Offshore Cranes for the Petroleum Industry

Seatrax often receives inquiries such as, “We need a 100-ton crane”. However, the maximum lift weight to be lifted only partially defines the crane size. In fact, because the offshore crane is fixed to the host structure, the distances from the crane at which various loads are to be lifted are more likely to determine the crane size rather than the maximum weight the crane will lift.

In addition, lifting a load off of a supply boat pitching in waves (offboard Lift) is treated differently than lifts from the deck of the installation the crane is mounted on (onboard Lift). Offboard/supply boat lifts have the potential to impose impact loads on the crane that are not experienced during onboard/deck lifts. Therefore, the offboard supply boat capacity is reduced from the onboard deck lift capacity.

The table below lists the API Specification 2C ratings for typical Seatrax crane models. The various columns are categorized by capacity or ratings basis, specifically:

Red column: the theoretical weight that can be suspended from the boom tip with the boom lengths and radii shown without exceeding the API 2C allowable stresses for onboard lifts from a bottom-supported structure.

Blue column: the maximum weight that can be lifted with the hoists and wire ropes normally supplied for the boom lengths and radii shown in full compliance with API 2C for onboard lifts from a bottom-supported structure.

Green column: the maximum weight that can be lifted in full compliance with API 2C, for the boom lengths and radii shown, for offboard supply boat lifts, with a significant wave height of 7 feet and a 30-knot wind, from a bottom-supported structure. The boom foot pin elevation is assumed to be 110 feet above the water.

Note: All tons are short tons of 2,000 lbs. All boom lengths are in feet. All radii are in feet.

Using the “100-ton crane” example, let's take a look at the red columns. The Seatrax S4820 carries the 100-ton structural rating at minimum radius with a 60-foot boom. However, if the purchaser wants to lift 100 tons from a supply boat, the minimum crane size from the green offboard columns will be in the range of the much larger Seatrax S9024, since that crane is the minimum sized model with at least a 100-ton rating.

It is also important to note that numerous parameters will cause the green offboard ratings in the table to increase or decrease. Examples of these parameters include boom length, significant wave height, wind forces and how high the crane is above the water. In the case of floating installations, accelerations and inclinations additionally impact both the blue onboard and green offboard columns.

For a purchaser to ensure they purchase the correct crane for the job, providing the following is the best way to start:

• The type of structure upon which the crane is mounted (i.e. bottom-supported, semi-submersible, ship-hulled, etc.)
• Boom length
• The magnitudes of the various loads and distances relative to the crane. (i.e. on the installation deck as well as how close the supply boats can be positioned)
• The environmental conditions the crane will operate in (i.e. significant wave height and wind)
• The height of the crane above the water

To contact Seatrax about a new crane, contact us here.