Lateral bearing characteristics of subsea wellhead assembly in the hydrate trial production engineering

Abstract

Conductor and suction anchor are the key equipment providing bearing capacity in the field of deep-water drilling or offshore engineering, which have the advantages of high operation efficiency and short construction period. In order to drill a horizontal well in the shallow hydrate reservoir in the deep water, the suction anchor wellhead assembly is employed to undertake the main vertical bearing capacity in the second round of hydrate trial production project, so as to reduce the conductor running depth and heighten the kick-off point position. However, the deformation law of the deep-water suction anchor wellhead assembly under the moving load of the riser is not clear, and it is necessary to understand the lateral bearing characteristics to guide the design of its structural scheme. Based on 3D solid finite element method, the solid finite element model of the suction anchor wellhead assembly is established. In the model, the seabed soil is divided into seven layers, the contact between the wellhead assembly and the soil is simulated, and the vertical load and bending moment are applied to the wellhead node to simulate the riser movement when working in the deep water. The lateral bearing stability of conventional wellhead assembly and suction anchor wellhead assembly under the influence of wellhead load is discussed. The analysis results show that the bending moment is the main factor affecting the lateral deformation of the wellhead string; the anti-bending performance from increasing the outer conductor diameter is better than that from increasing the conductor wall thickness; for the subsea wellhead, the suction anchor obviously improves the lateral bearing capacity and reduces the lateral deformation. The conduct of the suction anchor wellhead assembly still needs to be lowered to a certain depth that below the maximum disturbed depth to ensure the lateral bearing stability, Thus, a method for the minimum conductor running depth for the suction anchor wellhead assembly is developed. The field implementations show that compared with the first round of hydrate trial production project, the conductor running depth is increased by 9.42 m, and there is no risk of wellhead overturning during the trial production. The method for determining the minimum conductor running depth in this paper is feasible and will still play an important role in the subsequent hydrate exploration and development.