Offshore petroleum rigs/platforms: An overview of analysis, design, construction and installation

Jacket platforms are one of the most important and regularly used types of offshore structures for oil and gas extraction that have a big impact on the economy of the countries. In this paper, all aspects including design, analysis, construction and installing of the jacket type offshore structure, are summarized and classified. This type of structure is one of the specified platforms for shallow water, and for long term service, it also has the ability to carry large deck loads. This paper aims to present general guidance about the planning, design and construction of offshore jacket (template) platforms. Jacket platforms are fixed type platforms which are attached to the seabed using piles which provide stability against the wind, wave and current loads. Also, this type of offshore platform has a high initial and maintenance cost because of its exposure to corrosion, and cannot be reused after the end of its service period. Jacket platforms are most suitable for shallow water having no better alternative while it has the cost disadvantage for deep water.


INTRODUCTION
Offshore structures play a significant role in the economy of many countries, as offshore structures are mainly drilling platformers which used to extract oil and gas from the bottom of the seabed. This study gives an overview of the offshore jacket platform's characteristics, and investigate the stage of its design, construction and installation. "Jacket type platforms are one of the most important and common types of offshore structures, and it plays an important role in drilling and exploring the oil" (Zhang et al., 2012).
Fixed platforms mainly are used in the Persian Gulf, Caspian Sea, North Sea, Gulf of Mexico, and Alaska (Nouban, 2016). Where the water is shallow, the use of this type of platform is more compatible and economical to the use in depths generally ranging between 10 m to 200 m (Xiaojie et al., 2019). Also, Sadeghi (2008) gives a detailed guide for the design and construction of offshore platforms while focusing on the jacket type platforms.

Fixed platforms
This type of platform is attached and fixed to the seabed and have a very limited movement, whereas stability is one of its main advantages (Nouban, 2016). Also, there are many types of fixed platforms. For example, there are tower platforms and gravity platforms, in addition to jacket platforms. However, those platforms cannot be used in the deep water level because of the high risks, from the resulting deep-sea currents which made it uneconomical choice. The other types of fixed platforms like tension leg platforms can be used in deep water level up to 1500 m (Sadeghi & Tozan, 2018).

Moveable platforms
This type of platforms can change its location and move from one place to another. This category includes other types of platforms like Jack-up, Semi-submersible platform and floating drilling ship.

Floating production system (FPS)
Floating production system is a kind of movable platform. This type of platform requires dynamic positioning. The system stabilizes and ensures its location directly above the drilling site using electronic motors and sensors that sense the waves and currents' directions. It then pushes the platform in opposite directions. Also, this type of platform can be used in shallow water between 70 -250 m, plus has many advantages like low cost and reusability.

JACKET PLATFORM
Jacket platform is a kind of offshore structure, made of lots of steel cylindrical members with different diameter arranged in an oblique condition relative to the seabed (Zhang et al., 2006). Table 1 summarizes the advantage and the disadvantage of the jacket platform.

Prices
The weight of the steel and concrete offshore structure increases exponentially with the waterdepth, and this results in a high increase in costs (Reddy & Swamidas, 2014) as

Planning
A lot of factors must be taken into consideration when planning to construct a jacket platform like the location of the project, where selecting the best site holds great importance and affect the project in many aspects. Besides, the environmental conditions must be considered when planning the project, for example, the waterdepth, seismic conditions, wind directions, ice loads, current and weather. Also, "design, calculate, construct, and install coastal, port, marine, and offshore structures, environmental data are needed, in particular wave and wind data." (Sadeghi, 2007). It is difficult to start and construct the project in seasons that have bad weather like storms. However, the site location can be limited to the location of the oil and gas fields, so it is important to consider that in planning.

Analysis and design
There is a lot of required analysis for the Jacket type offshore platform where the main required analyses are listed below. Where the analysis, design recommendations and guidance can be found in design codes like AISC. •

Pile foundations design
Offshore type jacket platform is fixed to the seabed using long piles, with a minimum of 3 piles. The pile's depth depends on the soil type and the location of the offshore structure. However, the inclination of the piles has a big effect on the stability of the platform as the inclination is one of the main factors affecting the pile's behaviour where it was found that the optimal degree of inclination is about 5 degrees, and the by increasing it the instability of the soil surrounding it start to grow significantly in a wild scale (Aminfar et al., 2016).

Loads considered in the design
The offshore structure faces extreme marine environmental conditions in the sea with different types of unusual loads that land-structures do not face it. Therefore, it is a challenging task to analyse and design where the offshore structures can face a combination of wind, current and wave loads in addition to its self-white and live load at the same time. For that its crucial to have a very good and reliable design with high safety factors for a long service life period. As in this section, we will discuss the load's types that the jacket type offshore structure is exposed too.

Dead and live load
The dead load considered in design is the self-weight of the jacket platform and all permanent loads on top of it like the drilling rigs, oil and gas reservoirs and other excavation equipment. While as the live loads are the workers who stay on top of it.

Wind force
The wind force is one of the loads taken under consideration in the design as it may play a critical point and affect the structural stability where it is very critical in the marine environment.

Wave impact
Wave is one of the environmental conditions considered in analysis and design, and there are many methods used to get the waves data like using devices, simulation models and data collected from remote sensing by satellites. Additionally, many equations can be used in the analysis of waves like Brettschneider equations and spectrum wave to predict the characteristics of waves (Sadeghi, 2007) as spectral height and peak spectral period. These characteristics can be used with waterdepth data to determine the condition of waterdepths: shallow, transitional, or deep, and to determine the appropriate wave theory. https://www.londontechpress.co.uk

Current Load
Marine currents have the most influence and are the dominate load considered in the design, as most of the structure is located under the sea. This results in the push and drag force. Moreover, one of the common phenomena is the wave-current interaction, where the current has a big effect on the generation of waves and their direction (Zeng et al., 2019).

Earthquakes
The earthquake analysis of offshore structures has a significant difference from the onshore structures, and "the procedure must be modified to consider the fluid-structure and the soil-pile interaction effects" (Smith, 1996). Jacket offshore structures are directly attached and fixed to the seabed by piles, as the response of offshore platforms is completely dependent on the foundation (Asgarian & Agheshlui, 2009). Therefore, earthquake design is essential for the jacket platform. In contrast, the earthquake analysis is based on the historical records of the site and the location of it. Earthquake analysis is very important and must be considered with the effects of other loads because the maximum displacement of the load combination is more than the displacement of earthquake load alone (Bargi et al., 2011).

Materials used in the design
The main martial used to make jacket platforms is steel. Different grades of steel can be chosen (according to the design code) representing the main body and the foundation. Steel is used in a very high amount which increases the costs dramatically. The steel materials with the limitations are specified in the American Petroleum Institute (API). Additionally, other materials are used for corrosion protection like magnesium, aluminium and zinc.

Construction and fabrication
The fabrication and composition of the jacket platform are done onshore before transporting it to the site for installation. The main body is made of different diameter of cylindrical beams and column which are welded together. Also, bracings system is used to increase the stability and against earthquake as well as dampers.

Loud out, transportation and launching
Jacket platforms are usually manufactured onshore in a fabrication yard where it is usually slipped onto a cargo barge then transported to the installation site to be launched. A lot of consideration should be taken during design to avoid any uncontrolled motions during the launching process. The launching and upending of the platform are very critical parts of the project because it involves 40% of the total cost (Omdehghiasi et al., 2018).

Installation
The insulation process exposes the structure to a different type of loads that should be considered in the design. The structural sections should resist different types of launching, upending, up-righting and other installation stresses. The first step of the process is to place the mudmat steel plates on the seabed which designed to help in fixing the pile's angle, which will be hammered into the soil. The piles are divided into several sections, where the first will be hammered into the soil then the main body will be installed on it. The other sections will be welded together and hammered consecutively. In short "It consists of positioning and levelling the platform on the site and assembling the various components into a stable structure in accordance with the design drawings and specifications (Muyiwa & Sadeghi, 2007).

Corrosion protection
Durability is an essential issue in the offshore environment. Like every steel structure, jacket platform is exposed to corrosion which can further increase more in the marine environment due to the availability of factors like high humidity and sea salt that increase the electric conductivity of the seawater. Therefore, it is essential to use some corrosion resistance methods like cathodic protection by covering the steel surface by other metals like magnesium, aluminium or zinc. Those anodic metals will work as an alternative source of conductors and sacrifice themselves instead of the active part of the steel. However, in this method, the anode metal needs to be checked and maintained regularly.

CONCLUSIONS
In this paper, all aspects related to the construction of the jacket type offshore platform are summarized. It includes planning, analysing, designing, fabricating and integrating, transportation and insulation of the platform. Jacket platforms are wildly used in several applications like sea wind turbines, oil and gas, etc. However, our paper focuses on the drilling rigs for oil and gas platforms. It can be concluded that Jacket platforms have no better alternative in shallow water, having mare benefits and lower costs, while, in deep water is the opposite.