Self Elevating Units
30.06.2014 News Archive
Self-elevating offshore units are valuable options for locations with limited support infrastructure because they do not require a heavy lift vessel to install the “topsides” once the supporting structure is in place. Another reason why self-elevating units may be preferable is that they can be mobilized to a new location in a much more efficient way than so-called fixed towers or jackets.
Common components to all self-elevating units are as described below:
- Hull – This is the buoyant structure that initially holds the legs before they are lowered to the seabed and then serves as the work platform once the unit is at installation airgap.
- Legs – A minimum of three legs are required to have a stable platform, but many designs exist with as 4-6 and even 8 legs.
- Jacking/Holding System – The jacking system provides for the self-installation feature of the unit, as it allows relative movement of the hull and legs. The holding system may be integrated with the jacking system, or be a separate mechanism used when the legs and hull are to be kept at a set location (such as at installation airgap or during transit). Jacking systems can be electrical, hydraulic or pneumatic. One of the most common jacking systems consists of rack and pinions for relatively smooth jacking. Other jacking systems include pin and slot and the use of strand jacks.
- Footing(s) – Except for areas with very stiff soils, all units require a set of footings to prevent excessive penetration of the legs into the seabed. These footings can be suction cans, mats (a large piece connecting all legs), pads or so-called spudcans. Occasionally, suction or driven piles can be used to improve the capacity of the unit.
There are many kinds of self-elevating units, many of which have been in continuous operation for over three decades. Depending on their purpose and capacity, self-elevating units are referred to as jackups, liftboats or lift barges or jackup barges. There is no definitive distinction among these categories, but each of these units have unique features that are generally accepted as the differentiating factors in what the units are referred to as in the offshore industry.
The figures below show two of OVERDICK’s self elevating units. The first figure is of the recently completed BorWin beta MOAB, a proprietary design by OVERDICK installed in the German sector of the North Sea. As the figure shows, the MOAB is towed to the site while floating on its own hull. Once there strand jacks are used to lower the legs and footings to the mudline and then elevate the hull to the working airgap. The second figure is of the INNOVATION heavy lift jack up barge, designed by OVERDICK and completed in 2013. It is noted that due to its large crane capacity, this liftboat is also referred to as a heavy lift jack up vessel (HLJV). However; since it has a bow shape and is self-propelled, liftboat is also suitable designation.
Jackups are the most prevalent type of the self-elevating units. The most common type of jackup is a drilling jackup, although there are accommodation jackups and production jackups. Jackups are capable of working in harsh environments and in water depths up to 500 feet. The most common type of jackup has independent legs made up of square or triangular trusses, fitted with so-called spudcans that serve to reduce the soil bearing pressure once they are elevated. Many other types of jackups exist, including mat units, where all legs are connected to a large mat and move together, and jackups with tubular legs.
Self-elevating units can also be categorized as mobile vs movable. Mobile units are designed to move from work location to work location in a semi-regular basis, with relatively low planning/effort. On the other hand, movable units have the capacity to move from one location to another, but the move may require some preparation and effort, especially if piles or suction cans are used, or if the jacking system is removed after installation.
Some units are self-propelled and some are not. Typically, jackups and lift barges are not self-propelled and require towing to move from site to site. Usually, liftboats are on the move much more often than jackups and lift barges, as they are usually equipped with a propulsion system and sometimes with a DP (dynamic positioning) system. For this reason liftboats are usually designed with a bow shape aimed at improving transit speed. Jackups and lift barges usually have very blunt bow shapes.