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Spotlight

Jack-up Rigs vs. Platform Rigs

Tue, 01/22/2013 - 15:41

Jack-up rigs are one of two types of bottom-supported Mobile Offshore Drilling Units (MODUs) currently available for exploration and development of offshore oil and gas reservoirs. There are two types of jack-up rig: the independent-leg rig, which usually has three legs with a lattice construction, and the mat type, which usually has its legs attached to a large mat resting on the seabed. Jack-up rigs started being used in 1954 and are relatively motion-free because their legs are anchored to the seabed. At the same time, they offer mobility because they can easily be towed by tugboats or barges, although some have self-propulsion capabilities. Besides stability and mobility, modern jack-up rigs offer various advantages over platform rigs; they are easily upgraded by converting slot to cantilever units, their legs can be smoothly strengthened with more preload tanks, they have the ability to adapt to current environmental standards, they are relatively inexpensive to move, and due to their numbers, they are readily available worldwide.

Even though jack-up rigs were originally invented for shallow water, with a maximum water depth of 107m, modern technology has allowed companies to modernize and create jack- ups with the ability to drill and develop wells of up to 168m. However, as water depth increases beyond this level, semisubmersibles and drillships become a better option. After the catastrophic events of the Usumacinta Jack-Up in the Bay of Campeche in 2007, where a mat-supported jack-up up rig broke well valves that led to 21 reported deaths, Pemex only operates independent-leg rigs. Currently, the NOC operates 46 offshore drilling rigs; five semi-submersibles, five platform rigs, and 36 independent-leg jack-up rigs.

Platform rigs, unlike semisubmersibles, jack-up rigs, and drillships, are fixed atop offshore wells. This is usually achieved through a jacketed spar, tension leg platform (TLP), or gravity structure. Furthermore, unlike other exploration, development, and production infrastructure, most platform rigs are completely self-sufficient, with self-contained units that include power plants, crew accommodation, drilling equipment, life-saving equipment, and auxiliary services. However, there are various issues that need to be taken into consideration before deciding to use or build a fixed platform rig. The first and most important aspect is what specific type of fixed platform will be required: a modular fixed platform, a conventional standard platform, or a self-erecting, self-loading, and highly modularized platform.

Beside this decision, operators should always acknowledge whether the platform includes all the necessary items such as drilling mud, operator fixed items, liquids, portable tools, live loads, hook, setback, rotary table, storage, and expendable items like bulk casing and operator supplies. Additionally, since fixed platform rigs are static structures, operators should consider the cost of mobilization and construction, since once they are built they need to be broken down for shipment and mobilization. However, fixed platform rigs are cheaper to operate than jack-up rigs; therefore, operators usually opt for fixed platform rigs when there are multiple wells to drill, even though water depth, global jack-up availability and rates, and the mobilization cost and time are vital factors in the decision between using jack-up rigs and fixed platform rigs.