Enabling safe and reliable operations of subsea wellheads has a high priority in the global oil and gas industry. The objective of the current paper is to provide a novel method for bending moment estimates at the wellhead based on indirect moment measurements; this moment, together with fatigue properties are then used for fatigue damage estimation. Indirect bending moments are based on inclinations and accelerations measured by motion reference units (MRU) attached to blowout preventer (BOP), lower marine riser package (LMRP) and lower riser joint (LRJ) immediately above the lower flexible joint (LFJ). Also, required is the tension time history in the same period at the LRJ. The proposed methodology here can be implemented and integrated into a portal for data acquisition and visualisation. In order to validate the proposed method for indirect bending moment estimation, strain gages have been attached to a BOP and marine riser during drilling operations offshore Norway. Strain gage readings are transformed to bending moment which is used as reference (the so-called direct moment). The proposed method is used to calculate the moment indirectly, the so-called indirect moment. The resulting indirect moments agree very well with the direct moments.

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 campaig...

Approaches for derivation of joint design parameters in relation to multi-component environmental processes are briefly summarized. Within the context of structural design, this is frequently referred to as a “load combination problem”. It is focused on application of design contours for analysis of such load combinations. Contours of this type can be utilized for cases where the different load processes are both correlated and uncorrelated. Existing procedures for construction of such contours are summarized, and extensions to situations where the characteristic time scales for the different process components are widely different are outlined. Development of such tools based on application of “translation processes” (i.e. transformed Gaussian processes) is highlighted. Analysis of mechanical design formulations which involve several interacting response quantities is further addressed. Utilization of so-called contour response tubes within such a framework is discussed. An example of application is given for a cross-section at the upper end of a flexible riser configuration where the interaction between the tension and curvature processes needs to be taken into accounted.