A.K. Jain

Dynamic behaviour of square and triangular offshore tension leg platforms under regular wave loads

Among compliant platforms, the tension leg platform (TLP) is a hybrid structure. With respect to the horizontal degrees of freedom, it is compliant and behaves like to a floating structure, whereas with respect to the vertical degrees of freedom, it is stiff and resembles a fixed structure and is not allowed to float freely. The greatest potential for reducing costs of a TLP in the short term is to go through previously applied design approaches, to simplify the design and reduce the conservatism that so far has been incorporated in the TLP design to accommodate for the unproven nature of this type of platform. Dynamic analysis of a triangular model TLP to regular waves is presented, considering the coupling between surge, sway, heave, roll, pitch and yaw degrees of freedom. The analysis considers various nonlinearities produced due to change in the tether tension and nonlinear hydrodynamic drag force. The wave forces on the elements of the pontoon structure are calculated using Airys wave theory and Morisons equation, ignoring the diffraction effects. The nonlinear equation of motion is solved in the time domain using Newmarks beta integration scheme. Numerical studies are conducted to compare the coupled response of a triangular TLP with that of a square TLP and the effects of different parameters that influence the response are then investigated.

Nonlinear coupled response of offshore tension leg platforms to regular wave forces

Among the compliant platforms, the tension leg platform (TLP) is a vertically moored structure with excess buoyancy. The TLP is designed to behave in the same way as any other moored structure in horizontal plane, at the same time inheriting the stiffness of a fixed platform in the vertical plane. Dynamic response analysis of a TLP to deterministic first order wave forces is presented, considering coupling between the degrees-of-freedom surge, sway, heave, roll, pitch and yaw. The analysis considers nonlinearities produced due to changes in cable tension and due to nonlinear hydrodynamic drag forces. The wave forces on the elements of the pontoon structure are calculated using Airys wave theory and Morisons equation ignoring diffraction effects. The nonlinear equation of motion is solved in the time domain by Newmarks beta integration scheme. The effects of different parameters that influence the response of the TLP are then investigated.

Review of flexible risers and articulated storage systems

Abstract The anchorage system for mid-ocean loading or production consists of an articulated tower for mooring the tanker. Flexible risers are also essential components of the anchorage system. The present paper provides a state-of-the-art review on articulated storage systems and flexible risers, giving theoretical background for the development of computer software for the static analysis of flexible risers. In the state-of-the-art review for flexible risers, various analysis techniques for elastic lines and flexible risers under self-weight, current and wave forces are presented. The dynamic response of the flexible riser, including vortex-induced oscillations, is also outlined. The literature concerning the articulated tower and tanker is relatively scarce. Available works related only to dynamic responses of articulated towers. The combined response of tower and tanker is only studied by Chakrabarti and Cotter [(1978), Analysis of a tower-tanker system. In Proceedings of the 10th Offshore Technology Conference , OTC 3202, pp. 1301–1310] in a limited sense. The review of these works is summarised relevant to this paper. In the end, the static analysis of the flexible riser under its self-weight and current is presented using a finite difference approach. The problem essentially involves geometrical non-linearity, which is tackled with the help of an iterative solution based on modified Newton-Raphson technique. The theoretical formulation presented is being used to develop the computer software for the static analysis of the flexible risers.

Aerodynamic response of offshore triceratops

The common types of deep-water offshore structures have rigid connections, resulting in more stresses on members subjected to environmental loads. On the contrary, the relatively new offshore triceratops alleviates the encountered environmental loads by virtue of its innovative structural form and design. The top deck and the buoyant leg structure (BLS) are connected to each other by universal joints that permit transfer of translations from the BLS to the top deck but restrain transfer of rotations. The present study develops a mathematical formulation for the aerodynamic analysis of offshore triceratops and examines its response under regular waves in the presence of wind. Based on the numerical studies conducted here, it is observed that triceratops shows significant reduction in deck response, with no transfer of rotation from the BLS; the deck remains horizontal under the encountered wave loads. Lateral displacement of the deck is restrained due to the compliancy offered by the ball joints. The heave response is less in comparison with the surge response; this implies that the platform is stiff in heave degree-of-freedom, which is required for deep-water compliant platforms. A lesser response in the deck confirms that the deck remains horizontal even when the pitch response is significant in the BLS units. Offshore triceratops derives an advantage through the chosen geometric form for ultra-deep-water applications.

SEISMIC ANALYSIS OF OFFSHORE TRIANGULAR TENSION LEG PLATFORMS

Oil and gas production from deep-water offshore fields represent a major structural engineering challenge for the industry. The tension leg platform (TLP) is a well-established concept for deep-water oil exploration. It is necessary to design an offshore TLP such that it can respond to moderate environmental loads without damage, and is capable of resisting severe environmental loads without seriously endangering the occupants. Seismic analysis of triangular TLP under moderate regular waves is investigated. The analysis considers nonlinearities due to the change in tether tension and nonlinear hydrodynamic drag forces. The coupled response of TLP under moderate regular sea waves due to change in initial pretension in the tethers caused by seismic forces (vertical direction) is then investigated. Seismic forces are imposed at the bottom of each tether as axial forces. The tether tension becomes unbalanced when the hull is under offset position. The vertical component of seismic force is an important item to take into consideration, because it is directly superposed to pretension of tethers. The change in initial pretension due to the vertical component of the earthquake affects the response of the triangular TLP in degrees-of-freedom experiencing such forces. The tether tension varies nonlinearly when the platform is subjected to seismic forces caused by the El Centro earthquake and artificially generated earthquake using Kanai–Tajimis power spectrum. The response due to earthquakes varies with the intensity of the input ground motion. The seismic response of the triangular TLP exhibits nonlinear behavior in the presence of waves and it is non-proportionately influenced by the wave period and the wave height.

Dynamic Analysis of Buoyant Leg Storage and Regasification Platform Under Regular Waves

Buoyant Leg Storage and Regasification Platform (BLSRP) is the recent innovation in offshore structural engineering, which is essentially designed as storage and processing platform. The platform rests on six buoyant legs, which supports the deck through hinged joints. This is new hybrid conceptual design that restrains transfer of both rotational and translational responses from the BLS to the deck and vice-versa. BLSs are connected to the sea bed through taut mooring system. Numerical studies are carried out on the BLSRP at 600 m water depth, highlighting its dynamic response behavior under regular waves. Based on the studies conducted, it is seen that response of the deck is not influenced by wave action due to the presence of hinged joints. The proposed structural form proves to be effective in controlling the deck response in all active degrees-of-freedom. This improves the operational safety and shows high recentering capabilities even under large surge displacements.Copyright

Ringing Response of Offshore Compliant Structures

Ringing involves excitation of transient structural deflections at (or close to) structural frequency arising at the third-harmonic of incident wave field. Ringing waves are highly nonlinear containing strongly asymmetric transient waves and hence the shape of such waves becomes critical. This study develops mathematical formulation of impact waves responsible for ringing and examines their influence on offshore tension leg platforms (TLPs) with different geometric configurations located at different water depths. Based on the numerical studies conducted, it is found that the ringing response seen in pitch degree-of-freedom poses threat to platform stability. Also, undesirable response seen in stiff degree-of-freedom, like heave, emphasizes the necessity of the study for offshore compliant structures in deep waters, in particular.

Dynamic Response Behaviour of Stiffened Triceratops Under Regular Waves: Experimental Investigations

Among the various compliant type offshore structures, offshore triceratops is one of the recent innovative designs that is developed for exploration and production of oil and gas fields in deep and ultra-deep waters. Triceratops consists of a deck, which is supported by the Buoyant Leg Structures (BLS). Buoyant legs are connected to the deck by ball joints and held down in position by taut-moored tethering system. As buoyant legs are independently flexible, they need to be stiffened with the intermediate stiffeners to integrate them for counteracting the encountered lateral loads. Presence of ball joints restrains the transfer of rotational response from the BLS to the deck and vice-versa; only translational motion is transferred to the deck. Stiffened buoyant legs impose sufficient rigidity in the vertical plane, making it similar to that of a taut-moored Tension Leg Platform (TLP). The objective of present study is to show the reduction in transverse motions of offshore stiffened triceratops in operational sea state by increase in tethers stiffness (pretension). Hence, the study is focused on experimental investigations of a scaled model of a triceratops with stiffened buoyant legs under regular waves. Results show that the free-oscillation characteristics conform to a stable behavior of the platform under calm sea state. Deck exhibits lesser roll response under the chosen sea states, highlighting the advantage of ball joint between the deck and BLS units. However, the surge response is relatively high, which is controlled by stiffening the buoyant legs. This type of behavior is advantageous to upkeep more facilities on the deck and ensure comfortable operation during moderate sea states.Copyright