Part 6 Floating Structures - PIANC British · PDF file · 2017-03-03Part 6 –...

BS 6349: “Maritime Works”

Advancing on Firm Foundations 13th February 2017

BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne

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



BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne

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 2017

2



BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne

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



BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne

Designing for movement

• Shock absorption

units (replaceable)

• Clearances due to

rotation

• Replaceable

wearing components

4



BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne

Design of pontoons – stability

5



BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne

Basic ship stability

6

Metacentric height (GM) is a measure of

initial stability. The greater the metacentric

height, the greater is the restoring leaver,

and hence stability.



BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne

Righting lever (GZ) curves

7



BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne

Effect of free surfaces

8



BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne

Applying heeling moments

9



BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne

Notes: The six-column grid

IMO criteria for ships:

Area under the curve up to 30o should not be less than 0.55 metre-radians

Area under the curve up to 40o (or angle of downflooding) should not be less than 0.09 metre-radians.

The area between 30o – 40o should not be less than 0,03 metre-radians.

The righting lever GZ should not be less than 0.20m at an angle of 30o or greater.

Maximum righting arm should occur at an angle of heel preferable exceeding 30o, but not less than 25o.

10



BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne

Practical implications

11

Criteria based on large angles

of heel may not be appropriate



BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne


12



BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne

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



BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne


14



BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne

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



BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne

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 2017 16



BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne

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



BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne

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.

• ’.

Connections

18



BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne

Connections

19



BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne

Connections

Consideration of the various types of joint are used, for example:-

1. flexible rubber jointed units

20



BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne

Connections

2. Fixed hinged connection

21



BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne

Connections

3. Steel box connection option

22



BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne

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



BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne

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



BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne

Guides

25

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.



BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne

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



BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne

Loadings

27



BS

63

49

: P

art

6 F

loa

tin

g S

tru

ctu

res

Ga

reth

Eva

ns

& A

nd

rew

Jo

hn

sto

ne

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

28

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

Part 6 Floating Structures - PIANC British · PDF file · 2017-03-03Part 6 –...

Documents

Transcript of Part 6 Floating Structures - PIANC British · PDF file · 2017-03-03Part 6 –...