Evaluation of Wellhead Loads: An Alternative Approach
Authors: Roveri F.E. , Souza L.F.A. , Mourelle M.M. , Ellwanger G.B. 
Published: October 2008

A methodology is proposed to evaluate the wellhead loads due to the dtflling and completion phases for a subsea well. The loads transmitted by the drilling riser to the BOP stack at the flexjoint are determined using a static approach without the need of performing a detailed riser analysis. The flexjoint angle, as provided by the riser angle indicator at operation, is a key parameter in determining the shear and bending moment at the flexjoint and it synthesizes the riser response when subjected to a variety of loading conditions, such as environmental loadings, top tension, mud, static offset, etc.

The analysis takes Into account the angle of the well with regard to vertical (as-installed) and the soil nonlinear behavior. An incremental-iterative analysis is camkd outusing the Newton-Raphson method and an updated Lagangian beam element. Therefore the structureposition is updated at each iteration, accounting fix the heavy submerged weight of the BOP and LMRP.

The accurate determination of wellhead loads can lead to the design of more efficient structures. The proposed approach lessens the number of variables involved in the problem. As a result, safety is improved.

A parametric study is conducted using the proposed approach and the corresponding wellhead loadings are obtained. Parameters to be varied are over pull, water depth, mud, flexjoint angle and the angle of the weil withregard to vertical, A rigid model is also developed and the results are compaund with the flexible nonlinear model. The original computer model is improved to consider the presence of a drilling riser and some load cases are run in order to compare the results of the complete model with the results of the simplified flexible model approach.

Despite of being a static approach, its advantage is that the flexjoint angle to be input comes fem. field operations and this parameter encompasses much of the uncertainties involved in riser response, such as current velocities, hydrodynamic coefficients, etc.

INTRODUCTION

The determination of wellhead loads due to the drilling and completion phases for a subsea well has been a concern for the oil industry considering the move towpaths deeper waters123. Thorough study of the behavior of the wellhead system, blow-out preventer (BOP), lower marine riser package (LMRP) and riser when submitted to a variety of loads of different nature, is required in order to have an appropriate evaluation of the loads transmitted to the wellhead body by the upper part of the structure.

Consequently, there are numerous variables that must be taken into account in order to identify critical conditions associated with the operating envelope of the drilling vessel. A coupled analysis of the drilling riser and wellhead system requires information about the environmental data, platform motions, riser characteristics, etc. and frequently a dedicated computer program capable to handle the complex soil-fluid-structure dynamic interaction of the system.

A parametric study is often necessary to assess the effect of each variable on the results and a significant number of load cases is generated.