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Fixed steel and concrete gravity base structures
Many types of offshore structures are in service. Some are better suited to certain environmental and operational criteria; some are limited by availability of construction sites; and some are chosen simply by subjective preference of an owner/operator. Selecting a structure type is the first major structural design task after environmental and operational criteria have been defined and will sometimes require preliminary design of several concepts before a choice is made.
Fixed steel offshore structures
Jacket or template
The most common type of offshore structure in service today is the jacket (or template) structure, as illustrated in Fig. 1. The template was derived from the function of the first offshore structures to serve as a guide for the piles. The piles, after being driven, are cut off above the templates, and the deck is placed on top of the piles. The template is prevented from settling by being welded to the piles’ tops with a series of rings and gussets. Hence, the template carries no load from the deck but merely hangs from the top of the piles and provides lateral support to them.
Some companies prefer to place packers in the bottom of each template leg and to grout the annular space between the leg and pile from bottom to top. The structure and pile share the axial load from the deck and the compressive and tensile loads from the overturning moment produced by lateral wave loads. The grouted pile also provides additional strength to the tubular joints where horizontal and diagonal bracing is welded to the legs. Drawbacks to this system are the difficulty in ensuring that the grout is adequately placed and of sufficient strength to be counted in the analysis and the additional difficulty in platform removal.
Although both top-hung and grouted structures are loosely called templates, some prefer to call the latter a jacket to distinguish the difference in load path. This path is substantially different for the overturning moment as well as axial loads. The top-hung template requires that moment from lateral wave loads be transmitted up the structure to be resolved into axial pile loads. The grouted jacket has a direct downward load path for shear and moments.
When steel structures are designed for deeper water (in excess of 250 ft), pile-leg grouting is prevalent. Lateral wave loads that produce high base shear and overturning moments heavily influence deepwater jacket designs. Piles placed through the legs of the jacket are not always sufficient to transfer these loads to the soil; so “skirt piles” are added, normally in clusters around the corner legs. This adds a new dimension to the installation procedure. Pile guides are required up to water level, and a removable “follower” must be used during pile-driving operations. Grouting procedures for the skirt sleeve-to-pile must recognize that grout placement and inspection will be done remotely.
The offshore structure shown in Fig. 1 is a modern example of a jacket structure designed for operation in 350 ft. of water. The jacket structure is first placed on the seabed, and the foundation piles are driven through the pile sleeves (often called skirt piles) and grouted to form the support system for the structure. Often it is only necessary to provide piles through the legs, depending on the environment and soil characteristics. In these cases, the piles are either grouted or welded to connect the piles to the jacket and permit the topside and jacket loads to be transmitted to the piles and into the soil.
The production equipment and facilities are often called the topsides. To simplify installation and hookup at sea, the equipment and facilities are often placed in modules, which may weigh several hundreds to many thousands of tons. The modules are completely prefabricated and tested onshore prior to transportation and lifting onto the jacket deck(s) by offshore crane vessels.
The template or jacket structure is a steel space frame that supports, above water, a superstructure comprising one or more decks for production equipment and facilities needed to support and maintain production. The production tasks may include separation of oil, gas, water, and sand; treatment and measurement of oil and/or gas for sales; and treatment of water and/or solids for disposal.
Additional components of the topsides include:
- Utilities, i.e., electricity, fuel, instrument gas, power gas, water, and sewage
- Accommodations for personnel
- Work shops
- Control rooms
- Safety systems for hazard detection, protection, and escape.
- Flare boom for gas flaring (if necessary)
Often, a drilling derrick forms part of the equipment for drilling and maintenance of the wells. The accommodation/helideck facilities are situated as far from the potentially dangerous hydrocarbon process area as is physically possible.
To simplify installation and hookup at sea, the equipment and facilities are often placed in modules, which may weigh several hundreds to many thousands of tons. The modules are completely pre-fabricated and tested onshore prior to transportation and lifting onto the jacket deck(s) by offshore crane vessels.
The design of a jacket structure is a matter of:
- Determining the overall dimensions based on water depth and functional requirements, evaluating hydrodynamic loads caused by waves and currents
- Evaluating topsides and wind loads
- Sizing of the structure to meet state requirements for strength, fatigue, and serviciability
- Sizing of the appurtenances.
Design forces on jacket structures, shown as arrows in Fig. 2, can be calculated with specialized computer software available to the industry. The horizontal force from waves consists of drag forces from the kinetic energy of the water and inertia forces from the water-particle accelerations. An appropriate wave theory is used to calculate the water-particle velocities and accelerations. The total horizontal force is calculated by multiplying the projected area of the structural members with the water pressure. Jacket design is an iterative process because a number of design cycles are gone through before achieving the optimal sizing for a particular member to bear these horizontal forces. Most of the horizontal loads are directly related to the diameter of the tubular members and their locations within the structure.
The design of the foundation piles is also critical. There is significant interaction between the response characteristics of the jacket structure, its foundation piles, and the soil, such that pile-soil-structure interaction is explicitly catered for in the design recipe for jacket structures. Over the decades, the piles have grown in size (number, diameter, and length) in line with the jacket structures. Therefore, pile weight is an important part of total structural weight. For example, the Bullwinkle jacket structure weighed 44,800 tons, and the pile weight was 9,500 tons (i.e., the pile weights are a significant portion of the jacket weight, in this case, nearly a quarter of the structure weight). For small structures in shallow water, the pile weight may approach the weight of the jacket structure. The need for heavy hammers to drive large jacket piles has contributed to the development of semisubmersible heavy lift crane barges. Hydraulic hammers that operate underwater have superseded the early steam-driven hammers. A modern, high-energy hammer for 8-ft-diameter piles typically weighs around 160 tons.
Because the cost of piling is substantial, an alternative concept that has been developed is the suction pile, or bucket foundation, because its visual appearance is one of an inverted bucket. The suction pile is forced into the soil by the pressure difference over the bottom of the bucket as water is pumped out from within the bucket. While suction piles were originally developed to provide anchor points for single-point moorings, their use for jacket structures offshore Norway (Europipe 16/11E and Sleipner Vest) has encouraged their wider acceptance. The suction pile essentially comprises a plate, usually circular, and is surrounded and reinforced by a skirt. The Sleipner suction pile, for example, has a 45-ft-diameter plate with a 16-ft skirt depth.
Installation by lifting or launching
Without exception, the construction and installation of a jacket structure plays a central role in its design. Steel jacket structures are prefabricated onshore prior to transportation to site by a barge. Although jacket structures can be, and have been, designed as self-floating for transportation (with subsequent systematic flooding for installation), the most popular installation methods are lifting or launching. Small jackets may be lifted in place by a floating crane vessel. Larger jackets may require flotation devices to assist in their installation. The flotation devices are subsequently flooded to enable the jacket to sink slowly into its final resting place.
A typical sequence of steps involved in the installation of a jacket structure by launching is shown in Fig. 3. A jacket structure being launched is shown in Fig. 4 and is consistent with Step 3 in Fig. 3. The jacket structure is placed horizontally on a flat-topped barge and towed to site. At location, the jacket is launched off the barge, uprighted using a crane vessel, and allowed to sink vertically to the seabed. Once located on the seabed, foundation piles secure the structure. An examination of the steps in Fig. 3 reveals that a number of critical factors must be considered as part of the jacket design process:
- Weight of the jacket structure has to be less than the safe lift capacity at the construction facility if the jacket is lifted onto the barge rather than skidded.
- Jacket weight cannot exceed the capacity of the tow-out barge.
- Jacket has to be designed to withstand the loads involved during tow-out, transportation, and launching.
- Jacket has to be designed to float unassisted in the water following launch.
- Jacket is designed to be uprighted by a crane vessel, then sunk to the seabed with systematic flooding.
An alternative to launching the structure is to lift it in position. Today, vessels with a lift capacity up to 14,000 tons exist, allowing most jacket structures to be lift-installed in a cost-effective manner. Use of twin cranes on a single vessel or use of two crane vessels is often deployed to provide the required lifting capability. Fig. 5 shows a jacket structure being lift-installed using two crane vessels. Once the jacket is secured with its foundation system, the topsides structure can be installed as separate modules or as a single integrated unit; see Fig. 6.
Standards and guidelines
The benchmark design guidelines and standard for fixed steel structures is the American Petroleum Institute (API) RP2A, which was first published in 1969. In 2000, the 21st edition of API RP 2A was released. This edition reflects good engineering practice, knowledge, and experience gained by the industry over the past five decades. Equivalent practices exist in other countries such as the U.K. and Norway. In following the development of the technology related to fixed steel structures and the historic achievements, two additional observations are worthy of note.
The first observation relates to the industry’s ability to manage and maintain the structural integrity of existing installations during the service life of the platforms. In this regard, in the 1980s, Amoco pioneered the “assessment engineering” approach for its fleet of platforms in the UK sector of the North Sea. Today, this approach is embraced by the industry worldwide and is reflected in recommended practices, design codes, and standards. The assessment engineering approach is integral to the integrity management of offshore installations. The process, illustrated in Fig. 7, ensures the cost-effective life-cycle management of offshore structures.
Employing lessons learned
The second observation reflects the industry’s desire to put into practice the lessons learned from the offshore developments that have taken place worldwide. In the 1990s in particular, the industry undertook a “self-assessment,” particularly in light of the large number of undeveloped marginal fields in existence worldwide. This led to the concept of minimum facility platforms (MFPs), which aim to restrict the equipment and facilities on topside structures to the minimum required for production, minimize or eliminate the need for manned installations in light of improved flow-assurance technology, and minimize substructure arrangements without compromising robustness or safety.
A large number of novel and innovative MFP concepts exists at the present time, and sophisticated platform selection tools have been created to permit the most appropriate development options to be considered. Shell’s Skiff and Brigantine structures in the southern North Sea, for example, were designed to be installed using a jack-up rig with conductors placed through the legs and exploited as part of the foundation system. These platforms were also designed for sea access rather than helicopter access, thereby reducing the topsides facilities. The use of MFPs is set to grow over the next decade. The big prize for the industry is to develop MFPs that are self-installable, thereby removing the need for installation vessels.
Concrete gravity based offshore structures
While the vast majority of fixed offshore structures utilize steel-jacket substructures to support the topsides facilities, a number of offshore installations utilize a substructure manufactured from reinforced concrete. As the name implies, concrete gravity-based structures rely on their own weight to resist the lateral environmental loads. These types of structures were pioneered by Norway, consistent with their design expertise, construction facilities, and construction skills that leaned heavily toward concrete rather than steel. Norway’s fjords provided the ideal sites to permit construction of these large substructures. Further, these substructures provide the advantageous option of using the gravity base for oil storage. Fig. 8 shows an illustration of a concrete gravity-based structure (GBS). The topsides structure is similar to that for steel-jacket structures (i.e., it is either an integrated steel-deck configuration or is of modular construction with a module support frame). GBSs are constructed with reinforced concrete and consist of a cellular base surrounding several unbraced columns that extend upward from the base to support the topsides superstructure above the water surface.
An offshore GBS is large. Its size, and the large environmental forces, can cause design problems. The structural design requirements include the categories of material quality, strength, and serviceability. Most GBSs are designed for several functions, namely combined drilling, production, and oil storage. The design is targeted to offer least resistance to environmental loads while providing adequate support for the topsides structure. Typically, using a range of standards, the structure is designed to meet the criteria laid down for the ultimate progressive collapse, fatigue, and serviceability limit states. Prestressed concrete, as used for GBSs, provides good resistance to fatigue and corrosion. Prestressing is essential, because it permits the concrete to act in compression at all times.
Temporary loading conditions may very well govern the structural design. These temporary cases include:
- Construction in dry dock
- Construction in protected harbor
- Ballasting for deck installation
Often, the cellular base walls are not pressurized; consequently, they must be designed to resist the substantial hydrostatic pressure imposed during immersion. Coincidentally, the ballasting of the GBS for deck installation prior to towing to site is often regarded as an effective, full-scale, inshore pressure test prior to offshore installation.
From a geotechnical standpoint, the parameters considered in foundation design include the type/extent of contact between platform base and seabed; stability against overturning and sliding; skirt penetration and grouting, to provide lateral sliding resistance and protection from scour; settlement; effects of cyclic loading on the soil; and soil-structure interaction.
The foundation aspects are considered individually and collectively as part of the design process for a GBS. The design and construction of the huge concrete gravity base structures represent a remarkable achievement by the offshore industry. However, the development of fields in the future using this technology may be limited, primarily in light of the trend toward subsea wells with long tie-backs and floating production systems.
Construction and installation
The construction and installation of GBSs is entirely different from that employed for jacket structures. Fig. 9 shows a typical set of construction and installation steps for a GBS. As illustrated, the concrete bottom structure is constructed in dry dock. Once constructed, this is floated out and moored in a deepwater protected harbor. Completion by slip-forming of all the cellular base walls is undertaken in the harbor, followed by slip-forming of the towers in a continuous process; see Fig. 10. Once the towers are constructed and topped off, the whole structure is ballasted down to receive the topsides deck and modules. The completed platform is de-ballasted to a minimum draft for towing and is towed using tug boats to its final location and ballasted onto the seabed. The ballasting permits the skirts to penetrate into the seabed. Grouting is undertaken to fill any voids under the base. It can be observed that offshore hookup is minimized because most of the topsides equipment and facilities are commissioned onshore prior to placement on the deck.
- Lee, G.C. 1982. Design and Construction of Deep Water Jacket Platforms. In Behavior of Offshore Structures. Cambridge, Massachusetts: MIT.
- API RP2A, Recommended Practice for Planning, Designing, and Constructing Fixed Offshore Platforms, 21st edition. 2000. Washington, DC: API.
- Hard, C. 2001. Spoilt for Choice: How to Classify and Select Minimum Facility Solutions. Paper presented at the 2001 Conference on Minimal Offshore Facilities of the Future, League City, Texas, 9–11 October.
- Minimal Offshore Facilities of the Future. 2001. League City, Texas: PennWell Conferences and Exhibitions.
- Shell U.K. Exploration and Production: Design Report for Brigantine BG, report No. C239R003 Rev 1. 2000. Surrey, UK: MSL Engineering Ltd.
- O’Connor, P., Defranco, S., and Manley, B. 1999. Minimal Structures Open Global Production Opportunities. Offshore Magazine (January).
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