Direct And Indirect Measurement Of Well Head Bending Moments

摘要

During the past few years, there has been a focus on increased oil recovery, which has increased therndemand for drilling operations on subsea wells. Furthermore, the BOPs have become significantly largerrnin size. As a consequence, well head fatigue has been a growing concern for offshore drilling in the NorthrnSea.rnIn order to assess accumulated fatigue damage on a wellhead system, it is necessary to have somernknowledge of the load history. This can be established e.g. by use of global riser analyses. However, thisrnwill include some model uncertainty, and is known to be conservative in many cases. Improvedrnconfidence can be achieved by measuring the actual loads on the system, thus removing the uncertaintyrninherent in a numerical analysis. It is challenging to measure strains on a wellhead directly, however inrnsome cases it is possible to measure the strains close to the Wellhead, thus allowing for directrnmeasurements of the bending moments and axial forces. Statoil has been able to measure strains close tornthe WH on 3 different drilling MODUs. In one case this was accomplished by using strain gauges directlyrnon a spool piece below the BOP stack, in a second case by measuring the strains on the conductor casingrnand the surface casing below the housings, and in a third case in the form of flange face gap variationsrnon the BOP connector flange.rnThe direct method uses strain gauges or displacement measuring pins to assess bending strains. Thesernsystems are valuable in a sense that they provide an accurate measure of the bending moments. However,rnin some situations it is not possible to measure these loads directly, e.g. due to space limitations, or whenrnthe BOP support structure complicates the load path in the stack. Furthermore, it is difficult to measurernbending moments on the wellhead when the BOP has landed on a horizontal XT for completion. In theserncases indirect methods must be utilized instead. This paper presents a method for indirect measurementsrnof the bending moments on a wellhead.rnThe indirect method uses inclinometers on the lower riser adapter and BOP stack and allows forrncalculation of the bending moments in any cross-section below the lower flex joint. The benefit is that thisrnautomatically corrects for the height difference between measuring stations and the well head datum.rnPrevious work on this topic has focused on horizontal displacement on the BOP stack, see e.g. [5]. Ourrnresults indicate that BOP displacement is not a good indicator of well head moments. However, a highrncorrelation is observed between BOP stack inclination and well head moments. Statoil is currentlyrnpreparing papers concerning the theoretical background of these methods [6].rnStatoil is instrumenting and using both direct and indirect methods on several offshore drillingrncampaigns. Direct methods have been used to verify the indirect method, and the results show a goodrnagreement. The results are used to monitor BOP stack behavior and well head fatigue, giving importantrninput to the operational decision process.rnA measurement campaign on a semisubmersible drilling rig is used as an example case in this paper.