The technology used for offshore oil and gas production has always needed to be flexible and fast-developing, in order to meet the wide range of challenges that different environments can present. Overall, the most important requirement of this technology has been a working deck which is mounted on a larger structure that provides enough space for all necessary production equipment, like processing facilities to separate oil, gas, and water, as well as pumps, compressors, connections, and living space for workers on the rig.
When platform development was not so advanced, the well drilling would usually be completed before the production process began, to ensure the safety of workers. Additional equipment and accommodation would also usually be located on a separate structure for the same reason. However, as wells were constructed in ever deeper water, new types of platforms needed to be designed.
Deep-water structures are very high-cost, and it is therefore more economically viable to accommodate workers and equipment on a single platform. Coupled with improved safety practices and fire prevention, this means that is now commonplace for offshore development to be situated on just one structure.
The most fundamental requirement for offshore drilling is a platform from which the whole operation can be run. In most cases, this is done from a fixed platform, but in recent decades floating production facilities have been successfully developed, and these are becoming increasingly common. These floating production units are the most commonly used when it comes to deep-sea applications.
With fixed platforms, there are two basic types, both with a subset of variations for specific purposes. These two types are piled structures and concrete gravity structures (Figure 1). Neither has gained precedence over the other, mainly because of frequent changes in the cost of materials, equipment, and specialized labor needed to construct them, as well as changing demands from the offshore industry as to the size of the platform.
Each type has its own disadvantaged. Piled structures can be lengthy and costly to build, and during this period they are susceptible to damage from bad weather. They also lack oil storage capacity, and drilling and processing facilities and living space must be constructed only when the base structure has been fully completed. On the other hand, concrete gravity structures are more expensive to put together, and once the foundation has been put down, they are difficult to modify, meaning if the soil conditions change even slightly from those anticipated, there could be problems with the structure that are highly expensive to put right. In this article, it will describe basic details of piled offshore structure.
Piled Offshore Structures
Initial offshore structures were designed fairly simply, using large (24 in and 30 in) diameter tubular steel piles driven into the seabed using steam hammers. These structures were primarily used in areas with a soft, muddy seabed, such as the Gulf of Mexico, Lake Maracaibo, the Gulf of Paria between Trinidad and Venezuela, the South China sea offshore Brunei and Sarawak and the Arabian Gulf. Each platform needed either four, six, or eight piles, and once the pile driving operation was completed, the platform decks were then welded on top. Although the long piles would be penetrating thick layers of mud, they were still required to be driven straight and accurately. This need led to the development of the so-called “jacket” technique of pile-driving.
In this method, a frame is created onshore which is high enough for continuous conductor pipes to be attached. These pipes are then used to drive piles from above the surface into the seabed, while the jacket rests on the bed with little bearing pressures to ensure the jacket is stable and vertical throughout the pile driving process. The jacket is taken to the offshore site on a barge, before being launched into the sea. While it will initially float horizontally, by gradually ballasting specific leg members, it can be brought into an upright position, before additional ballasting is used to fix it into the seabed. Early structures only used the jacket as a pile driving construction tool, and not as a part of the structure itself.
As water depths increased, and top loads became more heavier and heavier, the jacket needed to not only be used for construction, but also be fitted to the structure itself. This meant that engineers had to come up with a way to drive multiple piles from around the base of each jacket leg. In the picture above, up to eight piles are driven in a cluster around each jacket leg, with sufficient clearance between piles. Clusters of piles act as a group, rather than alone. It is now possible to create platforms of fifty-plus piles, and with diameters of up to 72 inches.
Figure 3 shows the pile guide of a modern jacket for an offshore platform. Steel piping is driven through these holes before being cemented into place, so as to keep the fixed structure stable. When the platform is eventually removed, the pilings are cut through, allowing the platform to be towed away. However, this method of disposal results in a huge waste of steel. The piles can be cut using explosions, diamond wire cutting, or slurry jet cutting.
Numerous variations on this original design have been created. Even in the years after its introduction, engineers were already trying out concrete piles as a substitute for the steel tubulars, although they proved to be insufficient due to their inflexibility in length. This meant that they were unusable when faced with an unexpected variation in length. Further developments included the use of higher tensile strength steels for both piles and jackets, which made significant savings in steel weight. As depths increased, the problems of fatigue and stress intensification in the joint frames has become a greater issue, and a large amount of research has gone into this area. The jacket design became so large that launch barges began to take up a significant portion of the construction budget, which led to the idea of self-floating jackets that eliminate the need for the launch barge. Several structures of this type have been used in the Gulf of Alaska, where the ability to resist impact force from large masses of drifting ice was a particular need.
The launch-type structure has remained the most popular, though, and nowadays jackets weighing over 20,000 tonnes have been prefabricated and placed successfully. These structures are able to support dead loads of up to 20,000 tonnes, roughly equivalent to a live load imposition of around 40,000 tonnes. Thanks to these enormous installations, water depths of over 125 meters have been successfully drilled, and steel platforms fitted to the seabed with piling have been constructed in water depths of over 250 meters.
Figure 4 demonstrates the typical launch sequence for a steel substructure fixed platform. The jacket is floated out to the intended location by barge, and then allowed to sink. Steel pilings are subsequently forced around 100 meters into the seabed inside the jacket. The platform itself can then be safely lifted onto the top of the sunken structure by crane, usually in multiple sections.
The Compliant Tower
Once oil and gas development started to move into deeper waters of over 1500 ft, steel platforms needed more materials, which led to a steep increase in price. Compliant towers were therefore used as a practical solution to this problem. These tall structures are built from cylindrical steel rods, and are slender in shape. They are piled into the seabed in the way as a standard steel platform. They differ in that the base covers a much smaller space, which means that the narrow base can way by as much as 15 ft in extreme weather conditions.
Compliant towers (CT) are designed so that their upper regions are buoyant and have a high mass. This means that they have a very slow response to any great force. Usually, a 10 to 15 second wave cycle passes through the frame before it can respond, akin to a water reed in a river. The Bullwinkle platform (Figure 5) below is a typical example of a CT on a large scale.
Examples of Piled Offshore Structures
The largest platform ever built is named the Bullwinkle, and is located in the Gulf of Mexico. It comes in at 412 meters high, and Figure 5 shows it was being floated out to its location. The Bullwinkle boasts a light lattice type construction, something not normally seen in platforms built for use in the North Sea. The Bullwinkle would not survive rough North Sea conditions because this structure is not strong enough.
Figure 6, on the other hand, shows three North Sea Valhall Field platforms which are joined together. Most platforms worldwide are nowhere near the size of the Bullwinkle, and are instead relatively small. Platforms in shallow water are small, which means limited deck capacity. This requires multiple platforms to be grouped together to make a full field. In Figure 6, the structure on the left is used for accommodation and as a helideck. The middle structure is used for drilling wells, and passes fluids from the wellhead to the facility on the rightmost structure, the process platform.
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