Mitigating Wellhead Fatigue While Reducing HSE Risk, Deck Spread, Deployment Time, and Crew Size

Subsea wellhead systems have a design fatigue life that is expected to withstand the damage incurred from stress caused by cyclic loading during its operation. Wellhead fatigue is a critical factor when drilling offshore wells because the condition of the wellhead determines the length of time drilling activities can be carried out safely. The presence of the BOP on top of the wellhead affects fatigue life. Initially, these units were designed for 6-in. and 10-in. diameter casing and weighed slightly less than 1,400 lb. [ASME, 2003) Over time, BOPs evolved, and today's units are considerably larger and heavier than their predecessors, weighing in at approximately 400 metric tons. This increase in size and weight on the wellhead negatively impacts fatigue life. In recent years, the oil and gas industry has begun to look for ways to reduce wellhead fatigue to extend the life of the wellhead and expand the margins for safe drilling operations. A new ROV-operated Wellhead Load Relief (WLR) system, developed specifically to mitigate fatigue, uses special tensioners that are tethered individually to the BOP. Each tensioner contains a hydraulic spooling unit with a lock- and-pull mechanism that allows the ROV to tighten adjustable tethers subsea, pulling them from slack to a maximum tension of 35 metric tons. This approach to installation is a departure from common industry practice, which necessitates the configuration of the predetermined tether lengths on the topside. The ROV-operated WLR system described in this paper is a compact, high-capacity BOP tethering system suitable for both template and seabed anchoring. It provides a new and efficient way of tethering the BOP to avoid wellhead fatigue and delivers additional benefits that include minimized HSE risk, a smaller deck spread, decreased deployment time, and a smaller crew. The WLR system was operated subsea for the first time in December 2019. By precisely tensioning each tether and limiting the load transferred to the wellhead, the WLR significantly lessened wellhead fatigue, resulting in an almost complete halt in BOP movement. This new technology enables the operator to make optimal use of the fatigue life of the wellhead without compromising efficiency or safety.