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

Transcription

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

2 Topics for the presentation Wood Group Norway Arctic and cold climate design capabilities Assessment of Airgap and wave impact loads DNVGL OTG-13 DNVGL OTG-14 Impact on typical semi submersible MOU design Hydrodynamic aspects Structural 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 Arctic and cold climate design capabilities Experience from studies and FEEDs related to arctic and cold climate Winterization of semi submersible drilling units Impact assessment from floating ice onto floating structures Modification projects in arctic areas Personel with operation experience from drilling units in harsh environment Significant experience transfer from WG organization in Canada on subject matter 4

5 Air gap on semi-submersible MOUs High focus on subject matter after fatal incident in Dec 2015 All owners of semisubmersible MOUs operating in NCS has recently been forced to review the air gap analysis of their fleet in accordance with the latest DNVGL Offshore Technical Guideline (OTG) 13 If the unit is found to have negative airgap, a structural assessment of the resistance in the structure shall be carried out in accordance with the OTG- 14. A temporary waiver is given by DNVGL if the negative airgap is small. The consequence is that many owners see a huge impact on their units in terms of required strengthening of the existing hull to comply with the OTG- 14. Outcome is often questioned and seen as very unlikely, mainly related to the level of negative airgap from OTG-13 in combination with the single source take out of peak pressures from the OTG-14 5

6 OTG-13 Prediction of air gap Definitions 6

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 Change in assymetry factor is the governing driver For smaller MOU with shallow pontoons, air gap reductions are significant Surface elevation above shallow pontoon must be better understood Linear analysis gives unrealistic high surface elevations in such position Result not inline with model test 9

10 Surface elevation comparative study WG has recently compared the wave elevation prediction above pontoons between numerical results with model test results Wave elevation measured from sensors in several positions in a tank test is compared with numerical results in similar position 10

11 Surface elevation comparative study - results The results indicate that the linear numerical results are giving an overprediction and should be given a correction before other non linear corrections are added. Lower wave periods where diffraction part is significant may give worse airgap than large waves with higher period Current rules are lacking guidance how to compensate for such effects Availebility of model test results is key vital to understand the correct surface elevation for shallow pontoon designs 11

12 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) 12

13 OTG-14 Horizontal wave impact loads Beachline plot of max upwell is extracted from OTG-13 assessment Ratio of Vertical position/ max upwell is used to take out the peak pressures Determination of design pressure is thus only depending on the max upwell Thus to be precise in the max upwell prediction is highly essential Peak pressure above 1000 kpa is regular seen 13

14 OTG-14 Horizontal wave impact loads Failure criterias is depending on position and function of the structure External shell rupture is an absolute no-go Extent of strain allowable is determined by DNVGL-RP-C208, but can be discussed based on the function of the structure Deflection limits is generally governed by the function of the space inside. Cabin arrangements can not be impaired, neither critical utiliy systems 14

15 Structural aspects Typical challenges are; Existing girder design has too weak capacity in end connections Span of existing stiffners is too long to to resist the load. Add additonal girders Thin plates often get challenges related to out of plane bending with rupture as a consequence. Increase density of stiffener system Typical reinforcement may not be the effective structrual arrangement in a new design incorporating the load effect in the scantling rules 15

16 Reflections OTG-13/ Hydrodynamic aspect If the design demonstrate negative airgap by linear motion analysis; Understand which effects that is contributing to wave elevation and judge whether output is considered realistic with reference to tank test results if available If the pontoon are shallow and thus will affect the motion behavior, consider non linear time domain potential flow analysis to improve motion characteristic Wave periods other than the traditional large waves can be the driver for the wave elevation Availebility of tank test results are premium source of information OTG-14/ Structural aspects Always have the «total integrity» mindset to sort out what effect is most important to mitigate in the various areas Structural analysis of short duration high peak pressures has to be analyzed by non-linear time step FEA Design external shell bulkheads to absorb energy thru elasticity and not to add to much rigid structural element Design guideline must be developed to assist designers to conclude fit for purpose design as to carry out sophisticated FEA on regular basis is inefficient There should be a continued debate on the ULS/ALS terms especially related to these load effects Impact loads with the scale as currently seen will cause unpredictable damages, that is not impairing safety but will cause large scale repairs. This aspect needs to be seen in conjunction with the judgement of what the structure can take 16

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