April 2013

Hydrographic Survey of HK Electric’sHydrographic Survey of HK Electric s Submarine Natural Gas Pipeline

Presentation for Joint Gas Seminar - Ir Mario C.F. Lau, HK Electricese tat o o Jo t Gas Se a a o C au, ect c

Objectives of Hydrographic Survey:Objectives of Hydrographic Survey:● Check the integrity of the pipeline protections

● Measure the burial depths and locations of the pipeline

● Measure the changes in seabed levels along the pipeline route since the installation of the pipe in 2006route, since the installation of the pipe in 2006

● Identify obstructions third-party installed facilities and hazards● Identify obstructions, third party installed facilities and hazards along the pipeline route

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Hydrographic SurveyHydrographic Survey● Performed by EGS (Asia) Limited under the supervision of HK

El t iElectric● Survey period was between 2010 and 2011

● Checked the seabed level around the pipeline

● Map seabed features around the pipeline

● Measured the burial depth of the pipeline beneath the seabed

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About HK Electric’s Natural Gas PipelineAbout HK Electric s Natural Gas Pipeline● Nominal size: 500 mm

● Operation pressure at 80 ~ 90 Bar(g)

● Except the two landfalls, the whole pipeline was buried to 3 m below seabedbelow seabed.

● About 13 km of the pipeline is covered with rock armour● About 13 km of the pipeline is covered with rock armour(anchorage area), to protect it against damage by anchor drop and drag

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Location of surveys on Gas Pipeline• Hydrographic surveys were conducted along the 92 km longnatural gas pipeline

• From Cheng Tou JiaoShenZhen, PRCS e e , C

• To HK Electric Lamma PowerStation

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Methodologiesg

Three types of survey were performed:

Methodology Detection medium Information gained

Bathymetry survey Acoustic Underwater depth of seabed

Side scan sonar survey Acoustic Identify objects on seabed and details of seabed

Sub-bottom survey Acoustic Sediment structure and Sub botto su ey coust c Sed e t st uctu e a dburial depth of subsea natural gas pipeline

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Global Positioning System• GPS is used to locate the pipeline route.

GPS antenna mounted Pipe route shown on

7on survey vessel survey vessel monitor

Single Beam Echo SounderSingle Beam Echo Sounder

• Measures double way t it ti f dtransit time of sound wave signal reflected from the seabed.

• Water depth is computed by knowing the sound velocity inthe sound velocity in water.

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Multi Beam Echo Sounder● Transducers emit sound

wave to produce fan-pshaped coverage on both sides of the survey vessel.

● Produce a “swath” of soundings to provide full coverage of survey area.

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Sample Results of Bathymetry SurveySample Results of Bathymetry Survey

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Side Scan Sonar SurveySide Scan Sonar Survey

U hi h f d t id tif bj t th• Use high frequency sound waves to identify objects on the seabed.

• Transmitter and sensor of side scan sonar are mounted toa towfish that glides above the seabed. The towfish emitsa towfish that glides above the seabed. The towfish emitsfan-shape pulses and records the intensity of acousticreflections.

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Side Scan Sonar Surveyy

Tow fish

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Side Scan Sonar Survey - Towfishy

Location of sensorLocation of sensor

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Sample Results of Side Scan Sonar Surveyp y

Tyre

Pipe Trench

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Sample Results of Side Scan Sonar SurveySample Results of Side Scan Sonar Survey

h k f banchor mark of buoy

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Sample Results of Side Scan Sonar SurveySample Results of Side Scan Sonar Survey

• For better presentation,pcolours are addedto the imagecaptured by thecaptured by theside scan sonarsurvey.

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Sub-bottom SurveySub-bottom Survey• Use low frequency seismic source to identify and measure

i di t l b l b dvarious sediment layers below seabed.

• Offer information on burial depth of pipelineOffer information on burial depth of pipeline.

• Not applicable to the areas protected with rock berm.

• Seismic source – pinger is used for emitting a low frequencywave by vibration of piezoelectric substance.wave by vibration of piezoelectric substance.

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Sample Results of Sub-bottom Surveyp y

Seabed

~ 4 mTop of pipeline

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Problem EncounteredProblem Encountered● EGS found masking in some of the sections of the pipeline

● Consequently the burial depth could not be determined with confidenceconfidence

● Problematic sections are located near Ping Chau Po Toi and● Problematic sections are located near Ping Chau, Po Toi and Lamma Island

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Problem EncounteredProblem EncounteredPipeline reflectors are embedded in strong ringing signals from above

Pipeline Trench

~4 m

Figure: Data obtained in year 2006 at the same crossing

Figure: Data obtained in 2011, masking effect was observed

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Locations of Problem EncounteredLocations of Problem Encountered

• Green segmentsindicate positions with the masking effects

• Blue segmentsi di t tiindicate sections with rock armour

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Hypotheses for the Masking EffectHypotheses for the Masking Effect● EGS proposed two hypotheses for explaining the abnormal

hphenomenon

1) Irregularly deposited shells & shell fragments scattered above1) Irregularly-deposited shells & shell fragments scattered above some sections of the pipeline and forming water-filled voids

2) Gas released from benthic bloom and trapped underneath the seabedseabed

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Hypotheses for the Masking EffectHypotheses for the Masking Effect1st Hypothesis

● A layer of irregularly-deposited shells scattered above some sections of the pipeline trenchsections of the pipeline trench

● The silt/clay settled and covered on top of the shells● The silt/clay settled and covered on top of the shells

● That leaving small pockets of low density coarse sediments● That leaving small pockets of low density coarse sediments with voids underneath the shells

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Hypotheses for the Masking EffectHypotheses for the Masking Effect● These pockets might lead to the density contrast

● The density contrast would scatter the seismic signal, masking the results from greater depththe results from greater depth

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Hypotheses for the Masking EffectHypotheses for the Masking Effect2nd Hypothesis

● Gases released by a bloom of benthic organisms

● Marine micro-organisms blooming and fell back naturally to the pipeline trenchpipeline trench

• Microphotograph• Microphotograph of typical benthic animals (from wiki)

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Hypotheses for the Masking EffectHypotheses for the Masking Effect

Silt d l ( i d it ) b k fill d th i t h● Silt and clay (marine deposits) back-filled the pipe trench naturally and burying the organisms

● Gases formed during decay and trapped in the seabed

● Density difference between gas bubbles and surrounding silt/clay led to acoustic reflectivity contrastsilt/clay led to acoustic reflectivity contrast

● That caused the masking observation

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Verification of Hypothesis by Diving Sampling

● Diving sampling in three l t d l tiselected locations

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Sampling PointsSampling Points● In each location, 7 sampling points were chosen along the

li ti ith diff t ff tcross-line section with different offsets

● And at least one sample was collected at each point● And at least one sample was collected at each point

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SamplesSamples

● Sample collected bythe diver Shells can bethe diver. Shells can beseen clearly

Sample with shellf t dfragments and voids

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FindingsFindings● Chemical test of samples indicated that the hypothesis of gas

l d f b thi bl j t dreleased from benthic bloom was rejected

● Many collected samples had shell fragments and voids● Many collected samples had shell fragments and voids

Sample with shellfragments and voids

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FindingsFindings● The hypothesis of irregularly-deposited shells above the

i li f i t id t dpipeline forming water voids was accepted

● Voids trapped by irregularly deposited shells● Voids trapped by irregularly deposited shells

● It could be a cause for the poor seismic signal quality in these● It could be a cause for the poor seismic signal quality in these areas

● The density contrast would scatter the seismic signal, masking the signals from below

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ConclusionConclusion● Rock armour protections were found in place and intact;

● There was no movement of pipeline, both laterally and vertically; andvertically; and

● No seabed features were considered hazardous to the security● No seabed features were considered hazardous to the security of the pipeline.

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Thank You !

Equipment & FrequenciesEquipment & Frequencies

Multibeam echo sounding RESON Seabat 8125 multibeam echosounder system (455kHz)

Single beam echo sounding Knudsen 320M dual frequency echo sounder (200kHz/33kHz)g g q y ( )

Side scan sonar Edgetech 272-TD dual frequency side scan towfishSide scan sonar Edgetech 272-TD dual frequency side scan towfish (400kHz/100kHz)

i i i fili G i 210A l i (3 k )Marine seismic profiling Geoacoustics 5210A geopulas pinger system (3.5kHz)

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Estimates of AccuracyEstimates of Accuracy● Horizontal positioning of survey vessel +/- 1 m

H i t l iti i f di b MBES / 1 2● Horizontal positioning of soundings by MBES +/- 1 – 2 m● Horizontal positioning of seabed features by side scan sonar

+/ 3 5 m+/- 3 – 5 m● Sounding by SBES and MBES +/- 0.2 – 0.3 m● Horizontal positioning by seismic survey +/- 2 -3 m● Horizontal positioning by seismic survey +/- 2 -3 m● Depth estimation by seismic survey +/- 0.5 m

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Chemical TestChemical Test

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