Dynamic analysis of tensile base platform under impact load

Dissertation for Master's Degree in Civil Engineering, Offshore Structures Orientation

Abstract

Tension base platforms are among economic marine platforms for use in deep waters. The correct prediction of the platform's dynamic response to environmental loads is very important in its economic and safe design. In this research, the dynamic behavior of the tension base platform under the simultaneous effect of hydrodynamic forces and impact load has been investigated. Impact loads are considered as rectangular, semi-sinusoidal, triangular and semi-triangular and at different angles to the wave direction (surge direction). The maximum amount as well as the time of application of these loads are equal and their place of application is on the side column and at different depths compared to the free surface. Hydrodynamic forces have been obtained using the modified Morrison equation, and fifth-order non-linear Stokes theory has been used to calculate the kinematics of water particles. In addition, in this research, the influence of several factors such as the dependence of the degrees of freedom, large displacements of the platform, the fluctuation of the free surface of the water and the presence of the second-order term of the velocity in the drag force, which cause the problem to become nonlinear, have been considered. To solve the non-linear equations of platform motion in the time domain, Runge-Kuta numerical method of the fourth order has been used. The response diagram of the platform in different degrees of freedom is drawn in terms of time. According to these results, the rectangular impact load has the greatest effect on increasing the response of the structure. As the impact loading angle increases, the response in the sway and roll degrees of freedom increases and decreases in other degrees of freedom. Also, if an impact load is introduced into the columns at a depth greater than the free surface, the response of the platform increases greatly in the roll and pitch degrees of freedom, while there is no change in the response of the platform in other degrees of freedom. Introduction

1-1 Reasons for the review and the overall aim of the thesis

Given the existence of enormous energy resources in the seas and oceans, the importance of discovering and exploiting these resources is evident. With the reduction of energy reserves at shallow depths, the exploration and production of oil in deep water is a big challenge in the offshore industry. New oil and gas fields have been discovered in areas with great depth. Many of these fields are small and their economic development is doubtful considering the current technology. Attempts to produce and harvest oil and gas from such fields date back to the 1970s. The use of fixed structures for such areas is extremely uneconomical. The reason for this is that the increase in water depth, followed by more unfavorable weather conditions, increases the physical dimensions of fixed structures to provide the required hardness and resistance. In such a situation, the engineering solution is to use floating platforms and another category of platforms known as adaptive platforms [1]. Adaptive platforms can be assumed as a combination of fixed and floating platforms. These types of platforms are connected to the seabed by vertical braces. As a result, the behavior of these platforms against vertical forces is similar to fixed platforms, but against lateral forces such as floating platforms, they can have a large fluctuation range [1,2]. 

Tension base platform[2], which is called TLP in short, is one of the types of adaptive platforms that have been used in many deep water fields of the world as a drilling and extraction platform for oil and gas in the last few decades. This platform is supported vertically and its support system includes several vertical steel pipes. These pipes are located in each corner of the TLP columns and extend vertically to the sea floor. In technical terms, these tubes are called tendons [3]. This platform is installed in such a way that its buoyancy force is greater than its weight, and as a result, the harnesses always remain in tension. This platform is very suitable for use in deep water and the reason for this is the high cost of using fixed platforms in these conditions. It should be noted that the displacement of the platform body and the axial stiffness of the tendons are chosen in such a way that the natural period of the translational movement and the vertical angular movement of the platform is small, or in other words lower than the dominant period of the wave [1]. Perhaps the most important advantage of the TLP platform is the restriction of the vertical movement of the platform, which is very effective in the durability as well as the maintenance and care of risers, oil wells and restraints [3].

The traction base platform must safely withstand the force caused by environmental factors such as waves, wind, and current. In addition to the mentioned forces, the force caused by the impact should also be considered in the dynamic analysis of this type of platform.

The forces caused by the impact of ships on the platform or the impact of large sea objects such as icebergs are examples of impact loads that, although the probability of their application is less than other loads, but these forces, in turn, can have a great impact on the response of the platform and overshadow its stability [4].

The current research is to investigate the dynamic behavior of the square tension foundation platform against shock loads and in the presence of regular wave loads and currents. To do this, first, the equations of motion governing the dynamic behavior of the platform are obtained. This section includes the calculation of mass and additional mass matrices, damping, stiffness and the vector of external forces acting on the object. Finally, by solving the equations of motion, the responses of the platform in different degrees of freedom are obtained, and based on these responses, the dynamic stress in the tensile foundations of the platform is also investigated.

In this thesis, the first chapter introduces the work and the purpose of the project, and the background of the research conducted in the field of TLP is stated. In the second chapter, a brief explanation is given about the types of oil platforms and their differences. In the third chapter, a brief explanation is given about environmental loads on the structure as well as types of impact loads. In the fourth chapter, the equations of motion of the traction base platform are obtained and the method of solving these equations is explained. In the fifth chapter, the effect of different characteristics of the impact load, including the type, direction, amount and location of the impact on the response of the platform has been investigated. This work has been done with a numerical study for a tensile base platform with certain characteristics. The results are analyzed and compared with the results in the technical department. At the end of this chapter, suggestions for continuing the work are presented.

1-2 Overview of the history of the activities carried out

Tensional platforms can be considered as a new generation of adaptive platforms. The initial researches in the field of knowledge of this type of platforms started in the 70s. The results of this research were seen with the construction and installation of the first traction base platform in the North Sea, and after that, a lot of research was done to predict the behavior of these platforms more accurately. Among these researches, the following can be mentioned:

Pauling and Horton [4] [5] in 1970 presented a method for predicting the displacement of the tension base platform and the forces in the platform restraints under the effect of regular waves. In their study, they assumed that the hydrodynamic force acting on the structure is equal to the sum of the forces acting on the components of the structure (principle of linear superposition). In this research, all the members of the platform were considered cylindrical and their cross section was assumed to be small compared to their length and also the wavelength. The mutual influence of connected members and adjacent members was also neglected. Expressions related to the linearized drag force as well as the effect of water level fluctuations were neglected. The results of this research were in good agreement with the results of the laboratory models and the response of the platform as well as the stress in the waves changed linearly with the wave amplitude.

Angelides [5] and his colleagues [6] in 1982 investigated the effects of body geometry, force coefficients, water depth, initial stress and stiffness of restraints on the dynamic response of TLP. The floating part of the TLP was modeled as a rigid body with six degrees of freedom and the restraints were modeled as linear axial springs. The wave forces were evaluated using the modified Morrison equation in the displaced position of the structure and considering the effect of the change of the free sea level. Lyons [6] and his colleagues [7] in 1983 made comparisons between the results of hydrodynamic analyzes and two sets of large-scale experiments for TLP responses under wave excitation. The analysis and test results for the surge movement have a good agreement, but they differed about the tension of the restraints at certain frequencies of the waves. The Erie wave theory [7] was used and the opposition between the members was omitted. The non-linear damping was converted into a linear form by assuming the same energy loss in resonance. In 1983, Teigen [8] [8] investigated the response of the tensile base platform in different degrees of freedom for long-crest waves and short-crest waves in the majority of laboratory tests. He stated that the responses of the platform in the horizontal degrees of freedom and in the low frequency section are larger for waves with long crests compared to waves with short crests.