Nonlinear dynamic analysis and fatigue damage assessment for a deepwater test string subjected to random loads

Kang Liu,Ben-Rui Zhu,Yuan-Jiang Chang,Guo-Ming Chen,Bin-Bin Han,Xiu-Quan Liu

doi:10.1007/s12182-015-0063-4

Abstract

The deepwater test string is an important but vulnerable component in offshore petroleum exploration, and its durability significantly affects the success of deepwater test operations. Considering the influence of random waves and the interaction between the test string and the riser, a time-domain nonlinear dynamic model of a deepwater test string is developed. The stress-time history of the test string is obtained to study vibration mechanisms and fatigue development in the test string. Several recommendations for reducing damage are proposed. The results indicate that the amplitude of dynamic response when the string is subjected to random loads gradually decreases along the test string, and that the von Mises stress is higher in the string sections near the top of the test string and the flex joints. In addition, the fatigue damage fluctuates with the water depth, and the maximum damage occurs in string sections adjacent to the lower flex joint and in the splash zone. Several measures are proposed to improve the operational safety of deepwater test strings: applying greater top tension, operating in a favorable marine environment, managing the order of the test string joints, and performing nondestructive testing of components at vulnerable positions.

Highlights

In recent years, the deepwater drilling unit HYSY981, which is operated by the China National Offshore Oil Corporation (CNOOC), has drilled several exploration wells in the South China Sea (Xu et al 2013)
The results indicate that the amplitude of dynamic response when the string is subjected to random loads gradually decreases along the test string, and that the von Mises stress is higher in the string sections near the top of the test string and the flex joints
Several measures are proposed to improve the operational safety of deepwater test strings: applying greater top tension, operating in a favorable marine environment, managing the order of the test string joints, and performing nondestructive testing of components at vulnerable positions

Summary

Introduction

The deepwater drilling unit HYSY981, which is operated by the China National Offshore Oil Corporation (CNOOC), has drilled several exploration wells in the South China Sea (Xu et al 2013). The study of nonlinear vibration dynamics and fatigue damage in deepwater test strings is of great scientific and engineering significance. An equivalent composite model is often used to analyze deepwater double-layer hydrocarbon strings In this model, it is assumed that the pipes move together uniformly under external and internal loading, and the equivalent pipe bending and tension are shared based on the stiffness and the bearing area of the pipes. It is assumed that the pipes move together uniformly under external and internal loading, and the equivalent pipe bending and tension are shared based on the stiffness and the bearing area of the pipes This model can be used to analyze motion, but is likely to be inaccurate in fatigue damage evaluation (Harrison and Helle 2007). Vibration and damage in deepwater test strings are studied to provide guidelines for structural design and operation management

Nonlinear dynamic model

Wave-induced vibration mechanism

Dynamic response analysis

Upper flex joint Lower flex joint

Fatigue damage assessment

Analysis of contributing factors