Jin, Ruijia (Tianjin Research Institute for Water Transportation Engineering) | Xiong, Yan (Tianjin Research Institute for Water Transportation Engineering) | Wang, Yina (Tianjin Research Institute for Water Transportation Engineering)
This study is focused on influence of small-scale cylinders on the motion response under the wave action so that the response can be forecast more exactly. A mooring Truss Spar platform is calculated, and the numerical results demonstrate that the existence of the small-scale cylinders may lead to the decrease of the motion response of the platform, while the nature frequency will move to the low frequency. The phenomenon explains that the small-scale cylinders of the Truss Spar play an important role of damping and change the add mass of the platform, so in order to get the more accurate results of the motion response, the wave loads of the small-scale components unable to ignore.
Ocean structures, i.e., Truss Spar platform and Semi submersible platform can be divided into large-scale structures and small-scale cylinders via to the ratio of the cross-sectional dimension to the incident wave wavelength. Because the effects of different scales of structures on the wave field are different, different theories can be used to calculate the wave load. It is necessary to calculate the wave loads by diffraction radiation theory for large-scale structure since the coupling effect of structure and wave field needs to be considered (MacCamy and Fuchs, 1954). However, because the impact of small-scale cylinders on the wave field is negligibly small, Morison formula is always applied to calculate the wave load (Morison et al. 1950). Chung (2018) studied the Morison formula in practice and hydrodynamic validity recently.
Truss Spar platform occupies an absolute proportion in all Spar platforms, which is the main form of Spar platform today. Many scholars have carried on the relative studies of the Truss Spar platform. Chen et al. (2007) applied Morison formula to consider the truss structure only under the consideration of incident wave field, but didn't take into account the body changed the overall wave field. Li and Ou (2009) estimated the contribution of heave plates to heave hydrodynamic force of Truss Spar and predicted the heave response in extreme ocean conditions fleetly and effectively by the combination of experimental and numerical results. Shen et al. (2012) applied ITTC two-parameter wave spectrum numerically studied the heave motion of Truss Spar in random seas considering diffraction effect. Sadeghi et al. (2004) found that the existence of truss structure of Truss Spar platform affected the radiation damping and added-mass and thus affected its motion response.
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.
Offshore platforms are subjected to severe loads due to wave, current and wind. The floating structures are stationed in deep waters by using the mooring lines connecting them from the fairleads to ocean-bed. Considering the cost impact of mooring lines, it is essential to find the best possible mooring configuration for a given platform and met-ocean data, which reduces the responses of platform to an acceptable level. Hence, a study is conducted to find the effect of different mooring line pretensions on the dynamic responses of truss spar platform. The truss spar platform is modelled as a rigid body with three degrees-of-freedom i.e. surge, heave and pitch. The incident wave kinematics of water particles and wave forces on the structure are computed using the linear Airy wave theory and modified Morison equation respectively. The dynamic response analysis of spar platform is performed in the time-domain using the implicit Newmark Beta integration technique combined with the average acceleration method. The various mooring stiffness for different line pretensions are obtained by using quasi-static approach. Two numerical codes DATSpar and QSAML are developed in-house to conduct the dynamic analysis and quasi-static analysis respectively. It is observed that the dynamic responses of platform can be optimised by using mooring lines with relatively low pretensions. Although the platform dynamic motions are not significantly affected, its mean position varies for different mooring line pretensions.
In recent years, offshore explorations are increasing more towards deeper waters in order to meet the demand–supply equity. Many innovative floating platforms such as the articulated leg platform, semisubmersible platform, spar platform, tension leg platform, tethered buoy tower, etc. are proposed for deep waters. The spar platforms are among the largest offshore platforms in use for deep waters. They are kept stationed in the ocean environment by using mooring lines.
Pretension installed in the mooring lines has an appreciable impact on the design of floating structures as this tension at the fairleads shall be incorporated in the platform’s structural design. Thus, it is essential to find the effect of mooring line pretensions on the responses of platform. This will aid in installing an optimum pretension in the mooring lines and reduce the project cost without compromising on its performance (Bruno da Fonseca Monteiro et al., 2010).
Shen, Wenjun (Tianjin Research Institute for Water Transportation Engineering, National Engineering Laboratory for Port Hydraulic Construction Technology) | Gao, Feng (Tianjin Research Institute for Water Transportation Engineering, National Engineering Laboratory for Port Hydraulic Construction Technology) | Jianbing, Qu (China Classification Society Offshore Engineering Technology Center) | Zhao, Peng (Tianjin Research Institute for Water Transportation Engineering, National Engineering Laboratory for Port Hydraulic Construction Technology)
The motion equation of the Spar platform under stochastic wave and wind is established considering the static stability and drainage volume changes, including higher order nonlinear term and the effects of instantaneous free surface. Then a 2-DOF model is used to study the heave and pitch coupled dynamical response under the stochastic environment condition. Wind velocity and wave surface elevation are obtained by the method of superposition of harmonic in the time domain. Different conditions of wind velocity are calculated using the Runge-Kutta numerical iterative algorithm, and the results are compared with the response which is not considered the wind load. The results show that, wind load will not produce a resonance motion, but will increase both the heave and pitch amplitudes by coupling relationship between the 2-DOF. So besides the wave load, the wind load also plays an important role in the Spar motion response.
As the coupling between heave and pitch motion of Spar platform is strong, so the response of heave motion plays a very important role in the stability of the whole platform. Because the draft of Spar body is large, the high frequency part of wave excitation force on Spar body decreases with the increasing of water depth, and then the low frequency wave load plays a leading role in the heave motion. There will be heave resonance movement under the effect of swell, and the fixed but time-dependent. So the restoring moment is heave amplitude will be large, which make the volume and shape of Spar immersed in water change constantly. Displacement volume and metacentric height GM are not time-varying, and the parametric excitation motion may happen, which is the main reason of causing motion instability under severe sea condition. So it is important to study the heave-pitch coupling response of Truss Spar in stochastic waves, which can provide reliable theoretical basis for the design of it. T. Purath (2006) calculated a truss spar interacting with its mooring and riser system, during the numerical study, various hydrodynamic parameters which were crucial to the accuracy of predicting the global motions of the truss spar and tensions in mooring lines and risers were scrutinized. Zhao Jingrui (2010) calculated the nonlinear coupling dynamic response of a Classic Spar under regular wave, and the results show that, internal resonant and combination resonance response of heave-pitch motion can be suppressed by increasing system damping and adjusting the frequency ratio of heave and pitch. Shen Wenjun (2011) conducted a stochastics analysis of coupled heave-pitch motion considering the stochastic wave loads, it was found that, when the characteristic frequency of a stochastic wave is close to the natural heave frequency, the large amplitude pitch motion is induced under the parametric-forced excitation, which is similar to conclusion in the regular wave.
This paper provides an overview of the design and installation of Anadarko's Lucius Truss Spar. This spar is the seventh spar designed and delivered in the operator's fleet of offshore production facilities. The design and engineering of the spar and mooring systems were performed using the most current environmental and regulatory requirements for floating facilities. The spar hull was fabricated in Finland and traveled to the gulf coast by dry transport. There it was outfitted and then wet towed to Keathley Canyon Block 875 in preparation for upending and mooring installation in 7,100 ft of water. The offshore campaign for the installation of the hull and mooring system was successfully completed in October of 2013 in preparation for the Topsides set in February of 2014. This paper will summarize the design and fabrication of the Lucius spar as well as explore the project challenges for the delivery and installation of the facility, with special attention given to the issue of safety. The Lucius Spar is the operator's largest spar to date and uses the latest technology to moor the hull in very challenging terrain. The installation contractor for the hull and mooring system, provided extensive engineering and project management to plan and execute the installation safely.
Spar and semi-submersible platforms are proven deepwater production platforms. They have been used extensively in the Gulf of Mexico (GoM) to produce large deepwater developments. Spars have progressed from 600 m to 2,600 m water depth since 1995 and semi-submersibles from 450 m to 2,500 m since 1985. Topsides and hull sizes have grown to accommodate increasingly larger topside, riser, and mooring payloads. Extending these platforms to 4,500 m represents an almost 80% increase in water depth. The main technical challenges relate to the technical feasibility and installation of risers and mooring systems and their impact on the size and global performance of the hulls. Both aspects are examined in this paper for applications in the GoM.
The first aspect addresses technical challenges associated with designing production, injection, and export risers to handle the large hydrostatic pressure and limitations and/or gaps with regard to current lay vessel capabilities to install risers in 4,500 m water depths. The second aspect addresses the relative sensitivity of hull sizes and global performance of semi-subs and spars at these water depths for a common topsides and subsea architecture basis of design.
This paper presents the impact of extending design, fabrication, and installation of spar and semi-sub floating platforms to 4,500 m water depth. Field development scenarios, design basis and topside parameters (load, area, and height) are used to size hull, mooring, and risers to define the overall system. Proven, conventional hull designs and sub-systems technologies are used for both 2,500 m and 4,500 m water depths. Platform sizes and global responses in 2,500 m and 4,500 m water depths are compared. Technology gaps in meeting basis of design are identified. Trends, observations and relative sensitivities of the hull forms are captured. Enhancing and enabling technologies for risers and moorings are discussed. The 4,500 m hull sizes and key dimensions are compared with existing platforms to address availability of yards and vessels for construction and installation phases. Challenges related to fabrication, topside integration, mooring and risers installation in 4,500 m water depth for the two hull types are also addressed and contrasted.
This paper is based on a DeepStar study undertaken to evaluate innovative deepwater floating platforms and riser systems, with dry tree (DT) or direct vertical access (DVA) capability, for development of marginal fields and benchmarking with proven dry tree platform concepts [
The designs were developed for three regions (Gulf of Mexico, West of Africa, and Offshore Western Australia) and three water depths (3,000 ft, 6,000 ft, and 8,000 ft) for a common design basis with defined topsides and riser payloads, and specified design constraints. The estimates for novel designs were obtained from eight Concept Owners, and a comparative assessment was performed for 13 designs in three groups for nine combinations of regions and water depths. Coupled analysis was performed for most novel designs for GoM 6,000 and WoA 6,000.
The five field development themes with DT benchmarks, novel hull, novel riser, drilling or process will be described. The key design, global performance, and constructability features of alternative concepts will be discussed. The variability in key estimates for three groups will be identified. The novel hull designs evaluated enable integration of topsides at quayside or by floatover and provide ability to support DTs and long stroke RAM tensioners. For designs evaluated with a modular drilling rig and upto six TTRs, lower CAPEX is estimated.
The ranges of key design data, tensioner stroke, and global performance for each group have distinct variations. The novel riser design considered is Compliant Vertical Access Riser (CVAR), which is an offset riser with no tensioners, thus it enables use of semi-sub design with lesser draft. The development themes with wet trees and a drilling riser reduce the payload on hull.
Technology readiness level (TRL) varies for the novel concepts. The detailed comparative assessment undertaken showed high potential for standardization of hull designs. In ultra-deepwater regions with harsh metocean conditions, several of these novel hull designs provide enabling solutions.
This is the first major effort undertaken with evaluation of 13 alternative design cases with DT or DVA capability. The evaluations done for three regions would also be applicable in many other regions. The documentation of estimates in three groups identifies benefits from novel designs over benchmarks designs in each group, and distinct variations among groups. These results would help in selection of specific concepts for a region for further qualification and project readiness for development of worldwide marginal deepwater and ultra-deepwater fields.
Shen, Wenjun (Tianjin Research Institute for Water Transport Engineering, M.O.T.) | Tang, Yougang (Tianjin University) | Gao, Feng (Tianjin Research Institute for Water Transport Engineering, M.O.T.) | Jiang, Yunpeng (Tianjin Research Institute for Water Transport Engineering, M.O.T.)
The heave-pitch coupled equation of Truss Spar platform under random waves is established considering the static stability and drainage volume changes, including higher order nonlinear term and the effects of instantaneous free surface. The motion responses are calculated, and the impact of damping factor is analyzed. The results show that, when the characteristic frequency of stochastic wave is close to the heave natural frequency of heave, the large amplitude pitch motion is induced under the random parametric-forced excitation, when the significant wave height reaches a certain value. Increasing the heave damping or increasing the pitch damping is the most effective mean to suppress the pitch instability. Therefore, when design the platform, the number and the structure of heave plate and the reasonable arrangement of spiral side plates are important.
The truss spar is a proven floating production platform for a range of deepwater offshore oil and gas developments. From wells and manifolds on the seabed, catenary risers and umbilicals are routed from the keel of the platform to its topsides through steel pull tubes. Depending on the field architecture there may be a large number of pull tubes arranged within the truss in a complex, tightly-spaced array, where the spacing varies with elevation. Individual tubes may be fitted with strakes or other devices intended to suppress flow-induced vibrations. Fatigue of these tubes is an important consideration that influences their own design as well as riser and hull design. While relatively simple, semi-empirical programs do exist to analyze the fatigue due to vortex-induced vibrations (VIV) of isolated tubes in steady flows, none are generally available to analyze the complex fluid-structure interaction associated with this scenario. Existing models do not include the degradation of strake performance as a vibration suppressor in an arrayed, rather than isolated, configuration; buffeting of tubes in the turbulent wakes of the truss members and neighboring tubes; and, coupled fluid-tube interaction. Coupled computational fluid-structure inter-action (FSI) techniques - finite element (FE) solutions of a structural model coupled iteratively with computational fluid dynamics (CFD) solutions of a fluid model until convergence - are becoming progressively more feasible for addressing this type of problem as computing speeds and capacities continue to increase. This paper will demonstrate how one such computational coupled FSI has been used to deliver useful fatigue results in support of the design of a 35-tube array in a truss spar. It will also describe some key insights into the force-response mechanisms that would not have been discoverable via other analytical methods. Finally, it will make some qualitative comparisons to phenomena observed in physical model tests.
Although engineers often assume that waves and currents are collinear, they propagate in different (unparallel) directions, i.e. the incoming angle between waves and currents is not generally zero or 180° in reality. This paper makes a special effort to investigate how the different incoming directions affect motions of a Truss SPAR platform. In order to achieve this, the six degrees of freedom motion of the platform is considered with the wave and current loads evaluated by a slender body formulation. Various cases with different wave and current parameters are calculated and the effects of waves and currents on mean offset and oscillation motions of Truss SPAR are investigated.