Subsea Wellhead Fatigue Tracking Tool for HPHT Developments
Authors: Lu Wangming , Guan Xiaorui , Chase Thomas , Penaloza William , Mercan Bulent 
Published: April 2023

Many operators are currently dealing with the engineering challenges of high-pressure and high-temperature (HPHT) field developments which push conventional subsea technology beyond its limits. In many cases, a project specific risk assessment is required to determine the acceptability of wellhead fatigue response during drilling operations. To this end, a detailed subsea wellhead fatigue tracking tool for one of the industry's first HPHT applications in the Gulf of Mexico (GOM) was developed and is shared in this paper. Additionally, to facilitate wellhead fatigue tracking, a vortex-induced vibrations (VIV) fatigue damage indicator based on measured loop current profile and surface current speed is introduced.

The fatigue damage rates for drilling, completion and intervention operations are calculated using the hindcast metocean data (wave and current). The maximum allowable fatigue usage, or fatigue damage allowance, for each operation (e.g., drilling) and all events (e.g., drilling a hole) under that operation is determined considering the associated fatigue damage rates and operation time for each event. The actual fatigue damage rate is measured through an in-situ wellhead monitoring system and compared with the assigned fatigue damage allowance. In addition, a series of regression analyses between fatigue damage rate monitoring data and measured surface current speeds and profiles are performed.

An excel-based fatigue damage tracking tool was developed and used during HPHT drilling operations. This tool was shown to efficiently compare the measured fatigue damage to the fatigue allowance for each event thereby allowing the rig team to determine if excessive fatigue damage allowance was consumed during a specific operation, and if so, quickly implement mitigation methods to maintain sufficient fatigue usage for future operations. The tool is also highly sensitive to fatigue usage due to VIV during high loop current events or due to high waves. Regression analyses indicate that the current surface speed alone is less indicative of wellhead fatigue damage rate than the physics-motivated total current drag load (considering vertical profile throughout the column) to the riser and subsea stack.

In literature, several studies focus on wellhead tracking based on analysis or monitoring data. However, no effort has been made on a wellhead fatigue tracking tool combining upfront analysis results, operational data, hindcast and measured metocean data and monitoring data for the entire lifecycle of the wellhead. This paper highlights the first fatigue tracking tool that enables operators to track wellhead fatigue usage, compare that with the assigned fatigue allowances and facilitate decision making for ongoing and future operations on HPHT wells. A VIV fatigue damage indicator based on the total drag load is also described whichenables the rig team to evaluate the fatigue damage severity quickly and more reliably without the need of a comprehensive fatigue analysis or monitoring campaign.