suggestion the Thordon stave bearing in the Al Sadr was replaced by a Thordon Composite tube bearing an even more economical option that’s easier to install.
“We have experienced within our company that exchanging the bearing when we use complete bushes is far easier and quicker” says Visser. “Furthermore it is cheaper to buy three bush bearings than a whole bunch of stave elements.”
As of 2006 NMDC vessels had dredged 890 million cubic meters (1164 million cubic yards) which the company claims is enough to build a road 20 meters (65.5 feet) wide one meter (3.2feet) deep and 40650 kilometers (25250 miles) long–that would almost go completely around the earth at the equator.
The new Thordon bearing in the Al Sadr’s end shaft has big tasks ahead. By the end of 2010 NMDC aims to complete an AED 1.5 billion (US$313 million) contract by the Abu Dhabi Urban Planning Council (UPC) to construct the Mussafah canal. This canal will play a key role in the development of Mussafah Industrial City by accommodating ships with up to nine-meter (30-foot) draft to transit. And as of November 2009 the company was awarded a three-year AED 2.3 billion (US$626 million) project to construct four artificial islands in the Zakum marine oil field.
As for the now-retired bearing Thordon representatives continue to marvel over its longevity. It sustained most of its minimal damage from welding spatter when it was removed for inspection.
“Except for the first 200 millimeters (7.87 inches) at the mouth the rest is perfect” says Qureshi. “In my opinion it could have run another 10 years.”