This article is a brief summary of the types of cable trenching vehicles currently available and the general design drivers that relate to them.
Please note that this is a simple reference article and is not intended to be all-encompassing as there are a lot of variations on the themes!
Also, estimates for the trenching speed (especially for the ploughs) are very much based on ‘compliant’ seabed conditions. Trenching speeds can be far slower in variable or stiff clay.
Also, all pictures and links are gathered from the internet so are not the property of S&A
Subsea trenching in general
In the context of this article, by ‘trenching’ we mean the creation of a trench in the seabed using a subsea vehicle. That vehicle could create that trench using water jets, a plough share, a cutting wheel or cutting chain. The vehicle would be launched from a vessel such as a cable lay ship, a marine construction vessel, an anchor handler or something as basic as a platform supply vessel.
There are a few other methods of creating a trench such as ‘mass flow excavation’ or ‘vertical injector’ but in general they aren’t vehicles but tools suspended from a vessel.
Power cable vrs Telecoms cable
Without going into too much detail about the differences between power cable and telecoms cable its probably fair to say the big differences, in terms of trenching requirements, are:
Minimum Bend Radius (MBR): The MBR influences anything that touches the cable such as grabs, rollers, depressors and most importantly plough shares. In general, the bigger the MBR requirement the larger the vehicle. Telecoms cable tends to be up to 1.5 to 2m MBR whereas power cable goes up to 5m MBR making power cable trenchers, especially ploughs, a good deal bigger than their telecoms cable equivalents.
Outside Diameter: Telecoms cable diameters tends to max out at 200mm whereas power cable can be 300mm+
Repeaters: Long telecoms cables have repeaters spliced into the cable which have a diameter of up to 400mm. However, as the telecoms cable is a lot smaller diameter than the repeater trenching systems usually are set up to trench the cable and expand temporarily for the repeaters e.g. variable jetting sword width and repeater ‘flaps’ on the trailing edge of plough shares. Power cable trenching has no such issues as power cable does not feature repeaters.
Bundles: Power cables can be bundled together whereas telecoms cables are not
General Groups
In the context of this article, the cable trenching systems are:
- Cable Maintenance ROV
- Jet Trencher – free flying
- Jet Trencher – heavy duty
- Rock Cutter – Wheel
- Rock Cutter – Chain
- Jet Sled
- Cable Plough – telecoms
- Cable Plough – power (deck loading)
- Cable Plough – power (top loading)
Cable Maintenance ROV
Cable Maintenance ROV (CMROV) systems are typically found on most telecoms cable installation vessels. They are used for general workclass ROV tasks as well as cable related activities. These cable activities include post lay inspection, post lay burial, maintenance burial. CMROV systems are frequently carried as a secondary vehicle to support a plough system or as the sole vehicle on cable maintenance vessels.
Common features: Free flying ROV. Fitted with skids or tracks. Fitted with TSS cable tracking equipment. ‘Drop-down’ jetting swords with integrated depressor
Typical Specification:
Weight: 5Te – 10Te
Power: 150kW – 300kW
Max. trench depth: Up to 1.5m
Max. water depth: 3000m
Max. trenching speed: 300m/hr
LARS Requirements:
Side mounted ROV LARS skid with/ without TMS
Lifting umbilical winch
Jet Trencher – Free Flying
Jet Trenchers that free fly are found on cable installation vessels that operate in sandy conditions with shallow trenching requirements, e.g. inter array burial. They are used for general workclass ROV tasks as well as cable related activities. These cable activities include post lay inspection, post lay burial, maintenance burial. They are similar to Cable Maintenance ROVs but larger, more powerful and are usually the sole trenching system.
Common features: Free flying ROV. Fitted with tracks. Fitted with TMS cable tracking equipment. ‘Drop-down’ jetting swords with integrated depressor
Typical Specification:
Weight: 10Te – 50Te
Power: 300kW to 2000kW
Max. trench depth: Up to 3m
Max. water depth: Up to 3000m
Max. trenching speed: 300 – 600m/hr
LARS Requirements:
Side mounted ROV LARS skid with/ without TMS
Lifting umbilical winch
Jet Trencher – Heavy Duty
Jet trenchers that are for heavy duty activities are typically found on power cable installation vessels. They are used for trenching only. The vehicle is typically a platform style system that can have multiple functions (see rock cutter section)
Common features: Negatively buoyant. Fitted with tracks. Fitted with TMS cable tracking equipment. Feature multiple jetting pumps. ‘Drop-down’ jetting swords with integrated depressor. Bullet launched.
Typical Specification:
Weight: 50 – 200Te+
Power: 600kW – 3MW
Max. trench depth: Up to 3m
Max. water depth: Up to 1500m
Max. trenching speed: 500 – 1500m/hr
LARS Requirements:
Aft mounted A-Frame or crane launch
Umbilical winch
Lift winch
Rock cutting – Wheel
Rock cutting systems using a cutting wheel are modular attachments to heavy duty jet trencher vehicles. The rock cutting wheel module typically replaces the jetting components but will maintain some jetting features for cleaning the wheel. The rock cutting wheel is the most robust trenching system but is depth limited by the hub of the wheel.
Common features: Negatively buoyant. Fitted to a heavy duty tracked vehicle. Bullet launched. Removable wear tips.
Typical Specification:
Weight: N/A – see Jet Trencher Heavy Duty
Power: 500kW+
Max. trench depth: Up to 2m
Max. water depth: N/A – see Jet Trencher Heavy Duty
Max. trenching speed: 200 – 500m/hr
LARS Requirements:
Aft mounted A-Frame or crane launch
Umbilical winch
Lift winch
Rock cutting – Chain
Rock cutting systems using a cutting chain are modular attachments typically fitted to heavy duty jet trencher vehicles. The rock cutting chain module typically replaces the jetting components but will maintain some jetting features for cleaning the chain. The rock cutting chain is not as robust as a wheel system however it can dig deeper. A chain system is lighter than a cutting wheel and as such can be used on large free-flying trenchers provided the frame is sufficiently robust.
Common features: Negatively buoyant. Fitted to a heavy duty tracked vehicle. Removable wear tips.
Typical Specification:
Weight: N/A – see Jet Trencher Heavy Duty
Power: 500kW+
Max. trench depth: Up to 4m
Max. water depth: N/A – see Jet Trencher Heavy Duty
Max. trenching speed: 100 – 200m/hr
LARS Requirements:
Aft mounted A-Frame or crane launch
Umbilical winch
Lift winch
Jetting Sled
Jet sled systems are typically found on small scale, near shore cable installation vessels. They are generally very simple with minimal instrumentation and surveillance. The vehicle is not self powered so does not feature on-vehicle jetting pumps and requires a towing arrangement from the lay vessel / support vessel.
Common features: Negatively buoyant. Fitted with skids. Minimal instrumentation. Surface fed jetting system. ‘Drop-down’ jetting swords with integrated depressor. Crane launched.
Typical Specification:
Weight: 5Te – 10Te
Power: N/A, jetting supplied by vessel pumps (can be containerised diesel units)
Max. trench depth: Up to 5m
Max. water depth: 30 – 80m
Max. trenching speed: 600 – 1000m/hr
Typical tow force: 30 – 50Te
LARS Requirements:
Crane launch
Umbilical winch (if fitted with instrumentation)
Cable Plough – Telecoms Cable
Cable plough systems designed for telecoms cable are found on the majority of telecoms cable installation vessels. They are solely used for trenching. They can feature jetting systems however the jetting is not for parting the seabed as on trenching ROV systems. Jetting is to create a ‘bulb’ of expanded sand in front of the share making it easier for the share to pass through. Telecoms ploughs are towed by the lay vessel as part of a ‘simultaneous lay and burial’ operation. They can be launched and towed from a support vessel as part of a ‘post lay burial’ operation. however the plough will need to feature a ‘self loading’ system. They differ from a power cable plough in that they are smaller due to the cable having lower MBR and diameter restrictions. They can also feature systems for trenching repeaters such as repeater ‘flaps’ or guides along the trailing edge of the share.
Common features: Negatively buoyant. Fitted with articulating skids. Large ‘share’ for cutting the seabed with integrated depressor. Jetting fitted where seabed is expected to be sandy. Repeater trenching scheme.
Typical Specification:
Weight: 20Te – 40Te
Power: 30kW – 500kW (including jetting)
Max. trench depth: Up to 3.3m
Max. water depth: 3000m
Max. trenching speed: 1000 – 1500m/hr
Typical tow force: 80 – 130Te
LARS Requirements:
Aft mounted A-Frame
Tow/ Lift winch if drawbar based
Standard umbilical winch
Cable Plough – Power Cable (deck loading)
Cable plough systems designed for relatively small to medium diameter power cable are found on power cable installation vessels that intend to lay export cables. They are solely used for trenching. They generally feature jetting systems however the jetting is not for parting the seabed as on trenching ROV systems. Jetting is to create a ‘bulb’ of expanded sand in front of the share making it easier for the share to pass through. Power cable ploughs are towed by the lay vessel as part of a ‘simultaneous lay and burial’ operation. They can be launched and towed from a support vessel as part of a ‘post lay burial’ operation however the plough will need to feature a ‘self loading’ system. They differ from a telecoms cable plough in that they are in general larger due to the cable diameter and MBR requirement plus have a higher tow force.
Common features: Negatively buoyant. Fitted with articulating skids. Large ‘share’ for cutting the seabed with integrated depressor. Jetting fitted where seabed is expected to be sandy.
Typical Specification:
Weight: 45Te – 55Te
Power: 300kW – 500kW
Max. trench depth: Up to 3.5m
Max. water depth: 500m
Max. trenching speed: 1000 – 1500m/hr
Typical tow force: 130 – 150Te
LARS Requirements:
Aft mounted A-Frame
Tow/ Lift winch if drawbar based
Standard umbilical winch
Cable Plough – Power Cable (top loading)
A top loading power cable plough has all the same features as a typical power cable plough with the exception of the loading method: they load subsea to provide a potentially more ‘cable friendly’ loading operation. Loading subsea can be the prefered method for large diameter/ MBR power cables as deck loading forces the cable through a ‘S’ bend during launching which is something to be avoided if possible. Top loading is also intended to avoid lifting the product with grabs which would point-load the stiff and heavy product. There are a number of top loading ploughs in development by companies such as Nexans, NKT and Prysmian.
Common features: Negatively buoyant. Fitted with articulating skids. Large ‘share’ for cutting the seabed with integrated depressor. Jetting fitted where seabed is expected to be sandy. Top loading.
Typical Specification:
Weight: 45Te – 55Te
Power: 300kW – 500kW
Max. trench depth: Up to 3.5m
Max. water depth: 500m
Max. trenching speed: 1000 – 1500m/hr
Typical tow force: 130 – 150Te
LARS Requirements:
Aft mounted A-Frame
Tow/ Lift winch if drawbar based
Standard umbilical winch
Loading Considerations
CMROV, free flying trenchers and heavy duty jet trenchers are not launched with cable inside the vehicle. They land on the product that is resting on the seabed and proceed to jet the cable into the seabed. This is the most cable friendly trenching method. This implies that they can be deployed from the lay vessel or some other support vessel.
Rock cutting trenchers require the product to be loaded into the vehicle, an operation that is performed subsea using manipulators and articulating cable routes. This implies that they can be deployed from the lay vessel or some other support vessel.
Jet sleds and telecoms ploughs are loaded with cable before they leave the vessel and are launched with the cable captured within the vehicle increasing the potential for cable damage. This implies they can only be deployed from the lay vessel.
Power cable ploughs can be launched like a telecoms plough if the power cable is robust enough. When the power cable diameter or MBR is large the plough can be a risk to the cable as it passes through the plough during launching activities. The trend is to load the plough subsea using manipulators like a rock cutting trencher or to have the lay vessel lay product into the top of the plough (top loading). The ability to load subsea allows the plough to be used for both simultaneous lay and post lay burial operations while also reducing the risk of damage to the product.
Launch and Recovery Considerations
CMROV and free flying jet trenchers make use of standard ROV launch and recovery systems (LARS).
Jet sleds and lightweight ploughs are generally launched using a vessel crane. Heavy duty jet trenchers and self loading bullet launch ploughs can also be launched by a large vessel crane.
Ploughs in general make use of an aft mounted A-Frame as this is most convenient for the product routing and also the wire tire. Heavy duty jet trenchers also tend to be launched using the aft A-Frame as this makes use of existing vessel systems. A-frame launch systems are generally either a conventional construction A-Frame or a plough specific A-Frame.
In order to launch a plough or heavy duty jet trencher using a conventional A-Frame, the plough will need to be fitted with a bullet launch system. This also implies a separate lift winch and tow winch.
In order to launch a plough using a plough specific A-Frame the plough will need to be fitted with a drawbar system. This implies a combined lift and tow winch however that winch must be mounted at or routed to an elevated position to correctly route the tow wire.
Vessel Considerations
In order to operate a trenching vehicle from a vessel the primary vessel considerations are:
Available power: Any jet trenching system will require a significant amount of power. Depending on the size of the system, this can range from 300kW to 4MW. A plough system can be as low as 30kW however the trend is to fit jetting as standard which implies a power consumption in the range of 300kW to 500kW
Available tow force: Towed systems, primarily ploughs, require tow forces usually in excess of a typical lay vessel. 80Te is the minimum tow requirement for cable ploughs with 150Te being more typical as trench depths increase. It should be noted that this is continual tow load, not peak, as a plough can be at maximum tow load for extended periods.
Available deck space: Any A-Frame launched system will require a dedicated deck space below the A-Frame for storage, loading and maintenance. This area is usually high tonnage such as 10m^2. If the A Frame is plough specific, there is an additional deck spec requirement for when the A Frame is in the inboard location.
Tow winch location: Towing implies high tension wire ropes in which case it is not usual for a back deck to be off limits during towing. The placement of the tow winch and the route of the wire must take account of simultaneous operations that may occur during towing.
Launch and Recovery System (LARS) approach: If the trencher has its own LARS a space will be required along the port or starboard side of the vessel. If a plough has a drawbar the A Frame will need to be a plough specific ‘wide angle’ A Frame which is not as practical as a standard construction A Frame but has the advantage of allowing a drawbar system on the plough. If the plough or trencher has a bullet launch the standard construction A Frame will need a central lift point and a scissor frame.
Jetting vrs Ploughing
This graph illustrates a typical jet trencher vrs plough assessment in clay:
Operational Drivers
Information is indicative only
| Vehicle | Sand | Clay | Stiff Clay | Rock |
| CMROV | 0 – 1.5m | X | X | X |
| Trencher – Free | 0 – 2m | 0 – 2m | X | X |
| Trencher – HD | 0 – 3m | 0 – 3m | Marginal | X |
| Rock wheel | X | 0 – 2m | 0 – 2m | 0 – 2m |
| Rock chain | X | 0 – 4m | 0 – 4m | Marginal |
| Jet sled | 0 – 5m | X | X | X |
| Plough – Tele | 0 – 3.3m | 0 – 3.3m | 0 – 3.3m | X |
| Plough – Power | 0 – 3.5m | 0 – 3.5m | 0 – 3.5m | X |
Useful Info
Useful links:
Animation of new Nexans power cable plough (developed and constructed by S&A). This is the worlds first ‘top loading’ cable plough: https://www.youtube.com/watch?v=SJqGZwG2fl0
Beach landing video from VBMS that illustrates a plough in operation: https://www.youtube.com/watch?v=ulmA8HsTSyY
General SMD plough video: https://www.youtube.com/watch?v=QD0IXRRXWiA
Prysmium self loading plough animation: https://www.youtube.com/watch?v=fz-J0NgwKlU
General SMD free flying jet trencher animation: https://www.youtube.com/watch?v=6yM6AAFWg1w
LDT heavy duty jet trenching animation: https://www.youtube.com/watch?v=ZJDq5j-luTw
VBMS rock cutting animation: https://www.youtube.com/watch?v=UKp_0CBJm7E
If you would like to discuss trenchers, ploughs or related issues you can contact S&A by email using info@screaton.co.uk or via the contact form on our contact page
Screaton and Associates Ltd
Severn House, Mandale Business Park
Belmont, Durham, UK
DH1 1TH
