IntroductionIn approving a nominated towing vessel for an offshore tow, the surveyor should consider several factors. The size and loaded displacement of the towed vessel, the tow routing, the time of year, the expected wind and sea conditions (including the sail area presented by the loaded barge), the distance of the voyage, and the tug’s fuel, water and stores capacity are all important factors. Horsepower alone should not be the selection criteria.

Once a tug has been selected for the offshore tow, its towline must be sized according to the tug’s bollard pull. If a certified bollard test is not available, bollard

pull can be estimated at 27.5 pounds of pull per brake horsepower (BHP) of the main engines. The towing cable must have a breaking strength of at least 2.5 times the bollard pull of the tug.

Minimum requirements for towing gearThe table below shows industry general guidelines for matching the size of the towed barge to the horsepower of the tug. The estimated bollard pull, towline breaking strength and size of the towline are in accordance with the American Waterways Operators (AWO) Responsible Carrier Program minimum requirements for towing gear.

* long (UK) ton = 2240 lbs.

** short (US) ton = 2000 lbs.

Offshore Towing Gear Requirements

Tim DonneyGlobal Head Marine Risk Engineering

Allianz Risk Consultants (ARC)

+1.646.472.1432

tim.donney@agcs.allianz.com

www.agcs.allianz.com

Risk Bulletin

Number 36May 2011

Report published byAllianz Risk Consulting

Barge loaded displacement (long tons*)

Tug brake horsepower (BHP)

Bollard pull – estimated (tons**)

Towline breaking strength (tons**)

Towing cable size (inches diameter)

<1000 1000-2000 14-28 34-69 1.125-1.25

1000-1500 2000-2500 28-34 69-86 1.25-1.50

1500-3000 2500-3000 34-41 86-103 1.50-1.75

3000-6000 3000-4000 41-55 103-138 1.75-2.00

6000-12,000 4000-5000 55-69 138-172 2.00-2.25

>12,000 5000-7000special

considerationspecial

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Allianz Global Corporate & Specialty www.agcs.allianz.com

Recommendations – Tow cable, wire, and other accessories Towing cable sizes assume a tow cable made of Improved Plow Steel (IPS) or Extra IPS, with an Independent Wire Rope Core (IWRC) and construction of 6 in. x 19 in. or larger (flexibility), which should be galvanized or well coated with lubricant. The tow wire end must be provided with a spelter or thermo-set resin-poured socket, or splice eye with a thimble, sized to exceed the breaking strength of the tow wire. Towing bridles (each leg), terminal fittings on the barge, and all connections must have a breaking strength of at least 1.3 times the minimum required breaking strength of the towline. Towing bridles should be grade 2, or higher, forged integral stud link chain or IWRC galvanized wire rope.

The towing company shall provide copies of certifications for the main towline, emergency towline and all towing gear. The tug must maintain an inspection and maintenance log for the main towline, consistent with recommendations contained in USCG Navigation and Vessel Inspection Circular (NVIC) 5-92. This log should be available for the surveyor’s review.

ReferencesAmerican Waterways Operators (AWO) Responsible Carrier Program

US Coast Guard NVIC database (see NVIC 5-92)

Copyright © 2011 Allianz Global Corporate & Specialty AG. All rights reserved.

The material contained in this publication is designed to provide general information only. Please be aware that information relating to policy coverage, terms and conditions is provided for guidance purposes only and is not exhaustive and does not form an offer of coverage. Terms and conditions of policies vary between insurers and jurisdictions.

Whilst every effort has been made to ensure that the information provided is accurate, this information is provided without any representation or warranty of any kind about its accuracy and Allianz Global Corporate & Specialty cannot be held responsible for any mistakes or omissions.

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