A very large FPSO was to be towed from the builders yard transoceanically to another country where it would be completed. The W/S company was engaged by the owner to supply offshore marine services. A towing contractor had been chosen who had suggested a large single tug for the whole tow. The passage would transit numerous straits and would encounter heavy weather and high traffic densities at times. The W/S company had reservations about the tow contractor’s procedures and the final outcome was as follows:
• The bollard pull of the tug was more than sufficient for the design storm criteria, however the W/S company knew from previous tows with this style of FPSO that it had high resistance and was very difficult to handle in confined waters. The tow contractor did not know or was unaware of these features of the design.
• The constellation offshore marine services company, therefore, required arrangements for the main tug to bunker at sea, en route from an oilfield supply vessel as the owner did not wish to enter any port en route except in extremis.
• The offshore marine services company insisted on the services of an assist tug in those parts of the-route where navigational hazards and high traffic densities required slow speed manoeuvring in very confined spaces. A towing arrangement was fitted at the aft end to enable the tug to give braking or steering assist to the tow.
• At three points on the route where the tow was likely to encounter storms and coastal navigational hazards, such as limited sea room the services of an escort tug had to be engaged. The escort tug had to have a minimum of two-thirds the power of the main tug.
• Two complete sets of main and emergency tow gear were fitted. This enables two tugs to tow the vessel at any one time. The riding crew were supplied with high powered line throwing gear and high strength lightweight messengers for deployment of the extra tow gear in heavy weather.
The tow was successfully completed.