FPSOs

FPSO

FPSO

F

F

LOATI

LOATI

N

N

G

G

P

P

RODUCTION

RODUCTION

S

S

TORAGE &

TORAGE &

O

Espadarte

Espadarte

Elevation

Elevation

Overhead Structure Swivel Stack Superstructure Main Bearing Turret Riser I- tubes Moonpool Lower Bearing Chain Hawse

Espadarte

Turret Cut

Turret Cut

-

-

Away

Away

Riser Tie-in Spools Riser Termination Deck Riser I-tubes Riser Bell Mouth Chain Hawse Chain Jack Chain Support Arch

Espadarte

Espadarte

FPSO UTILITY

• Proven floating concept – ship

– Can survive any non-ice sea environment

• Large topside area for production

– Allows for horizontal integration – safety by distance

• Large storage capacity for periodic export

– Normal 1-2 week production storage capacity

• Proven offloading capability in tandem or

side-by-side

– High tandem offload thresholds assure continuous production

HULL SIZE DRIVERS

• Required storage capacity - Shuttling philosophy

– dedicated tankers - parcel size – tankers of opportunity

• Topsides

– required deck area – stability

• Environment

– freeboard in fully loaded conditions (“green water”) – minimum draft in ballasted conditions (“slamming”) – size of the mooring system (anchor leg / turret)

• Location of the accommodation • Location & size of the turret

– internal – external

CONVERSION vs. NEW BUILD

• Design field life

– at the end of the life of the field, the FPSO should not be older than 30-35 years

• Environment

– the fatigue resistance of a tanker is suitable for a 25-year deployment in West Africa

– in harsh environment, ships rules do not provide adequate fatigue resistance for long term deployment

• Project planning

– conversion: 14 - 24 months (topsides & mooring on critical path) – new construction: 24 to 30+ months (hull on critical path)

• Costs

– CAPEX in favor of conversion (but cost and schedule somewhat difficult to predict accurately)

– new construction costs (and schedule) can be reduced by accepting an early design freeze date on topsides

RULES & REGULATIONS

• Shipbuilding vs. Offshore Standards

– piping

– selection of materials

– accommodations

– control & safety systems

• Safety

– IMO MODU as a reference

– SOLAS limited to specific items not covered by

MODU

– MARPOL with unified interpretation for

FPSO’s

FPSO STATION KEEPING AND

OFFLOADING

Multidirectional environment Single Point Mooring (SPM)

Significant wave Height (m)

– Turret – Norm any

– Buoy / Yoke – Old < 8-10 – Tower/Yoke – Shallow < 5-6

Offloading – Tandem or side-by-side

Directional environment spread mooring – Multiple mooring lines tied directly to vessel

Significant wave Height (m)

– Bow on waves < 6-7

– Beam on waves < 4

Offloading – Through separate SPM, side-by-side* or tandem* *Safety issues

GLOBAL FORCES AND MOTIONS

• Global forces and resultant mooring motion – Wind (Steady) Steady offset

– Wave (Variable) Variable offsets – Current (Steady) Steady offset

• Global forces and resultant vessel motion

– Wind surge, sway, yaw and roll (heel)

– Wave surge, sway, yaw, heave, roll and pitch – Current surge, sway and yaw

GLOBAL FORCES AND MOTIONS

• Variable wave forces occur at:

vessel natural period and at FPSO/Mooring natural period

– Wave inertial hydrodynamic forces at periods of 7 to 20 sec induce large loads on FPSOs but small mooring forces.

– Wave drift forces which result from random wave group

sequences create mean and

variable forces on the FPSO. The variable horizontal force is

troublesome as it occurs at periods near to the FPSO/mooring natural period.

PM JONSWAP

WAVE FORCES

First Order (WF) Second Order (LF) Pierson-Moskowitz – Hs=6.7m Tp=15s

σ

_WF

=0.3m

σ

_LF

=9.3m

1.0E7 0.0 1.2E5 0.0

FPSO with mooring is simply a single

degree of freedom mass spring system

with a natural period ω. When second order wave

force excitation occurs at this period, large horizontal

motions are experienced limited only by damping. F

X

WIND

• Wind load prediction – OCIMF, wind tunnel or windage areas not from wave model basin tests.

• Windage areas can be very large owing to Topsides especially in ballasted condition.

• Wind can be designing in areas such as West Africa, when the vessel is spread moored.

• Plays a role in wave drift damping.

• How to best describe wind: a) uniform, b) speed and duration, c) wind spectrum, d) recorded time series.

• If we use wind spectrum or time series, combination of wind + wave both being random would have to be

CURRENT

• Current load prediction on hull – OCIMF, current basin, analytical (future CFD).

• Current load prediction on riser and mooring – current basin (scaling problems), analytical (many good

programs).

• Knowledge of current down to the fully loaded draft is important for current forces on the FPSO hull (say first 25m).

• Interaction with wave important in wave drift force.

• Knowledge is important throughout the depth for mooring and riser system.

• Profile is important both in magnitude and direction. • Plays a significant role in wave drift damping.

SOURCES and DERIVATION

of DAMPING

Y Y Potential Wave drift Y Y Potential Radiation Y Y Drag Wind Y Y Drag Current Y N Drag Mooring line/riser Fully coupled derivation Quasi-static derivation Physics Source

Mooring and riser damping derivation is an iterative process (damping depending on amplitude, which itself depends on the damping). To solve this problem, fully coupled methods should be used.

COUPLED HULL, RISER AND

MOORING ANALYSIS

• Fully coupled analyses are required to capture all interactions influencing mooring loads.

• Computer time for fully coupled design prohibitive from a design point of view.

• Alternate method:

– Make a short coupled run.

– Derive proper behavior / damping from this run. – Use this in an uncoupled quick analysis program. – Do probabilistic analysis.

NUMERICAL PREDICTIONS

• Main limitations

– Mooring: Yaw and fish-tailing,

– 1st _{order motions: Roll prediction and damping}

– Risers: VIV 0 . 0 0 . 3 0 . 6 0 . 9 1 . 2 1 . 5 W a v e f r e q u e n c y ( R a d / s ) 1 1 0 2 3 4 5 7 2 3 4 5 7 2 3 4 5

Roll motion RAO [°/m]

R a d i a t i o n d a m p i n g o n l y B i l g e k e e l d a m p i n g

IMPACT on DESIGN & COST of

METOCEAN UNCERTAINTIES

Anchor leg tension Mild environment – shallow water Mild environment – deep water Harsh environment – shallow water Pretension 10% 50% 15% Wind 30% 15% 20% Current 10% 10% 5% Mean wave 25% 6% 13% Slow-drift 20% 11% 17% Wave freq. + dynamic 5% 8% 30% Total 100% 100% 100% Anchor Leg Tension

IMPACT on DESIGN & COST of

METOCEAN UNCERTAINTIES

Horizontal mooring force Mild environment – shallow water Mild environment – deep water Harsh environment – shallow water Wind 34% 50% 24% Current 10% 13% 6% Mean wave 27% 12% 15% Slow-drift 22% 15% 22% Wave freq. + dynamic 7% 10% 33% Total 100% 100% 100% Horizontal Mooring Force

CONCURRENT ENVIRONMENT

CONDITIONS

Wave height and period

0.2 0.4 0.6 0.8 1.0 1.2

Wave frequency [Rad/s]

0e0 2e4 4e4 6e4 8e4 1e5

Surge drift force [N/m^2]

Drift force JONSWAP - Tp=13s JONSWAP -Tp=16s 0 5 10 15 20

CONCURRENT ENVIRONMENT

CONDITIONS

Wave with current influence on wave drift force

0.0 0.3 0.6 0.9 1.2 1.5

Wave frequency [Rad/s]

-1e4 3e4 7e4 1e5 2e5 2e5 Surge QTF [N/m^2] No current 1m/s collinear current 1m/s opposite current

METOCEAN IMPACT

on OPERABILITY

Areas having persistent crossing wave, wind or current

conditions can cause operational problems

•

Cross current (or wind) vessel orientation to wave

can lead to roll induced problems with:

1. Crew

2. Process