Next-Generation HP/HT Subsea-Wellhead-System-Design Challenges and Opportunities
Authors: Carpenter Chris 
Published: January 2017

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 25643, “Next-Generation HP/HT Subsea-Wellhead-System-Design Challenges and Opportunities,” by Jim Kaculi, Dril-Quip, prepared for the 2015 Offshore Technology Conference, Houston, 4–7 May. The paper has not been peer reviewed.
Moving to higher-capacity wellhead systems for high-pressure and high-temperature (HP/HT) environments will require a larger mandrel and conductor-casing size to accommodate the high loads encountered during drilling and production operations for normal-, extreme-, and survival-loading conditions. Numerous analytical studies with 3D finite-element analysis (FEA) and other advanced tools have been performed in an effort to determine the changes needed, and an innovative solution is presented for next-generation subsea-wellhead equipment.
Introduction
The numerous functions that a wellhead system performs are enabled by a large number of parts and subassemblies with complex geometries and interfaces and complicated load paths. The only way to have a true understanding of the wellhead and have confidence in the design safety margin is by performing verification analysis and validation testing of rated, extreme, and survival capacities at the system level, and not only at the component and subassembly level. Testing only at rated conditions is insufficient to verify design margins and eliminate the possibility of unexpected events that can occur at higher load magnitudes.
Current industry standards that guide the design of wellheads require some general testing to be performed at rated capacity for components and subassemblies. However, unlike the standards for casing connectors below the mudline, there are no requirements to test the wellhead at a system level, nor to test the system to the limits (yield capacity) of the equipment, which would confirm the design margins. Casing connectors are tested per requirements that subject the connector to combined load testing of tension, compression, internal pressure, external pressure, and bending at ambient and elevated temperatures. It is logical that testing similar to casing-connector tests be performed on wellhead systems.
New Wellhead-System and Connector Design
The typical wellhead systems currently used in the industry include mandrels with 27- and 30-in. outer diameter (OD), and conductor casings are as large as 36-in. OD. However, these configurations may not be able to resist the high load magnitudes and load combinations expected in HP/HT applications. Moving to higher-capacity wellhead systems requires larger mandrel and conductor-casing sizes capable of enduring the high loads encountered during various drilling and production operations. A new and advanced design concept of the wellhead connector and mandrel was developed to accommodate the HP/HT loads and industry demands. A survey of operators and contractors combined with forecast load magnitudes obtained from analysis studies on HP/HT applications led to a 35-in. wellhead/mandrel design with sufficient structural capacity to meet the industry needs for future decades of drilling and production (this design is described in detail in the complete paper). This mandrel and connector design concept is shown in Fig. 1.