Srinivasan Chandrasekaran

Potential-flow-based numerical study for the response of floating offshore structures with perforated columns

Perforated piles are used in near-shore breakwaters for coastal protection and deep water platforms for oil exploration. Emerged and submerged perforated cylindrical structures reduce wave–structure interactions and scouring problems considerably, but their use on a floating structure is limited. Mathematical model is developed to introduce perforated walls in floating structures and method of solution is discussed. Eigenfunction expansion approach is used to derive velocity potentials based on which hydrodynamic forces on perforated cylinders are obtained. An example tension leg platform having perforated caissons is chosen; length of the perforated region and its location on tension leg platform are varied to determine their influence on its motion. For increase in perforation length, fluid–structure interaction reduces, causing decrease in surge and pitch responses. However, increase in perforated length in the lower part of column does not significantly increase these responses; heave remains relatively unaffected regardless of the length of perforation in the columns.

Dynamic Analysis and Design of Offshore Structures

This chapter deals with the evolution of platform and various types of offshore platforms and their structural action under different environmental loads. The newly evolved structural forms and their discrete characteristics are discussed in this chapter. This chapter also gives the reader a good understanding about the structural action of different forms in the offshore. An overview of the construction stages of offshore plants and their foundation systems is presented.

Offshore triangular tension leg platform earthquake motion analysis under distinctly high sea waves

TLPs are compliant structures designed to withstand moderate loads without damage and severe loads without seriously endangering the occupants. Dynamic behavior of TLPs under distinctly high sea waves in the presence of both horizontal and vertical seismic excitations is examined and method of analysis is discussed. Seismic forces imposed at tether bottom make tether tension unbalanced when the hull is under offset condition. Tether tension varies nonlinearly under vertical seismic excitations generated using Kanai-Tajimi ground acceleration spectrum. Analytical studies conducted on triangular TLPs show that this tension variation is much higher than the regulation values indicating the necessity for examining them for seismic safety. Clearly, the peaks seen in the response of all active degrees-of-freedom occurring near to the average sum frequencies of waves and input ground motion is a significant influence of seismic excitations on TLP tethers under high sea waves. The numerical results obtained also verify/establish the fact that TLPs built in deeper sea locations show significantly lesser response to the combined wave and earthquake loading.

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.

Response analyses of offshore triceratops to seismic activities

Offshore triceratops is one of the alternate structural forms that is desirable for the deep-sea oil exploration. It consists of a deck and buoyant leg structures (BLS) that are positively buoyant and the platform is position-restrained by tethers. Ball joints, which connect the deck and the BLS units, restrain the transfer of rotations between them. These platforms experience a significant tether tension variation under the vertical seismic excitations that are caused at the seabed. In the present study, response analyses of triceratops to seismic activity in the presence of waves are examined. Records of the El Centro earthquake and the artificially generated earthquake using Kanai–Tajimi (K-T) power spectrum are considered for the study. Earthquake excitations are imposed on the tethers to determine the dynamic tether tension variations caused by the seismic excitations. Triceratops is then analysed numerically for these tether tension variations in the presence of regular waves. Based on the numerical studies conducted, it is seen that the heave and pitch responses of the BLS units and the deck are significantly high under these tether tension variations. It is also seen that the response amplitude operator of the deck in the pitch degree of freedom is lesser than that of the BLS units. This study quantifies the response variations of the triceratops under seismic activities, which is useful for the design and development of new generation offshore platforms for deep waters.

Hydrodynamic Response of Tension Leg Platforms with Perforated Members

Tension-Leg Platforms (TLPs) are commonly preferred offshore structures for the deep-water oil exploration. Their reduced response to the encountered waves is achieved by their compliancy. This innovative structural design dampens the vertical motion (heave) of the platform but the large horizontal movements (surge, sway and yaw motion) cause inconvenience to the people on board, though the platform remains stable for the operational sea state. Coastal and offshore structures are constructed with the protective perforated layers mainly to reduce the direct impact caused by the waves. Present study highlights the detailed experimental investigations carried out on the scaled model of the TLP with the perforated members under regular waves. Based on the experimental investigations, it is seen that there is a significant reduction in the dynamic response of the TLP with the perforated columns. Encompassing column members with the perforated outer cover is seen as one of the effective method of retrofitting offshore structures to improve their serviceability.

Response Behaviour of Perforated Cylinders in Regular Waves

In order to reduce the direct wave impact, coastal and offshore structures are often constructed with one or more perforated layers. Several permeable breakwaters and docks have been deployed in coastal protection measures to reduce direct impact due to encountered wave loads and to reduce wave reflection in front of these structures as well; compared with the traditional ones, such structural forms are found to be more economical. Such structures result in lesser surface fluctuation in harbours due to the low reflection, which is vital for loading and unloading of ships. Deploying permeable breakwaters posses other advantages namely: i) increasing water circulation; ii) retaining water quality and iii) enhances coastal protection. Emerged and submerged perforated cylindrical structures reduce wave-structure interactions and scouring problems considerably, but their use on floating structures is scarce in the literature. This study is focused on detailed experimental investigations carried out on impermeable inner cylinder encompassed by a larger outer cylinder with perforatios along its length. By varying the porosity and perforatio diameter, their influence on the hydrodynamic response of the cylindrical member is highlighted through the current study. The conclusions on the comparison of forces in the cylinder with and without perforated cylinder coverage are presented.Copyright

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.

Power Generation Using Mechanical Wave Energy Converter

Ocean wave energy plays a significant role in meeting the growing demand of electric power. Economic, environmental, and technical advantages of wave energy set it apart from other renewable energy resources. Present study describes a newly proposed Mechanical Wave Energy Converter (MEWC) that is employed to harness heave motion of floating buoy to generate power. Focus is on the conceptual development of the device, illustrating details of component level analysis. Employed methodology has many advantages such as i) simple and easy fabrication; ii) easy to control the operations during rough weather; and iii) low failure rate during normal sea conditions. Experimental investigations carried out on the scaled model of MWEC show better performance and its capability to generate power at higher efficiency in regular wave fields. Design Failure Mode and Effect Analysis (FMEA) shows rare failure rates for all components except the floating buoy.

Axial Force-Bending Moment Limit Domain and Flow Rule for Reinforced Concrete Elements Using Euro Code

Performance-based design approach of special moment-resistant reinforced concrete (RC) framed structures demands a thorough understanding of axial force—bending moment (P—M) yield interaction of RC elements, particularly when the structure is subjected to seismic loads. Latest design approach includes desirable features of both ultimate strength and working stress to ensure a suitable ductile deformation response. This demands a detailed understanding of nonlinear response of P—M interaction. A complete set of analytical expressions for P—M yield interaction are proposed in a closed form by defining the limit boundary with ten sub-domains based on Euro Code currently in prevalence. P—M interaction relationships are also verified for plastic flow-rule in two main sections namely: (i) tension failure resulting in yielding of steel; and (ii) compression failure resulting in crushing of concrete. The conventional limit P—M domain is described based on Euro Code as long as the plastic strain increment becomes nearly normal to the yield domain over the part of bending response in the presence of axial force. The verified flow rule shows a close agreement in all sub-domains of tension failure, while does not qualify in few of the sub-domains of crushing failure. Practical examples of RC sections are chosen to illustrate the influence of different parameters namely: (i) cross-section dimension; (ii) percentage of tension and compression reinforcements; and (iii) properties of constitutive materials on the P—M boundary. Mathematically developed P—M interaction model is capable of identifying the damage mechanism of different sub-domains in RC sections; damage identification is made on the basis of strain profile of concrete and reinforcing steel.