Air gap on semi-submersible MOUs under DNVGL Class. Current & Future Design Practice

Transcription

1 Air gap on semi-submersible MOUs under DNVGL Class Current & Future Design Practice

2 Topics for the presentation Wood Group Norway Motivation DNVGL OTG-13 DNVGL OTG-14 Case study Impact on semi submersible MOU design

3 Wood Group Norway Wood Group Norway AS part of Wood Group 500 engineers with multidiscipline background Three main business streams: Floaters, Brownfield and Greenfield 1977 Nordraug 1984 Sedco Phillips SS" "T. Finder" "Byford Dolphin 2001 GVA 4000M Bingo Transocean Searcher 1980 Treasure Finder" "Nordraug Nortrym" C. Kirk Rhein Jr. COSLProspector "T. Hunter" Dyvi Stena" 1995 Borgholm Dolphin "T. Finder 2009 "Safe Holmia Drillmar" 1999 Eirik Raude 2013 "Borgsten Dolphin COSLPioneer Bideford Dolphin HYSY COSLInnovator 1998 Island Innovator Nortrym 1983 Nortrym" 1993 Blackford Dolphin COSLPromoter 1981 Borgland Dolphin T. Swan "Ross Rig 2014 Borgny Dolphin 2013 Safe Scandinavia Safe Caledonia 1978 Polymariner 1996 Byford Dolphin 2000 Leiv Erikson Floaters Key competence Upgrades and conversions New build design Drilling technology Floating systems and concept studies

4 Motivation Accident on MODU 30 th December 2015 We ve concluded that similar types of waves can occur. So, a learning point from this accident is that horizontal forces from waves is something that can occur within a hundred-year storm, and it needs to be taken into consideration for design. And that has not been done before. Mr. Ernst Meyer, DNV GL director classification 4

5 Motivation Outcome of the incident on the Norwegian Continental Shelf PSA investigation concluded that the unit was adequately assessed at that the time of the accident following the applicable rules and regulations at the time of construction, however air gap analysis should take into account all effects including horizontal wave impact when the unit has negative air gap DNVGL issued two technical guidelines a) To assess the air gap and (OTG-13) and b) In case of negative air gap how to assess the vertical structures of the semi submersible (OTG-14) Currently there is a high degree of uncertainty on the consequences amongst the stakeholders Governments that Share Harsh Environment O&G Offshore Areas Petroleum Safety Authorities Maritime Authorities Classification Societies Semisubmersible Owners, Operators and Designers General public 5

6 DNVGL OTGs released in September

7 OTG-13 Prediction of air gap Main changes with respect to common practice New contributions to air gap Mean inclination Operational condition if not on even keel Ballast uncertainty (1 deg) Low frequency motions Roll / pitch resonant motions Revised Wave asymmetry factor due to nonlinearity in wave shape 7

8 Significant contribution for wave asymmetry factor Common practice prior to OTG -13 a = 1.1 (1.2) According to OTG-13 a = 1.2 a = 1.3 depending on location and direction of waves 8

9 Impact of OTG-13 OTG-13 directives generally give a reduction of the MOU air gap For smaller MOU with shallow pontoons, air gap reductions are significant Comprehensive model tests are suggested to be carried out in order to properly document air gap 9

10 OTG-14 Horizontal wave impact loads Objective is to provide a guideline for the loads to be used to document structural and floating integrity of MOUs that are subject to horizontal wave impact with the deck in the design conditions for MOUs with negative air gap Assessments should be done in a holistic way and with the objective of ensuring the global integrity of the unit The exposed or sensitive areas to be evaluated Sides of the deck box Columns Equipment, pipes, lifeboat support structures and appendages Peak pressure loads can be extracted based on level of negative airgap (from OTG-13) 10

11 OTG-14 Horizontal wave impact loads Main changes with respect to common design practice High peak dynamic pressures are introduced as a governing design load In order to assess deflections and capacities of the main bulkheads, non-linear FEA need to be introduced during the design Windows and other weak/non-ductile points need to be removed 11

12 Case study - geometry Deck box typical structure S355 steel or equivalent grade Bulkheads: PL12 - PL15 PL20 Girders: T400x200x12x25 Stiffeners: HP200x10 12

13 Case study - methodology Assessed comparative examples: Static approach 400 kpa peak pressure load Minimum DNVGL requirement on plates and stiffeners Dynamic explicit approach Static approach 1000 kpa peak pressure load Large deformations are expected Dynamic explicit approach 13

14 Case study load application Static setup model Dynamic explicit setup pulse load 14

15 Case study results static/dynamic for 400 kpa Static model Max. deformation: 111 mm. Dynamic model Max. deformation: 137 mm. 15

16 Case study results static/dynamic for 1000 kpa Static model - CONVERGENCE NOT REACHED Showing partial results ( 80% of target load 800 kpa) Max. deformation: 276 mm. Dynamic model Max. deformation: 420 mm. 16

17 Case study - conclusions Static analysis Deformations 400 kpa load 111 mm kpa load (unconverged at 800 kpa) 276 mm. Solver predicts fully recoverable deformations: unit recover its original geometry shape after the impact: Wrong conclusion Totally wrong prediction of damages the use of an static approach leads to inaccurate, un-conservative an erroneous conclusions, in best cases. Dynamic explicit analysis Deformations 400 kpa load 137 mm. [+23% if compared to static approach] 1000 kpa load 420 mm. [+150% if compared to static approach] Solver predicts a very small recovery of the original shape. Large permanent deformations (as observed on actual events). Accurate prediction of damages after the impact results could be used for further analysis. 17

18 Impact on semi submersible MOU design OTG-13 Avoid negative air gap by Increasing static air gap Avoiding shallow pontoons Improve motion behavior to avoid non-linearities Execute comprehensive model tests to improve statistics OTG-14 Design external shell bulkheads taking into account energy absorption properties Slamming model tests with pressure panels Prepare for more complex structural analysis including non-linear FEA 18

19 Thank you