Fixed platform rigs

As the name indicates, this type of rig is located on a fixed structure previously installed at the well location. The structure may be a fixed jacketed platform, spar, tension leg platform (TLP), or gravity structure; whatever it is, the rig sits atop it.

Overview

Fixed platforms may have as few as 3, or more than 50 well conductors. Generally, the drilling rig is not a permanent part of the fixed structure. However, on some occasions, the unit is left on the platform for future workovers or additional drilling, if removing it is uneconomical. Most units are complete, self-contained units that include their own power plant, accommodations, drilling equipment, life-saving equipment, and auxiliary services. However, some do not have their own power plant and obtain power from the platform’s generators, which are usually powered by produced natural gas. On large, central field platforms that have their own living quarters, the rig may not have its own accommodation facilities. In this case, the life-saving equipment (e.g., lifeboats and gas-detection, fire-fighting, and communication systems) is part of the fixed platform. Most fixed platforms have their own craneage, but, usually, it is not big enough to load or unload the components of a conventional platform rig. Most modern platforms are built to American Petroleum Inst. (API) standards, allowing movement of a standard API-configured platform rig from platform to platform with little or no modification.

Types of fixed-platform rigs

There are three types of fixed-platform rigs:

Fixed Platform Rig Type	Description
Conventional Standard Platform Rigs	These rigs are not self-erecting, not particularly modular in construction, are heavy, and are built to API well spacing standards, so they can work on a wide range of platforms. They usually require a derrick barge or a large platform crane to load and erect. They may take 2 to 4 weeks to erect, and their dry weight will probably exceed > 5,000 kips. These rigs are usually self-contained, and can include over a 1 million lbm of derrick and traveling equipment
Self-erecting, Self-loading, and Highly Modularized Rigs	These rigs are set up to go from platform to platform quickly. Generally, they take up much less space, and their dry weight (750 to 1,250 kips) is considerably less than that of conventional standard platform rigs. Unfortunately, most of these rigs have limited hook and traveling-block capacity, and sometimes do not have all the auxiliary equipment, such as bulk tanks, large liquid-mud-storage capacity, and emergency power. They are particularly attractive for in-casing workovers and out-of-casing redrills. A few of the larger modular rigs have hook load ratings of 1 million lbm but also have compromised weight and ease of mobilization. Modular rigs first appeared in the late 1980s and early 1990s. They generally do not have modules weighing more than 30 tons, have a self-erecting “leap frog” crane, contain modules that can be transported on any standard-sized workboat, and can be completely rigged up or down in 2 to 3 days
Modular Fixed-Platform Rigs	These rigs, having gained popularity recently, are site-specifically designed and constructed to be placed on deepwater spars and TLPs. They are very compact and lightweight (Fig. 1). Their mobilization and rig-up time is much more than that of standard modular rigs. Because modular rigs are generally not self-erecting, cost and total rig-up/rig-down time are issues

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  Fig. 1—Example of highly specialized and site-specific modular fixed-platform rigs used on spars, deepwater fixed platforms, and TLPs. This unit is on a TLP in the GOM. Courtesy Helmerich & Payne Intl. Drilling Co.

Considerations for fixed-platform rigs

The first consideration in using a fixed-platform rig, usually controlled by the operator, is whether the platform is large enough and has a high enough load bearing to place and work the rig. This includes:

• The space and dry weight of the rig itself
• Wet weigh
  • Mud
  • Operator fixed items
  • Liquids
  • Portable tools, etc
• Live loads
  • Hook
  • Setback
  • Rotary
• Storage
• Expendable items like bulk casing and operator supplies

Generally, a four-pile structure is the smallest fixed structure that a conventional standard platform can be placed on and work efficiently. Usually, the second consideration is the mobilization method and cost. Numerous platform rigs, when broken down for shipment, cannot fit on a standard workboat, and a derrick barge is required. Modular rigs can usually fit on a workboat.

Why would someone want to use a fixed-platform rig? Generally, their day rate is considerably less than that of a jackup, assuming that the platform is in accessible jackup water depth and that there are enough wells to warrant the mobilization cost. The decision to use a jackup or standard platform rig is usually controlled by the number of wells to be drilled; the more wells there are to drill, the more attractive the platform rig becomes. Of course, the platform water depth, availability of a suitable jackup, metocean, and the mobilization cost and time of either unit are also factors. In shallow water, less expensive jackups are available; however, a platform rig will be more economical in deeper water. Market conditions at the time of use are usually the driving economic force. Another alternative to a platform rig is a tender assist drill (TAD); however, availability will be a problem because there are so few units, especially semi TADs. Environment may be an issue for monohull tenders. With semi TAD hulls, environment should not be an issue.

With a semi TAD, operating efficiency is higher. Studies have shown that a tender with very large load-carrying capability and space availability is operationally very attractive. A number of operators have stated that they think a semi TAD is 10 to 25% more efficient as controlled by workboat transit time, weather, specific type of wells being drilled, and space/weight limitations of the platform and platform rig.

There is no standard, easy answer for all situations to specifically recommend a specific rig type. With the appearance of extended-reach wells (ERWs), the required loads and space are becoming so great that a cantilevered jackup or TAD sometimes becomes a more attractive alternative than even a large standard platform rig.

See also

History of offshore drilling units

MODU types

Fixed steel and concrete gravity base structures

PEH:Offshore_Drilling_Units

Noteworthy papers in OnePetro

Robert W. Ruhe and Phil Griffin 1970. Adaptation of Fixed Platform Rig Instrumentation to Floating Vessels, Offshore Technology Conference, 22-24 April. 1308-MS. http://dx.doi.org/10.4043/1308-MS

External links

American Petroleum Inst. (API)