8.6.3 Buoyancy and weight forces

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Transcript of 8.6.3 Buoyancy and weight forces

  • Southern Pipeline: Physical Process Assessments

    ASR Marine Consulting and Research 178

    FL = CL An w (u2/2g) (8.1)


    CL = empirical lift coefficient

    An = projected area of solid body normal to flow

    w = specific weight of water

    g = gravitational acceleration

    u = magnitude of flow velocity

    Based on the Reynolds number of the flow regime and information from the literature,

    the lift coefficient for the present situation would likely be in the range of 0.3 to 1.

    Substituting in to the equation above (An = 1.2 m2, u = 1 ms-1, w = 1024 kg/m3) and

    using a range of CL values (from 0.3 to 1.0) yields a lift force of between 185 and 614

    N/m of pipe length.

    8.6.3 Buoyancy and weight forces

    The buoyancy of the pipeline per metre of pipe length should also be considered. In

    general the buoyancy force is equal to the weight of the fluid displaced. This is

    expressed as:

    FB = w V (8.2)

    w is the specific weight of water and V is the volume of water displaced.

    For an empty pipe with an outside diameter of 1.2 m, the buoyant force is equal to 45.2

    kN per meter of pipe length. This will be reduced by the weight of the pipe but the wall

    thickness is not known at this stage. The buoyant force is in opposition to the weight of

    the fluid being carried inside the pipe (in this case wastewater) and the weight of the

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    pipe which both act vertically downward. Since the weight of the wastewater will be a

    function of the interior diameter (ID) of the pipe and because wastewater should be

    slightly less dense than sea water, the net force (excluding the weight of the pipe) will

    be upwards, without the use of a concrete weight coat on the pipe. The final

    magnitude of the buoyant or gravitational force will then be dependent on the pipe

    material, its weight and wall thickness, but the excess buoyancy will need to be offset

    by a concrete weight coat.

    8.6.4 Scour

    Generally some amount of scour is a desirable end result for a pipeline laid on the sea

    bed. However, if the pipeline is laid across bed material of differing erodibility, the result

    may be a free span of pipe that is not in contact with the seabed. This is a potentially

    dangerous situation that could lead to pipeline failure. In general, the Coastal

    Construction Manual recommends a simple relationship between maximum scour

    depth and pipeline diameter for situations where the current velocity exceeds the

    critical velocity for bed erosion. This relationship is given as:

    Sm = 0.7 D (8.3)

    where Sm is the maximum scour depth in metre and D is the pipeline diameter in


    8.6.5 Wave impact

    The most likely locations where wave forces or scour may be come problematic are at

    the locations where the pipeline emerges from the estuary onto dry land or in the

    intertidal zone. In these locations and under certain circumstances, the wave heights

    could become a significant fraction of the water depth and either break or generate

    significant impact forces on the pipeline. For the case of wave impact, the force

    exerted on the structure can be expressed as:

  • Southern Pipeline: Physical Process Assessments

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    FU = CU Az w (w2/2g) (8.4)


    CU = laboratory derived slamming coefficient

    Az = projected area of solid body in the horizontal plane

    w = vertical component of flow velocity at the level of the object

    w = specific weight of water

    g = gravitational acceleration

    The Coastal Construction Manual cites values of CU acting on horizontally mounted

    cylinders (i.e. a pipeline) to be in the range of 4.1 to 6.4. Under the range of conditions

    expected at the site (wave height = 0.2 m, period = 2 sec) vertical velocities could

    range up to 0.45 m/s. Assuming a CU of 5 and the quantities listed above, the force

    acting on the pipeline would be approximately 622 N per meter of pipe length.

    Additional scour protection and reinforcement might be considered for the section of

    the pipe that might fall in these conditions.

    8.6.6 Free spans

    Due to differential bed erosion or bathymetric highs and lows, the pipeline may end up

    with sections that are not directly supported by the seafloor in a free span. Free spans

    can be problematic for two reasons. First, the pipeline must be analyzed for the

    additional stresses that include the bending moment and tensile forces that will be

    generated at the mid-point of the free span. Second, fluid flow due to currents and

    waves can lead to oscillatory motion of the pipe. Lift forces can be generated in a

    downward direction if there is a narrow space under the pipe where water can flow. If

    the flow is oscillatory (i.e. from waves) then alternating positive and negative lift can

    be generated resulting in a vibrational motion that can lead to failure. The frequency of

    this flow-induced oscillatory motion should be de-tuned from the frequency of the free

    span. This level of analysis requires information on the pipe material to be used and

    cannot be done here. Overall, it is preferable for the pipeline to be laid at a depth where

    risk of free spanning is minimised.

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    8.7 Summary and Recommendations

    Two submarine harbour-crossing pipeline route options have been assessed for the

    worst case assumption that the pipeline is, or becomes, exposed. Both routes are

    predicted to alter the hydrodynamics and sediment transport in the main channel, but

    only at a very local scale. However, the northern route near the railway bridge (Option

    5B) is found to be superior in relation to flow alterations, sedimentary impacts and

    visual appearance. One negative influence of the southern pipe (Option 5C) is the

    identified potential for muds to build up in the zone between the pipe and railway

    causeway. These muds may degrade the inter-tidal zone, change the biota and

    potentially lead to polluted mud contamination in the future. The long crossing over the

    inter-tidal sand flat could also weaken the natural bonds in the harbour bed

    sediments. The southern route is a difficult traverse over multiple channels and sand

    banks, and could interfere with boating safety.

    The northern route (Option 5B) with the pipeline placed along the existing railway

    causeway is recommended for these reasons and because negligible environmental

    effects are anticipated along this route, noting that the biological effects of direct burial

    of the seabed by the widened causeway is not considered in this report. The beach

    crossings at the western and eastern ends of the pipeline are considered in Chapter


    8.8 Shoreline traverse of the proposed pipeline

    In the next chapter, the effects of a proposed walkway to cover the pipe as it traverses

    south at the top of the beach on the western side of the channel (Figure 8.15) is

    considered. Likely physical and social impacts of this route include:

    Build up of muds between the pipe and the natural shoreline

    Changed wave and current climate along the shoreline

    Changed natural character of the existing shoreline

    Difficulties associated with water access for the public

  • Southern Pipeline: Physical Process Assessments

    ASR Marine Consulting and Research 182

    Potential boating/anchoring hazard at the northern extremity

    Figure 8.15. Proposed pipeline routes

  • Southern Pipeline: Physical Process Assessments

    ASR Marine Consulting and Research 183

    9 Walkway along the western shoreline beach

    9.1 Introduction

    URS provided the latest design for the proposed western shoreline walkway, which

    will serve the dual purpose of a promenade and a structure to cover the pipeline as

    it traverses north along the upper section of beach towards the railway bridge

    (Figure 9.1). The walkway is to be constructed of rock as an embankment. In

    addition, shoreline sand nourishment (some 3000 m3) is proposed for the beaches

    along the western side and this was incorporated into the bathymetry grids.

    Numerical modelling was undertaken to determine the secondary effects of the

    proposed walkway on the channel circulation and sand banks. The effects on the

    upper beach or the direct effect of burial by the structure are not considered.

    Specifically, the questions to be addressed are:

    what effect will the walkway have on the harbour circulation and on existing

    sand banks, particularly the sand bank to the north between First Ave and

    the railway bridge (Figure 9.2); and

    will there be an impact on the restaurant to the immediate north of the bridge

    (Figure 9.3)

    will there be toe scour at the foot of the walkway structure and what are the

    implications for structure stability

    A fine scale numerical model of the region was developed to predict the changes to

    currents and inferences are drawn in relation to the likely impacts on the sediment

    transport. Sediment transport modeling to determine the long-term effects or the rate

    of scour along the