|About Cranes: API SPEC 2C
| About the API's crane specification.
API Spec 2C is the specification for the design and manufacture of offshore cranes. To purchase a copy, go to www.global.ihs.com/standards.cfm. Key in "api spec 2c" at document number. Last checked it was $119.00 USD. You can always get one through www.api.org. We suggest that you buy a hardcopy, and not the aggravating single print electronic version.
What makes API Spec 2C cranes different? Some of the many differences include:
- Impact loading
- Offshore cranes are exposed to impact loading during off-board lifts since the load may be falling with the sea while the hook is rising quickly in order to snatch the load off the supply vessel, hopefully just when the vessel deck is at the top of a wave.
- Support Structure Motions
- To say the least, it is very complicated to provide crane ratings for cranes which move up and down and sideways and tilt and are subject to wind and impact loads, all simultaneously.
- Pedestal Mounted
- Pedestal mounted cranes can not tip over and thus can be unwittingly overloaded. Crane pedestal and supporting structures employ design factors 50% greater than the weakest link in the crane structure.
- Bearing Strength
- As of the 1983 edition of API 2C, offshore cranes require strength factors on the order of 5:1 to ensure the crane slew ring bearings (or bolts) do not fracture under impact loading.
- Side & Off-Loading
- Offshore cranes are exposed to horizontal loading on off-board lifts due to supply vessel drift, wind, crane tilt, etc.
- Cranes usually utilize multiple parts of line and thus have a block and tackle type of
arrangement. The hook will have sheaves (pullies) about which the rope will pass. The crane boom tip will have a similar arrangement of sheaves. A common and very dangerous condition exists when the hook block is hoisted all the way up to the crane boom tip and makes contact. In such a case damage usually occurs but what is worse is that the rope may part and allow the hook block, along with the lifted load, to fall. 2C requires that means to prevent this (or at least alarm) are provided. But overrides are allowed. This situation should be taken seriously. Operators often have several different areas which compete for their attention. This situation can occur in unexpected situations such as when telescoping out a crane boom or when booming (luffing) down if the winch is not located on the boom itself. Most suppliers have their own peculiar device with different strengths and weaknesses. Some are better than others.
- Personnel Lifting
- Offshore cranes are required to provide for personnel lifting. Often this is the preferred method of personnel access to the platform. It may also be the safest, as unlikely as this seems.
- Automatic Set Brakes Except Swing (No Free Fall)
- All primary motions (hoist, luff, swing) must automatically stop when controls are set free. Swing motions are exempt from this requirement when "free swing" is specified. This is the default provision for most suppliers. The intention is for the boom to freely move from side to side as the supply boat drifts, preventing potentially damaging side-load to the boom. This can cause a problem (of booms drifting to the side) on ships that tilt or on non-level platforms. If you anticipate a problem, then specify automatic setting dynamic swing brakes which use the motor to gently stop the swing motion.
- Emergency Load Lowering
- Provision for lowering the load should power be interrupted, for at least one winch, is required. (Why only one winch - what if it is the wrong one?) Remember that no free fall is allowed. Thus if the engine or such fails while a load (such as personnel) is suspended brakes will automatically set and not allow the load to be lowered. Provisions usually involve a method to safely disengage brakes and allow a controlled lowering of the load.
- Boom Hoist Secondary Mechanism
- Boom (luff) winches have a secondary dog (ratchet & pawl) to catch should the brakes slip. Hydraulic rams have a secondary lock valve to catch if a hose should burst.
- Marine Environment
- Offshore cranes are designed with the marine environment in mind and usually employ
features such as seal welding everything and use of hydraulic power transmission.