1 00:00:20,990 --> 00:00:28,410 friends welcome to the sixth lecture on the online course title offshore structures under 2 00:00:28,410 --> 00:00:38,180 special loads including fire resistance design today in the sixth lecture we will talk about 3 00:00:38,180 --> 00:00:55,100 the continuation of new generation offshore platforms . 4 00:00:55,100 --> 00:01:02,629 in the last lecture we already said that the offshore structures . are essentially governed 5 00:01:02,629 --> 00:01:36,619 by form base design . we also said that the lateral loads imposed by waves . wind current 6 00:01:36,619 --> 00:01:57,460 etcetera which are conventional loads . are alleviated by large i should say relative 7 00:01:57,460 --> 00:02:05,170 displacement phenomena . 8 00:02:05,170 --> 00:02:25,740 that is the structures are made compliant it means flexible . so that they have relative 9 00:02:25,740 --> 00:02:51,580 displacement which enables them to counteract the lateral loads . now the question comes 10 00:02:51,580 --> 00:02:59,650 why one is interested to revise the existing compliant structural forms on the other hand 11 00:02:59,650 --> 00:03:06,650 what is the necessity . for the formation of new generation platforms let us answer 12 00:03:06,650 --> 00:03:18,180 this question by understanding the structural action of let say a t l p . we all know that 13 00:03:18,180 --> 00:03:27,890 if they are three axis . under specific point call this as x y and z 14 00:03:27,890 --> 00:03:38,610 we know that the surge is a displacement degree along x and sway is the displacement degree 15 00:03:38,610 --> 00:03:49,320 along y . and heave is the displacement along z direction we also know rotation about x 16 00:03:49,320 --> 00:04:03,250 axis is roll rotation above y axis is pitch and rotation above z axis is yaw . interestingly 17 00:04:03,250 --> 00:04:12,140 these six degrees of freedom are clearly divided into two sets one is flexible degree and other 18 00:04:12,140 --> 00:04:17,410 is stiff degrees let say what are these flexible degrees we 19 00:04:17,410 --> 00:04:24,479 do agree that flexible degrees of freedom are those whose time periods are very very 20 00:04:24,479 --> 00:04:39,659 large because we know omega is root of k by m is also two pi by t . so for larger period 21 00:04:39,659 --> 00:04:50,430 . omega will be lesser for lower omega for the same mass stiffness will be lesser for 22 00:04:50,430 --> 00:05:00,069 a system whose stiffness is lesser we call that as flexible so the degrees of freedom 23 00:05:00,069 --> 00:05:11,629 where . the periods are very large are flexible degrees so we do agree in a t l p surge sway 24 00:05:11,629 --> 00:05:21,830 and yaw have periods ranging somewhere from seventy seconds to one twenty seconds 25 00:05:21,830 --> 00:05:32,490 when i talk about the other set of degrees like heave roll and pitch these periods vary 26 00:05:32,490 --> 00:05:42,020 anywhere from two to five seconds so lower the period higher the stiffness so stiff degrees 27 00:05:42,020 --> 00:05:57,469 could be heave roll and pitch whereas flexible degrees could be . surge sway and yaw please 28 00:05:57,469 --> 00:06:06,529 pay attention to this figure . based on this figure one can easily understand now surge 29 00:06:06,529 --> 00:06:15,449 is along x and sway is along y and yaw is a rotation is a rotation above the z direction 30 00:06:15,449 --> 00:06:26,569 so one can now say very clearly tlps are flexible . in 31 00:06:26,569 --> 00:06:40,999 horizontal plane let say x y plane because surge sway and yaw motions which are marked 32 00:06:40,999 --> 00:06:50,039 on the x y plane have high periods whereas on the vertical plane the responses marked 33 00:06:50,039 --> 00:06:58,169 could be heave could be roll and pitch which are very low period therefore it is very stiff 34 00:06:58,169 --> 00:07:08,650 in vertical plane . . therefore the original idea which as conceived 35 00:07:08,650 --> 00:07:24,850 in offshore platform design was a hybrid concept . where flexibility . is introduced in horizontal 36 00:07:24,850 --> 00:07:37,831 plane . whereas stiffness . was ensure about the vertical plane that is the very interesting 37 00:07:37,831 --> 00:07:44,430 reason why this design was conceived if you look at one specific degree of freedom let 38 00:07:44,430 --> 00:07:47,259 us say heave response in the vertical plane . 39 00:07:47,259 --> 00:08:06,710 we need to limit the heave response because of operational convenience . . to reduce the 40 00:08:06,710 --> 00:08:27,779 change intension in tendons because this may cause further fatigue failure also to reduce 41 00:08:27,779 --> 00:08:45,790 the consequences please understand this term of change in buoyancy caused by added mass 42 00:08:45,790 --> 00:08:51,410 now one can ask me a question that ok operation convenience because heave motion happens some 43 00:08:51,410 --> 00:08:57,410 on vertical plane if heave response is very high one will not be able to do the drilling 44 00:08:57,410 --> 00:09:04,550 operation so this is acceptable . reduction in change intension because that's 45 00:09:04,550 --> 00:09:10,680 how the change intension is imposed because of heave motion you want to reduce that change 46 00:09:10,680 --> 00:09:17,139 intension therefore you can reduce the fatigue failure this also acceptable for one is not 47 00:09:17,139 --> 00:09:23,519 able to agree and easily understand to reduce the consequences caused by the added mass 48 00:09:23,519 --> 00:09:31,810 contribution so i want you to pay attention to the articulated towers and the guide towers 49 00:09:31,810 --> 00:09:56,269 where the recentering capability . was dependent on buoyancy 50 00:09:56,269 --> 00:10:07,430 so recentering was faster induce . a single point failure . 51 00:10:07,430 --> 00:10:15,410 may be in the universal joint . or may be in the spud can or maybe in the hinged connection 52 00:10:15,410 --> 00:10:28,720 . so this effect was to be avoided therefore people felt that i want to retain the stiffness 53 00:10:28,720 --> 00:10:41,029 in the vertical plane however the whole design of hybrid concept of tlp was slightly violated 54 00:10:41,029 --> 00:10:47,300 in spar platform because this spar platform if you look at the heave periods the heave 55 00:10:47,300 --> 00:10:55,300 periods can go slightly higher than now let say twenty to forty seconds and of course 56 00:10:55,300 --> 00:11:10,579 power platform did not receive any support from the mooring lines . . whereas tlps where 57 00:11:10,579 --> 00:11:19,639 slowly dependent on the restoration happened by tendons 58 00:11:19,639 --> 00:11:27,640 so people conceived different idea in a spar boy by violating slightly this norm on a vertical 59 00:11:27,640 --> 00:11:35,769 plane by keeping it heave slightly flexible but however in both this concepts of new designs 60 00:11:35,769 --> 00:11:46,940 which evolved in eighties till two thousand ten etcetera they were some difficulties . 61 00:11:46,940 --> 00:12:04,130 let see what is the difficult . . 62 00:12:04,130 --> 00:12:19,160 one difficulty is there is a clear band separation of frequencies one is higher one is lower 63 00:12:19,160 --> 00:12:30,410 higher in sense very high and lower in sense very low though people said interestingly 64 00:12:30,410 --> 00:12:41,190 this is an advantageous feature because higher periods with low frequencies enabled easy 65 00:12:41,190 --> 00:13:07,629 installation the system could remain a float even when the tether is damaged . . 66 00:13:07,629 --> 00:13:21,040 so therefore top side is saved but there is a main issue here by having this clear band 67 00:13:21,040 --> 00:13:30,110 separation of frequency you are making the platform viable . 68 00:13:30,110 --> 00:13:43,300 sensitive to both wind and waves because we all know wind is a low frequency phenomena 69 00:13:43,300 --> 00:13:57,889 . whereas wave is slightly a high frequency phenomena compared to wind . 70 00:13:57,889 --> 00:14:06,290 so now you are making the platform sensitive . to both these so you have to design to counter 71 00:14:06,290 --> 00:14:11,560 act both these kind of frequencies and make the design save you understand the problem 72 00:14:11,560 --> 00:14:19,750 now the complexity of making the platform sensitive to both bands of frequencies essentially 73 00:14:19,750 --> 00:14:26,279 arrows because of the hybrid concept conceived with the design of offshore platforms that's 74 00:14:26,279 --> 00:14:36,589 the first issue the second issue could be . people are now focusing on only the rigid 75 00:14:36,589 --> 00:14:44,009 body motion now one may ask me a question so what is the rigid body motion the whole 76 00:14:44,009 --> 00:15:03,589 platform for example a tlp . for example a spar boy . . which may have 77 00:15:03,589 --> 00:15:11,879 or may not have a mooring line it doesn't matter has rigid body motion that the whole 78 00:15:11,879 --> 00:15:31,050 system is displaced both by rotation and translation at c g of the mass 79 00:15:31,050 --> 00:15:46,000 so on the other hand there is no difference between the substructure element and super 80 00:15:46,000 --> 00:15:58,290 structure element . under the global response now what does this statement mean let say 81 00:15:58,290 --> 00:16:13,189 a tlp as a water level here . i would call this . the column and the pontoon as substructure 82 00:16:13,189 --> 00:16:20,509 element the one which is above water maybe including the derrick including etcetera will 83 00:16:20,509 --> 00:16:27,410 be a super structure element both are them undergo the same global response measure about 84 00:16:27,410 --> 00:16:35,279 the c g of the platform so what does it mean i have a wave action which is happening on 85 00:16:35,279 --> 00:16:44,360 the tether and the members which is transform to the hull or to the depth so the wave response 86 00:16:44,360 --> 00:17:02,410 . which happens on the column member on the pantoon or the deep draft caisson in case 87 00:17:02,410 --> 00:17:27,459 of spar . is transformed . to the hull this 88 00:17:27,459 --> 00:17:35,610 is undesirable . this we don't need because the wave response happening on the substructure 89 00:17:35,610 --> 00:17:41,309 should not be transformed to the hull now the question why it is so if the substructure 90 00:17:41,309 --> 00:17:50,070 response not transformed to the hull it would be always better that the hull or the top 91 00:17:50,070 --> 00:18:18,900 deck can remain operational even under . larger responses of the members under water . so 92 00:18:18,900 --> 00:18:36,880 the request the requirement is can we isolate the super structure from the supporting sub 93 00:18:36,880 --> 00:18:44,020 structure before we sake can we do it or not what would 94 00:18:44,020 --> 00:18:56,070 be the advantage if you do this the advantage if you do this . could be wave force causing 95 00:18:56,070 --> 00:19:15,140 response on the substructure . elements will need to only undergo local displacements they 96 00:19:15,140 --> 00:19:34,290 will not transfer or influence the deck due to wave action so that advantage we get alternatively 97 00:19:34,290 --> 00:19:53,740 wind forces acting on the super structure which can cause high moments on the deck c 98 00:19:53,740 --> 00:20:09,679 g . will now not influence . the sub structural elements . 99 00:20:09,679 --> 00:20:17,330 so this concept is not new in conceiving structural geometry we design this concept is otherwise 100 00:20:17,330 --> 00:20:23,629 being used in structures under earthquake engineering what we call as base isolation 101 00:20:23,629 --> 00:20:37,659 . so the idea here is isolate the sub and super structure by a media by a layer which 102 00:20:37,659 --> 00:20:49,830 then does not transformed the undesirable activities or responses vice versa by doing 103 00:20:49,830 --> 00:21:05,590 so we have one main demerit . what is the demerit the system should remain . stiff in 104 00:21:05,590 --> 00:21:12,740 heave degree of freedom we already said why it is required to the 105 00:21:12,740 --> 00:21:31,620 main stiff in heave degree because it saves the platform . from fatigue failure . it ensures 106 00:21:31,620 --> 00:21:54,240 a gentle recentering . under the action of added mass what we call as variable submergence 107 00:21:54,240 --> 00:22:10,000 the catch here is it ensures a gentle recentering . which inform confirms safe operability of 108 00:22:10,000 --> 00:22:17,510 course we also know if the system is more and tendons supported it will reduce fatigue 109 00:22:17,510 --> 00:22:31,080 failure probabilities . so we need to ensure even in the new concept of design that stiffness 110 00:22:31,080 --> 00:22:36,940 should be maintained in heave degree can we have a platform of this type which has been 111 00:22:36,940 --> 00:22:43,230 conceived by people in offshore engineering which is still in the research development 112 00:22:43,230 --> 00:22:48,710 stage but no such platform in constructed in prototype physically but lot of research 113 00:22:48,710 --> 00:22:55,299 work is happening around the world to see the feasibility of study this kind of geometry 114 00:22:55,299 --> 00:23:01,260 and it's possibility and viability for the an ultra deep waters that's what we say as 115 00:23:01,260 --> 00:23:04,980 . new generation platforms now let us see what are these platforms all 116 00:23:04,980 --> 00:23:23,409 about when you talk about new generation platforms . we need to keep in mind that the deck responses 117 00:23:23,409 --> 00:23:57,840 should remain local . what do you mean by local so deck responses 118 00:23:57,840 --> 00:24:17,500 vertical plane . . so let us see the next lecture how this idea 119 00:24:17,500 --> 00:24:41,899 can be conceptualized thank you very much .