Design of Floating Structures CE 5710

Lecture 6B Spar Design Sept. 19, 2006 Dr. John Halkyard E1A-07-15 Tel 6516 2153 cvehje@nus.edu.sg

SPAR SIZING

Oryx Neptune

SPAR PROGRESSION

Cell Spar Truss Spars

Classic Spars Also FDPSO

Basic Arrangements and Terms Classic Truss

DRY TREE

DRY TREE UPPER STEM EXTENSION DAMAGE CONTROL BULKHEAD

MWL

EL (+)50'-0"

EL (+) 50'-0" VOID VOID DAMAGE CONTROL BULKHEAD

MWL

Freeboard

CHAIN JACK

VOID

VOID

VOID

VOID

275'-0"

BUOYANCY CANS STRAKE

RISER BUOYANCY CAN VOID VOID BUOYANCY CAN GUIDE LEVEL

VOID

VOID

VARIABLE BALLAST TANK FAIRLEAD MOORING CHAIN TOP TENSIONED RISER

FAIRLEAD

STRAKE VARIABLE BALLAST TANK

MOORING CHAIN

275'-0"

STEM GUIDE

HEAVE PLATE (TYP.)

35'-0"

EL (-)530'-0"

Soft Tank

EL (-) 505'-0"

Keel
SCR

KEEL TANK FOR FLOATOUT & FIXED BALLAST

FIXED BALLAST TANK

(fixed ballast)
STEEL CATENARY RISER (SCR) TOP TENSIONED RISER

TOP TENSIONED RISER

25'-0"

290'-0"

Midsection

LOWER STEM EXTENSION

240'-0"

VOID

VOID

Hard Tank

VOID

VOID

Chain Locker
DRY TREE

VOID
EL (+)50'-0"

VOID

DAMAGE CONTROL BULKHEAD

MWL

Dk 10 Dk 9 Dk 8 Dk 7 Dk 6 Dk 5 Dk 4

VOID

VOID

DAMAGE CONTROL BULKHEAD

MWL

VOID

VOID

VOID

VOID

VOID

VOID

275'-0"

BUOYANCY CANS STRAKE

VOID

VOID

VOID

VOID

VARIABLE BALLAST TANK FAIRLEAD MOORING CHAIN TOP TENSIONED RISER

VOID

VOID

STRAKE

VARIABLE BALLAST TANK

275'-0"

FAIRLEAD MOORING CHAIN TOP TENSIONED RISER

Dk 3 Dk 1

EL (-)530'-0"

FIXED BALLAST TANK SCR TOP TENSIONED RISER

35'-0"

275'-0"

BUOYANCY CANS

Setting the Spar Configuration Start with the Risers Size the Centerwell Provide Buoyancy to Support all the vertical loads: Topsides Outfitted Hull Weight SCR Vertical Loads Mooring Vertical Loads Variable Ballast Fixed Ballast Risers that are not supported on buoyancy cans Provide adequate stability

Main Spar Input Parameters

WT Topsides Weight (Fixed + Variable) ZT Topsides VCG (above spar deck) AT Wind Area for Topsides Zw Centroid of Wind Area (from Waterline) Topsides Eccentricity Depth (for mooring and risers) Number of Risers & Slots Spacing of Risers Allowances for future risers.

ZT

ZW

Water Depth

KF

Fmoor

Fmoor Vertical Mooring Load KF Fairlead Elevation above keel* Friser Riser Vertical Load KR Riser Elevation above keel Environment Survival Wave, Wind & Current

Friser

* - Actually design variable

Main Spar Constraints

4.5 m for equipment

ZT

Air Gap

Max Diameter for Construction Largest spar D=45 m Typical D = 33 m Max length and hull weight for dry transport (150 m and 25,000 t) Check Heavy Lift Vessels Max Draft in Horizontal for float-off (10 m) Minimum deck clearance (air gap) Can use two piece transport for large spars. Air Gap >= 1.15*Hs (1st cut) Hmax = 1.86*Hs Amax = 0.93*Hs

Water Depth

KF

Fmoor

Friser

Main Spar Design Parameters

CW Centerwell Width HHT Hard Tank Depth FB Free Board T Draft D Diameter Compartment Sizes Variable Ballast Fixed Ballast

CW

FB HHT (Hard Tank Depth)

Variable Ballast Fixed Ballast D

Sizing the Wellbay

Number of riser slots?
5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 0 500 1000 Water Depth, m 1500 2000 Wellbay Riser Spacing, m

Single or Dual Casing Risers? Required Top Tension Factor? Workover Strategy? Buoyancy can capacity and allowance for damage? Arrive at can diameter and length? Minimum well spacing = can diameter + allowance for grillage (.3 to .4 m) Space allocation for drilling, SCRs and moonpool for ROV launching Arrive at a centerwell size

Well Spacing

Suggestion: Use 3.6 m for depth < 1500m; 4.3m for depth > 1500 m

Spar Weight Categories

Topsides (Fixed and Variable) Hull Hull Outfitting Ballast Variable Ballast - Fixed External Loads Mooring Risers (initial and future)

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Buoyancy Must Balance the Weights and the Hull Must Be Stable!

Spar Weight Categories

Topsides (Fixed and Variable) Hull Hull Outfitting Ballast Variable Ballast - Fixed External Loads Mooring Risers (initial and future)

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Buoyancy Must Balance the Weights and the Hull Must Be Stable!

Topsides Parameters

12

1951 m2 5448 tonnes

35 m

Units here are English. Weight in kips (1000 lb, .454 tonne)

Wind Force
Wind Force
Select different block areas based on elevation, shape
5

13

45 deg

z5

z4

2 1

z2 z1

z3

0 deg

z6

Block 1 2 3 4 5 6 7

Desc z center Ch (ABS) Length Height Area Area Corre Adj Area Cs CsChArea Force/Ur**2 Lower Decks 21.8 1.1 100 3.6 360 1 360 1 396 242 Rig 27 1.1 30 7 210 1 210 1 231 141 Quarters 30 1.1 20 10 200 1 200 1 220 134 Process 28 1.1 20 8 160 1 160 1 176 108 Derrick 55 1.3 15 40 600 0.6 360 1.25 585 357 Hull 7.6 1 21.9 15.2 334 1 334 1 334 204 Deck Supports 17.5 1.1 21.9 15.2 333 0.6 200 1 220 134 2 2 Total Force/Ur Force at 0 deg 1187 N/(m/s) 2 2 Equivalent Area = Force/(wgUr ) 1942 m Centroid of Force 30.2 2 Total Force/Ur2 Force at 45 deg ~ 1.2* Force at 0 deg 1424 N/(m/s) 2 Use this in sizing of the hull >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> Equivalent Area = Force/(wgUr2) 2331 m

Spar Weight Categories

Topsides (Fixed and Variable) Hull Hull Outfitting Ballast Variable Ballast - Fixed External Loads Mooring Risers (initial and future)

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Buoyancy Must Balance the Weights and the Hull Must Be Stable!

Hull Weight Estimating

Design Spiral Start with Guess for Hull Size, Compartments, etc. Make approximate weight estimate based on Area of plate, or Volumetric Weights Perform buoyancy and stability check (initial GM Target 4 6 m for spar; Natural Period in Pitch 40 75 sec) Update size and approximate weight Once size is OK determine scantlings and actual weight including margins based on local strength (check against previous designs) Redo buoyancy and stability check until successful Check global strength and fatigue later in design! Hull Weight is based on local strength (pressure).

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See MODU Rules, Course Notes!

INTERNAL STRUCTURE - EXAMPLE

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Hull Compartments
Section 9 8 A A 7 6 5 B B 4 A-A 3 Typical Section Section at Waterline Waterline

17

2 1

B-B

Design Head (Local Pressure)

Waterline

18

Design head is Static + Dynamic (for operational waves)

Structural Design
Idealization of Outer Shell as Flat Panels Struts reduce girder span

19

Outer shell treated as flat plate

Weight Summary from Scantling Design Program (example)

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Sect. 9 8 7 6 5 4 3 2 1 Totals:

Length, m Outer Shell Inner Shell Decks 9.1 120.8 0.0 53.4 15.2 191.3 271.1 38.0 14.9 318.3 0.0 63.8 14.9 376.7 0.0 79.9 14.9 438.6 0.0 92.9 14.9 500.2 0.0 233.6 118.9 2084.9 0.0 0.0 11.0 310.3 0.0 172.1 1.2 20.8 0.0 367.0 0.0 4361.9 271.1 1100.8

Weight, tonne Primary BHCenterwell Secondary Struts Totals 16.0 45.1 0.0 10.7 245.9 52.5 132.8 0.0 13.7 699.5 55.1 168.2 0.0 30.0 635.5 67.2 200.3 0.0 45.4 769.5 73.9 235.2 0.0 57.4 898.0 79.9 266.8 0.0 77.0 1157.5 0.0 0.0 0.0 0.0 2084.9 176.0 91.8 0.0 24.9 775.0 13.7 5.7 0.0 0.0 407.1 534.1 1145.9 0.0 259.0 7672.8

Weights - By Area

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Y=9.6*h+75.4
600 500 Weight, kgf/sq m 400 300 200 100 0 0 10 20 30 40 50 60 70 80 Design Head, m

Y=6.2*h+90

Outer Shell Deck Primary BHD Centerwell

Add all panel weights and add struts and non-watertight bulkheads if applicable.

Weights By Volume

22

300 250 Weight, kgf/m 3 200 150 100 50 0 0 10 20 30 40 Design Head, m 50 60 70 80

y = 96.547e0.0157x

Using volumetric factors is easier but less accurate that using area factors.

Weights of Midsection & Truss

Midsection in classic spar designed from minimum head (6.1 m) Control of ring frames is allowable deflection during construction, usually less than about 50 mm! Truss design is controlled by global bending at maximum pitch angle. Assume static loads at 10 deg angle Select truss members for lateral gravity loads

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Spar Weight Categories

Topsides (Fixed and Variable) Hull Hull Outfitting Ballast Variable Ballast - Fixed External Loads Mooring Risers (initial and future)

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Buoyancy Must Balance the Weights and the Hull Must Be Stable!

Minimum Variable Ballast

Deck Eccentricity Future Risers Compensating a Flooded Compartment Overcoming buoyancy of stored oil over water (if any)

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Minimum Variable Ballast Deck Eccentricity

Balance Deck Eccentricity

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Deck Eccentricity, FB

Variable Ballast Fixed Ballast

Ballast = Wtopsides

Centroid of Ballast,

Minimum Variable Ballast Damaged Compartment

If a single compartment is flooded, MODU Rules Require the ability to restore original draft and trim in order to initiate repairs. Typically a damageddepth of 25 50 ft is assumed, include the equivalent amount of ballast from asingle flooded compartment (one quadrant of the buoyanct hull).

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Spar Weight Categories

Topsides (Fixed and Variable) Hull Hull Outfitting Ballast Variable Ballast - Fixed External Loads Mooring Risers (initial and future)

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Buoyancy Must Balance the Weights and the Hull Must Be Stable!

Outfitting Weight

29

1st Cut Outfitting weights are about 20% of the hull structural steel!

Parametric Model for Sizing

Desired Outcome(s) = Function of (size, weight, environment, etc.)

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Keep iterating until the desired outcome is reached!!

Spar Design Process

Select Input Parameters and Centerwell Size for Risers Select Mooring Fairlead Elevation Select A Trial Hull Size (draft, diameter, hardt tank depth) Estimate hull weight by approximate method Calculate Imbalance of Weight and Buoyancy. If excess weight, increase diameter or hard tank depth If excess buoyancy, add fixed ballast Calculate heel angle for 100 year storm If less than target (5) reduce diameter or hard tank length If greater than target increase diameter or hard tank length If target ismet perform detailed weight estimate and check Iterate on this procedure for optimum design

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SPAR SIZING
WIND LOAD THE CENTRAL CRITERIA IN SPAR SIZING IS TO BALANCE THE OVERTURNING MOMENT DUE TO WIND WITH THE RESTORING MOMENT DUE TO BG (100 Yr Storm)
BHard FCurre FWind WTop
sides

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Topsides

WHard WOutf

Tank

MWL

Tank

nt itting able B allast

MOORING LOAD

FMoori

ng

WVari

Hull
FMoori
ng

Var Ballast

BTrus WTru
ss

Max Heel Angle ~ 5

WSoft Tank WFixe

d Balla st

FSCR

Fixed Ballast

SPAR SIZING
WTopsides FWind

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K Pitch = GM GM = KB KG + I
BG ~ 5 8m

MWL

BHard Tank

BM < 1m

B
FCurrent

WHard Tank

WOutfitting WVariable Ballast

FMooring

FMooring

BG is the dominant restoring moment The waterplane effect (BM) is not significant The spar is unconditionally stable The center of gravity is always below the center of buoyaancy Flooding will not cause instability Broken mooring will not cause instability Broken riser will not cause instability
BTruss WTruss

WSoft Tank WFixed Ballast FSCR

Hull Geometry Example ISAP

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Draft = 198 m (650 ft) Diameter = 21.9 m (72 ft) Freeboard = 16.8 m (55 ft) Centerwell = 9.8 m x 9.8 m (32 ft) Hard Tank Depth = 67 m (220 ft)

Buoyancy and Internal Water

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CW

FB A

T B Fixed Ballast D

BUOYANCY = w

DT

Weight Summary

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Hull Weight = 8424 t (18567 kips) Outfitting Wt = 1993 t (4389 kips) Topside Wt = 5448 t (12000 kips) Fixed Ballast = 1123 t (2474 kips) Var ballast = 1486 t (4876 kips) Internal Water = 56617 t (124708 kips) Vertical Loads = 1072 t (2361 kips) Total = 76896 t (169375 kips)

Including Internal Water makes no Difference to Hydrostatics!

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Weight w/o Water Vertical Loads

Disp w/o Water BM KB+BM-KG GM*DISP

With Internal Water Without Internal Water Wt KG Wt Kg 42306 493.1 167014 314.9 2361 310.0 2361 310.0 44666 483.5 169375 314.8 B KB 44666 522.1 169375 325 1.5 0.4 40.1 10.6 1791014 1791014

Whether you include internal water or not does not make any difference to the hydrostatics. It does matter for the hydrodynamics, however.

Disp 44666 kips (20278 t) 169375 kips (76896 t)

Oil Storage Spar

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the centerwell is extended through the midsection, the shell is strengthened to withstand the differential pressures generated when the storage compartments are filled with oil, the amount of fixed ballast is increased to account for the lower center of buoyancy

BAR STIFFENER

EL (+)50'-0"
VOID VOID

DAMAGE CONTROL BULKHEAD

MWL

VOID

VOID

EL (-)20'-0"
VOID VOID

EL (-)70'-0"
OIL
VOID VOID

BUOYANCY CANS STRAKE

275'-0"

EL (-)120'-0" VARIABLE BALLAST TANK

RING FLANGE RISER STEM

EL (-)170'-0"

EL (-)220'-0" TOP TENSIONED RISER FAIRLEAD MOORING CHAIN

RISER

A-A

BOLT HOLES

EL (-)292'-0"

A OIL

EMULSION LAYER

SCR HOUSING PULL-IN GUIDE

STAB-IN TIE BACK SCR HOUSING

~EL (-)585'-0" EL (-)615'-0" EL (-)650'-0"

SCR

BOTTOM OF SPAR

35'-0"

395'-0"

FIXED BALLAST TANK SCR

KEEL JOINT SLEEVE

RISER

STORAGE SPAR INBOARD PROFILE

TOP TENSIONED RISER

Distribution of Loads for Spars

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TYPE OF SPAR TRUSS TRUSS TRUSS CLASSIC TRUSS TRUSS TRUSS CLASSIC DISPLACEMENT 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% 100.0% TOPSIDES 24.7% 23.8% 30.6% 27.7% 26.2% 27.7% 26.4% 26.3% STORAGE IN HULL 1.4% 0.7% 2.0% 1.2% 0.0% 0.0% 0.0% 0.0% VARIABLE BALLAST 5.5% 10.1% 0.0% 4.6% 6.1% 6.9% 4.9% 7.2% FIXED BALLAST 16.5% 18.6% 26.4% 15.5% 23.3% 21.2% 23.6% 0.0% SCR LOAD 0.8% 3.1% 2.1% 0.0% 2.2% 3.7% 2.4% 1.4% TTR LOAD 12.2% 0.0% 1.4% 0.5% 0.0% 0.0% 0.0% 0.0% MOORING LOAD 5.0% 6.2% 3.4% 3.6% 7.0% 6.2% 7.1% 2.4% OUTFITTED HULL WEIGHT 33.9% 37.5% 34.2% 46.9% 35.2% 34.3% 35.5% 62.7%

Excercise
Consider example spar in this lecture 72 ft (21.9 m) dia 220 ft (67 m) hard tank depth Etc. Add 1000 tonne to deck weight (same VCG and wind area) . Total 6448 tonnes Increase hard tank depth and/or diameter to achieve the same GM What is new hull weight (approximately) Hint: you mayhave to increase fixed ballast as well. Hmax = 75 ft Period = 13 sec.

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