A Simplified Methodology for Comparing Fatigue Loading on Subsea Wellheads

Holden, Harald; Bjønnes, Pål; Russo, Massimiliano

doi:10.1115/omae2013-11529

Cited by 4 publications

(1 citation statement)

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“…This method is based on methodology from [1] and [4]. The contributions from LFJ moment, riser tension and inertia are often difficult to determine with precision.…”

Section: Indirect Methodsmentioning

confidence: 99%

Direct And Indirect Measurement Of Well Head Bending Moments

2015

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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...

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“…This method is based on methodology from [1] and [4]. The contributions from LFJ moment, riser tension and inertia are often difficult to determine with precision.…”

Section: Indirect Methodsmentioning

confidence: 99%

Direct And Indirect Measurement Of Well Head Bending Moments

2015

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Integrity Assessment of Offshore Subsea Wells: Evaluation of Wellhead Finite Element Model Against Monitoring Data Using Different Soil Models

Russo¹,

Zakeri²,

Kuzmichev

et al. 2016

Journal of Offshore Mechanics and Arctic Engineering

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Understanding well conductor–soil interaction mechanism plays an important role in well integrity assessment. Evaluation of field data obtained from monitored offshore wells can provide valuable insights into the matter. This paper presents a comparison of the numerical results obtained from a full three-dimensional (3D) finite element (FE) analyses model of the blowout preventer (BOP), wellhead (WH), conductor, and surface casing versus the field measured data obtained during drilling operations. Sensitivity studies were also performed for several parameters that were considered important in the local well response analysis under observed sea state conditions. The conductor–soil analyses were simulated using the Winkler spring p–y curves obtained by two different approaches: American Petroleum Institute (API) RP2GEO and a recently developed model by Zakeri et al. The results of bending moments in conductor and surface casing have indicated good agreement between FE model results and the field measurements.

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Full Scale Measurements of Cyclic Bending Moments on Subsea WellHeads on the Norwegian Continental Shelf

Grytøyr¹,

Reinås²,

Eilertsen³

et al. 2016

Day 1 Mon, April 11, 2016

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Wellhead Fatigue is a growing well integrity concern when installing subsea wells, especially with exposure to harsh environments, like e.g. North Sea or East Canada, and extreme environmental loads, using deep water drilling rigs in shallow water. Consequently, subsea instrumentation has been employed in several projects to document the actual load levels experienced by the subsea wellheads during drilling. There are no standard methods for such instrumentation and measurement processing, but in these cases the bending loads on subsea wellheads during offshore drilling have been measured according to ref. 11/ and methods presented at OTC 2015, ref. /5/, and OMAE 2015, ref. /3/. The actual load levels presented here have never before been published, and are of an unprecedented magnitude. Previously, state-of-the-art riser analyses have been found to be conservative when comparing to full-scale measurements offshore. Recent drilling campaigns in shallow water on the NCS have shown that in some cases the analyses are no longer conservative as they match the actual loads measured. Cycle-range histograms have been established from the measured WH bending moment time series. In some cases the dynamic bending moments are excessively large, even in moderate storm conditions. The loads can be a significant fraction of the static capacity of a typical subsea wellhead. Not sufficient to threaten the structural strength of the wellhead, but representing a significant challenge for the fatigue capacity. Three important findings have been made from resent measurements campaigns in shallow water in the North Sea. The dynamic bending moments acting on the subsea wellhead may be exceedingly large in shallow water.The magnitude of the dynamic bending moments is not primarily governed by the vessel and vessel motions, but also by the drilling package, i.e. BOP and riser. The dominant effect is the direct wave loading on the riser and BOP system itself.There seems to be a shift in the relation between predicted loads and actual loads at some given water depth. For deeper waters, riser analyses are usually found to provide conservative results. However, the riser analyses may not necessarily be conservative when moving into shallow water. This is believed to be related to the first riser mode coinciding with the region with most wave energy, i.e. 8–10 seconds.Consequently the margin of safety in safety-by-design excerisises may vary with waterdepth The relation between load levels and selection of drilling package is described in more detail. Modern sixth generation drilling rigs with large Blow Out Preventers (BOP), will get a double penalty. The large height of the BOP increases the bending moments at the wellhead. In addition, the riser tension is increased by the large submerged weight of LMRP. Finally, if the BOP stack itself is set in dynamic motion, it can potentially give a dynamic amplification of the loads. This paper provides important new information to assessments of subsea well integrity in shallow water.

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A Simplified Methodology for Comparing Fatigue Loading on Subsea Wellheads

Cited by 4 publications

References 0 publications

Direct And Indirect Measurement Of Well Head Bending Moments

Direct And Indirect Measurement Of Well Head Bending Moments

Integrity Assessment of Offshore Subsea Wells: Evaluation of Wellhead Finite Element Model Against Monitoring Data Using Different Soil Models

Full Scale Measurements of Cyclic Bending Moments on Subsea WellHeads on the Norwegian Continental Shelf

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