水下着陆

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.

Operational Challenges

  • Limited operational weather window to reach required vertical landing speed.

  • Structures with landing speed limitations. Exceeding these might cause overstress within the structures.

  • Landing of structures with tight angular tolerance.

  • Rapid heeling of vessel during landing of heavy loads.

Cranemaster Benefits

  • Reduction of landing speed.

  • Possibility to maintain tension in wire during landing phase.

  • Reduction of peak loads in case of re-lifting.

  • Prevention of rapid heeling of vessel during landing of heavy loads.

Cranemaster® is reducing landing speed for seabed structure.

Subsea landing speed with and without Cranemaster®

Manifold toal area: 1300m2 • Manifold weight: 829T • Significant wave height (Hs): 1.5m • Wave period (Tz): 8sec.

Two 700T Cranemaster® units are used in parallel. Actual observed forces and movements corresponded well to the Orcaflex simulation.

Without Cranemaster® (to the right):
Maximum landing speed = -0,7 m/s
Maximum crane tension = 2000 ton

With Cranemaster® (to the left):
Maximum landing speed = -0,1 m/s
Maximum crane tension = 840 ton
No slack observed in the rigging

Subsea landing of structure using Cranemaster®

Source: Subsea 7, Chevron

Notice the stroke.

Notice the complete lack of vertical motion on the object.

Key considerations

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.

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.

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