Studying the coupled axial and lateral oscillations of the drilling riser under conditions of irregular seaways

Orest Slabyi

doi:10.15587/1729-4061.2018.132661

Abstract

Construction of an improved mathematical model of the axial and lateral oscillations of the in the plane of action of the velocity vectors of the fluid flow washing the was considered. This model makes it possible to study the stress-strain state of the with simultaneous impact on it from the sea and the change in the force of tensioning its upper end. In addition, the model specifies the force effect exerted on the by the washing fluid flowing in it.Based on the developed mathematical model, a simulation model of operation of the drilling ship – rope-type tensioning system of the – riser system was created in the Modelica modeling language and a series of numerical experiments were performed at various levels of seaways. The obtained results show that the proposed model produces 22‒40% higher calculated values of the amplitude of lateral oscillations and 10‒25 % higher calculated values of the bending moments in critical sections compared with the results of the classical model of lateral oscillations. The greatest difference between the simulation results was observed with moderate seaways. With a growth of seaways, the difference between the two models decreases. Proceeding from the obtained results, it is not recommended to neglect the effect of variation in time of the forces tensioning the in applied problems of studying operation in conditions of slight sea.

Highlights

The complexity of well-drilling under deep sea conditions is primarily connected to overcoming powerful and highly mobile water masses. This is predetermined by sea currents and seaways that cause drifting and pitching of a floating drilling rig and deform marine risers
Construction and study into theoretical models has remained the main method for studying the dynamics of operation of risers
In view of impossibility of describing simulation models in the Modelica language using differential equations with partial derivatives for constructing models of riser sections described by equations (1), (3)–(8), the numerical method of lines (NMOL) [19] was used

Summary

Literature review and problem statement

The complexity of well-drilling under deep sea conditions is primarily connected to overcoming powerful and highly mobile water masses. Where ER is the modulus of elasticity of the riser material; IR is the axial moment of inertia of the riser cross section; Te(z) is the “effective” tension force in riser sections [13]; ρR is the weight of a unit of the riser length; w(z, t) is the value of lateral displacement of riser sections; CM is a coefficient of inertial component in Morison’s equation; CD is the resistance factor in Morrison’s equation; ρw is sea water density; De is the outside diameter of the riser; ww(z ,t) is the velocity of the point of the stream passing over the riser This model was used in [4, 5] to study operation of the riser under irregular seaways and the action of sea currents. Z1 =li where li is the coordinate of the i-th connection and (i+1)-th sections of the riser

Conducting simulation of operation of the system under study

Single

Findings

Conclusions