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This is Part 1 of a brief introduction to self-elevating units.
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 specific names is associated to specific features that are generally accepted in the offshore industry.
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.
For a more complete introduction to jackups, the reader is referred to a paper I co-authored in 2005 entitled JACK UP UNITS – A TECHNICAL PRIMER FOR THE OFFSHORE INDUSTRY. The figure below, from this PRIMER shows the two main modes of operation (afloat and elevated), along with the transitions for a jackup, common to (most) self-elevating units.
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 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 hole or the use of strand jacks.
Footing(s). Except for areas with very stiff soils, all units require a set of “spread” 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.
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.
The pictures below show a rendering of a liftboat design by 3DENT, and an actual liftboat – the BLUE TITANIUM (built by Baker Engineering).
The complete reference to the Jackup Primer is given below.
Vazquez J.H., Michel R.P., Alford J.H., Quah M. and Foo K.S. JACK UP UNITS – A TECHNICAL PRIMER FOR THE OFFSHORE INDUSTRY PROFESSIONAL. Published by Bennett & Associates and Offshore Technology Development, 2005.