Bechtel Process Engineers rule of thumb and CE Process checks
10 MISCELLANEOUS
A. Large Production & Processing Platforms 1. Specific Process Considerations
The following process considerations are relevant to the platform layout:
a. the layout should reflect a logical progression of the process. This should then minimize major pip ing requirements. Other equipment with small diameter piping, can be located where necessary e.g. glycol regeneration package.
b. Related equipment should be modularized to allow maximum hook up and precommissioning to be achieved, e.g. compressor with associated knock out drum and aftercooler.
c. Requirements for pump NPSH should be established e.g. export pumps.
d. Pump suction lines should have no vapor pockets and be of minimum length.
e. Systems based on gravity drainage should be identified.
f. Requirements for future equipment should be identified. to allow incorporation into initial layout.
g. Allowances for straight piping runs should be made where appropriate e.g.
compressor suction lines, metering runs.
h. Compressor suction lines should have no liquid pockets.
i. Two phase lines should be of minimum length and with no pockets to minimize potential slugging.
j. Two phase lines from coolers should free drain to the knock out drum.
k. Vapor lines in wet Carbon Dioxide or corrosive service should have no liquid pockets.
l. Filter separator/KO drums located upstream of TEG contacting/acid gas treating units should be located close to prevent hydrocarbons or millscale from entering the contactor.
m. Flare lines should slope to the KO drum with no liquid pockets.
n. Locate control valves in bubble point liquid service so that there is no possibility of flashing at the valve inlet.
2. Preliminary Space Estimate
On the assumption that a modularized concept is used, the following factors can be used to assess deck area requirements for preliminary layout studies.
3. Seawater System:
A. Carbon Steel Service
Although Carbon Steel is fine for service in low velocity seawater at ambient temperatures, the corrosion rate increases rapidly with temperature and agitation.
At 122 ° F (50 ° C) and above, the corrosion rate in agitated seawater is greater than 50 mils/year.
At velocities over 15 ft/sec, turbulence may greatly accelerate the corrosion rate by eroding away the protective film. This occurs frequently at heat exchanger tube inlets, U bends, and piping elbows.
At more extreme velocities, erosion – corrosion will occur. At 39 ft/sec, the corrosion rate reaches over 200 mils, year.
B. For Seawater Waterflood systems:
Typical specifications to prevent formation plugging with solids, downhole corrosion, and bacteria growth are as follows.
1. Solids – Remove 97% of all solids greater than 5 microns size.
2. Oxygen – Remove to a maximum level of 10 ppb(wt) 3. Sterilization – Chlorination and/or UV sterilization required.
4. Portable Water systems:
a. Based on Ekofisk experience, provide the following potable water quantities for preliminary design:
1. 200 to 250 liters (53 to 66 gallons) per day per person for personnel who live on the platform.
2. 100 to 150 liters (26 to 40 gallons) per day per person for day workers and visitors who do not live on the platform.
b. Based on Ekofisk experience, provide the following potable water storage preliminary design:
1. For platforms with potable water makers, provide a minimum of 2 days storage.
2. For platforms which depend solely on hauled potable water, provide a minimum of 3 days storage.
Module type Area Ratio*
Set by well pattern/drilling requirments
*Ratio of Major Equipment footprint area to total module area.
B. Water and Steam Systems
1. Approximate break point for steam pressure at which silica becomes a problem with vaporization and deposition on turbine blades is at 500 psig.
2. The evaporation rate on a cooling tower is dependent on the amount of water being cooled and temperature differential. For each 10 ° F temperature drop across the tower, 1 % of the recirculation rate is evaporated. In other words, 0.001 times the circulation rate in gpm times the temperature drop equals the evaporation rate is gpm.
C. Economics
1. Capex Ratio Exponents
For Processing Plants and Ancillaries Same No. of Units
COST 2=(SIZE OR CAPACITY 2/SIZE OR CAPACITY 1)0.5(COST1) Unit number change required for new capacity.
USE 0.6 exponent
Infrastructure (Camps, Warehouses, Maintenance facilities) USE 0.3 exponent
2. Capex Factors From Major Equipment Cost Installed Cost
Onshore 2.5 x (Major equipment cost) Offshore 5.0 x (Major equipment cost) (excludes deck and jacket costs) 3. Annual Operating Costs
[excludes fuel and depreciation]
Onshore; 3% of Capital cost Offshore; 5% of Capital Cost
4. Capex (Total) Remote Area LNG Plants
[One Train} $ 2x109 per 2x106 MTY [1990 BASIS]
Note: Cost Reductions via technology offset regulatory increases.
Use 5%/Yr esc. in general costs.
MTY = Metric Tonnes/Year D. Hydrates
1. Hydrates generally form at 50 to 60 ° F.
2. Expect 1 ° F depression for each 1% methanol in liquid drainage. (Methanol content may be estimated with a hydrometer)
3. Hydrate Control
Add a margin of 50% to calculate hydrate inhibitor injection rates.
4. Typical hydrate inhibitor concentrations:
MEG: 70 – 80 wt% at inlet and 60 wt% in solution outlet MeOH: 98 wt% at inlet and 90 wt% in solution outlet
5. Glycol inhibitor loss estimate is 1 lb/MMSCF plus 200 ppm (v) in liquid hydrocarbon.
6. MeOH will melt hydrates already formed. MEG will not.
E. NGL Expander Plants
If the CO2 in the feed gas to the cryogenic plant is in excess of 0.25 mol %, be sure to check for CO2 solidification in both the liquid and vapor phases immediately downstream of the expander and in the top four stages of the demethanizer.
F. Miscellaneous Plant Systems
Instrument air – As a preliminary estimate for instrument air requirements for feasibility study design, use 0.5 to 0.75 scfm per control instrument.
G. Liquified Natural Gas (LNG) Plants
1. For preliminary estimates of LNG plant design inlet volume for a premised LNG delivery to ships for transport, use a 93 plant availability factor.
2. Mercury occurring naturally in some natural gas streams is extremely corrosive to aluminum heat exchangers used extensively LNG plants processes. Plan to check for mercury in feed gas up front in any project.
H. Gas Processing – Simulation Guidelines
Include a 5 ° F margin from the sales contract hydrocarbon dewpoint temperature to accommodate uncertainties in process simulator to predict dewpoint.
I. Offshore Pipeline Gas Specifications
1. Hydrocarbon dewpoint temperature sho uld be 10 – 20 ° F below minimum operating temperature in pipeline for the operating pressure range to prevent liquid drop out.
2. Water dewpoint temperature should be 10 – 20 ° F below minimum operating temperature in pipeline for the operating pressure range to prevent free water drop out.
3. Maximum allowable platform discharge temperature is typically 90 – 125 ° F.
J. Offshore Crude Oil Specifications
1. Max RVP 10 psia for export to tanker/storage.
2. Salt content 70 to 200 mg/1 (sales spec to pipeline) 3. Water max 2 wt% (sales spec to pipeline)
4. BS&W 0.5 vol% maximum ; 0.1 vol% average (sales spec to pipeline) K. Wind Loadings
Flat Surfaces P=0.004 x V2
Cylindrical Members P = 0.6 x 0.004 x V2
N. Platform Deflection
Maximum deflection of platform due to wind and sea is:
deflection = Span / 400
Where: span = distance from cellar deck to mud line, Ft.
O. Kinetics
For a second order reaction in a constant volume reactor, if 95% of the reactants react in one time, it will require 20 more time units to react 95% of the remaining reactants.
P. Storage, Vessel Capacity
A. Vessel Capacity: Capacity (gallons)=(Diameter, ft)2+2) x Length, inches.
Q. Pipeline Volume:
(Diameter of pipe, inches)2 = Barrels/1000 feet (Results are approximately 3% high) R. Pressure Vessels
In general, the maximum operating pressure (MOP) of a process pressure vessel is established from the maximum internal or external pressure at which the vessel operates while fulfilling it’s normal function.
In general, for vessels subject to internal pressure only, the maximum allowable working pressure (MAWP) of a process vessel can be arrived at by adding the greater of 10% or 10 to 25 psi to the maximum operating pressure.
Economic L/D ratios for pressure vessels generally fall in the 2 to 5 range where L = shell seam length and D = inside diameter, both in feet.
S. NACE Requirements
“Material shall be selected to be resistant to Sulfide Stress Cracking (SSC) or the
environment should be controlled if the gas being handled is at a total pressure of 65 psia or greater and if the partial pressure of H2S in the gas is greater than 0.05 psia."
T. Pressure Waves (e.g. water hammer) magnitude of PW in lbs f/in2 psi
PW = a x d x vd/(144 x g)
a = velocity of sound in the fluid fps d = density of fluid, lb mass/ft3 vd = velocity decrease, i.e. velocity
before change less the velocity after change, FPS
g. = conversion factor = standard acceleration due to gravity, 32.174 fps2
U. Insulation Types in cold service, since it is water absorbent.
General material.
Mineral Wool - 0.35 0.52 - 60 to 1,900 Noncombustible
0 0
70 (high) Good workability Cellular Glass 0.27 0.42+ 0.65 - -290 to 1,200 Noncombustible
0 0.7 (low) Significant deterioration in acids and in various organics. Generally, is replacement for Polystyrene 0.18 - - 0.23 at 75 °F -40 to 275 Combustible,
although sometimes self-extinguishing
- (low) Excellent workability at low temperatures (below freezing point of water)
Polyurethane 0.14 0.25 - - -100 to 220 May be a fire risk 1.6 (low) Significant deterioration in acids and various organics,
but resists water and vapors.
75 (high) High water absorbency, but will dry out. Good workability
Perlite - 0.47+ 0.58 - 60 to 1,500 Noncombustible
0 0
16 (medium) Good workability
°
+Manufactures’ data
V. Absolute Pressure of Atmosphere at Height ‘H’ feet above Sea Level P = P1 (1 – 0.00000687H)5.256
P1 = pressure at sea level – pisa
Density: W = W1 (1-0.00000687H)4.256 W1 = density of air at sea level