“WE NEEDED TO CONSTRUCT A landfall for a wind-farm project: power cables from offshore turbines had to come ashore under a beach.” Stuart Stephens is Special Projects Director for Stockton Drilling, which specialises in coastal and near-shore operations. “Direct Pipe seemed the obvious solution,” he says.
The project in question is the US$3.6bn Beatrice Offshore Windfarm Landfall (BOWL) situated in the Moray Firth, an inlet of the North Sea off the Scottish coast, northeast of Inverness. It comprises 84 wind turbines producing 584MW and powering a staggering 450,000 homes.
“We would launch a Direct Pipe TBM from the beach, jacking the steel conduit-pipe behind it; and when TBM and pipe were 450m offshore, we would steer it up to the seabed, disconnect the TBM from the pipe, and recover the TBM out of water. Our steel conduit would be laid from land out to sea and the cable could be threaded through it. “And then we would refurbish the TBM and use it again for a second landfall that the wind farm needed, 50m further along the beach. It seemed a nice and natural way to do it. When we contacted Herrenknecht to discuss the project we were surprised to find that the method had never been tried before.”
Herrenknecht had not designed Direct Pipe as part of a system that retrieved expensive TBMs from the soft alluvium of the North Sea, according to Joachim Engelhardt, head of sales support at the company and who helped devise the technology a decade ago. But when he received the call from Stockton, possibilities became realities. And landfalls have proved an ideal application for the innovative technology.
Pipe-jacking pushes short lengths of generally concrete pipe from an excavation horizontally into the ground using jacks thrusting against the end of the pipe section. When one section has been pushed in, the next section is lined up behind it and pushed in its turn. That can be seen as the forerunner of Herrenknecht’s Direct Pipe system. Engelhardt explains the genesis:
“This method combines the advantages of microtunnelling and HDD technology. In just one step, a prefabricated pipeline can be installed and the required borehole excavated at the same time.
“Around 2006 or 2007, we started building the pipe thruster, which is one of the key elements of Direct Pipe,” he says. “It grips the steel pipe from the outside and pushes it forward into the ground.” Since it grips the pipe around the circumference, rather than needing an end to push against, it can operate on much longer sections of pipe; in particular, steel pipe, cheap and easily available, can be used. “Initially the pipe thruster was developed as a tool to assist in horizontal directional drilling (HDD), for when a borehole has been drilled and the lining is then pulled through. The thruster can either pull out stuck pipelines or assist by pushing from one end while you are pulling from the other.
“That was the initial development. Soon after we said ‘we have this Pipe Thruster, why do we not put a microtunnelling machine on the front of the steel pipeline and it could do pipe-jacking tasks but with longer steel pipe sections. Mining out the ground at the front would make it a lot easier for the jacks to push. Then, we would have the best of both worlds.”
Thus the Direct Pipe method was born. The pilot project was a crossing of the River Rhine in Germany – which was perhaps ambitious for a first project. “The Rhine is not one of the largest rivers around, but it is sizeable, and the ground conditions are not easy. You find gravel and other tricky conditions, but it was the right project to do because we wanted to prove first of all that the technology works in difficult or in challenging ground, and second, that you can also do it on longer distances.
“As you know, in standard pipejacking you can do lengths of maybe 150 or 180 metres. With some added gimmicks you can go to maybe 250 or even 300 metres, but that is typically the limit for smaller diameters. Beyond that you must use concrete pipe, of larger internal diameter, say 1.8m or 2m; that way you can go to a 1,000m length or more. But we wanted to prove that we can go that far with 48-inch (1,220mm) steel pipe. It worked well, and we went on from there.”
With the Rhine safely crossed, Direct Pipe established itself as a standard technology, at any rate for onshore use. “It is steerable, in the same way that a TBM is steerable,” says Engelhardt. “It is possible to do curves with pipe-jacking, in fact the majority of standard pipejacks are in a straight line from start shaft to pick-up shaft, A to B. But more and more planners are eliminating intermediate shafts by opting for curved drives. Also, DirectPipe is steerable up, down or sideways to perform three-dimensional curves.”
The excavating cutting head is controlled remotely by umbilical cord. “Two slurry lines supply it with feed water, light bentonite going in, heavyweight slurry coming out to a separation plant at the launch site. And you also have the communications cable, the power cable, and everything that a standard TBM needs to operate it.
“It can handle diameters up to 60 inches (1,524mm) – which is the largest diameter that we can grip with the Pipe Thruster. The smallest size used to be 30 inches (762mm), but we now are able to go down to 24 inches (610mm).”
For the gas industry, the system has many benefits. “A gas pipeline operates under pressure. To install it by pipejacking you usually get steel pipes in 12m–18m lengths and transport them to the job-site where you have to weld them together, a section at a time, in a deep excavation at the launch site, squashed up against your ram jacks. You can test your welds, you can X-ray them to see that they look good, but you cannot actually pressure-test the joint. You can only do that test once the entire length of pipeline has been installed and is underground; and if you find a leak at that stage, all you can do is pull the whole lot out and start again. So you are putting your whole project into the ground untested, which is something that makes contractors and project owners very nervous. They do not like it at all.
“But, if you do the same job with Direct Pipe, you pre-weld a section 200 or 300m long, or even 500m if you want. You do that above-ground; and you can actually pressure-test that section before you put it in the ground.”
It has advantages over HDD as well: “With HDD, you bore the hole, then pull the product pipe through. With Direct Pipe you do effectively the same but in one operation rather than two. Direct Pipe also has the advantage of being able to operate in non-cohesive soils, such as sands and gravels, whereas HDD struggles with these.”
And for Stockton Drilling’s landfall operations, HDD would have been problematical, as Stephens explains: “With HDD, the bore remains unsupported until you have finished excavating and can pull your lining or cables through. That is fine in ground that is self-supporting, but our landfall was in cobbles, sand and soft alluvium, and would have collapsed. Direct Pipe sleeves the tunnel as it advances, which is a very real advantage.”
So, here was a technology discovering a use. Stockton Drilling and Herrenknecht worked closely together to make it work. The essential new ingredient was the remote disconnect module, fitted to the back of the TBM, that would separate the TBM from the steel conduit out at sea. “We discussed that with Stockton Drilling. They would have to use and deploy it so we made sure they were comfortable and secure with the design,” says Engelhardt.
“It was somewhat hard to work out. The important thing in offshore scenarios is that the simpler the design, the more reliable it will be. The challenging part was realising which parts of a standard microtunnelling outfall do not apply to Direct Pipe, and which parts of standard Direct Pipe do not apply to this sub-sea outfall. The actual disconnect module was operated by umbilical cable.”
“When it came to that part of the operation, we just pressed a button in the control cabin on-shore, and the disconnect happened,” says Stephens.
But before that stage was reached they had a lot of tunnelling to do. “The beach is an SSSI – a Site of Special Scientific Interest, so we had to get permissions and so forth. We were working 100m back from the high tide line, on an ancient, raised beach.
“We were able to lay out roughly 100m strings of helical steel pipe behind our launch excavation, supported on rollers. We only had to do four sets of welds to make up our whole distance. There is an impressive amount of flexibility in welded steel pipe, so it was able to curve down into the launch pit for the thruster jacks to grip and push on. The vertical radiuses in the tunnel were 1,000m. The outside diameter of the pipe was 1.2m, and we had 750t of jacking force. The geology was primarily alluvial, of pebbles and cobbles; however, towards the end we drove through a bed of weathered sandstone.
“Our drilling fluids were registered with the Centre for Environment, Fisheries and Aquaculture Science (CEFAS) as Posing Little or No Risk to Environment (PLONOR), so the muck could be spread on local farms that wanted it.
“When we reached our destination, 450m offshore, we steered the TBM, still with its pipeline behind it, up onto the seabed. There was a bulkhead door at the back of the TBM. We filled the tunnel with water to equalise pressures; then, operated from the control cabin, some rather lovely Herrenknecht hydraulics and electronics cut the umbilicals, closed the bulkhead door and separated the TBM from the steel pipe. Then, it was just a question of lifting the steel TBM, all 28t and 28m of it, up from the seabed.”
The main problem was the shallow depth of water. “We exited 5m below LAT – Lowest Astronomical Tide, the lowest that can be expected normally to occur. Our fear was that the TBM might be too heavy for the ooze to support it and that it might just follow the bedrock but stay lying on top of it half buried or more in the ooze. In the event, we could actually see it from the surface. Divers went down to put slings around it. We excavated a shallow pit in the ooze for them using an airlift, which operates by suction; but the airlift needs to be fully submerged to work properly, and part of the time it was not. But they fitted the slings, and a crane on a jack-up barge raised the TBM, using quite a substantial spreader beam from Modulift. Then it was taken by multi-cat to Buckie Harbour for refurbishment before the second run. The refurbishment really consisted in little more than painting. The only change suggested by that experience was to put tungsten carbide inserts into the cutterhead. The cutters had emerged worn, though not alarmingly so; but if the second run had encountered more abrasive conditions then we might have had problems so we took that precaution.
“Each drive took about 90 days; we were ten months on site in all, including set-up and downtime. Once the conduits were in place, we cast a slab for a winch at the launch site. A crew from the cable company stationed a drum of cable on a vessel offshore and towed it through with a messenger wire using the winch. At the end of it, everyone was quite impressed with the operation.”
That was the world first and, as a result, huge opportunities have opened up. Its major selling point is that it sleeves the tunnel as you advance, says Stephens, so it can be used in alluvial soil and very shallow ground that is not self-supporting; and that is the main reason why the energy markets and anyone who needs landfalls are expressing so much interest. Landfall locations that can be easily mined by HDD are limited. Direct Pipe makes many more sites feasible for bringing cables and pipelines ashore.
“It is less risky than pipe-jacking, because if you hit an obstacle, even if it is 500m out, you can simply retract all your piping back to the surface, and that too is a fantastic benefit and selling point; pipejacking in contrast is a strictly one-way process.
“And it works in very shallow depths as well. You need only five metres of cover, and that excites the wind-farm people because heat loss effects mean they can use cables of smaller thermal capacities – in other words, of smaller diameter.
“Elsewhere on a previous project we put a 500m cofferdam into the sea. That was expensive and it could not have been done here because of the environmental designations. So this was the right technique in the right place. Any other solution would have entailed risks that no-one really wanted to accept.
“We have been quietly beavering away explaining to clients that you can actually now undertake landfalls in non-supporting soil where HDD is not suitable.” And the explaining is paying off: “It is being used elsewhere all over the world, on around five or six projects, in my understanding.”
Which Engelhardt confirms: “And you do not have to use a floating crane to retrieve your TBM,” he says. “A simpler and easier alternative is to use floatation bags and just float and tow it to the nearest harbour for a land-crane to take it out of the water. That is being done on some of those other projects.”
“We are currently evaluating ten more landfalls for windfarms in the UK on sites where HDD is risky, which is why the project owners came and asked us about Direct Pipe,” says Stephens. “People are starting to see the benefits, which is really good. There is a vast potential workload coming in.”