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.
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.
In some cases, where the tow will last for over 12 hours, then an Extended Field Tow may be necessary. This is a tow where the unit is never more than 12 hours away from a safe port of call, in the event of adverse weather conditions. During this process, the Jack Up unit is afloat on its own full with the legs raised, in much the same way as a Field Tow. An Extended Field Tow can take multiple days to complete. The same motion criteria applies for an Extended Field Tow as for a standard Field Tow, and the same preparations will take place. The only addition is that the weather must be carefully monitored for the duration of the tow.
A Wet Ocean Tow is one which takes place with the hull afloat, which lasts over 12 hours, and which does not come within the requirements for an Extended Field Tow. In these cases, additional precautions will usually have to be made, since the motion criteria for Wet Ocean Tows are more strict than with Field Tows. These extra precautions might include adding some extra leg supports, cutting or lowering the leg so that it is shorter, and ensuring all of the equipment and cargo in and on the hull is fully secure.
On the other hand, a Dry Ocean Tow involves transporting a Jack Up unit aboard another vessel. Rather than being afloat, the unit is secured onto the other vessel as deck cargo. The motion criteria for the unit is therefore dependent on the motions of the vessel being used to transport the unit. This means that the precautions to be taken with regards to supporting the unit’s legs will vary depending on the transportation vessel. However, in most cases the legs will be retracted as far as possible into the hull of the unit so that the hull can be kept as lows as possible to the transportation vessel’s deck, and to cut the amount of cribbing support. Another essential precaution which only needs to be taken during Dry Ocean Tows is supporting the Jack Up’s hull. Strong points (i.e. bulkheads) need to be supported by cribbing, and often portions of the hull will overhang the edges of the transportation vessel. These overhanging sections may be exposed to wave impact, which will increase the amount of stress put on the hull. Furthermore, if the legs overhand, then the bending moment on the hull (which is increased by vessel motions) can be significant. It is therefore important that proper calculations are made to prevent the hull from lifting off the cribbing under the expected tow motions.
Arriving on the location
Once the Transit Mode stage has been completed, the Jack Up unit is referred to as being in “Arriving On Location Mode”. During this time, the unit will be secured from transmit mode, and preparations begin to jack up the unit into its Elevated Mode. These preparations include removing any wedges which may be in the leg guides, initializing the jacking system, and removing any of the leg securing mechanisms which may have been installed for transit. When the latter step is completed, the weight of legs is transferred to the pinions.
Soft pin the jack up legs
In the case of independent Jack Up units which will be operating next to Fixed Structures, or when they will be operating in a difficult area with bottom restrictions, then it may be necessary to temporarily position the unit somewhere away from its final location. This is known as “Soft Pinning” the legs in a “Standing Off” location. To do this, at least one of the legs are lowered until the bottom of its spud can is only just touching the soil. This is in order to provide a “stop” point during the Arriving On Location process. At this stop point, all of the necessary preparations can be made before moving the unit to its final location. These precautions will include running anchor lines, powering up any thrusters on the unit, and coordinating with assisting tugs.
Final location approaching
Regardless of whether a Jack Up unit makes a stop at a Soft Pin location or remains on course directly to the final jacking up location, there will need to be some way of positioning the unit properly for ballasting or preloading operations. With an independent leg Jack Up unit, the holding position is achieved by traveling to the location with all three legs lowered to just above the seabed. Once the unit reaches the final location, the legs are then lowered so that they can securely hold the rig on location without the need for tugs. Mat type Jack Up Units will either be held on location by assisting tugs, or else drop spud piles to hold the unit securely on location until the mat is ballasted and lowered.
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