As subsea structures get larger and more complex, landing speed of subsea structures becomes an increasingly important topic. Cranemaster has been used extensively to reduce motion for subsea lifting operations. Boom tip movements are picked up by the stroke of the Cranemaster unit and thereby limiting the load movements.
- Reduction of landing speed.
- Increased horizontal stability during landing.
- Possibility to maintain tension in wire during landing phase.
- Reduction of peak loads in case of re-lifting.
- Ship stability – prevention of heeling during landing of heavy loads.
- Cranemaster will be more efficient for loads with high drag forces and large added mass such as manifolds, mud mats, protection covers and suction anchors. It will be less effective for objects with a small horizontal area such as spool systems.
- Since the forces preventing the object from movements are dependent of velocity and acceleration, the performance of Cranemaster will decrease with increased wave periods.
- Cranemaster will itself have a resonance frequency which may be close to the wave periods. This may cause increased motion and velocity, and pre-calculation and/or system simulations are therefore important especially for objects with low drag.
For objects with a large horizontal drag area, the landing speed can be significantly reduced by using Cranemaster. In this Orcaflex simulation, a manifold with a total area of 1300 m^2 and a dry weight of 829T is landed on the seabed. Two 700T Cranemaster units are used in parallel. Significant wave height Hs = 1,5 m and wave period Tz = 8 sec. As seen from the video, maximum landing speed in this example is -0,7 m/s without Cranemaster, and -0,1 m/s with Cranemaster. The maximum crane tension is reduced from around 2000T down to 840T, and no slack is observed in the rigging. The lift was conducted using Cranemaster, and actual observed forces and movements corresponded well to the simulations. (Simulation replay speed is set to 500%)
CRANEMASTER TUNING AND PERFORMANCE
Cranemaster should have a flat spring versus stroke curve to minimize spring forces. Dampening forces from Cranemaster should be carefully controlled.
A reduction in landing speed from 0-95% can typically be obtained for a well-designed Cranemaster setup. The efficiency of Cranemaster is dependent on the average stiffness being less than the sum of forces acting on the load. As a consequence, Cranemaster units used for passive heave compensation have large accumulators to reduce the average stiffness.
The performance of Cranemaster will depend significantly on the hydrodynamic properties of the structure/object. The most important parameter is the vertical drag. A large drag area will give high reduction in landing speed.