Modeling and Measuring Dynamic Well Intervention Stack Stress
- Edward Allen Smalley (CTES/Varco) | Kenneth Ray Newman (CTES/Varco) | Rodney Stephens (BP America)
- Document ID
- Society of Petroleum Engineers
- SPE/ICoTA Coiled Tubing Conference and Exhibition, 12-13 April, The Woodlands, Texas
- Publication Date
- Document Type
- Conference Paper
- 2005. Society of Petroleum Engineers
- 4.2.4 Risers, 1.10 Drilling Equipment, 1.3.2 Subsea Wellheads, 4.3.4 Scale, 3.2.2 Downhole intervention and remediation (including wireline and coiled tubing), 4.5 Offshore Facilities and Subsea Systems
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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.
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 devices
Sensitivity of the field stress measurements provided by the system
Confirmation 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.
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