Jacket structures are the base for fixed offshore platforms used to extract hydrocarbons from seas and oceans. A jacket consists of a self-supporting skeletal tower comprising leg members and bracing. The structure is secured to the sea floor by piles extending through guide sleeves.
Different corrosion zones affect the jacket module differently. For accurate assessment, a proper method should be employed to determine the base shear capacity.
Structural analysis is an important part of the design process. It involves modeling the structure and analyzing forces to determine stresses, buckling, and displacements. This is done using structural software. The analysis can be performed either linearly or nonlinearly. In the latter, nonlinear dynamics and fluid-structure interaction are incorporated, and plasticity and kinematic strain hardening effects are taken into account.
A sensitivity study is also conducted to evaluate the effects of different parameters on the structural performance. This helps in identifying the critical parameters and limiting the design scope. Finally, a life cycle cost (LCC) analysis is done to estimate the expected cost of the platform over its lifespan.
The structural analysis also considers the actions of the jacket structure during its transportation from the fabrication yard to the installation site and during sea transportation, mating, installation, and hook-up. The design should be capable of surviving these accidental loads as well as the environmental and functional loads.
The geotechnical analysis involves the evaluation of soil conditions at the proposed location of the structure. This is usually accomplished by drilling and sampling a core of soil. The data obtained are used to determine the soil resistance developed on the jacket leg extensions and lowest level of horizontal bracing.
In addition to linear static analysis, a nonlinear dynamic analysis is normally carried out. This includes fluid-structure interaction, large displacements, plasticity and kinematic strain hardening effects. It also considers pile-to-jacket leg annulus interaction.
Moreover, a modal analysis is also done to capture the natural frequencies of the structure. Lastly, time history dynamic analysis is performed to increase the accuracy of the results. This involves considering the axial force and flexural anchor forces of the jacket elements as random variables over a critical direction. The damage index is then determined based on the performance function of tension and compression. The results are then used to perform a structural check.
Design of Pile Foundations
When a jacket platform is constructed, the geotechnical engineer explores soil conditions at the location of the foundation by drilling and sampling. This information is used to provide a basis for the structural design of the foundation elements.
The design of pile foundations is important in jacket platforms because they transfer the loads of the structure to the underlying soil or rock layers. These are usually weaker than the structure above, so it is necessary to have strong support underneath. There are several types of pile foundations that can be used, including end-bearing piles and friction piles.
Energy piles are designed to be driven into a soil layer with a high bearing capacity, such as the transition layer between a hard and soft stratum. Their shape is typically a long shaft with a rough surface to increase frictional resistance. Driven piles are installed by striking them repeatedly with a hammer, while bored and cast-in-place piles are created by drilling and pouring concrete while gradually withdrawing the drill tool.
Design of Jacket Legs
After fabrication of jackets, they are loaded onto barges or a heavy lift ship and transported to site. They can then be skidded or lifted upright into saddles for the final positioning of the legs on the seabed. Bracing members are then welded to the legs and scaffolding is used for access to the top interfaces.
During installation, a berthing dolphin can be made integral with the jacket to minimize the amount of piling on site. This can also help with buoyancy control of the jacket during its launch sequence as it reduces the required ballast load. A rubber diaphragm closure is usually incorporated in the jacket leg to prevent water entering the annulus between the pile and the jacket leg during jacket flooding operations.
Three-legged jackets have the advantage that they have fewer piles to drive into the seabed, so noise and environmental impact is minimised. However, they are not as stable as four-legged jackets and could collapse if one of the legs fails. Therefore, they require more extensive structural analysis than four-legged jackets.