Cost-Effective Validation of HP/HT Subsea Equipment Using Seawater Hydrostatic Head
Authors: Feder Judy 
Published: February 2020

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper OTC 29413, “Validation of Cost-Effective Design Methods Using Hydrostatic Head for High-Pressure/High-Temperature Applications,” by Parth Pathak and Nicholas Katsounas, OneSubsea, prepared for the 2019 Offshore Technology Conference, Houston, 6–9 May. The paper has not been peer reviewed. Copyright 2019 Offshore Technology Conference. Reproduced by permission.

Validation testing of subsea equipment designed for high pressure/high temperature (HP/HT) applications is necessary but can be extremely expensive and infeasible. The complete paper presents a practical approach for validating design-verification analysis for subsea equipment, using a representative pressure valve block to correlate finite-element analysis (FEA) predictions for strain changes with actual measured changes. The design methods use the guidelines in technical report API 17TR8, and load cases per API 17TR12.

Introduction

Current editions of API standards covered in API Subcommittee 17 (API SC 17) for subsea production equipment are geared toward designing the equipment for its absolute internal working pressures. Also, on the basis of previous regulatory requirements for the offshore industry, until 2014 it was not advised to take advantage of the external seawater hydrostatic head and other external pressures to design certain types of subsea equipment covered under API SC  17. Thus, equipment per its corresponding API standards was designed to have an absolute rated working pressure always greater than the well shut-in tubing pressures.

Three API technical reports published in 2014 and regulatory guidelines for the Gulf of Mexico published in 2017–2018 provide design guidelines for depth-adjusting subsea equipment specific to its working seawater depths, and detailed verification and validation guidelines for designing HP/HT equipment—that is, with absolute internal pressure rating exceeding 15,000 psi and temperatures exceeding 350°F. These guidelines can be used to develop subsea equipment rated to working pressures greater than 15,000 psi. The complete paper details the step-by-step methodology of combining the API 17TR12 and API 17TR8 design verification guidelines to depth-adjust typical existing 15,000-psi-rated subsea equipment for HP/HT use.

Validation tests for external pressures for entire equipment assemblies can quickly become impractical and infeasible for most of the equipment covered in API SC 17 because of the sizes and complexity of the equipment. The authors believe such testing is not entirely necessary because testing for external pressure of critical and representative components, which are pressure-containing and -controlling, can be sufficient to validate the applicability of the external pressures for the equipment. Furthermore, they contend that for larger components for which hyperbaric chamber testing might not be feasible, comprehensive FEA can be used to validate stresses and deflections as long as the FEA methods and results are appropriately validated. The complete paper presents the various tests that were performed covering typical subsea equipment and are deemed sufficient to validate the verification methods used to depth-adjust using external pressures.

The actual tests performed were part of an HP/HT development for the completion and workover riser (CWOR) system for a potential application that required depth-adjusted working pressure higher than 15,000 psi. The work performed was to validate the design and verification of the equipment manufactured by the services provider and cannot be applied to other manufacturers or engineering houses.