When preparing for a ro-ro operation it is important to know your environment (tides, depths, type of quay/jetty) as this has an impact on equipment selection (number of ballast pumps, size of the barge) and type of ro-ro operation (with ramp, with steel plate, grounded barge, floating barge).
We differentiate between 5 basic types of ro-ro operations;
- roll-on (roll-off) to free floating barge (tidal)
- roll-on (roll-off) to free floating barge (non tidal)
- roll-on (roll-off) via steel plates (non tidal)
- roll-on (roll-off) to barge fixed aground
- beach landing, barge fixed aground
Figure 1:Note: On 12 September 2013, DNV (Det Norske Veritas) and GL (Germanischer Lloyd) merged into DNV GL, forming the largest classification society with headquarters in Oslo, Norway.
It is important to point out the difference between the two summations of these 5 ro-ro types as they may seem identical at first glance but there is a distinct difference which could cause confusion. The DNV classes of load-out (figure 1) are based on tidal limitations with reference to pumping capacity or pumping requirement whereas the basic 5 ro-ro operations have expanded that and includes the operational environment.
The two main differences being that DNV does not differentiate between using ro-ro ramps or using a steel plate as the transition between shore and barge, because from a pumping requirement it makes no difference. It makes however an operational difference in the allowable vertical barge movement, which is much less when using steel plates.
DNV also does not differentiate between a grounded barge at a jetty and a beach landing where a jetty is absent and has to be constructed.
Note: Even though the list in figure 1 is titled “load-out classes” and the DNV 0013/ND document is called “guidelines for load-outs”, leaving the impression that only load-outs are covered (excluding load-ins), section 1.2 states that load-ins are also covered by this document.
Load-out onto free floating barge with tidal conditions
Figure 2:Ideally, when performing a load-out and the draft of the barge increases, the upcoming or rising tide will bring the barge deck back to the desired level, in line with the quay. This delicate balance has to be monitored throughout the entire load-out. If the cargo is rolled onto the barge too fast and the tide cannot catch up, the barge deck will end up below the quay. Likewise, if the cargo is rolled onto the barge too slow, the barge deck will end up above the quay, as the rising tide will not stop and wait.
Figure 3:Does this mean that the load-out can be performed without any ballasting if the rising tide provides sufficient buoyancy? The answer to this question is a resounding “No”. Let us assume that the load-out is performed on the bow of the barge. As the load is transferred from shore to barge, the bow of the barge submerses further into the water. At the same time the stern raises (draft at the stern reduces) and the barge will end up with a certain amount of trim. Trim being defined as a difference in draft between bow and stern. The length of the transporter determines the allowable trim, you cannot run out of stroke. DNV 0013/ND document section 14.2.5 recommends not to plan beyond 70% of the transporter stroke. Generally during a load-out the barge is maintained at near zero trim.
Knowing that the barge moves (pivots) about the Center of Flotation (COF), for the sake of convenience, we assume that the barge in question is perfectly symmetrical at the bow and the stern, and that the COF is located exactly at the longitudinal centerline of the barge. This means that if the draft at the bow increases by 30 cm (1 ft), the draft at the stern decreases by 30 cm (1 ft).
Note: The actual Center of Flotation can be found in the hydrostatic particulars and differs with the draft as the barge in reality is not symmetrical. Column 5 in figure 2.
Every ton of weight transferred to the barge at the bow needs to be compensated with water pumped into the ballast tanks at the stern in order to keep the barge at near zero trim Figure 5. From the total weight of the barge (incl. ballast and cargo), the average draft can be read from the hydrostatic tables (column 1) in figure 2.
Figure 4:The trim of the barge can be calculated with the following formula;
trim = (displacement * ( COG - COB ) / MT1)
Equation 1
where;
- displacement is equal to the total weight of the barge, cargo, crew, equipment etc
- COG and COB are the Center of Gravity and Center of Buoyancy in longitudinal direction
- MT1 is the moment to change trim 1 inch, column 7 in figure 2
Notes: The trim is affected by the term (COG-COB), as the COG get closer to the COB the barge will show less trim. We cannot influence the COB but we can influence the COG (the combined COG) by means of ballasting. If we manage to ballast the barge in such a way that the COG gets close (or even lines up with) the COB, the barge will remain at near zero trim.
MT1 stands for Moment to Change trim by 1 unit (either 1 inch or 1 cm depending on other units in the hydrostatic tables). In figure 2 MT1 is Moment to Change Trim 1 inch.
It is important to check that the unit for the displacement (metric ton, long ton etc) is the same unit as for the MT1 (metric ton * meter / cm, long ton * ft / inch).
With the above information (average draft and trim) we can determine the draft at the bow and stern with the below two formulas;
Figure 5:As the axles move beyond the COF something changes. These axles now place load on the barge beyond the pivoting, the COF, and contribute to the draft at the stern. As we have been pumping ballast water into the stern tanks, we now need to start pumping water out of the stern tanks as the axles beyond the COF take over this function.
At the same time, there is less and less weight placed on the bow of the barge. We have been relying on the tide to keep the barge in line with the quay but as there is now less weight on the bow, we need to replace this weight with ballast water. In short, when the cargo moves beyond the COF, the stern requires de-ballasting while the bow requires ballasting in order to keep the barge at near zero trim. The principle of moment about the COF applies Figure 6.
Figure 6:Once the cargo is in its final location (often required to be confirmed by a marine warranty surveyor) it is common that both bow and stern tanks contain a certain amount of water. The exact amount of water depends on the required or preferred draft Figure 7. When a barge is being prepared for its voyage it is often trimmed with the bow slightly higher than the stern (some 1 to 2 degrees) as it aids its hydrostatic behavior and reduces fuel consumption of the tugboats.
Figure 7:Load-out onto free floating barge without tidal conditions.
The above case covered the load-out onto a barge in an area where tidal conditions apply. Such a load-out is subject to two constraints;
- load-out is subject to (low) tide and is to be completed within 6 hours
- rising tide provides buoyancy and reduces ballasting requirements
So, what happens in an area where tides are absent, such as on a river or lake, how is a load-out performed under these conditions as there is no tide to provide the needed buoyancy.
Let’s review this step by step. If there is no tide, then the load-out is not subject to any tidal movement. This means that the load-out can therefore be performed at any time of the day (or night). Secondly, if there is no tide, then there is no reason to complete the load-out within 6 hours as there is no falling tide after 6 hours. Those are two positive aspects of performing a load-out in a non-tidal area.
Figure 8:How do we deal with the lack of increased buoyancy from a rising tide? As with a load-out in a tidal area, the first activity is to pre-ballast the barge, so it lines up horizontally with the quay. See Figure 8. To keep the barge horizontally lined up with the quay we must maintain the draft at the same level as during pre-ballast condition. When the first axles roll onto the bow of the barge the draft at the bow increases. To ensure that the draft remains equal, and does not increase, weight has to be removed from the barge as the axles roll onto it. This can only be achieved by pumping out water from the bow that was pumped into the barge for pre-ballasting. For every ton of weight that rolls onto the barge, a ton of water has to be pumped out to keep the barge horizontally lined up with the quay. See Figure 9.
Figure 9:This is an interesting phenomenon as this means that the barge should hold quite a bit of water in pre-ballasted condition. Indeed, the barge should hold at least an amount of water that is equal to the weight of the cargo that is rolled onto it. Ideally this should be a bit more as ballast tanks cannot be emptied completely, a 5% residual volume is a fair assumption. If there is less water in the ballast tanks than the weight of the cargo that is rolled onto the barge, you will eventually run out of water in the ballast tanks and you will not be able to complete the load-out as the barge draft will increase and horizontal line up of the barge with the quay is disturbed. This can have serious negative effects on the load-out performance.
One note needs to be made here. When in a situation where the barge ballast tanks are empty and the cargo has not yet completely rolled onto the barge, there is a way to recover from this situation. As there is no tide, it is relatively easy to roll the cargo off the barge again while filling the ballast tanks again. There are no time restrains from a tidal point of view.
As far as the hydrostatic behavior of the barge goes, this is quite a bit easier than compared to a load-out with tidal conditions as we are now dealing with only one draft. Consequently, we are dealing with only one fixed Center of Floatation (COF) instead of a varying COF due to varying drafts.
During the execution of a load-out it is next to impossible to keep the draft at a fixed value. A load-out is a dynamic process that is performed in a dynamic environment (waves, other water traffic etc). These constant motions have an effect on the transporters; they reduce the effective usable stroke of each axle. That is why it is important to keep the barge and the quay horizontally lined up at near zero trim. The transition between the barge and the quay can consist of a ramp or a steel plate. Typical ramp height is 30 cm (1 ft) and this thickness result in a reduction of the effective transporter stroke. In case of a steel plate (typical thickness 1.0 to 1.5 inch) obviously has much less impact on the transporter stroke. The drawback of using steel plates is a much smaller degree of allowable barge movement in vertical direction.
It is imperative that a load-out is prepared and engineered to an extend that prevents and avoids unexpected situations, such as running out of ballast water, as mentioned above. In practice however, a load-out is not an exact science. When performing a load-out onto a reasonable size barge, it is not practical to aim for a draft within 1 cm or 0.5 inch. The dynamics of the surrounding environment simple prohibits this. Therefore, in case of using a steel ramp as the transition between the quay and the barge, the ramp is used as an indicator.
As mentioned before, the barge is pre-ballasted so that it is lined up horizontally with the quay. In reality the barge is often ballasted to a few inches above the quay. When the ramps are positioned, they show an air gap between the underside of the ramp and the quay. This air gap is used as an indication of how much the cargo can be rolled onto the barge as the air gap will decrease during this motion. When the air gap has been decreased to virtually nothing it is time to let the ballast operation increase this air gap again while the transport does not move forward. With sufficient experience, the rolling onto the barge and maintaining a certain air gap can be a simultaneous task.
Figure 10 shows that the cargo has passed the Center of Flotation (COF). In this figure, it is shown that the bow still holds sufficient water for de-ballasting if there would be a deviation from the expected situation. At the same time, we can see that the stern has started with de-ballasting as the cargo has passed the COF, which causes the stern to increase in draft if de-ballasting would not commence.
Figure 10:When the cargo has reached its final location and assuming that the Center of Gravity (COG) is lined up with the longitudinal and transverse centerlines of the barge and assuming that the barge shape on the bow and stern is identical, the amount of ballast water in the stern and bow tanks would be identical as well.
At this point it could be stated that the load-out has been successfully completed. However, the operation has not yet been completed as the transporters still have to be removed from underneath the cargo back onto the quay. This may be seen as a minor detail, but this is not always the case. Let us assume that the cargo has been rolled onto the barge via a 30 cm (1 ft) steel ramp using SPMT’s with a minimum deck height of 1.2 mtr (4 ft) and a maximum deck height of 1.8 mtr (6 ft). The cargo has been set onto sea fastening stillages of 1.65 mtr (5.5 ft). There is 1.65 – 1.2 = 45 cm (1.5 ft) of space left between the transporter deck and the underside of the cargo, the transporter has therefore this 45 cm of vertical maneuvering space or stroke. There is also 30 cm (1 ft) of ramp height that the transporter needs to negotiate, this needs to be deducted from the available transporter stroke. So, there is only 45 cm – 30 cm = 15 cm (6 inch) of stroke available to maneuver the transporter from underneath the cargo back onto the quay. As you can see, even during this part of the operation it is important that the barge is vertically lined up with the quay in order to retrieve the transporter combination. It may even be required to ballast the barge to achieve this. See Figure 11.
Figure 11:
