Usefull for introduction to semi submersible Platform.

2. Definition
A semi-submersible (semi submerged ship) is a specialized
marine vessel with legs/pontoons for buoyancy to float,
and weight to keep the structure upright, which used in
a number of specific offshore roles as,
• offshore drilling rigs,
• heavy lift cranes,
• oil production platforms,
• safety vessels.
They are designed with good stability and sea keeping
characteristics.

5. Semi-Submersible
• Exploratory and production purposes;
• floating structure;
• towed to the site,
• ballasted and moored;
• large vertical columns connected to large
pontoons/legs,
• columns support deck structure and equipment.

6. Semi-Submersible
• Water depth 200m to 3000m,
• Offers large deck space,
• Exceptional stability and sea keeping,
• Withstand rough waters,
• Highly flexible and efficient,
• Can be moved from place to place.

8. Applications
• Mobile offshore drilling units (MODU) or
Semi–Submersible Drilling Units (SSDU)
• Semi-Submersible crane vessels (SSCV)
• Offshore support vessels (OSV)
• Offshore production platforms

9. Semi-Submersible Drilling Units
(SSDU)
Based on the way the rig is submerged in the
water, there are two main types
• Bottle-type semi-subs and
• Column-stabilized semi-subs

10. Bottle-type semi-subs
• Hull below the drilling deck is Bottle-shaped.
• Hull can be submerged by ballasting.
• Originally bottles below the rig were completely
submerged, resting on ocean floor.
• As time progressed bottles were only partially
submerged.
• Offers exceptional stability for drilling operations.

11. Column-Stabilized semi-subs
• More popular design
• Two horizontal hulls are connected via cylindrical
or rectangular columns to the drilling deck above
the water
• Smaller diagonal columns are used to support the
structure
• Mooring lines and dynamic positioning help to
keep the semi-sub on location

12. Mobile offshore drilling units (MODU)

14. Semi–Submersible Drilling Units SSDU classified
by IMO

15. Semi-Submersible crane vessels (SSCV)
• De-ballasted during transit; only part of the
lower hull is submerged.
• The vessel Ballasted during lifting operations.
• High stability allows them to lift extreme high
loads

16. Semi-Submersible crane vessels (SSCV)

17. Offshore support vessels (OSV)
Semi-submersibles are used as number of offshore
support vessels because of their good stability, large
deck areas, and variable deck load (VDL).
 Construction support vessel,
 Diving Support Vessel (DSV),
 Offshore Safety support vessel,
 Offshore accommodation platform,
 Offshore Multiservice Vessel (MSV),
E.g.- Ocean Odyssey – Converted semi-submersible
drilling rig used as a rocket launch pad.

19. Offshore production platforms
• When oil fields were first developed in offshore
locations, drilling semi-submersibles were converted
for use as combined drilling and production platforms.
• As progressed to deeper water and harsh
environments, purpose-built production semi-
submersible platforms were designed.
• Offers very stable and cost effective platforms.
• Mostly converted semi-submersible vessels.

21. Mooring patterns and Dynamic
positioning
• Spread Mooring patterns and Dynamic positioning
used to keep the floating rig in place
• Mooring is similar to multiple anchors
• Chosen depending on shape of unit and sea conditions
• Dynamic positioning uses different motors or
propulsion units to counteract motions of water
• Dynamic positioning system is guided by telemetry
signals from beacons, satellite information and the
angular movements of a cable.

22. Semi–Submersible Drilling Units – SSDU
Advantages Disadvantages
1. Semi-submersibles can achieve good
(small) motion response and, therefore,
can be more easily positioned over a well
template for drilling.
2. Most stable of any floating rig.
3. Semi-submersibles allow for a large
number of flexible risers.
4. Provides large deck area.
5. Transportation from location to location
is easy.
6. Can be reused and converted to other
support vessels.
1. High initial and operating costs.
2. Pipeline infrastructure or other means is
required to export produced oil.
3. Building schedules for semi-submersibles
are usually longer than jack-up rigs.
4. Limited deck load (low reserve
buoyancy).
5. Structural fatigue.
6. Expensive to move large distances.
7. Limited dry-docking facilities available.
8. Difficult to handle mooring systems and
riser in rough seas.

23. Thank You
Questions?