How do I calculate the tail load in an upending operation?

Take moments about the tail attachment using horizontal lever arms. If the attachments and the centre of gravity all sit on the vessel axis, both arms carry the same cosine of the inclination, the cosines cancel, and the share does not change as the vessel rises. On the worked 80 t vessel the tail carries 314 kN flat, is still carrying 301 kN at 45 degrees and 270 kN at 75, and only lets go in the last few degrees. A tail crane sized on the assumption that its load fades away through the lift is sized wrong.

Do I need a tail crane, or will a dolly do?

It depends on how much control you need over the tail's position and on what the tail runs on. A tail crane lifts and positions the tail, and it brings a hook check and a tailing attachment check with it. A dolly lets the tail roll with a low horizontal resistance, and it needs a rated dolly, a flat clean surface and a plan for what happens if it jams. Dragging on the ground needs no equipment and puts a substantial horizontal force into the arrangement plus a surface that has to survive being dragged over.

What friction value should I use at the tail?

A band, not a value, and the operation should be acceptable across all of it in both directions. Too much resistance and the tail does not move when it should, so the vessel bends and the main crane is pulled off its radius. Too little and the tail runs away. Skids on steel, skids on timber, a dolly with good bearings and a dolly that has stood in a yard are four different bands, and weather moves all of them.

When is the tail released during an upend?

At an angle that comes out of the calculation rather than out of judgement on the day: near vertical, when the tail's share is small enough that releasing it does not disturb the vessel. The plan needs three things - the angle, a named person who confirms it with a way of measuring, and what happens if it is released early, because the vessel then swings horizontally towards the main crane's plumb line with energy in it.

Why does the tailing attachment govern the operation?

Because it is loaded far out of its own plane for most of the rotation, and a plate out of plane is a cantilever with a small section modulus. On the worked 80 t vessel the tailing attachment reaches 98 percent while the main hook is at 78 and the tail hook at 65. It is the normal result, it is why tailing lugs are thick with large returned welds, and it is the main argument for using a trunnion at the tail instead.

Tailing lift calculation: crane, dolly, friction

The short answer

The tail load does not decay as the vessel rises. On a colinear arrangement the cosines in the two lever arms cancel exactly; on the worked vessel it goes 314 kN flat, 301 at 45 degrees, 270 at 75, and only then collapses.
Which tailing arrangement you choose changes what has to be checked at all, not just the numbers. A tail crane brings a hook check and a tailing attachment check that a dolly does not.
On the worked vessel the tailing attachment governs the whole operation at 98 percent, ahead of both cranes. That is normal, and it is why tailing lugs are heavy.
Friction at the tail is a band, not a number, and the honest response is a wide declared range that the operation survives all of.
The transfer point is a discrete event with its own load case. It needs a stated angle, a person who confirms it, and a plan for releasing early.

What tailing is, and the four ways to do it

Something has to hold the bottom of the vessel while the top comes up. The choice between a crane, the ground, a dolly and nothing changes the whole calculation.

An upend starts with a vessel horizontal and ends with it vertical. Through the rotation the bottom end has to be supported, restrained, or dragged, and there are four ways to arrange that.

01What holds the tail?
A second crane
The tail is lifted and its position is controlled. Adds a hook check and a tailing attachment check, and adds a second crew. The most controlled and the most expensive.
The ground
The tail is dragged, and friction is a real force in the balance. No second crane, and a horizontal force that has to go somewhere, plus a surface that has to survive being dragged over.
A dolly or skate
The tail rolls, and the horizontal force drops sharply. The middle answer, and it needs a dolly rated for the load, a surface it can run on, and a plan for what happens if it jams.
Nothing, after transfer
The vessel hangs free from the main crane. The end state. The tail attachment is unloaded and the body's own bending has largely gone.

Choosing the arrangement is the design decision, and it is usually made on what is available rather than on what it costs. The rest of this article prices the difference.

The tail force, by hand

Moments about the tail attachment, with horizontal lever arms. The result is not the smooth decay most people picture: the tail holds nearly all of its load until the vessel is almost upright, and then loses it in a few degrees.

Take a vessel with both hooks vertically above their attachments. Moments about the tail attachment give the top share, and the tail is the remainder:

Moments about the tail attachment

P_{top} d_{att} = W d_{cog}

$d_{att}$: is horizontal distance from the tail attachment to the top attachment
$d_{cog}$: is horizontal distance from the tail attachment to the centre of gravity

Both lever arms are horizontal, which is why the inclination cancels out of the ratio.

Now look at what happens to those two horizontal distances as the vessel rotates. If the attachments and the centre of gravity all sit on the vessel axis, both are the along-axis distance multiplied by the cosine of the inclination - so the cosines cancel and the share does not change with inclination at all.

That is worth pausing on, because the intuition is that a rising vessel unloads its tail progressively. It does not. On this vessel the tail attachment sits 1.2 m off the axis, so the two arms are not exactly proportional and the share drifts slowly; then, in the last few degrees, the two plumb lines converge and the tail lets go almost at once.

For the worked vessel - 782.0 kN, top attachment at 28.0 m and tail attachment at 0.6 m from the tail, centre of gravity at 17.0 m, solved by the engine at each inclination:

Tail vertical force through the rotation782.0 kN vessel, attachments 28.0 m and 0.6 m from the tail

Three readings.

The tail crane is loaded for the whole operation, not just the start. 313.9 kN with the vessel flat, and still 300.8 kN at 45 degrees and 269.8 kN at 75. A tail crane is not doing progressively less work as the lift proceeds, and it cannot be released early on the assumption that it is.

The main crane is sized at the end. It takes the whole vessel at vertical, whatever the tail is doing.

Both are near their worst at the same time, for most of the rotation. That is the opposite of the usual assumption, and it is why the tail crane has to be sized on the flat case and then kept there rather than being treated as a helper that fades out.

The horizontal force is the other half. Whatever holds the tail also has to resist it moving horizontally as the vessel rotates, and that force is friction on the ground, rolling resistance on a dolly, or a horizontal component in the tail crane's rigging.

The four arrangements, run

Same vessel, same cranes, same attachments. What changes is which checks exist.

Vessel Upending & Tailing Lift Calculator · computed at page render

Tail crane

The controlled case. Both cranes carry the vessel and the tail attachment is loaded through the rotation.

Main hook loadworst at 90 degrees	782.0kN
Main hook utilisation	78.2%
Tail hook utilisationworst at 0 degrees, flat	64.5%
Tailing attachmentat 71.1 degrees out of plane - governs	98.2%
Body bending	22.5%

Governing check: Tailing lug at its governing inclination (delegated)98.2% utilisationPass

The tailing attachment governs the whole operation, ahead of either crane. That is the normal result and it is worth expecting rather than discovering.

Open this example in the calculator

Vessel Upending & Tailing Lift Calculator · computed at page render

Tail dragged on the ground

No second crane. The tail slides, and the declared friction band is 0.35 to 0.6.

Main hook utilisationunchanged: at vertical the main takes the vessel regardless	78.2%
Tail hook checkthere is no tail crane in this arrangement	not evaluated
Tailing attachment checkthe tail does not hang on rigging here	not evaluated
Body bendingagainst 22.5% with a tail crane	23.6%

Governing check: Main hook load vs declared capacity (governing over the sweep)78.2% utilisationPass

Two checks disappear, and that is not the same as two problems disappearing. The tail is still carrying force; it is being carried by the ground and by whatever is between the vessel and the ground, and that is now outside this calculation and inside somebody else's.

Open this example in the calculator

Vessel Upending & Tailing Lift Calculator · computed at page render

Tail on a dolly

The same arrangement with the tail rolling instead of sliding, and a declared resistance band of 0.05 to 0.15.

Main hook utilisation	78.2%
Body bendingagainst 23.6% dragged	23.5%
Declared resistance bandagainst 0.35 to 0.6 dragged	0.05 to 0.15

Governing check: Main hook load vs declared capacity (governing over the sweep)78.2% utilisationPass

The dolly's value is the horizontal force it removes, and that force does not appear in this table because it was never the vessel's problem. It was the ground's, the tail crane's, or the main crane's, and it is precisely the force that pulls a crane off its radius.

Open this example in the calculator

The main crane is identical in all four, because at vertical it carries the vessel regardless of how the tail was handled. Everything that differs is at the tail.

The friction assumption

It is a band, and a design that only works at one value in the band is a design that depends on a number nobody measured.

Whatever the tail slides or rolls on, the horizontal resistance is genuinely uncertain, and it is uncertain in both directions.

Too much resistance and the tail does not move when it should. The vessel bends, the main crane is pulled off its radius, and the tail attachment sees a horizontal force nobody planned for.

Too little resistance and the tail runs away. The vessel accelerates through the rotation and arrives somewhere quickly.

Both failure modes are real and they need different responses, which is why a band matters more than a value:

Skids on steel plate, dry: a broad band, and it changes with rust, paint and grease.
Skids on timber or soil: higher, more variable, and it changes with weather.
A dolly with good bearings: low, and it depends on the surface being flat and clean.
A dolly that jams: back to the sliding case, suddenly.

The design has to work across the whole declared band. If it only works at the middle, the middle is a number nobody measured, and the operation depends on it.

Three controls follow:

Declare the band, not the value. With a reason for its width.

Plan for the jam. What happens if the dolly stops rolling is a load case, and on a long vessel it is a severe one.

Give the tail a way to be pulled or held. A tirfor, a tugger or a second machine on the tail is cheap insurance against the tail doing neither of the things it was supposed to.

The transfer point

The moment the tail stops carrying anything. It is a discrete event in a continuous operation, and it needs a stated angle and a person.

Near vertical, the tail's share approaches zero and the vessel can hang from the main crane alone. That transfer is a decision somebody makes, and three things have to be decided before the day.

At what angle. The tail is released when its load is small enough that releasing it does not disturb the vessel. That is an angle, and it comes out of the calculation rather than out of judgement on the day.

Who confirms it. One named person, with a way of seeing the angle. Not a general instruction to release when it looks right.

What happens if it is released early. The vessel swings towards the main crane's plumb line, which is a horizontal movement with kinetic energy in it. On a long vessel that is a serious event, and the plan has to say what the exclusion zone is at that moment.

Vessel Upending & Tailing Lift Calculator · computed at page render

After transfer: free hanging

The end state, with the tail released and the vessel hanging from the main crane alone.

Main hook utilisation	78.2%
Tailing attachmentnothing to check at this stage	unloaded
Body bendingagainst 22.5% during the rotation	3.2%

Governing check: Main hook load vs declared capacity (governing over the sweep)78.2% utilisationPass

Note the body bending, which has almost gone. A vessel hanging vertically from one point carries its own weight axially, which it is very good at. The bending case that mattered was back near horizontal, at the start of the operation.

Open this example in the calculator

What a tailing operation needs recorded

Eight things beyond a normal critical lift plan, and three of them exist only because the tail is not a crane.

Additional to a normal critical lift plan

01The tail force through the rotationNot one number. The tail crane is sized at the start and the main at the end, and both are checked across the sweep.
02The tailing arrangement, namedCrane, ground, dolly or a combination. It decides which checks exist.
03The declared resistance bandA range with a reason for its width, and the operation acceptable across all of it.
04What the tail runs onThe surface, its condition and who prepares it. A dragged tail is a ground bearing problem with a moving load.
05The jam caseWhat happens if the tail stops moving, and what force that puts into the vessel and the cranes.
06The transfer angleA number, with a way of measuring it and a named person who confirms.
07The early-release caseThe swing towards the main crane's plumb line, and the exclusion zone that covers it.
08The tailing attachment's out-of-plane rangeOn the drawing where a rigger can see it. It is the check that usually governs.

Six ways a tailing operation goes wrong

Three are about the tail force and three are about the transfer.

1. The tail crane sized at the midpoint. The tail force is largest with the vessel flat, which is the first minute of the operation.

2. A single friction value assumed. It is a band, and the design has to survive all of it in both directions.

3. The jam case not considered. A dolly that stops rolling turns a low-resistance arrangement into a high-resistance one instantly.

4. The transfer angle left to judgement. It comes out of the calculation, and it needs a person and a means of measuring.

5. The early-release swing not planned. The vessel moves horizontally towards the main crane's plumb line, with energy.

6. The tailing attachment checked in its own plane. It is loaded far out of plane for most of the rotation, and on the worked vessel it governs the entire operation.

Common questions

How do I calculate the tail load in an upending operation?: Take moments about the tail attachment using horizontal lever arms. If the attachments and the centre of gravity all sit on the vessel axis, both arms carry the same cosine of the inclination, the cosines cancel, and the share does not change as the vessel rises. On the worked 80 t vessel the tail carries 314 kN flat, is still carrying 301 kN at 45 degrees and 270 kN at 75, and only lets go in the last few degrees. A tail crane sized on the assumption that its load fades away through the lift is sized wrong.
Do I need a tail crane, or will a dolly do?: It depends on how much control you need over the tail's position and on what the tail runs on. A tail crane lifts and positions the tail, and it brings a hook check and a tailing attachment check with it. A dolly lets the tail roll with a low horizontal resistance, and it needs a rated dolly, a flat clean surface and a plan for what happens if it jams. Dragging on the ground needs no equipment and puts a substantial horizontal force into the arrangement plus a surface that has to survive being dragged over.
What friction value should I use at the tail?: A band, not a value, and the operation should be acceptable across all of it in both directions. Too much resistance and the tail does not move when it should, so the vessel bends and the main crane is pulled off its radius. Too little and the tail runs away. Skids on steel, skids on timber, a dolly with good bearings and a dolly that has stood in a yard are four different bands, and weather moves all of them.
When is the tail released during an upend?: At an angle that comes out of the calculation rather than out of judgement on the day: near vertical, when the tail's share is small enough that releasing it does not disturb the vessel. The plan needs three things - the angle, a named person who confirms it with a way of measuring, and what happens if it is released early, because the vessel then swings horizontally towards the main crane's plumb line with energy in it.
Why does the tailing attachment govern the operation?: Because it is loaded far out of its own plane for most of the rotation, and a plate out of plane is a cantilever with a small section modulus. On the worked 80 t vessel the tailing attachment reaches 98 percent while the main hook is at 78 and the tail hook at 65. It is the normal result, it is why tailing lugs are thick with large returned welds, and it is the main argument for using a trunnion at the tail instead.

Sources

Every document below is linked at its publisher or regulator. Xarpis reproduces no standard text; where a clause is named, the identifier is given so you can find it in your own copy.

HSE lifting equipment guidanceLifting equipment at work: planning and organising lifting operations
UK Health and Safety Executive · free to read
The regulator's own plain-language account of what planning a lifting operation means in UK law: who is competent to plan it, what a plan has to address, and how it scales from a routine repetitive lift to a one-off complex one. Free, short, and the closest thing to an official answer to 'what has to be in a lift plan'.
DNV-ST-N001Marine operations and marine warranty
DNV · paid document
The marine warranty standard behind most offshore lift factor sets: dynamic amplification, skew load and consequence factors, and the load cases a marine operation is planned against. Widely applied onshore by contract even though its scope is marine.
ASME B30.5Mobile and Locomotive Cranes
ASME · paid document
Construction, installation, operation, inspection and maintenance of mobile cranes in the US, including load rating and the requirement to operate within the manufacturer's chart. It governs the machine; the ground it stands on is 29 CFR 1926.1402 and the calculation is yours.
29 CFR 1926 Subpart CCCranes and Derricks in Construction
US Occupational Safety and Health Administration · free to read
The whole US construction crane subpart, free in full: ground conditions, assembly and disassembly, power line clearance, operator qualification, signals, inspection and multiple-crane lifts. The index page, because the duty a reader needs is usually two sections away from the one they searched for.

Run the check properly

Reading about a calculation is not the same as being able to hand one over. These tools produce the traceable record.

Lifting Trunnion Design Calculator Lifting Lug Calculator Crane Ground Bearing Pressure & Outrigger Pad Calculator

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