Offshore storage

1. OFFSHORE STORAGE
1

2. Need for Offshore Storage at Site
• For an Early production system (EPS)
• To permit production from marginal wells that do
not justify pipeline construction
• To permit production in deep water where pipeline
construction is not feasible
• To serve as temporary storage in case of pipeline
shutdown
• To permit temporary storage for oil destined for
terminals other than the nearest shore terminal
2

3. Types of Offshore Storage
• Floating storage
• Fixed storage
• Semi-submerged storage & loading buoy
Floating Storage varies with “Anchoring System”. Old
converted tankers, with a fixed anchoring system
were first to be used. Concrete floating storage was
developed to avoid corrosion. A design termed
“RMS” is a rotating , Single-Point-Mooring (SPM),
large capacity concrete tank, using the water
displacement principal.
3

4. • Fixed Storage is placed on sea floor, either completely submerged
or emerging above the sea level. Materials of construction may
be either steel or prestressed concrete or a combination of both.
• Semi-submerged Storage & Loading Buoy is usually a slender
vertical floating body. A typical example is the “Spar-Type
Structure”. This type of structure is anchored to the sea bottom
with a swivel joint.
General overview of production schemes is required in order to
place storage in its proper context. Oil production has three main
components:
- Production
- Storage &
- Loading
Some production schemes combine two or more of parts. For
example, a concrete gravity platform may combine production &
storage or a SPAR Type structure may combine storage & loading.
4

5. Differentiated system i.e. a system with 3
separate components of production, storage
& loading, has following advantages:
• Separate structures may be mobilized at the
same time
• Replacement of any one component is easier
• The risk of system downtime is reduced
Integral system consists of all three production
system components in one structure.
5

6. Floating Storage – Marginal Fields
Oil fields in water depths of up to about 1000 ft. may
use any of these storage systems:
• Converted tankers or barges
• Floating Production Storage Off-Loading System
(FPSO)
• Converted Semi-Submersible Drilling Rig
For depths greater than 1500 ft; storage systems using
a converted or special purpose semi-submersible rig
may be used.
6

7. Separate Systems in Large Oilfields
Structural System Advantages Disadvantages
PRODUCTION
Medium Water Depth (200 to 300 m)
Conventional Steel Jackets -Many experiences
-Large payload
- Long installation time
Concrete Gravity -Successful experiences in
N. Sea
- Large payload
-Short installation time
- Construction site (deep
draft)
Steel Gravity -Large payload
-Short installation time
- Construction site (deep
draft)
Articulated Column - Compliant Structure -Smaller payload
-Critical ‘U’ – joint at base
7

8. Structural System Advantages Disadvantages
Deep Water Depth (> 500 m)
Guyed Tower -Compliant Structure - Relatively small payload
Purpose-Built S - Submersible -Compliant Structure
-Relatively easy installation
- Riser design
Tension Leg Compliant Structure -Relatively small payload
-Tension piles or large
gravity base
-Riser design
Storage (Medium & Deep
Water)
Converted Tanker - Cheap & easy installation - Unstable in rough weather
Spar Buoy - Stable in rough weather
Submerged Tank - Stable - Sensitive to sea floor
conditions
8

9. Structural System Advantages Disadvantages
Loading (Medium & Deep
Water)
Single Anchor Leg Mooring -Relatively easy mooring
- Riser protected in column
Catenary Anchor Leg
Mooring (CALM)
- Relatively easy mooring - Riser design
Direct Tanker Mooring to
Storage Tanker (alongside
or in tandem)
-No separate structure
-Required for loading
- Relative motion of two
tankers
9

10. SYSTEM FOR MARGINAL FIELDS
PRODUCTION
Medium Water Depth (200
to 300 m)
Converted Tanker - Successful
-Used as storage system
- Sensitive to weather
Converted SS Drilling Rig - Successful
Deep Water Depth
(>500m)
Converted or purpose built
SS rig
-Compliant structure
-Relatively easy installation
- Riser design
Tension Leg -Compliant structure
- Field tested
- Relatively small payload
-Tension piles or large
gravity base
- Riser design
10

11. INTEGRAL SYSTEM (BUILT WITH OIL STORAGE AS PART OF STRUCTURE)
Medium Water depth (200 to 300 m)
Concrete gravity -Successful in N Sea (no
direct tanker loading)
-Large Payload
- Construction site (deep
draft)
Steel Gravity - Successful in Loango (no
direct tanker loading)
- Large payload
- Construction site (deep
draft)
Articulated Column - Compliant structure -No field experience
-Critical ‘U’ joint at base
-Relatively small payload
Deep Water > 500m
Guyed Tower -Compliant structure
-Field tested
- Relatively small payload
Concrete Floating (SS) - Compliant structure -No field experience
- Large mooring force
-Relatively small payload
11

12. 12
Tension Leg
a. Storage at Base
b. Storage at midway &
legs attached to base
c. Storage at legs & no
base but moored with
wire rope
-Compliant structure
- Field tested (X)
-
-Smaller wave force
- Intermediate support at
riser
-Improved dynamic
characteristics
- Conventional catenary
mooring system
-Relatively small pay load
-Riser design
- Tension piles or large
gravity base

13. Converted Barge
It is usually a converted
tanker. It is moored so it is
free to swing around 360 arc.
The barge will weather- vane
as directed by wind & wave
forces.
Converted barge is moored to
a monobuoy anchored to
bottom of sea. First mooring
system was of calm type. This
led to development of ‘single
buoy storage system’ using
rigid arms for permanently
mooring vessel or barge to
buoy. Other variations
include use of “SALM” with a
rigid yoke connecting vessel.
Fig. shows typical Barge
operation – Production
Facilities
13

14. Floating Production Storage & Off-loading System (FPSO)
Typical FPSO is fully integrated, in as much as production from one or
more wells usually enters processing train positioned on deck of
tanker through lines crossing a rigid yoke that connects tanker to
moored buoy. Several stages of oil & water separation using
process vessels designed for service in moving environment
separate O, G & W. Part of gas is used for power tanker
boilers/generators. Produced water is collected in dedicated
tanker compartments for treatment & disposal. Export quality
crude is then metered onboard & transferred via tandem
offloading to shuttle tankers.
System may use a tanker or barge to serve as production, storage &
offloading vessel. This system has all of the major service
components of a conventional SS platform.
Fixed Storage consists of mainly the following:
• Differentiated Fixed Storage steel or prestressed concrete tank
• Integrated submerged or prestressed concrete tanks i.e. part of a
production platform unit
14