This paper presents a simplified way of comparing the fatigue load on different subsea wells. The simplest comparison is done by accumulating the number of days BOP has stayed connected to the wellhead. The wellhead fatigue load is however heavily dependent on the vessels used, water depth and weather while connected to the well. An equation for deriving a benchmark load factor for each operation phase for a subsea well is proposed. This benchmark load factor takes into account the water depth, metocean season of the operation, BOP height and weight, and the stiffness of the marine riser lower flex joint. This benchmark load factor will represent a standard number of days with a BOP connected, correcting for some known effects. The goal has been to define a measure of ‘BOP days’ that accounts for the water depth, operational season, and BOP particulars. A base case (one MODU, 100 m water depth, and all year operation), equating to one standard BOP day, has been chosen as the reference for all cases discussed. The validity of the benchmark load equation will be shown through a comparison with 31 different global riser analyses intended for wellhead fatigue. For each of the 31 data sets, time domain load analysis is done for all sea states in the wave scatter diagram. The different analyses covers different rigs, water depths and two operational phases (with or without subsea XT installed). To enable a large scale comparison of the bench mark factor, an approach where the fatigue load is summarized using the bending moment standard deviation on the wellhead datum is presented. This methodology is then compared to four full fatigue calculations using a typical subsea wellhead fatigue capacity. Then the simplified fatigue calculation is performed for all 31 global riser analyses. The calculated damage is then compared with the corresponding bench mark formula in each case. Finally it is shown how this benchmark load formula has been implemented into the Statoil WellSpot database as a fatigue load criticality screening tool for the different Statoil subsea wells. It is further shown how this can be used as a tool during planning of future operations, and how to prioritize wells where a detailed fatigue analysis is recommended.

Operators in the North Sea have recently strengthened their efforts in documenting the integrity of subsea wellhead systems. As a part of this effort, fatigue damage estimation of subsea wells in service has been performed. Fatigue damage estimation on subsea wells due to drilling riser dynamic loads was carried out by the use of analytical model results. The applied analytical methodology is based on a decoupled approach, where global load analyses and local stress calculations are carried out prior to a SN based fatigue accumulation. Applying such methodology on safety critical systems the analytical philosophy should ensure conservative fatigue damage. For cases where the fatigue calculations returned unfavorable estimates, one corrective action has been to measure the actual riser response and to monitor the development of fatigue damage closely. For this purpose a methodology for fatigue estimation based on measured riser response was needed. In this approach of estimating the fatigue damage, the global load analysis results are replaced by measured dynamic load time series. By combining direct riser response measurements with local stress calculations, a revised SN based fatigue accumulation can be performed. The fatigue damage derived from measured riser response is compared to the fatigue damage based only on analytical results. From this comparison the conservatism in the analysis methodology for the global riser response is shown to be significant. As this method relays on measurements, it will only yield historical fatigue damage and at best it can return updated fatigue capacity usage on the fly. Forecasting fatigue damage still have to be established based on global riser analyses results, resulting in a conservative forecast. This paper suggests an updated methodology using actual measured response to both asses fatigue damages of historical operations and forecast fatigue damages based on historic operations. By cycle counts of measured response time series (one hour response) a link between this cycle count and the coexistent significant wave height and spectral peak period can be established. This relationship between observed weather and measured response is representative for the rig and riser system on which the measurements were performed. Then forecast and measurements of the weather conditions can be used to estimate the historical damage and the future fatigue damage respectively. The paper will present results from the suggested approach by use of examples from a real North Sea well in shallow water.

Estimating wellhead loads from lower stack motion measurements is a practical and cost-effective approach. In this paper, a new method is proposed, which is based on system identification techniques rather than Newtonian mechanics, thus omitting reliance on uncertain and variable quantities such as lowerflex joint stiffness/damping, riser and drill pipe tension etc. The proposed method is simple and easy to apply, while maintaining accuracy. Both simulation and real-world measurement data are utilized to demonstrate and evaluate the method.

The scope of this paper is to show how digitized, structured data, as a combination of design data, operational data, riser analyses and measured riser response can be used to enable drilling operations on a subsea exploration with well challenging soil or/and harsh environmental conditions. The structural integrity of the well foundation and soil support has been verified by combining the structured data obtained from measurements with the design information. A sensor system has been fitted to the riser and BOP on mobile drilling units to monitor soil and structural integrity. The combination of pre-operational assessments and monitoring during operations has been carried out for 7 consecutive drilling campaigns, with two different semi-submersible drilling rigs. The work presented in this paper will give a comparison between the measured response and the up-front design analysis, and show show to combine the design information with operational data and measured response to enable future operations. During the design phase of a subsea exploration well a wide range of design assumptions must be considered. The range in high and low estimates for parameters such as soil support or riser and BOP mass and damping forces may be considerable. In general conservative parameters must be selected, leading to worst case scenarios, which again may lead to limited operational windows, introduce high cost mitigating actions or in worst case prevent operations from being carried out. This paper will present the benefit of using actual measured response and operational parameters back into the design loop when planning upcoming drilling campaigns. Structured operational data and measured response is used to improve analysis models; which lead to reduced conservatism. For cases of re-entry on an existing exploration well with heavier equipment, the measured soil support can be used to rule out some worst-case scenarios, and enable the upcoming operation. A case example of an exploration campaign enabled by the design loop will be shown.

During drilling and well intervention (DWI) operations today operating limits are normally given as limiting wave height, and sometimes wave periods. The resulting diagrams are often not directly comparable with weather information received on the rig and the final decisions are often based on subjective assessment of wave height and period. The paper will present how BP, on the newly developed Skarv field in the Norwegian Sea, through thorough planning in the engineering phase has implemented a system where operating limits are specified based on directly measurable parameters such as rig heave and upper and lower flexjoint angles. How weather forecasting can be translated to give the rig crew direct forecasting of the limiting vessel or riser responses (e.g. flexjoint angles or heave), will also be presented. It will be shown how this allows for improved operational planning and support from onshore. Over the last years requirements for oil companies to be able to document the structural integrity of their subsea assets, including wells, has increased. On the Norwegian Continental Shelf (NCS) there has been a particular focus on fatigue loading in the wellhead structure, including the upper sections of casing and conductor, due to loads induced by the riser and BOP during DWI operations. There have been cases where the design fatigue life of a wellhead system limits the number of days one can perform operations with a rig on a given well. This in term affects future oil recovery rates as the well fatigue life may not be sufficient to allow for side step drilling or intervention work required to maintain an optimal production from the well. The paper continues to present how BP on the Skarv field, stores and utilizes the measured lower flexjoint response to track and document well integrity. It will be demonstrated how the return on investment of a drilled well can be improved by documenting actual fatigue loading from each operation on a well compared to conservative design calculations. BP has addressed the above issues in a way that is likely to set a new standard for drilling and intervention operations in the North Sea in the future. 4Subsea AS has provided the engineering and instrumentation services that formed the basis for this paper.