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2.0 Description of Vessel Types
2.1 Tugs
Tugs are usually small but extremely powerful boats whose main role is to pull or push much larger
vessels, non self-propelled units, or other floating structures. Their scope of work includes towage, salvage and
assistance. Modern tugs have also fire fighting, ice-breaking and oil dispersal capabilities. However, their main
offshore use is for towing barges, structures and rigs as well as being used for the accurate positioning of
offshore Jackets and Jack-Up drilling rigs. Many of the newer built tugs are designed with Azimuth Stern
Drive (ASD), Voith Schneider or other sophisticated propulsion systems so as to be highly manoeuvrable. Tugs
for offshore use range between 4,000 and 20,000 BHP.
2.2 Anchor Handling Tugs
Anchor Handling Tugs (AHT) are essentially tugs fitted with anchor handling winches enabling the vessel
to perform anchor handling operations and the towing and positioning of drilling rigs. They are shorter than
AHTS vessels and do not carry cargo but, due to their increased maneuverability, are often also used for
anchor handling during pipe laying operations and construction projects where the vessel length can be
critical.
2.3 Anchor Handling Tug Supply Vessels
Anchor Handling Tug Supply (AHTS) vessels combine the role of supply vessels and anchor handling
tugs. They are the backbone of offshore operations and constitute the largest proportion of offshore vessels,
being approximately 30% of the current total offshore fleet. Not only do they deliver supplies such as deck
cargo, water, fuel, dry bulk and mud to oil rigs and platforms, they are specially designed to provide anchor
handling services, towage duties and in some cases also serve as an Emergency Towing Rescue and Recovery
Vessel (ETRRV). Some modern AHTS vessels are also equipped for fire fighting, rescue operations and oil
recovery to enable them to have a more multi-role capability.
Winches are fitted for towing and anchor handling, with an open stern to allow the decking of anchors.
With the increase in deep-water exploration, it is necessary for vessels to be designed with higher horsepower
and a larger beam to be able to handle the heavier gear required to operate at such depths. Vessels in this
category range from 4,000 to 27,000 BHP and work in different areas of the world, although the smaller ones
often do not perform any anchor handling.
Vessels operating in the North Sea typically range from 150 to 270 tonnes bollard pull (BP), are suitable
for harsh weather environments and have winches with 350 to 500 tonnes line pull. However, in view of the
Bourbon Dolphin incident which occurred in April 2007 when the AHTS vessel capsized during rig move
operations resulting in the loss of eight crew members, it is envisaged by several shipowners that there will be
an increasing demand for larger vessels of minimum 265 tonnes BP and 20m beam.
2.4 Supply Vessels
Platform Supply Vessels (PSV) are designed specifically to transport cargo to offshore oil rigs and
platforms and when converted can perform a variety of tasks to support offshore operations. PSVs are similar
to AHTS vessels, except that PSVs do not have winches, have a closed stern and generally have large decks.
Supply vessels range from under 1,000 tonnes deadweight to around 4,500 tonnes deadweight.
Cargoes are mostly carried both above and below deck, in order to maintain the stability of the vessel.
Drilling pipes and general cargo in containers are carried above deck for efficiency and safety of transport
whilst bulk cargo used in the drilling process, such as mud and cement, fuel and water, are carried in tanks
below deck with the cargo subsequently pumped up to the rigs and platforms.
Some PSVs have been purpose built or converted to undertake a particular job such as seismic survey,
flexible pipe laying or repair operations and may have helidecks to enable passenger transport to and from the
vessel, especially in remote locations.
2.5 Barges
Offshore barges are flat-bottomed, non self-propelled units of shallow draft, designed for the transport of
heavy or voluminous cargo on deck. Barges range in length from around 30m to 200m and need to be towed
to and from location. Some barges are submersible so cargo can be floated on and off and then transported in
a ‘dry tow’ mode with a tug towing the barge.
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