Marine studies for the design and construction of offshore pipelines

MARINE STUDIES FOR THE

DESIGN AND CONSTRUCTION OF OFFSHORE PIPELINES David R. Miller

Vice President; Daniel, Mann, Johnson, &: Mendenhall Los Angeles, California

ABSTRACT

The design and construction of submarine pipelines involves special investigations and studies to understand marine environmental conditions which affect the location, installation methods, and design techniques for undersea lines. The paper will discuss application of marine geology, oceanography and marine engineering studies to sub-marine pipeline problems. Types of submarine pipelines will be re-viewed and special application problems of design and installation will also be reviewed.

INTRODUCTION

Submarine pipelines are being used with increasing frequency by civil engineers. Applications are found in marine disposal of wastes, submarine aqueducts, marine gathering lines, fuel loading lines and power plant cooling water intakes. In the past, a principal deterrent to the use of submarine pipelines has been the cost of installation and a scarcity of available information on marine environmental design. However, with the development of new construction methods and equip-ment and rapid advances in marine research, submarine pipelines now can be more economically used for transportation and/ or disposal of liquids and gases.

Hitherto, the offshore areas have been the province of the marine scientist and through his efforts a large fund of general knowledge about the sea has been accumulated. However, the civil engineer, in designing a submarine pipeline, is faced with the problem of deter-mining in detail the marine environmental conditions in which the line is to be placed and must translate and apply the fund of general scien-tific knowledge into specific design conditions. The engineer must often carry out extensive offshore studies and even engage in special applied research in order to have sufficient information upon which to base the design of an important project.

GENERAL MARThlE ThlVESTIGATIONS

PrograTIls for TIlajor offshore projects, usually involve at least two phases. The first phase of a study prograTIl is concerned with a broad area which encoTIlpasses all possible project alternatives withiJ1 its boundaries. The investigations carried out during this phase are general in nature and have as their objective an understanding of the broad environTIlental conditions so as to identify constraining factor 5 and to provide a basis for the choice of a liTIlited nUTIlber of alternatiVe 5 to be studied in detail in the next prograTIl phase.

The general TIlarine investigations for waste disposal projects " TIlay involve very large areas. The Hyperion Sewerage Marine Inve atl.-gation ProgrciTIl for the City of Los Angeles, for exaTIlple, covered 0- e study area of over 100 square TIliles. The studies for this program p.o-v been reported in detail by Stevenson et al (1956). The following parO--graphs will outline specific activities included in an oceanographic survey of this type.

BathYTIletry. An overall bathyTIletric chart of the study area is 3 0 essential requireTIlent. It will be used aTIlong other studies and will p ee_ the initial TIlap for the definition of study route alternatives. Only r<3.-:r ly will existing hydrographic charts be adequate for this purpose. T p e

1 TIlap is prepared froTIl fathoTIleter traverses with radar position cont:ro ~ The fathoTIleter traces should be corrected for tidal variations, and :£ r e quent check points on the fathoTIleter travers es are desirable. The topographic details of the sea floor are contoured with an interval 0:£ one fathoTIl on a chart scale of 1 :40,000.

BottoTIl SediTIlents and SubTIlarine Geology. An investigation 0:£ 1;

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type and lateral distribution of bottoTIl sediTIlents will provide TIluch

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forTIlation on the general geologic structure of the study area. Apr 0 -

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graTIl of bottoTIl saTIlpling is carried out using orange peel and "snapg

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saTIlplers. The saTIlples of the bottoTIl sediTIlents are exaTIlined as to

grain size, sorting, percent silt and clay, percent organic TIlatter, -t:;i -TIlineral constituents, sphericity and roundness of particles. The ve ~ "1::7_1e

cal distribution of bottoTIl sediTIlents is deterTIlined to the extent pos G

::J-by coring TIlethods.

~ The results of the bottoTIl sediTIlent study are plotted on the ba £7 __ TIlap and an exaTIlination of this TIlap along with the basic data will p r c::7 in vide an indication" of the direction and average velocity of bottoTIl

flo

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:In the study area and areas of erosion and deposition can be identified. .£?= addition, the aTIlount of organic TIlatter contributed to the sea floor f~ present discharges can be deterTIlined along with areas of buildup.

Tidal Characteristics and Currents. The characteristics of the tides can usually be determined from existing data. However, in cer-tain instances where tidal forces are the dominant factor in producing water motion, then magnitude and variations of flow must be deter-mined. Non-tidal currents are studied with particular emphasis placed on the determination of current patterns and the vertical distribution of currents. Wind currents are studied in relationship to general investi-gations of climatology. Stevenson (1960) has outlined relationships between wind current velocities and surface currents. Techniques of current measurement have been described by Wiegel (1964) and by Sverdrup et al (1942). Because of the variable nature of nearshore currents, a complete description of current patterns cannot be made until currents have been studied through a one-year cycle.

Salinity and Temperature Distribution. Determinations of the vertical and horizontal distribution of salinity and temperature are particularly important for waste disposal projects. The salinity is used to determine water densities which affect direction and magnitude of sewage flow. Water temperature measurements also aid in density determinations and considerations of thermoclines and internal waves. Sea and Swell. Determinations of significant wave characteris-tics are particularly important in understanding mixing effect of near-shore water and water motion in shallow areas. A field investigation should include observations of sea and swell period, wave height, di-rection and wave length. In many cases, actual information may be lacking or difficult to obtain and hindcasting must be used to obtain sig-nificant data. Techniques for wave hindcasting have been developed by Burt and Sauer (1948).

Other Parameters. In addition to the factors discussed above, other parameters are frequently studied in marine investigations. Among these are transparency and color and biological investigations which are useful in determining the distribution of water contaminants. GENERAL MARINE INVESTIGATION PROCEDURES

The procedures for field and laboratory observations in oceano-graphic studies have been compiled by Gorsline (1958) of the staff of the Allan Hancock Foundation of the University of Southern California. Other general techniques for observations and collections at sea have been described by Sverdrup et al (1942). An important factor in the successful implementation of offshore studies is the use of a vessel properly equipped for field sampling and data collection and preferably for on-board laboratory testing.

DETAILED ENGINEERING STUDIES FOR MARINE PROJECTS GENERAL

After the cOITlpletion of the general ITlarine investigations, the project engineer ITlust interpret the results and narrow the nUITlber of alternatives to be further studied. The engineer will probably have carried out other preliITLinary engineering studies in parallel with the ITlarine research which assist in the paraITleters for the cOITlparison ITlodel of alternatives. These preliITLinary engineering studies for sub-ITlarine pipelines projects have been outlined by the author (1958) to include general considerations of alignITlents and terITlinal constraints, structural and hydraulic design factors, and construction considerations.

The culITlination of this effort is a recoITlITlendation for a single route or liITLited nUITlber of alternatives to receive detailed study. In order to ITlake final designs, the engineer ITlust secure additional infor-ITlation which would include the following:

AlignITlent and profile along the exact route of the pipeline Soils conditions along the route

Long-terITl stability of the bottoITl and nearshore regions MaxiITluITl wave forces and currents that would affect the anchorage and stability of the pipeline.

Distribution and character of ITlarine organisITls which ITlay attack the pipe or pipe coating.

Oceanographic conditions affecting construction ITlethods. Other special studies for ocean outfall as outlined herein-after.

HYDROGRAPHIC SURVEYS

The first engineering requireITlent for the detailed study of a sub-ITlarine pipeline is for a carefully controlled, accurate profile of the sea floor along the alignITlent where the pipe will be placed. It is iITlportant to have the survey as close as possible to the actual line of the pipe. For ITlost projects the profile can be traced using a fathoITleter in a launch with control provided froITl two onshore triangulation stations equipped with directional theodolites and radios. The author (1958) has described techniques for such offshore surveys where observations of up to eight ITliles have been ITlade. In these surveys it is important

to accurately calibrate the fathometer and to correct the trace for tidal variations. Careful attention must be paid to the establishrnent of the tidal datum and its relationship to land datum used. Othe1' sys-tems for offshore surveys have been reported by Brown (1960), Cass

(1960), and Poirot (1959). General techniques of hydrographic surveys

are described in the U. S. Coast and Geodetic Survey Manual, Adams

(1942).

While the techniques described above have been successful for surveys extending up to 10 miles offshore, longer pipelines may re-quire new methods. Bradbury (1961) has described the use of elec-tronic position location equipment for surveys as great as 200

ro

i1es offshore.

Marine Geologic and Foundation Conditions. While the study o~

bottom sediments and submarine geology may have been carried out ln

the general marine investigations phase of a program, once the

align-ment is known, the designer must have more specific information

about bottom conditions to be encountered along the line. A program

of field investigations would be as follows:

1. Review of fathograms along alignment and sediment data.

2. Investigation of onshore surface geology to deterrn.ipe rela-tionship to offshore conditions.

3. Investigation of geology and nature of bottom sediITJ-ents along the pipeline alignment. Information concern:iPg the surface sediments and rock outcrops to be obtained by direct observation of the bottom by diving geologist s. In-formation concerning the nature and thickness of s"L1-bsurface

sediments to be obtained by a series of jet probes a.1ong the alignment.

4.

Underwater still pictures to be taken of the bottom other pertinent features.

and

5. During investigations of the bottom sediments, obs e rva-tions to be made of marine biologic forms to ascertain the prevalence of marine boring organisms.

Geological Traverses. The geology and descriptions of ~he de-tailed submarine topography are made by geologists using Self _Cc:::>ntained Underwater Breathing Apparatus (SCUBA) as they are being to~ed over the sea floor by a survey boat. This technique permits measu- ::rement of details of the bottom much more accurately than previously po s sible b

e-cause the observer is able to make direct observations of the ;::jf:eatures

The surveying boat with diving geologists aboard is positioned on a predetermined line by land-based surveyors using transit control. .

When the boat is on station, the diving personnel are signaled by rad10 to drop over the side. The position of the boat is then recorded along with the time the divers went over the side.

The divers descend directly to the bottom and at the descent pos-ition make observations of the bottom micro-topography, sediment diS-tribution and thickness, take underwater pictures when possible, and sample the bottom sediment or rock. Upon completion of these observa-tions they would signal the surface by means of a tow line and the boa.t would move ahead towing the geologists slowly along the traverse. The boat is kept on line by radio-given directions from shore-based survey-ors. The time at the beginning and end of movement along the traverse is recorded underwater by the diving-geologists along with the times of significant changes in bottom conditions. Thes e notations are later correlated with distance along the traverse and used to determine the relative positions of significant features. Upon completion of a tow, which may have a duration of approximately 15 to 30 minutes, the geol

-ogists make another series of detailed observations as they had at the beginning of the dive. They then return to the surface and record their findings on data sheets. At the same time as the geologists are mak-ing these detailed observations, the survey boat would move to a posi-tion directly over the divers (indicated by the bubbles from their divipg equipment) and its position is determined by the surveyors.

This method was used to make continuous underwater geological

investigations along the entire length of the proposed pipelines betwee:O the mainland and the Islands of Coche and Margarita.

Jet Probes. In addition to the geological traverses, jet probes are made along the propos ed pipeline routes in selected areas which are covered with sedimentary overburden. Jet probing permits the geologists to determine the thickness of bottom sediments, and the general type of sedimentary material found beneath the sea floor in

to

e

area of the probe. The probe itself consists of a four-meter long pip

e

of 1. 90 cm. diameter which is attached to a hose furnishing high -pressure water from a pump located on the tending boat. A portable, constant-displacement pump capable of supplying water with a pres su ~ e of 1 O. 6 kg / cm2 (150 psi) has been us ed for these surveys.

With experience, a geologist using the jet probe can tell whethe ~

the pipe is jetting through sand, gravel, shells or clay, or hitting bed -rock by the sound and feel of the pipe. The results of the jet probe survey are summarized in large scale profiles of the detailed geolo gy-along the proposed pipeline alignment and in geological profiles and descriptions.

Underwater Photography. Many of the features seen underwater have no surface counterpart and it is, therefore, difficult to convey their exact nature to persons not familiar with the sea floor. Under-water photographs often help to remedy this lack of direct contact. Pictures of significant features on the bottom are, therefore, taken whenever possible on a survey. Unfortunately, the underwater world is not as favorable for photography as is the land. Visibility during a sur-vey can be limited by the water turbidity. For this reason large scale features are difficult to photograph; only those features which are small enough to be encompassed at relatively short distance from the camera could be recorded.

Foundation Investigations. In areas where unstable sedimentary soils are traversed and in the near shore regions where the pipeline may be deeply buried, more detailed foundation investigation may be required. For the former conditions, special probes of the sediments are made using piston coring devices. The coring apparatus us ed for these investigations has been described by Brown (1957). Samples taken therefrom enable the determination of the density of bottom sedi-ments which affects the selection of pipe weight coatings. For the near-shore areas, borings should be taken and undisturbed samples obtained. The boring rig is set upon a portable tower and drilling casing is used. Near the surf zone the portable tower is hazardous to handle and while borings can be taken from shore there generally is a region where it is impos sible to secure borings.

Geophysical techniques are sometimes used to map sub-bottom sediment horizons. Bechmann et al (1960) described a geophysical survey of a Chesapeake Bay crossing and indicated the possibility of using such equipment at depths exceeding 1600 ft. Chambers (1965) outlined the application of a tool used for oil field seismic exploration to submarine pipeline foundation investigations. The method is called a High Resolution Sparker Survey and its use provides an idea of the continuity of layers on the sea floor, with special value obtained from its ability to note conditions between cores.

SPECIAL STUDIES FOR OCEAN OUTFALLS

The disposal of sewage and other wastes into the sea involves ex-tensive marine studies. These studies seek to understand the effect of the waste on the marine environment, and, conversely, the effect of the marine environment on the waste. The latter objective usually is stated in terms of the ability to achieve certain standards designed to limit marine pollution in receiving waters.

Pearson (1955) and (1960) has summarized much of the available

information on waste disposal in the marine environment. AIvy, Miller

& Lawrance (1957) have described the exhaustive studies carried out for the design of the large Hype rion ocean outfall for the City of Los

Angeles. As part of that program, Terry (1956) prepared an annotated

bibliography on bacteriology, oceanography and marine geology as they

relate to the disposal of waste material into the sea. DESIGN P ARAME TERS

Marine studies for the design of ocean outfalls must furnish the engineer information on certain design parameters involved in the

for-mulation of dilution requirements. These parameters fall into four

main categories:

1. Determination of bacterial survival in sea water.

2. Dilution of the sewage field by jet mixing of the sewage

effluent into the sea water.

3. Diffusion and dispersion of the sewage field.

4. Mass transport of the sewage by wind action, waves and

local currents.

The evaluation of the bacteriological factors in sewage disposal has been made by Rittenberg (1958) and Gunnerson (1959) reporting on

the results of studies for the Hyperion Project. An important finding

of these studies was that the bacterial content of sewage discharges var-ied according to degree of treatment and that the sewage discharge from

a particula.r locality had individual characteristics. It was also para

-doxically found that pre-treatment of sewage effluents might adversely

affect their viability characteris tics in sea water.

The dilution and diffusion of sewage through jet mixing and the

design of special diffusers has been well established by Rawn et al

(1960) and others and a further discussion of these factors is not

pre-s ented herein.

Diffusion and Dilution. Despite the many theoretical studies of

eddy diffusion, turbulent mixing, a thermal convection and diffusion,

the outfall designer will usually decide to conduct field experiments to

verify the actions under the actual site conditions. These field

experi-ments involve the determination of sewage field dilution at varying

distances from the point of discharge. Dilution can be measured by

chemical, physical or bacteriological means. Chemical methods of

salinity, or other chemical constituents of the sea water. Tests of density and the use of dyes are physical means of determining dilution, and bacteriological means involve counting coliform or other type of bacteria present in the sewage.

Radioactive Tracer Studies. Most chemical and physical methods

of dilution determination are unable to measure dilutions in excess of 300. However, radioactive tracers can be used for determinations of up to 10,000 to 1.

The Hyperion Project featured a special radioactive tracer exper-iment. This experiment used 20 curies of radioactive scandium (Sc-46).

This tracer element had been procured from the U. S. Atomic Energy

Commission National Laboratories at Oakridge, Tennessee. The Sc-46

was added to ten gallons of water in a mixing tank at the treatment plant

and this mixture was fed at a constant rate into the effluent over a peri-od of one hour. The activity of the Sc-46 was such that dilutions up to 1 to 10,000 could be determined, yet at no time was the maximum

per-missible concentration exceeded. The test was conducted under the inspection of state and local health authorities and was found to be com

-pletely safe.

Radioactivity was detected in the boil over the Hyperion outfall approximately 1 hour after its introduction into the outfall, and

measure-ments were continued in the boil for another hour until the radioactivity

started to decrease. Two dye path tags were started, about one hour

apart, while the radioactivity was at its peak. A series of patterns,

spaced at appropriate intervals were then traversed by the oceanograph -ic survey vessels, making radioactive measurements while underway to

delineate the extent and position of the radioactivity radioactive field at various time s. Along with this radioactivity travers e, an auxiliary survey vessel was making chemical and bacterial tests throughout the

field.

The prime purpose of the radioactive tracer study was to determine

actual factors of dilution of the sewage field and to provide additional

data on the disappearance of coliform organisms. Through the experi-ment, the survey team was able to compare observed and calculated coliform concentrations resulting from the study. Subsurface

radio-activity measurements were taken at the profile stations and were used to determine the depths of the sewage field.

Current Studies. With the exception of the summer season when

strong thermal gradients can keep the sewage fields submerged, the

effluent will rise to the surface layer of the ocean and be carried,

dis-persed, or both, by the currents in this upper layer. Under ideal

shore all the time and under these conditions, very little treatment of the sewage would be necessary. However, very few sewage disposal sites approach this ideal condition. When the surface currents are directed on shore at least a portion of the time, then some combination of treatment, diffuser and length of outfall is necessary. Of these fac-tors, the length of outfall is the most easily varied to fit local conditions, and it becomes necessary to determine the rate and frequency of shore-ward transport.

Two methods are possible for the determination of this shoreward transport. One method relies upon the use of drift cards which are introduced at selected locations in the survey study area and which drift in the direction of the prevailing current. When picked up either on shore or at the end of a selected time period, they provide an indication of the net drift taking place during that period. These cards are best used in areas which are ringed by heavily used beaches and thus the

card return can be substantial enough to make the results significant.

This type of study as indicated shows the overall net effect of the cur

-rent, but it does not necessarily trace the actual path of the drift cards and thus the actual sewage field. Another method which allows the actual motion of the sewage field to be more closely followed uses short-term velocities obtained by current meter and float methods, and then statistical analyses are made which take into account variation of cur-rent direction with time.

Subsurface currents influence the amount of diluting water that can be brought to the diffusers. At certain times of the year, the sewage field will stay submerged; therefore, the dispersal of the field will be subject motion. Theoretically, the flow at depth should be slower and in a different direction from the surface current in accordance with the Ekman spiral relationship. These deep currents can be studied by cur-rent meter and float measurements to confirm the variations in direc-tion and velocity with depth. These current studies are statistically analyzed and net averages are compiled to indicate time to shore under IniniInuIn design conditions. Thes e findings are then us ed as the appro-priate parameter in the design equations. Generally, conservative values are used to insure the soundness of the final results.

SUBMARINE PIPELINE DESIGN AND CONSTRUCTION When adequate and complete marine studies and investigations are available, the submarine pipeline designer can approach his task in Inuch the saIne way as he would a conventional pipeline, having knowl-edge of the forces to which the line will be subjected, the conditions under which it will be installed and its functioning under construction and operation. These Inarine studies also can provide needed

back-ground for the contractor to enable him to understand the effect of sea

conditions on his work.

It can be dangerous to undertake a large marine pipeline project

without having adequate marine studies and investigations. Equipment

and methods are now greatly improved and marine specialists are

avail-able to serve as consultants to advise on the conduct of required studies

and the interpretation of their results. With current emphasis on more

oceanographic research and new equipment, submarine pipelines will be

able to reach further and further offshore. Depths even up to 10, 000

feet can be traversed and the economic potential of marine disposal of

wastes can be achieved without undue disruptions or degradation of the

marine environment.

REFERENCES

Adams, K. T. (1942). Hydrographic Manual; U.S. Department of

Com-merce, Coast and Geodetic Survey, Special Publications No. 143.

Alvy, R. R. , Miller, D. R., and Lawrance, C. H. (1957). Ocean

Outfall Design; Hyperion Engineers, October 15, 1957.

Beckmann, W. C., Drake, C. L., and Sutton G. H. (1960). SDR

Sur-vey for Proposed Chesapeake Bay Crossing: Proceedings of the

American Society of Civil Engineers, Journal of the Surveying

and Mapping Division, July 1960.

Bradbury, J. T. (1961). New Miniaturized Raydist Equipment for

Off-shore Oil Exploration: Offsho re, March 1961.

Brown, R. J. (1957). Soil Mechanics Important in Marine Pipeline

Construction: The Oil and Gas Journal, Vol. 5S, No. 23,

pp.10S-115.

Burt, W. U. , and Sauer, J. F. T., Jr. (194S). Hindcasting technique

provides statistical wave data: Civil Engineering, Vol. 18, No.

14, pp. 47-49.

Chambers, Pete (1965). The Blue Dolphin Pipeline - An Offshore Texas

Milestone: Pipeline Engineer, No. 11, Vol. 37, p.43.

Cass, J. R., Jr. (1960). Computer, Radios Speed Survey; Engineering

Gorsline, D. S. and staff (1958). A Field and Laboratory Manual of Oceanographic Procedures: MiIneographed report, Allan Hancock Foundation, University of Southern California.

Gunnerson, C. G. (1959). Sewage Disposal in Santa Monica Bay: AIner-ican Society of Civil Engineer, Transaction, Vol. 124, 1959, p.823. Miller, D. R. (1958). Report on Studies and Investigations for the

Sub-Inarine Aqueduct for Margarita and Coche: Daniel, Mann, Johnson

& Mendenhall de Venezuela.

Miller, D. R. (1958). Report on Marine Investigations for the SubInarine Aqueduct for Margarita and Coche: Daniel, Mann, Johnson, &

Mendenhall de Venezuela.

Nuclear Science and Engineering Corp. (1956). Report to Hyperion Engineers, Radioactive Tracer Study of Sewage Field in Santa Monica Bay: Pittsburgh, Pa., June 29, 1956.

Pearson, E. A. (1960). Waste Disposal in the Marine EnvironInent: Pergamon Press.

Pearson, E. A. (1955). An Investigation of the Efficacy of Submarine

Outfall Disposal of Sewage and Sludge: Report to California Water Pollution Control Board, DeceInber, 1955.

Poirot, J. W. (1959). Hydrographic Surveying froIn a Helicopter; Civil

Engineering, February 1959.

Rawn, A. M., Bowerman, F. R., and Brooks, N. H. (1959). Diffusers

for Disposal of Sewage in Sea Water; Journal of Sanitary

Engineer-ing Division, American Society of Civil Engineers, Vol. 86, No.

SA2, March 1960, pp. 65-105.

Rittenberg, S. J. (1956). Final Bacteriological Report: Report to Hyper-ion Engineers, Allan Hancock Foundation, University of Southern California, SepteInber 1956.

Stevenson, R. E., Tibby, R. B., and Gorsline, D. S. (1956). Oceano -graphy of Santa Monica Bay: report to Hyperion Engineers. Stevenson, R. E. (1960). Wind Drift in the Waters of the Southern

California Shelf: Water & Sewage Works, April, pp.146-150. Sverdrup, H. 0., Johnson, M. W., and FleIning, R. H. (1942). The

Oceans, Their Physics, Chemistry and General Biology: Prentice

Terry, R. D. (1956). An Annotated Bibliography on Bacteriology, Oceanography and Marine Geology as They Relate to the Disposal of Waste Material into the Sea; Allan Hancock Foundation, Univer-sity of Southern California, Aug. 3, 1956.

Wiegel, R. L. (1964). Oceanographical Engineering; Prentice Hall, p. 336.

Figure 3. Near Shore Trestle for laying Hyperion Ocean Outfall for Effluent Discharge, 5 miles 144" diameter

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