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Hydrodynamic Response for a Deepwater Truss Spar in Wind, Wave and Current

Author: LongZuoZuo
Tutor: LiWei
School: Shanghai Jiaotong University
Course: Design and manufacture of ships and marine structures
Keywords: Truss Spar mooring and riser system dynamic response mooring broken heave plate Floatover Installation
CLC: U674.381
Type: Master's thesis
Year: 2012
Downloads: 160
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Abstract

With the offshore oil exploitation moving forward to deep sea gradually, hydrodynamic problems have become more complicated and important than in the shallow sea. To avoid causing harm in engineering fields, and ensuring the safety of the deepwater floating structure’s production processes and people’s lives, such hydrodynamic problems must be studied and analyzed thoroughly. The deepwater floating platforms consist of floating structure, mooring line and riser systems where there are two types of scales. One is the large-scale object such as floating structures, the other is the small-scale object such as mooring lines and risers, called the flexible components. The flexible components have a profound effect on the platform motion in deepwater, and the greater the water depth, the more significant influence. Therefore, coupled analysis method in time-domain has become an important tool for hydrodynamic characteristics of the platform and mooring/riser system considered as a whole.On the basis of reviewing the history of various forms of offshore platforms and hydrodynamic theory, this thesis considered the hydrodynamic characteristics for deepwater Truss Spar and its floatover installation in Wind, Wave and Current. The research contents are mainly as follows.Chapter III deals with the coupled-dynamic characteristics of the Truss Spar with mooring/riser systems in irregular wave, wind and current environments. Results show that surge, pitch and heave motion and mooring line tension are characterized by low frequency, so it can avoid the resonance between platform and waves. when the risers exist, the dynamic characteristics of the platform and its mooring system will change, and especially the amplitudes of the motion and tension spectrums will reduce significantly when TTR exists. After coupling the riser, the performance of platform and mooring system will be more comprehensive. When some mooring lines are broken under extreme sea conditions, the influence on the dynamic characteristics of the platform and its mooring system is remarkable, and the influence is different for different type of risers. Therefore, the evaluation and analysis on such coupled-dynamic characteristics have great significance when some mooring lines are broken under different sea conditions in engineering practice.Moreover, the effect of the heave plate parameters on the platform hydrodynamic characteristics is also analyzed. Results indicated that the heave added mass and RAO of the platform can increase with the increment of the distance between the heave plates and perforated area, however increase with the decrease of heave plates’thickness. When the perforated area is constant, the heave added mass and RAO of the platform will increase with the decrease of hole radius.In Chapter IV, the coupled-dynamic characteristics where the topside of the Truss Spar is installed by use of the Floatover method in wave, wind and current environments are considered. There are several substantial advantages to installing integrated topside onto a Truss Spar using floatover method, particularly for large topsides which exceed the single lift capacity of the available heavy lift derrick barge fleet. These advantages include schedule and cost savings for the integration and commissioning of modules on land rather than at sea. To his end, two states are considered for such a floatover installation of the Truss Spar topside. Results show that in the transportation state, the catamaran system has a heave natural period around 8 sec., a surge and roll natural period around 11 sec.; for heave and roll, beam seas produce the largest motion response, for surge, head seas produce the largest motion response. In the installation state, Truss Spar and two barges are considered as a whole, for heave, surge and pitch, head seas produce the biggest maximum motion, for roll and sway, beam seas produce the biggest maximum motion.

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