Offshore Semi-Submersible Platform Engineering

Srinivasan Chandrasekaran

• Publisher: CRC Press
• Year: 2020
• ISBN:
  9781000292992
• Edition: 1
• Topic: Biowissenschaften
• Subtopic: Ökosysteme & Lebensräume

Offshore Semi-Submersible Platform Engineering presents a primer on the analysis and design of semi-submersible platforms, in particular, while also covering general analysis and design guidelines of offshore compliant platforms. It introduces general structural designs and also examines the details of the various environmental impacts that act upon them, such as fatigue, fire, collisions, and water waves.

Features

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• Provides thorough coverage of the dynamic analysis and design of semi-submersible platforms

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• Assists readers through detailed analysis methods using MATLAB® as well as other computer programs used to carry out structural analysis

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• Explains impact loading and dynamic response through numerical analysis and examines the various factors that affect semi-submersibles

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• Presented in a coursework teaching style, the content is explained in a step-by-step manner using color figures, photos, screen shots, and illustrations, thereby enabling students, researchers, and practicing engineers to carry out analysis with ease

Offshore Semi-Submersible Platform Engineering serves as a practical guide for upper-level students and graduates of various engineering disciplines, for example, naval architecture, and structural, mechanical, pipeline, and offshore engineering. Further, it can also be used as a reference for practicing professionals, as the book covers a broad range of scholarships and applications.

1 Introduction

1.1 Ocean Environment

The ocean environment is highly complex as it generates various types of loads upon the structures constructed in the sea. However, wave-structure interaction, which dominates the geometric design of offshore structures, is a well-known and clearly understood phenomenon. Wave-structure interaction has been successfully taken into consideration in the design principles of ships, and then later extended to the design of offshore structures. While conventional design of structures deals with gravity loads in static design and lateral loads in the dynamic model, load combinations that act on offshore structures are complex and novel. A large variety of environmental loads that act on offshore structures include wave loads, wind loads, current, ice loads, and impact loads. Several complexities that are present in the ocean environment make these load computations highly mathematical and uncertain. As geographical phenomena control the amplitude and period of these loads, say, for example, wave loads, they are estimated using different idealized theories and empirical relationships.

1.2 Environmental Loads

Environmental loads continuously interact with offshore structures in the form of waves, wind, current, or earthquakes. Ocean structures are repeatedly loaded, and these cyclic loads are capable of inducing fatigue damage to the structure over time. Among all these loads, two that are of primary concern are wave and wind loads; the former is a high-frequency phenomenon in comparison with the latter. Offshore structures are also prone to seismic loads when located near tectonic zones (Sigurdsson, 1988). Unlike the conventional design procedures, loads that act on offshore structures should be assessed in challenging the strength of the structural members (Chandrasekaran and Ajesh, 2019); many of the loads pose a challenge to the structural stability. This is because they act while the structure is in a floating condition. Most of the compliant structures have inherent stability only through their geometry and not by the material and member strength.

1.2.1 Wave Loads

The most important of all environmental loads is the wave load. Waves play a critical role in the design of offshore compliant structures due to the complications involved in the hydrodynamic behavior of the platform in open sea conditions. Wave analysis can be performed either by the design wave concept or the statistical approach. In the design wave concept, a regular wave is defined using the wave height (H) and the wave period (T). The wave forces are then calculated using the appropriate wave theories. Water particle kinematics are calculated as a function of sea-surface elevation using the potential theory; waves are assumed to be long-crested. Various wave theories are developed to include a wide range of wave parameters; the most common theory, which is widely used, is Airy’s linear wave theory. In the statistical approach, random waves are generated using the appropriate wave spectra, which is site-specific. The most probable maximum wave force is then computed using the linear wave theory. A statistical approach is highly essential to accurately assess the dynamic behavior and the fatigue strength of the structure. It helps arrive at a response spectrum that defines the maximum expected response within a particular interval of time.

The motion of the water particles induces wave loads on the offshore structures, which, in turn, results in their dynamic behavior. Wave loads on offshore structural members are computed based on the size of the structural members encountering the wave loads. Slender members with a diameter (D) to wavelength (L) ratio of less than 0.2 will not influence the wavefield; therefore, wave loads can be calculated using Morison’s equation. But, large-diameter members interfere with the wavefield; in such cases, wave loads are calculated using diffraction theory. Generally, ocean waves are random but can be represented as a regular wave described by a deterministic approach.

As the waveform in each cycle in a regular wave is the same, wave theories describe the characteristics of a typical cycle; it remains invariant for other cycles. Two significant parameters are the period (T) and height (H) of the waves and water depth (d). A sample time history of a regular wave is shown in Figure 1.1.

Figure 1.1 Typical regular wave (2 m, 5 s).

Based on the structural waveform, waves can be classified as regular waves and irregular or random waves. Initially, the transfer of energy causes capillary waves, which grow to form irregular waves with different amplitudes and periods depending upon the wind speed and direction. If the wind blowing over the ocean surface has a constant wind velocity, then the generated irregular wavelets will grow to a fully developed sea with constant wave amplitude and period. Such waves are referred to as regular waves, and these waves exhibit a sinusoidal motion. Because the actual wave formation is complex, the following mathematical formulations are always used to model the ocean surface with regular waves.

1. Airy’s two-dimensional small-amplitude linear wave theory

2. Stokes theory

3. Solitary wave theory

4. Cnoidal theory

5. Stream function theory

Airy’s theory, which assumes linearity between the wave height and kinematic quantities, is commonly used in the theories listed above. The regular waves are usually defined by their wave height (H) and wave period (T), as shown in Figure 1.2. For determining the wave forces on offshore structures, the wave surface profile should be idealized. Any one of the above mentioned appropriate wave theories should be used to compute the water particle kinematics. The applicability of wave theory is based on a wide range of parameters, including the wave height, water depth, and wave period. It is not always possible to select the wave theory precisely suitable for the selected condition. Airy’s small-amplitude linear wave theory is valid for deep water conditions where (d/gT²) > 0.8, and Stokes theory should be used when (H/gT²) > 0.04.

Figure 1.2 Wave parameters.

The most straightforward wave theory is​​​​​ Airy’s linear wave theory, or small-amplitude wave theory. According to this theory, the waveform has a sinusoidal profile. This theory also provides the kinematic and dynamic amplitudes as a linear function of wave amplitude or wave height. Thus, the normalized amplitude value is unique and invariant to the wave amplitude. It helps to represent the response of the offshore structures as a normalized value. The normalized responses, as a function of wave height, are called the transfer function or the Response Amplitude Operator (RAO). This method is simple and predicts the extreme response of the structures.

However, in reality, ocean waves are a combination of a set of waves with different frequencies and directions; they appear as irregular or random waves (Chandrasekaran and Anubhab, 2004; 2005). Random waves are represented by wave energy density spectra, which describe the ocean wave’s energy content. It is spread over a wide frequency ranging from zero to infinite value, but waves are found to be concentrated on a narrow band. Their statistical parameters characterize random waves. Sea states are represented by the significant wave height (H_s) and zero-crossing periods (T_z). Different sea states used to describe random waves are summarized in Table 1.1.

Table 1.1
Characteristics of random sea states

Sea state description	Significant wave height Hs (m)	Zero-crossing period Tz (s)	Wind velocity (m/s)
Moderate	6.5	8.15	15
High	10	10	35
Very high	15	15	45

Several spectral models are available for use in the design of offshore structures. These models are derived based on the observed properties of the ocean and are empirical. The frequency characteristics of the real sea conditions influence the spectral formulation. The most commonly used spectral formulas include the P-M spectrum...