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DNV-OS-A101 Safety Principles and Arrangements
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SECTION 2
Design Principles and Accidental LoadsSec.2
A. General
Sec.2
A 100 Objective
Sec.2 A
101 Application of these design principles is intended to establish
an acceptable level of safety, whilst promoting safety improvements
through experience and available technology.Sec.2
A 200 Application
Sec.2 A
201 The principles and requirements shall be applied throughout
the project lifecycle, beginning in the concept phase, and reviewed
and updated through detailed design and construction. The principles
shall also be applied with respect to subsequent modifications.Sec.2
B. Design Principles
Sec.2
B 100 Main principles
Sec.2 B
101 The following general principles shall be applied throughout
the concept and design phases of the unit or installation.
Sec.2 B
102 The unit or installation shall be designed and constructed
with sufficient integrity to withstand operational and environmental
loading throughout its lifecycle.
Sec.2 B
103 Systems and structures shall be designed with suitable functionality
and survivability for prevention of, or protection from, design
accident events affecting the unit or installation. Refer also DNV-RP-C204
- Design Against Accidental Loads.
Sec.2 B
104 Effective escape, shelter and evacuation facilities shall
be provided to safeguard all personnel, as far as practicable, at
all times when the unit or installation is manned.Sec.2
B 200 Additional requirements
Sec.2 B
201 In meeting the main design principles in 100, the following
requirements shall be applied:- The design shall be sufficiently
robust to tolerate at least one failure or operator error without
resulting in a major hazard, or damage to the unit or installation.
- Suitable measures shall be provided to enable timely
detection, control and mitigation of hazards.
- Escalation to plant and areas that are not affected
by the initiating event shall be avoided.
Sec.2
C. Design for Accidental Loads
Sec.2
C 100 General
Sec.2 C
101 The provisions given in C and D are
based on international practice, experience with offshore designs and
results obtained by various risk assessments carried out on offshore
units. For relatively standardised designs (e.g. typical drilling
units) the prescriptive requirements given in these standards are
intended to anticipate the most likely hazards which may be encountered.
Sec.2 C
102 Each project shall, however, consider the applicability of
the generic load approach used in D with respect
to the intended application and operation in order to identify,
where applicable, hazards associated with non-standard design or
application.Guidance note:
For example generic collision load is based on a supply vessel
size of 5000 tons. In applications where supply vessels are of much
larger size this will need to be accounted for in defining the collision
load.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.2 C
103 For complex or non-standard applications a more comprehensive
assessment shall be carried out. Guidance on carrying out such an
assessment is given in Appendix B.Sec.2
D. Generic Design Accidental Loads
Sec.2
D 100 General
Sec.2 D
101 The prescriptive requirements given here and elsewhere in
DNV offshore standards are intended to take account of accidental
events which have been identified through previous risk studies
and through experience.
Sec.2 D
102 The requirements are based on consideration of the integrity
of the following main safety functions:
| — | integrity of shelter areas |
| — | usability of escape ways |
| — | usability of means of evacuation |
| — | global load bearing capacity. |
Sec.2 D
103 The selection of relevant design accidental loads is dependent
on a safety philosophy considered to give a satisfactory level of
safety. The generic loads defined here represent the level of safety
considered acceptable by DNV, and are generally based on accidental
loads affecting safety functions which have an individual frequency
of occurrence in the order of 10-4 per
year. This will normally correspond to an overall frequency of 5
x 10-4 per year as the
impairment frequency limit.
Sec.2 D
104 The most relevant design accidental loads are considered to
be:| — | impact loads, including dropped
object loads and collision loads |
| — | unintended flooding |
| — | loads caused by extreme weather |
| — | explosion loads |
| — | heat loads. |
Sec.2 D
105 This standard is intended to address the above design accidental
loads. Other additional relevant loads that may be identified for
a specific design or application will need to be separately addressed.Sec.2
D 200 Dropped objects
Sec.2 D
201 It is assumed that lifting arrangements comply with Sec.3 F with regard to location of
cranes and lay down areas and with respect to lifting operations
over pressurised equipment.
Sec.2 D
202 It is assumed that critical areas are designed for dropped
object loads as defined in 203 and 204.Guidance note:
Typical critical areas normally include, accommodation, workshops,
storage areas for pressurised gas, areas with hydrocarbon equipment.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.2 D
203 The weights of the dropped objects to be considered for design
of the structure are normally taken as the operational hook loads
in cranes.
Sec.2 D
204 The impact energy is normally not to be less than:| E | = |
|
| M | = | mass of object (tonnes) |
| g0 | = | 9.81 m/s2 |
| H | = | drop height in air (m) |
Guidance note:
The impact energy at sea level is normally not to be taken
less than 5 MJ for cranes with maximum capacity more than 30 tonnes.
The impact energy below sea level is assumed to be equal to the
energy at sea level.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.2
D 300 Collision loads
Sec.2 D
301 The kinetic energy to be considered is normally not to be
less than:| — | 14 MJ (Mega Joule) for sideways
collision |
| — | 11 MJ for bow or stern collision |
corresponding to a supply vessel of 5000 tonnes displacement
with impact speed v = 2 m/s.
Sec.2 D
302 The impact energy is given as:
| M | = | displacement of vessel (t) |
| a | = | added mass of vessel, normally assumed as 0.4
M for sideways collision and 0.1 M for bow or stern collision |
| v | = | impact speed (m/s). |
Sec.2 D
303 It is assumed that the unit or installation is not operating
in a shipping lane. In such case a more detailed assessment of relevant
collision loads shall be carried out.
Sec.2 D
304 Where a unit is operating in tandem with a shuttle tanker,
special precautions shall be taken to minimise possibility of collision,
or the design is to take account of collision loads.Sec.2
D 400 Unintended flooding
Sec.2 D
401 The design sea pressure on watertight subdivisions (bulkheads
and decks with compartment flooded) shall for accidental damaged
condition be taken as:
| hb | = | vertical distance in m from the load point
to the damaged waterline. |
Sec.2
D 500 Loads caused by extreme weather
Sec.2 D
501 Characteristic values of individual environmental loads are
defined by an annual probability of exceedance equal to
10-2 (for Ultimate limit states,
ULS) and 10-4 (for Accidental
limit states, ALS).Sec.2
D 600 Explosion loads
Sec.2 D
601 Requirements given in this standard are applicable to hydrocarbon
gases. Where hydrogen, ethylene or acetylene is used in large quantities
special consideration shall be given to explosion loads.Guidance note:
The overpressure values quoted in this section are based on
studies with ethane, propane, butane, condensate and crude oil vapour.
Methane or evaporated LNG values may be slightly lower.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.2 D
602 In a ventilated compartment the explosion load given by the
explosion overpressure and duration is mainly determined by the
relative ventilation area and the level of congestion.Guidance note:
For compartment volumes of approximately 1000 m3 and relative ventilation
area of about 0.5, ignition for stochiometric gas mixtures is expected
to lead to pressures of approximately 100 kPa (1 barg) in cases
with medium level of congestion. High level of congestion may increase
the pressure with a factor of 2 to 3. Larger volume also tends to
increase the pressure.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.2 D
603 The design overpressure in a ventilated shale shaker room
with volume less than 1000 m3,
with moderate congestion, may be taken as 200 kPa (2 barg), combined
with a pulse duration of 0.3 s, unless a more detailed assessment
is carried out.
Sec.2 D
604 The design overpressure in connection with explosion on open
drill floor area may be taken as 10 kPa (0.1 barg), combined with
a pulse duration of 0.2 s, unless a more detailed assessment is
carried out.Guidance note:
Where an explosion passes through a large number of stored
pipes (more than 10 x 10), local overpressure may rise considerably.
Stored pipes should therefore be stored with a minimum air gap between
pipes.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.2 D
605 Where a more detailed assessment is carried out in evaluating
the explosion load in a vented compartment, the considerations given
in 606 to 609 shall be accounted
for.
Sec.2 D
606 The vent area, Av,
can be taken as the sum of free opening areas and blowout panel
areas (i.e. light weather cladding) provided the static opening
pressure of the panels is less than 5 kPa (0.05 barg). The relative vent
area, a, is given as Av or
Volume2/3.Guidance note:
The duration of the positive phase pressure pulses is expected
to vary from 0.2 s for fairly open compartments to 1 s for quite
closed compartments. The structural response may in many cases be
estimated on a static basis.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.2 D
607 If panels or walls are intended to give explosion relief by
failing, a maximum pressure up to 2 to 3 times their static release
pressure can still be expected within the compartment.Guidance note:
This is the case only if the total ventilating area of those
panels and walls is large enough to be the dominating factor. For
large and congested compartments local pressures may give a greater
load.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.2 D
608 For compartments where the length to diameter ratio, L/D,
is greater than 3, the long flame acceleration distance available
tends to result in higher pressures. The diameter can be estimated
as D = ÖA where A is the smallest cross-sectional
area. L is the greatest dimension of the compartment.
Sec.2 D
609 Where it is possible for an explosion to propagate from compartment
to compartment and for tunnels and chutes where explosion venting
can be foreseen at one end only, detailed investigations shall be
carried out.
Sec.2 D
610 Design explosion overpressure in a completely enclosed compartment
handling hydrocarbons (e.g. STP or STL rooms) shall be subject to
special attention. The following principles shall apply:- Generally, bulkheads that need
to remain intact after an explosion, e.g. towards storage tanks,
shall be designed for an overpressure of 400 kPa (4 barg) and a
pulse duration of 1 s. This assumes that the safety features required
elsewhere in this standard are in place, i.e. control of ignition
sources through area classification, provision of suitable ventilation,
gas detection and emergency shutdown systems.
- If a lower explosion design pressure is used this must
be justified through more detailed assessment.
Sec.2 D
611 It is assumed that the process plant is designed with a suitable
blowdown system and deluge system in accordance with a recognised
code (e.g. DNV-OS-E201), in order to avoid possible pressure vessel
rupture.
Sec.2 D
612 Doors for pig launchers shall be oriented in a direction where
inadvertent opening would result in minimal damage.
Sec.2 D
613 For process areas on open deck covering a relatively small
area (e.g. 20 m x 20 m) an explosion overpressure of 30 kPa (0.3
barg) may be used in design combined with pulse duration of 0.2
s.Guidance note:
The overpressure values quoted in 613-615 for process plant
on deck assume that the plant area is congested.
Volume blockage ratio (possible blocking volumes vs. total
volume considered) may be used as a measure of congestion.
Volume blockage ratio less than 0.05 may be considered as
not congested.
The values quoted also assume that the process area is not
enclosed by walls or roof.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.2 D
614 For process areas on open deck covering a medium size area
(e.g. 20 m x 40 m) an explosion overpressure of 100 kPa (1.0 barg)
may be used in design combined with pulse duration of 0.2 s.
Sec.2 D
615 For process areas on open deck covering a larger footprint
than in 614 an explosion overpressure of 200 kPa (2.0 barg) may
be used in design combined with pulse duration of 0.2 s.
Sec.2 D
616 Design shall as far as possible aim to minimise the possibility
of gas build up.Guidance note:
Where a solid process deck is used, the location of possible
leak sources below this deck should be minimised.
Similarly, for internal turret designs the number of leak
sources within the enclosed sections should be minimised.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.2 D
617 The following items shall be designed to withstand the specified
design overpressure:| — | protective walls |
| — | structures capable of blocking escape ways |
| — | safety systems (and control lines) |
| — | structure supporting hydrocarbon containing equipment. |
Sec.2 D
618 Typical design values are summarised in Table D1. This shall
be read together with the reservations in the text of D.Guidance note:
Accurate predictions of explosion overpressures are dependent
on numerous variables and therefore specific analysis with use of
actual project details is recommended.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.2 D
| Table D1 Nominal Overpressures |
| Area no. | Offshore Installation | Area 1) | Design Blast
Overpressure 2)
(barg) | Pulse
Duration
(s) | 1 | Drilling rig | Drill floor with cladded
walls | 0.1 | 0.2 | | 2 | Drilling rig | Shale shaker room with strong walls,
medium sized | 2 | 0.3 | | 3 | Mono-hull FPSO | Process area, small | 0.3 | 0.2 | | 4 | Mono-hull FPSO | Process area, medium 3) sized with no walls or roof 4) | 1 | 0.2 | | 5 | Mono-hull FPSO, large | Process area, large with
no walls or roof | 2 | 0.2 | | 6 | Mono-hull FPSO | Turret in hull, STP/STL room
with access hatch | 4 | 1 | | 7 | Production platform, Semi-sub | Process area, large with
no or light walls, 3 storeys, grated mezzanine and upper decks 5) | 2 | 0.2 | | 8 | Production platform, fixed | Process area, medium sized, solid upper
and lower decks 6),
3 storeys, 1 or 2 sides open | 1.5 | 0.2 | | 9 | Integrated Prod/Drilling | Process area and drilling
module each medium sized on partly solid decks, 3 storeys, 3 sides
open | 1.5 | 0.2 | | 10 | Integrated Prod/Drilling | X-mas tree/wellhead area, medium
sized with grated floors | 1 | 0.2 | Notes:- All areas, with the exception of #1
and #6, are considered to be congested. Designs intended
to give an even more compact process area than typical offshore
practice are expected to have larger nominal overpressures.
- All leak rates are considered, but for explosions, large
leaks (e.g. 50 kg/s) dominate the explosion risk.
- The terms small, medium, large for process area are
not specifically defined, however examples of typical footprint
sizes are given in the text above
- A process area with more than one level is expected
to have somewhat higher overpressure if the footprint area is the
same.
- A design with solid mezzanine and upper decks would
result in a higher nominal overpressure
- If all decks are grated the nominal overpressure is
expected to be lower.
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Sec.2
D 700 Heat loads
Sec.2 D
701 Where the living quarters are exposed to a heat load below
100 kW/m2 a
passive fire protection rating of A-60 is considered sufficient
for the surface facing the source of the heat load. For heat loads
above 100 kW/m2 H-rated
protection shall be used.Guidance note:
For standard design drilling units the passive fire protection
requirements of the IMO MODU Code will be considered as acceptable.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.2 D
702 Where radiation levels at lifeboat stations exceeds 12.5 kW/m2, radiation protection shall
be provided.
Sec.2 D
703 Heat loads as a result of blowout during drilling operations
will primarily be a function of blowout rate, hydrocarbon composition,
and distance. The relationship related to impact on safety functions
is shown in Figures 1 and 2.
Fig. 1 Distance to impact levels as function of blowout rate.
100% methane
Fig. 2 Distance to impact levels as function of blowout rate.
75% oil
and 25% gas
Guidance note:
These Figures may be used to assess a specific design with
respect to the safety criteria specified in 701, 702 and 703.
The 100% gas case should be used in assessing shallow
gas blowouts and production from gas reservoirs. The gas or oil
mixture case should be used for production from oil or condensate
reservoir.
Consideration may be given to duration of load with respect
to ability to move off position.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Sec.2 D
704 For drilling in water depths less than 100 m the potential
effects of subsea blowouts must also be considered.
Sec.2 D
705 For production units or installations, heat loads in connection
with ignition following loss of containment of hydrocarbons shall
be taken as follows, unless otherwise documented:- In areas with both gas containing
and oil or condensate containing equipment, critical items shall
be designed to withstand a jet fire (250 kW/m2) for 30 minutes and a pool
fire (150 kW/m2)
for the following 30 minutes.
- In areas with only oil or condensate containing equipment,
critical items shall be designed to withstand a pool fire (150 kW/m2) for 60 minutes.
- In areas with only gas containing equipment, critical
items shall be designed to withstand a jet fire (250 kW/m2) for 30 minutes.
Sec.2 D
706 The following critical items shall be designed to withstand
the specified design heat load:| — | protective walls |
| — | structures supporting hydrocarbon pressure vessels |
| — | structures capable of blocking escape ways |
| — | essential safety systems |
| — | main structure. |
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