Tech Feature
Water Injection Dredging
A New Era of Flexibility for Water Injection Dredging
Water injection dredging has been in use for 30+ years. Some things have changed; others not so much.
By Wendy Laursen
Tiamat works by agitating the sediment using water jets and using suction pumps to extract the agitated sediment, placing it higher into the water column than traditional WIDs.
Image courtesy of Haven DredgingThe days can pass slowly for the operator of a water injection dredger. At a steady pace of 1-2 knots, they can sail all day, all week, without seeing the sediment they are moving other than on an echo sounder.
With this type of dredging, water is jetted into the sediment at low pressure to fluidize it and create a density current of fluid mud that progresses along the slight downward sloping bottom contours while not mixing with the water column itself. It is carried either towards a nearby stream or to a nearby depression to settle and to be moved later on in a more condensed way.
One major benefit of water injection dredging (WID) is that it is very fuel efficient relative to its net output. Full prototype tests in the Netherlands have proven that a WID induced density current can sustain itself for distances up to five miles and more, and it can have a net in situ production capacity of about 1,000 to 2,000 m3/hr.
Small ports use the method for maintenance dredging, and the flexibility of the technique means it can be used for the port’s entrance and basin, and around jetties, vessels and underwater infrastructure such as cables and pipelines.
However, as leading operator Van Oord points out, a suitable bathymetry is required, and production rates decrease with increased sand grain size and transport distance.
Industry veteran Marco Pluijm confirms that knowledge of the physical conditions such as bathymetry, sediment characteristics, morphology and actual behavior are essential for successful deployment of WID, more so than for regular hopper work. “WID is a more delicate method with less impact towards its perimeter, so it demands adequate knowledge.”
Companies such as Van Oord and Jan De Nul have been conducting successful operations around the world for decades. However, the technique is not used much in the Netherlands itself these days mainly due to fading experience and knowledge levels of the Dutch government and other authorities, says Pluijm. It is, however, widely applied in Germany and the UK by subsidiaries of Dutch and Belgian contractors.
Van Oord’s first dedicated WID, Jetsed, was built in 1987, and it has been joined by a variety of other vessels to meet demand. After a spate of newbuildings earlier this decade, the company now has a new generation leading the way forward.
The first two vessels, Maas and Mersey, delivered in 2021, feature energy storage and dynamic positioning. They can be pre-programmed to perform more efficiently than earlier vessels. Another two similar vessels will be delivered in 2024.
In 2021, IHC America delivered the Osprey to North Carolina State Ports Authority. The vessel was built at St Johns Shipbuilding in Florida, and it features an automated measurement system so the operator can follow production levels in real time.
Jan De Nul added Pancho and Cosette, newbuildings from Neptune Marine, in 2022. At 27.5m long, the ultra-low emission vessels are more compact than the company’s other WIDs but they still have a depth range of over 20m.
Damen has delivered a range of solutions including dedicated WIDs, tug/WID designs and modular, self-supporting dredging systems that can be placed on the deck of a workboat or barge. The yard has also delivered a variant that injects air as well as water in the sediment with the aim of increasing the uptake of sand particles.
Currently under construction is a Shoalbuster 2711 WID for UK Dredging. The vessel will enable the company to dredge and thus extend the interval between hopper dredging operations, thereby increasing efficiency and lowering emissions and costs.
In place of the usual submersible jets, the WID will use a fully electric Damen E-DOP450 dredge pump on the “plough” that houses the water jets. The pump, the largest in Damen’s DOP dredge pump portfolio, has a 4,000m3/h capacity and is designed to provide optimal efficiency and low operational costs. Ivo van Gelder, D&P engineer – dredging, says the pump is more robust than a water pump, making it particularly suitable for operation in high-sand environments.
The vessel will be equipped with a hydraulically operated A-frame. When the WID not in the use, the A-frame will tilt, enabling the plough to be stored on the vessel’s aft. The plough can be dismounted and stored on the quayside.
The hybrid vessel will not have a battery energy storage system. “The vessel will sail during the dredging operation very slowly, so the full power of the main engines won’t be used by the propulsion system and can also power the dredging pumps,” says van Gelder. However, the vessel will have a DC link so that an energy storage system could replace a generator in the future. The vessel is scheduled to be delivered in Q1 2025.
In 2023, a new hydrodynamic technology emerged from the UK. Haven Dredging’s Tiamat is a fully submerged towable modular unit that can be attached to most small workboats using an A-frame. Tiamat works by agitating the sediment using water jets and using suction pumps to extract the agitated sediment, placing it higher into the water column than traditional WIDs. On reaching the water column, the sediment is put into suspension where it is highly dissolved, allowing it to be easily redistributed using the tide and current.
“Tiamat’s use of the water column means the sediment can be moved greater distances, and as a result, the rate of reconsolidation is much longer than when using WID,” says Jake Storey, executive director of Haven Dredging.
“The key markets for Tiamat are muddy ports, harbors and rivers. However, due to the modular attributes of Tiamat, it can be deployed into a wider variety of environments than WID,” says Storey. He is in discussions with reservoir operators: accretion is reducing global reservoir capacity at an alarming rate, he says. There is also interest from the offshore renewable sector, where Tiamat can solve some of the trench dredging challenges experienced during offshore construction. Storey has also had enquiries about using Tiamat to move uncompacted sand short distances.
Like WID operations though, the operator is unlikely to see any sediment movement from the vessel, and operations proceed at a leisurely 1-2 knots.
