offshore marine services on preparation and towing of barges

the preparation and towing of barges are :

1. When cargo barges are loaded out with modular cargo, the design engineering for the carriage of that item will have specified a load draught/ trim for the barge involved.
It is always wise to check the draughts on the barge and compare them with the calculated draughts.
Investigate any significant discrepancies and if the barge is deeper than calculated, although not overloaded (below loadline marks), bear in mind the effects of heavy rolling.
This is especially true in the case of cargoes which have significant overhang each side since this may cause the cargo to’ contact wave tops at an earlier point than the design work calculated.

2. Ballast tanks on cargo barges should be either full or stripped empty without any excess free surface.
If possible, inspect or sound every tank and compare the actual ballast disposition with that set out in the design parameters for the loaded condition.

3. The decks of large cargo barges suffer considerable abuse as complex support frames and seafastening structures are welded on and then cut off.

4. Investigate the deck plating, especially at deck edge and at transverse and longitudinal bulkhead areas. Weakened plating is not always apparent without careful visual checking.
Incautious use of grinders and gas cutting equipment can remove quite significant amounts of metal, resulting in, splitting and cracking of plating in way of stiff connections such as bulkheads.

5. It is often the case that a tug arrives to tow a loaded unmanned barge with little or no time to allow the tug master and chief engineer to examine their charge. If possible, the tug master and his engineer must thoroughly check the tow before departure and they should have written instructions and some practice at starting power plants, operating bilge
pumping systems, operating powered retrieval winches and any other systems that the barge might have.
A set of drawings of the barges tank layout ballast disposition and all other pertinent documentation should form part of the “package” given to the tug master.

5. Some large cargo barges behave in the most unexpected fashion when towed at particular trims or speed. High speeds sometimes achieved by large oilfield towing units have resulted in severe damage to barge hulls. This high speed can result in a continuous vibration set up in the hull which on one particular class of carrier, specifically the large semi-submersible “dumb” cargo carriers, causes splitting and cracking of transverse bulkheads in way of the deck and bottom plating connections.

6. On long tows with unmanned barges, the tug should use periods of fine weather to go across by boat to the barge and carry out a routine check of the tow gear, barge draught and trim, ballast disposition, cargo security and general machinery status.

7. Unlike most other areas of marine activity, the vessels of the offshore oilfield, particularly large semi-submersible drilling units, work ships and other “industrial” function craft,seem to neglect watertight integrity with regard to hatches, doors, their closing devices and many other mechanisms designed to seal off spaces from sea ingress. This comes
about mainly because of the lack of awareness of the vessel’s “industrial” crew.
Doors are distorted, vent flaps knocked off, clips on hatches and doors are allowed to rust-up or become unworkable. When preparing a vessel for a passage, it is always wise to check the “tightness” of doors, hatches or vents from the inside after closing them.
Bolted manhole covers are frequently left without their full complement of nuts and bolts and even closed without the use of jointing compound – if doubt exists as to the fitness of a closing device, then hose test it.
Another often neglected area on large semi-submersibles is the fitness for operation of chain locker dewatering systems. These spaces can house very large accumulations of mud and debris from mooring chains which, if allowed to build up, chokes the locker pump out lines – only regular cleaning and testing can avoid the problem.

8. On some6 semi-submersible- drilling units, it is the practice to carry out in field and interfield moves at “deep draught” – that is, close to operational draught.
This comes about because the allowable deck load at transit draught is less than that permitted at operating
draught. Discharging equipment wastes time and charterer’s money.
Therefore, the unit is moved, albeit at half or less speed than at transit draught, in such condition.
Although not inherently dangerous provided the stability criteria are complied with (storm and normal operating), this practice can lead to a situation where so much load is on board that ballasting either up or down presents the operational staff with a situation of the unit being over her allowable stability limits as soon as significant free surface occurs
which is inevitable during ballast operations.
The great success of the moored semi-submersible as a work platform comes about partly with the aid of the motion “damping” effect of the mooring spread and even when heavily loaded ballasting up and down within both draught and stability limits is not usually a problem.
In the transit condition with the mooring system stowed quite small changes in draught and trim due to ballasting can have spectacular effects.
One particular class of semi submersible having the tendency to oscillate backwards and forwards over an arc of
10 to 15 degrees which is most difficult to stop.

Finally, the task of towing or moving a large semi-submersible at full operational draught results in high drag forces and requires some tugs to have to tow at high power just to achieve modest progress. This is not good moving practice. It is hard on the tow gear and leaves small margins for safety factors. Sea states, tug motion and high line tension can
result in sudden peak loadings which part the gear at the most unexpected times.
With the submersible at transit draught making good progress, towing forces are not usually very great and there is some degree of harmony between tow and tug in motion response, whereas in moderate sea states with the barge at deep draught almost no motion occurs while the tug may be likened to a salmon leaping around on a fisherman’s line – when the leap is high enough and violent enough, the fishing line breaks.

9. During the transit of manned units between work locations the operational crew, that is the industrial crew, as distinct from the marine crew, may have considerable maintenance,repair, refurbishment or preparation work to complete on equipment and systems.
While most of this work is essential in order for the vessel to carry out her commercial function, it is vital that it be co-ordinated and controlled by those in charge of the marine operation.
Moving large heavy items (which abound in the oilfield) by skidding, pulling, lifting etc are the norm, but failing to take account of vessel pitch, roll or heave, failing to resecure correctly after moving the item, failure to warn ballast control staff of the movement taking place, neglect of good rigging practice and neglect of safe working procedures has caused,
and continues to cause, severe damage to equipment and people almost all of could be avoided by good planning and communication.
The function of both supervisory staff in all departments on board and the duties of the deck “patrol” are for effort and activity to be carried on such a scale that the vessel proceeds to her destination in all respects fit to work and remains at all times fully prepared to cope with the hazards of the sea.

10. On manned barges, the marine department should organise routine regular “patrols” or rounds of the whole vessel. The function of this patrol is to ensure that hatches, doors and closing devices are kept properly secured, that pump rooms and spaces within the hull are watertight, that life saving firefighting and emergency gear is in place, that lighting,
statutory signals and lights are operating, that cargo and equipment seafastenings are tight and secure, that unauthorised activity is not taking place and that, in all respects, the vessel is “seaworthy”.
The patrol should take place on a regular basis at intervals determined by weather, activity and manpower availability. At the least, a four-hourly patrol is desirable with the frequency increased depending on circumstances.
Many specialist oilfield vessels carry large items stowed externally to the vessel’s main deck. For example, anchors stowed on racks, buoys stowed in saddles hung overside.
Pennants to anchors, mooring gear fendering systems etc. The security of all such items must be regularly checked along with deck stowed and internally stowed equipment.
Good briefing, good supervision and rotation of duty for the patrol amongst a number of men will ensure that their vital role is not dulled by endless repetition.