Rack Phase Difference (RPD) is a difference in the elevation between rack teeth of the chords of any single leg of a

jackup rig A jackup rig or a self-elevating unit is a type of mobile platform that consists of a buoyant hull fitted with a number of movable legs, capable of raising its hull over the surface of the sea. The buoyant hull enables transportation of the unit ...

with open

truss A truss is an assembly of ''members'' such as beams, connected by ''nodes'', that creates a rigid structure. In engineering, a truss is a structure that "consists of two-force members only, where the members are organized so that the assembla ...

-type legs. This type of jackup vessel operates with a

rack and pinion A rack and pinion is a type of linear actuator that comprises a circular gear (the '' pinion'') engaging a linear gear (the ''rack''). Together, they convert rotational motion into linear motion. Rotating the pinion causes the rack to be driven ...

drive system, as opposed to the pin-hole system found on jackups rigs with tubular legs. The legs are mostly triangular though some with rectangular designs can be found. The chords are connected via a network of bracings to reinforce the leg structure. When a jackup positions its spudcan - i.e. the shoe mounted at the bottom of the leg - onto the

seabed The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as 'seabeds'. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...

the mass of the vessel pressing down onto the

will cause a

reaction force As described by the third of Newton's laws of motion of classical mechanics, all forces occur in pairs such that if one object exerts a force on another object, then the second object exerts an equal and opposite reaction force on the first. The th ...

. If the seabed is inclined, or the spudcan does not travel straight through the soil due to the consistency of the soil layers encountered during leg penetration, the forces acting on the chords will be unequal, which will cause a chord - and the rack attached to it - to move ever so slightly up or down. This displacement is the Rack Phase Value (RPV), and can be either positive or negative. The sum of the absolute values of the RPV is the Rack Phase Difference. This relative displacement between the racks causes additional loading on the leg members, which induces additional stresses between * the rack and

pinion A pinion is a round gear—usually the smaller of two meshed gears—used in several applications, including drivetrain and rack and pinion systems. Applications Drivetrain Drivetrains usually feature a gear known as the pinion, which may ...

teeth that are meshing during the jacking operation, which increases rack and pinion wear * the rack teeth and the upper and lower leg guides, which increases rack wear on the tips * the welded connections between bracings and chords The RPD limit should be clearly defined by the designer of the jack-up rig, and is subject to narrow design and as-built

tolerances Engineering tolerance is the permissible limit or limits of variation in: # a physical dimension; # a measured value or physical property of a material, manufactured object, system, or service; # other measured values (such as temperature, hum ...

. The better the design tolerances are followed during construction, the lower the RPD will likely be during jacking operation1s, and the less wear the system will encounter. Excessive loading may result in failure of structural members in case the RPD reaches more than the allowable value. * cracks in welds between structural members * buckling of bracings * shearing off of rack and pinion teeth * deformation of the leg guide plate supporting structure * damage on the drives (e.g. internal planetary gear damage, damage on

bull gear This page lists the standard US nomenclature used in the description of mechanical gear construction and function, together with definitions of the terms. The terminology was established by the American Gear Manufacturers Association (AGMA), under ...

s, shearing off of shaft keys, ...)

Reasons

# Eccentricity of Spudcan centre due to uneven ground condition # Sliding of leg

References

{{Reflist Oilfield terminology Oil platforms