OCIMF's RECOMMENDATIONS
This is a short extract from “OCIMF’s RECOMMENDATIONS FOR SHIPS’ FITTINGS FOR USE WITH TUGS”

We recommend all interested to purchase the book from OCIMF since there is a lot of important information.

Order Address
WITHERBY & CO. LTD
32/36 Aylesbury Street
London EC1R 0ET, England
Tel no: +44 (0)20 7251 5341
Fax no: +44 (0)20 7251 1296

E-mail: books@witherbys.co.uk

www:witherbys.com

Escorting

Escorting is becoming a widely used method of risk management at various ports and harbours worldwide. It is usually, although not exclusively, used for tankers. It differs from the normal use of harbour tugs in that the escort tug is in attendance and may be made fast (active or tethered escorting) for an extended distance, with the escorted vessel making way at speeds of typically 5 to 12 knots.

The usual purpose of escorting is to assist the tanker in the event of steering and/or propulsion failure. Within the limits of tug safety, the escort tug can take the way off the tanker, alter its heading or both. Crucially, it can slow the tanker down so that it becomes easier to control.

It is important to note that with the escorted vessel underway, and the tug applying a towing force at an angle to the ship's heading, the water flow against the hull and the tug's own displacement will generate hydrodynamic forces. When combined with the propulsion forces these may give a resultant (steering) force in the towline far exceeding the static bollard pull of the tug.

PREAMBLE

Safe Working Loads (SWLs) are expressed in metric tonnes rather than the technically correct unit of force Newtons (N) to avoid conflict with recommendations contained in Mooring Equipment Guidelines.

(Note: I metric tonne force = 9.81 kilo Newtons).

Recommendations for the Tanker Owner Recommendations for the Tanker Owner

Tankers over 20,000 dwt but under 50,000 dwt to provide:

  • a chock (fairlead) arrangement, with suitable reinforcement, having a minimum SWL of 100 metric tonnes: and
  • a strong point arrangement, with suitable reinforcement, having a minimum SWL of 100 metric tonnes when used with a single eye towing line or grommet.
Tankers of 50,000 dwt and above to provide:
  • a chock (fairlead) arrangement, with suitable reinforcement, having a minimum SWL of 200 metric tonnes; and
  • a strong point arrangement, with suitable reinforcement, having a minimum SWL of 200 metric tonnes when used with a single eye towing line or grommet.

The following recommendations assume that the strong point is not incorporated in the Emergency Towing Arrangement. In such cases:

  • the minimum safety factor of major components and supporting structure to be a minimum of 2 times the SWL rating;
  • the chock to be located on the stern, as close as possible to the centre line of the vessel. (If the emergency towing arrangement is used, the strong point should be located so as to facilitate towing from either side of the stern and to minimise the stress on the towing system
    the strong point to have a minimum diameter of 600mm and a minimum height of 300mm.
  • minimum distance from strong point to chock to be 4.0 metres. It is recognised that this may be difficult to achieve on vessels of less than 50,000 dwt but is aimed at ensuring that the eye splice of the towing line sits inboard of the chock. I f the distance from strong point to chock is less than 4.0 metres, the tug should be advised accordingly.
  • each fitting to be clearly marked by bead weld outline with its SWL. The SWL to be expressed in metric tonnes (letter 't') to avoid any confusion;
  • fixed gear such as strong points, chocks, foundations and associated vessel supporting structure to be demonstrated as adequate for the loads imposed. The ship should hold a copy of the manufacturer's type test certificate for the fittings or a certificate confirming that the fittings are constructed in strict compliance with a recognised standard that specifies design load, safety factor and load application. The ship should also hold a certificate attesting to the strength of the strong points. chocks, foundations and associated vessel supporting structure substantiated by detailed engineering analysis or calculations and an inspection of the installation. Both certificates should be issued by an independent authority (such as a Classification Society). The equipment should be subject to periodic survey and be maintained in good order: