Design of Offshore Structures For Extreme Ice Conditions Prepared - - PowerPoint PPT Presentation

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Design of Offshore Structures For Extreme Ice Conditions Prepared - - PowerPoint PPT Presentation

Aug 23, 2022 41 likes •393 views

CJK ENGINEERING Design of Offshore Structures For Extreme Ice Conditions Prepared for SNAME 2017 Arctic Section by John Fitzpatrick P. Eng. CJK Engineering Ltd. jfitzpat@telus.net Graphics by Jakub Ciring and Associates Inc.

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CJK ENGINEERING

Design of Offshore Structures For Extreme Ice Conditions

Prepared for SNAME 2017 Arctic Section

by John Fitzpatrick P. Eng. CJK Engineering Ltd. jfitzpat@telus.net Graphics by Jakub Ciring and Associates Inc. jakubciring@shaw.ca

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  • Why a Gravity Based Structure (GBS) will not

work in 100 to 200 meters of water under extreme tabular ice conditions!

  • Two alternative solutions for permanent platforms

in 200 meters of water.

  • First: Gravity Fendered Structure (GFS).
  • Second: Rock Island Structure.
  • Costs for both concepts.

Overview

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Transfer Equations for Energy to Force

Non-deformable Structures: F = 2 E2/3 σ1/3 / (H/D)2/3 Large potential for error in σ and H/D Deformable Structures: F S = E = MgH No potential for error

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Energy vs. Force, depending on shape & crushing strength of the ice feature

5,000 10,000 15,000 1,000 5,000 MN MNm Hibernia 10,000 2,500 MN 10,000 MN

700 million tonnes Ice Island @ 0.22m/s

10-2, 100m 10-2, 200m 10-4, 100m 10-4, 200m

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GBS vs. 100m Thick Tabular Ice

E = 1,700,000 Tm = 17,000 MNm

F = 2 x (17,000)2/3 x 41/3 / 0.12/3 = 10,000 MN (“Flat”) F = 2 x (17,000)2/3 x 11/3 / 0.42/3 = 2,500 MN (“Bump”)

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GBS vs. 100m Thick Tabular Ice

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GFS Concept

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GFS for 200m water depth – principal particulars and design energy parameters

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Size comparison with existing large structures

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GFS vs. size of the Design Tabular Ice Island, 3000m in diameter and 100m thick

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Timeline of absorption of energy: 0 sec.

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Timeline of absorption of energy: 90 sec.

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Timeline of absorption of energy: 180 sec.

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Timeline of absorption of energy: 275 sec.

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Timeline of absorption of energy: 390 sec. Interaction and force transfer complete

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Timeline in plan view: 0 sec., 700 million tonne Ice Island contacting

Fender at 0.22m/s

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Timeline in plan view: 390 sec., Ice Island stopped, after displacing Fender by 56m horizontally and 17m vertically.

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Energy to Force for Deformable Structures

F S = E = MgH (68,000t÷2)×50m = 0.5 ×700,000,000t × 0.22^2÷9.81=100,000t×17m =1,700,000 tm No potential for error in Base Shear if Mass and Velocity are known. F maximum = 68,000 metric tonnes = 680MN Base overturning moment = 12,000,000 tonne meters

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Fender response to ‘rogue wave’ embedded in 15m significant sea state

400 800 1200 1600 2000 t = 0 sec

60,000 45,000 30,000 15,000

  • 30,000
  • 45,000
  • 60,000

40 30 20 10

  • 10
  • 20
  • 30
  • 40

Departure [m] Base Shear Force [ T ] 10 min 20 min

  • 15,000

See detail

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Wave induced Fender motions. 15m wave Steady State plus Transient ‘Rogue’impact

20 40 60 80 100 120 140 160 180 sec. 10 20 30 m 10 20 30 m 10 20 30 m 20 40 60 80 100 120 140 160 180 sec. 20 40 60 80 100 120 140 160 180 sec.

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  • H=15m, T=16 secs, WL=400m
  • Fender mass (including annulus water) = 3,240,000 tonnes
  • K=15,000kn/m, Cm = 1.5, Cd=0.7
  • Fender natural period 160 seconds
  • Resonance not possible
  • Steady state total Base Shear = 30,000 tonnes
  • Confused irregular sea state = 50,000 tonnes
  • Wave Base Moment = 8,000,000 tonne meters
  • Steady state departure +/- 6m.

Wave Loads and Input to dynamics program

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15m high (Steady State) Regular Wave action 0 sec.

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15m high Regular Wave action – 4 sec.

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15m high Regular Wave action – 8 sec.

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15m high Regular Wave action – 12 sec.

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15m high Regular Wave action – 16 sec.

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  • In the event of a blowout the inner annulus of the Fender

could act as an effective oil containment boom with a capacity of 15,000,000 bbls. The Fender itself could contain an additional 5,000,000 bbls.

  • The Fender provides a 200m wide approach channel for

tankers in which to approach and load from the platform

  • The inner annulus could provide a safe haven and mooring

location for ice breakers or supply ships under 100 meters in length. Additional benefits

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GFS Cost Estimate (excluding topsides)

Steel quantity input: Base 150,000T 1st Step 40,000T 2nd Step 20,000T Pillar 50,000T Deck 40,000T Subtotal 300,000T Fender 100,000T Total 400,000T

Total steel: 400,000T x $4,000/T = $1.60 Billion 250 cables x 100m x $1,100/m = $30 Million + $10 Million for Ends = $0.04 Billion Ballast: 300,000m3 x $200/m3 = $0.06 Billion Capital cost subtotal $1.70 Billion Towing & installation: 300days x $1 Million/day = $0.30 Billion Contingency: $0.50 Billion Grand total $2.5 Billion

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Rock Island for 200m water depth - principal quantities and design energy

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Concept of Rock Island for 200m water depth

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Rock Island construction sequence using 10 new self- propelled, ice strengthened Rock Barges, each with 15,000 DWT capacity

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  • Virtually no limit to the amount of energy that can be
  • absorbed. An indentation of 25m would absorb some

10,000,000 tonne meters of energy or 6 times more than the 10,000 year design energy.

  • The central platform should not have a draft any deeper

than 30m as this could result in overload.

  • The central platform needs to be surrounded by a

protective berm

  • The Rock Island cannot be built with sand due to

quantities and instability. (D50 ≈ 350 kg ≈ 600 mm Φ ). Comments on Rock Island

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Rock Island Cost Estimate (excluding topsides)

Input for the cost of rock: Open pit mine to provide 60,000T/day $35/m3 Transport & loading/ conveyer system $10/m3 Fleet of 10 self propelled ice strengthened barges, 15,000DWT each + assistance $35/m3 Total unit cost for rock $80/m3

Octagonal platform 100m x 110m x 90m, with 3,000,000 bbl. storage $0.60 Billion Berm: 30,000,000 m3 x $80/m3 $2.40 Billion Grand total $3.0 Billion Construction time 3 to 4 years

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Conclusions

  • Gravity

based non-deformable structures are not feasible in extreme tabular ice conditions.

  • Alternative ``deformable`` solutions

exist for reasonable costs.

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Thank you for your attention.