An example of initial desing of a Platform supply vessel. Derive the main dimension of a PSV base on the operational specifications

 

 

1. Cargo Capacity

1.1. Deck cargo

The cargo on deck should not exceeded 2450 tonnes.
The center of gravity of the deck cargo is 1 m above the main deck; thus, the side walls of the deck should be 2,00 m to safely accommodate this load.

1.2. Tank Cargo

The PSV will transport a variety of liquids. The volume, density and total mass of each liquid is shown on the table below. The total tank capacity should be 3150 m³. The tanks are located below the deck. Drawing 4 shows the layout of the tanks, and the location of the discharge point for each liquid

 

Tank CargoVolumeDensityWeight
Fresh water 600 m3 1000 600 t
Fuel 800 m3 860 688 t
Liquid mud 400 m3 2800 1120 t
Brine 300 m3 1500 450 t
Base oil 150 m3 850 128 t
Glycol 100 m3 1120 112 t
Methanol 100 m3 800 80 t
Cement (dry) 100 m3 2400 240 t
Ballast/drill water 600 m3 1000 600 t

 

1.3. Deadweight

Deck cargo: 2450 tonnes

Tank cargo: 4018 tonnes

Deadweight: 6468 tonnes (Tank cargo + Deck cargo)

2. Vessel main dimensions

The main vessel dimension are summarized in the table below. The derivation of though dimensions follows next

Length (L): 84 m
Beam (B):r 20 m
Max Draft (T): 6.2 m
Min Draft (T): 1.7
Depth to main deck (D): 8 m
Clear deck length 60 m
Clear deck area 1164

 

2.1. Deck Dimensions

The deck should be long enough for storing 4 stacks of pipes.
The pipe length is 40 ft (12,20 m). A small allowance of 0,30 m is considered for the practically uneven stacking of pipes (fig. 01). Thus, we assume that the length of a stack is 12,50 m.

The clearance between the stacks is 2,00 m
Thus, the total length of the deck should be 60,00 m
and the clear deck area is 84*(20-0.6) = 1164 m²

2.2. Max draft

For the maximum draft we assume that the tanks are full (4018 tonnes) and the deck has the maximum allowable load (2450 tonnes).

First we assume a value T for the draft and we calculate the corresponding lightweight and displacement. For that displacement we calculate the draft. We repeat the process until the assumed draft is equal to the calculated

Lightship weight = 1562 tonnes (Lightweight = L x B x T x 0.15)
Deadweight = 6468 tonnes (Deadweight = Cargo + Fuel + Ballast)
Displacement = 8030 tonnes (Displacement = Lightweight + Deadweight) 

Δ=(1025/1000)·L·B·T·0,75 (tonnes) Assuming block coefficient Cb=0,75
Max Draft = 6.2 m (greater than 6 m) ?? Less not possible

GM=KB+BM-KG 3.54 m quite high ??
KB = 3.11 m Distance of CoB from keel (Draft/2)
BM = 5.96 m I/Displacement
KG = 5.53 m Distance of CoG from keel (2450*9+4018*4+1562*4)/8030

2.3. Min draft

For the minimum draft we assume that all tanks are empty except from the ballast tanks and that the deck cargo is zero.

First we assume a value T for the draft and we calculate the corresponding lightweight and displacement. For that displacement we calculate the draft. We repeat the process until the assumed draft is equal to the calculated

Lightship weight = 1562 tonnes (Lightweight = L x B x T x 0.15)
Ballast = 600 tonnes
Displacement = 2162 tonnes (Lightweight + Ballast)
Min Draft = 1.7 m

3. Propulsion and positioning

Minimum loaded speed: 13 knots (7m/s)

In order to specify a propulsion system that can achieve this minimum speed and has the necessary parts for DP, we refer to similar vessels (in dimensions and displacement) and checked their propulsion system.

Such systems should have the following parts:

  • Main Engine set 1: 2 x 2000 kW-1800RPM
  • Main Engine set 2: 2 x 2000 kW-1800RPM
  • Total kW: 8000 kW

Propulsion Aft: 2 x 2500 kW, frequency driven electric motor, fixed pitch

Bow tunnel thruster: 1 x 900 kW, frequency driven electric motor, fixed pitch

Bow Azimuth swing-up: 1 x 900 kW, frequency driven electric motor, fixed pitch

Main Generators set 1: 2 x Siemens 2028kVA/440V/60Hz

Main Generators set 2: 2 x 2028kVA/440V/60Hz

Emergency Generator: 1 x 200kW, 3x440V, 60Hz

DP-system-control-monitorA DP system is required for the positioning of the vessel. Such a system comprises of position reference sensors, wind and other sensors, motion sensors and gyro compasses, computer system to calculate the required steering angles and thrusters output and the thrusters as specified previously.

4. Safety features

Two features that can improve the safety of the vessel are:

4.1. Dual redundant dynamic positioning

Such a system will greatly reduce the risk of the vessel crashing on a platform due to a failure at a part of the DP system. In case of failure the system will immediately switch-over to the standby part.

Such a dual system can have:
- dual DP controller unit
- dual operator stations
- dual high-speed data network
- dual interface units for the connection of sensors and position reference systems

4.2. Advance video and monitoring system

Such a CCTV system will transmit video from outside and inside cameras which can be used to:
- assist the captain in handling the vessel more accurately
- monitor watertight doors
- observe potential hazards and inform the crew