Active/Passive Motion Compensating Crane For Handling A Remote Unmanned Work System
Authors: Stricker P.A. 
Published: October 2008

ABSTRACT
This paper describes the concept, analysis, design, and sea trials of an active/passive ten ton capacity motion compensating crane for deploying a remote unmaned work system. The same concepts can be employed to make possible the handling of tethered loads, including ship to ship transfers, in high sea states without the risk of cable failure or excess payload pendulation. Motion compensation is achieved by driving the boom up and down, while the ship is heaving, such that the boom tip remains substantially stationary with respect to a fixed point on earth. The combination of active and passive boom control resulted in significant savings in power consumption over a purely active system while providing excellent motion compensation. The crane; mounted at the stern of a salvage tug, has a reach of 30 feet (9 m), a boom tip stroke of 24 feet (7.3 m), and can deploy the vehicle over the side or stern. A traveling saddle on the boom positively restrains the payload during deck handling operations while moving the load inboard or outboard. The cable storage reel accommodates over 23,000 feet (7000 m) of Kevlar wound electromechanical cable, and the entire crane can be disassembled into modules for air transportability. Performance data is provided for a sea state four operation, based on computer simulation predictions.

INTRODUCTION
The safety and performance of offshore operations are severely limited by wave-induced vessel motions, and as a result work is often suspended during heavy weather. Curtailment of operation can be very costly, and possibly intolerable, as in the case military operations.
In 1973, a requirement for a motion compensating crane was identified for launching, recovering and 6. handling a tethered underwater work system to 20,000 feet (6000 m) depth. Analysis using digital computer simulations indicated that the existing technology of passive systems would not meet the performance criteria. Therefore, the development of an active system was initiated, with the prototype being completed and ready for sea trials by early 1977.

SYSTEM REQUIREMENTS
The general arrangement of the remote unmanned work system is shown in Figure 1. Our basic requirement was to provide the "Motion Compensation Rotating Gantry Crane" which enables the work system to operate successfully in a Sea State 4. The key specifications for the cable and payload are given in Table 1. In addition to being extremely long, the Kevlar cable is very fragile, having little resistance to abrasion, flexure and fatique. The crane, therefore, must treat the cable "gently" in all facets of operation: deck handling, launch and recovery, and deep submergence. As a result, the following primary requirements have been established.The cable must be stored at low tension. This necessitates a separate traction winch and storage reel.All bends must be large diameter, and inside sheave surfaces must be soft and smooth.Cyclic bending of the cable for motion compensation cannot be tolerated; this rules out a "constant tension" winch in favor of an oscillating boom.