The Transit Mode occurs when a Jack Up Unit is being transported from one location to another. The unit could either be afloat on its own hull (known as a “wet tow”), or be carried as cargo aboard another vessel (a “dry tow”). Each of these modes will be described in detail below.
In terms of preparation for each transit mode, the main areas that need to be dealt with are the leg support, hull support, the watertight integrity of the unit in general, and properly securing all cargo and equipment so that it does not shift during transit. The units legs will need to be raised so that they do not come into contact with the seabed, but they do not need to be fully retracted in most cases. By keeping parts of the legs lower than the hull baseline, the jacking time is reduced, and leg inertia loads are reduced due to tow motions and stability is increased thanks to lessened wind overturning. In addition, slightly lowering the legs can help to improve the hydrodynamic flow around the open leg wells and reduce tow resistance. Regardless of the exact position of the legs during towing, it is essential to check their structure at the leg/hull interface, to ensure that they are able to withstand the gravity and inertial loads experienced during the tow.
Wet tow
A “Field Tow” takes place when the Jack Up unit is being transported afloat on its own hull, with the legs raised, and where it is only being transported a fairly short distance to its new location. Since this move is only a short distance, it is easy to predict the weather and sea state during the tow. This means that the preparation for a Field Tow is usually less intense than for a longer tow. Field Tows do not last longer than 12 hours, and need to satisfy particular requirements with regards to motion criteria. This criteria, which is expressed as a roll/pitch magnitude during a certain period, limits the inertial loads on both the legs and the leg support mechanism.
Figure 1 – Wet-tow of Mærsk Innovator (Maersk Drilling, 2014)
