PIPELINE SPANNING-CONCLUSION SPANNING CAN BE PREVENTED, CORRECTED IN DEEPER WATER

Marvin M. Beckmann, James R. Hale, Craig W. Lamison
Brown & Root Inc.
Houston

Analysis and correction of subsea pipeline spans are becoming more important as pipelay operations move into the deeper waters of the U.S. Gulf of Mexico.

The initial article of this series (Part 1, OGJ, Dec. 9, p. 56) showed how careful route selection may prevent spanning.

This concluding article sets forth methods for eliminating spans during design or correcting those which occur despite careful design. The information and practices described are based on recent Brown & Root Inc. experience with pipeline spanning in the Gulf of Mexico.

As defined in Part 1, correcting may involve placement of sand bag supports under the unsupported spans or jetting out high spots supporting the pipe.

ALLOWABLE SPAN

During detailed design of a pipeline, a conservative maximum allowable span should be determined. Limitations on span length fall into two categories: static stress related and vortex-shedding resonance related.

STRESSES DEFINED

At the design stage, allowable spans may be determined with back-calculations of bending stresses based on an allowable code stress.

Bending stress is frequently related to span length by an assumption that the maximum moment in a span is equal to (WsL2)/10 where: Ws = submerged weight of pipe; L = span length.

This relationship applies because it is consistent with the moment in a continuous beam.

The moment produced falls between that for a fixed-end beam and a pinned-end beam. In addition to the dead load, horizontal bending stresses should be added vectorially. These include route curvature and hydrodynamic loads from currents and waves.

Span lengths are determined this way for the appropriate design conditions, which are usually installation, hydrotest, and operation.

Once a value is calculated, an allowable span is selected that is less than the calculated value and an even multiple of 20 ft (one half of a 40-ft pipe joint). This allows divers or an ROV easily to compare the actual span length to the allowable span length.

Although this approach only approximates actual conditions in the field, it is the best that can be done at the design stage.

This situation results from the following factors:

• The actual location of the pipeline may vary from the planned location, thus changing the anticipated bottom topography.

• Span-support conditions are influenced not only by bottom topography but also by soil characteristics. These latter may vary markedly along the pipeline and are usually not known in much detail.

Based on the anticipated route and soil conditions, the likelihood of spans exceeding this value can be determined with a suitable computer program, such as the American Gas Association (AGA) pipeline stability program.

For this determination, the pipe would be modeled on the anticipated topography with foundation properties reflecting the local soil characteristics. Because this process can be time consuming, areas with smooth topography are not usually analyzed.

Such an analysis performed for the AEDC (U.S.A.) Inc. pipeline (Part 1, OGJ, Dec. 9, p. 55) predicted that approximately six spans might require correction. Number, location, and severity of such spans agreed well with actual spans which occurred during pipelay.

For conservative reasons, correcting the AEDC spans was chosen.

Another option, however, is to analyze the actual spans and determine if they in fact exceed the code requirements. This was done in part for the AEDC pipeline to determine if it could be flooded before correction of the last spans.

At this point, if the bottom topography is accurately known beneath the spans and for several joints on either side, the unknowns are eliminated.

The pipeline location is now fixed, and the pipe geometry at the ends of the spans can be modeled to reflect the actual soil conditions. Thus, if analysis shows the stresses to be within the allowable for a given span, the values of span length specified during the design may be ignored and the span accepted.

SHEDDING RESONANCE

The second limitation governing allowable span length results from vortex shedding-induced resonance. Lock-in or synchronization of vortex shedding may occur when the frequency of eddies being shed in a current is similar to a natural frequency of the pipe span.

A resonating span can undergo significant deflections and associated stresses. Because of the cyclical nature of the stress, fatigue can occur.

Among the various causes of currents are storms or hurricanes, loops or eddies of major deepwater ocean currents, tides, and waves.

Over the time that resonance is developed, most of these causes generate essentially steady currents. The exception is wave action.

Wave-particle velocities vary in magnitude and direction over a relatively short time. Thus, they do not contribute to extended periods of resonance in the same manner as steady currents.

Although wave-particle velocities contribute to vortex shedding, no means are available to quantify the effects accurately. Also, it is overly conservative to include a steady-velocity component equal to the waves' maximum particle velocities.

Thus, the generally accepted approach is to ignore the wave-generated vortices for on-bottom pipeline spans.

Vortices are shed in two ways. At lower current velocities, vortex shedding is symmetrical. Vortices are shed simultaneously from both sides of the pipe.

At higher velocities, a vortex is shed from one side of the pipeline followed by a vortex shed from the other in an alternating pattern (Fig. 1).

Symmetrical shedding causes the pipeline to vibrate in line with the flow direction.

Asymmetrical shedding, however, causes two components of vibration. The component in the direction of the flow, in-line motion, is the same as for symmetrical shedding; the other, at right angles to the flow, is called cross-flow motion."

The in-line impulse occurs in the same direction with every vortex. The cross-flow impulse alternates direction.

In-line excitation is thus at a frequency twice that of cross-flow excitation and has a smaller motion amplitude and stress. For this reason in-line resonance is of less concern than cross-flow motion and is sometimes neglected.

PREVENTING RESONANCE

Three approaches exist to deal with lock-in of vortex-shedding resonance:

1. Avoid it entirely; that is, specify a longest allowable span shorter than what can support resonance in the worst-case current. (Shorten any spans which exceed allowable.)

2. Accept resonance with sufficient analysis to confirm that pipe is not overstressed or excessively fatigued.

3. Use other measures (such as spoilers) to prevent vortex resonance from occurring.

Analysis of pipeline spans for susceptibility to vortex-shedding resonance is performed according to the guidelines of DnV Rules for Submarine Pipeline Systems.1

Avoiding unacceptable spans or increasing the allowable span requires examination of the following parameters which affect vortex-shedding resonance:

• Strouhal's number

• Reynolds number
  • Flow velocity and kinematic viscosity

  • Total diameter

• Natural frequency
  • Pipe characteristics, dimensions, mass

  • Span end conditions (between fixed-fixed and fixed-pinned beam)

  • Tension

  • Damping

  • Span length

Little can be done to alter the Reynolds number of the flow or the first three parameters affecting the natural frequency.

The structural damping of a bare steel pipeline in water can be at least doubled by applying concrete weight coating. This may be a suitable option for increasing allowable spans at the design stage.

Concrete coating may have the added benefit of causing the pipeline to settle and conform to the seabed. This will reduce the actual span length.

Span length may be directly reduced after pipelay by a variety of methods.

Flooding the pipeline may reduce the span length just as described earlier for weight coating.

Trenching may be used to change the bottom topography, or supports may be introduced along a span to shorten it.

FATIGUE CHECK

If the first approach is followed with a limit on in-line spans, cross-flow vortex shedding will also be acceptable. Or, in-line span resonance may be allowed, with a stress-fatigue check performed for spans in the region of in-line resonance from onset of in-line resonance to the onset of cross-flow resonance.

Likewise fatigue may be examined for peak cross-flow resonance greater than the range of in-line motion.

For proper evaluation, fatigue should include the sum of all dynamic effects, not just vortex shedding. For example, oscillations resulting from wave loadings or fatigue from installation will reduce the fatigue life available for vortex-shedding resistance.

Sufficiently detailed data are not usually available for such an exhaustive fatigue check. It is often possible, however, to check the vortex-shedding component alone for a more limiting condition. This shows whether a given span will be significantly affected.

An example would be a span that is acceptable for a 10-year return period current and for cross-flow conditions for the 100-year return period. But it exceeds the allowable span length for a 100-year storm based on in-line resonance criteria.

For these conditions a relatively simple check could be performed. The frequency of vibration, amplitude of deflection, corresponding stress, and current duration could be calculated for the 100 year in-line condition.

With this information, the fatigue life would be determined with a cumulative damage approach (such as the AWS X-curve)2 and conservatively compared to the expected total duration of storms greater than the 10-year storm.

If adequate safety for the full design life is not provided but failure is not imminent, another option is available. The span may be monitored periodically rather than rectified.

Given time, natural settlement, scour, and sedimentation may reduce the span below the allowable length, thus alleviating the need for rectification.

STRUCTURAL ANALYSIS

Accurate prediction of the extent of pipeline spanning for a given situation requires a structural analysis tool with the following capabilities:

• Irregular seabed or foundation geometry

• Tensioned beam formulation

• Elastic foundation or soil strength

• Sufficient pipe length (minimum 2,000 ft suggested)

• Sufficient boundary conditions to model end conditions (pipe angle and tension).

In addition to these capabilities, interpreting the analyses can be greatly simplified by a good post-processing tool. The tool should graphically display spans and calculate resulting span lengths and heights off-seabed.

This type of analysis has been conducted for Conoco's Jolliet project (OGJ, Nov. 11, p. 60) and for AEDC's MC 486 project.

Inputs to the analyses include seabed bathymetry and soil strength along the pipe route, pipe parameters (diameter, wall thickness, and submerged weight), and the estimated residual tension from installation. The outputs include the pipeline configuration and stresses, as well as span information (span lengths and height off seabed).

Except for bottom topography, the most important input parameter is residual installation tension. This parameter is a function of the installation method.

For tow methods, it is a function of bottom friction, tow length, and the tow weight. For surface installation methods, pipe tension at the seabed is a function of pipe weight, water depth, and pipe exit angle (the angle the pipeline leaves the installation vessel).

In deepwater, where the catenary equation closely approximates the actual pipelay parameters, the expression for bottom tension is given by the following:

Tb = Ws

D(cos[A])/(1 - cos[A])

where:

Tb = Tension at seabed

D = Water depth

A = Pipeline exit angle from installation vessel

Bottom tension (Tb) as a function of pipe exit angle (A) is shown in Fig. 2 for various pipe weights and water depths. In addition, Fig. 3 shows how a particular pipe span is affected by variations in tension.

Fig. 3a shows that with tension doubled, the span length is more than doubled; with tension tripled, the span length is more than tripled.

The effect of soil strength on a particular pipe span is shown in Fig. 3b.

The figure illustrates a case in which a variation in soil strength of an order of magnitude affects the span only slightly (approximately 20%). It should be noted that Fig. 3 is for specific sets of conditions; similar variations in tension and soil strength may be more or less dramatic for other conditions.

SPAN AVOIDANCE

If survey data indicate unacceptable spans during the design of the pipeline, various steps may be taken to address the problem. As previously mentioned, the most obvious solution is to reroute the pipeline wherever possible considering any given physical and economical constraints.

Other approaches include reducing pipelay tension, increasing pipe submerged weight, releasing pipelay tension in vicinity of spans, using flexible pipe in areas of extreme irregularity, using jet bury barge to presweep areas of irregularity to smooth seabed along route (water depth constrained), allowing for the use of preinstalled supports, and increasing pipe grade or wall thickness.

The following elaborates on these approaches.

Tension in the pipe will tend to lengthen a span by decreasing deflection. During pipelay, use of tension is necessary for this purpose because in the sagbend the deflection is not limited by any supports.

Once the pipeline is laid, however, unlimited deflection is impossible because the pipe will eventually contact the seabed. Maximizing the pipelay angle and using the least acceptable amount of tension will reduce on-bottom span lengths.

Likewise, in areas of predicted spanning, additional weight may be added to the pipe to make the span conform to the bottom. As previously mentioned, if concrete is used to provide additional weight, it will also provide additional damping thus increasing the allowable vortex-governed span length.

One approach which has been considered is the introduction of a mechanism in the line to relieve tension locally in the area of expected spanning (Fig. 4).

A slack piece of flexible pipe matching the pipeline size is installed within an oversize casing which is flanged to the pipeline. During the installation the pipelay tension is transmitted through the casing.

Once the pipe is on bottom the casing flange bolts are cut or released to allow the flexible pipe to straighten and relieve the tension in the pipeline.

Laying the pipeline in segments with free ends for later subsea connections would accomplish the same purpose.

For long sections of irregularity, entire segments of flexible pipe may be used. This was the approach eventually selected for Jolliet because flexible pipe was already being specified for the TLP catenary risers and deepwater section of the pipeline.

Another approach is to modify the bottom topography rather than the pipeline. If limited areas of irregularity are reliably identified, the pipelay corridor in these areas may be preswept with a jet bury barge to level the seabed. Care must then be taken to lay the pipeline within the leveled right of way.

In shallow water, supports are sometimes installed for pipeline crossings from the front of the pipelay vessel or an immediately preceding vessel. A similar technique might be employed for rectification of shallow water spans during pipelay.

In deep water, accuracy of placement would be more difficult. In such instances grout-bag supports could be attached to the pipeline by divers just after the pipe leaves the laybarge. The supports would have to be designed for the maximum anticipated span heights.

Once the pipeline is on bottom, the bags would be filled with grout by an ROV or diver-made connections. Some excess bags would need to be installed to ensure support of the entire span. Bags which ended up in areas of no spanning would not be filled.

Finally, acceptable span lengths may be obtained by an increase in the strength of the pipe with either a higher grade material or thicker wall.

SPAN CORRECTION

Measures taken during the design of a pipeline may be insufficient to prevent unacceptable spanning. This is the result of the previously discussed difficulty in predicting spans and the possibility that spans will develop from scour after the pipeline is in operation.

If post-lay or inspection surveys discover unsupported spans on an existing pipeline, these spans must be evaluated.

As has been mentioned, spans discovered during such surveys may be compared to allowables calculated during the design. Alternately, they may be analyzed based on site-specific survey and environmental data to determine their acceptability.

Spans may also disappear in time as a result of both sedimentation and settlement. Borderline spans may only require periodic monitoring to determine if in-filling or settlement is reducing the spans.

Examining the potential for bed-load transport will help indicate the possibility of sedimentation. Dynamic analysis to estimate settlement due to wave loads may be performed with the AGA's pipeline stability program.

The following alternatives exist to rectify existing unacceptable spans: sand or grout bags, inflatable grout supports, auger supports, piled supports, frame supports, jetting or trenching, forced settlement (overweights, vibration), backfill, and enhanced sedimentation (artificial seaweed).

BAGS; JETTING

Sand-cement or grout bags placed by divers are a common means of span rectification in shallow waters. Although common, such supports have been known to settle or tumble as a result of scour beneath them.

A more sophisticated approach uses fabric structures which attach to the pipeline and are then inflated with grout to form a monolithic support conforming to the pipe and the seabed.

If scour or an unusually soft bottom is anticipated, a grouted fabric mattress may be installed first as a foundation for the support (Fig. 5).

Proven methods exist for installing these devices in deep water with an ROV or atmospheric diving suit.

Another approach (Fig. 6) consists of auger anchors screwed into the seabed and clamped to the pipeline. Such anchors can be used to provide both support and anchoring of the pipeline against movement resulting from environmental loads.

A similar approach employs piles which may be either drilled and grouted or driven into the seabed.

In some instances, a support frame may be built to clamp to the pipeline. Such a frame can be installed in deep water if necessary.

Fig. 7 shows a frame concept proposed by Snamprogetti. This is an hydraulically engaged support that adjusts to variations in water depth and slope. A simpler support could be purpose built if span geometry is known.

Another technique that has been employed in shallow water is the use of post-lay trenching equipment. At the ends of a span, the seabed is lowered, This allows the pipeline to rest continuously on the seabed.

While this approach is valid in any water depth, common air-eduction jetting equipment is limited by depth. Water eduction has been used in the North Sea to extend the range of jet bury equipment.

SETTLEMENT, SEDIMENTATION

Two approaches use forced settlement to reduce or eliminate spans.

The first is similar to the design approach in which pipeline weight is increased in the predicted area of spanning by use of concrete coating. This forces conformance with the seabed topography and sinks the pipeline into the seabed at the ends of the spans. Using this approach after the pipeline has been laid requires that overweights be placed on the pipeline. These may be precast concrete shapes or grouted fabric mattresses (Fig. 8).

Care must be taken to ensure that the pipeline will not be unacceptably stressed when this approach is used.

A related approach is to flood the pipeline. Since the weight may significantly decrease when product replaces water in the line, spans require rechecking once the pipeline is in operation.

A second approach may be used in soft soils which involves vibrating the line to induce settlement into the soil. This could be done by attachment of a vibrating device to the outside of the pipeline with an ROV.

Alternately, vibrations may be induced by an internal pig. The pig could be eccentrically weighted and spiral through the pipeline to induce vibrations, or it could use a mechanical device to induce vibration.

Artificial backfill has been used to correct spans as well. Rock dumping from hopper vessels has been done in the North Sea. Pumping of a sand slurry or dumping of sand could be used as well.

If sand is used, care must be taken to prevent the sand liquefied by the emplacement technique from floating the pipeline to the seabed. This may result in a worse span.

If the backfill does not in-fill beneath the pipeline, however, this method may have unintended consequences. The material may place a weight surcharge on the line, increasing stresses.

Finally, a span can be alleviated by encouraging sedimentation to fill in the spanned depression. If significant bottom transport of granular material exists, this material can be trapped by use of a matrix of artificial seaweed made from polypropelene or other buoyant fibers (Fig. 9). Burial of the pipeline may even be achieved.

ACKNOWLEDGMENT

The authors wish to thank their management for permission to publish this article.

REFERENCES

1. Rules for Submarine Pipeline Systems, Appendix A2, Det norske Veritas, 1981.

2. "Structural Welding Code," American Welding Society (AWS), AWS D1.1.

Copyright 1991 Oil & Gas Journal. All Rights Reserved.