AHTS Anchor Handling – The Basics for Newcomers

Every anchor handling operation puts tens of tonnes of tension across a working deck where people stand. The margin between routine and fatal is thinner than anywhere else in commercial maritime operations.

Introduction

Anchor handling is described, correctly, as the most dangerous routine job in the offshore industry. Not the most dangerous emergency. Not the most dangerous abnormal event. The most dangerous thing that gets done regularly, on purpose, to a plan.

The distinction matters. Emergency response allows adrenaline and deviation. Routine work breeds familiarity. Familiarity on an AHTS aft deck, where wire tensions can exceed 300 tonnes and the geometry changes by the second, is the precursor to every serious casualty the industry has recorded.

This article is written for newcomers to anchor handling — cadets, junior officers, ABs transferring from conventional tonnage, and shore-side personnel who need to understand what happens on that stern. It covers the vessel layout, the core operations, the vocabulary, the environmental constraints, and — plainly — the specific mechanisms by which people are killed. It is not a procedural manual. It is a framing document. The procedures come from the vessel’s own Anchor Handling Manual and the rig’s mooring analysis. This article explains what those documents assume the reader already knows.

Nothing here replaces time on deck under supervision. But time on deck without context is just exposure to risk without understanding it.

Contents

• 1. The AHTS Aft Deck — Layout and Function
• 2. The Vocabulary
• 3. Running an Anchor
• 4. Recovering an Anchor
• 5. Preloading
• 6. Chasers, Grapnels, and Pennant Work
• 7. Heave, Weather, and Operational Limits
• 8. The Casualty Record — What Went Wrong and What Changed
• 9. What Kills on an AHTS Stern

1. The AHTS Aft Deck — Layout and Function

The AHTS is purpose-built around one task: controlling heavy objects under tension over the stern while maintaining vessel heading and stability. Every significant piece of deck machinery exists to manage that tension or guide it safely.

Main Winch (Anchor Handling Winch)

Positioned forward of the working deck, typically below deck level with the drum accessible from deck and the control stand on the bridge or a local station. This is the primary tension member. It pays out and heaves in the work wire — the steel wire that connects the vessel to whatever is being handled. Brake holding capacity and rendering characteristics define the operational envelope of the vessel. The winch does not just pull. Its most critical function is controlled release.

Tugger Winches

Smaller winches, port and starboard, used for messenger lines, positioning pennants across the deck, guiding wires onto the roller, and general deck work. They are not rated for anchor tensions. Using them beyond their capacity is a known killer.

Shark Jaws

A hydraulic clamping device set into the deck, forward of the stern roller, designed to grip chain or wire and hold the full working load. The shark jaws allow the main winch to be disconnected or re-rigged while the load is held at the deck. They open and close under hydraulic control, typically from the bridge. When they are closed on a chain under 200 tonnes of tension, the person who commanded them closed has accepted responsibility for every link in that chain and every tonne of that load sitting on the vessel’s stern.

Shark jaws that do not close fully, or that are closed on the wrong part of the catenary, have ended careers and ended lives.

Tow Pins (Stern Pins / Karm Forks)

Vertical retractable pins, port and starboard, aft of the shark jaws. They guide and restrain the wire or chain laterally, preventing it from sweeping across the deck as the vessel yaws. Karm forks — the self-locating, hydraulically operated type — are now standard. Older vessels with manual pins required crew to be physically near the wire to set them. The casualty record from that era is grim.

Stern Roller

The large-diameter roller at the transom over which wire, chain, and anchors pass. Its diameter, freeboard height, and SWL determine what can be brought aboard. The roller is the transition point between the seabed catenary and the deck. Everything that goes wrong tends to go wrong here or just forward of here.

The deck itself is kept deliberately clear. There are few obstructions, few places to shelter, and very little that cannot be reached by a parting wire. This is by design. A clear deck is a survivable deck — if the crew are not on it when something fails.

2. The Vocabulary

Anchor handling has its own lexicon. Misunderstanding a single term during a live operation has direct consequences. The following are non-negotiable for anyone stepping aboard.

Work Wire: The main steel wire rope from the AHTS winch drum. This is the vessel’s primary connection to the anchor system. It takes the full working load.

Pennant Wire: A length of wire, typically with eyes at each end, used to connect the work wire to the anchor or chain. Pennants are rigged, connected, and disconnected on the aft deck — often the most exposed phase of the operation.

Chaser Pennant: A wire pennant fitted with a chaser — a heavy steel ring or collar that rides along the rig’s mooring line to locate and engage the anchor. The chaser is sent down the catenary to find what cannot be seen.

Polyester Pennant: A synthetic pennant used in some mooring configurations, particularly with polyester mooring lines. Lighter, easier to handle, but with different stretch and snap-back characteristics that must be understood.

PCP (Permanent Chaser Pendant): A pendant permanently rigged on the rig’s mooring line, designed to be picked up by the AHTS to facilitate anchor recovery without deploying a separate chaser. When it works, it simplifies the operation significantly. When it is fouled, corroded, or incorrectly positioned, it creates an additional problem to solve under tension.

Catenary: The curve formed by the mooring line between the rig and the seabed. The shape of the catenary determines the horizontal and vertical load components at the AHTS stern.

Grapnel: A multi-pronged hook dragged across the seabed to snag a mooring line or pennant that cannot be picked up directly.

Messenger Line: A lightweight line used to pull heavier wires or pennants across from the rig to the AHTS or vice versa.

Every one of these terms will be used under noise, in poor weather, over a radio with variable quality, between a rig crew and a vessel crew who may not share a first language. There is no room for approximation.

3. Running an Anchor

Running an anchor means deploying it from the rig to the seabed at a predetermined location. The AHTS takes the anchor from the rig, transits to the target position, and lowers it to the seabed while paying out the rig’s mooring line.

The sequence, simplified: the rig crane or chute presents the anchor. The AHTS connects via the work wire, takes the weight, and secures the anchor on or over the stern roller. The vessel then moves away from the rig on a specified heading while the rig pays out chain and wire. At the target location, the anchor is lowered — sometimes set by the vessel’s bollard pull, sometimes left for preloading later.

This is the operation most newcomers see first. It appears controlled. It is controlled — until the vessel drifts off heading, the anchor fouls on the roller, the chain jumping off the wildcat generates a shock load, or the rig’s winch brakes render unexpectedly.

Running an anchor is the simplest anchor handling operation. It is not a simple operation.

4. Recovering an Anchor

Recovering is the reverse: the AHTS connects to the rig’s mooring line, the rig slacks the line, the vessel heaves the anchor off the seabed and brings it back to the rig or onto its own deck for transit.

The critical phase is break-out — the moment the anchor releases from the seabed. Suction, burial depth, and soil type determine the break-out load, which often exceeds the steady-state holding load by a significant margin. The vessel must be positioned, tensioned, and stable for this peak. If the anchor breaks out suddenly, the vessel surges forward and the deck goes momentarily slack before re-tensioning. That dynamic cycle — peak load, sudden release, slack, re-tension — is where snap loads originate.

Once the anchor is off the bottom and in the water column, it is a pendulum. It swings with the vessel’s motion. A fifteen-tonne anchor swinging under a stern roller in a two-metre swell is not a controlled object. It is a projectile on a wire.

5. Preloading

After an anchor is set, the mooring line must be tensioned to a specified proof load to verify the anchor’s holding capacity before the rig relies on it. The AHTS provides this bollard pull.

The vessel connects to the mooring line, builds up power, and holds the required tension for the required duration — typically monitored via the vessel’s towing winch tension readout and verified against the rig’s mooring analysis.

Preloading places the AHTS under sustained high stern load. This is where static stability becomes critical. The pull is acting at the stern roller — above the waterline, aft of the centre of flotation. It trims the vessel by the stern and, if the vector has any lateral component, induces a heeling moment. On a vessel already loaded with chain, anchors, or deck cargo, the residual stability margin may be less than the master assumes.

Preloading is the operation where the vessel feels most stable and is least so.

6. Chasers, Grapnels, and Pennant Work

Not every anchor recovery begins with a convenient PCP waiting to be picked up. In many cases — corroded PCPs, buried mooring lines, lost surface buoys — the AHTS must locate and engage the mooring line blind.

A grapnel run involves dragging a grapnel hook along the seabed across the expected line of the mooring. When the grapnel snags the line, the vessel heaves in carefully to confirm the catch, then rigs a chaser or recovery pennant. This is slow, imprecise work. The grapnel may snag subsea infrastructure, pipelines, umbilicals, or debris. Situational awareness of the seabed — from ROV surveys and rig mooring plots — is not optional.

Deploying a chaser means sending a weighted ring or collar down the catenary of the mooring line. The chaser slides down the wire or chain under gravity and its own weight until it reaches the anchor shackle, at which point the AHTS heaves in and the chaser locks onto the recovery point. In theory. In practice, chasers jam on kinks, corroded links, clump weights, and mid-line connections. A jammed chaser on a tensioned mooring line is a problem with no easy answer and several dangerous ones.

Pennant connections — making up and breaking out shackles, inserting and removing pins, passing eyes over the roller — are the moments when crew must be on the aft deck near tensioned equipment. These are the moments of maximum human exposure. Every effort in modern anchor handling practice is directed at minimising the number of these moments and the number of people present for them.

7. Heave, Weather, and Operational Limits

Anchor handling operations are governed by weather limits specified in the rig’s mooring analysis and the AHTS operating procedures. These limits typically reference significant wave height, peak period, wind speed, and current — but the parameter that matters most on the aft deck is vessel motion. Specifically, heave and pitch at the stern roller.

A stern roller heaving through two metres changes the tension in the catenary dynamically. Wire that was carrying 150 tonnes at the bottom of the heave cycle may carry 250 tonnes at the top. That 100-tonne dynamic load is not felt gradually. It arrives in the time it takes the stern to rise. Every component in the system — wire, shackle, shark jaw, roller bearing, winch brake — must tolerate not the mean load but the peak dynamic load, repeatedly, for hours.

Weather windows are planned. They are also optimistic. The forecast says 2.0 metres significant; the vessel experiences 2.5. The operation is three-quarters complete. The temptation to continue is enormous. The operations superintendent wants the rig moved. The vessel’s day rate is running. The crew are fatigued and want it finished.

More anchor handling casualties have occurred in marginal weather than in genuinely bad weather. Bad weather stops operations. Marginal weather just raises the stakes.

Swell period matters as much as height. A long-period swell under an otherwise calm sea produces slow, powerful heave cycles that load the system heavily with little visual warning. Short, steep seas are uncomfortable and obvious. Long swells are deceptive. They feel manageable on the bridge. They are not always manageable at the stern roller.

8. The Casualty Record — What Went Wrong and What Changed

The anchor handling casualty record is the worst in the offshore marine sector. Multiple total vessel losses. Multiple fatalities in single events. The causes recur with depressing consistency: snap-back from parting wires, capsize under stern loading, loss of stability during tow or preload, and crew struck by moving equipment on the aft deck.

The loss of Bourbon Dolphin in 2007, with eight lives, was a watershed. The investigation revealed a cascade of failures — operational pressure, unclear command authority between vessel and rig, inadequate stability management under complex stern loading, and a mooring configuration that exceeded the vessel’s assessed capability. The vessel capsized and sank in minutes.

The loss of Stevns Power in 2003 and the loss of Maersk Puncher‘s crew member in separate incidents reinforced the same lessons. The industry had normalised risk levels on AHTS sterns that would not have been tolerated in any other offshore context.

What changed:

• NWEA / NOG Guidelines for Safe Anchor Handling: Now in wide use across the North Sea and adopted internationally. These prescribe operational weather limits, minimum stability criteria during operations, crew muster and exclusion zones on the aft deck, and defined authority for the vessel master to stop operations.
• Dynamic Stability Assessment: Post-Bourbon Dolphin, the requirement to assess vessel stability not just in static loaded condition but under dynamic stern loading — including the effect of the towline vector on GZ curves — became standard. Computer-based tools now model real-time stability against actual wire tensions and angles.
• Defined Weather Limits: Operations now carry explicit stop criteria based on significant wave height, heave at the roller, and wind speed — not subject to override by the rig or the operations superintendent.
• Authority of the Master: The master’s absolute authority to cease anchor handling operations was reinforced in regulation and contract. This sounds obvious. It was not always honoured in practice before the casualties forced the issue.

The guidelines work — when followed. They are paper when ignored.

9. What Kills on an AHTS Stern

This section exists because newcomers need to hear it plainly, without euphemism.

The primary killing mechanism on an AHTS aft deck is the release of stored energy from tensioned wire or chain. A steel wire rope under 200 tonnes of tension contains enough stored elastic energy to be lethal at distances far beyond the length of the exposed wire on deck. When it parts — at a shackle, at a worn section, at a kink, at the shark jaw — the broken ends accelerate to speeds that the human body cannot survive.

There is no reaction time. There is no ducking. There is no PPE that helps.

A parting wire does not give warning. It gives consequences.

The specific failure modes:

• Wire parting under tension: The most common fatal event. Caused by overload, fatigue, corrosion, kinking, or shock loading. The failed end whips through any space it can reach. Snap-back zones on AHTS decks are mapped for this reason, but snap-back does not follow neat diagrams when the wire has wrapped around a pin or roller.
• Shackle or fitting failure: A shackle pin backs out under cyclic loading. The eye of a pennant deforms and slips. The anchor’s connecting hardware fractures. The result is identical to wire failure — sudden, total release of stored energy.
• Shark jaw failure or inadvertent opening: The chain or wire held in the jaws is released uncontrollably. It runs aft over the roller under the load of the catenary. Anything between the jaws and the roller goes with it.
• Caught by moving wire or chain: A foot placed inside a bight of chain. A hand on a wire that suddenly tensions. A body between a moving pennant and a fixed structure. The wire does not care. The chain does not stop.
• Capsize: The vessel itself fails. Excessive stern pull, asymmetric loading, free surface effect from ballast transfer during operations, or a combination that was not modelled. The vessel goes over. The aft deck crew are in the water under the hull in seconds.

The mitigation is straightforward in principle and relentless in practice: minimise time on the aft deck. Minimise the number of people on the aft deck. Never stand in a snap-back zone. Never step over a tensioned wire. Never trust a fitting that has not been inspected. Never assume the shark jaws are holding just because the panel says they are closed. Never accept a task that requires crew exposure during peak tension phases.

The master who refuses to send people aft until the wire is slack and the system is confirmed secure will run slower operations. That master will also bring the same number of crew home at the end of every trip.

Closing Reality

Anchor handling is the offshore sector’s most lethal routine activity. It has been made significantly safer by hard-won regulation, better vessel design, and operational guidelines written in the aftermath of deaths. But the fundamental hazard — enormous stored energy in steel wires and chains managed over an open deck — has not changed and cannot be engineered away entirely.

For any newcomer: the aft deck of an AHTS under tension is not a place to learn by watching from close quarters. It is a place to learn by understanding the system, respecting the exclusion zones, and absorbing the casualty record until the reality of stored energy in wire is not an abstract concept but a felt constraint on every decision.

The wire does not know it is a routine operation.