OTC-28778-MS

On-Bottom Stability Design of Pipelines and Umbilicals on SeabedSusceptible to Scour: A Multi-Faceted Approach

Bassem Youssef and Dermot O'Brien, Atteris Pty Ltd

Copyright 2018, Offshore Technology Conference

This paper was prepared for presentation at the Offshore Technology Conference held in Houston, Texas, USA, 30 April–3 May 2018.

This paper was selected for presentation by an OTC program committee following review of information contained in an abstract submitted by the author(s). Contents ofthe paper have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material does not necessarily reflect anyposition of the Offshore Technology Conference, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the writtenconsent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations maynot be copied. The abstract must contain conspicuous acknowledgment of OTC copyright.

AbstractPipelines and umbilicals are essential elements in any offshore hydrocarbon development. Ensuring the on-bottom stability of pipelines and umbilicals during their operational lifetime is an integral part of the designof offshore hydrocarbon developments.

During storm events, pipelines and umbilicals can be subjected to significant hydrodynamic loads.Analysis approaches to assess the on-bottom stability of pipelines have evolved over time to arrive atcurrently accepted pipeline stability requirements. Early simple approaches relied on determining theminimum pipeline specific gravity that satisfied the force balance between the hydrodynamic loads andthe pipeline submerged weight and the soil resistance (typically considered as a Coulomb friction factor).Recent more detailed approaches have been developed using finite element modeling of a three dimensionalpipeline, time domain storm loading and dynamic soil resistance modeling (See Youssef and O'Brien Ref.17 for example). Project costs can be significantly impacted where pipeline or umbilical self-weight isinsufficient to ensure the on-bottom stability requirements and secondary stabilization is deemed necessary.However, for seabed material susceptible to scour, there is still more engineering work to be done to achievemore economic pipeline on-bottom stability designs. On-bottom stability recommended practice DNVGL-RP-F109 (Ref. 7) does not take into account the effects of seabed instability on the on-bottom stability ofpipelines.

Seabed materials in many regions around the world are known to be mobile. Seabed mobility can occur athydrodynamic conditions significantly lower than the hydrodynamic conditions that would cause instabilityof a pipeline or umbilical resting on a flat seabed.

While seabed scour process can have a significant influence on the stability of pipelines and umbilicals,the scour processes are relatively random and time dependant. This complicates the task of predicting theexact evolution of the scour process and accounting for the impact of scour on the stability assessments.Moreover, the extent of scour and pipeline self-burial is dependent not only on the magnitude of thehydrodynamic loading, but also on the duration of exposure and the hydrodynamic loading history. As statedby DNVGL-RP-F114 (Ref. 8) on the effects of seabed scour on the on-bottom stability "This is a complexprocess, and design methodologies taking these effects into account have not yet been fully developed".

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The on-bottom stability approach presented in this paper is based on an understanding of how seabedscour processes develop in the presence of the pipelines and the resultant impact on the pipeline stability.Numerous field observations and other evidence of pipelines and umbilicals becoming self-buried duringtheir operational lifetime due to seabed scour have consolidated the need to account for scour in theon-bottom stability analysis. This approach accounts for the interaction between seabed scour, pipelineself-burial and on-bottom stability to better predict the pipeline on-bottom stability during design, and toeliminate or reduce the need for any secondary stabilization.

The approach has been built up and validated through: 1) offshore field observations and recordsof pipelines and umbilicals, 2) Physical testing results of pipe models on erodible soil, 3) Theoreticalapproaches that predict scour and pipeline self-burial mechanisms, 4) Absolute stability analysismethodology of DNVGL-RP-F109 (Ref. 7).

The objective of this paper is to outline a methodology that can be applied to allow seabed scour to beincorporated into on-bottom stability assessments for new pipelines. It should be noted that seabed mobilitymay also affect other design considerations such as free spanning, pipeline thermal expansion, axial walking,buckling and fatigue analyses. However, these analyses are not part of the scope of this paper.

Background and Literature ReviewOn-bottom stability design methodologies of the recommended practice DNVGL-RP-F109 (Ref. 7)consider pipeline stability on a stable seabed. For cohesionless soils susceptible to scour and sedimenttransport, this is not the case. Considerable research activities including academic and Joint Industry Projects(JIP) have been conducted on the topic of pipeline stability on mobile seabed to better understand and assessthe phenomenon of pipeline self-burial. For examples of such research, see Jas et al. (Ref. 11); Cheng et al.(Ref. 3); Draper et al. (Ref. 6) and Griffiths et al. (Ref. 10). In sandy soils, the development of seabed scourunderneath the pipeline will in most cases enhance the stability of the pipeline. Once the pipeline sags intothe scour trench, the pipeline is partially sheltered from wave and current hydrodynamic loads. Moreover,the pipeline benefits from the increase in the passive soil resistance when the scour trench backfills withsediments, which usually occurs at the end of the scour process and in calmer sea states.

On-the-other-hand, scour around secondary stabilization structures, such as concrete mattresses or gravityanchors, can impose unnecessary risk to the pipelines. Figure 1 presents two views of severe scour thatoccurred at a concrete mattress location. The concrete mattress has been used to prevent abrasion and toprovide stability at a crossing between two umbilicals. Seabed scour occurred underneath the concretemattress shorter side causing the concrete mattress to sag and effectively ‘hang’ on one of the umbilicals.Such an incident highlights the need to assess seabed scour behavior when any new structure is introducedto the seabed.

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Figure 1—ROV Snapshot of Scour around Secondary Stabilization

Pipelines laid directly on the seabed will experience initial embedment. The pipeline initial embedmentvalue is function of many parameters. Among these parameters are: pipe submerged weight, dynamic actionsduring the pipe laying process and soil bearing capacity (DNVGL-RP-F114, Ref. 8). Figure 2 presents aschematic of scour evolution along a pipeline. In this case the pipeline initial embedment post laying isassumed to be 1% of the pipeline diameter (D). Due to seabed irregularity (case a), there are gaps wherethe pipeline is not in contact with the seabed. Amplified shear stresses at these gaps can be sufficient toinitiate seabed scour beneath the pipeline (Sumer and Fredsøe, Ref. 14). Inaddition, for a case of ideallyflat seabed (case b), due to wave hydrodynamic load irregularity; there will be points along the pipelinewhere hydrodynamic loads are high enough to trigger seepage underneath the pipeline due to the pressuredifference between points x and y in Figure 2-b (Section 2-2). This seepage can cause piping failure to thesoil material. The piping failure would as a consequence create points along the pipeline where scour caninitiate (Sumer et al., Ref. 16).

The presence of the pipeline itself on the seabed creates a disturbance to the hydrodynamic flow aroundthe pipeline which leads to shear stress amplification and seabed scour around the pipeline. Once a scourhole has initiated, the scour process would evolve underneath the pipeline until the equilibrium scour depthis achieved (Sumer and Fredsøe, Ref. 14). At the same time, scour will propagate axially along the pipelinein both directions. The rate at which the scour evolves depends on the magnitude and ratio of the wave tocurrent velocities, duration of hydrodynamic loading and soil properties. The typical process of scour andself-burial of a long pipeline section can take up to 2-5 years as noted by Leckie at al. (Ref. 12). However, thisprocess would depend on the pipeline rigidity and the pipeline outer diameter in addition to the parametersmentioned above. It is fair to conclude that a small diameter flexible pipeline would experience self-burialmuch more easily and in a shorter period than a large diameter rigid pipeline.

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Figure 2—Scour Evolution along a Pipeline

As the scour extends along the pipeline, the reaction loads at the span shoulder will also increase leadingthe pipeline to penetrate further into the seabed at the span shoulder location. This may occur along withpipeline sagging at the middle of the span length (See Figure 3). A more flexible pipeline or umbilical wouldmore readily sag into the scour trench. Adjacent spans may also integrate forming larger span once theseparating span shoulder scours or collapses. This would further increase the reaction on the span shouldersand accelerate the pipeline sagging into the scour trench.

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Figure 3—Seabed Scour Trench Formation underneath the Pipeline

From a hydrodynamic loading perspective, the horizontal and vertical forces on a pipeline within thespan are much less than the hydrodynamic loads on a pipeline touching the seabed. Moreover, for a pipelinein a span configuration, during part of the wave period, the lift loads will be acting downward (AGA, Ref.1; Sumer and Fredsøe, Ref. 15). This would tend to improve pipeline on-bottom stability as the downwardlift load would apply more pressure on the span shoulder, increasing resistance to lateral movement.

The above scenario assumes the pipeline remains laterally stable while the scour evolution processevolves leading to the gradual pipeline self-burial. However, there is also another credible scenario that thepipeline displaces laterally outside the initial scour trench before experiencing self-burial. The later scenariowill result when the pipeline self weight is insufficient to keep the pipeline in position during the scourprocess, or the hydrodynamic loading has built up very rapidly such that there is insufficient time for thescour process to fully develop.

During a typical storm event development there is a period during which the storm wave height ramps-up from pre-storm wave height to the storm peak value. This ramp-up period can vary between 12 to 36hours as reported by Draper et al. (Ref. 6) based on measurements data of various tropical storms in theNorth West Shelf region of Western Australia. The rate of increase in the amplitude of equivalent near-bed wave velocity can range between 10−6 ms−2 and 10−5 ms−2. While the rate of increase in the amplitudeof the current velocity is about 10−3 ms−2. Therefore, during such a storm, a pipeline on the seabed wouldexperience a gradual increase of near seabed velocities rather than a sudden peak storm velocity.

Draper et al. (Ref. 6) have performed experiments reflecting the above two scenarios through physicalmodeling of a pipeline model in the O-tube flume facility of the University of Western Australia (for moredetails about the O-tube facility, refer to Cheng et al., Ref. 4). In Draper et al. work, 12 different experimentswere conducted to investigate pipeline scour and self-burial phenomena for different wave to current ratios,initial embedments, rates of increase of the loads and pipeline bending properties. The pipeline model usedis 196 mm in diameter and extended across the full O-tube width of 1.0 m.

The two test cases of interest that will be discussed here were conducted using a load control methodology.The load control test simulates the self-weight of the pipe while allowing for horizontal and verticalmovement. Consequently, it was possible for the pipeline to sink into the seabed due to scour and to capture

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stability directly, while also allowing the pipeline to displace due to any hydrodynamic forces that exceedthe available soil resistance.

Test 1 (PRS-04): In this test the current velocity increased rapidly at a rate of 2E−2 ms−2 and pipelinespecific gravity (SG) 1.5. Scour started to develop underneath the pipeline but as the test progressed thepipeline displaced laterally. In this test the rate of loading was sufficient to move the pipeline laterally beforethe scour developed significantly. The increase in the loading rate that has been used in this test is higherthan actual field observations and presents an extreme condition. However, the test result highlights thescenario of a pipeline loosing stability before the development of the scour and the pipeline self-burial.Figure 4 presents two snapshots of Test 1.

Figure 4—Two Snapshots of Test 1 (PRS-04). Pipeline moved Laterally beforeExperiencing Significant Scour. Current is from Left. (Adopted from Draper et al., Ref 6)

Test 2 (PRS-01): In this test, the testing conditions are as per Test 1 except that the current velocityincreased at a rate of 2E−3 ms−2 which is 10 times less than velocity increase rate used in Test 1. Duringthis test seabed scour initiated under the pipeline. As the test progressed; the scour underneath the pipelinecontinued to develop and propagated inwards towards the middle of the pipeline model. The pipeline beganto lower vertically into the trench as the trench scour evolved further. Eventually the pipe lowered morethan a full diameter relative to its initial position on the seabed, and the scour trench backfilled toward theend of the storm period. Figure 5 presents six snapshots of Test 2 showing the evolution of the scour processand the pipeline self-burial.

OTC-28778-MS 7

Figure 5—Six Snapshots of Test 2 (PRS-01) Pipeline Sinking intothe Seabed. Current is from Left. (Adopted from Draper et al., Ref 6)

Two points of particular interest from this test are as follows: 1- The trench depth developed underneaththe pipeline is more than the pipe model outer diameter. This is due to the fact that the pipeline was saggingwhile the scour process is in progress (see Figures 5-b to 5-d). 2- Backfilling of the scour trench started tooccur once the pipeline sagged into the scour trench completely below the exposed seabed level. This canbe confirmed from the shape of the sediments deposited under the pipeline in Figure 5-e.

Scour Induced Burial of Existing ServicesOperational pipelines and umbilicals usually undergo regular inspections, using ROV surveys for example,to confirm that the pipeline conditions are as per the intended design. Such surveys are an integral part of

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maintaining pipeline integrity and ensuring the pipelines continue to perform their function as required.During these surveys, any seabed scour, pipeline self-burial and pipeline free spanning are typically recordedand reported.

It is often the case that the recorded span lengths and start and end points change from survey to another.This phenomenon is usually referred to as "span migration". Such data is very important to understand thebehavior of the pipeline-seabed interaction in real offshore conditions. However, very little of this type ofdata is available in the public domain.

This paper presents typical examples of pipelines and umbilicals installed on the North West Shelf ofWestern Australia which have experienced significant embedment during their operational lifetime Theflowlines and umbilicals were installed directly on the seabed. The specific gravity of the flowlines andumbilicals were typically designed to satisfy the absolute stability requirements for 100 year return periodmetocean conditions without consideration of seabed scour.

Table 1 shows the structural details of the pipelines and umbilical, and the duration between installationand inspection years.

Table 1—Three cases of pipeline and umbilicals in the NWS of Australia

Parameter Case 16-inch Flowline

Case 24-inch Flowline

Case 3Umbilical

Outside Diameter, mm 224.3 155.6 122.0

Dry Weight, kg/m 94.54 45.90 31.70

Submerged Weight, kg/m 54.04 26.41 19.70

Specific Gravity, - 2.33 2.35 2.60

Duration between Installation and Inspection, years 8 8 8

The flowlines and umbilical inspection has revealed that the flowlines and umbilical have experienceddifferent levels of self-burial along considerable sections of the routes. There are also some locations wherefree spanning are reported which indicates that scour processes are on-going. Figure 6 presents the burialstatus along sections of the two flowlines routes.

Figure 6—6-inch and 4-inch Pipeline Burial Status

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The burial status of the flowlines and umbilical during inspection are summarized as follows:Case 1: 6-inch flowline is found to be more than 75% buried along majority of the flowlineroute with some sections where the flowline is shown to be spanning.Case 2: 4-inch flowline is reported to be 100% buried along almost the entire route except the500 m section next to the manifold tie-in location the flowline burial status is shown to be varybetween 100% buried and spanning.Case 3: 122 mm umbilical is reported to be 100% buried along almost the entire umbilical routewith some sections the umbilical only partially buried.

Figure 7 presents six snapshots of the ROV inspection of the flowlines and umbilical along with indicativeburial percentages.

Figure 7—ROV Survey Snapshots of the Flowlines and Umbilical

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Seabed Scour CalculationsGeneral seabed scour calculations provide an indication of the critical velocity at which the seabed willstart to experience instability remote from any seabed structures. Sumer and Fredsøe (Ref. 14) provides thecritical water particle velocity (Equation 1) that cause seabed instability. The critical velocity expressionis based on equating the stabilizing gravitational force acting on the soil grain and the critical shear stressexerted on the same soil grain.

(1)

where (g) is the acceleration of gravity, (SS) the relative grain density, (D50) soil parameter and (fw) soilfriction factor which can be defined from Equation 2

(2)

where (AW) is the orbital semi-diameter of water particles (AW = UmT / 2π) and (KB) the equivalent sandroughness parameter (KB = 2.5 D50).

Once the pipeline is installed on the seabed, it will disturb the hydrodynamic flow around the pipeline andenhance the potential for localized seabed scour. Assessment of the onset of scour underneath the pipelinecan be calculated using the equations of Sumer and Fredsøe (Ref. 14). Onset of scour is affected by theinitial embedment depth (e) and the diameter of the pipeline (D). The critical velocity for onset of scour incurrent only conditions is defined as follows:

(3)

where (Ucr) is the critical current velocity for onset of scour, (s) soil specific gravity, (n) the soil porosityand (e/D) the pipeline embedment level.

For wave only conditions Sumer and Fredsøe (Ref. 14) define the critical wave velocity as a function ofthe pipeline embedment level (e/D) and the Keulegan-Carpenter (KC) number :

(4)

The value of the right hand side of Equation 4 can be determined using the graph presented in Sumerand Fredsøe (Ref. 14). The graph is based on tests performed for a range of KC values and embedmentlevels up to 40%. The graph might be extrapolated for values corresponding to embedment levels above40%. However, these values should be used as indication only because of the uncertainly in the extrapolatedvalues. As noted from Equations 3 and 4; the higher the pipeline embedment the higher the critical velocityrequired to initiate seabed scour.

The expected equilibrium scour depth that can be developed underneath the pipeline is presented inSumer and Fredsøe (Ref. 14) for different wave and current flow conditions, and is presented in Figure 8.The graph provides the equilibrium scour depth considering a pipeline fixed in position (i.e. the pipelinedoes not sag in to the scour trench). However, when the pipeline or umbilical sag into the scour trench, thepipeline may force increased scour depth to occur as can be seen from the test results presented in Figure5. This graph might present indicative value for the possible equilibrium scour depth.

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Figure 8—Equilibrium Scour Depth, Wave and Current Condition (Adopted from Sumer and Fredsøe, Ref. 14)

Stability CalculationsThe absolute stability method of DNVGL (Ref. 7) provides the equations required to estimate the pipelinesubmerged weight required to achieve on-bottom stability requirements. As seen in the physical testingresults (see Figure 5), once scour initiates beneath the pipeline, the pipeline is expected to rest in an openV-shape scour trench. The scour trench will tend to be backfilled with sediments as the storm subsides andin subsequent less onerous metocean events. However, for the worst case scenario, the onbottom stabilityanalysis should not take any advantage of the passive soil resistance, and should assume that the pipelineneeds to overcome an open V-shape scour trench to displace laterally outside the scour zone.

Figure 9 shows the force balance diagram along the scour trench slope. By considering the force balancediagram, and ignoring the vertical component of the horizontal load (Fh sin(θ)) in the force balance diagrama conservative expression of the lateral horizontal forces can be defined as:

(5)

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Figure 9—Force Balance Diagram along the Scour Trench Side Slope

The side slope angle (θ) is conservatively considered to vary linearly between 0° for no embedment toa maximum of 30° for a 50% pipeline embedment or greater.

New Stability ApproachThe on-bottom stability approach presented in this paper is only applicable to pipelines and umbilicals onseabed susceptible to scour. Before commencing the on-bottom stability analysis following this approach,it must be confirmed that the seabed soil is prone to scour and the pipeline self-burial behavior is expected.This can be achieved by obtaining evidence of previous scour and self-burial cases of existing pipelines orumbilicals within the field. In cases of new fields, the soil conditions along the pipeline route should beassessed through geotechnical sampling and laboratory testing to confirm that the seabed material along thepipeline route is cohesionless and prone to scour. Furthermore, the approach is not applicable to cases withhydrodynamic conditions known to build up suddenly or over a short period of time.

The new stability approach can be applied to estimate the pipeline or umbilical minimum self-weightrequired to achieve absolute stability considering seabed scour. The approach also can be used to determinethe minimum embedment level that is required to achieve absolute stability for a range of metoceanconditions.

Two sets of velocities are used in this approach:

1. Stability threshold velocity (Vst): which is defined as the maximum velocity that the pipeline with acertain embedment level can withstand before displacing laterally.

2. Scour critical velocity (Vsc): The minimum velocity required to initiate scour underneath a pipelinewith a certain level of embedment.

For the pipeline to be stable during the seabed scour process and to achieve self-burial; the stabilitythreshold velocity must be higher than the scour critical velocity at all pipeline embedment levels. Thestability threshold velocity can be back-calculated from the absolute stability calculation for a pipelinein an open V-shape trench, as discussed in the previous section. While the scour critical velocity can becalculated using Sumer and Fredsøe (Ref. 14) onset of scour equations presented previously in this paper.The methodology is outlined in the flowchart presented in Figure 10.

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Figure 10—New Approach Outline

The following numerical examples present the results of this approach to on-bottom stability calculationsfor the 6-inch flowline and the 122 mm umbilical presented in previous section of this paper. The designmetocean data and the flowline and the umbilical properties are presented in Table 2.

Table 2—Numerical Example Data

Parameter 6-inch Flowline Umbilical

Outside Diameter, mm 224.3 122.0

Specific Gravity, Original Design, - 2.33 2.60

Specific Gravity, New Methodology, - 1.50 1.60

100 year Return Period design conditions

Max Perpendicular Wave Velocity, m/s 0.48 0.45

Wave Period, s 13.45 13.45

Steady Current Velocity @1m ASB, m/s 0.72 0.72

Current Attack Angle, degree 90 40

Soil Conditions

Friction Coefficient, 0.5 0.5

Median Particle Size D50, mm 0.2 0.2

Submerged Unit Weight, N/m3 8000 8000

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The stability threshold velocity of the flowline and the umbilical considering the original design specificgravity (SG of 2.33 and 2.60, respectively) are shown to be much higher than the scour critical velocityfor all embedment levels and higher than the 100 year return period metocean conditions. Hence both theflowline and the umbilical are expected to be fully stable during the scour and self-burial processes. Theseresults agree with the ROV inspection findings presented in previous section.

Considering the flowline and the umbilical specific gravity presented in Table 2 (SG of 1.50 and 1.60,respectively), the stability threshold velocity is calculated for consecutively increasing embedment levelsof the flowline and the umbilical. As shown in Figure 11 the stability threshold velocity is higher thanthe scour critical velocity for all the umbilical embedment levels. The results indicate that the umbilical isexpected to be stable while seabed scour is developing underneath the umbilical. Furthermore the figureindicates that the most critical embedment level for stability is when embedment levels are between 10%and 30%, because in this region the margin between the stability threshold velocity and the scour criticalvelocity is lowest.

Figure 11—Example of Umbilical On-bottom Stability Analysis Considering Seabed Scour

Similar results are presented in Figure 12 for the flowline. As shown in Figure 12 the stability thresholdvelocity is higher than the scour critical velocity for all the flowline embedment levels. The results indicatethat the flowline is expected to be stable while seabed scour is developing underneath the flowline. For 20%embedment level, the scour critical velocity is marginally less than the stability threshold velocity, indicatingthat stability at this embedment level is critical. For this case it may be decided to increase the flowline SGto provide a higher margin of safety between the stability threshold velocity and the scour critical velocity.

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Figure 12—Example of Flowline On-bottom Stability Analysis Considering Seabed Scour

As indicated in previous section, the onset of scour graph presented in Sumer and Fredsøe (Ref. 14)is based on embedment levels up to 40%. The critical velocities calculated for embedment levels above40% are based on extrapolation of the onset of scour graph of Sumer and Fredsøe and should be used asindicative values.

Considering the results presented in the above examples, it is clear that the traditional on-bottom stabilitymethods are conservative, and including the beneficial effects of seabed scour in the analysis would lead tomore economic and still safe on-bottom stability designs. In the worked example, umbilical specific gravitycould be reduced from 2.60 to potentially as low as 1.60 with demonstrated stability in the 100 year returnperiod metocean conditions. Such a reduction would yield a significant cost saving in the manufacture andinstallation cost of the umbilical. Similarly for the worked flowline example, flowline specific gravity couldbe reduced from 2.33 to potentially as low as 1.50.

The new design methodology can be performed combined with any statistical analysis method (See forexample Youssef el al. Ref. 18) to account for the uncertainty in the design input parameters.

ConclusionThis paper introduces a new approach for on-bottom stability analysis that accounts for the effect of seabedscour. The new approach reflects a more realistic offshore hydrodynamic-pipe-soil interaction approachaccounting for seabed instability, and provides a means of demonstrating acceptable on-bottom stability forumbilicals and flowlines in erodible seabed areas with a significantly lower specific gravity than currentapproaches which assume the seabed is stable.

The approach has been built-up and validated through: 1) analyzing numerous offshore field observationsand records of self-buried pipelines in seabed areas susceptible to scour 2) physical modeling test dataavailable in the public domain for pipeline self-burial as a result of seabed scour 3) theoretical calculationsthat predict scour mechanisms 4) absolute stability method of DNVGL-RP-F109 (Ref. 7) which is slightlymodified to reflect a pipeline in an open scour induced trench.

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Validation of the approach has been performed considering actual offshore small diameter pipeline andumbilical cases (diameter less than 250 mm) which have experienced significant self-burial. The validationtest results have matched the real offshore self-burial conditions and confirmed that similar diameterpipelines with lower specific gravity would have experienced similar self-burial behavior. Applicability ofthe presented methodology to larger diameter pipeline is not yet confirmed.

This approach is only applicable for pipelines and umbilicals on seabed susceptible to scour, and wherethe build up of hydrodynamic design loading is gradual. A well-developed understanding of the seabedscour conditions along the entire pipeline route is required to confirm that the seabed material is susceptibleto scour and that self-burial can be expected.

The approach should be used with caution, given that it is not endorsed by an industry recommendpractice. The approach is intended for users with full knowledge, understanding and experience of thehydrodynamic-pipe-soil interaction and the seabed scour behavior.

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15. Sumer, B. M. and Fredsøe, J., 1997. Hydrodynamics around Cylindrical Structures. vol. 12,World Scientific Publishing, London.

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