Significant Wave Height (H1/3) or (HS)
The wind creates the waves, the stronger the wind, the higher the waves.
Significant wave height is the average of the height of the 33 percent highest waves over a period of 20 minutes.
The size of the highest single waves can be twice the significant wave height.
Effective Significant Wave Height (H1/3 Eff) method compliant to EN13852-1:2013.
It is the (downward) vertical velocity of the load’s supporting deck (vD) at the instant of load pick up from the vessel which has impact on the dynamic load applied to the crane, and which therefore also has impact on the rated capacity of the crane for off-board lifting.
vD 1/3 is defined as the average velocity of the one-third highest peak vertical (downward) deck velocities in a 20-minute record of vessel movements.
As for H1/3, a complete continuously and accurate 2 Hz record for the vD 1/3 should be over a period of 20 minutes in all relevant lifting points and converted to H1/3 Eff to fully comply to the crane standard and MetOcean standard.
Accurate information from the DMM may allow lifting whereas the traditional sea state method would normally overestimate the motions and not allow lifting operation.
Using the DMM system will increase safety as well as operational availability to all offshore lifting operations.
In particular during heavy lift operations, where the risk is higher and margins for error are smaller, there is no room for guessing.
The system has several stand-alone modules for various operations; DMM-Personnel, DMM-Relative Motion, DMM-Deck View, DMM-Simulator Mode and DMM-Seakeeping.
The system saves time and money and helps to reduce the environmental costs by the ability to make more accurate decisions.
DMM - Ease of use
The DMM system is designed for usability, ensuring that the interface has elements that are easy to access, understand, and use.
Conducting a safe crane operation requires experience and accuracy.
As the DMM system allows operation in a wider range of weather conditions, it is crucial that accurate data is presented in an intuitive manner.
The main display consists of three elements – deck area, Effective Significant Wave Height and a real time heave trend curve.
The grid of the vessel deck changes colour according to the set wave limits for the crane.
Red area indicates unsafe lifting conditions, green area indicates safe lifting conditions and yellow is a warning of approaching unsafe conditions.
The traditional method, using wave buoys, radars or manual observation of the sea state, does not take any further consideration to the following:
• The accuracy of the estimated / measured Significant Wave Height (H1/3)
• The variation in wave type and period
• The variation in downward velocity at different deck positions
• The variation in shape, stability and loading condition of the vessel
• The variation in direction of the waves in relation to the vessel heading
By using the DMM all the above mentioned uncertain factors are eradicated.
With the DMM system, the Vessel Officer and Crane Operator can make safe decisions based on facts.