Challenges Associated with HPHT Intervention Equipment
Authors: Skeels H. Brian , Broussard Dane , Byrd Michael Brian 
Published: October 2008

A project has been progressing for better part of two years developing an all encompassing deepwater, large bore completion riser system for installation of vertical and horizontal wet Christmas trees. Although the higher pressure is a formidable exercise, weight and metal usage exercise, the unique and challenging aspect of the project has been overcoming all of the issues surrounding the high temperature aspects of the equipment.

The design of the riser system's various pressure containment packages was influenced by:Using current technology status with respect to metals derating, thermal life cycle issues, and selection of nonmetallic materials for 350 degree F temperature rating.Determining the maintenance replacement frequency cumulative usage of lower temperature rated non-metallic materials used for short-term durations at elevated temperatures, based on thermal life cycle curves.Dealing with a patchwork of Industry specifications and recommended practices addressing various aspects of component design, raw material fabrication, assembly and test as technology catches up with requirements. The paper will address the design and project management cost issues, the level of understanding of HPHT issues using project "risk management techniques to balance current capital investment against future upgrades and the likelihood of these requirements occurring again from a market perspective.

Introduction

Wells drilled beyond 25000 feet TD will, by nature, require HPHT equipment. Many of the current deepwater to ultra deepwater plays in the Gulf of Mexico are demonstrating the need for HP systems and intervention equipment to support their installation and maintenance. HT applications are more sporadic and therefore less market driven. Nonetheless, either application demands a technically challenging and often equally costly system. The biggest problem is to technically balance three divergent parameters: size, materials, or new configurations.1 Ultra deepwater workover/completion riser systems also have to deal with weight and external stresses associated with deploying in water depths beyond 6000 feet and large throughbore sizes of five inches or better to satisfy the latest generation of subsea trees and their high flow rate capacities. Second, deeper and larger requirements pose higher demands on rig tensioning requirements and stress levels in the riser pipe and their connectors. Higher pipe and connector stress equates to a need higher strength materials or thicker walled designs. Both are challenging in that more exotic materials and or thicker wall designs create more complicated fabrication processes, especially if manufacturing processes have to work within the limited boundaries imposed by fatigue and stress corrosion cracking. Third, radical new approaches in process, procedure, or hardware configurations are counter to the efforts in reliability and standardization the Industry is pushing for.

Riser System

The workover completion riser system is a high-pressure monobore configuration with options for separate paths to circulate wellbore and completion fluids. The monobore riser configuration has been in use for the last five years working on both horizontal and vertical subsea tree installation and intervention.3 The system is made up of five distinct entities: the lower riser workover package (LWRP), the riser pipe conduits, the tensioning interface, the surface tree (or terminal head), and the control system that operates it.