What is Leg Punch Through (Rapid Penetration) for Jack Up Rigs ?

Leg punch through which is one of the biggest dangers of a Jack Up is happened when a jack up leg or more legs rapidly penetrates into formation underneath a spud can(s). This will result in damage in legs, loss of balance of a rig, and potential harm people life.  Whether higher environmental loads or larger water depths, this increases demand on Jack Ups, leads to higher elevated weights, and creates more pronounced consequences of a punch. As depth increases, so does a typical soil’s bearing capacity. Soil strength also sees a rapid reduction when a soil layer is underlain by a weaker layer. The weaker soil eventually gives way as the spud can reaches the interface and the jacking system can’t cope with the speed at which the support of the leg moves downwards; the leg isn’t able to maintain the hull level. The hull then sways after it rotates and bends the legs. Relative to the supports, a weight shift occurs and to maintain equilibrium the required footing reaction increases. This process will only end when any hull buoyancy (from the hull now in the water) or the soil’s bearing capacity allows an equilibrium to be reached.

Figure 1 - Soil Bearing

Figure 1 – Soil Bearing, Apvandenberg.com. (2008).

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Site Specific Assessment and Class Approval for Jack up Rigs

When it comes to safety for Jack Ups, there is a rigorous process in place including International Bodies (International Codes), Class Societies (Class Rules), National Legislation (Shelf States), and National Maritime Legislation (Flag States). Although they don’t require a flag, having one enables them to move freely across international waters. When this is planned, the Maritime Authority regulations must be adhered to according to the Flag State (the country whose flag they are flying).

Normally, each drilling unit will be registered with a Flag State Governmental Administration. Essentially, the role of this administration is to set the requirements for each registering unit and these requirements generally follow the lead of the International Maritime Organization (IMO) since these are internally-agreed suggestions. More recently, the responsibility of verifying compliance has been awarded to classification societies and they also issue Marpol certificates, Loadline, and Tonnage.

Moving away from the flag, there are some governments who also request all drilling units to adhere to their own safety regulations when working in their waters. For example, the United Kingdom has a Health and Safety Offshore Division which ensures the safety of all drilling equipment in UK waters.

In terms of classification societies, these are largely independent and have no relation to any other party and this is important because their sole priority is the quality of the Jack Ups. In addition to authorities, these societies could be the owners, insurance underwriters, charterers, sub-contractors, building yards, and finance institutions.

Figure 1 - Jack up Rigs in a Shipyard (gcaptain.com, 2018)

Figure 1 – Jack up Rigs in a Shipyard (gcaptain.com, 2013)

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Basic Knowledge of Jacking Operation for Jack up Rigs

Jacking, which is one of operation modes for jack up, is a specific way to raise up a hull of a jack up rig so this article will describe about the basic of jacking and preloading operation.

Jacking Operation

Mat jack up units (figure 1) jack the mat to the seabed, in accordance with the ballasting procedure. When the mat is fully lowered, the hull is then jacked out of the water. After this is complete, the unit will then proceed to preload operations. It is vital that all independent leg units also perform preload operations before being jacked to the design air gap. The majority of independent leg units lack the capacity needed to fully elevate the Jack Up unit while the preload weight is still aboard. In these cases, the next step will be to jack the hull out of the water until a very small air gap is achieved- this should be at the very most 5 feet. Once this has been reached, then preload operations can continue.

Figure 1 - Mat Footing Jack Up Rig

Figure 1 – Mat Footing Jack Up Rig

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Basic Knowledge of a Jack Up Rig Transit Mode

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)

Figure 1 – Wet-tow of Mærsk Innovator (Maersk Drilling, 2014)

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Elevating System of Jack Up Rigs

jacking-system-jack-up-rig

This article describes basic data with regard to the jack up rig elevation system, guides, pinion chord and leg fixation.

Jacking System

Firstly, let start with the jacking system , to lift and lower the hull, every Jack Up will have a special mechanism and the most basic of these is called ‘pin and hole’ (Figure 1); discrete leg positions will be used for the positioning of the hull. In the market today though, most Jack Ups will use a ‘rack and pinion’ (Figure 2) system to allow for continuous operation.

Figure 1 - Pin and Hole (Hercules offshore,2018)

Figure 1 – Pin and Hole (Hercules offshore,2018)

 

Figure 2 - Rack and Pinion

Figure 2 – Rack and Pinion (sagta.com,2018)

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