Seatrax Inc. Offshore Marine Cranes Basics

An offshore crane is defined as a pedestal-mounted elevating and rotating lifting device used to transfer materials or personnel to or from marine vessels, barges and structures, according to API Spec 2C, a set of standards used to manufacture marine cranes. These marine applications include bottom-supported, floating platform and ship-hulled vessels used in production and drilling operations, shipboard applications and heavy-lift applications.

The offshore oil and gas industry was established in 1954 with the first fixed platform installed near Morgan City, Louisiana in the Gulf of Mexico. The only cranes available for use on these early platforms were existing land-based construction machines. Unlike these cranes, though, offshore cranes are fixed structures incapable of moving with their loads. However, customers often make inquiries along the lines of, "We need a 100-ton crane". The next question becomes, "What is the customer's definition of a 100-ton crane?"

A crane's capacity depends on multiple factors a purchaser must supply to its crane manufacturer. These include the structure upon which the crane is mounted (i.e. bottom-supported, semi-submersible, ship-hulled), the environmental conditions in the platform's location and the load's location in relation to the structure.

Crane Terminology*

Auxiliary (whip line or fast line): the secondary rope system capable of lifting a lower capacity than the main block
Boom: connected to the upper structure and supports the hoist tackle
Boom hoist: raises and lowers the boom
Boom suspension: the collection of wire ropes, sheaves, shafts blocks and other rigging components used to support the boom
Cab: where the operator maneuvers the crane's controls
Gantry: a frame to which the boom support ropes are reeved
Kingpost: connects to the platform and is the centerline of rotation for the upperstructure
Pedestal: the substructure the upper structure is mounted on
Revolving upperstructure: the rotating frame structure and the operating machinery mounted

*Definitions provided by API Spec 2C

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Duty Cycles

Offshore cranes can also be classified by duty cycle or the frequency of their use. The duty cycles include:

Production Duty: annual operating time of 200 hours, typically on bottom-supported production platforms

Intermediate Duty: annual operating time of 2,000 hours, typically on bottom-supported or floating platforms with temporary rigs

Drilling Duty: annual operating time of 5,000 hours, typically on MODUs or floating structures that operate full time

For more information on the difference between drilling duty and production installations, review our technical papers: Drilling Duty vs. Production Installations and Seatrax Drilling Duty vs. Typical Production Duty Cranes.

Production Duty

Intermediate Duty

Drilling Duty

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Methods of Crane Support

For many years, cranes were supported on their foundations by roller bearings at their bases and are still commonly used on production duty applications. However, the kingpost design is the modern method for supporting high use cranes and/or those on floating installations and is the preferred arrangement for drilling duty and floating applications.

Roller Bearing-Type Cranes

The revolving superstructure bolts on top of the roller bearing

Roller bearing subject to full overturning moment from weight of load, the crane itself and boom

Failure of bolts or the slew roller bearing can and has led to loss of operator, crane from its mount

On floating applications, the bearing is subjected to load cycles from the crane self-weight and installation motions even when the crane isn't in use

Replacement is costly and difficult to accomplish offshore

Seatrax Kingpost-Type Cranes

Revolving superstructure fits over and revolves around the kingpost

Crane cannot separate from mount because of slew bearing failure

No bolts = no bolt failure

Bearings are easily replaced in the field using simple hand tools

For more information on roller bearing versus kingpost cranes, review our technical paper Support Bearing Comparison: Ball vs. Kingpost.

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Winches/Hoists

Seatrax Draw-Works Type Band Brake Hoist for High Duty Cycles

Reduction gearing and brakes are mounted externally

Drum is driven by a solid, one-piece hardened alloy shaft and hoist drum spline connection

No periodic internal inspections are required or recommended

The hydraulic dynamic brake is at the hydraulic motor input, and the band brake acts directly on the drum, so no single failure in either brake can render both braking systems ineffective

For more information on Seatrax hoists, review our technical paper Why Seatrax Doesn't Use "Braden"-Type Winches.

Conventional Production Duty "Works in the Drum" Winches

Planetary reduction gearing and static brake mechanisms mounted inside the drum barrel

Winch must be removed from its mount and completely disassembled for inspection or replacement of the brake, gearing and other load transmitting components

Manufacturers of this type of hoist typically mandate periodic removal and internal inspection

Both the hydraulic dynamic brake and the mechanical brake are located at the input, so a single internal failure in the driveline can render both brakes ineffective

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Hydraulic System Types

Fixed Displacement

Offshore crane mounted on fixed structure typically operates for 150-300 hours per year

Marine crane used to support drilling operations typically accumulates 3,000-5,000 hours per year

Fixed displacement gear type hydraulic system works well for many years on fixed platform given low cumulative hours

Because of the cumulative hours and number of hours run during drilling cycle, fixed displacement gear type hydraulic system provides limited service life and performance on a drilling support crane

Variable Displacement

Variable displacement (medium flow) advantages over fixed displacement include:

–Provides up to 50%+ faster operational speeds

–Provides 400%+ greater hydraulic component life

–Improved controllability for the operator

–Quieter operation. (Approximately 72 dba vs. 85-90 dba in the cab w/open wingdeck)

–Combine the advantages above to translate to greater safety

For more information, review our technical paper Fixed vs. Variable Displacement Hydraulic Systems.

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Boom Support Types

Continuously Reeved

The suspension wire rope runs from sheaves in the gantry directly to sheaves in the boom tip. The advantage of this approach is that it puts less dynamic loading into the sheaves, but it requires longer wire rope, making rope replacement cost more.

Pendant and Bridle

The suspension wire rope runs from sheaves in the gantry to a suspended sheave cluster which attaches to the boom tip with static links or cables. The advantage is that it requires less wire rope, but heavy duty steel sheaves and bearings should accompany this arrangement.

For more information, review our technical paper Seatrax Boom Suspension Systems.

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*Technical information provided by API Spec 2C and Seatrax Technical Papers. To learn more about API, API Spec 2C and other manufacturing standards, visit API's web site at www.api.org.


Crane Terminology*
Auxiliary (whip line or fast line): the secondary rope system capable of lifting a lower capacity than the main block Boom: connected to the upper structure and supports the hoist tackle Boom hoist: raises and lowers the boom Boom suspension: the collection of wire ropes, sheaves, shafts blocks and other rigging components used to support the boom Cab: where the operator maneuvers the crane's controls Gantry: a frame to which the boom support ropes are reeved Kingpost: connects to the platform and is the centerline of rotation for the upperstructure Pedestal: the substructure the upper structure is mounted on Revolving upperstructure: the rotating frame structure and the operating machinery mounted *Definitions provided by API Spec 2C back to top

Duty Cycles
Offshore cranes can also be classified by duty cycle or the frequency of their use. The duty cycles include: Production Duty: annual operating time of 200 hours, typically on bottom-supported production platforms Intermediate Duty: annual operating time of 2,000 hours, typically on bottom-supported or floating platforms with temporary rigs Drilling Duty: annual operating time of 5,000 hours, typically on MODUs or floating structures that operate full time For more information on the difference between drilling duty and production installations, review our technical papers: Drilling Duty vs. Production Installations and Seatrax Drilling Duty vs. Typical Production Duty Cranes.

Production Duty	Intermediate Duty	Drilling Duty
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Methods of Crane Support
For many years, cranes were supported on their foundations by roller bearings at their bases and are still commonly used on production duty applications. However, the kingpost design is the modern method for supporting high use cranes and/or those on floating installations and is the preferred arrangement for drilling duty and floating applications.
Roller Bearing-Type Cranes The revolving superstructure bolts on top of the roller bearing Roller bearing subject to full overturning moment from weight of load, the crane itself and boom Failure of bolts or the slew roller bearing can and has led to loss of operator, crane from its mount On floating applications, the bearing is subjected to load cycles from the crane self-weight and installation motions even when the crane isn't in use Replacement is costly and difficult to accomplish offshore	Seatrax Kingpost-Type Cranes Revolving superstructure fits over and revolves around the kingpost Crane cannot separate from mount because of slew bearing failure No bolts = no bolt failure Bearings are easily replaced in the field using simple hand tools For more information on roller bearing versus kingpost cranes, review our technical paper Support Bearing Comparison: Ball vs. Kingpost.
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Winches/Hoists
Seatrax Draw-Works Type Band Brake Hoist for High Duty Cycles Reduction gearing and brakes are mounted externally Drum is driven by a solid, one-piece hardened alloy shaft and hoist drum spline connection No periodic internal inspections are required or recommended The hydraulic dynamic brake is at the hydraulic motor input, and the band brake acts directly on the drum, so no single failure in either brake can render both braking systems ineffective For more information on Seatrax hoists, review our technical paper Why Seatrax Doesn't Use "Braden"-Type Winches.
Conventional Production Duty "Works in the Drum" Winches Planetary reduction gearing and static brake mechanisms mounted inside the drum barrel Winch must be removed from its mount and completely disassembled for inspection or replacement of the brake, gearing and other load transmitting components Manufacturers of this type of hoist typically mandate periodic removal and internal inspection Both the hydraulic dynamic brake and the mechanical brake are located at the input, so a single internal failure in the driveline can render both brakes ineffective
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Hydraulic System Types

Fixed Displacement Offshore crane mounted on fixed structure typically operates for 150-300 hours per year Marine crane used to support drilling operations typically accumulates 3,000-5,000 hours per year Fixed displacement gear type hydraulic system works well for many years on fixed platform given low cumulative hours Because of the cumulative hours and number of hours run during drilling cycle, fixed displacement gear type hydraulic system provides limited service life and performance on a drilling support crane	Variable Displacement Variable displacement (medium flow) advantages over fixed displacement include: –Provides up to 50%+ faster operational speeds –Provides 400%+ greater hydraulic component life –Improved controllability for the operator –Quieter operation. (Approximately 72 dba vs. 85-90 dba in the cab w/open wingdeck) –Combine the advantages above to translate to greater safety For more information, review our technical paper Fixed vs. Variable Displacement Hydraulic Systems.
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Boom Support Types
Continuously Reeved The suspension wire rope runs from sheaves in the gantry directly to sheaves in the boom tip. The advantage of this approach is that it puts less dynamic loading into the sheaves, but it requires longer wire rope, making rope replacement cost more.	Pendant and Bridle The suspension wire rope runs from sheaves in the gantry to a suspended sheave cluster which attaches to the boom tip with static links or cables. The advantage is that it requires less wire rope, but heavy duty steel sheaves and bearings should accompany this arrangement.

For more information, review our technical paper Seatrax Boom Suspension Systems.
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*Technical information provided by API Spec 2C and Seatrax Technical Papers. To learn more about API, API Spec 2C and other manufacturing standards, visit API's web site at www.api.org.

Cranes 101