Giant subsea ploughs are playing a role in laying pipelines in the offshore oil and gas sector. Berenice Baker reports
While the crane, the digger and the dumper truck are instantly recognisable, their subsea cousins, the mechanical leviathans that lay our ever-expanding network of oil and gas pipelines, remain the unsung heroes of heavy engineering.
Few systems are larger or more impressive than the snappily named PL3: a subsea plough developed by Northumberland-based offshore specialist IHC Engineering Business.
The 205-ton plough — along with a related backfill system that covers cables once they have been laid — is currently being fitted to Far Samson: a ship that is on long-term lease from its owner Farstad by Italian oil-and-gas contractor Saipem.
Custom built over 18 months and designed to withstand the rigours of the offshore engineering world for around 10 years, the two machines are rated to operate up to 1,000m beneath the waves.
Explaining the valuable role played by pipeline ploughs, Mike Crosby, engineering business consultancy manager, said: ‘Typically, clients would like to bury [the pipeline] below the seabed to offer some degree of protection. Uncovered, they can be subject to damage from things such as ships’ anchors, items dropped from vessels and seabed stability problems. There are strong currents on the seabed and, if it’s sandy, the profile changes over time.’
Further problems can be caused by differential cooling. He added: ‘If you’re passing hot oil through a pipeline, on the seabed, the top cools quicker than the bottom and pipelines can buckle. Hydrates can also cause blockages in certain pipelines in these conditions.’
Depending on soil conditions, the plough can dig up to 2.5m-deep trenches in a single pass, travelling between 1km/h and 3km/h. ‘Sometimes you need to pull the pipe up to, say, 1.8m on your first pass and then go back, picking the pipe up again, and have another go,’ said Crosby. ‘If you’re in stiff clays or very dense sands, then the force required to make a deep trench can be quite large. Sometimes it’s quicker pulling hard and going slowly and sometimes it makes more economical sense to do it in two bites.’
The ploughs tend to be paired with a vessel with a large bollard pull — a measure of the strength of a towing vessel. Far Samson recently achieved the world record for bollard pull at 423 tons, matching the PL3, which is designed for a peak pull of 400 tons. A more typical pipeline plough support vessel would have a bollard pull of 250-300 tons. Crosby said that the new systems could potentially be used on the proposed Nordstream project: the longest gas pipeline to date at 1,220km, linking Russia to Germany, bypassing some of the Baltic states.
Typically, the pipeline is pre-laid on the seabed. For a large export line, pipe sections come in 12m lengths that are welded together and laid on the seabed by a pipe-lay vessel. The plough is brought in and rolled over the top of the pipe, which it picks up with pipe grabs fitted at the front and rear — the PL3 can pick up 100 tons. The pipe rollers are then closed beneath it, supporting the pipe.
The two halves of the plough share are then closed together to form a cutting tool. Shaped like traditional plough shares, the blade splits in two, each half inclined at 30o from the horizontal to give it a sharp point. It is also inclined from port to starboard to push the soil out of the trench.
The vessel then moves ahead of the plough and starts pulling. In a process known as ‘transitioning’, the plough goes from the ocean surface to the required trench depth, steering along the pipeline as it goes. The pipe is then released in a reverse action to being picked up.
At the end of the ploughing process, the pipe lies in the trench with two spoil heaps either side. The backfill plough is lowered over the top of the trench and opens up from its folded position to form two scraping blades that span about 24m in width. It is then towed along by the support vessel and returns most of the soil back over the top of the pipe.
A key difference of the BPL3 over previous backfill ploughs is that the front skids run on undisturbed seabed outside the spoil heaps generated by the trenching plough. The skids fold hydraulically into a much shorter space than previous backfill ploughs, so it can be launched and recovered in the correct orientation. Previous ploughs had to be deployed facing backwards and then undergo a complex mid-water operation to turn them into the correct orientation.
The PL3 features more than 60 sensors, including those that detect the position of every hydraulic cylinder and the load being applied to the pipe to ensure that no damage is being caused. It has 10 light-emitting diode (LED) lamps, six low-light cameras, three colour cameras and two object avoidance sonars front and rear. The sonars give a profile of the seabed, a measure of the trench depth achieved and an image of the spoil heaps to make sure there are no problems when it comes to backfill. There are also altimeters for depth, a compass for direction and transponders to give the plough’s position relative to the host vessel.
As ploughs get larger and heavier, two problems arise: not only is a larger crane and an A-frame needed to lift them on and off the support vessel, but a particularly soft seabed simply will not support its weight. ‘In soft-soil conditions, we have to put buoyancy tanks onto the plough,’ said Crosby. ‘We lower an airline down with the plough and, using a compressor on the surface, fill these tanks with compressed air to increase the buoyancy under the sea.’
Since the forces on the plough increase with size, they are constructed using extra-high-grade steel with a yield of up to 690MPa to keep the structure as light as possible. The electronics pod is shielded from the pressure of the ocean at depth, but the rest of the structure uses pressure compensation. All the junction boxes and hydraulic valve tanks are filled with oil and all the steel structures can flood with water.
Another problem with going deeper is that, as the power cables get longer, there are increased power losses and the voltage used has to be increased. The new plough, which operates at 4.5kV, uses a buoyant power umbilical sheathed in polyethylene that has a density less than water so the cable floats, eliminating the need to tie floats on the umbilical to keep it away from the tow wire, making launch and recovery faster.
Although launch and recovery are labour-intensive operations, once the ploughs are on the seabed, they are usually operated by a pilot and a co-pilot in co-ordination with the ship’s captain. With smaller pipelines, the ploughs have to be steered manually but on larger pipelines, the pipe is quite stiff so the plough can follow it to some extent. Operators also ensure that the load put on the pipe during on-the-roll retrievals is kept below the allowable limits, balancing the front and rear loads while trenching.