Trimming System

Hi to everyone,

I have a situation with a challenging task for which I may need your help.
Assume a vessel who needs to have a trimming system in order to fulfill her purpose.

Ship particulars:
LBP: 150 meters
Draft (T): 11.0 m
LCF: 72.0 m
Tank LCG: 141.0 m (tank full)
Tank LCG: 140.0 m (even keel)
Tank LCG: 139.0 m (tank empty/dead water level)


Limits (classification societies):
maximum flow speed throughout the trim channels: 4.0 m/sec
maximum pressure in the trim tank: 1.0 Bar

The vessel is designed with one (1) trim tank and two (2) trim channels open to the sea, all located in the fore ship. The openings to the sea are about 2.2 x 2.2 meters.
One of the challenges is to have these opening watertight; so far, this is very demanding.
One of the ideas is to have the entire tank acting as an airlock, with water levels sensors for say a minimum and a maximum tank level; the purpose of these sensors is to counteract the effect of lowering/raising the water level in the tank when the ship is heading at full speed in open waters.
Another thing to say is that the dead water level is bellow the water line; so the airlock should sustain a reasonable level to avoid unwanted trim of the vessel by not having the trim channels opening to the sea, watertight;
and, the tank is so arranged that level keel should be obtained for with the water level in the tank at ship's water line/draft (+sinkage/-emergence). Much appreciated any idea you may spare for this point.
By choosing watertight cover for these openings, I assumed the upthrust equal to a column of water having 1.025*2.2*2.2*13 = 64.5 tonnes/cover (where 13.0 is ~85%*Depth, 1.025 sea water density).
Haven’t calculated this value for ship’s acceleration. Perhaps you may have a hint on how to do this.
Nevertheless, the force acting on this cover without acceleration is quite big.
Since the amount of water to be displaced if significant and the time requirements are demanding, the pumps are out of discussion; needless to say the high speed pumps have inherit inertia when full flow stop/full flow reverse; add here the time required for the fluid itself to stop at the limit were the pumps can run again without damage.
Remaining feasible are the blowers. In this respect, I would need some help regarding some makers.

The aim is for equal trim bow up / bow down: say 0.8 m, in total 1.6 change in trim at FP.
Obviously, this trimming should meet a time criteria: roundabout 25 seconds (half period)
The water to be displaced cannot be equal for bow up and bow down due to effect of lost buoyancy and MCT values which are not the same, since the vessel is not wall sided.

Bow up:
Volume of water needed to be displaced for bow up (Vbow up): ~850 tonnes
Discharge (Qbow up) = Volume for trim / Time needed for discharge
Qbow up = Vbow up / time, thus Qbow up = 34 m3/sec = 2040 m3/min
Flow velocity in the trim channels (v) = Qbow up / Cross Sectional Area of the Trim Channels
vout flow = 3.51 m / sec

Bow down:
Volume of water needed to be displace for bow down (Vbow down): ~950 tonnes
Discharge (Qbow down) = Volume for trim / Time needed for discharge
Qbow down = Vbow down / time, thus Qbow down = 38 m3/sec = 2280 m3/min
Flow velocity in the trim channels (v) = Qbow down / Cross Sectional Area of the Trim Channels
vinflow = 3.93 m / sec
Whereas,
Cross Sectional Area of the Trim Channels, A = 2.2 * 2.2 * 2 = 9.68 m2
Both velocities are smaller than 4.0 m/sec as required by the classification societies.

Therefore, the blower units/system should be designed for max. 38 m3/sec.
The blower are runing under certain limits, especially pressure difference.
The smaller the pressure difference, the bigger the efficiency of the system.

What I didn’t considered in my calculation are the losses:
- the trim channels are square type
- due to the construction itself, the trim channel is angled at 90 degrees in the area of the opening to the sea, therefore losses; no chance to have them directly under the trim tank, neither way.
- losses within the equipments (valves, blowers, etc) – but these should be accounted when selecting the blowers, therefore with margins
- vapour pressure/compression pressure
- other losses

Having the details above, I would welcome your expertise over the pros and cons for the calculations I run.
Furthermore, I would appreciate some direction for the right approach with respect to calculating the losses in such system.


If I forgot something, I'll be around; just let me know.

Any support is welcome.


Thank you,

CNi

 

No one?
Perhaps doesn't make sens to any of you.

Let me know.

I would need some help.

Thanks.