Z-Drive Escort Tug Operating Modes

Abstract

The suitability of various types of tugs to perform indirect towing has been a lively subject of debate, mainly in the international arena, and increasingly in North America. In the United States, this is the result of the enactment of well-meaning laws to improve the safety of seaborne oil transportation in environmentally sensitive areas such as Puget Sound and San Francisco Bay. Escort by means of a tug tethered to the stern of a tanker to permit rapid response to loss of propulsion or steering is often promoted as being the best way to accomplish the goal of such legislation. The development of indirect towing has made it possible to satisfy the desire to combine environmental safety with reasonable transit speeds. Indirect towing uses steerable propulsion to present the profile of the hull to the direction of advance of the tanker. Simultaneous braking and steering forces result. At elevated speeds, extremely high line pulls can be achieved, but with some risk to the towing vessel as it is in a girthing situation at all times. Research conducted by Aquamaster-Rauma Ltd. reveals that vessels equipped with independently controlled steerable propulsion systems (i.e., Z-drives) are capable of other means of producing arresting and steering forces with reduced risk of placing the escorting tug in a potentially dangerous situation. In this paper, these methods are discussed with comments on the situations where each mode applies.