DMM 100
Deck Motion Monitor: The approved `Effective Significant Wave Height` for offboard lifting.
Why DMM?
In offboard lifting there is a risk of re-entry of the crane’s load by the moving deck causing a slack crane wire. As the deck moves downward faster than the hoisting speed it will result in a detrimental pull, which may overload and damage the crane and the load.
During personnel transfer there is a risk for exceeding the landing acceleration limits for humans.
DMM displays if the deck velocities are within safe limits for and will increase safety as well as operational availability to offshore lifting operations.
System Overview
Deck Motion Monitor (DMM) displays the Effective Significant Wave Height and real time waves on the vessel deck and provides accurate wave input for the crane’s safe load indicator.
DMM is the approved method to the traditional Significant Wave Height (H1/3) described in the Offshore Cranes Standard EN-13852 Table B.2 – Load supporting deck velocity VD (m/s)
DMM measures the effective vertical deck velocity vD 1/3 and translates it to `Effective Significant Wave Height` (H1/3 Eff) in each spot on the load supporting deck. H1/3 Eff derived of the DMM is allowed used as Wave input to the cranes Safe Load Indicator determining the cranes off-board lifting capacity.
As the DMM allows operation in a higher wave condition, it is crucial that accurate data is presented in an intuitive manner.
The crane’s wave limit for the next lift is entered and displayed, the color-coded vessel deck area indicates if the next lift will be within limits (green) close to limit (yellow) or over limit (red) The grid changes color according to the set wave limits for the crane.
The H1/3 Eff value of selected deck area is displayed, and you may toggle between a 20 minutes or real time heave trend curve for the selected deck area.
The Wi-Fi signal status between vessel and optional crane cabin mounted DMM 100C is indicated on the display.
If a deck camera is connected, this may be viewed on board the vessel and in cranes with 100C.
Saving time, energy and money with DMM 100
With DMM you can safely expand the weather window for offshore crane operations.
When the sea builds up you can continue longer and when it calms down you can start earlier, reducing the Waiting on Weather time.
DMM is 5 cm accurate in any weather condition and there will be no more estimation. Clients have reported 70% less waiting on weather using the H1/3 Eff method onboard PSV`s in the North Sea.
Case study by DNV comparing H1/3 (Hs) to H1/3 Eff (DMM)
A case study onboard an 80-meter PSV performed by DNV in 2007, comparing H1/3 (Hs) to H1/3 Eff (DMM), that is traditional Wave Radar to DMM, concludes that there is a considerable discrepancy between experienced vessel motions and traditional sea state methods using remote wave radars/buoys, as these do not take any consideration to the following uncertainty factors:
- Accuracy of the estimated/measured Significant Wave Height (H1/3)
- Variation in wave type and period
- Variation in downward velocity at different spots on the load supporting deck
- Variation in shape, stability and loading condition of the vessel
- Variation in direction of the waves in relation to the vessel heading
DMM eliminates the above uncertainty factors
DMM-Personnel
When transferring people between an offshore unit and a moving vessel deck there is a risk for accelerations which may cause whiplash injury during the landing or take-off phase.
The DMM-Personnel displays if the vertical accelerations vD 1/3 in m/s for the selected carrier is within safe limits for personnel lifting and landing in the selected deck area. Different Frogs or baskets have different motion limits which may be selected by DMM the operator.
Deck View - WEB Service for shore based logistic personnel
The optional Deck View – WEB Service offers camera images from the vessel deck as well as screen shots from the DMM deck grid.
A shore-based server receives 2- or 10-minute data via vessel email. Data is stored for 7 days.
By subscribing to this service, you will get an overview of the supply vessel deck, the weather conditions and surroundings. The deck crew is not recognizable.
DMM approval history
The DMM method was developed through DNV Crane Department Bergen in 2007/2008 through the project “Better use of Offshore Crane Capacity / Safer Offshore Lifting” as per DNV/ Report No. 08-BGN-4067 including detailed technical specifications.
The main purpose of the project was to provide the crane operator with more accurate information regarding the impact of the sea state on a crane’s rated capacity when lifting from or to the deck of a supply vessel.
DMM measure effective vertical deck velocity vD 1/3 and translates it to Effective Significant Wave Height (H1/3 Eff) for each spot on the load supporting deck.
The load supporting deck velocity has impact on the dynamic load applied to the crane, and the rated lifting capacity of the crane.
H1/3 Eff derived of the DMM is allowed used as Wave/ Sea Condition input to the cranes Safe Load Indicator/Load Chart for off-board lifting to determinate the off-board lifting capacity.
The information can be radioed from the vessel to the Crane Operator or online via an optional Wi-Fi radio link to a DMM-100C operator station mounted in the crane cabin.
The DMM H1/3 Eff method was approved and recommended for use on the 15.02.2007 by the Working Group 5 (Product) within Technical Comity CEN/TC 147 «Cranes-Safety»
And is the primary basis for the alternative method replacing the measured Significant Wave Height (H1/3) described in the Offshore Cranes Standard EN-13852 from 2015 Table B.2 – Load supporting deck velocity VD (m/s).
As an alternative to the measured H1/3, used to calculate the load supporting deck velocity VD, an `effective significant wave height` derived directly from measured movements at the spot of the deck where the load is lifted from or landed onto, may be used.
