Guoming Chen

Envelopes for connected operation of the deepwater drilling riser

Abstract The common critical criterion and nonlinear search method were adopted for the study of connected operation envelopes of deepwater drilling riser and a riser-wellhead-conductor integral finite element model was established. The combination parameters of drilling platform offset, current speed and slip joint stroke were used to determine the riser operability envelopes. The results show that the drilling envelope has an upconing shape and is limited by lower flex joint angle when the surface current speed is low (less than 1.0 m/s). In downstream direction, when the current speed increases, the rotation angle of the lower flex joint increases and the allowable maximum offset of the platform reduces, but in upstream direction, the conditions will be opposite. When surface current speed exceeds 1.0 m/s, the drilling envelope is limited by upper flex joint angle. When it is in upstream direction, the increase of current flow will increase the rotor angle of the upper flexible joint and reduce the drilling envelope rapidly. The non-drilling envelope and emergency disconnect sequence (EDS) actuation envelope are mainly subject to the maximal equivalent stress of the conductor and they will drift towards the upstream direction with the increase of current speed. In addition, through the analysis on influence factors of connection window operation of riser, the tensile force at the top could be increased and drilling fluid density could be reduced properly, so that the drilling envelope could be expanded.

Analyses and countermeasures of deepwater drilling riser grounding accidents under typhoon conditions

Abstract For deepwater drilling riser system facing grounding risk in the South China Sea, a mechanical analysis model of deepwater drilling riser grounding accident was established. Focusing on drilling riser grounding accidents of a deepwater floating drilling platform in the South China Sea, mechanical characteristics and grounding accidents of riser were analyzed. And the riser retrieving window was determined. The results showed that the shape of the riser system after riser grounding is catenary. The maximums of bending moment and von Mises stress are located at lower flex joint. Platform moon pool, tensioner and lower flex joint are damaged due to severe bending deformation in the upper and lower drilling riser system after riser grounding accidents. The riser may fracture if drilling platform continues to move toward shallow water. Drilling platform displacement and riser lift height must be within the riser operation window during riser retrieving, or else the riser may fracture and the LMRP may suffer further damage. Residual riser and LMRP can be retrieved in a fixed position when the drilling riser system is separated from seabed.

Multistring analysis of wellhead movement and uncemented casing strength in offshore oil and gas wells

This paper presents a theoretical method and a finite element method to describe wellhead movement and uncemented casing strength in offshore oil and gas wells. Parameters considered in the theoretical method include operating load during drilling and completion and the temperature field, pressure field and the end effect of pressure during gas production. The finite element method for multistring analysis is developed to simulate random contact between casings. The relevant finite element analysis scheme is also presented according to the actual procedures of drilling, completion and gas production. Finally, field cases are presented and analyzed using the proposed methods. These are four offshore wells in the South China Sea. The calculated wellhead growths during gas production are compared with measured values. The results show that the wellhead subsides during drilling and completion and grows up during gas production. The theoretical and finite element solutions for wellhead growth are in good agreement with measured values and the deviations of calculation are within 10%. The maximum von Mises stress on the uncemented intermediate casing occurs during the running of the oil tube. The maximum von Mises stress on the uncemented production casing, calculated with the theoretical method occurs at removing the blow-out-preventer (BOP) while that calculated with the finite element method occurs at gas production. Finite element solutions for von Mises stress are recommended and the uncemented casings of four wells satisfy strength requirements.

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

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.

Drift-off warning limits for deepwater drilling platform/riser coupling system

Abstract The drift-off dynamic model of deepwater drilling platform and riser coupling system was established. An analysis method on drift-off warning limits of deepwater drilling platform and riser coupling system was proposed, and a deepwater drilling platform/riser system was taken for case study. The analysis model of deepwater riser, wellhead and conductor coupling system and the drift-off dynamic model of platform were established respectively. The drift-off dynamic solver of deepwater platform was developed. The coupling dynamic characteristics and coupling effect of the deepwater drilling platform and riser system were analyzed in combination with example, and the analysis method for drift-off warning limits was described. The results show that: the riser load acting on platform plays a driving role in the platform drift-off in the initial drift-off stage, and begins to inhibit the platform drift-off gradually as the drift-off displacement increases; During the platform drift-off, the transient response speed of upper riser parameters is high, while the transient response of lower riser parameters presents an obvious hysteresis effect; As the current speed increases or water depth decreases, the drift-off warning limits of deepwater drilling platform/riser coupling system decrease and the deepwater drilling riser should be disconnected earlier.

Analysis on the Operation Fatigue of Deepwater Drilling Riser System

Abstract: Fatigue is one of main failure modes of deepwater drilling riser system. Analysis models of wave induced fatigue and vortex induced fatigue of deepwater drilling riser are established according to the riser connection, installation and hang-off operations. Characteristics of wave induced fatigue and vortex induced fatigue of riser system in different operation modes are studied. The influence of each operation fatigue on the combined fatigue is also identified. The results show that wave induced fatigue damage and vortex induced fatigue damage of upper riser are large in installation and hard hang-off modes. The fatigue damage of riser system in soft hang-off mode is less than that in hard hang-off mode. The combined fatigue damage of the upper and lower riser is large. The lower riser is under the influence of wave induced fatigue and vortex induced fatigue, while the upper riser is mainly under the influence of wave induced fatigue, especially that in installation and hang-off modes. The fatigue damages of riser in installation and hang-off modes have a great influence on the combined fatigue of riser and cannot be neglected in riser fatigue analysis.

Wind-induced response of large offshore oil platform

Abstract In connection with wind sensitivity for the towering and hollowed-out structures (drilling derrick, crane, etc) of large offshore oil platform, the wind-induced response of towering structure was studied. By the similarity criteria, high frequency force balance tests for the large offshore oil platform under 0–360° wind directions were carried out, and the spatial distribution model of fluctuating wind load acting on platform was presented. Also, the characteristics of wind-induced vibration and the changing rule of gust loading factors were obtained precisely through wind-induced assessment in all directions. The results show that: the RMS (Root Mean Square) of the fluctuating across-wind load is about 10% of the fluctuating along-wind load on the platform; the vibration is mainly focused on the towering and hollowed-out structure like derrick, and the RMS of the across-wind acceleration is about 55% of the along-wind acceleration; the towering derrick has a big dynamic magnification of fluctuating across-wind load. The across-wind load can not be neglected in wind resistance design of large offshore oil platform, also the wind-induced response on the top/bottom of derrick and the magnification of fluctuating across-wind load of towering structure should be mainly considered.

Notice of Retraction Dynamic wave-loading fatigue reliability analysis of deepwater drilling riser

Deepwater drilling riser is a major component of offshore drilling system that connects drilling platform with subsea wellhead. The primary damage mode of a drilling riser is fatigue failure due to oscillatory environmental loads. Riser fatigue is sensitive to varieties of uncertainties, such as structural loads and material properties. It is vital to evaluate the fatigue failure using probabilistic approach. This paper focuses on dynamic fatigue reliability of deepwater drilling riser under random waves together-with drilling platform motion. Several parameters that contribute meaningfully to the uncertainty of riser fatigue are firstly considered. A sensitivity study is then conducted to identify critical random variables. It is found that S-N parameter A, response amplitude operator (RAO) of drilling platform and Palmgren-Miners index Δ have a pronounced influence on the uncertainty of fatigue damage in splash zone. Subsequently, dynamic reliability analysis model for wave-loading fatigue is established in the form of second-order polynomial. Monte Carlo simulation (MCS) is used for solving the established model. In the calculations, fatigue reliability at different service time can be determined.