Lift plans and rigging basics — sling angle, centre of gravity, and the reasons to stop

A piping supervisor is not the appointed person for lifting operations, does not select the crane and does not write the load chart. What they do is work to the outputs of all three, every day, usually several times a day, and the decisions that go wrong are rarely the crane’s.

A close view looking up at a crane hook block with two round sling legs running down from the hook at a wide spread angle to shackles at the top of a large pipe, the legs clearly far from vertical against a bright overcast sky. — The legs are nowhere near vertical, so neither of them is carrying half the load. How much more is the arithmetic below.

This topic is the supervisor’s share of it.

What a lift plan has to say

Before anybody touches a spool, four things have to be known and agreed, and the first one is the one most often guessed.

The weight. Not estimated by eye. A fabricated spool with a valve in it is routinely half as heavy again as people guess, and the guess is always low.

The centre of gravity. Where it actually is, not where the spool looks balanced.

The method. Sling type, configuration, lifting points, and the hardware between the hook and the load.

The people. Who is the operator, who is the slinger, who is the signaller, and who may stop it. Named, out loud, before it starts.

Sling angle, which is the arithmetic that matters

This is the one piece of rigging arithmetic worth knowing by heart, because it is the one that is wrong on site most often, and because the error always runs in the dangerous direction.

The intuition people carry is that two legs share the load, so each leg takes half. That is true only when the legs are vertical, and on a spool with two lifting points at the same height they are never vertical. They spread, and as they spread the tension in each leg rises — quite slowly at first, then sharply.

The same 1,000 kg load slung on two legs at four angles from the horizontal. At 90 degrees each leg carries 500 kg, a factor of 1.0. At 60 degrees, 577 kg, a factor of 1.155. At 45 degrees, 707 kg, a factor of 1.414. At 30 degrees, 1,000 kg, a factor of 2.0, so each individual leg carries the entire weight of the load. — At 30° each single leg is carrying the whole load. Two legs, and neither of them is sharing anything.

The relationship, with the angle measured from the horizontal as the rating tables do:

leg tension = (load ÷ number of legs) ÷ sin(angle from horizontal)

Which gives the factors worth remembering:

Angle from horizontal	Factor	On a 2,000 kg load, per leg
90° (vertical)	1.000	1,000 kg
60°	1.155	1,155 kg
45°	1.414	1,414 kg
30°	2.000	2,000 kg

At 30° each leg is carrying the whole load, which is why 30° is treated almost everywhere as the practical floor. Below it the numbers get ugly very quickly.

Centre of gravity on a spool

A spool is an awkward lift in a specific way: its weight is concentrated at discrete points rather than distributed along it.

A long-radius elbow, a flange pair, a valve — each is a lump, and the lumps are usually at the ends. So the centre of gravity moves further from the midpoint than an eye estimate suggests, and the consequences arrive in two places.

While slinging. If the hook is not over the centre of gravity, the spool tilts as it comes off the stand. That is not merely untidy — it is the moment the sling can slide, the moment the load swings, and the moment somebody puts a hand out.

At the hardware. Angular loading is what an offset centre of gravity produces, and angular loading is what rigging hardware is least tolerant of. An eye bolt loaded off-axis loses most of its rating. A shackle side-loaded does the same. Both will look entirely normal while doing it.

The reasons to stop

Short, and worth being explicit about with a crew, because a stop authority nobody has ever used does not exist.

The load does not behave as expected as it is taken up — it tilts, swings or the sling slides.
The weight turns out to be different from the plan.
Wind rises beyond what the tag lines can control.
Anybody enters the exclusion zone.
Visibility between the signaller and the operator is lost.
Anything at all is not as briefed.

Anybody in the team may stop a lift, and that includes the slinger and the banksman, not only the supervisor. The reason is practical rather than procedural — the person who first sees the problem is very often the one standing closest to it and holding the least rank.

What stays with the appointed person

For clarity, because the boundary gets blurred and a supervisor who takes on the wrong half of it is exposed:

Theirs	Yours
Crane selection and configuration	Knowing the weight of what you are asking to be lifted
Load chart, radius, ground bearing	Preparing the load — bevels protected, nothing loose on it
Outrigger and mat requirements	Access, exclusion zone and who is in the area
Authorising the lift plan	Briefing your crew on it, and stopping if it changes

The failures in practice are almost always in the right-hand column.