The practical considerations of carrying out different types of surveys by Offshore Survey Companies Dubai
Tug approval off shore surveys
Mobile offshore units – Towing or Moving
Operations Planning Checklist
Barge Towing Operations
Case studies and problem areas
The jack-up drilling unit of off shore company
Dry Transport of a JACK-UP drilling unit
JACK-UP drilling unit – Unusual load
Towing Connection Problems
Watertight Integrity and Fitness to tow problems
Tugs and Towing Gear
Practical Knowledge required
Towing vessel requirements and selection
Determining the power of oilfield tugs
Voyage or move Average Speeds
Consumables – Towing Vessels
Strength Requirements for Towing Gear
The Preparation and Towing of Barges
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 sea fastening 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.
6. 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.
7. 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.
8. 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.
9. On some semi-submersible- drilling units, it is the practice to carry out in field and interfiled 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 is 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 a 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.
10. 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 which 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.
11. 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 lifesaving fire fighting 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 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.
Towing Gear – General notes on Rigging
The following notes are applicable to rigging and use of towing gear on both semi-submersibles and cargo barges.
They are not in order of importance, but all towing gear should be inspected and used only after having regard for the following points which are reiterated because they are vital for success:
1. Is the gear suitably sized to the tow?
2. Is the towing vessel matched to the tow’s gear?
Points to Note
1. Try and determine the age and usage pattern of tow gear fitted to any particular unit. Wire rope deteriorates when immersed for long periods and the bridles of semi-submersibles, if made of wire spend long periods hanging unused. The area at the vessel’s load waterline on these wire ropes is liable to suffer severe corrosion damage – not visible until they part.
2. As a general policy, wire rope bridles should be removed from service and thoroughly examined or replaced every two years and after long hard weather towages.
3. Chain bridles and bridle components made up from chain should be inspected at least annually. Chain link/link wear down measured and the whole section rejected if link wear exceeds 10% the link bar diameter. Chain bridles and sections should be installed without twists as far as possible and so far as possible avoid installing joining links except at terminal points.
4. Towing brackets of whatever construction, pad eyes, Smit bracket or lugs should bee thoroughly checked for wear on the whole of pad eyes, weld integrity of connection of the bracket of the eye to the vessel’s structure by means of non-destructive testing, (magnetic particle or ultrasound) and by x ray if doubt exists. This inspection should be biennial. An examination for distortion or damage or deterioration of the primary structure behind the towing bracket should also take place at the same. time as the examination of the bracket.
5. The lead of the tow pennant or bridle leg (chain) from its securing point to the outboard of the deck should be such as to as far as possible avoid significant side loading on the outboard fair lead. The shoulders of the fair lead should be smooth, round and undamaged no matter what type is fitted and they should be thoroughly checked as in point (4) above.
6. In every case where a tow bridle leg or two pennant passes through a fair lead from the secure point, the fair lead must be of the closed type that is, there must be either a heavy bar by which the pennant or bridle leg cannot jump out of the lead. In the case of a closed Panama type lead, this obviously does not apply.
7. Where a tow bridle leg or two pennant either chain or wire passes over the outboard edge of the deck of the vessel, the edge should be well rounded so as to avoid a sharp “nip”. This “roller” should be at least three to five times the diameter of the wire or chain which passes over or around – the larger the diameter the better.
Bollard Pull Certification and Trials
In order to establish the pulling capacity of a towing vessel, a physical test of its capability may be carried out. This test is referred to as the bollard pull trial or test.
It may be carried out when the vessel is a new building, when re-engined or has new rudder propellers fitted or for the purposes of satisfying a specific requirement of a contract of towage. During bollard pull tests, the towing winch machinery may also undergo a variety of trials.
There are a number of methods and criteria for carrying out the test, but the basic aims are as follows:
1. Establish the maximum pulling capacity of the vessel when the engines are working at the manufacturer’s recommended maximum continuous RPM. In addition it is normal to test a new vessel at 110% Power Output, in other words at 10% overload in order to establish a maximum bollard pull value.
2. Establish the pulling force exerted at various settings of engine speed, propeller pitch or engine configuration.
3. Test the towing winch brake holding capacity with the winch drum full of wire and the vessel worked up towards maximum bollard pull.
4. Test the winch quick release mechanism when pulling at some predetermined, bollard pull, usually 30% to 40% of maximum power. This test may be in two parts:
• Testing the quick release mechanism when the winch is hauling in; and
• When the winch is riding on the brake.
5. Test any tow winch spooling gear. This test is carried out with the spooling gear engaged and the wire at an angle of about 60% off the boat’s centreline. The boat should be pulling at about the maximum continuous bollard pull.
6. Test the strength of fixed gog wire, guide pins or towing pods. This test is carried out with the tow wire at about 60 degrees each side of the boat’s centreline and pulling at or near her maximum bollard pull.
It is obvious that all the components, pulling points, tow wires, shackles and fittings should be certified and examined to ensure that they are strong enough to sustain the maximum expected loads. An adequate safety factor of 10% to 20% is normal. An adequate Safety Factor of at least twice the maximum calculated loading is usually applied.
The Offshore Survey company conditions required for the test are briefly as follows:
1. The clear water distance between the stern of the boat and the point of attachment of the tow line should not be less than twice the boat’s length (BP) and ideally between 300 and 350 metres.
2. The water depth should be at least twice the boat’s maximum draught.
3. The test cell should have an accuracy of about ± 2% and have a certificate of calibration and remote reading and recording mechanism for the test cell is highly desirable.
4. The weather conditions should be calm with minimum wind, sea and current.
5. The boat should be at her normal operational trim and draught.
Please fill in the form on the right side for more information about
Offshore Survey Companies Dubai.