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Valve and Actuator Technology for the Offshore Industry
Published in Karan Sotoodeh, Coating Application for Piping, Valves and Actuators in Offshore Oil and Gas Industry, 2023
The fourth type of platform is semi-submersible, which is a special type of platform with legs or pontoons that provide buoyancy for the platform to float, and weight to keep the structure upright. Columns in the platform are used to provide stability, and pontoons connect the columns together. This type of platform, illustrated in Figure 5.6, can be used for multiple purposes, such as offshore drilling rigs and oil production. It is noticeable that the old semi-submersible platforms could have multiple purposes, but the newly designed, semi-submersible platforms are specifically designed for a unique purpose. As illustrated in the figure, this type of platform has large columns that are connected to large pontoons. A semi-submersible platform has some advantages, such as providing a large deck, exceptional stability on the sea and ocean, the possibility of being moved from one location to another, etc. This type of platform can be used for oil field development as deep as almost 3,000 m. All facilities, including valves, actuators and piping systems, are located topside on the top or deck of the jacket, as illustrated in Figure 5.6.
Semi-Submersibles
Published in Srinivasan Chandrasekaran, Offshore Semi-Submersible Platform Engineering, 2020
Semi-submersibles also possess superior construction and installation features and the convenience of being able to be towed to different locations (Chandrasekaran and Uddin, 2020; Odijie, 2017; Zhu and Ou, 2011). Since the 1980s, there has been extensive research done on the improvement of the hull-design of the semi-submersibles for obtaining a better response under combined environmental loadings. Semi-submersibles are specialized marine vessels, used for various operations as exploratory drilling rigs, safety vessels, oil production platforms, and crane vessels. Unlike fixed platforms that derive their strength from member and material strength, semi-submersibles derive their strength from being highly compliant in the horizontal plane. For any floating structure, all the six-degrees-of-freedom remain active under the action of waves. There are translational degrees-of-freedom along the X, Y, and Z axes, which correspond respectively, to surge, sway, and heave, respectively; rotation about each of these axes are termed roll, pitch, and yaw, respectively. Out of these six degrees-of-freedom, natural periods in surge, sway and yaw motion of a semi-submersible are distinctly different and larger than heave, roll and pitch. Semi-submersibles need to be highly versatile. In the horizontal plane, they need to be flexible, while in the vertical plane, they need to behave stiffly due to their large buoyancy.
Deep Sea Development
Published in Shashi Shekhar Prasad Singh, Jatin R. Agarwal, Nag Mani, Offshore Operations and Engineering, 2019
Shashi Shekhar Prasad Singh, Jatin R. Agarwal, Nag Mani
Beyond 6,000 ft, TLPs become impractical because of the amount of steel needed for the tendons that moor the platform to the ocean bottom. Spars have virtually no water depth limit and are designed specifically as a dry tree unit. However, their size is limited by their cylindrical hulls, which constrain the available deck space, thereby pushing designers to stack the decks vertically. A conventional semi-submersible platform offers the optimum amount of deck space for safer operations and payload flexibility that a spar cannot, however, it has too much vertical motion for a dry tree to operate safely. Several semi-submersibles operate in depths exceeding 6,000 ft; however, all of them use subsea trees. Unlike subsea trees that are installed at the seabed and deliver hydrocarbons to a surface platform through a flexible production riser or a metallic riser such as a steel catenary riser (SCR), a dry tree uses a rigid riser system known as a top-tensioned riser (TTR) locked onto the subsea wellhead at the seafloor and to the dry tree at the platform deck, thereby making it very sensitive to movement. Although the DTS hull is designed to reduce motion caused by ocean forces, it is the motions that move the platform up and down that are of the highest concern. To compensate for excessive vertical motion current, DTS concepts are relying on proven riser tensioner technology [19] (Figures 6.46 and 6.47).
Numerical simulation and experimental study on perforated heave plate of a DeepCwind floating wind turbine platform
Published in Ships and Offshore Structures, 2023
Wei Wang, Cheng Zhao, Panpan Jia, Zhiqiang Lu, Yonghe Xie
Drawing from the design classifications of floating offshore platforms in the oil and gas industry, floating wind turbines can also be categorised into four main types: spar-buoy, tension-leg platform (TLP), semi-submersible (column stabilised), and pontoon-type (barge-type) (Wang et al. 2010). Compared with other types, the semi-submersible platform has the advantages of a flexible setting water depth and convenient assembly and maintenance. Some countries are limited by their geographical location and ocean conditions, and thus semi-submersible platforms have become the first choice. The semi-submersible platform has a shallow draft, and the main body of the platform is located near the free surface of the ocean. The position and stability of an offshore wind turbine are maintained through a balance of gravity, buoyancy, and catenary tension. The heave motion of a semi-submersible platform can be affected by the heave plate, amplitude of the heave and pitch can be effectively reduced, and pitching motion can be improved (Jonkman 2007).
A study of the wave impact loads on a fixed column-slab combined structure with a GPU-accelerated SPH method
Published in Ships and Offshore Structures, 2022
Gangjun Zhai, Jianyu Bao, Zhe Ma, Yefeng Yang, Hee Min Teh
With the gradual exhaustion of onshore energy, offshore oil and gas resources have been explored to a greater extent. In recent years, semi-submersible offshore platforms have been widely used in offshore energy exploitation and production as a result of the wide range of sea conditions and the strong mobility and excellent stability of these platforms. However, under complex wave conditions, extreme wave conditions such as tsunamis and storm surges will seriously affect the safety and stability of the platform. Bea et al. (1999) carried out research on wave impact on an offshore platform deck in the Gulf of Mexico; the critical maximum load that the platform could withstand was determined. Faltinsen et al. (2004) focused on the wave impact phenomenon of ship marine structures and systematically reviewed the slamming problem. In the process of a wave impacting a marine structure, wave climbing, near-field interference, and deck slamming phenomena often occur, further exacerbating the complexity of the impact process. Figure 1 shows the Borgholm Dolphin installation unit in the North Sea east of Aberdeen, which was subjected to extreme waves during a storm in January 2015. The waves climbing the columns further impact the lower part of the platform, and the instantaneous severe impact causes considerable damage to the semi-submersible offshore platform. Therefore, it is necessary to study and simulate such severe impact processes through experiments and numerical simulations in the lab.
Investigation on hydrodynamic performance of a two-module semi-submersible offshore platform
Published in Ships and Offshore Structures, 2022
Yuji Miao, Xiaoming Cheng, Jun Ding, Chao Tian, Zhengwei Zhang
The past several decades have witnessed the development of various types of offshore vessels and structures in the ocean engineering, among which the submersible platforms play an important role in offshore engineering owing to their outstanding sea keeping performances. Semi-submersibles are used for various purposes in offshore and marine engineering, such as drilling vessels, production platforms, accommodation facilities, tourist vessels and so on (Sharma et al. 2010; Odijie et al. 2017). In addition, the semi-submersible platforms can be used as the basis for on-site investigation of ocean environmental conditions, such as extreme wave conditions and wave spectra (Mas-Soler et al. 2018).