Harstad is not the end of the world but you can see it from there, a real frontier area. From this area above the polar circle exploration and development has been lead in the Norwegian and the Barents seas. Exploration wells are being drilled in the now opened former disputed areas, was it worth the fuss? "Technology forum about the Arctic in the Arctic" has always been the slogan of the SPE Northern Norway Workshop. In March 2019, this two-day biannual workshop will raise the stakes, broaden the scope, and showcase all the latest success in the region.
Haro, Marco Polo Espinoza (Korea Research Institute of Ships, KRISO) | Hwang, Sung-Chul (Korea Research Institute of Ships, KRISO) | Nam, Bo Woo (Korea Research Institute of Ships, KRISO) | Cho, Seok-Kyu (Korea Research Institute of Ships, KRISO) | Sung, Hong-Gun (Korea Research Institute of Ships, KRISO)
A series of numerical simulations were carried out to investigate the current generation performance in deepwater ocean engineering basin (DOEB) of KRISO. In the numerical simulations, unsteady Reynolds averaged Navier-Stokes (RANS) equations were solved to study the flow characteristics of the current generated in the basin. Three different current profiles, i.e. uniform, Gulf of Mexico (GoM) and Brazil profiles, were directly generated by applying the same current generation system as the DOEB and their accuracy was examined. In addition, an optimization algorithm based on cost function is newly proposed to control the inlet and outlet velocities of the multiple culverts for the desired current velocity profile. Through various numerical simulations, discussion are made on free surface elevation, effect of initial ramping operation, effect of false bottom plate, and full 3D simulation.
Nowadays, large amount of the total global production of oil and gas is composite of offshore resources which only may be reached with the help of new floating structures such as Floating Production Storage and Offloading (FPSO), Tension Leg Platform TLP, and SPAR platform, among others. Offshore systems are exposed to extreme environmental conditions for example wind, current and waves.
Current accounted largely in the total load of offshore systems with deep draft such as SPAR platforms. Risers experience vortex induced vibrations (VIV) due to current loads. Floating systems are affected by different current loads based on its depth and location in the world. Current velocity profile in Brazil, North Sea, Gulf of Mexico, Caspian Sea, Persian Gulf, and Yellow Sea among others have been monitored and studied. Characteristics of current velocity profiles were recognized at each particular site.
Hydrodynamics performance of offshore structures due to current loads are investigated through scaled model test. Current loads are modeled mainly in towing tank by towing the model at constant speed and ocean basin through a local jet and other techniques; however, previous method may model only uniform current profile and realistic current profiles cannot be generated. Therefore, some institutions and universities built a new kind of facilities which can generate more realistic environmental loads. Characteristics of these new facilities are described in Alho et al. (2008), Lu et al. (2010), and Bucher and de Wilde (2008). Representative numerical and experimental studies performed are presented next.
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.
Vortex shedding of multi-column floating structures is complex due to the wake interaction between front and rear columns. The vortex shed from upstream columns will impinge upon the downstream columns and change their pressure distributions on the surface, which sequentially affects the dynamic response of vortex-induced motions (VIM). This paper tries to reveal the mechanisms of the vortex shedding, wake interference, and their impacts on the VIM of a paired-column semi-submersible by means of computational fluid dynamics (CFD). In the present work, a scaled model (1:54) of paired-column semi-submersible (PC-Semi) is studied. The CFD solver used in this paper is an in-house CFD code naoe-FOAM-SJTU, which is developed on top of the OpenFOAM framework. Turbulent flows around the geometry are modeled by delayed detached-eddy simulation (DDES). Meanwhile, the motions of the model are constrained in the horizontal plane and obtained by solving six-degrees-of-freedom motions equations. Numerical simulations at different current headings and reduced velocities are performed. The overall motion responses of the structures are evaluated. Vortex shedding process and wake impingement on downstream columns are also discussed. These preliminary results show how the vortex shedding process and wake impingement influence the VIM characteristics of a multi-column floating structures.
Vortex shedding is a common physical phenomenon on flow past bluff body. It is a consequence of boundary layer separation, which is caused by the reduction of velocity in the boundary layer, combined with a positive pressure gradient. The vortex shedding will generate periodic pressure fluctuation on alternate sides of the bluff body. For long and thin cylindrical structures, the pressure fluctuation should lead to vortex-induced vibrations (VIV). In ocean engineering, the vertical columnar shaped floating structures will suffer similar excitations, which is called vortex-induced motions (VIM). VIM is very complicated due to the involvement of flow separation, rigid body motion, mooring stiffness and other physical properties of the system. Understanding the physical principle of VIM is vital to engineers to avoid mooring line fatigue failure.
Liu, Dongxi (Shanghai Maritime University) | Wang, Jin (COTEC Offshore Engineering Solutions, Shanghai Jiao Tong University) | You, Yunxiang (Shanghai Jiao Tong University) | Tang, Wenyong (Shanghai Jiao Tong University)
Based on the oil-water displacement principle, Wang and Luo (2016) developed a new type of offshore oil storage system. This storage system can be applied to a conventional Spar platform or any other platforms. However, there is the potential for the hydrocarbon contamination of the discharged ballast water, for the formation of oil/water emulsion and for heat transfers between the hot oil and the cold water in the wet storage system. When wet storage system is applied to a floating platform, the sloshing of oil-water interface in the storage tank may exacerbate above-mentioned potential problems. Extensive tests, using crude oil and seawater, were performed in a 1:40 scale model of the wet storage system developed by Wang and Luo (2016). The objective of the experiment is to test and prove the feasibility of oil-water displacement principle in floating platforms, to demonstrate the superiority of the present wet storage system, and, finally, to investigate the mechanism of oil-water heat transfer in the vertical cylindrical storage tank. The model is mounted on a sloshing test rig, which could simulate the motion of the floating platform in the practical sea conditions. Movement speed of oil-water interface in the model was to simulate the anticipated movement speed in the proposed storage system and the repeated cycling of all operations. Operating temperature of the incoming crude oil and incoming seawater are 60 °C and 10 °C, respectively. In addition, in order to investigate the effect of liquid sloshing on the process of oil-water displacement, two different cases were done at static condition and at sloshing condition, respectively. In each case the interface was recycled up and down forty times to far exceed any anticipated recycling in the actual storage tank in one year. The important observations of the present work are listed below: The displaced water can be discharged to the sea directly without any treatment; There is no chance for emulsion to form at the oil-water interface; The effect of stratified liquid sloshing on the process of oil-water displacement is negligible; The heat of hot oil is mainly transferred to the exterior of the storage tank.
Votsis, Renos A. (Cyprus University of Technology (CUT)) | Michailides, Constantine (Cyprus University of Technology (CUT)) | Tantele, Elia A. (Cyprus University of Technology (CUT)) | Onoufriou, Toula (Cyprus University of Technology (CUT))
A huge number of marine (coastal and offshore) structures of different types, floating or fixed, and of various sizes have been constructed and installed worldwide. The state of a marine structure during its life cycle must remain in the domain specified in design, although this can be altered by normal aging due to usage, by the action of the environment, and by accidental events. In recent years the field of smart technologies (structural and environmental monitoring), including sensing and control, has been growing at a fast rate. The selection of an effective method for observing the marine structure’s performance and integrity, as well as their control is of paramount importance aiming to give a diagnosis of the ‘health’ of the examined marine structure In the present paper an insight into the monitoring technologies that exist for observing the performance of marine structures is presented. Critical information on monitoring marine structures with regard to the existing technologies, the measured quantities and applications on different types of marine structures, that are used in both offshore (e.g. oil and gas structures and systems, offshore renewables) and coastal (e.g. jetties, pipelines, breakwaters) areas are presented and discussed.
About seventy percent of the earth’s surface is covered by oceans, which offer essential elements for the life on earth like mineral resources, energy and food and provide means for transport and infrastructure. Oil and gas are the dominant sources of energy in our society. Twenty percent of these sources are recovered from reservoirs beneath the seabed while new sources to be exploited being sought in deeper waters and in new offshore areas. However, the oceans are an inexhaustible resource of various renewable energy sources such as wind, waves, currents and thermal difference in seawater. Meanwhile, high technology marine structures are developed inherent with new requirements and design specifications. Marine structures can be categorized based on the industrial technology that they will be used; namely, oil and gas, renewable energy, aquaculture, environmental protection and docking.
This paper presents a comprehensive dynamic analysis of a marine spar platform with various mooring system configurations. From a practical viewpoint, the mooring system configuration is managed by reel-motor devices that change cable lengths while keeping all cables under tension. The spar platform is anchored to the seabed by twelve mooring cables (in six cable bundle arrangements), and the domain that the cable-driven spar platform can be within is called the platform effective area. The analysis is based on a global frame of reference at the seabed and a local frame of reference at the platform center of gravity. Under the context of rigid body dynamics, the averaged values of the mooring cable tension are calculated through the use of a second norm measure. The platform dynamic response under unidirectional harmonic water waves and changeable submerged depths is investigated over the entire spar platform effective area. The minimum platform natural frequency at each location within the effective area is used as a measure of the platform degree of rigidity.
Spar floating marine platforms are often used for offshore operations such as oil and gas exploration and production and wind energy harvesting. A spar platform consists of a floating structure that is connected to a heavyweight spar and anchored to the seabed by a cable-based mooring system. The first spar platform in the oil industry was installed in the North Sea in the 1970s and used for oil storage and offloading (Bax and de Werk, 1974; Van Santen and de Werk, 1976). While the floating platform is exposed to the environmental loads, the mooring system has the objective of retaining the floating structures’ location. Commonly used mooring systems consist of three cables (Karimirad and Moan, 2012; Jeon et al., 2013; Muliawan, Karimirad and Moan, 2013; Muliawan et al., 2013; Si et al., 2014; Kim et al., 2014; Yu et al., 2015), four cables (Downie et al., 2000; Chen et al., 2001; Sethuraman and Venugopal, 2013), nine cables (Zhang et al., 2007; Zhang et al., 2008; Montasir and Kurian, 2011; Montasir et al., 2015), and twelve cables (Wang et al., 2008; Yang et al., 2012).
A number of researchers analyzed the dynamic response of spar platforms through the use of different numerical and experimental techniques. The spar platform motion was investigated by Ran et al. (1996) through the use of a higher-order boundary element method, and they compared their numerical results with the measurement data, showing good agreement. Jha et al. (1997) obtained an analytical prediction for wave drift damping and viscous forces that influence the dynamic response of spar platforms. The effect of nonlinear sea waves on the dynamic response of a spar platform was investigated by Anam and Roesset (2002) through the use of the hybrid wave, stretching, and extrapolation models. Using Morison’s equation, Anam et al. (2003) studied the differences between the time domain analysis and frequency domain analysis in predicting the spar platform slow drift response.
The Paired-Column Semi-Submersible platform(PC Semi) is a deep-draft semi-submersible (DDS) designed by Houston Offshore Engineering (HOE). It has 8 columns instead of 4 columns compared with conventional DDS. In this paper, the VIM characteristics of PC Semi was numerically investigated by our in-house CFD solver naoe-FOAM-SJTU. Drag test and free-decay tests was carried out first, then VIM tests at different reduced velocities were performed. Shear-stress transport based delayed detached-eddy simulation (SST-DDES) was used for modeling the massively separated turbulent flows. To illustrate the benefit of SST-DDES model, an extra unsteady Reynolds-Average Navier-Stokes (URANS) simulation was computed for drag test. Drag, flow fields and vortical structures were compared with those obtained by SST-DDES. For VIM tests, the dynamic overset grid technique was applied to handle the motions of platform. Results showed that the current CFD approach is applicable and reliable for VIM and can be an alternative for model testing.
The origins of semi-submersible floating production systems can date back to 1960s. Till today semis are widely installed in offshore oil and gas fields all over the world. In deep water oil drilling production environment, semis are often favored for their low dynamic response to waves in surge and sway. While for conventional semi, large wave induced heave motion is always a great challenge to the strength and fatigue of its riser and mooring system. To overcome this issue and reduce heave motion, one common improvement is to increase the draft of platform. Deep draft semi-submersible (DDS) is of course such an attractive solution and preferred by offshore industry. The deep draft hull forms can mitigate large heave motion. However, it introduces a new issue, the vortex-induced motions (VIM). VIM is actually the result of transverse fluctuating pressure caused by periodic vortex shedding on long vertical hull of platforms. The periodic sway motions will also accelerate the fatigue failure for riser and mooring system.
Xu, Liangbin (Research Institute) | Zhou, Jianliang (Research Institute) | Li, Chaowei (Research Institute) | Sheng, Leixiang (Research Institute) | Wang, Yu (Research Institute) | Hao, Xining (Research Institute)
This paper provides a coupling dynamic method in which the combined excitation of vessel motion, uniform current, surface wave and internal solitary wave are considered. Then, a case shows that internal solitary waves largely increase envelopes of the deformation and stress of a riser system; and a new concept, “hysteresis effect”, adds the operation time of emergency disconnection. At last, the paper proposes a detailed procedure that dealing with platform positioning during different internal solitary wave spreading. In this procedure, normal operation time and connected mode of a riser system can be maintained for more time by early-warning and pre-deviate the platform.
In the South China Sea(SCS), a large amount of oil and gas resources has been successfully explored during the last three decades. However, the sea state in the SCS is very harsh because of some typhoons, currents and waves. Particularly, internal solitary waves(IWs) occur frequently in the CSC but was not well studied in the previous papers.
IW is one kind of internal gravity waves, that oscillate within a fluid medium rather than on its surface. In oceans, it exists mainly by two conditions: (1) Stratified seawater with density differences, i.e., the density must increase continuously or discontinuously with water depth due to changes of temperature and/or salinity. (2) Disturbance of the seawater, e.g., by currents that flowing over a rugged seabed, by a moving submarine navigation or by a marine earthquake(Cai et al., 2011). An IW often has large wavelength (from dozens to hundreds of kilometers), long period(from dozens of minutes to several hours) and large amplitudes(from several to hundreds of meters). For example, in the western CSC, IWs have amplitudes of 170 m, half widths of 3 km, and phase speeds of 2.9 ± 0.1 m/s(Klymak et al., 2006). Not only the CSC, IWs also appear over continental shelf regions and where brackish water overlies salt water at the outlet of large rivers, such as the Estremadura Promontory off the West Iberian Coast(Magalhaes and Silva, 2012) and Mascarene Plateau of the Indian Ocean(New et al., 2013).
To improve the hydrodynamic performance of conventional semi-submersible platform (SEMI), the large individual column was divided into several small columns, and the hydrodynamic performance of the SEMI with multiple small columns (MSC) was numerically investigated in this paper. For a comparative study, a series of MSC SEMI models together with a conventional SEMI model were established and the displacement of them were kept constant. Detailed parametric studies were carried out in frequency domain to investigate the influence of column number and column diameter on structural hydrodynamic performance of MSC SEMI. The numerical results indicated that the concept of multiple small columns can not only weaken the hydrodynamic load on structure but also improve structural hydrodynamic performance significantly as expected.
The recent emphasis on deep and ultra-deep water hydrocarbon exploration and production, the floating systems, such as the tension leg platform (TLP), Spar platform, and semi-submersible platform (SEMI), are progressively replacing conventional gravity systems to exploit oil and gas in deep water (Low and Langley, 2007; Jiao, 2007). Different from the gravity system, the motion response of floating system is the limiting factor. The motion response of TLPs and Spars are very small, especially in heave direction, due to the tendons of TLP and the deep draft and damping members of Spar (Zeng et al., 2013). However, the TLP tendon design and installation become challenging when the water depth beyond about 1500m (Mansour, 2009; Williams et al., 2010). The Spar's available deck space is constrained by its hull diameter, and thereby pushing designers to stack the deck vertically to provide enough space (Jacobs, 2013). In addition, the topside-hull integration cannot be performed at quayside due to the deep draft of Spar. The topside and hull should be wet towed to the installation site and an extra derrick barge is needed to upending the hull and install the topside. These operations not only introduce greater risks but also incur costly on-site hook up and commission (Chen et al., 2008).