COPEDEC VI, 2003, Colombo, Sri Lanka
NEW GUIDANCE FOR WAVE FORCES ON JETTIES IN EXPOSED LOCATIONS by 1
2
3
by K.J. McConnell , N.W.H. Allsop , G. Cuomo and I.C. Cruickshank 1
INTRODUCTION
1.1
Background
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Trade activities of coastal nations rely on jetties for berthing of vessels for the loading and discharge of cargo. Traditionally, these facilities were constructed in sheltered locations or sheltered by breakwaters hence hydraulic loadings were relatively small. In recent years there has been increased demand for development of large single use industrial terminals (especially those for Liquid Natural Gas (LNG), and Liquid Petroleum Gas (LPG)) which require deep water and sheltered berths for larger vessels, but do not necessarily need shelter to the approach trestles carrying the delivery lines. These terminals are often required in remote locations where there is no wave shelter, no existing infrastructure and the construction of new protective breakwaters for the whole facility may not be cost effective. Therefore, in many instances the jetties and/or their approach trestles are being constructed in exposed locations without breakwater protection. Views of a typical jetty approach trestle are shown in Figures 1 and 2.
Figure 1: Typical exposed jetty
1 Senior Engineer, HR Wallingford, Howbery Park, Wallingford, Oxon, UK, OX10 8BA, Tel: +44 (0)1491 822304, Fax: +44 (0)1491 832233, Email:
[email protected] 2 Technical Director, Coastal Structures, HR Wallingford, UK & Visiting Professor, University of Southampton 3 Marie Curie Visiting Research Fellow, University of Rome 3, c/o HR Wallingford, UK 4 Principal Engineer & Project Manager, HR Wallingford, UK 1
COPEDEC VI, 2003, Colombo, Sri Lanka
Figure 2: Typical approach trestle Other examples of exposed jetties include small jetties on open coasts in tropical regions serving small fishing communities, ferry services and emergency access to remote locations. For most of their design life, the environmental conditions may be benign but occasionally cyclone and hurricane conditions hit, putting the exposed jetty under significant hydraulic loading. 1.2
Wave loadings
Of particular concern in these locations is the risk of occurrence of wave forces on the jetty superstructure and the likely magnitude of such forces should they occur. As well as being important for the design of structure elements, these loads need to be considered when assessing the potential for damage to equipment located on approach trestles and jetty heads. There are also potential environmental risks arising from damage to exposed jetty facilities, particularly those carrying oil or other hazardous materials. Existing guidance on such loadings mainly derives from the offshore industry. In this field an approach termed the 'air gap' approach is generally adopted for platform design. Following this approach, the maximum wave crest elevation is predicted for the design condition and the deck (or soffit) level is located at an allowance or 'air gap' above this elevation to ensure a low probability of occurrence of wave forces on the superstructure. The 'air gap' approach is often adopted in the design of shore connected trestles and jetties, however the design of structures in this environment may be dictated by other constraints which prevent the adoption of this method. Constraints may include vessel freeboard at berth, the need for loading / offloading and tidal range, all of which dictate practical deck levels to ensure efficient operations. In addition there may be considerations such as material costs, member sizes and construction methodology. In such cases there may be a risk of wave loads on the structure. Methods available to the designer for prediction of the forces are limited, complex to apply and practical guidance for their use is not readily available.
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COPEDEC VI, 2003, Colombo, Sri Lanka
1.3
The "exposed jetties" research project
In response to the demand for design guidance for predicting wave forces on jetties, a research project entitled 'hydraulic design of exposed jetties' was undertaken at HR Wallingford funded by the UK government. The project was guided by a Project Steering Group from industry, including designers, contractors and owners. These research studies reviewed existing knowledge and undertook a new series of model tests to evaluate loads on deck elements and provide new guidance that could be readily applied by the design engineer. For the purposes of the project, an exposed jetty was defined as: "A solid vertical or open piled structure, possibly with cross-bracing, providing a berth or berths constructed in a location where wave forces have a significant influence on the design" "These structures can be remote from the land in deep water (where the influence of shallow water is small) or in exposed locations such as marginal quays (where the influence of shallow water impacts are more significant)" 2
MODEL TESTS
2.1
Model set-up and test conditions
Following a review of available literature and methods for prediction of wave forces, a series of model tests were designed. The tests are described in more detail in Tirindelli et al (2002). The model test section comprised a typical jetty head on cylindrical piles constructed from downstanding cross-beams and a solid deck, contructed at a scale equivalent to 1:25. The model design was developed in consultation with the Project Steering Group to ensure that it was representative of typical real structures, such as the jetty head shown in Figure 3.
Figure 3: Typical jetty head (courtesy Kier)
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COPEDEC VI, 2003, Colombo, Sri Lanka
Figure 4: Physical model in wave flume The model was located in a 2-dimensional wave flume capable of generating random waves, Figure 4. Within the superstucture of the model, two beam and two deck elements were fitted with force transducers, see Figure 5, which recorded force measurements at a sampling frequency of 200Hz. During testing it was clear that there could be strong 3-dimensional flow effects around the structure, particularly as the structure deck was inundated. As a result, an additional series of tests was completed with panels fixed to each side of the deck to prevent 3-dimensional inundation of the structure. This provided data for the 2-d scenario which allowed 3-d effects to be quantified and also provided a scenario that was more comparable with some of the prediction methods available which concentrated on 2-d scenarios. In addition, a third test series was undertaken with the deck superstructure inverted such that the underside was a flat deck. This configuration did not include side panels. Thus three configurations were tested as follows: • • •
Configuration 1 - deck with downstand beams Configuration 2 - flat deck Configuration 3 - deck with downstand beams (as for configuration 1) with side panels to limit 3-d flow effects.
The test programme covered a range of wave conditions and relative water and deck levels, summarised in Table 1. Parameter Hs (m) Tm (s) Water depth, h (m)
Model 0.1 - 0.22 1 - 3 0.75, 0.6*** 0.06 - 0.16* 0.01 - 0.11**
Clearance, cl (m) Wave height to clearance ratio, Hs/cl Wave height to water depth ratio, Hs/h Relative water depth, h/Lm Sampling frequency (Hz) Notes:
Prototype (at 1:25) 2.5 - 5.5 5 - 15 18.75, 15*** 1.5 - 4* 0.25 - 2.75* 1.1 ñ 18 0.13 ñ 0.33
0.1 200
* Configurations 1 & 3, ** Configuration 2, *** Configuration 3 only
Table 1: Range of test conditions 4
0.48 40
COPEDEC VI, 2003, Colombo, Sri Lanka
LEGEND
LB1 CB1
CB2
CB3
CB4
LB = Longitudinal Beams
LB2
Waves B1
CB5
CB = Cross Beams
D1
B2
B = Beam Elements
D2
D = Deck Slabs A
B
C
LB3
D
A B C D = Force Transducers
LB4 27.50 / 1100 6.50 / 260
6.50 / 260
6.50 / 260
6.50 / 260 Down-standing cross beams (1.50 x 1.50 x 25.00) (60 x 60 x 1000)
7.50 / 300
25.00 1000
Down-standing longitudinal beams (2.50 x 2.50 x 27.50) (100 x 100 x 1100) 7.50 / 300
Deck slab 7.50 / 300
Slender element (1.50 x 1.50 x 5.00) (60 x 60 x 200)
Deck element (0.5 x 5.00 x 5.00) (20 x 200 x 200)
dia = 2.50 / 50
Figure 5: Underside of model deck showing measurement elements Note: dimensions given as prototype (model)
2.2
Preliminary analysis
The time series from the various force measurements were processed to extract a number of key force parameters. These were identified for each force 'event' which occurred as a wave hit the structure. One such event is shown in Figure 6, which defines the various force parameters, defined as: Fmax
Impact force (short duration, high magnitude)
Fqs+, v or h
Maximum positive (upward or landward) quasi-static (pulsating) force
Fqs-, v or h
Maximum negative (downward or seaward) quasi-static (pulsating) force
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COPEDEC VI, 2003, Colombo, Sri Lanka
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Fmax
Force (N)
6 Fqs+
4 2 0
Fqs-
-2 -4 76.5
77
77.5
78
78.5
79
79.5
Time (s)
Figure 6: Definition of force parameters (model units) The extracted force parameters were then processed to derive the force at 1/250 level for each test, that is the average of the highest 4 loads in 1000 waves. For most test conditions, many waves will have generated loads, so F1/250 is relatively well supported. For a few tests however, there may be relatively fewer loads contributing to F1/250 defined in this way, and the measure may be less stable. All the results presented in this paper are based on F1/250. Preliminary analysis of the results and comparison with predictive models is discussed in Tirindelli et al (2002). The results of the analysis demonstrated that methods available (eg. Kaplan (1992, 1995), Shih & Anastasiou (1992)) may underpredict wave forces on jetty components. An example comparison is shown in Figure 7 for seaward deck elements.
80
Measured Kaplan
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F1/250 (N)
60 50 40 30 20 10 0 0
0.05
0.1
0.15
0.2
0.25
Hs (m) Figure 7: Comparison of measured and predicted uplift forces on jetty deck elements, after Tirindelli et al (2002) (model units) 3
RESULTS
3.1
Discussion on presentation of results
Following on from the analysis described in Tirindelli et al (2002), the data were processed and presented in dimensionless format. A range of dimensionless parameters were considered for 6
COPEDEC VI, 2003, Colombo, Sri Lanka
presentation of the results, in order to provide some useful means of using the data for force prediction. Firstly a means of non-dimensionalising the forces was considered. From the perspective of the designer, it was considered that the force measurements might be most usefully be presented as a function of a force value that can be easily calculated from design information. A notional or 'basic wave force' F* is therefore defined. F* is calculated based on the predicted maximum wave crest elevation, ηmax, whilst assuming no (water) pressure on the reverse side of the element. F* is calculated separately for vertical and horizontal forces. F*v is defined by a simplified pressure distribution using hydrostatic pressures, p1 and p2, at the top and bottom of the particular element being considered. F*h is calculated assuming a uniform pressure p2 over the base of the element. F*v and F*h are defined in Figure 8, and can be calculated as follows:
∫ ∫
F *v =
bw bl
p 2 ⋅ dA ≅ bw ⋅ bl ⋅ p 2
ηmax
F *h =
∫ ∫p bw
hyd
⋅ dA = bw ⋅ (η max − c l ) ⋅
cl
c l + bh
F
*
h
=
∫ ∫p bw
(1)
hyd
⋅ dA = bw ⋅ b h ⋅
cl
p2 for η max ≤ c l + bh 2
(p1 + p 2 ) 2
for η max > c l + b h
(2)
(3)
where
p1 = [ηmax ñ (bh+cl)]·ρg
(4)
p2 = (ηmax ñ cl)·ρg
(5)
and p1, p2 bw bh bl cl ηmax
pressures at top and bottom of the element element width (perpendicular to direction of wave attack) element depth element length (in direction of wave attack) clearance (distance between soffit level and still water level, SWL) maximum wave crest elevation (relative to SWL).
Figure 8: Definition of 'basic wave forces' F*v and F*h
In order to derive the maximum wave crest elevation, ηmax, the maximum wave height, Hmax, must be calculated. A method is given by Goda (1985) for a range of conditions and by Battjes & Groenendijk (2000) for shallow foreshores. The maximum wave crest elevation, ηmax, can then be calculated from Hmax using various non-linear wave theories. In deep water, a simple approximation for ηmax is given 7
COPEDEC VI, 2003, Colombo, Sri Lanka
by Stansberg (1991). This gave good agreement with Stream Function Theory and Fenton's Fourier theory for the range of conditions tested, however for shallower water depths the more sophisticated approaches should be used. The dimensionless forces, Fqs/F*, are presented against the dimensionless parameter (ηmaxñcl)/Hs, which describes the incident wave conditions and geometry. When written as (ηmax/Hs)ñ(cl/Hs) this parameter describes the relative elevation of the wave crest (ηmax/Hs), often between 1.0 and 1.3, then the relative excess of the wave over the clearance (cl/Hs). Over the test range, relatively little effect of either wave steepness or relative depth was detected in these data, although that conclusion may be specific to the relative size of the test elements considered. The following forces were analysed and are discussed in this paper:
• • • •
vertical upward acting force, Fvqs+ caused by slam on the underside of the deck or beam vertical downward acting force, Fvqs- caused by inundation of the deck or beam, which can persist after the wave has passed beneath the structure horizontal landward force, Fhqs+ caused by the wave front hitting the beam horizontal landward force, Fhqscaused by the wave hitting the back of the beam, most likely due to the wave being trapped by the deck substructure
It should be noted that the discussion in this paper concentrates on slowly-varying or quasi-static forces (Fqs). Shorter duration impact forces, Fmax, as defined in Figure 6, were also processed and are discussed briefly in this paper. Further discussion of these results will be given in Cuomo et al (2003). In some cases forces experienced by the outer, seaward measurement elements differed to those experienced by the internal elements, which were influenced by the deck configuration. In some cases beams and deck elements showed significantly different behaviour and for some elements there was a clear influence of 3-dimensional effects. The influence of each of these factors was assessed and the data sorted such the the influence of these parameters could be identified.
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COPEDEC VI, 2003, Colombo, Sri Lanka
3.2
Vertical quasi-static forces
Vertical loads on the seaward beam and deck elements were found to be relatively unaffected by the configuration of the test structure, and were similar in magnitude for both element types. These can therefore be considered together, see Figures 9 and 10 for upward and downward acting forces respectively. It is worth noting that the smooth deck tended to give lower element loads that the deck with downstanding beams. 3.5 Seaward elements - downstand beam configuration
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Seaward elements - flat deck configuration
Fvqs+ / F
*
v
2.5 2 1.5 1 0.5 0 0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
( ηmax - cl ) / Hs
Figure 9: Vertical (upward) forces on seaward elements
0 -0.5
-1.5
Fvqs- / F
*
v
-1
-2 -2.5 Seaward elements - downstand beam configuration
-3
Seaward elements - flat deck configuration
-3.5 -4 0
0.2
0.4
0.6
0.8 1 ( ηmax - cl ) / Hs
1.2
1.4
1.6
1.8
Figure 10: Vertical (downward) forces on seaward elements
Conditions for the internal elements are more complex, with the deck and beam elements showing different trends. The results for upward and downward loads on the internal deck element are shown in Figures 11 and 12 respectively. Upward loads were not obviously influenced by 3-d effects, however local 3-dimensional effects did significantly influence downward loads, resulting in larger loads than the simplified 2-d scenario.
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COPEDEC VI, 2003, Colombo, Sri Lanka
It is worth noting that the flat deck configuration also experienced lower downward forces, most likely due to the fact that this configuration was represented simply by turning the deck over and the resulting upstanding beams will have blocked 3-dimensional flow effects over the measurement element to some degree. 9 Internal deck
8 7
Fvqs+ / F
*
v
6 5 4 3 2 1 0 0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
( ηmax - cl ) / Hs
Figure 11: Vertical (upward) forces on internal deck
0 -0.2 -0.4
Fvqs- / F
*
v
-0.6 -0.8 -1 -1.2 -1.4 -1.6
Internal deck - 3-d effects Internal deck - 2-d effects
-1.8 -2 0
0.2
0.4
0.6
0.8
1
1.2
1.4
( ηmax - cl ) / Hs
Figure 12: Vertical (downward) forces on internal deck
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1.6
COPEDEC VI, 2003, Colombo, Sri Lanka
Vertical wave forces on the internal beam are also complex, but the loss of some test data resulted in a less clear trend than that identified for the deck element. Upward and downward forces are shown in Figures 13 and 14, respectively. 3 Internal beam 2.5
Fvqs+ / F
*
v
2 1.5 1 0.5 0 0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
( ηmax - cl ) / Hs
Figure 13: Vertical (upward) forces on internal beam
0
Fvqs- / F
*
v
-0.5
-1
-1.5
-2 Internal beam -2.5 0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
( ηmax - cl ) / Hs
Figure 14: Vertical (downward) forces on internal beam
Some general observations can be made for vertical forces for all of the test elements:
• • • •
For (ηmaxñcl)/Hs > 0.8, F*v seems to give a safe estimation of Fvqs+ For (ηmaxñcl)/Hs
