Offshore Oil and Gas Platforms for Deep Waters

Offshore Oil and Gas Platforms for Deep Waters Atilla Incecik Department of Naval Architecture, Ocean and Marine Engineering University of Strathclyde, Glasgow, UK (atilla.incecik@strath.ac.uk)

Summary of World Energy Demand

Energy Supply

Global Deep Water Discoveries

Deep Water Oil and Gas Platforms

Main Factors to be considered in Concept Selection: Production Volume Environment Water Depth Distance to shore or infrastructure The number of drilling centres required to drain the reservoir The well intervention frequency Cost Strategic reasons

Well Configuration and location options: Surface well completion Seabed Well Completion tied back to o adjacent surface facility o remote surface facility

Development Concept Options: TLP FPSO SPAR Semisubmersibles Subsea Tieback

Main Platforms Features TLPs Custom designed for site application Single drilling centre Surface completed wells Integral Drilling / Workover Facilities No oil storage Tensioned rigid risers for production Flexible or steel catenary for import or export Sensitive to top side loads Relatively long development schedule

FPSOs New-build or tanker conversation Remote wells, normally completed subsea Drilling / workover requires specialist vessel Integral oil storage and offloading Flexible risers Insensitive to topside load Short development schedule

SPARs Custom designed for site specific application Single drilling centre/surface completed wells/integral workover, or Remote wells completed subsea/workover by specialist vessel Integral oil storage or No oil storage Tensioned risers, flexibles or steel catenary risers Medium development schedule

Semisubmersibles: New-build or conversion Remote subsea wells with workover by specialist vessel Wells below with integral drilling/workover facilities No oil storage Sensitive to topside load Flexible risers Short to medium development schedule

Subsea Tie back to Shallow Water Custom designed for site specific applications Multiple drilling centres Remote subsea wells Well workover by specialist vessel No oil storage, oil is exported from host platform Short development schedule Hydraulic performance of long flowlines is key design issue

Page 118 Global Marine Trends 2030 Offshore energy sector Floating offshore platforms Fig. 100 Number of floating platforms Source: University of Strathclyde 2010 270 Total platforms in 2010 2 42 3 48 19 51 2 27 16 58 2

Challenges Mars TLP Medusa Spar Typhoon TLP Thunderhorse Petronius tower

Hydrodynamic Design Challenges Short crestedness Extreme waves/spatial wave characteristics Vortex Induced Vibrations/Vortex Induced Motions Multiple body interactions Coupled systems (offloading) Wave impacts Green water Non-linear wave effects Shallow water waves Dynamic Positioning CFD applications Model testing of Deep Water Offshore Platforms

Development of simulation tools for coupled systems

Development of analysis tools to predict the occurrence and impact of green seas on FPSO s and FPUs

Wave Impact Loading Wave Probe Load Cell 6 10 2 CL 8 4 9 1 C L Wave 11 3 7 5 Lower Deck Plan

Prediction of Loading and Response due to Non-linear Waves

Experimental investigation into motion control of Truss Spars

Numerical and experimental studies to simulate loads and motions during installation of SPARs

Model Testing of Deep Water Offshore Platforms

Waves Excitation on Floating Platforms 1st order forces at wave frequency (WF) 2nd order forces mean wave drift forces forces at sum frequencies (HF) forces at difference frequencies

Wind Excitation on Floating Platforms Mean wind forces Fluctuating wind forces due gusts in the wind field Vortex induced vibrations/motions (VIV/VIM)

Current Excitation on Floating Platforms Mean current forces Fluctuating current forces Vortex induced vibrations/motions (VIV/VIM)

Dynamic Responses Motions Mean offset, WF, and LF Mooring Forces Mean, WF, LF and HF (for TLPs)

Dynamic Responses Vessel Natural periods (s) Surge Sway Heave Roll Pitch Yaw FPSO >100 >100 5-12 5-30 5-12 >100 SemiSub >100 >100 20-50 30-60 30-60 >100 Spar >100 >100 20-50 50-100 50-100 >100 TLP >100 >100 <5 <5 <5 >100

Dynamic Responses 1.4 1.2 Heave Amplitude/Wave Amplitude (m/m) 1 0.8 0.6 0.4 0.2 0-0.2 Spectrum FPSO SEMISUB SPAR 0 10 20 30 40 50 60 70 80 Period (s)

Dynamic Responses Horizontal Motion WF LF WF LF M Horizontal motions are obtained from the solutions of coupled equations W WF M LF M M 70 330 2000 Water Depth (m)

Sources of Low Frequency Wind Damping Damping Wave Drift Damping Wave radiation Viscous Hull Damping Viscous mooring line and riser damping Friction between the mooring lines and sea bed

Low Frequency Damping for an FPSO 90 80 Series1 Series2 Series3 Series 1: 70 m water, mooring only Series 2 : 860 m, mooring only Series 3 : 860 m water, mooring and riser 70 ITEM: 1. Wind Damping 2. Wave Drift damping in 2.0 m/s current % of Total Damping 60 50 40 30 3. Hull Damping in 2.0 m/s current 4. Mooring and riser damping in still water 5. Mooring and riser damping in 2.0 m current 6. Mooring and riser damping in 2.0 m/s current and 8 m regular waves 20 10 0 1 2 3 4 5 6 Item

Model Testing of Deep Water Offshore Platforms

Model Testing of Deep Water Offshore Platforms Properly scaled system Prerequisites : All mooring lines to be included, correctly scaled with respect to mass, elastic and geometric properties. Results: Hull forces, motions, and mooring system loads can be obtained, including mooring line tension

Model Testing of Deep Water Offshore Platforms Simplified catenary modelling i.e. simplified or omitted lines Prerequisites : Careful documentation of modelling approximation by means of verified theoretical models or special calibration tests must be provided to demonstrate adequate damping and stiffness properties Results: Hull forces and motions can be obtained

Model Testing of Deep Water Offshore Platforms Catenary mooring lines replaced by horizontal lines Prerequisites : Correct restoring force characteristics needs to be documented Results: Since the mooring line damping is not modelled the motions will be too large. Wave drift forces on the hull can be obtained

Model Testing of Deep Water Offshore Platforms Dynamically controlled mooring lines Prerequisites : Documentation of the winches ability to simulate the dynamic mooring characteristics. Results: In principle it should be feasible but it will be very difficult to represent the instantaneous dynamic effect of wave frequency motions on line tensions. Hull forces and first order motions can be obtained but low frequency motions and mooring line tension cannot be obtained easily.

Model Testing of Deep Water Offshore Platforms Passive Hybrid Systems Truncated parts of the mooring lines and risers are represented by a system of springs, masses and dampers. In a passive hybrid system the horizontal mooring stiffness can be modelled correctly whereas damping due to mooring lines and mooring line dynamics cannot be obtained accurately.

Model Testing of Deep Water Offshore Platforms Active Hybrid Systems Truncated parts of the mooring lines and risers are represented by computer controlled actuators that can work in model scale and in real time. In an active hybrid system the dynamic mooring line behaviour can be simulated, including the damping and mooring line sea-bed friction characteristics

Concluding Remarks For the offshore oil and gas industry, the challenge is to continue to provide for the future energy needs of the world s consumers from resources in difficult locations in ways which are environmentally responsible, safe and economically viable. This lecture presented the factors in the selection of a concept design for deep water offshore oil and gas field developments, and the recent research and development activities and challenges in hydrodynamics analysis of deep water platform designs.