Tullaroop Outlet Tower Screen Guide Rehabilitation

Diver Richard Sanderson prepares to position Xcalibre and the rack

Xcalibre and the Rack....

I have always found drilling concrete underwater a difficult task. Even though we were equipped with hydraulic drills and diamond tipped drill bits the jobs were physically demanding and maintaining accuracy with a hand held drill is extremely difficult. When quoting contracts requiring drilling I was often beaten by my competition leaving me wondering what the secret is. I decided to find out. The next job we quoted required one hundred and sixteen, 16mm. chemical anchors to be drilled and fitted horizontally to secure some trash racks. My competition won the job at about half my quoted price. Dam! We have to get better at this. But then came the leveller. We were asked to do a quality surveillance of the job and found that only 34 of the anchors were fitted and of those only about 16 were actually effective. The rest were drilled on an angle or not drilled deep enough. Yes, they hadn’t learnt to drill concrete either.

A Boroko ultathermic cutter is used to remove the old cast iron rail

When the next job come along it was a beauty. It required forty-eight 24mm. holes to be drilled horizontally through 30mm. of cast iron then 190mm. into reinforced concrete. There was no margin of error allowed as new stainless steel rails had to be bolted onto the structure and the rails were pre drilled. To make matters worse the water was at a cool six degrees Celsius, the visibility minimal and blue green algae present. Actually the drilling was the second part of the job. The first required us to remove about a metre of silt from the base of the inlet tower and then cut out 20 metres of imbedded cast iron rail to make way for the new stainless steel rails.

When I approach a difficult job underwater I usually start from the viewpoint of how would I do this job if it were on land so I started my research by looking at surface drilling. The answer is you definitely have to use diamond drills. But diamonds ain’t diamonds. You have to select the right type for your job and the quality varies enormously with the price. The variables include the hardness of the diamond, the size of the diamond chips and the composition of the matrix they are imbedded in. The matrix is highly specialised and generally includes Tungsten, Silver, Cobalt, Chromium and brass. Just to make it even more interesting in specialised drills the diamond can be Tungsten coated. With the help of a company specialising in surface drilling we opted for two types of drill. One specifically for drilling through the cast iron and a second one for the concrete. This proved to be the right choice as although the concrete drill can drill through the cast iron the specialised drill completed the hole in half the time. First part of the problem solved.

A Hydraulic drop saw is used to clean up the raw edges

The next thing that is absolutely critical is drill speed. All drills have an optimum speed to maintain a preferred number of metres per second across the face of the drill. Too fast and you will wear the drill out too quickly and too slow and the drill goes blunt because you don’t crack the diamonds and expose new sharp faces. So now its back to testing our hydraulic power pack. I called in our local hydraulic experts and had them fit a speed controller to our unit then test the flow and pressure at all adjustments of the controller. We do it all again on our various hose combinations. Armed with this information I set about selecting the hydraulic drill. We already owned a Stanley hand held drill but it is not big enough and it is hand held. We are going to need something that will be more accurate. Wouldn’t you think this would be a standard underwater tool? Well if it is I couldn’t find it. So again I looked at what was used on the surface. I found a beautiful hydraulic drill called Xcalibre in England. This drill will drill up to 100mm. core holes and matched the flow and pressure of our power pack. In Germany I found what I call "The Rack". This is a conventional drilling rig made of modular components consisting essentially of two rails, one horizontal and one vertical, with precision cut teeth. Running on the rails is the drill holder which is wound in or out or up and down with a hand spindle. The feature that got me was that the vertical rail could be manufactured to our height so it would fit into the recess in the tower specifically for our job. I also opted to take the horizontal rail which can drill 360 degrees around the vertical rail. Best of all it is made specifically for Xcalabre. To our knowledge this is the only one of its kind currently in Australia.

A diver prepares for a Q.A video survey

The job started uneventfully. The silt was removed with a 100 mm. dredge and we got straight into the cutting. Cast iron is not as easy to cut as steel and consumed a surprising amount of Boroko rods and Oxygen. Even though we purposely undercut there were still some small sections protruding slightly proud of the wall. These were removed with the hydraulic drop saw fitted with a diamond blade. The new rails were positioned then it was time to drill. We removed the horizontal rack and just used the vertical rail however we fitted it horizontally in the recess in the wall. Now the experts had told us that because we were drilling underwater we would not need to use water on the drill. Wrong. It quickly became apparent we needed water as the first drill melted in the hole. Sure we were under water and that should keep it cool and wet, but pressurised water down the centre of the drill has another important function. It washes out the overburden and prevents the drill clogging up. Once the drill clogs up, the water can’t get to the cutting face and the drill heats up and melts. The problem was how do we get pressurised water when we were out in the boat? The answer was a fairly simple one. We connected the drill to the water pump of our hot water suit. Problem solved. I had expected to be able to drill up to three holes an hour, which included resetting the drill for each hole. When everything went well we managed up to five holes per hour but when the winds blew gale force and the Blue Green Algae reduced the visibility to about 100mm, we slowed down to only one and a half holes an hour. 

Testing the trash rack fit

With the holes drilled we set the chemical anchors and tightened them. Then it was time to test fit the gates. Here we found the engineers had not allowed for the length that the chemical anchors protruded and they fouled the gates so all 48 had to be cut off flush with the nuts. Normally this would be pretty straightforward however, as the drop saw would not fit into some of the more awkward spaces more than half of the 25mm. stainless steel bolts had to be cut with a hacksaw. What a fun five hours. With the gates now fitting perfectly we took a quality control video and demobilised.

The entire job took us 15 days, 47 dives totalling 81 hours of bottom time and a total of 600 man hours. That’s really all it takes to learn how to drill concrete.

For more photos on this contract please visit our photo gallery or select the link below:

http://www.descend.com.au/training/gallery/PhotoGallery.asp?whichcategory=Concrete%20Drilling%20Underwater&AreaID=17