What is a fallpipe vessel?
Rock dumping vessels, also called stone dumping vessels are designed specifically for dumping rocks on the seafloor. They have reinforced hulls to withstand the weight of the rocks and can range from large bulk-carrying vessels to smaller precision vessels that deliver rock to help with erosion control. There are three main types of rock dumping vessels: side-discharge, also called side stone installation vessels (SSIV) or side stone dumping vessels (SSDV); crane; and fallpipe. Side discharging vessels load up with rocks and once positioned, simply dump them over the side. Crane vessels pick up loads of rock with a crane and deliver them with more precision. Fallpipe vessels are the most advanced, using an underwater pipe with an ROV at the end to deliver rock to the sea floor with high precision, even dumping rock underneath structures.
“DP Side Stone Dumping Vessel PompeР“Р‡”. Image from Offshore Fleet.
Why dump rocks in the ocean?
Rock can be used to help prevent or restore shore erosion, bolster wind turbines, and protect underwater structures. Flowpipe vessels can “be used to ballast the legs of offshore platforms, and this work can involve, for example, pumping a mixture of water and rock into the legs.” (Boskalis) The most common use of rock dumping vessels is to deliver rocks and gravel to cover and protect underwater pipes and cables. According to Wärtsilä, “Offshore rock placement is applied mostly to stabilise and protect pipelines, flowlines and power cables.” The rock covering protects underwater pipelines and cables that can be damaged by any number of hazards including shark bites and spying, but the most common danger is fishing gear and anchors that can damage underwater lines. Underwater pipe and cable repairs can be extremely costly, and protecting them is a high priority. Balmoral Comtec recently released a new cable protection and stabilization system to keep underwater cables from moving from everyday ocean currents as well as storms, drastically extending the life of the cables without the need for additional covering.
How do they precisely control the rock?
In shallow waters, side dumping and crane vessels can be used due to the close proximity of the target. In deeper waters, fallpipes are usually preferred because of their advanced precision. However, staying on station near a wind turbine or directly over a cable 1000 meters below the ocean surface takes a specialized system. Most of the modern rock dumping vessels have dynamic positioning systems for this purpose. DP systems use computer controlled thrusters underneath the vessel to not only keep it in place against wind and waves, but also allow it to move with accuracy over the underwater structures as the rock is released. Dynamic positioning systems can be DP1, DP2, and DP3, each increasing the redundancy for additional safety. OneStep Power tests DP2 and DP3 vessel systems for fault-ride through ability, or the vessel’s ability to survive a fault in the power system and stay on station. Check out this video from Van Oord on their dynamically positioned flexpipe vessels.
What type of rock is used?
Many types of rock can be used from concrete pieces, gravel, and large stones up to 800 kg. Highly dense rock like the eclogite seen below may be used, although this is rarer than more commonly available rock or crushed concrete. Any heavy rock that will help minimize movement and protect the underwater structures from weather and anchors can be used.
“Eclogite piece from Norway with a garnet (red) and omphacite (greyish-green) groundmass. The sky-blue crystals are kyanite. Minor white quartz is present, presumably from the recrystallization of coesite. A few gold-white phengite patches can be seen at the top. A 23 millimetres (0.91 in) coin added for scale.” Image from Wikipedia.
How big are rock dumping vessels?
Fallpipe vessels operate with much more precision than side dumping and crane rock dumping vessels and are usually used in deepwater dumping operations. The larger the vessel, the more efficiently it can operate with a larger load of rock and fewer trips to shore. The Van Oord vessel Stornes is the largest fallpipe vessel in the world. It started operation in 2011 and has a revolving projection transporter belt with the ability to discharge 2,000 tons of rock per hour. The 175 meter long ship has a deadweight of 26,648 tons. It has a DP2 dynamic positioning system with approximately 8,000 KW of propulsion coming from three bow thrusters at 1,500 KW each and two retractable thrusters at 2,200 KW each. Rock installation can be done at depths of up to 1,350 meters of depth and is expandable to 1,800 meters. Van Oord has a fleet of three rock installation vessels, including the Nordnes and the subsea rock installation vessel Bravenes. According to Van Oord, “Bravenes is equipped to perform three different types of rock installation, namely the fallpipe through the moon pool, a fallpipe over the side, or a tremie pipe over the side. This third ship configuration allows the vessel to install rock very close to offshore platforms, such as monopiles.” Nordnes is a flexible fallpipe vessel similar to Stornes but slightly smaller.
Van Oord Stornes vessel. Image from Van Oord.
As the internet connects us around the world and wind farms continue to grow, rock dumping vessels like these will be in high demand to cover cables and secure monopiles. The team at OneStep Power is excited to keep these fascinating vessels running safely.
We hope you enjoyed this article on rock dumping vessels - Happy Friday!