Sun, L. (Dalian University of Technology, Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration) | Ding, Y. F. (Dalian University of Technology) | Zheng, J. T. (Wuhan NO.2 Ship Design Institute) | Zong, Z. (Dalian University of Technology, Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration) | Liu, C. F. (Dalian Ocean University)
Offshore platforms exerted by long-period flow may experience large vortex induced motion (VIM), which will induce fatigue damage of chains and risers connected. To study this problem, the discrete vortex method (DVM) is employed here. The vortex-induced vibration of twodimensional elastically-supported cylinder at high Reynolds numbers is computed to verify the DVM model. Furthermore, the VIMs of a low-mass-ratio Spar platform and a simplified semi-submersible platform are both simulated to estimate the hydrodynamic efforts induced by different parameters. Based on the analysis of the characteristics of wakes and motion response, the mechanism of VIM has been partly revealed, and some conclusions have been obtained.
Marine structures used for oil and gas drilling or resource exploration in deep water, such as semi-submersible and Spar platforms, are exerted by currents, inducing alternate vortex shedding and unsymmetrical wake field. The unbalance pressure acting on the body causes structural vibration, which is so called vortex-induced vibration (VIV) (Smith, et al., 2004). However, as for Spar and semi-submerged platforms, their entire hydrodynamic character that causes large transverse displacement, accompanied with intense vortex shedding when they are located in flows with high Reynolds numbers. i.e., Re>105 (where Re=UD/v is the Reynolds number, and U, D and v are the free stream flow speed, the characteristic length of the platform and the kinematic viscosity of the fluid, respectively), which is called Vortex-induced motion (VIM).
Traditional experiments of VIV often choose cylindrical risers as models. Motion mode can be set as self-excited and forced vibration to analyze the relation among different parameters. Feng (1968) and Williamson (1999) conducted VIV experiments of a cylinder in wind tunnel and water tank, respectively, and both concluded that the structural response is related to the mass ratio. Stappenbelt, et al. (2007) studied the VIM response characteristics of a cylinder with low mass ratio in experiment. To investigate the influence of Reynolds numbers, Sarpkaya (1995) analyzed the loads and wakes of a forced-vibration cylinder with various Reynolds numbers. Through large scale model tests, Roddier (2009) drew the conclusion that in low reduced velocity, the VIM of Spar platform is not sensitive to the change of Reynolds numbers. In fact, the transverse movement is larger than the stream-wise movement. Rosetti (2012) figured out that the sway amplitudes of two-freedom motion was always larger than that of single-freedom and was closer to the reality through model tests in three tanks with different specifications. With the utilization of helical plates and other attached structures, the three-dimensional effect should be considered. Cueva (2006) conducted model test on single-cylinder Spar platform to study the influence of the three-dimensional effect and justified that it's non-ignorable. Similar to VIV, it's also observed that the lock-in range and lock-off range did exist in VIM of Spar. For different sectional geometry, the VIM of semi-submersible platforms shows different characteristics. Rijken (2008) carried out the experiment of VIM on one semi-submersible platform with four square columns in high Reynolds numbers, and the results showed that the platform moves along its diagonal line during the “lock-in” region and the transverse amplitudes are approximate to the stream-wise amplitudes in high reduced velocity. Bai (2013) obtained the conclusion about the relationship between the sway motion and the reduced velocities together with the incident angles through the test on a typical semi-submerged platform DDS. The test also stressed the importance of yaw motion during the research on response of VIM.
Liu, Weiwei (COTEC Offshore Engineering Co., Ltd.) | Wang, Jin (COTEC Offshore Engineering Co., Ltd.) | Huang, Jia (COTEC Offshore Engineering Co., Ltd.) | Liu, Yang (COTEC Offshore Engineering Co., Ltd.) | Li, Yang (COTEC Offshore Engineering Co., Ltd.)
In recent years, Spar platform has become one of the most attractive deepwater development concepts due to its superior stability and strong operability suitable for dry-tree drilling and production in a wide range of water depth from 300m to 3000m. A new concept Spar platform, namely Spar Drilling Production Storage Offloading or SDPSO, combining the advantages of the deep-draft classic Spar configuration with the capability of oil storage, will be studied in this paper. This new concept Spar or SDPSO doesn't rely on a subsea pipeline system for oil export, thus significantly improves the flexibility of technical solutions and decrease the overall cost for deepwater marginal field development. Unlike conventional ship-shaped FPSOs, the SDPSO uses the oil-over-water or oil-water displacement method for oil storage. This paper first will provide an overview of the oil storage and offloading systems of the SDPSO. In general, two methods can be used for offshore oil storage loading and offloading, namely oil-gas displacement method (commonly used in a conventional FPSO) and oil-water displacement method. Due to its economical advantage and efficiency, the oil-water displacement method has been widely used in fixed gravity based structures in the North Sea and offshore Canada. In principle, the oil- water displacement method for oil storage and offloading is simply based on the natural separation of oil and water by gravity as the density of oil is lower than that of sea water. Since the SDPSO is a floating platform, the wave induced motions of the floater may cause certain effects on the oil-water interface mixture and potential pollution risk in the water discharged into the sea when the sea water is displaced out by oil in the storage tank. This paper will discuss typical process flow diagrams (PFD) of the oil storage and offloading systems of the SDPSO and provide a new system design which not only retains the advantages of oil-water displacement method for oil storage and offloading, but also eliminate the potential risk of environmental pollution from the displaced sea water discharged into the sea.
A new concept Spar-FPSO is proposed for deepwater oil field development integrating the advantages of the deep-draft Spar concept and for oil storage. Unlike conventional ship-shaped FPSOs, the Spar FPSO uses the oil-over-water storage method or oil-water displacement technology for oil storage, which is similar to that used in the fixed gravity-based platforms.
In order to investigate the VIM’s influence to mooring line tension and the platform motion characters, towing model test and wave basin model test are done. Based on model test conclusions, some corrections are added to numerical model, and perform a good consistency between numerical results and model test values. And from a full scale numerical stimulation, the VIM‘s effect on the mooring tension and Spar-FPSO’s offset are discussed.
A simple mooring line fatigue sensitivity analysis about VIM’s influence is done by using API simple summation method. VIM phenomenon lead to a more complex mooring tension distribution, higher offset motion and more serious mooring line fatigue problem. In order to better understand VIM character, more studies are needed.
Cylindrical structures exposed to a current create alternating eddies, or vortices, at a regular period. Figure.1 show how these eddies appear in the downstream wake of a cylinder. When the vortex shedding period close to the natural period of platform, “lock-in” phenomenon may appeared. The VIM may lead two aspects to mooring system of Spar1:
Firstly, inline pulsing drag force and transverse lift force would increase mooring line tension load;
Secondly, the long period response would increase mooring line fatigue damage.
With Spar platform taken into operation since 1990s, many studies are done on Spar VIM. By monitoring, the Genesis Spar has experience as large as 40% its diameter VIM, which is two times compared with design value 17% diameter (David W. Smith, 2004)2. Mehernosh Irani(2004) 3 described Spar VIM model test method , given a conclusion that model test is an effective method to analysis Spar VIM. By towing model test, uniform current flow could be simulated approximately. Un-uniform current flow could be simulated by using wave basin model test. Radbound Van Dijk(2003) 4 describe a VIM model test of Truss Spar and compared platform VIM behavior with and without strake, found the strake could reduce VIM motion amplitude effectively. Weiwen Zhao (2014) 5 using CFD analysis Spar’ VIM under uniform current flow, shows a good consistency with towing model test results.
Poll, Philip (Houston Offshore Engineering) | Park, Y.C. (Williams Field Services) | Converse, Robin (Williams Field Services) | Godfrey, Dan (Williams Field Services) | Gian, Michael (Gulf Marine Fabricators)
Historically, Spar hulls have been compartmented using both flats and radial bulkheads. These hulls were fabricated into quarter or half sections and joined into full cylinder sections then into the full hull length while in a horizontal orientation. This approach left considerable fabrication work to be performed after assembly, disadvantageously, due to the horizontal position.
The configuration of the Gulfstar FPS reduces the number of radial bulkheads by locating the flats closer together, thus opening up the option to fabricate full cylindrical sections upright with open tops for ventilation and access during fabrication. Rotating each cylindrical section for final assembly was not required until all possible work, inside and out, on each section had been completed. This fabrication plan leaves only structural and systems tie-ins between sections once blocks are rotated and set in place.
Vertical fabrication of sections starts an overall structural arrangement philosophy to develop a configuration that is not only repetitive and efficient but also "fabrication friendly." The idea for improving constructability permeated other aspects of structural design. The purpose of this paper is to summarize some of the ways in which the Spar structure was arranged to improve fabrication and to evaluate the expected improvement with actual fabrication experience. This paper presents the genesis of the basic framing then culls out and discusses some notable lessons learned.
The information and results presented in this paper are applicable to engineers, designers and fabricators when evaluating structural arrangements and fabrication options for stiffened plate hulls, particularly Spar hulls. Cost and schedule benefits can potentially be achieved by incorporating some of the lessons learned presented in this paper.
Internal solitary waves (ISWs) can impose significant loads on offshore structures. During the past several decades, ISWs have led to a series of incidents. With large draughts, spar platforms can be affected by ISWs; therefore, the ISW effect should not be neglected during the design, installation and operation in areas where ISWs can occur. However, there is a gap in research into this area with only a few preliminary studies published so far.
In this paper, a series of experiments are carried out with a spar platform model in a large-scale stratified tank at Shanghai Jiao Tong University to investigate interaction characteristics of ISWs. Based on a two-layer ISW theory, a simplified theoretical model is established for predicting the ISW loads on a spar platform. It is shown that the horizontal ISW loads consist of drag and inertia components, which can be calculated using the Morison's equation, while the vertical loads are mainly the vertical Froude-Krylov force, which can be obtained by integrating the ISW-induced dynamic pressure over the spar bottom.
By fitting theoretical results with experiments, specific formulas are established to determine two empirical coefficients in Morison's equation under ISW conditions, which are different from traditional methods. The numerical results from this method show good consistency with experimental results on both the amplitude of the loads and the time varying characteristics. This research provides a practical theoretical model for predicting ISW loads on spar platforms.
Williams Field Services (WFS) use of the classic Spar hull form for the deepwater field development projects is the first classic Spar hull since ExxonMobil installed the Diana/Hoover Spar in 2000. Since that time, 14 Truss Spars and one Cell Spar have been installed. Learnings from those Spars along with increases in the metocean condition since 2000, significant increases in topside payload during detailed design, support of the five initial SCRs on a porch at the keel plus the decision to fabricate the hull in a US graving dock provided new challenges for the Naval Architects from design through to hull installation.
The paper discusses the key drivers, constraints and criteria that had to be reconciled into a floating system with acceptable global motions, acceptable horizontal trim and robust characteristics for changes in topside payload and CGs. Examples of these sometimes conflicting design requirements include: the increased hull freeboard which was driven by the new metocean condition which then led to a higher topside VCG, a 30% increase in the topside's maximum operating payload between the end of FEED and midway into final design yet the depth and length of the graving dock dictated that these changes had to be accommodated without increasing either the diameter or draft of the hull. This paper presents the challenging results of the model tests and their impacts on the design of the mooring system as well as the very close tolerances in trim and stability for the tow out of the graving dock and then to site.
As the development of model ocean engineering techniques, Spar off-shore platforms have been widely used in the area of deepwater drilling. Vortex Induced Motions (VIM), as a common phenomena of Spar platforms exposed to flow, is one of the main factors that affect the lifecycle of offshore platforms and should be avoid as much as possible in the design stage. Two common effective ways to mitigate VIM are the configuration of helical strakes and the adjustment of mooring line stiffness. The former could change the flow pattern in the vicinity of Spar hull and the latter can change the eigenfrequency of platforms in still water to avoid resonance frequency. There have been many investigations on Spar VIM both numerically and experimentally. In this paper, VIM of bared cylinder and straked Spar are compared numerically in uniform current at model scale and at different Reynolds numbers. Fundamental study of VIM is done by comparing motion amplitude at different reduced velocity. To predict the motion of Spar, a spring model is employed. To capture the detailed eddy information of the flow, Large Eddy Simulation (LES) is applied. All the simulation are done at a model scale (1:60).
Between 2005 and 2010, three major events led to a significant increase in design demands of deepwater field developments in the Gulf of Mexico (GoM): Category 5 hurricanes Katrina and Rita (2005), The Macondo well blowout (2010), and The development of deeper, tighter, more remote reservoirs
Category 5 hurricanes Katrina and Rita (2005),
The Macondo well blowout (2010), and
The development of deeper, tighter, more remote reservoirs
These events have resulted in increased metocean criteria, new safety regulations and functional requirements associated with producing deeper, higher pressure and lower porosity reservoirs. This paper will examine and contrast the design impacts on Tension Leg Platform (TLP), Semi-submersible and Spar floating platforms before and after these events. The overall impact of these new requirements on topsides, hull, station-keeping and riser systems of recently sanctioned TLP, Semi-submersible and Spar platforms will be compared with pre-2005 sanctioned platform analogues to demonstrate the resulting impacts on platform size and cost.
Increased demands of post-2005 sanctioned GoM floating platforms have resulted in higher deck elevations, greater topsides payload, more robust station-keeping systems and larger hull displacements. Further, the feasibility of proven risers and well systems is challenged by the higher wave induced motions associated with greater design and survival sea-states and high pressure reservoirs. The design impacts of pre- and post-2005 sanctioned TLPs (Mars A, Olympus), Semi-submersibles (Atlantis, Jack St. Malo) and Spars (Tahiti, Lucius) on topsides, hull, station-keeping and riser systems will be compared and differences explained.
This paper will enable Operators and platform designers to: Appreciate the magnitude of impact on size and cost of floating platforms of post-2005 requirements, Understand the relative impacts on the three major hull types commonly used for GoM developments, and Update analogues and norms used in benchmarking and concept selection
Appreciate the magnitude of impact on size and cost of floating platforms of post-2005 requirements,
Understand the relative impacts on the three major hull types commonly used for GoM developments, and
Update analogues and norms used in benchmarking and concept selection
Description of the Proposed Paper:
The scope is to study the hydrodynamic response of the spar (39m diameter and 198.1m draft) using WAMIT. The analytical expression proposed by Tao et al., is used to estimate the viscous damping for the spar with the disc at the bottom. The disc increases the added mass and damping. The spar with single disc configuration is studied varying the disc diameter. The spar with twin disc configuration is studied varying the spacing of disc. The disc diameter and spacing is obtained for the permissible heave response of 5m and pitch of 4 degrees considering the swell (regular wave height of 2m and wave period of 25s) and sea (regular wave height of 20m and wave period of 14s) states on the east coast of India.
The results can be used to design the spar with single or twin disc. The disc diameter and spacing of the disc can be determined for the site specific swell or sea wave height and period.
Results, Observations, and Conclusions:
The heave and pitch responses are observed to be within limits for single disc configuration with disc diameter equal to 1.4 times spar diameter and twin disc configuration with disc diameter equal to 1.3 times spar diameter and disc spacing equal to 0.4 times disc diameter.
Significance of Subject Matter:
1. The effect of single disc with varying diameter and twin disc with varying spacing on spar response is predicted.
2. The effect of viscous damping in addition to radiation damping is studied.
3. The effect of swell wave is found to be the significant for survival condition.
This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 24511, "Deepwater Floating Production Systems in Harsh Environments: A Look at a Field Development Offshore Norway and the Need for Technology Qualification," by T.S. Meling, Statoil, prepared for the 2013 Offshore Technology Conference Brasil, Rio de Janeiro, 29-31 October. The paper has not been peer reviewed. Copyright 2013 Offshore Technology Conference. Reproduced by permission.
Deepwater field developments are regarded as standard technology in several areas of the world, but in harsher environments, extreme design loads and increased fatigue loading become more challenging. In particular, risers and mooring solutions are vulnerable to loading conditions in harsh environments. Development solutions that work well in more-benign environments may not work as required in deepwater harsh environments such as those offshore Norway.
Floating production systems in harsh environments have long been in operation, but have been limited to water depths of 400 to 500 m. Deepwater floating production systems have been in operation in benign environments such as offshore Brazil, west Africa, and the Gulf of Mexico (GOM) for almost 2 decades. After Hurricanes Katrina and Rita, the design level for the GOM has been increased, and one can argue that the GOM is no longer considered a benign environment. But tropical-storm areas such as the GOM have significantly less dynamic loading in general than harsh-environment areas such as offshore Norway.
Currently, Statoil and its partners OMV and ConocoPhillips have sanctioned the gasfield development Aasta Hansteen in 1300-m water depth in the Norwegian Sea. Field developments are moving into deeper waters in some of the harshest conditions in the world.
Concept Selection: Deepwater Floating Production in Harsh Environments
Aasta Hansteen is a rich gas field located 300 km from shore. It consists of three reservoirs, and the drainage strategy is pressure depletion. Dry trees offer no benefits; therefore, the field will be developed with subsea trees, seven in total, tied back to a floating production facility. There is also a limited amount of condensate present.
The plant processing capacity is higher than needed because the development opens a new area; the production facility will be used as a hub. The treated gas will be exported through a new approximately 500-km-long pipeline to Nyhamna for final processing and further export (Fig. 1), and the produced water will be cleaned and disposed of at sea. However, the small volumes of condensate caused some challenges for concept selection. Economically, the condensate is more of a burden than a value because a solution is required for the limited production. Export through the pipeline was not feasible from a flow-assurance standpoint; and, because the closest existing facility with storage capacity is 150 km away, a separate condensate- export pipeline was not attractive either. Therefore, local storage of 25 000 m3 of condensate and offloading was required at the lowest possible cost.