Heavy seas influence riser designs
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The marine riser is the direct link betwen the drilling vessel and the ocean floor. With the expanded economic and political incentives for exploration, the search for hydrocarbons has been extended into increasingly hostile environments. These environments are imposing stresses that may exceed the design limits of an existing marine riser system. One situation that results in high stress is the storm hangoff of the riser and the lower marine riser package (LMRP). To prevent riser failure, the LMRP is unlatched and the riser is hungoff to ride out the storm. If a severe storm coincides with a high-velocity current, the resulting stresses could part the riser. The ideal solution to this problem is to avoid the costly purchase of an improved riser system, yet ensure the survival of the existing system subject to the severe environment loading. There are a few methods to reduce the stress level. One such method is to utilize the advantages of a variable buoyancy system. The two basic riser buoyancy systems are the fixed and variable concepts. Each concept is described. The marine riser is subjected to two stresses: axial compression and moonpool loadings. A variable buoyancy system on the bottom of the risermore » is the most desirable for the prevention of axial compression. To prevent moonpool loading, the riser's hanging weight should be increased, the hydrodynamic drag reduced, the riser physically restrained from contacting the side of the moonpool, and the riser dropped below the moonpool. A variable air-can buoyancy system remains the most practical method to increase the hanging weight of the riser. (DP)« lessKeywords:
Drilling riser
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Offshore ultra deepwater field is being promising as the future of oil and gas reserves. However, the development of ultra deepwater field posed many challenges, in particular, on the selection of the riser concept. Long suspended length of riser will significantly increase the vessel payload. High external hydrostatic pressure on the riser will increase the probability of collapse failure. Large dynamic motions of the vessel and large vessel offset yields potential buckling issues at the touch-down-point (TDP). In addition, potential fatigue problems due to vessel motions and soil-riser interactions also present at TDP area. Large current speed in deepwater field might also lead to vortex induced vibration (VIV) which eventually will contribute to significant fatigue damage for particular riser sections. By looking into these challenges, it is very important to select the most appropriate riser concept for the ultra deepwater field. Catenary Offset Buoyant Riser Assembly (COBRA) as newly developed hybrid riser concept offers a solution to overcome the challenges in ultra deepwater field. In general, COBRA consists of a catenary riser section with a long-slender sub-surface buoyancy module on top which is tethered down to sea bed via two mooring lines. The catenary section from top of the sub-surface buoy is connected to the floater by a flexible jumper. This flexible jumper can effectively absorb the floater motions, which give significant improvements for both strength and fatigue performance on the overall system. As a hybrid riser concept, this concept offers cost effective solution by avoiding all the expensive bottom assemblies that normally needed for a hybrid riser concept. This paper focuses on COBRA riser concept specifically for Santos Basin Central Cluster region at 2200 m water depth. It is observed that there is common sudden change phenomenon on the current direction in Santos Basin area. The effect of bidirectional current is analyzed, and the comparison with unidirectional current is discussed thoroughly. The analyses are focused on the global strength design performance under extreme environmental load and global fatigue design performance of the riser due to wave induced and VIV induced. The results clearly indicate that COBRA riser concept has a robust design and it is feasible for 2200 m water depth, in particular for Santos Basin Cluster Region area. It is also shows that COBRA riser has sufficient strength performance even for extreme bidirectional current.
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Jumper
Drilling riser
Winch
Subsea
Wellhead
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Drilling riser
Mode (computer interface)
Seabed
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The production riser is a critical component of most offshore petroleum production systems. The riser acts as a physical connection between the subsea wells and floating oil and gas production facility. The transference of production and reservoir injection fluids and other wellhead control commands are exchanged between the subsea system and platform thru the riser. In the literature, many different riser systems have been studied and analyzed for use in ultra deep water applications. The Steel Catenary Riser (SCR) is attractive in terms of ultra deep water depth conditions. This system is comprised of a rigid steel pipe installed in a free catenary shape, and the weight of the riser usually limits its use by requiring larger capacity and much more expensive floating production platforms. Catenary shaped risers with lighter material such as aluminum seem to be an alternative which greatly reduces the riser weight thus allowing the use of smaller floating platforms and ships. The main objective of the present paper is to present and to discuss procedures involving analysis of the operation and design of petroleum production riser systems, and make comparison between catenary riser system with steel pipe and other with lighter material. Systems are analyzed to water depth up to 3000 meters. Hydrodynamic loading due to currents and waves along with floating platform motions are included. Parametric results are presented to identify a feasible solution for very deep water depths with focus on a feasible sytems for pre-salt reservoirs.
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Wellhead
Subsea
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The term ‘riser recoil’ refers to the situation when the lower end of a top tensioned riser is released, and the riser is lifted up by the riser tensioner and/or top motion compensator system on the supporting vessel. The elastic energy stored in the riser is then released, and the riser ‘recoils’. This paper focuses on the case of planned disconnect. Recoil of Marine Drilling Risers has been the subject of several research papers over the past two decades. Some examples are listed in references [2] through [7]. Completion and Work Over (CWO) risers are unique in the sense that they may be simultaneously connected to both the riser tensioner system and the top motion compensator system of a drilling vessel. A Marine Drilling riser, on the other hand, is only connected to the riser tensioner system. Typically the riser tensioner system has a stroke of ± 8–9 m, whereas the top motion compensator system has only ± 3.5–4 m. It is imperative that the connector is lifted clear of the subsea structure in order to avoid damage to the equipment after the riser has been disconnected. The operating window for planned disconnect of CWO risers is severely limited by the available stroke of the top motion compensator. One of the purposes of the disconnect analysis is to establish the maximum wave height at which there is still sufficient clearance between the connector and the subsea structure after disconnect. Previous experience has shown that this may be the governing limitation for workover operations. The current industry practice is to use a regular wave approach in the analysis. The wave frequency is varied in order to find the maximum response, and hence one is actually searching for the extreme response, without paying attention to the probability that this will occur. In this paper a new method is presented, where the analysis is based on an irregular wave approach and the Monte Carlo technique, using time-domain simulations. Acceptance criteria are established based on a stochastic analysis, and are based on target levels of probability of exceedance. The results are documented through a case study of a typical CWO riser system connected to a semi-submersible in typical North Sea environmental conditions. The semi-submersible and the CWO riser system are exposed to both regular and irregular waves. Comparison of the resulting allowable wave height indicates that using the approach presented here with irregular waves will give a considerable increase in the operating window, and the resulting operability, compared to a regular wave analysis.
Drilling riser
Subsea
Workover
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Typical riser analysis for a marine drilling riser using the conventional composite modeling scheme will have only one line of elements mathematically representing riser pipe and auxiliary lines. The mass and weight of the riser system may be properly modeled. It is up to the analyst to include auxiliary lines’ stiffness, bending and/or axial. The rule of thumb is to ignore them if the riser is not of load-sharing design, otherwise include them. However, even with stiffness included in the model, the composite modeling scheme will not capture the load path correctly. Parametric studies were performed to develop a modeling scheme for a load-sharing marine drilling riser. This paper compares the results by the newly developed modeling scheme with the composite model. Significantly different tension patterns between the two models are observed.
Drilling riser
Tension (geology)
Parametric model
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Riser recoil analysis for drilling at a harsh environment, deepwater site is particularly challenging. Recoil analysis results are used in conjunction with results of other riser analysis to develop a practical and safe riser management plan for drilling with a dynamically-positioned drillship at a site where rapidly-developing, high seastates occur frequently. Details of the recoil analysis are discussed along with brief references to other riser analyses that were carried out. The recoil analysis showed that criteria to avoid contact between the LMRP and BOP stack after disconnect and to limit slack in the riser tensioner ropes governed the selection of top tension settings. These recoil criteria also governed the maximum sea state in which the riser could remain connected. Recoil analysis was an important part of the effort to develop a practical and safe drilling riser management plan for a challenging location.
Recoil
Drilling riser
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With the development of deep water drilling engineering, marine riser has become the important equipment. With the increase in water depth, the failure of marine riser is very serious, the vibration is the main reason. According to the actual situation, the model of marine is set up, the rule of lateral vibration is obtained. The result is helpful to avoid the phenomena of resonance of marine riser under wave loads.
Drilling riser
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When the oil exploration and exploitation moved away from the shore to deeper ocean, the riser interference became an important issue to be addressed during platform design. It became so important, particularly, when a great numbers of rigid risers are installed on the platform. Those kinds of risers are particularly used at Spar buoy and TLP platform. A conservative approach for riser interference design can result on the oversize platform or to reduce the number of wells hanging on it. On the other hand, a non conservative approach can bring serious problems, like collision of risers, which can cause a dent on the riser wall and it may reduce its fatigue life. In order to identify the hydrodynamic behavior of one riser placed in the vicinity of other one, Petrobras R&D Center carried out some experimental and numerical studies for getting their hydrodynamic behavior when they are aligned and spaced by 2, 3, and 5 times diameter.
Buoy
Spar
Drilling riser
Numerical models
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A disconnected riser suspended from a drillship may damage itself or the drillship during storm conditions or high currents. Evaluation of operational limits for long disconnected risers must account for both axial and lateral problems. Frequency domain riser analysis methods are used to predict operational limits for a typical 6000 foot riser with air can buoyancy modules. The effects of venting the air can buoyancy modules on storm survivability of the riser are evaluated for the 6000 foot riser as one method of increasing storm survivability of long disconnected risers.
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Drilling riser
Deepwater drilling
Towing
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