1. Introduction

This is a preliminary concept design of an FPSO supposed to explore the oil fields in Norwegian Sea.
The following key points were considered while designing this project:

  1. Extra space on deck has been kept spare for future modules for change in production or utilities.
  2. Lay down area and H2S removal modules are kept well within range of main crane radius.
  3. Keeping flare module far away from accommodation area and helipad is kept as close as close to accommodation area.
  4. The flow lines at sea bed are manifold in order to reduce the number of risers coming to turret.
  5. Mooring is done through external turret in order to tackle the harsh weather effects of Norwegian Sea.

2. Field data

  1. Recoverable oil = 480 million bbls
  2. Plateau production = 83000 bopd
  3. GOR = 425 cubic feet/barrel
  4. Crude gravity = 30 degrees API at 65 degrees F
  5. Number of producing wells = 16
  6. Number of water injection wells = 4

3. Environmental data:

  1. Water depth = 800m
  2. 100 year wind velocity = 52m/s from NW
  3. Wave condition = defined by the following scatter diagram:
  4. 100 year surface current =1.2m/s from
  5. Annual downtime for maintenance = 10 days
  6. Weight of the process plant = 10300 t

1. Field life:
Considering a typical production profile and 355 days of production per year, the following data has
been computed :
year
oil per day
(bopd) Bbl of oil per year
1 27500 9762500
2 60000 21300000
3 70000 24850000
4 78000 27690000
5 83000 29465000
6 83000 29465000
7 83000 29465000
8 83000 29465000
9 83000 29465000
10 80000 28400000
11 75000 26625000
12 70000 24850000
13 65000 23075000
14 60000 21300000
15 55000 19525000
16 50000 17750000
17 45000 15975000
18 40000 14200000
19 35000 12425000
20 30000 10650000
21 25000 8875000
22 20000 7100000
23 15000 5325000
24 8000 2840000
Total Production 469842500
**we included 355 production days because of 10days downtime
As shown we can see that we have the maximum production during five years and a near value for
other two years. Then the production decreases by 5000 bopd. This behaviour is shown in the
following graph:
The production is stopped after 24 years, having recovered 469842500 barrels of oil. The production
is stopped at this point because we are producing 8000 bop, value close to the 10% plateau
production.

2. Storage capacity:

The required storage capacity of the FPSO, based on the plateau production and a shuttle tanker
turnaround of 13 days, is calculated in the following way:
CAP_fpso = Prod_max x Shuttle_ta + 10% reserve .
Considering that the tanks cannot be filled more than the 98%, we have:
Storage capacity = plateau production * 1.1*shuttle tanker turnaround (days)*1.02 = 1210638
barrels
This quantity can be converted in cubic meters using the following:
Storage capacity (m3) = storage capacity (barrels) * 0.159 = 192491m3
0
10000
20000
30000
40000
50000
60000
70000
80000
90000
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
graph
graph
3. Main dimensions:
To derive the main dimension we have referred ourselves to existing vessels, but considering the
storage capacity, taking 30m as the maximum length for each tank.
These graphs show tankers main dimension, but they are also suitable for FPSOs’ due the similarity of
shapes.
We knew that we needed 192491m3 of storage. This volume has to be filled with oil, which specific
gravity can be calculate by mean of the following formula:
Specific gravity =
= 0.876 tons/m3
To calculate the deadweight the following procedure has been adopted:
The steel weight has been calculated using the following formula:
SW = WB * L * B * D
Max volume to store crude including 10% reserves 192491 m^3
**max 1day production x 13days x 10%reserve x 1.02(for2% extra volume) x 0.159(for bbl to m^3)
Specific gravity of oil 30 deg. API at 65 F 0.876 tons/m^3 {141.5/(131.5+Degrees API)}
Total weight of the crude 168623 t
Water treatment 20000 t
Fuel for generators 15000 t
H2S removal 500 t
Miscellaneous 4000 t
TOTAL DEADWEIGHT 208123 t (168632+20000+15000+500+4000)t
Weight of topsides process plant 10300 t (given)
Ship Data
The main dimensions of the ship are calculated with an iterating procedure:
The FB value has been taken 6.4m because of the harsh environmental conditions. The weight of the
topsides has been considered as well in this calculation.
The B/T ratio amounts to 2.83, falling into the expected range.
Due to the hostile environment in the North Sea, a ship shape with a high freeboard at the bow has
been decided for the FPSO in order to reduce green water on the deck and slamming effects on the
bow.

4. Tanks:

In total 7 cargo tanks have been placed on the vessel. Each cargo tank has a length of 30m and is
divided into 5 different parts: a central part and 2 lateral part on each side. The arrangement is
shown in the figure below:

  • Length, L 300 m
  • Depth, D 27 m
  • Breadth, B 50 m
  • Steel weight , Sw 58900 t {(0.12 x LBD) + topside steel wt}
  • Dead weight ,Dw 208123 t ( crude + treated water + fuel + H2S + misc)
  • Total ship wt, Ts 267023 t ( Sw + Dw)
  • Volume Displaced, V 260510.2 (Ts/1.025)
  • Max Draft , Tmax 21 m (V/LxBx0.876)
  • Free Board , Fb 6.1 m (Fb > 6.0m) OK
  • B/Tmax ratio 2.390 ( 2.833 < 3) OK

Ship Dimensions

The central and the lateral tanks are used for crude storage, though the fore tank (i.e. Tank7) is
divided into 2 parts, corresponding to the 70% and to the 30% of its dimensions. The 70% is used to
store oil, while the remaining 30% is used as a slop tank.

There are two main ballast tanks: the fore peak tank and the aft tank. To achieve the 100% of ballast
on boar these two are filled with water and also the double bottom and the wing tanks. This
arrangement has been chosen in order to give enough stability to FPSO in case we have no crude
loaded. If we have 100% ballast on board and only the 10% of crude, considering no water treatment
during ballasting, the following conditions would be experienced:

5. General arrangement

The general arrangements are shown in the annexure File. The turret has been positioned on the
external part of the bow for 2 main reasons:
1) It is easier to weathervane, with a minimum use of the thrusters
2) Less longitudinal reinforcement is needed
The living quarters have been positioned towards the stern for 2 main reasons:
1) Lower vertical motions
2) Large distance from the swivel (area of high hazards)
Thrusters are provided to allow to helicopter to land without being disturbed by smoke. The
lifeboat is placed close to the living quarters to provide a fast escape route in case of emergency.
The flare is at the bow, as farer as possible from the living quarters. No ventilation is needed
because enough height is provided to expel dangerous gases. The reduced number of risers is
achieved by manifolding. The manifolds used are the template type, to have higher protection from
fishing or dropped objects.
The risers used are flexible to withstand the ship motions

6. Height of the bow

The height of the bow has been estimated considering the maximum significant wave height, taken
from the scatter diagram. This wave height amounts to 15m, therefore at the bow the FB has been
increased of 7,5 metres, also providing this part with drainage holes to discharge the green water
present on deck as shown in the Longitudinal View drawing provided in the annexure. This particular
height has been chosen because we think that with this FB we have enough protection, also
considering that the topsides modules are almost 4 metres above the main deck. The support
structure of the topsides is a truss structure.

7. Natural period of roll

The first step consists of determining the position of the COG. Its position has been determined in
the fully loaded condition, considering separately hull, tanks and topsides.
To calculate the position of the COG of the lightship the following approximation has been used:
Vertical position of the COG of the steel hull = 0.057 DS
The longitudinal and transversal centre of gravity of the steel hull has been calculated considering
the idealized case of perfect symmetry, therefore:
YG =0
XG= amidship
DS = D+ 0.008D we use DS instead of D to take into account the sheer and the hatchway volume.
The centre of gravity of the tanks has been calculated per each tank and then the resultant centre of
gravity has been taken out. Regarding the 7th tank it is, in a first approximation, considered 100% full
and the contained liquid is considered to have the same density of oil. The procedure adopted is