SeaBase: The Flexible MOB Alternative in Trondheim, Norway

1. SeaBaseä The Flexible MOB Alternative VLFS Trondheim, Norway Oct. 29 2004 Erik Pettersen Moss Maritime

2. The SeaBase concept consists of • Three semi-submersible Main Modules, connected by • Two Flexible Bridges

3. 1500 m runway length, with takeoff and landing capabilities for C-17 Globemaster

4. DESIGN CRITERIA DESIGN GOALS C-17 SEASTATE 6 20,000 Troops 460,000 m2 700,000 Barrels TRESHOLD VALUES C-17 SEASTATE 6 10,000 Troops 320,000 m2 700,000 Barrels 12 Knots 40 Years AIRCRAFT : AIR OPERATIONS : ACCOMMODATION : EQUIP. STORAGE : FUEL STORAGE : (AIRCRAFTS & MIL. EQUIP.) TRANSIT SPEED : DESIGN LIFE :

5. FLIGHT DECK LENGTH : 1,528.5 m FLIGHT DECK WIDTH : 171.0 m FLIGHT DECK HEIGHT : 44.5 m (ABOVE WATER) DISPLACEMENT TRANSIT : 1,444,000.0 tonnes OPERATION : 2,721,000.0 tonnes MIAN DIMENSIONS

6. Highlights of the SeaBase concept • Three semi-submersible Main Modules, giving small wave induced motions • Two Flexible Bridges that comply with the motions of the Main Units with elastic structural deformations while inducing moderate stresses • The units are connected together by means of simple fixed type connectors to form a seamless flight deck

7. Analysis Tools State of the art analysis tools from the offshore industry • Linear Regime: SESAM program package (Det Norske Veritas): • WADAM hydrodynamic (sink source, based on WAMIT) • SESTRA linear dynamic solutions • POSTFRAME and POSTRESP fatigue and statistical post-processing • Non-linear Regime: ASAS (WS Atkins) • WAVE-NL: Morrison type calculations with non-linear drag • ASAS-NL: Non-linear time domain dynamic calculations

8. Original Geometry (Spring 1998)

9. Original model proved the objectives of the flexible concept, yet there was potential for improvements • Improve motions during normal operation • Reduce dynamic amplifications by increasing the damping of the structure • hydrodynamic damping • structural damping

10. Main module, Operation Heading: 0 deg Period: 16 sec Original geometry Base case

11. Main module, Operation Heave Response Original geometry Base case Heave RAO (m/m ) Wave period ( ) Heave RAO ( m/m) Wave period ( )

12. Main module, Operation Pitch Response Original geometry Base case Pitch ROA (deg/m ) Wave period ( ) Pitch ROA (deg/m ) Wave period ( )

13. Natural Frequency, Torsional Mode

14. Natural Frequency, Parallel Pitching

15. Natural Frequency, Pitching End Units

16. Dynamic Response Main objective is to prevent normal periods in the wave excitation area, generally considered impossible for complex structures. The Flexible Bridge response is “tuned” by the following two methods: • Adjust the flexibility to keep as many as possible of the eigenmodes out of the excitation area • Reduce dynamic amplification of the remaining eigenmodes by means of structural damping

17. Wave Spectra vs. Eigenmodes - Base Case Eigenperiods Energy Density Period (sec)

18. Wave Spectra vs. Eigenmodes - Base Case with Reduced Stiffnes Eigenperiods Energy Density Period (sec)

19. Flexible Bridge Base Case

21. Typical Damping Element • Linear or non-linear fluid viscous damper assumed • Maximum required damping force of about 2000 tons is same order of magnitude as existing applications • Overall size of unit will be about 1.5 m diameter, 2.0 m long [1,0.1] Typically 0.4 - 0.5

22. SeaBase Energy (kW)

23. SeaBase Energy (kW)

24. Flexible Bridge Base Case - Seastate 6, all periods

25. Flexible Bridge Base Case - Seastate 8++, all periods

26. Fatigue Damage Assumption: 35 percent of life in operation, 35 percent in survival (stand alone). Fatigue budget for each site is 1/8 in operation and 1/8 in survival. Numbers below give fatigue damage relative to this budget. BASE CASE MAX DAMAGE RATIOS OPERATION SURVIVAL SITE Site 1 – North Atlantic 5.1 – Damage 3.37 - Damage Site 2 – Western Pacific 0.79 – OK 0.47 – OK Site 3 – Arabian Sea 0.33 – OK 0.18 - OK Site 4 –Sea of Japan 0.18 – OK 0.10 – OK SN assumption: Cast joints

27. Conclusions from the structural calculations • Maximum stresses in Seastate 6 (Hs=6 m) are acceptable • Maximum stresses in Seastate 8++ (Hs=14 m) are high, but within the capability of the structure with minor modifications • Fatigue exposure at all actual sites except the North Atlantic site is small. Continuous operation in the North Atlantic for more than one year will require special measure with respect to inspection and repair • Connector forces are of a magnitude that can be handled by means of simple fixed type connectors

28. I.e. • SeaBase does not require disconnection when conditions exceed Seastate 6 • SeaBase can stay connected at all actual sites and all times except when encountering tropical cyclones or North Atlantic winter storms • There is still room for structural optimizations that will improve the fatigue endurance in North Atlantic environment

29. Connection between the units: • shall be able to disconnect in severe weather conditions • shall be able to re-connect in moderate weather conditions (dynamic problem) • shall have sufficient Ultimate Limit Strength • shall have acceptable fatigue properties. • Advantage with SeaBase compared to some other MOB solutions: • the connector can be fixed, without flexibility or damping when finally connected • the flexibility and required damping can be distributed over a large volume in the flexible bridge.

30. Re-connection sequence.