2.019 Design of Ocean Systems

Lecture 2

February 11, 2011 Typical Offshore Structures

Platforms: – Fixed platform –FPSO – SPAR – Semi-Submersible – TLP (Tension Leg Platform)

Key Components: – Platform on surface (with production facility, gas/oil storage) –Risers – Mooring – Subsea system – Pipelines or transport tankers

© Centre for Marine and Petroleum Technology. All rights reserved. This content isexcluded from our Creative Commons license. For more information, see http://ocw.mit.edu/fairuse. Semi‐Submersible Platform Photo courtesy of Erik Christensen, CC-BY.

Tension‐Leg Platform (TLP) Gravity‐Based Platform © SBM Atlantia. All rights reserved. This content is excluded from our © source unknown. All rights reserved. This content is excluded Creative Commons license. For more information, see http://ocw.mit.edu/fairuse. from our Creative Commons license. For more information, see http://ocw.mit.edu/fairuse. FPSO (Floating, Production, Storage, Offloading)

ƒ Movable ƒ Versatile for use in smaller reservoirs ƒ Relatively cheap in cost

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– Topside –Hull – Mooring/riser system

Courtesy of Linde AG. Used with permission. Design of FPSO vs. Tankers/Ships

Tankers/ships: FPSO:

ƒStorage ƒStorage ƒStability ƒStability ƒSpeed ƒNo Speed requirement ƒMotion (seakeeping) ƒMotion (seakeeping) ƒCost – Limited riser top-end displacement – Production – Offloading – Equipment installation ƒFreeboard – Green water – Bow impact ƒMooring/risers – Drift motion – slowly-varying motion ƒReliability Basic Design Requirements of FPSO

ƒ Requirement for a turret mooring to allow the FPSO to weathervane and minimize environmental loads on the mooring system

ƒ Selection of suitable hull size and form with good motion characteristics

ƒ Freeboard

ƒ Production facilities design to minimize motion downtime

ƒ Hull size to provide adequate buffer storage to minimize shuttle tanker offloading downtime

ƒ Hull structure design (strength and fatigue)

ƒ Human safety Hull Size and Form Major requirements: ƒ Crude oil storage ƒ Deck space ƒ Sea-Keeping performance ƒ Availability of tanker hulls for conversion

Typically, ƒ Weight of lightship (i.e. steel): 13-16% of displacement ƒ Weight of topside: 7-8% of displacement ƒ Cargo deadweight (i.e. oil storage): ~ 75% of displacement ƒ Weight of risers/moorings: depending on type and number of riser/mooring lines

In general, ƒ Hull steel weight: Length/depth ratio of hull and hull size ƒ Deck space: Length X breadth ƒ Mooring loads: Breadth/length ƒ Stability: Breadth/depth ƒ Sea-keeping (primarily pitch): Length ƒ Green water on deck: Freeboard ƒ Bottom slamming forward: Ballast capacity ƒ Bow wave impact: Bow fineness Structure Configuration

Double sides /bottom:

ƒMinimize risk of oil spilling due to collision ƒSingle or double bottoms are OK for FPSO ƒ2m width of wing tank for trading tankers

Bulkhead spacing:

ƒMinimize free surface effects due to partial-filled tanks ƒLimits on maximum tank size to keep oil spillage below a certain volume in the event of tank rupture ƒStructure support of production/utility system ƒMore tanks to allow safe inspection and maintenance while not interrupting operation

Typically, the cargo tank region is subdivided by a centre-line bulked and four or more transverse bulkheads. Ballast System

The capacity and location of ballast tanks should be designed to ensure that with crude oil storage tanks empty or part-fill, the FPSO draft and trim meets requirements on:

ƒ stability ƒ maximum trim for production equipment operation ƒ minimum draft forward to prevent bottom slamming. Turret Mooring/Riser System

Internal Turret: ƒSmaller riser head displacement due to pitch motion ƒEasy for maintenance ƒLarge deck space ƒLonger ship length

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External Turret: ƒNo effect on ship hull length and deck area ƒInconvenient for maintenance ƒLarger riser head displacement due to pitch motion

Courtesy of Linde AG. Used with permission. Estimate of Heave and Pitch Natural Periods

For a barge, length L, width B, draft T

Heave:

m + ma CBT Tnh =2π =2π (1 + 0.4B/T ) s K s Cwg

C = /(LBT ): block coefficient; C = S/(LB): waterplane coefficient B ∇ w

Pitch: 2 2 12T R2+ B m(R +Ra) L2 4 Tnp =2π ρg H 2π g ∇ ≈ q r H: metacentric height in longitudinal direction R: radius of gyration Ra: radius of gyration of pitch added mass Stability Requirements

Intact stability Damage stability

Hull Structure and Deflection

Shear requirement Sagging/hogging moment requirements Deflection requirement Hydrostatic Pressure and Forces

z x

Wetted Surface S dS Normal Vector n

Image by MIT OpenCourseWare.

Fz = ρg dVolume ZZZ Hydrostatic Pressure: p=p ρgz = ρg submerged volume a − × Fx =0 F =0 ~ y Force: F = S ρgz~ndS − Mx = ρg ydVolume Moment: M~ = ρgz(~x ~n)dS V RR− S × ZZZ My = ρg xdVolume RR ZZZV Mz =0 Hydrostatic Stability — Fully Submerged Body

Waterline

Force: ρg Volume

Center of buoyancy B G Weight

Image by MIT OpenCourseWare.

Moment Equilibrium: G and B are in a vertical line Restoring moment M = ρg GB sin θ under a rotation θ r − ∇ × Stable: G is under B (i.e. GB > 0) Unstable: G is above B (i.e. GB < 0) Hydrostatic Restoring Moment —SurfacePiercing Body

No change in submerged volume Total restoring moments:

Upward force from submerged wedge M =(ρgI GBρg )θ rx − t − ∇ x It = ρg ( + GB)θx B − ∇ G ∇ = ρg GM θ − ∇ × × x Downward force from emerged wedge

Image by MIT OpenCourseWare. For equal displacement volume, the waterline contribution to restoring moments: Metacentric height for transverse rotation: Mrx = ρgItθx (heel) It − GM = + GB M = ρgI θ (trim) ∇ ry − l y Moments of inertia of waterplane area: • Stable if GM > 0

2 2 • Compared to the submerged It = y dA, Il = x dA Aw Aw bodies, the presence of free Center RRof flotation (COF): RR surface increases the restoring xdA =0, ydA =0 moment. Aw Aw RR RR Righting Arm

Heel angle Metacentre

GZ: righting arm Centre of gravity

Centre of buoyancy Centre of before heel buoyancy after heel

Righting arm GZ: Image by MIT OpenCourseWare. righting moment GZ = = GM sin θ GMθ − displacement × x ≈ x GZ Curve

GM at 1 Radian Heel ) m

( 2.0

m r a

g n i t

h 1.0 g i R

0.0 20 40 60 80 Heel angle (deg)

Image by MIT OpenCourseWare.

A floating structure is STABLE for heel and trim respectively when:

GMt > 0(heel)

GMl > 0(trim) Wind Overturning Moment

Image by MIT OpenCourseWare. 1 2 F = ρ C C V A M = F h wind 2 air s H 10 wind wind × s Cs: shape coefficient, C =1.0 for large flat surface CH: height coefficient, CH=1.0 for FPSO P V10: wind velocity at 10m above sea surface A: Project area of the exposed surfaces in the vertical or the heeled condition h: vertical distance between center of wind force and center of resistance (by mooring lines, etc) Evaluation of Large Angle Hydrostatic Stability

© Centre for Marine and Petroleum Technology. All rights reserved. This content is excluded from our Creative Commons license. For more information, see http://ocw.mit.edu/fairuse. Wind heeling arm = (wind moment) / displacement

Safety Criterion: Mmax ≥ Mw

Mmax: maximum righting arm Mw: wind moment Dynamic Stability Criterion:

Area B Righting arm Downflooding angle

Area A Second intercept

Heel angle

Image by MIT OpenCourseWare.

Erighting moment 1.3(or1.4)Ewind − ≥ Eright moment = Area B − Ewind =Area A Hydrostatic Stability Analysis of Ships and FPSOs

Intact stability analysis: 6

• Compute GZ curves for various Ballasted loading conditions 4 • Compute heeling moments by Loaded wind, etc

• Compare righting lever, GZ, to 2 heeling lever Righting arm Slack Damaged stability analysis: 0 0 20 40 60 80 • Both wind loading and a degree Heel angle of flooding are assumed Image by MIT OpenCourseWare. • Flooding due to waterline GZ curve for a FPSO damage and/or flooding due to burst ballast • Evaluate the stability as in intact stability analysis • Typical rule requirements: 50 knot wind and side or bottom damage ABS MODU (Mobile Offshore Drilling Unit) Stability Regulations

Intact Stability: • Design wind speed: 100 knots • The angle of inclination should be no greater than 25 degree • Dynamic stability criterion: Erighting moment 1.3(or1.4)Ewind − ≥

Damage Stability: • Design wind speed: 50 knots • The angle of inclination should be no Minimum extent of watertight integrity provided.

greater than 25 degree Righting moment (>_ Overturning moment, θD >_ θ1) • Horizontal penetration should be at least 1.5m First Intercept • Longitudinal damage extent should Moment be (1/3) L2/3 or 14.5m, whichever is less Overturning moment Angle of • Damaged compartments are downflooding completely filled Inclination about θ θ θD • Downflooding angle must be larger 2 1 the critical axis than the first intersection of the Image by MIT OpenCourseWare. righting moment and heeling moment MIT OpenCourseWare http://ocw.mit.edu

2.019 Design of Ocean Systems Spring 2011

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