Dynamic Analysis and Control of Heave Compensation System for Offshore Drilling Operation based on Multibody Dynamics

Abstract

In this paper, dynamic response analysis and control of the hoisting and heave compensation system of an offshore drilling rig was performed during drilling operations of a drilling rig under wave loads. The drilling rig is composed of an offshore drilling system and a semi-submersible, and the heave motion of a semi-submersible by the ocean environment affects drilling operations. Therefore, a heave compensation system is needed to control the position of a drill bit, which is a boring tool used for drilling, so that the bit would remain stationary and unaffected by the heave motion. In order to carry out the dynamic response analysis, the dynamics kernel was developed based on mutibody dynamics. To construct the equations of motion of the multibody system and to determine unknown accelerations and constraint forces, the recursive Newton-Euler formulation was adapted. The recursive formulation is the method which uses a set of generalized coordinates and allows easy additions of generalized forces. Functions of the developed dynamics kernel were verified by comparing with other commercial dynamics kernels. The hydrostatic force, hydrodynamic force, and control force were considered as the forces that act on the semi-submersible and the heave compensation system. In contrast with the commercial dynamics kernels, the developed kernel can deal with the hydrostatic and hydrodynamic forces. A nonlinear effect was implemented by calculating the hydrostatic force of the semi-submersible at the instantaneous position and orientation. The conversion between Euler angle and angular velocity was considered for exact expression and calculation of the instantaneous orientation. The linearized hydrodynamic force was converted from the frequency domain to the time domain by the convolution integral. The added mass and damping coefficient in the frequency domain were calculated using WADAM, a commercial software. To determine the control force of the heave compensation system, pneumatic and hydraulic systems were used, and PD control algorithm was adopted. It was desirable to determine initial position and orientation of the semi-submersible in static equilibrium state before the dynamic simulation. Therefore, a static analysis was performed to determine the initial position and orientation of the semi-submersible. In particular, the partial derivative coefficients of the hydrostatic force with respect to the instantaneous position and orientation of the semi-submersible were derived as the explicit terms associated with the change in the planar area and the submerged volume of the semi-submersible. With the initial position and orientation, a numerical simulation was performed for the analysis of the dynamic response and the constraint forces, which can be utilized for the mechanical design. A drill string was modeled considering a 12,000m drill pipe, a 200m heavily walled drill pipe, and a 100m drill collar. For the numerical simulation, Horizontal motions were assumed to have been controlled by mooring and dynamic positioning system, and a regular wave with a 3m amplitude and a 10 sec period was applied. The simulation results showed that the amplitude of the heave motion of the crown block and of the semi-submersible were about 0.03 m, about 0.86 m, i.e., Efficiency of the heave compensation system was around 96%. The simulation confirmed that the maximum dynamic constraint force of 1,780 kN was exerted on a base joint of a cylinder for the drill string compensator. The calculated constraint forces could serve as reference data for the design of the mechanical system. The data set of the simulation results were provided to shipyards; and are under validation, the simulation results are expected to be also used for the determination of the severity ranking for the risk assessment.