A new concept design of heave compensation system for longer life of cables

Conventional heave compensation system consists of fixed sheaves and moving sheaves supported by spring. However, this system has several inherent problems. Typical one is the wear of cable. When the operator stops paying in/out of the cable and when the ship is heaving, the same parts of the cable are always running over the moving and fixed sheave blocks of the heave compensator, and this motion damages the cable seriously. Other problem is the sheave efficiency. This nominal sheave efficiency can easily be calculated as N-th power of single sheave efficiency where N is the number of sheaves. However, this calculation is focused only on the loss by the friction of the cable on the sheave, but it does not include the bending stiffness of the cable which consumes energy and reduces the total sheave efficiency. Heave compensator shows seemingly good works for shallow water operation of ROV, but shows little works at deep water even at the same sea condition. We know that the vehicle is still heaving by some significant level of amplitude even when the compensator is seemingly working well. The reason of the little works at deep water is that the heave compensation is done by the spring effect of the cable. In order to solve these problems, a new concept design of heave compensation system is proposed here. This new system is a type of armed sheaves system, and consists of three or four sheaves connected by two long arms with one sheave located at the hinge position of the two arms. One end of the upper arm is pin-jointed to the structure, and the other end of the lower arm is suspended by spring to support the weight of the cable and the vehicle. As the maximum compensation stroke shall be long, say 4m to 10m, long hydraulic cylinder will be very difficult to apply as this spring, but wires with one-layered winch drums driven by high-torque/low-speed pump/motor will be applicable. The feature of this system is that the cable runs over the sheave by very short length, and the total sheave efficiency becomes very high, and also the life of the cable becomes fairly longer. The lower end of the arm can accommodate a docking system, and this whole system can become a launch/recovery system with heave compensation function. Active heave compensation technique can very easily be adapted to the system.