Recent observations of the sea state that result in the undesirable events confirm the presence of extreme waves like freak waves, which is capable of causing irreparable damages to offshore installations and (or) create inoperable conditions to the crew on board. Knowledge on the extreme wave environment and the related wave-structure interaction are required for safer design of deep-water offshore structures. In the current study, typical long crested extreme waves namely: i) New Year wave at offshore Norway; and ii) Freak wave at North Sea are simulated using the combined wave model. Dynamic response of the Tension Leg Platforms (TLP) under these extreme waves is carried out for different wave approach angles. Based on the analytical studies carried out, it is seen that the TLPs are sensitive to the wave directionality when encountered by such extreme waves; ringing type response is developed in TLPs which could result in tether pull out. © 2013 ANAME Publication. All rights reserved.

Srinivasan Chandrasekaran

Department of Ocean Engineering

Koshti Yuvraj

Fulltext

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Journal of Naval Architecture and Marine Engineering

Process of deploying wireless sensor network in offshore structures for structural health monitoring is presented in this paper. Offshore structures are subjected to environmental loads with high uncertainties that are capable of causing damage during their service life. Preventive measures to safeguard the offshore platforms from such damages are not only innovative but also inevitable. Structural damages can be readily identified by monitoring their response behavior deploying dense array of sensors. Deploying wireless structural health monitoring (SHM) system ensures reduced installation time, compact design and cost reduction in comparison to that of the traditional wired systems. Wireless sensor network architecture for monitoring offshore platforms, proposed in the present study will acquire data using network of sensor nodes and transmits to the base station for post-processing. Present study compares the structural responses of offshore tension leg platform under different postulated failure cases through experimental investigations on scaled model. Acquired data are also hosted in the webpage after analyzing and post-processing. The alert monitoring system, which is an integral part of SHM triggers alert messages is an indigenous design. © 2018, JASNAOE.

Journal of Marine Science and Technology (Japan)

Health monitoring of tension leg platform using wireless sensor networking: 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. © 2018, Harbin Engineering University and Springer-Verlag GmbH Germany, part of Springer Nature.

Journal of Marine Science and Application

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

In offshore structural engineering, buoyant leg storage and regasification platform (BLSRP) is one of the recent innovative structural forms that is archived to suit industrial requirements. Proposed platform consists of a deck, which is connected to six buoyant leg structures (BLSs) through the hinged joints while BLS units are connected to the seabed using taut-mooring tethers. The conceived structural form is a hybrid concept, which restrains transfer of both rotational and translational responses from the BLS units to the deck and vice versa. The main advantage is the improved functionality in terms of increase in the storage and regasification capacity of liquefied natural gas. Experimental investigations are carried out on a scaled model of BLSRP (1:150) under regular waves for two different wave approach angles. Free vibration tests are carried out on the scaled model to estimate the natural periods; results show that BLSRP resembles a tethered compliant structure like tension leg platform (TLP) except showing relatively higher stiffness in the yaw degree-of-freedom. Geometric design of BLS units ensures a good recentring capability of the deck in all the translational degrees-of-freedom. Lesser heave response of deck, in comparison to that of the BLS units ensures comfortable and safe operability. Attempted study is a prime-facie towards design and development of offshore production and process platforms that can reduce the cost of oil and gas exploration. © 2016 Informa UK Limited, trading as Taylor &amp; Francis Group.

Ships and Offshore Structures

Dynamic analyses of buoyant leg storage regasification platform (BLSRP) under regular waves: experimental investigations

Response control of offshore tension leg platforms (TLP) using a passive damper is studied. The study is attempted to control the large surge motion of a TLP. Tuned mass damper (TMD) is used for the control purpose as it effectively controls the motion of the primary structure under the lateral loads. Experimental investigations are carried out on a scaled model of TLP with TMD under regular waves to examine the effectiveness of TMD in controlling the surge response of TLP. TMDs with two different mass ratios, which are tuned to that of the structural characteristics of TLP model, are used in the study. Based on the studies conducted, it is seen that there is an effective control of surge response in certain bandwidths of wave periods under the chosen mass ratio. It is also seen that the presence of TMD controls heave and pitch response as well in addition to exhibiting reduction in tether tension variation. Attempted study is novel in terms of its application to offshore compliant structures although TMDs are commonly used in the response control of onshore structural systems. © 2016 Informa UK Limited, trading as Taylor &amp; Francis Group.

Response control of tension leg platform with passive damper: experimental investigations

Offshore tension leg platforms (TLPs) is often subjected to critical environmental loads that cause serious damage; Mars TLP in Gulf of Mexico, damaged by hurricane Katrina in the year 2005 is an example. Existence of external perforated cover reduces hydrodynamic forces on the existing members caused by direct wave impact. Although many studies confirm this fact but variations in the water particle kinematics along the water depth needs a detailed investigation to verify the phenomenon. Geometric configuration of the column member of the damaged MARS TLP is modeled on a 1:100 scale with an external perforated cover. Variations of water particle kinematics of the column member encompassed by perforated cover are presented through detailed computational fluid dynamics (CFD) analysis. Traces of the horizontal water particle velocity along the water depth are derived for a wide range of sea states for various chosen geometric configuration. Results of the numerical studies show that there is a significant reduction in the water particle velocity, which verifies the fact of reduction in forces on the column members. The research reported in this paper has a direct application in the repair of damaged column members by encompassing with outer perforated cover. © 2014 American Society of Civil Engineers.

Journal of Performance of Constructed Facilities

Numerical study on geometrical configurations of perforated cylindrical structures under regular waves

Certain class of offshore structures exhibit highly intense nonlinear behavior called springing and ringing. Dynamic response of compliant structures like Tension Leg Platforms (TLP) under impact and non-impact waves responsible for ringing and springing phenomenon is of large interest to marine engineers. This paper develops a mathematical formulation of impact and non-impact waves and discusses the method of analysis of TLPs of triangular geometry under these wave effect; response of square and equivalent triangular TLPs are compared. Heave response in square TLPs show bursts but there are no rapid build-ups; gradual decays are seen in most cases looking like a beat phenomenon while such results are not predominantly noticed in case of equivalent triangular TLPs. Ringing, caused by impact waves in pitch degree-of-freedom and springing, caused by non-impact waves in heave degree-of-freedom in both the platform geometries are undesirable as they pose serious threat to the platform stability. Analytical studies conducted show that equivalent triangular TLPs positioned at different water depth are less sensitive to these undesirable responses thus making it as a safe alternative for deepwater oil explorations. © 2011 IOS Press and the authors. All rights reserved.

Journal	Journal of Naval Architecture and Marine Engineering
ISSN	18138535
Open Access	No