The rigging module treats the lift as a rigid-body statics problem. The pick crane holds the top lifting point or points, the tail crane holds the tailing lug, and both hook lines stay vertical. The vessel is turned about the tail pin from horizontal (0°) to vertical (90°) in the angle steps the user lists; the default list is 0, 15, 30, 45, 60, 75 and 90°. At each angle the code solves the split of the weight between the two hooks, then the load on each lug.
Two weights, two routes
The same statics is run twice. Once on the erection weight W_E, unfactored, which feeds the ASME BTH-1 checks (impact is inside the design factor N_d there). Once on W_L = K_L·W_E, with K_L the overall load factor combining impact and safety (Moss: 1.5 to 2.0), which feeds the Moss / AISC checks and the shell bending. A K_L below 1.5 raises a warning; below 1.8 the tool warns that the Moss / AISC checks do not reach the F_y/3 basis of ASME B30.20.
Pick and tail load
Every point is placed by its axial position z from the base and its offset e in the lift plane (positive upward while the vessel lies horizontal). Rotating the vessel by θ gives the horizontal coordinate of a point as below. With X known for the tail pin (T), the pick point (L) and the centre of gravity (G), the moments about the tail pin give the pick load; the tail takes the rest.
With the pick point and the centre of gravity on the axis this reduces to Moss 10-4, T = W·cos θ·L2 / (cos θ·L1 + sin θ·L4), where L1 is the axial distance from tail pin to pick point, L2 the distance from centre of gravity to pick point, and L4 the offset of the tail pin, entered signed (the code comment equates L4 to the tail offset e). Off-axis pick points or centre of gravity need no extra formula, which is why the code uses X rather than Moss's lengths.
Load on each lug
- Per lug: f = load of the end / number of lugs sharing it. Along the vessel axis f·sin θ, across the axis in the lift plane f·cos θ.
- Sling spread at the top: radial load f·tan β (β the sling angle from vertical), as Moss 10-4. Two tail lugs whose slings converge to one hook at α from vertical: sling f/cos α, with f·tan α toward the lift plane.
- Side allowance: a fraction of the lug load (Moss: at least 10 %) acting out of the lift plane. Lug resultant = √(f² + lateral²).
Two cases are flagged rather than hidden. If the centre of gravity passes beyond the pick point the tail load is clamped to zero and the vessel is shown hanging from the pick lugs (tail slack). If the centre of gravity falls behind the tail pin the pick load goes negative, and the warning says the arrangement cannot hold the vessel. If the pick point is straight above the tail pin the split is undefined and that step is skipped.
Shell bending at the first pick
Longitudinal shell bending is largest at θ = 0. The code takes M from the factored weight at the centre of gravity (Moss case 1) and, separately, from the weight spread uniformly over the vessel length on two supports (cases 2 and 3, moment sampled along the length at 400 intervals). The stress is the thin-ring section formula:
Default lift, W_E = 400 kN, K_L = 1.8
| Geometry | centre of gravity z = 6000, tail pin z = 300 and e = −1400, pick z = 14000 on the axis (all mm) |
| θ = 0° | P = 400·5700/13700 = 166.4 kN, T = 233.6 kN |
| θ = 30° | X_T = 959.8, X_L = 12124.4, X_G = 5196.2 mm; P = 400·4236.3/11164.5 = 151.8 kN, T = 248.2 kN |
| θ = 60° | X_T = 1362.4, X_L = 7000, X_G = 3000 mm; P = 116.2 kN, T = 283.8 kN |
| θ = 75° | P = 22.4 kN, T = 377.6 kN (largest tail load of the list) |
| Tail lug at 30°, one lug | along the axis 124.1, across 215.0, side 24.8 kN; resultant √(248.2² + 24.8²) = 249.5 kN on W_E, 449.0 kN on W_L |
| Shell at θ = 0, W_L = 720 kN | M = 720000·8000·5700/13700 = 2.397·10⁹ N·mm; D_m = 2420, t = 20: σ_b = 4·2.397·10⁹/(π·2420²·20) = 26.1 MPa |
Above θ = 76.2° (tan θ = 5700/1400) the centre of gravity is behind the tail pin and P goes negative: the tool warns, and the angle list should stop there or the tail pin offset be revisited.
Limits
- Rigid vessel, vertical hook lines, quasi-static. No dynamic amplification beyond K_L (or N_d on the BTH-1 route), no wind, no crane or boom geometry.
- The shell bending formula is the Moss ring-section stress; it does not include local stress at the lugs or the weight of attachments beyond what is in W_E.
- Angles are the listed steps only; the governing step is the largest lug resultant among them, not an interpolated maximum.