Knuckleboom Crane 2.0
The subject of discussion is a knuckle boom crane. The load bearing members of the crane have to be analyzed and designed to meet the following requirements:
Steel at very low temperatures will experience a reduction in ductility which means that it is more likely to fracture upon impacts. Temperature changes will also cause stresses that increase the risk of cracks, expecially in welds. In general, materials with an FCC-structure are better suited for cryogenic applications than those with BCC-stucture.
There are steel types specifically made for low temperature application like A537 Gr. A and A537 Gr. B which can be used in applications down to -60 °C and have a yield strength of 345 Mpa and 414 Mpa, respectively. Typical uses are offshore drilling platforms, storage tanks, and earthmoving equipment .
According to DNV's standard for lifting appliances , a safety factor against excessive yielding of 1.10 should be used (Load factor = 0.96 and material factor = 1.15). This means that for A537 Gr. B, the maximum allowed stress is 376 Mpa.
In relevant standards for lifting appliances, several load cases are described depending on where the crane is supposed to be used. Most load cases account for dynamic effects by simply multiplying the load by a dynamic factor. In our case however, dynamic effects of the ship and wind loads will be also considered. The loads are defined as:
SG + SM + SW, max
SG : Loads due to the weight of the crane itself.
SM : Inertia forces due to motion of the vessel .
SW, max : Wind loads.
Further, the inertia forces due to motion of the moving vessel are defined in three independent directions in relation to the vessel: Vertical, longitudinal, and transversal. The magnitude of these forces are dependent on the size of the ship, but typical values for 180 m ships are:
aV = 0.6g = 5.9 m/s^2
aT = 0.7g = 6.9 m/s^2
aL= 0.3g = 2.9 m/s^2
Finite Element Analysis
The strength of the crane is analyzed according to DNV's standard for marine lifting equipment No. 2.22 Load case III which takes into account wind, nominal load, acceleration of the vessel, and more.
The resulting crane has a total horizontal reach of 12.4 m, lift capacity of 1240 kg, and is made to be as small as possible in the folded state.