Direct And Indirect Measurement Of Well Head Bending Moments
Authors: Lindstad Halvor Borgen , Grytøyr Guttorm , Russo Massimilliano 
Published: April 2015

During the past few years, there has been a focus on increased oil recovery, which has increased the demand for drilling operations on subsea wells. Furthermore, the BOPs have become significantly larger in size. As a consequence, well head fatigue has been a growing concern for offshore drilling in the North Sea.

In order to assess accumulated fatigue damage on a wellhead system, it is necessary to have some knowledge of the load history. This can be established e.g. by use of global riser analyses. However, this will include some model uncertainty, and is known to be conservative in many cases. Improved confidence can be achieved by measuring the actual loads on the system, thus removing the uncertainty inherent in a numerical analysis. It is challenging to measure strains on a wellhead directly, however in some cases it is possible to measure the strains close to the Wellhead, thus allowing for direct measurements of the bending moments and axial forces. Statoil has been able to measure strains close to the WH on 3 different drilling MODUs. In one case this was accomplished by using strain gauges directly on a spool piece below the BOP stack, in a second case by measuring the strains on the conductor casing and the surface casing below the housings, and in a third case in the form of flange face gap variations on the BOP connector flange.

The direct method uses strain gauges or displacement measuring pins to assess bending strains. These systems are valuable in a sense that they provide an accurate measure of the bending moments. However, in some situations it is not possible to measure these loads directly, e.g. due to space limitations, or when the BOP support structure complicates the load path in the stack. Furthermore, it is difficult to measure bending moments on the wellhead when the BOP has landed on a horizontal XT for completion. In these cases indirect methods must be utilized instead. This paper presents a method for indirect measurements of the bending moments on a wellhead.

The indirect method uses inclinometers on the lower riser adapter and BOP stack and allows for calculation of the bending moments in any cross-section below the lower flex joint. The benefit is that this automatically corrects for the height difference between measuring stations and the well head datum. Previous work on this topic has focused on horizontal displacement on the BOP stack, see e.g. [5]. Our results indicate that BOP displacement is not a good indicator of well head moments. However, a high correlation is observed between BOP stack inclination and well head moments. Statoil is currently preparing papers concerning the theoretical background of these methods [6].

Statoil is instrumenting and using both direct and indirect methods on several offshore drilling campaigns. Direct methods have been used to verify the indirect method, and the results show a good agreement. The results are used to monitor BOP stack behavior and well head fatigue, giving important input to the operational decision process.

A measurement campaign on a semisubmersible drilling rig is used as an example case in this paper.