Anchor-handling tug supply vessels, or AHTS vessels, are specialized workboats that move and set the huge anchors used to hold offshore drilling rigs and floating platforms in position. They combine high engine power, strong winches, open stern decks, and precise navigation to control loads that can weigh many tons. This matters because a rig must stay nearly fixed even when wind, waves, and currents push on it from different directions.
Safe anchor handling protects the crew, the vessel, the rig, and the seafloor equipment below.
Key Facts
- Bollard pull measures a vessel’s steady pulling force, often given in tonnes or kilonewtons.
- Weight force is W = mg, where m is mass and g is about 9.8 m/s^2.
- Line tension must be controlled so the anchor wire or chain does not exceed its safe working load.
- A catenary is the curved shape made by a heavy chain or wire hanging under its own weight.
- Anchors hold best when the pulling line is close to horizontal at the seabed, not steeply upward.
- Mooring systems balance forces from wind, waves, current, and platform motion so the rig stays on station.
Vocabulary
- Anchor-handling tug supply vessel
- A powerful offshore vessel designed to move anchors, tow rigs, carry supplies, and handle heavy mooring lines.
- Bollard pull
- The maximum steady pulling force a vessel can produce while tied to a fixed point.
- Mooring line
- A chain, wire, or rope that connects a floating structure to an anchor or seabed attachment.
- Winch
- A powered drum system used to reel in, pay out, and tension heavy wire, rope, or chain.
- Seabed anchor
- A heavy or embedded device placed on or in the ocean floor to resist the pull of a mooring line.
Common Mistakes to Avoid
- Treating the anchor like it only depends on weight is wrong because many offshore anchors hold mainly by digging into the seabed and resisting horizontal pull.
- Assuming a vertical line gives the best holding power is wrong because anchors usually hold better when the pull is nearly horizontal along the seafloor.
- Ignoring dynamic loads is wrong because waves, vessel motion, and sudden line jerks can create tensions much larger than the steady towing force.
- Confusing bollard pull with vessel speed is wrong because bollard pull describes pulling force at very low speed, not how fast the vessel can travel.
Practice Questions
- 1 An anchor has a mass of 12,000 kg. Using g = 9.8 m/s^2, what is its weight in newtons?
- 2 An AHTS vessel has a bollard pull of 180 tonnes-force. If 1 tonne-force is about 9.8 kN, estimate the pulling force in kilonewtons.
- 3 A rig anchor line becomes very steep as it pulls upward from the seabed. Explain why this can reduce the anchor’s holding ability and what an anchor-handling vessel might do to improve the situation.