Subsea Wellhead Drilling Loads: Comparing Direct Versus IndirectMeasurements of Wellhead Bending Moment

Abstract

Monitoring of wellhead bending loads on subsea wells during operations with subsea blowout preventers (BOPs) deployed from mobile drilling rigs has gained widespread acceptance as a standard practice in recent years. The main purpose of such monitoring is to maintain structural well integrity (wellhead fatigue) and to support operational decision-making. Additionally, the requirements for such monitoring systems have become common in contractual specifications for new drilling rigs, particularly for operations in harsh environmental conditions.

Market-available monitoring systems can be categorized into two distinct types: direct and indirect load monitoring systems. Direct load monitoring systems typically measure bending loads directly using strain measurements positioned near the BOP connector. Indirect measurements on the other hand utilize motion sensors of the BOP and riser, then apply motion equations to infer the bending loads at the wellhead connector. The objective of this paper is to compare results from monitoring campaigns using both these two methods, and from this try to quantify the difference in quality of the two methods.

The paper will examine the uncertainties associated to both direct and indirect measurements and their underlying assumptions. Following this, actual monitoring data from multiple campaigns collected using sensor configurations enabling both approaches will be employed to compare the two methods. The dataset includes 20 monitoring campaigns, totaling around 800 days of BOP monitoring in harsh environments on the Norwegian continental shelf. These campaigns cover a variety of drilling units and water depths.

This study will conduct a detailed comparison between direct and indirect load measurement methods across multiple campaigns. Subsequently, key results from all the campaigns will be analyzed to quantify the overall differences between the two measurement approaches.

Indirect measurements will typically rely on the well rotational stiffness to obtain the wellhead loads. An additional observation from the work is that the stiffness of subsea wells turns out to be direction-dependent, which in turn affects the accuracy of indirect measurements.

The objective of this study is to support the selection of appropriate monitoring systems. Typically, indirect measurement systems are more cost-effective than direct systems, with the primary tradeoff being in measurement accuracy. The findings aim to assist decision-makers in making informed choices regarding the most suitable monitoring approach.

Authors

Harald Holden
Lead Engineer Drilling at 4Subsea

Ali Cetin
Tech Lead Data Lab at 4Subsea

Vetle Kjærstad
Team Lead Operations Drilling Monitoring at 4Subsea

Jan-Erik Hygen
Drilling Engineer at 4Subsea