Splash zone crossing
SPLASH ZONE, THE CRANEMASTER HOME GROUND
Lifting through the splash zone is a key application area for Cranemaster. Many hundred spool systems, mud mats, manifolds and suction anchors have crossed the splash zone successfully supported by a Cranemaster unit.
- Slamming forces, added mass and drag of the object can increase the forces transferred to the crane and the object by many times the weight of the object.
- Snap loads caused by wave or boom-tip motion may be very large.
The general principles and issues are thoroughly described in Recommended Practice DNV-RP-H103 from April 2011.
With Cranemaster mounted, these forces will be removed or reduced significantly. The main functionality of Cranemaster is to decouple the movement of the boom tip from the movement of the load. The spring force of Cranemaster will support in avoiding slack wire during the splash zone transfer, while excessive forces in crane or load due to wave and boom tip movements will be removed by a combination of the spring and dampening forces of the Cranemaster unit.
OBJECTS FOR WHICH PARTICULAR BENEFITS ARE OFFERED
- Objects with a large horizontal surface independent of their weight, e.g. mud mats, protection covers and manifolds.
- Objects which have a large difference between weight in water and weight in air, e.g. suction anchors and closed pipes.
Significant reductions of forces in load and crane may be obtained by using Cranemaster in the splash zone. The video below shows and example of an Orcaflex simulation with and without Cranemaster for a manifold structure. The dimensions of the structure is 10m x 10m x 7m, weight in air is 50T,and the structure is lowered through the splash zone at a speed of 0,3 m/s. Significant wave height (Hs) in the simulation is 2.5m with wave period Tz =6 sec. As seen, the stability increases with Cranemaster, and the DAF is reduced from around 5.0 to 1.1. The minimum tension in the crane wire is not acceptable without Cranemaster.
CRANEMASTER SELECTION AND TUNING
- The maximum stroke of Cranemaster should be longer than the boom tip movement.
- Cranemaster should have a spring response corresponding to the maximum allowable forces on load and crane.
- Cranemaster should have dampening forces tuned according to the expected boom-tip and characteristic vertical water particles accelerations.
- A slack wire may be acceptable since the snap load will be picked up by the Cranemaster unit. However, stability of the load may be a concern and it should therefore generally be avoided.
- A full stroke is not acceptable since the Cranemaster unit then will give no protection. A safety margin of 5%-15% of maximum stroke is recommended.