Monday, January 28, 2013

Surveyor Guide Notes for Towing Arrangement Survey

The objectives of these entry is to ensure safety at sea, prevention of human injury or loss of life, avoidance of damage to the environment, in particular to the marine environment, and to property through providing minimum recommendations for the organization, planning and execution of ocean towages and the design of associated equipment.

In cases, where particular circumstances or factors signify an increased risk to the tow, or where the risk cannot be evaluated on the basis of seafaring and nautical knowledge and experience alone, the owner of the towing vessel, owner(s) of the towed object or the towing master/tug master should apply for survey in accordance with MSC/Circ.884 Guidelines for Safe Ocean Towing, dated 21 December 1998 or by a competent organization or authority, as appropriate.

1. The towing equipment should be designed according to the below mentioned recommendations and recognized standards. The towing arrangements should be suitable for the particular tow and of adequate strength.
2. The towing vessel should be equipped with a towing winch.
3. It is recommended that the towing winch brakes should have an appropriate static holding capacity to that of the documented minimum breaking load (MBL) of the largest towline to be used. The holding capability should be calculated for the outermost towline layer on the winch drum at which towing will be performed.
4. The design and scantling of the towing winch, including supports, should be capable of withstanding the breaking load of the main towing wire rope without permanent deformation.
5. It should be possible to release the tension on the winch drum(s) in an emergency and in all operational modes. The end attachment of the towing wire rope to the winch drum should be of limited strength, thus forming a weak link in case the towline has to be run out. After an emergency release the winch brakes should revert to normal function without delay. It should also be possible to carry out the emergency release sequence (emergency release/application of brakes) even during a black-out.
6. It is recommended that on board towing vessels, whenever practicable, the winch should be fitted with equipment for measuring the tension in the towline. This equipment should, as a minimum, record the mean tension and the tension peaks, and the information should be displayed in the wheel house.
7. Means should be provided to spool the towline effectively on the drum(s).
8. Towline protection sleeves, or other means should be provided to prevent the towlines being damaged by chafing or abrasion. There should be no sharp edges or obstructions at the stern of the vessel that may damage the towlines during operation. A sufficient number of spare towline sleeves should be carried on board.
9. An appropriate length for the towline should be determined using established criteria. Where no such criteria have been established, the minimum required length (L) of the main towline should be determined from the formula;
L= (BP/BL) x 1800 m
where:                     BL = Documented breaking load of the towline,
                                                     BP = Continuous bollard pull
10. All wire ropes in use should have the same lay (i.e. right hand, left hand, etc.).
11. The minimum documented breaking load (MBL) of the main towline should generally be in accordance with the following table.

Bollard pull (BP) (tones)                             <40                         40-90                                >90
Minimum Breaking Load (MBL in tones)  3.0 x BP               (3.8-BP/50) BP                   2.0 x BP

12. A spare towline satisfying all requirements for the main towline should be kept on board the towing vessel.
If the towing winch is equipped with two drums the spare towline should preferably be stored on the winch drum, readily available for use.
The alternative is to have a spare towline which should be in position and so arranged to ensure that transfer to the main towing drum is easily, quickly and safely effected.
In case of two towed objects whereby two independent towlines (main and spare) are to be connected, an extra spare towline should be on board, arranged as specified above.
13. All wire rope terminations should be hard eyes, i.e. reinforced thimbles or spelter sockets except for the end connection to the drum on the towing winch.
14. All connecting items like shackles, rings, etc., should have an ultimate load bearing capacity of minimum 50% in excess of the documented minimum breaking load (MBL) of the towing arrangement to be used.
15. If fiber rope pennants are used, the pennants should be in a sound condition and the minimum breaking load of any fiber rope pennants should not be less than:
- 2.0 times the tow line MEL, for tugs with bollard pulls less than 50 tones;
- 1.5 times the tow line MEL, for tugs with bollard pull greater than 100 tones; and
- linearly interpolated between 1.5 and 2.0 times the tow line MEL for tugs with bollard pull between 50 and 100 tones.
Fiber rope pennants should be of grommet construction and be terminated with hard eyes, and should not normally be connected directly to the apex of the towing bridle.
16. The towing vessel should be equipped with sufficient spare equipment to completely replicate the towing arrangements, unless found impractical.
17. Inspection of the towline should be carried out on completion of each towing operation. The results of the inspection should always be recorded as a basis for decision on future inspection programs. The inspection should also be noted on the towing log (Appendix B).
18. No part of any towline arrangement should be used for the towing operation if:
- the reduction in cross sectional area due to wear, abrasion, corrosion and broken wires exceeds 10% or there is severe kinking, crushing or other damage resulting in distortion of the rope structure;
- end sockets or other towline terminations such as thimbles, etc., are damaged, deformed or significantly corroded.
19. If relevant, Gog ropes or alternative arrangement should be provided to prevent athwart ship pull, and to facilitate retrieving of the towline. The arrangement should be remotely operated from a safe position. A spare Gog rope should be carried on board.
Here you have some quick tips for to calculate the towing arrangement:
1 BHP = 30 pound/2240 = bollard pull in ton.
Rope = Bollard pull plus 2 1/2 ton.
Bridle SWL = Bollard pull plus 50 ton.
Shackles = 3 times bollard pull.
Working load = twice bollard pull.

Noble Denton Technical Policy Board Guidelines for Marine Transportation's:
This guide is very comprehensive