Tower of power Pumpcrete draws upon technology and previous experience to create massive concrete gravity structure

Pumpcrete draws upon technology and previous experience to create massive concrete gravity structure

Mention construction of an offshore oil platform and visions of a rig destined for the Middle East, the Gulf of Mexico, or the North Sea probably come to mind. However, in the industrial seaport of Argentia, Newfoundland, a huge concrete structure is taking shape that, once joined with its wellhead platform, will be extracting oil and gas for owners Husky Energy some 350 kilometres off that very same Newfoundland coast.

Heading up the concrete pumping facet of the West White Rose Project, as it is known, Canadian firm Pumpcrete is using a pair of placing booms and a slipform system to help make that happen. By project’s end, the company will have pumped more than 78,000 cubic metres (102,000 cubic yards) of concrete, and the structure will rise to a height of 146 metres (480 feet). A small fleet of tugs will then tow it to its designated location in the Atlantic, where a combination of water and ballast will allow it to take its place, rising up off the ocean floor awaiting attachment of the deck and the start of production.

Grave situation

Work on the West White Rose Project’s concrete gravity structure (CGS) is being conducted in a purpose-built graving dock which — testimony to the size of the CGS itself — measures 110-metresby-130-metres (at the base with a 1:1 slope) by 25-metres deep. When work is complete, the dock will be flooded to allow the structure to float, and an earthen berm will be removed, opening the dock to the port for movement of the completed structure. According to Dave Moriarty, Pumpcrete’s special projects manager, work on the CGS began with the mass pour of a 75-metre diameter, 1.2-metre-thick base slab.

“This was really the most basic part of the entire project,” said Moriarty. “Other than the incredible amount of rebar extending from it, the slab was a basic 13,000-cubic-metre pour using a pair of Schwing SP 8800-E stationary pumps feeding Schwing SDB 35 placing booms seated atop octagonal masts. The combination of the 35-metre booms and the masts themselves — a pair of six-metre sections joined to give us 12 metres of height — afforded us all the reach we needed. We poured the slab in five separate sections: the centre and four quadrants, and did it in 400-millimetre lifts.”

Seawater ballasting pipework was also cast into the base slab to allow the CGS, when completed, to be flooded, towed, and ballasted to install it in position on the Grand Banks southeast of Newfoundland.

Giving them the slip

With the base slab completed, Moriarty said they moved on to the base caissons which rise from the surface of the slab upward for 46 metres. Each of the hollow caisson sections has 500-millimetre-thick walls. Together they are designed to disperse enough water to allow the structure to float, but also to provide stability as it sits on the ocean floor. In order to streamline production and minimize the risk of horizontal joints during the pour, traditional concrete forms were bypassed in favour of a slipform system from Gleitbau/Salzburg.

“We’ve used the slipform approach on a couple of similar projects in the past and it really served us well,” he said. “This structure is heavily reinforced and we are constantly flying rebar in, so having to move a placing boom around to access all areas of the caisson would have made things difficult. Also, because of our geographic location, the winds can get extremely strong at times. If we had the placing booms feeding traditional formwork, we would be needing to regularly shut them down for safety. With the slipform system, since we generally know the prevailing wind direction, we are able to use the completed sections of the structure to keep the booms sheltered, minimizing the need to shut down for wind.”

All jacked up

It’s important to note that, because of the slipform system design, Pumpcrete’s placing booms are not placing concrete into the form itself. Instead, they are pumping through slickline to the uppermost deck of a three-deck configuration, where a series of valves directs concrete to six hoppers. One of the team’s operators remotely monitors the distribution of the concrete via video cameras, and controls a number of functions, including the output from the batch plant, the speed of the pump, and the automatic floats in each of the six hoppers located on the slipform.

Laborers, equipped with one-quarter-cubic-metre Gar-Bro concrete buggies, take the concrete from discharge chutes on the hoppers and feed the slipform system, which is incrementally raised using a hydraulic jacking system.

“The form is on jacks and we pour in lifts,” said Moriarty. “The first pour here was a 300-millimetre lift and, once that was done, we did a series of 150-millimetre lifts until we got to the top of the form. By the time we’ve completely filled the form, enough time will have passed that the concrete at the bottom has already achieved its initial set. With that confirmed, a series of jacking rods raises the form. That continues until we get to our desired elevation.” The first pour here was a 300-millimetre lift and, once that was done, we did a series of 150-millimetre lifts until we got to the top of the form. By the time we’ve completely filled the form, enough time will have passed that the concrete at the bottom has already achieved its initial set. With that confirmed, a series of jacking rods raises the form. That continues until we get to our desired elevation.”

He added that, because they can only pour as fast as the setting concrete allows, working in this manner is not a race to see how quickly they can get things done, but rather an exercise in consistency so that they can raise the system at a steady pace.

“We are not concerned with high flow rates” he said. “If we jack too fast, concrete that hasn’t set up properly will seep out the bottom of the form. We’ve really got the process down at this point. Running two slips, we went from the top of the base slab up to six metres in about five days and subsequently completed from there up to 46 metres in about 30 days.”

Big push

As mentioned, concrete is being pumped by a pair of Schwing SP 8800-E stationary pumps, which Pumpcrete has installed in a small structure located at the lip of the graving dock. Moriarty said doing so not only keeps the pumps cleaner, it also affords some protection in cold weather and makes any necessary maintenance a lot more bearable. The pumps, specifically modified for Pumpcrete by Schwing Bioset, feature a 2020-7 pump kit with 200-millimetre cylinders and a two-metre-long stroke, as well as a pair of 300 HP motors.

“We could have easily ordered those pumps with a 600 HP motor in each, but opted instead for the pair of 300s to provide a level of redundancy,” said Moriarty. “In that way, should anything fail — and to this point nothing has — we have another motor that can still run that system and production will not be impacted.”

While the distance from the stationary pumps to the placing booms is significant, Moriarty said they have had no issues in that regard either.

“When we did the base slab, I had upwards of 300 metres of five-inch pipe laid out at different points — that was our longest push so far,” he said. “However, I’ve been doing test pours in preparation for pumping the conical slip up to the 146-metre elevation. Although there is nothing currently onsite that is that high, I need to confirm that we can pump that distance. So, I’ve been laying system out on the ground and have pumped as much as 550 metres of line. The 8800 had no problem at all with that, so I’m confident we’ll be fine.”

Wetter the better

Getting the mix design to where they needed it for pumping proved to be one of the more challenging aspects of the West White Rose project for Pumpcrete. Moriarty estimates that they spent about a month working with a contracted mix developer and ready-mix supplier (Capital Ready Mix/ LaFarge Partnership) to fine-tune the mix to get it the most pump-friendly.

The end result of that effort is a semi-lightweight, high-slump concrete with a very low water-to-cement ratio, a Sika plasticizer, and a 13-millimetre (halfinch) Stalite aggregate in it.

“When developing a mix in a lab environment, things can seem great,” he said. “But putting it through a pump and a long length of line can be a whole different issue. That’s particularly true in this case in which we are dealing with a semi-lightweight concrete with a coarse aggregate. Because of the low water-to-cement ratio, it’s easy to end up driving all the moisture into the aggregate, so having a good pre-soaked material is critical for us. In addition, the air that we have in the concrete also needed to be dialed in because density is a critical component of this lightweight concrete mix in order to maintain as much buoyancy as possible.”

Running with scissors

The test pours are particularly important, given that Pumpcrete will not be using the placing booms for the conical section of the CGS, opting instead to feed a standpipe that will rise along with the slipform system.

“We worked with engineers from SDP (SNC-Lavalin, Dragados and Pennecon), the consortium heading up this project, to come up with an arrangement that will work best for us,” said Moriarty. “To add pipe as the form rises, we will be using a ’scissors’ setup, modeled somewhat after the approach Schwing uses on their S 31 HT boom pump. So as the structure goes up and we need to add an additional two-metre section of pipe, I can simply shut off a valve, disconnect the scissor pipe from the stand pipe, raise it, insert the section of pipe and start the process again.

“The level of cooperation between everyone involved — SDP, our equipment suppliers, the tradespeople — has been great. When the CGS is completed and taken out to sea, we should all be proud of what we’ve accomplished out here.”

The structure is scheduled for a 2020 completion and will be operational by 2022.