Crane Wiki : what wikipedia has to say about cranes in general.
IMCA m171 2013 : an interesting article by the International Marine Contractors Association targeted to shipboard cranes.
Offshore cranes rotate on pedestals. The pedestal is affixed to the supporting base (platform, vessel, etcetera). Pedestal-mounted cranes do not tip over. Since they do not tip over, this means that they can be significantly overloaded without warning. Because of this, API crane pedestal and supporting structures employ design factors up to 50% greater than the crane structure.
GROSS OVERLOAD PROTECTION
Offshore cranes can experience unbounded loads should they accidentally snag on a supply vessel. Therefore, they are provided with means to mitigate operator fatality. Means, which vary with the offshore crane code, may include automatic or manual gross overload protection devices or intentional failure of a less critical part.
Cargo on a heaving, surging and swaying supply vessel falls as the vessel follows the waves. The operator must snatch the load quickly. Thus, offshore cranes are exposed to impact loading during such off-board lifts as the rope slack suddenly becomes taught while the supply vessel is falling and the hook is racing upward.
Crane bases may be out of level. Even for platforms, API 2C requires cranes to be designed for at least 0.7 degrees of total tilt (0.5 list + 0.5 trim).
FLOATING CRANE BASE MOTIONS
Floating crane bases tilt out of level causing horizontal loads on the crane. Floating cranes have speed and accelerations due to wave motion. Accelerations include vertical heave and horizontal surge/sway. The vertical boom tip speed imparts additional impact forces on off-board lifts.
OFFLEAD AND SIDELEAD
During off-board lifts, the supply vessel may drift away from the crane and drift sideways. This imparts horizontal loading to the crane which may have a significant detrimental effect, especially at high boom angles.
DIFFERENT RATINGS DEPENDING UPON THE LIFT
API 2C cranes must have separate ratings for on-board lifts, off-board lifts and personnel lifts. See SWL for more information.
The speed of hoisting the hook is significant for offshore cranes. Higher hook speeds are required for off-board lifts in order to ensure that a workboat will not strike the cargo after the cargo is lifted. Higher speed increases the impact loading for off-board lift and may have a significant effect.
Wave height affects the motion of floating crane bases. Wave height affects the motion of the workboat for off-board lifts. The workboat deck falls with a higher rate of speed for higher waves. The workboat will drift more upon wave strikes causing higher offlead and sidelead horizontal forces on the crane. See Sea State for more information.
Wind pressure pushes the cargo away and/or to the side. Wind pressure also directly pushes the crane boom away and/or to the side. These together impart additional horizontal loading on the crane.
FIRE AND EXPLOSION HAZARD
Oilfield cranes are often located in areas exposed to flammable gas. In such cases, special equipment is required which may be extremely expensive.
Offshore cranes are designed with the marine environment in mind and usually employ features such as seal welding, means of capturing oil leaks and anti-corrosion materials and coatings. Special steels are used in order to prevent brittle fracture of structures under impact loading at low temperatures.
Design codes for offshore cranes include:
API Spec 2C Offshore Pedestal-mounted Cranes
EN13852 Offshore Cranes
DNV Lifting Appliances
ABS Lifting Appliances
All of the standards require approximately the same installation load test of up to 1.25 x SWL.
Of the above crane standards, only API Spec 2C requires design authentication in the form of prototype strain gage testing at Cv x SWL or, alternately, heavy lift load testing at 2 x SWL.
API is very different from the other standardization bodies in that it offers a monogram license.
For more information on API monogram program, see API Monogram.
For more important code differences, see:Introduction to API 2C
LIFT TYPE AND SWL RADIUS
Different lift types have different impact loading on the crane or different safety margins. See SWL for more information.
Motion of the base upon which the crane is mounted will reduce lifting capacity. See Crane Bases for more information.
The heelpin is the pivot upon which the crane boom (jib) rotates. Elevation is the vertical distance from sea level. The crane heelpin elevation is significant. Higher elevation requires more wire rope to be spooled on the hoist drum, which reduces hoist performance in a large way. Low elevations for offboard lifts can significantly increase offlead/sidelead angle.
MACHINERY LIFE / DUTY CYCLES
The usage of offshore cranes varies considerably. High frequency use of heavy loads requires much heavier duty (and very much more expensive) machinery. High frequency use of lighter loads requires much less heavy duty equipment. Low frequency use of light loads requires much less expensive equipment. Consult API Spec 2C for standard categories of usage. Often corrosion or under-usage is a primary failure mode and heavy duty equipment may be much more expensive to maintain because of this. Select your duty cycle with care.
SHORT DURATION HORSEPOWER
Simply put, doubling the speed doubles the horsepower required for a given load. To optimize the use of power and minimize cost, many owners will rig the crane for few line parts and fast/low load operations. This arrangement should be specified at purchase so the crane is configured appropriately and so multiple load charts are provided.
LONG DURATION THERMAL HORSEPOWER
Engines and power transmissions are usually rated for high power use for a short period and then a low power period to allow cool down. This is how most cranes operate. However, if it is known that heavy loads will be demanded without intermittent rest periods, higher horsepower prime movers must be provided and hydraulic oil coolers will be needed. This increases cost and maintenence.
Offshore cranes are generally critical to operations and are often subject to abuse and severe environmental exposure. A crane with fewer gadgets and ameneties will be much more reliable and much less expensive to purchase and operate. Do everyone a favor and keep it simple.