Modeling and Measuring Dynamic Well Intervention Stack Stress

Abstract

This paper reports the field results obtained from application of a system that provides both pre-job modeling capabilities and real-time monitoring of maximum stress levels in the entire intervention stack, from the wellhead to the injector assembly.In addition, the paper documents the dynamic movement capabilities recently incorporated in the model and validation of the model calculations. Introduction Reference 2 discusses an intervention riser safety system which has become known as the ? (Zeta) Safety System.This paper documents further development and testing that has been done with this system.The system is composed of two basic components:?model - a numerical dynamic simulation model which models the stresses in an intervention stack.?gauge - a lubricator spool, instrumented with fiber-optic strain gauges, is placed in the intervention stack. It measures axial force, internal pressure, and bending moments in the spool. The initial coiled tubing (CT) field application of this safety system was performed to satisfy several primary objectives, including:Validation of modeled calculations versus field data measured by independent devicesSensitivity of the field stress measurements provided by the systemConfirmation that system design and calibration is sufficiently robust for routine field applications The ability to accurately model dynamic movement of two independent structures was driven by increased utilization of floating structures (TLPs and Spars) being deployed in deepwater projects.The tethered topside structure typically exhibits some amount of horizontal displacement in a figure-eight pattern as a result of wave motion, with the wellhead exhibiting a similar displacement pattern but with differing frequency and amplitude.The intervention stack may experience increased stress levels when each end of the rigid lubricator/riser assembly is attached to these two independently-moving bodies.A dynamic modeling capability incorporated in this model addresses these field conditions. In addition, offshore intervention stacks are becoming taller to accommodate offshore floating structure size, and often pass through multiple deck surfaces that constrain lateral stack movement.This can create a condition whereby conventional safety limits are exceeded.While counter intuitive, removal of lateral stack constraints may actually increase the safety of a given stack.Another finding is that the maximum stack stress may occur in situations where no CT hanging weight is applied to the stack. The pre-job modeling capabilities of the system are used to optimize intervention rig-up design and to determine the probability of exceeding pre-set safety limits during the operation.During the field operation, real-time stress values provided by the system enable informed decisions, rather than a judgment call, to be made if maximum stress levels are approaching unsafe limits.

Advanced Casing Design With Finite-Element Model of Effective Dogleg Severity, Radial Displacements, and Bending Loads

“…The model involves an iterative solution, in which wall-contact forces are successively adjusted through use of elastic-radial-stiffness forces that are updated to constrain the inner string within the outer wellbore to within a specified tolerance. The FEA model can also be applied in dynamic mode through use of a finite-difference timestep scheme (Smalley et al 2005).…”

“…The model has been applied with a real data-acquisition system for prejob design and real-time stress analysis of coiled-tubing-intervention stacks in Smalley et al (2005) applications have been the modeling of buckling and stresses, while snubbing workstrings into the surface apparatus of conventional snubbing units and also wireline deflection during riserless subsea-well interventions (Newman et al 2006). Application has also been made to drillstring hookloads and dynamics during jarring (Newman and Procter 2009).…”

Field Experience Using an Intervention Wellhead Structure Modeling and Monitoring System

“…• Buckling -an instability occurs in the stack which causes it to buckle catastrophically. The Euler buckling load type calculations tend to be inaccurate and nonconservative (Smalley 2005) when used for analyzing intervention stack structures. A dynamic finite element analysis (DFEA) is best suited for this type of analysis, especially when there is movement in the structure due to a floating vessel.…”

“…The Zeta DFEA stack analysis software, (Smalley 2005), was used to analyze the wellhead and initial stack described above. Many design iterations were performed before a final stack design was agreed upon.…”

“…Extensive stack modeling was performed during the job planning and equipment selection process. A dynamic finite element stack modeling package (Smalley 2005) was used to perform this analysis. The API 6AF bending limits were added to this analysis software because these flange bending limits became the most limiting aspect of the job.…”