A comparison of ship-mounted and cage-mounted passive heave compensation systems

Tethered marine systems experience large tensile loads in their tether when operating in rough seas. Heave compensation systems can be used to reduce these loads and increase the safe operating sea states. In this work, a discrete representation of a possible heave compensator is developed and added to a finite-element model of a deep-sea ROV system to investigate the performance of ship-mounted and cage-mounted compensation systems. Numerical simulations are performed for operating depths ranging from 3280-16,400 ft (1000-5000 m) and a range of compensator stiffnesses. Both ship and cage-mounted systems reduced the natural frequencies, rms cage motion and rms tension, and extended the operating sea state of the ROV. During extreme seas, the cage-mounted compensator effectively eliminated all snap loads. However, the compensator's characteristics must be carefully chosen because a poorly designed compensator can exacerbate operational problems. [S0892-7219(00)00903-1].