Buoyant Legs Research Articles

Combined Freak Wave, Wind, and Current Effects on the Dynamic Responses of Offshore Triceratops

Offshore structures are exposed to various environmental loads, including extreme and abnormal waves, over their operational lifespan. The existence of wind and current can exacerbate the dynamic response of these structures, posing threats to safety and integrity. This study focuses on the dynamic responses of offshore triceratops under different environmental conditions characterized by the superimposition of freak waves, uniform wind, and current. The free surface profile of the freak wave was generated using the dual superposition model. The numerical model of the offshore platform designed for ultra-deep-water applications was developed using the ANSYS AQWA 2023 R2 modeler. Numerical investigations, including the free decay tests and time-domain analysis under random sea states, including freak waves, were initially carried out. Then, the combined effects of freak waves, wind, and current were studied in detail under different loading scenarios. The results revealed the increase in structural response under the freak wave action at the focus time. Wind action resulted in a mean shift in responses, while the inclusion of current led to a pronounced increase in the total response of the platform, encompassing deck and buoyant legs, alongside the tether tension variation. Notably, considerable variations in the response were observed after freak wave exposure under the combined influence of wind, freak wave, and current. The results underscore the profound effects induced by wind and current in the presence of freak waves, providing valuable insights for analyzing similar offshore structures under ultimate design conditions.

Offshore Triceratops With Elliptical Legs Under Postulated Failure of Tethers

Offshore Triceratops are classified as form-dominant structures possessing the novelty of counteracting the lateral loads with material strength and geometric form. The deck is supported by the buoyant legs, which are positioned on the seabed using taut-moored tethers. Ball joints connect the legs with the deck and help partially isolate the deck to remain horizontal under waves and wind. While offshore structures commonly have tubular legs, the current study investigates Triceratops with elliptical legs under the postulated failure of tethers. Detailed dynamic analysis under postulated failure conditions is carried out using ANSYS AQWA, while the LIBRIUM analysis is used to obtain the equilibrium position of the platform under intact and postulated failure conditions. Results show that under postulated failure conditions, the peaks of the power spectral density function of the pitch response are seen closer to its natural frequency and the input wave frequency. Ball joints restrain this rotation from transferring to the deck, conforming to one of the advantages of its existence. The fatigue life of tethers calculated for different load cases under intact and postulated failure conditions helps understand the stability conditions of the platform.

Dynamic analysis of offshore triceratops supporting wind turbine: Preliminary studies

Offshore triceratops, the recent innovation in deep water compliant platforms, is primarily designed to withstand lateral forces through its geometric shape. The uniqueness of the platform is the presence of the ball joints between the legs and deck, which partially restrain the transfer of rotations from the legs to the deck and vice-versa. However, displacements such as surge, sway, and heave motions are transferred, ensuring a rigid connection between the legs and the deck. Efficient operations of offshore wind turbines are more dependent on the support systems on which they are mounted. Increased stability and reduction in stress concentration in rigid connections are desirable. Nonlinear dynamic response analysis is carried out in FAST by coupling the frequency response of the platform obtained in ANSYS AQWA with that of the HydroDyn module of FAST. The current study investigates a fully coupled three-dimensional hydro-aerodynamic model of triceratops mounted with a horizontal axis wind turbine. Unsteady Blade Element Momentum theory (BEM) is used to estimate the aerodynamic loads, which encompasses the effect of wind shear using a power law and spatially coherent turbulence. In contrast, Morison equations are used to estimate the hydrodynamic loads on the platform. After the preliminary proportioning of the platform, Response Amplitude Operator (RAO) plots are drawn to illustrate the partial motion transfer between the deck and the buoyant legs. Based on the preliminary studies, it is seen that the environmental loads do not impose instability, reinforcing the dynamic stability of the platform. Frequency responses for operating and parked conditions illustrate the coupling between the degrees of freedom and the influence of the rotor motion of the wind turbine on the platform deck. Tether tension variation is assessed in all three legs for the operational sea states to check the safety standards for a compliant system to avoid tether pull-out. The presented study is prima facie to encourage the suitability of triceratops as floaters to support the wind turbine under moderate sea states. Highlights This study is focused on new-generation offshore triceratops as support system for wind turbine Prelimiary dynamic anlaysis of coupled action of the supprting system and wind turbine are presented Use of ball joints help partial isolation of the deck and restrain transfer of moment from the turbine shaft to the supporting system, which is a novelty Infleunce of rotor motion of the triceratops is illustarted to highlight the advantage of complinacy of trirceratops This study disucsses only the performance assessment and not the design perspectives

Dynamic response of offshore triceratops under regular waves with tether postulated failure

Triceratops are innovative offshore platforms that consist of a deck, three buoyant legs, and ball joints. Their deck is supported by buoyant legs, which, in turn, are restrained on the sea bed using taut-moored tubular tethers. Ball joints partially isolate the deck from the buoyant legs by restraining the transfer of rotations from the legs to the deck. Triceratops platforms exhibit a monolithic structural action in the vertical plane and a hybrid dynamic behavior resembling tension leg platforms and spar buoys. The present study investigates their dynamic response in regular waves. Response amplitude operators (RAOs) are estimated for intact and postulated failure conditions of the tethers. Based on detailed numerical investigations, it is seen that the pitch response of the deck increases significantly under the postulated failure of tethers, which are otherwise absent due to the presence of ball joints. Marginal change in surge response, even under tether failure, confirms the suitability of triceratops for ultra-deepwater installations. The fatigue life of tethers before and after postulated failure conditions is compared using the S-N curve approach and noticed that the adjacent tethers’ fatigue life reduces significantly under postulated failure conditions of tethers.

Tether response of offshore Triceratops under hurricane conditions

The form-dominant approach is increasingly adopted in offshore platform design; triceratops is one of the very recent examples. These platforms are position-restrained by taut-moored tethers, which govern the dynamic response under extreme environmental loads. Due to the partial isolation of the deck from the buoyant legs in the presence of ball joints, sub-structure analysis is more critical. The present study examines the tether response of triceratops in 2400 m water-depth under hurricane conditions caused by 10-yr, 100-yr, and 1000-yr Gulf of Mexico (GoM) storms. The numerical studies show that the Peak-Wave case governs the dynamic tension variation of tethers for the chosen hurricane conditions. For the 100-yr and 1000-yr storms, they are about 15% and 45% higher than 10-yr storms, respectively. However, the maximum axial tensile stresses are about 8% of the yield, ensuring a no-rupture condition of the tethers. The service lives of the tethers for 10-yr storms are about one-third that of 100-yr and 1000-yr storms. A positive minimum tension for all load cases confirms no slackening of the tethers.

Dynamic analyses of triceratops under Hurricane-driven Metocean conditions in Gulf of Mexico

The form-dominance approach is efficient in designing deep-water offshore platforms; one of the structural forms conceived in the recent past is a triceratops, which consists of three buoyant legs connected to the deck by ball joints. The buoyant legs are connected to the sea bed by a set of taut-moored tethers under high initial pretension. Dynamic response of Triceratops under Hurricane-driven Metocean conditions in the ultra-Deepwater depth of 2400 m is assessed under 10-year, 100-year, and 1000-year storms. Based on the numerical studies, it is found that the responses in all active degrees of freedom increase drastically with the increase in the severity of Hurricane conditions. The Peak Wave Case governs the Surge response, whereas the Sway response is governed by the Peak Current Case for the Wind and Current heading considered in the present study. The maximum Surge and Sway responses are only about 3% of the total water depth, and the maximum Heave is about 2.6% of the draft under extreme conditions. The rotational responses in the deck are negligible as ball joints partially isolate the deck. The recentering of the platform is observed as gentle, and deck responses are stable and periodic for different Hurricane conditions.

Motion Responses Analysis for Tidal Current Energy Platform: Quad-Spar and Catamaran Types

One approach to support floating tidal current turbines is by using a moored catamaran, a barge type platform. Considering its low draft, one might expect that it performs best at typical straits with sea states of small wavelets to small waves. The problem is that the high rotational motion responses of the catamaran due to wave loads tend to reduce the turbine performance. This paper looks for a possibility to deteriorate these rotational responses by introducing a platform with four buoyant legs referred to as a quad-spar considering its good stability performance. The platforms are moored by four catenary cables as their mooring system. The motion response modeling was undertaken by Computational Fluid Dynamic (CFD) simulation based on three-dimensional potential flow theory. Considering sea states of straits with typical tidal current energy potentials, the environmental load was set on random wave with the significant wave height, Hs, of about 0.09 to 1.5 m and the wave period, T, of about 1.5 to 6 s corresponding to the wave frequency, ω, of about 1.1 to 4.2 rad/s. This study found that lower motion responses can be satisfied by the quad-spar, in which its yaw, roll and pitch responses are on average about 5%, 44%, and 38%, respectively, compared to those of the catamaran. This result indicates that the quad-spar is more effective in reducing rotational motion responses needed to keep a high performance of the tidal current energy system.

Dynamic response of offshore triceratops with elliptical buoyant legs

Oil and gas exploration in ultra-deepwaters necessitates innovative geometric forms for offshore steel structures. Offshore triceratops is one of a new steel compliant structures found suitable for ultra-deepwater applications. It consists of three buoyant legs attached to the deck by ball joints, which partially isolate the deck from the buoyant legs and constitute the novelty. The present study discusses a detailed numerical analysis of triceratops with buoyant legs of elliptical cross section. The focus is to assess the hydrodynamic diffraction characteristics of the platform caused by the change in cross section of buoyant legs from a conventional tubular one to the elliptical ones. Three elliptical sections with varying eccentricities are considered in the dynamic analyses to assess the response behavior of triceratops under regular waves. The results showed an increase in the total force in the buoyant legs in sway degree of freedom with the increase in the eccentricity of the cross section. Besides, reduced transverse vibrations and increased stability are also observed in both the deck and elliptical buoyant legs. Based on the numerical results, it is recommended that elliptical buoyant legs with eccentricity close to 2 are highly suitable for offshore triceratops in ultra-deepwaters.

Offshore Triceratops Under Impact Forces in Ultra Deep Arctic Waters

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.

Parametric studies on the impact response of offshore triceratops in ultra-deep waters

This paper addresses the parametric studies on the impact response of offshore triceratops subjected to low-velocity impact. Triceratops is the object of the present impact study which is one of the new generation offshore complaint platforms with three buoyant legs connected to deck by ball joints. The innovative geometric form and compliant characteristics distinguish triceratops from conventional offshore platforms. The buoyant legs in triceratops are usually designed as orthogonally stiffened cylindrical shell structures. Numerical studies are carried out using Ansys Explicit analysis solver to predict the impact response and local damage of buoyant leg of triceratops. From the impact force time history obtained through explicit analysis, the hydrodynamic time response analysis is carried out in Ansys Aqwa to predict the response of deck and buoyant legs. Parametric studies are mainly focussed on the effect on the location of the indenter, size of the indenter, type of indenter, number of stiffeners and strain-rate definition. Force displacement curves are reported along with detailed numerical observations.

Dynamic analyses of offshore triceratops in ultra-deep waters under wind, wave, and current

Compliant offshore platforms are sensitive to aerodynamic loads. Triceratops is one of the new generation compliant platforms consisting of a deck, which is connected to buoyant legs through ball joints. Ball joints partially isolate the deck from buoyant legs by transforming translational motion and restricting the rotational motion. Buoyant legs are connected to the seabed using taut-moored tethers. This study is carried out to assess the dynamic response of triceratops under the wind, wave and current loads at a water depth of 2400 m in different sea states. Numerical analysis is carried out using ANSYS AQWA under moderate, high and very high sea states. Results of studies show that aerodynamic load resonates response of deck in different degrees of freedom under moderate and high sea states. Pitch response of the deck is insignificant, verifying the satisfactory design of the conceived geometry. Statistical analyses show that the response increases with the increase in wave height and wind speed, but lesser than a surge in all sea states. Presence of current induces a shift in the mean position of surge response, and the platform oscillates at this new mean position; this shift increases with an increase in the sea states. Pitch response is significantly reduced under the combined wind, wave and current loads.

ICE-INDUCED response of offshore triceratops

Increase in drilling operations in the Arctic region intuits to examine the suitability of platforms under ice loads. This paper examines the dynamic response of triceratops under continuous ice crushing. Effect of ice loads on one and adjacent buoyant legs are examined under different ice velocity, crushing strength and ice thickness. Based on the numerical investigations carried out, it is seen that the response of deck in active degrees-of-freedom is nonlinear, showing higher response with the increase in ice crushing strength. Ice loads cause a shift in the mean response. Limit ice load acting on the buoyant legs increases with the increase in the ice thickness. With the increase in ice crushing strength, set-down increases along with the increase in the mean value of surge; mean and total yaw response also increases with the increase in the ice crushing strength. Increased deck response under ice loads is due to transfer of translational response from buoyant legs to deck by ball joints. However, pitch response of the deck is lesser than that of buoyant legs, verifying the restraint offered by ball joints to transfer rotation. It may be an added advantage of triceratops under ice-infested sea loads. Though this may not be the only sufficient requirement, certainly no transfer of rotational response from buoyant leg to the deck enables a comfortable working environment. From the analysis of triceratops under lock-in ice condition, it is found that the response of the deck and buoyant legs is bounded and the resonance build up is not seen in any degrees-of-freedom due to damping present in the structure, which is an additional advantage of a triceratops.

Tether analyses of offshore triceratops under ice force due to continuous crushing

Oil exploration and production in ultra-deep ice-infested waters demands an innovative structural form to withstand the ice loads. Offshore triceratops is one such new-generation innovative, compliant platforms, which show good adaptability in extreme sea state conditions even under the combined action of wind, waves, and current. As tethers are the crucial components in compliant platforms like a triceratops, it is of paramount importance to study the effect of dynamic ice loads on tether tension variation. The main aim of this study is to carry out the tether tension analysis under the ice force action in different ice sea states. The ice force time history is developed from the continuous crushing ice spectral model, and numerical studies are carried out using Ansys Aqwa by applying the ice force on buoyant legs. The study reveals that the tether tension variation is less than 10% even under extreme ice sea state. The parametric studies are also carried out to assess the effect of ice velocity, ice crushing strength, and ice thickness on tether tension variation. It is found that the maximum stress developed in the wires of tethers is very less compared to the yield stress of the material, ensuring no tether failure. However, a large number of stress cycles may lead to fatigue failure of tethers. Hence, fatigue analysis is also carried out to assess the fatigue damage and service life of tethers. The results show that fatigue damage increases from normal to extreme ice sea state. The results are also compared with open-water load case. Studies carried out under lock-in ice condition shows the increase in the fatigue damage and reduction in the service life of the tethers.

Mathieu stability of offshore Buoyant Leg Storage & Regasification Platform

Increasing demand for large-sized Floating, Storage and Regasification Units (FSRUs) for oil and gas industries led to the development of novel geometric form of Buoyant Leg Storage and Regasification Platform (BLSRP). Six buoyant legs support the deck and are placed symmetric with respect to wave direction. Circular deck is connected to buoyant legs using hinged joints, which restrain transfer of rotation from the legs to deck and vice-versa. Buoyant legs are connected to seabed using taut-moored system with high initial pretension, enabling rigid body motion in vertical plane. Encountered environmental loads induce dynamic tether tension variations, which in turn affect stability of the platform. Postulated failure cases, created by placing eccentric loads at different locations resulted in dynamic tether tension variation; chaotic nature of tension variation is also observed in few cases. A detailed numerical analysis is carried out for BLSRP using Mathieu equation of stability. Increase in the magnitude of eccentric load and its position influences fatigue life of tethers significantly. Fatigue life decreases with the increase in the amplitude of tension variation in tethers. Very low fatigue life of tethers under Mathieu instability proves the severity of instability.

Structural Health Monitoring of Offshore Buoyant Leg Storage and Regasification Platform: Experimental Investigations

Offshore platforms are of high strategic importance, whose preventive maintenance is on top priority. Buoyant Leg Storage and Regasification Platforms (BLSRP) are special of its kind as they handle LNG storage and processing, which are highly hazardous. Implementation of structural health monitoring (SHM) to offshore platforms ensures safe operability and structural integrity. Prospective damages on the offshore platforms under rare events can be readily identified by deploying dense array of sensors. A novel scheme of deploying wireless sensor network is experimentally investigated on an offshore BLSRP, including postulated failure modes that arise from tether failure. Response of the scaled model under wave loads is acquired by both wired and wireless sensors to validate the proposed scheme. Proposed wireless sensor network is used to trigger alert monitoring to communicate the unwarranted response of the deck and buoyant legs under the postulated failure modes. SHM triggers the alert mechanisms on exceedance of the measured data with that of the preset threshold values; alert mechanisms used in the present study include email alert and message pop-up to the validated user accounts. Presented study is a prima facie of SHM application to offshore platforms, successfully demonstrated in lab scale.

Tether analyses of offshore triceratops under wind, wave and current

Offshore triceratops is new-generation offshore platform, whose novel geometric form is advantageous and safe in ultra-deep waters. Deck is partially isolated from the buoyant legs by ball joints. Buoyant legs are anchored to sea bed using taut-moored tether with high initial pretension. Compliant nature of the platform induces degree of flexibility with large time periods; they fall within the range of wind-excitation frequency, making them vulnerable under aerodynamic loads. As tethers are crucial component in compliant structures such as triceratops, this study aims at investigating the effect of combined action of wind, wave and current on dynamic tension variation caused in tethers. It is followed by fatigue life prediction under different sea states. Statistical analyses of tether tension variation show that it is periodic in nature; maximum number of cycles is found to be in low sea state. Presence of current enhances energy of tension spectrum significantly. There is an increase in service life of tethers in the presence of wind under different sea states; but presence of current reduces the service life of tethers.

Dynamic analyses and preliminary design of offshore triceratops in ultra-deep waters

More adaptable geometric form of offshore platforms, counting on the benefits of form-dominant design, is effective to encounter various environmental loads. Offshore triceratops is a new-generation offshore platform, whose conceptual design showed good degree-of-adaptability to ultra-deep-water conditions. Deck is partially isolated from the buoyant legs by ball joints by allowing transfer of partial displacements of buoyant legs to deck but restraining transfer of rotational responses. Prior to the suitability assessment of triceratops for ultra-deep waters, detailed dynamic analysis on the preliminary geometric form is necessary as a proof of validation for design applications. Current study discusses a detailed numeric analysis of triceratops at 2400 m water depth under regular and irregular waves; preliminary design of both buoyant legs and the deck is also presented. Buoyant Legs are designed as stiffened cylinders and the deck is designed as the integrated truss system. In compliant structures, the role of tethers is of paramount importance. Hence, the stress analysis and fatigue analysis of the tethers are also carried out to assess the service life of the structure. Presented study shall aid offshore engineers and contractors to understand suitability of triceratops, in terms of design and dynamic response behaviour.

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

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.

Ringing response of offshore triceratops

Triceratops is a new generation offshore platform, proposed for oil and gas exploration in ultra-deep waters. Deck is supported by buoyant legs, but isolated from each other by ball joints. Buoyant legs are taut-moored to the seabed using tethers with high initial pretension. The presence of ball joints makes the platform less sensitive to the encountered environmental loads. The present study investigates transient response of triceratops under distinctly high sea waves through numerical studies. Spectral analysis is carried out to estimate the ringing response, which is highly nonlinear. While ringing waves are simulated with parametric variation, JONSWAP spectrum is used to simulate the time series of large irregular waves causing ringing response on triceratops. Based on the numerical studies carried out, a gradual decay is seen in the response of buoyant legs looking similar to that of a beat phenomenon. Deck response shows a significant reduction in rotational degrees-of-freedom than that of the buoyant legs due to the presence of ball joints, which aids the platform to remain operational even under high sea states. Statistical analyses on tether tension variation show that ringing caused by strong asymmetric waves is responsible for causing impact load on the platform.

Mathieu stability of offshore triceratops under postulated failure

ABSTRACTOffshore triceratops is one of the new generation compliant platforms, whose structural form inherits salient advantages in alleviating environmental loads. Deck is connected to buoyant legs through ball joints, which transfers only translational motion but restrains rotations. As the buoyant legs are taut-moored, dynamic tether tension variations under the wave loads can cause Mathieu-type instability. The present study examines the stability of triceratops through postulated failure case by increasing payload on the deck. Detailed dynamic analyses under wave loads are carried out through numerical modelling of triceratops, while increased payload intuits postulated failure cases. Phase plots are interpreted to assess the stability of the deck and tethers are assessed using Mathieu stability. It is seen from the results that increased payload causes tether instability. In addition, platform was also found to be unstable prior to that of tether instability, under the postulated cases considered for the study.