Jacket refers in the oil and gas exploration and production to the steel frame supporting the deck and the topsides in a fixed offshore platform (https://www.2b1stconsulting.com/jacket/).
Basic Performance and Characteristics
There are multiple types of offshore platforms depending on the applications and the depth of the water. Fixed platforms are generally used for explorations at shallow water production. Jacket platform, one of the fixed platforms which are often used for production platform, has good adaptability, high safety, and high reliability. It also has the advantage of simple structure and low cost and has become the primary structure form in the exploitation of offshore oil and gas resources for more than half a century. The steel jacket platform is fixed to the seafloor by means of piling. Since its first use in the 6-m-deep waters of the Gulf of Mexico in 1947, the steel jacket platform has been rapidly developed. By 1978, its depth had reached 312 m. So far, there are about 2,000 large- and medium-sized jacket-type offshore platforms built in the world, and the working water depth has reached nearly 500 m.
Jacket Platform Development History
The world’s first fixed offshore platform was built in 1887; it was installed in one oil field in California, USA. But it is a wooden trestle structure. After World War II, many advanced scientific and technological achievements were put into use for marine and offshore development. In 1947, the world’s first real steel jacket platform was successfully installed at a depth of 6 m in the Gulf of Mexico, creating a new era of offshore development. Since then, the offshore platform has been rapidly developed, and jacket platform has become the dominating structure of the offshore platforms in the middle shallow sea. At the end of the 1970s, steel jacket platforms were installed in the sea site with more than 300 m. By 1990, a huge jacket platform with a height of 486 m was successfully installed at the site with water depth of more than 400 m in the Gulf of Mexico. Jacket platform has gradually expanded to deeper water area and harsher marine environments over the past years. Jacket platforms are mainly designed and built for exploration and exploitation of marine resources, mainly for oil and natural gas resources. So far, there are about 2,000 large- and medium-sized jacket-type offshore platforms existing in the world, and the working depth has reached to 500 m.
Features of Jacket Platform Structure
There are many types of steel jacket offshore platforms, and the cost-effect determines the choice of platform type, for example, typically in the deepwater area, using all fully functional, self-contained monolithic platforms. In shallow waters, multiple separate platforms with different functions are mostly welcomed, for example, supply platform, drilling platform, production platform, living platform, auxiliary platform, etc.
The jacket platform has three main structural parts including topside module, jacket, and pile foundation.
The topside module, also called deck structure, includes platform decks, bulkheads, deck pillars, and truss structures, of which primary function is to provide enough space for drilling or production at sea to deploy drilling or recovery equipment, auxiliaries, various kinds of living equipment, and helicopter lift. The topside module includes two kinds of structural forms, namely, the monolithic and block structure.
All deck structures are in principle composed of three-dimensional steel components. The main load-bearing components are divided into three forms, plate girder, box girder, and truss. Plate girders and box girders consist of a pallet. The girders that make up the truss are tubular or profiled (I-beams, channel.). In the case of monolithic structures, the equipment is installed after the construction of the structure; in the block construction, the deck foundation is constructed in prior, and then the module is transferred and fixed on the jacket structure and foundation.
Jacket is the supporting structure of the jacket platform. Jacket structure is a steel or steel welded frame and is actually a three-dimensional flat panel rack or plane truss consisting of a three-dimensional space truss structure. These trusses are mainly composed of large-diameter pipe piles (chords), small-diameter pipe, small-sized profiles, and horizontal, vertical, and diagonal strut composition. Tubular node structures are formed at the intersection of the tubular members. General tube nodes are chord (namely, larger diameter tube member in the tube node) wall thickening or other measures to be strengthened. The main role of the jacket structure is to support the topside module and the platform for offshore installation and construction. The jacket’s legs are as piling positioning and orientation. In addition, jacket can also be used to connect the ship, in order to facilitate the supply boat off the platform.
Deepwater Jacket Platform
Main Challenges in Jacket Platform Application
Ductile collapse. With the increasing of height of jacket platform, ductile collapse has becoming one of its major failure modes. Because the complexity of large jacket platform, its ductility in different directions is versatile. In the direction with high collapse redundancy, more loads will transfer through this path, and more structural components can effectively bear the load and thus improve the anti-collapse ability of the platform.
Ultimate strength. Ultimate strength is one critical issue when designing and constructing a jacket platform. Under severe sea states, wave, wind, and current will induce gigantic loads on the jacket platform. The ultimate load-bearing capability needs to be estimated accurately during the design stage. The calculation of ultimate strength often includes the consideration of plastic-elastic characters of the steel material of the jacket platform. In addition, another issue is also often considered, post-collapse strength, which fully considers the material plasticity to assess the strength of the jacket platform after collapse happens.
Foundation failure. Failures of pile foundation and the ground foundation have become one of the main control factors of the ultimate bearing capacity of the platform. When the platform reaches the limit state, the double plastic hinge is formed on the upper part of the pile leg, resulting in the cause of great reduction of ultimate bearing capacity of the whole platform.
Wave slamming. The design of the platform should ensure that the deck air gap has sufficient safety margin. When the wave reaches a certain height, the jacket and the upper block connection unit have to endure the wave slamming effect and can be plastically deformed. When the plastic deformation expands to form a plastic hinge, it will cause the upper part of the platform to roll over and collapse, which is consistent with the observed platform failure mode after typhoon. Therefore, it is necessary to consider the effect of the wave load on the deck in the safety assessment of the existing offshore platform (especially in the case of serious settlement).