Challenges of Designing HP/HT Equipment
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 109999, "The Challenges of Designing HP/HT Equipment," by Andrew Forsyth, SPE, Expro; Brian Saucier, DeepMar; Daniel More, Expro; Ray A. Stawaisz, Chevron; Dave Gill, Stress Services; and Ray Younger, Prospect, prepared for the 2007 SPE Annual Technical Conference and Exhibition, Anaheim, California, 11-14 November. The paper has not been peer reviewed.
The full-length paper presents an overview of the main design activities for a completion and intervention landing-string system. The functional- and structural-design challenges encountered because of the high operating pressures and temperatures are reviewed. Analysis tools were used to quantify the observations found during testing, and subsequently to assist in determining a design solution. A key aspect of the design process was viewing the design and operations from a holistic perspective by evaluating the design throughout its operational life.

Introduction
The full-length paper details the technical challenges of developing a high-pressure/high-temperature (HP/HT) completion and intervention landing-string system for the Tahiti field in the Gulf of Mexico (GOM). Because the project presented some specific operational challenges, traditional analytical methods were insufficient for the design process and a comprehensive test and qualification program was required to validate that the equipment proposed was fit for purpose. The full-length paper presents an overview of the design methodology adopted and how the results from finite-element modeling assisted in validating the test program.
Fig. 1 shows a typical subsea landing-string configuration within a blowout preventer (BOP). The hydraulic-controlled functions of the subsea test tree (SSTT) are the following.To provide a dual-barrier well-isolation system during completion and intervention operations.To allow coiled tubing (CT) and wireline access into the well and to cut CT and wireline in an emergency situation without compromising pressure containment.To disconnect the upper landing string from the SSTT by means of a latch mechanism.To retain hydrocarbons in the landing string in the event of shear or planned disconnect.To provide a method of closure of the BOP rams on special joints for HP well integrity.
The SSTT comprises the latch assembly, dual fail-safe closed ball-valve assembly, and a slick-joint assembly to facilitate closure of the BOP pipe rams.
With the introduction of subsea horizontal trees in the mid-1990s, landing-string systems were developed for well-completion and -intervention operations. Early landing strings were designed to operate at maximum differential pressures and temperatures of 10,000 psi and 250°F, respectively. With the discovery of significant hydrocarbon reserves in deep water, the challenge to develop economically viable equipment had been set. Along with the requirement for HP/HT applications, the locations and environment that landing strings are required to operate in have become significantly more arduous. The HP/HT landing string also was required to be controlled with a subsea electrohydraulic control system for the deepwater application as opposed to earlier systems that were operated hydraulically from surface.