ABSTRACT:

This paper focuses on the evaluation of the loads transmitted by a drilling riser to the wellhead during either the drilling or completion phases of a subsea well in deep waters. Two subjects are considered: (1) the development of the equations for assessment of the wellhead loads based on the forces acting at the lower flexjoint, considering a static behavior and (2) the monitoring of the loads during the operation. Like other parameters participating in the operating window, such as the upper and lower ball joint/flexjoint angle and the combined (e.g. Von Mises) stress in the riser, the allowable wellhead loads may pose constraints on the drilling and completion operations. Disconnection would be necessary to take place if the permissible loads were exceeded. In the first part, the forces acting on the wellhead are evaluated considering the as-installed tilt angle of the well with regard to vertical and the additional tilt of the stack due to the soil lateral flexibility. The riser system including the BOP stack and wellhead is divided into two sub-systems:

  1. the drilling riser above the lower flexjoint and

  2. the BOP stack and wellhead system, from the lower flexjoint downwards.

The key parameters for the approach are the lower flexjoint angle and the BOP stack tilt angle. In the second part, the development of a procedure for real time evaluation of wellhead forces during the drilling or completion phase operation based on monitoring of the BOP stack and the lowermost riser joint angles is presented. Safety would be significantly improved through the assessment of such loads in real time.

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