Dynamic Analysis of Semi-submersible Under the Postulated Failure of Restraining System with Buoy

Srinivasan Chandrasekaran,Syed Azeem Uddin,Mubarak Wahab 한국강구조학회 2021 International Journal of Steel Structures Vol.21 No.1

Coupled dynamic analyses of a deep-water Semi-submersible platform, in the South China Sea region, is carried out under postulated damage of the restraining system for both 10 and 100-years return period events. Under the combined action of wind, wave, and current loads, motion responses of Semi-submersible at 1500 and 2000 m water depths are analyzed in timedomain. Dynamic tension variations in the mooring lines are investigated for a fatigue failure using the S–N curve approach. Inclusion of a submerged buoy in the mooring system resulted in a marginal increase of the response due to a reduction in the restoration force of the mooring lines; submerged buoy also resulted in additional damping. The results of numerical studies showed an increase in tension in the mooring lines, which are adjacent to the damaged ones, causing reduced fatigue life. With the inclusion of submerged buoy in the mooring system, there is a considerable decrease in tension variation in mooring lines, increasing fatigue life. Failure of a mooring line causes an increase in tension of the adjacent mooring line, but not valid under all circumstances. It is seen from the studies that despite the postulated failure induced in a mooring, the adjoining line remains unaff ected due to a steady coupling motion of the platform.

Response of triceratops to impact forces: numerical investigations

Chandrasekaran, Srinivasan,Nagavinothini, R. Techno-Press 2019 Ocean systems engineering Vol.9 No.4

Triceratops is one of the new generations of offshore compliant platforms suitable for ultra-deepwater applications. Apart from environmental loads, the offshore structures are also susceptible to accidental loads. Due to the increase in the risk of collision between ships and offshore platforms, the accurate prediction of structural response under impact loads becomes necessary. This paper presents the numerical investigations of the impact response of the buoyant leg of triceratops usually designed as an orthogonally stiffened cylindrical shell with stringers and ring frames. The impact analysis of buoyant leg with a rectangularly shaped indenter is carried out using ANSYS explicit analysis solver under different impact load cases. The results show that the shell deformation increases with the increase in impact load, and the ring stiffeners hinder the shell damage from spreading in the longitudinal direction. The response of triceratops is then obtained through hydrodynamic response analysis carried out using ANSYS AQWA. From the results, it is observed that the impact load on single buoyant leg causes periodic vibration in the deck in the surge and pitch degrees of freedom. Since the impact response of the structure is highly affected by the geometric and material properties, numerical studies are also carried out by varying the strain rate, and the location of the indenter and the results are discussed.

Curvature Ductility of RC Sections Based on Eurocode: Analytical Procedure

Srinivasan Chandrasekaran,Luciano Nunziante,Giorgio Serino,Federico Carannante 대한토목학회 2011 KSCE JOURNAL OF CIVIL ENGINEERING Vol.15 No.1

Correct estimate of curvature ductility of reinforced concrete members has always been an attractive subject of study as it engenders a reliable estimate of capacity of buildings under seismic loads. The majority of the building stock needs structural assessment to certify their safety under revised seismic loads by new codes. Structural assessment of existing buildings, by employing nonlinear analyses tools like pushover, needs an accurate input of moment-curvature relationship for reliable results. In the present study, nonlinear characteristics of constitutive materials are mathematically modelled according to Eurocode, currently in prevalence and analytical predictions of curvature ductility of reinforced concrete sections are presented. Relationships, in explicit form, to estimate the moment-curvature response are proposed, leading to closed form solutions after their verification with those obtained from numerical procedures. The purpose is to estimate curvature ductility under service loads in a simpler closed form manner. The influence of longitudinal tensile and compression steel reinforcement ratios on curvature ductility is also examined and discussed. The spread sheet program used to estimate the moment-curvature relationship, after simplifying the complexities involved in such estimate, predicts in good agreement with the proposed analytical expressions. Avoiding somewhat tedious hand calculations and approximations required in conventional iterative design procedures, the proposed estimate of curvature ductility avoids errors and potentially unsafe design.

Offshore Triceratops Under Impact Forces in Ultra Deep Arctic Waters

Srinivasan Chandrasekaran,R. Nagavinothini 한국강구조학회 2020 International Journal of Steel Structures Vol.20 No.2

In the recent years, offshore oil drilling and production is moving towards ultra-deep Arctic region which demands an adaptable structural form. Apart from the environmental loads, offshore structures in Arctic region will also be subjected to impact forces arising due to ship platform collision. Such loads may endanger the safety of the platform due to the combined effect of reduced temperature and impact forces on the material and geometric properties of the structure. Thus, there is a need to understand the behaviour of offshore structures under impact forces in low-temperature conditions. Offshore Triceratops is one of the recent new-generation compliant platforms proved to be suitable for ultra-deepwater applications. The main aim of this study is to assess the response of triceratops under impact forces in Arctic environment numerically. As the buoyant legs of triceratops are susceptible to impact forces arising from ship platform collision, the numerical model of a buoyant leg is developed using Ansys explicit dynamics solver. The impact analyses is then carried out with rectangular box-shaped indenter representing the stem of a ship, under both ambient conditions and Arctic temperature (− 60 °C) and the local response of the platform is studied through force deformation curves and stress contours. In order to study the global response of the platform, the numerical model of triceratops is developed in Ansys Aqwa solver and analysed under the action of impact load time history obtained from explicit analysis of buoyant leg. The impact load on the buoyant leg resulted in the continuous periodic vibration of the platform. Furthermore, parametric studies were also carried out to investigate the effect of indenter velocity, size, and location on the impact response of triceratops under Arctic temperature, and the results are discussed.

Nonlinear response of stiffened triceratops under impact and non-impact waves

Chandrasekaran, Srinivasan,Nassery, Jamshed Techno-Press 2017 Ocean systems engineering Vol.7 No.3

Dynamic response analysis of offshore triceratops with stiffened buoyant legs under impact and non-impact waves is presented. Triceratops is relatively new-generation complaint platform being explored in the recent past for its suitability in ultra-deep waters. Buoyant legs support the deck through ball joints, which partially isolate the deck by not transferring rotation from legs to the deck. Buoyant legs are interconnected using equally spaced stiffeners, inducing more integral action in dispersing the encountered wave loads. Two typical nonlinear waves under very high sea state are used to simulate impact and non-impact waves. Parameters of JONSWAP spectrum are chosen to produce waves with high vertical and horizontal asymmetries. Impact waves are simulated by steep, front asymmetric waves while non-impact waves are simulated using Stokes nonlinear irregular waves. Based on the numerical analyses presented, it is seen that the platform experiences both steady state (springing) and transient response (ringing) of high amplitudes. Response of the deck shows significant reduction in rotational degrees-of-freedom due to isolation offered by ball joints. Weak-asymmetric waves, resulting in non-impact waves cause steady state response. Beat phenomenon is noticed in almost all degrees-of-freedom but values in sway, roll and yaw are considerably low as angle of incidence is zero degrees. Impact waves cause response in higher frequencies; bursting nature of pitch response is a clear manifestation of the effect of impact waves on buoyant legs. Non-impact waves cause response similar to that of a beating phenomenon in all active degrees-of-freedom, which otherwise would not be present under normal loading. Power spectral density plots show energy content of response for a wide bandwidth of frequencies, indicating an alarming behaviour apart from being highly nonlinear. Heave, being one of the stiff degrees-of-freedom is triggered under non-impact waves, which resulted in tether tension variation under non-impact waves as well. Reduced deck response aids functional requirements of triceratops even under impact and non-impact waves. Stiffened group of buoyant legs enable a monolithic behaviour, enhancing stiffness in vertical plane.

Fatigue Assessment of Offshore Triceratops Restraining System Under Hurricane-driven Metocean Conditions

Srinivasan Chandrasekaran,Bharat Shah,Yogesh J. Chauhan 한국강구조학회 2023 International Journal of Steel Structures Vol.23 No.1

Compliant offshore platforms use a form-dominant approach to achieve the required compliance in ultra-deep water. They are position-restrained by taut-moored tethers, which undergo considerable stress cycles during their service life, making fatigue assessment essential. The present study investigates triceratops’ restraining system at 2400 m water depth under hurricane-driven Metocean conditions. The fatigue assessment uses two approaches: T–N curve and the S–N curve. As the numerical studies show, the 10-year hurricane return period governs the fatigue life of tethers. In specific terms, peak wave and peak wind cases govern the fatigue life of tether-2, whereas the peak current case governs tether-1 under hurricane conditions. Further, the unfactored fatigue life estimated using the S–N curve is about 60% higher than that of the T–N curves. However, the factored service life estimated using both the approaches with recommended safety factors matches closely. The presented study will aid the offshore engineers in preliminary designing a restraining system for the novel triceratops.

Steel Coped Beam with Functionally Graded Material for Offshore Topside: Numerical Investigations

Chandrasekaran Srinivasan,Thennavan M. 한국강구조학회 2024 International Journal of Steel Structures Vol.24 No.3

The use of rolled steel sections is typical in off shore topside, while recent developments are focused on coped sections to accommodate service lines and fi re hydrants. Coped beams are better alternatives without compromising the load-bearing capacity and shear strength. On the topside, the use of coped beams is helpful in creating a more orderly, safe, and undisturbed workspace for laying service mains and fi re hydrants; otherwise, they pose a general threat to the topside functional safety. Coped beam replaces the stack of service mains without compromising on the strength requirements of the web. Enhanced safety and improved aesthetics of the topside design are additional advantages. The present study investigates a coped beam with functionally graded material (FGM) to accommodate the services mains in a safer mode without losing its strength; corrosion resistance is also investigated as an additional requirement for any leak occurring from the mains. A coped beam of FGM and X52 steel is investigated under conventional loads and compared for their strength and serviceability requirements. Parameters such as cope length and depth are varied as per the design requirements. FGM coped beams showed higher load-carrying capacity even under the larger coped sections. Under deeper web sections, the cope depth and length have a signifi cant impact on the buckling and load-carrying capacity. For all the chosen coped sections, FGM beams registered a signifi cant reduction in buckling stress compared to X52 steel beams in the coped region, indicating the former is a better candidate for such applications.