Part 6 Floating Structures

Transcription

1 Part 6 Floating Structures Brief update on the work to date Gareth Evans PhD. BSc C.Eng FICE FIStructE MBCS An explanation on the stability requirements for pontoons Andrew Johnston B.Eng. (hons) C.Eng MRINA A presentation on Marina pontoons John Berry B.Eng. C.Eng. MICE 1

2 Update on Part 6 The work has involved a considerable amount of research in order to bring this part of the code up to date. The initial draft is approximately 50% complete The current projection is to have the work completed by the end of

3 Fatigue issues on pontoon connections FEM modelling using elasto-plastic analysis Requires more detail in the standard Requires better detailing Use replaceable components Use energy absorption systems 3

4 Designing for movement Shock absorption units (replaceable) Clearances due to rotation Replaceable wearing components 4

5 Design of pontoons stability 5

6 Basic ship stability Metacentric height (GM) is a measure of initial stability. The greater the metacentric height, the greater is the restoring leaver, and hence stability. 6

7 Righting lever (GZ) curves 7

8 Effect of free surfaces 8

9 Applying heeling moments 9

10 Notes: The six-column grid IMO criteria for ships: Area under the curve up to 30 o should not be less than 0.55 metre-radians Area under the curve up to 40 o (or angle of downflooding) should not be less than 0.09 metre-radians. The area between 30 o 40 o should not be less than 0,03 metre-radians. The righting lever GZ should not be less than 0.20m at an angle of 30 o or greater. Maximum righting arm should occur at an angle of heel preferable exceeding 30 o, but not less than 25 o. 10

11 Practical implications Criteria based on large angles of heel may not be appropriate 11

12 Practical implications 12

13 Design risk assessment: A statement on the intended use. The environmental design criteria used, including return periods and limiting environmental conditions. The design loads including any load restrictions and how these are controlled. Limiting responses to loads such as minimum freeboard, maximum walkway gradients, and evacuation criteria. Cyclic loading and natural frequency; fatigue of critical elements. Suitable stability criteria including the effects of moorings or adjacent pontoons. 13

14 Practical implications 14

15 Design risk assessment Accidental damage criteria (flooding, collision loads, mooring line failure). Inspection and maintenance cycle requirements including the stability case for delivery and maintenance. Environmental impact and pollution containment measures (sewage and fuels). Requirements for lifesaving appliances and navigation aids. Any ballasting requirements. 15

16 Marina Pontoons Typical marina style pontoons consist of a deck, most probably a timber system on a steel galvanised frame which is supported on a system of floats. The floats may either be a poly block encased in concrete, a set of plastic boxes or perhaps a poly block with a simple GRP covering. The essence of stability is the Metacentric height which needs to be positive to be stable. The nearer to 0 the more tender and more ie easily capsizeable the unit. BS6349 pt 6 did give some guidance on allowable limits, Table 9 Dealing with Pontoons the range given is 1.0m to 15m. At +15m the unit being very stable 16 February

17 Stability Analysis of the Metacentric height can be simply ascertained in considering pontoons as opposed to vessels as the waterplane area will remain relatively constant over the heel range that can be expected in service. The procedure as set out in the existing code will be included in the proposed new version. In looking at heel and stability the YHA guide specifies a minimum reserve freeboard to the floats of 50mm. This is another area where the new code will seek to provide guidance, however! As an anecdotal point in relation to a set of pontoons used for a well known on water boat show, the pontoon providers were asked about the design live loads and what would happen if this were exceeded. The answer was the people will start to get wet feet as the pontoons are loaded to a point where the buoyancy of the floats are exceeded at which point people will tend to back away and the pontoon deck will return to its normal freeboard In that case the footprint of the pontoon walkways and fingers provided an overall stable platform and with sealed floatation units the loads are spread along the walkways and if some floats are effectively sunk they haven't lost their inherent buoyancy. What this does highlight is the need to adequately design and specify the connections between the individual pontoons 17

18 Connections Connections for Pontoons used in the Marina type environment Pontoons used in the Marina environment often rely on the interconnection between units to provide the stability, utilising the buoyancy of the adjacent floats on either fingers or walkways. Typical Marina Pontoon Units This requires the designers to assess the forces that will be transmitted across the joints and the type of joint for the location and exposure. The joints are under constant stress due to the motion of the units flexing. Often the stability of an single run of walkway pontoons is very tender the MGyy often very small. The effects of either concrete floats or plastic can also have significant effects on the stability of the unit. However stability is not usually an issue once connected to a system of units.. 18

19 Connections 19

20 Connections Consideration of the various types of joint are used, for example:- 1. flexible rubber jointed units 20

21 Connections 2. Fixed hinged connection 21

22 Connections 3. Steel box connection option 22

23 Connections 3. Concrete Pontoons The detailing of the connections to this sort of unit needs careful consideration in the way the loads from the connection elements is transposed into the concrete itself such that fatigue cycles on the concrete cannot lead to failure of the concrete surrounding the reinforcing elements 23

24 Guides Mooring of pontoon units Typical piled installation Various types of mooring details are available to the designer from:- 1. Piles 2. Anchor and Chain 3. Elastomeric types 4.Wall guides For types 1 and 4 the motion of the units needs to be considered. If too much heel occurs or the heave / roll response is to great then the pontoon pile guide can put considerable loads on the pile or guide and could cause the system to bind or be damaged. 24

25 Guides Elastomeric types can be useful in some locations but consideration of water depth / tidal range needs very careful attention to ensure the angles of moorings always stay reasonably the same and that the tension variation is within the capacity of the system. 25

26 Pontoon Loadings & Stability The principal factor in safe operation for all concepts on floating structures is stability, ie the ability to withstand overturning forces or moments and return to a normal attitude after removal of the unbalancing loads Also in respect to loading it is considered reasonable on pontoons with service ducts to look at the applied load over the central section not necessarily the full pontoon width. A comparison of loads noted from various well respected publications already in existence is shown below:- 26

27 Loadings 27

28 Loadings considered for inclusion in new Code Item Flotation Stability Un Restricted Access 5 kn/m2 2.5 kn/m2 Restricted Access 2 kn/m2 1.5kn/m2 Marina s: Secondary walkways 1.5 kn/m2 1.5 kn/m2 Fingers 1kn/m2 1kn/m2 For flotation apply over deck plan and for stability over the parts of the deck giving worst load distribution ie ½ width for heeling, but not under gangways for structural analysis 28