SUBSEA MINING – MAJOR CHALLENGES

Dr. Ir. Sritama Sarkar

Saipem UK

CONTENTS

Seafloor Minerals Major Challenges Potential Subsea Miners Subsea Cutting Production and Power Subsea Cutting Forces, Weight and Stability Gathering Challenges Environmental Impact Transport Challenges Locomotion Challenges Launch and Recovery Challenges

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SEAFLOOR MINERALS 3

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Offshore Diamonds

Polymetallic Nodules

•Established industry •Maximum water depth 400m •Placer pockets in overburden (sand) overlaying clay/ bedrock

•Extensive R&D since 1970s •Ni, Cu and Co •Water depth 5000m to 6000m •Abyssal plains

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Methane Hydrates

Cobalt Crusts

•Exploration •Water depth 500m to 2000m •Soft sediments in Arctic environment •Slope and rise sediments in continental margin

•Research •Water depth 400m to 4000m •Volcanic island arcs/ Volcanic sea-mounds – as very thin crusts

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Seafloor Massive Sulphide (SMS) Deposits

•Exploration and planned commercial exploitation •Cu-Zn-Ag-Au` •Water Depth 1500m to 5000m • Very hot hydrothermal fluids, slightly acidic • Edges of tectonic plates

MAJOR CHALLENGES 7

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SUBSEA MINING PROCESSES

Gathering (Hydraulic)

Transport (Hydraulic)

Cutting

Mechanical Cutting

Hydraulic Cutting

Locomotion

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Seafloor Mining Challenges Vs. Minerals

Minerals Processes

Offshore Diamonds

SMS Methane Hydrates

Mn Nodules

Co Crust

Cutting

+ (Insignificant

cutting)

+++ +++

(Phase dissociation)

No cutting ++

(Very thin crust)

Gathering ++

(Hydraulic) +++

(Hydraulic)

+++ (Phase

dissociation)

+ (Mechanical + Hydraulic)

+++ (Hydraulic)

Transport (Hydraulic)

+ ++ +++ +++ +++

Locomotion + +++ ++ ++ ++

Relative Rating in Terms of Challenges More ‘+’ sign, more difficulty

POTENTIAL MINERS 10

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POTENTIAL CUTTERS: DREDGING INDUSTRY

Dredge cutter And Dredge pick

Cutter Suction Dredger

Trailing Suction Hopper Dredger

Drag Head

Courtesy: Van Oord, Vosta LMG

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POTENTIAL CUTTERS: DREDGING INDUSTRY

Courtesy: EEM (P) Ltd., India

• Surgical cutting, resulting in less environmental impact, Ref. Sarkar, M.K. 2012 • Possibly suitable for Cobalt Crust (Thin)

Transverse Cutter: Rotavator

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POTENTIAL CUTTERS: LAND MINING

Road header Radial pick

Point attack pick

Continuous Miner

Courtesy: Sandvik

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POTENTIAL CUTTERS: LAND MINING

Road Miner

Surface Miner

Courtesy: Trencor and Wirtgen

Position of drum cutter important for cutting force and stability of miner.

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SUBSEA MINER CONCEPTS

Gathering Machine

Courtesy: IHC

Courtesy: Aker

Mining the Seafloor with Robots Ref.: www.iaarc.org/news/a_news_2012_09_11.pdf‎

Bulk Miner

Preparatory Machine

CUTTING PRODUCTION AND POWER

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Average production = f(External factors, Manageable factors) Instantaneous production = f(Average production, Uncontrollable factors)

CUTTING PRODUCTION

CUTTING PRODUCTION (SOLIDS)

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Based on Nautilus Minerals estimated Annual Production = 1.8Million Tonnes (dry) from

Solwara 1

Ref: http://www.nautilusminerals.com/s/Home.asp

NAT005_Solwara_1_Offshore_Production_System_Definition_and_Cost_Study_Rev_3_21_June20

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CUTTING PRODUCTION(SOLIDS) – CASE STUDY

Production Envelope - Example

CUTTER DRIVE POWER

Dynamic factor(DF): Ratio between peak and average cutting force For hydraulic drives: Hydraulic efficiency and gear box efficiency Hydrodynamic efficiency: Hydrodynamic drag of rotating cutter Recirculation efficiency: Regrinding of excavated material

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CUTTER DRIVE POWER

Instantaneous Production = Specific Energy =

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SPECIFIC ENERGY (LAND): EVAN’S MODEL

Specific Energy (Subsea) = f(In-situ UCS, Hyperbaric Factor)

Disturbed sample: Decompression effect on UCS

Hyperbaric factor = f(Rock properties, Cutting parameters, Cutting tool geometry and lacing)

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ROCK FAILURE: SUBSEA CONDITIONS

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CUTTER DRIVE POWER: GENERIC EXAMPLE

Calculation Approach

Required Ins. Solids Production

(m^3/hr)

Cutter Drive Power for Sp. En. =

10kJ/m^3

Cutter Drive Power for Sp. En. =

20kJ/m^3

Optimistic Approach

Min Max

130 200

900 1500

1900 3000

Conservative Approach

Min Max

135 220

1000 1600

2000 3300

Cutter drive power determines weight of the subsea miner

If specific energy increases challenges in subsea power technologies (voltage levels etc.)need to be addressed

Design of Launch and Recovery System is dependent on weight of the subsea miner

Option for multiple subsea miner

CUTTING FORCES, WEIGHT AND STABILITY

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24 F_h: Horizontal Force, F_v: Vertical Force, F_a: Axial Force

CUTTING FORCE: DREDGE CUTTER

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Single Roadheader can’t meet the production requirement

Roadheaders are critical in yaw stability and

sliding stability

CUTTER POWER TO WEIGHT: ROADHEADER

Courtesy: Sandvik

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Approximate Cutter Drive Power: Miner Weight Ratios (Air) Continuous Miner = 4:1 Surface Miner = 6:1

CUTTER POWER TO WEIGHT: DRUM CUTTERS

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RESULTS – DRUM CUTTERS

Courtesy: Sandvik

Largest subsea vehicles: Rock Trencher RT1 (Deep Ocean)

Weight = 200Te (Air) Total power = 2.3MW Total cutter power in 3 chain cutters = 250kW, 2*400kW Operating depth = 500m

Pipeline Plough PL3 (Saipem) Weight = 240Te (Air)

Machine WeightPeak Force (Hor.

And Vert.)

(Te) (Te)

240 to 400 20 to 35

480 to 820 40 to 70

Machine WeightPeak Force (Hor.

And Vert.)

(Te) (Te)

160 to 270 20 to 35

300 to 550 40 to 70

10MJ/m^3

Cutter Drive Power

(kW)

900 to 1600

Continuous Miner (Cutter Power : Weight = 4:1)

20MJ/m^3 1900 to 3300

Sp. Energy

Sp. Energy

Surface Miner (Cutter Power : Weight = 6:1)

Cutter Drive Power

(kW)

10MJ/m^3 900 to 1600

20MJ/m^3 1900 to 3300

Compressive strength and specific energy of cutting very important

Laboratory tests

In-situ tests

Uncertainty in cutter drive power estimation reduced by accurate determination of ore properties

Cutter drive power and cutter mounting position determines weight of the subsea miner

Stability of subsea miner controlled by cutting forces generated and machine design parameters

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SUMMARY – CUTTING

GATHERING CHALLENGES

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The rock mass properties and cutting methodology determines particle size distribution of excavated ore

The suction velocity should be sufficient to gather as much as possible of the excavated ore

Environmental impact should be minimum

Suction velocity should match the riser transport velocity – hence impacts the design of riser pump(s) and diameter of riser

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GATHERING CHALLENGES

ENVIRONMENTAL IMPACT

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Habitat assessment

Plume generation due to cutting and gathering devices – impact on species existing in mine area

Noise generation

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ENVIRONMENTAL SENSITIVITY

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TURBIDITY MEASUREMENT

Courtesy: Dr. Mridul Kumar Sarkar, Dr. Neil Bose, Dr. Suhong Chai, Dr. Kim Dowling Australian Maritime College, Tasmania; EEM (P) Ltd. , India; University of Ballarat, Australia

TRANSPORTATION CHALLENGES

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Involves transportation of the excavated ore as a slurry from the seabed to the surface support vessel

Research work – ongoing on vertical transport of slurry by various universities and research institutes

Variation in the slurry density will change the catenary shape of the riser (if flexible riser used)

With the variation in the slurry density additional forces will be transferred to the connection point with the subsea miner

Riser wear needs to be investigated

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TRANSPORTATION CHALLENGES

LOCOMOTION CHALLENGES

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Topography of the terrain

Slope of the terrain

Rock – Reduced friction of wet rock, e.g. SMS and Cobalt crusts

Soft sediments – Bearing Capacity, e.g. Methane hydrates, Polymetallic nodules

Proper assessment of cutting forces necessary for sliding stability assessment

Additional devices to balance cutting forces when friction is low

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LOCOMOTION CHALLENGES

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OTHER LOCOMOTION DEVICES

Cutter module

Suction mouth

Dipper Ladder boom

Eductor pump

Rotating yoke

Leg

Foot

Ref: Dr. S Sarkar, Dr. N. Bose, Memorial University of Newfoundland, Canada and

Dr. M.K.Sarkar, EEM (P) Ltd. India 2002-2007

Ref: Dr. M.K. Sarkar, EEM (P) Ltd. , India Dr. N. Bose, Australian Maritime College, Tasmania, 2008- 2012

Courtsey: IHC Deep Sea Mining

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LAUNCH AND RECOVERY SYSTEM

Support Vessel

Umbilical

Riser System

Subsea Miner Lifting Wire

Courtesy: Saipem, Wellstream (GE), Seatools, Certex

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Dynamic subsea component – since the subsea miner is continuously moving

The subsea miner needs to be recovered frequently for scheduled/ unscheduled maintenance

Static subsea system

Once installed the subsea structure is not recovered

Flowlines

Risers

Jumpers

Transfer

Lines

LARS Requirement: Oil and Gas

LARS Requirement: Subsea

Mining

Courtesy: Wellstream and Nautilus Minerals

Support vessel – DP operated or moored

Weight of subsea miner and DAF will control design of LARS

Management of the riser during operation and movement of the support vessel in the mine

Shape of riser determined by environmental load, slurry density and movement of subsea miner – pipeline integrity

Accurate estimation of weather downtime

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LAUNCH AND RECOVERY SYSTEM:CRITICALITIES

Wave, Current

Design of suitable devices for in-situ determination or ore properties

Wear estimation of cutter tooth and riser system

Robotic change of cutter tooth subsea

Riser integrity monitoring during subsea mining operation

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FOOD FOR THOUGHT

Wave, Current

QUESTIONS ???

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