After you have completed this chapter, you will:
• be familiar with tugs and towing gear;
• understand the calculations required for bollard pull and fuel requirements for towage; and
• understand the requirements and preparation of towage.
PRACTICAL KNOWLEDGE REQUIRED
Much of the work of the warranty surveyor will involve tugs and towing operations. In oilfield operations it will also involve anchor handling and positioning operations using multiples of tugs. Although some texts are available on the subject the best teacher will be experience and no amount of book study or case study will be a substitute. Field conditions are usually at variance with those experienced in the base office and this is never more true than when sent to a distant location to approve a towing operation involving a tug and barge or drilling rig whose outfitting, equipment and fitness leave a lot to be desired.Trying to fit the requirements to conduct the operation safely and meet the guidelines and the obvious commercial pressure from the client can be a fine balance where experience is the most important
element. There are many circumstances where the C of A should not be issued but it is better to look for a practical solution to a problem rather than simply refuse to offer assistance or advice.
TOWING VESSEL REQUIREMENTS AND SELECTION
The responsibility for the selection and provision of towing vessels suitable for the operations under consideration may rest with a number of authorities, the vessel’s owner, the vessel charterer, the cargo owner in the case of high value barged cargoes and may also be influenced by regulations laid down by statutory bodies, port states, underwriters’ requirements or class society rules. No matter who selects the towing vessels the following basic factors will be applied in selecting the towing
• The size type and characteristics of the object to be towed
• ·The geographical area …etc
• The availability ..etc
• The propulsion capability ,if any ,of the object to be towed
• The commercial factors such as …..etc.
Bearing in mind these factors, the towing vessel or vessels must be chosen bearing in mind a basic set of criteria which will specify the minimum available towing force to be available.
This minimum towing force is the subject of some debate and the following criteria are commonly used:
• Criteria 1 – Minimum Towing Force
‘Suggested by Commander WJ Milwee of MJ Milwee Associates Inc for cargo barge towing.
The tug or tugs assigned to a particular tow must be able to maintain a speed of 2 knots enroute under the following conditions:
Wind speed from ahead 25 knots
Current from ahead 1.5 knots
Significant wave height 11 feet
The tug or tugs should be able to maintain zero speed under the following conditions:
Wind speed from ahead 50 knots
Current speed from ahead 1.5 knots
Significant wave height 25 feet
• Criteria 2 – Minimum Towing Force A well known classification society requires:
Sufficient towing force available to hold any vessel under tow at zero speed against the following conditions:
Wind speed from ahead 40 knots
Current speed from ahead 1 knot
Significant wave height 16 feet
• Criteria 3 – Minimum Towing Force A well known warranty survey company requires that:
Sufficient towing power is available to hold the vessel being towed at zero speed against the following conditions:
Wind speed 44 knots
Current speed 4 knots
Significant wave height 25 feet
It is also notable that some port state authorities require that those in charge of planning and organising the movement of specialist oilfield vessels around their coasts require minimum tow speeds in calm weather.
For example, local regulations in one area require that:
• jack-up units must have sufficient tow power to maintain 3 knots; and
• semi-submersibles must have sufficient tow power to maintain 5 knots.