The electronic pdf versions of the documents found through http://www.dnv.com/ are the officially binding versions. Copyright Det Norske Veritas.
![[Table of Contents]](/dwicons/b_toc.gif)
|
DNV-OS-C201 Structural Design of Offshore Units (WSD method)
|
Sec.1
C. Definitions
Sec.1
C 100 Verbal forms
Sec.1 C
101 Shall: Indicates a
mandatory requirement to be followed for fulfilment or compliance
with the present standard. Deviations are not permitted unless formally
and rigorously justified, and accepted by all relevant contracting
parties.
Sec.1 C
102 Should: Indicates a
recommendation that a certain course of action is preferred or particularly
suitable. Alternative courses of action are allowable under the
standard where agreed between contracting parties but shall be justified
and documented.
Sec.1 C
103 May: Indicates a permission,
or an option, which is permitted as part of conformance with the
standard.Sec.1
C 200 Terms
Sec.1 C
201 Accidental condition: When
the unit is subjected to accidental loads such as collision, dropped
objects, fire explosion, etc.
Sec.1 C
202 Accidental loads: Loads
which may occur as a result of accident or exceptional events, e.g.
collisions, explosions, dropped objects.
Sec.1 C
203 Atmospheric zone: The
external surfaces of the unit above the splash zone.
Sec.1 C
204 Cathodic protection: A
technique to prevent corrosion of a steel surface by making the
surface to be the cathode of an electrochemical cell.
Sec.1 C
205 Characteristic load: The
reference value of a load to be used in the determination of load
effects. The characteristic load is normally based upon a defined
fractile in the upper end of the distribution function for load.
Sec.1 C
206 Characteristic strength: The
reference value of structural strength to be used in the determination
of the design strength. The characteristic strength is normally
based upon a 5% fractile in the lower end of the distribution
function for resistance.
Sec.1 C
207 Characteristic value: The
representative value associated with a prescribed probability of
not being unfavourably exceeded during the applicable reference
period.
Sec.1 C
208 Classic spar: Shell
type hull structure.
Sec.1 C
209 Classification Note: The
Classification Notes cover proven technology and solutions which
is found to represent good practice by DNV, and which represent
one alternative for satisfying the requirements given in the DNV
Rules or other codes and standards cited by DNV. The Classification
Notes will in the same manner be applicable for fulfilling the requirements
in the DNV Offshore Standards.
Sec.1 C
210 Coating: Metallic,
inorganic or organic material applied to steel surfaces for prevention
of corrosion.
Sec.1 C
211 Column-stabilised unit: A
floating unit that can be relocated. A column-stabilised unit normally
consists of a deck structure with a number of widely spaced, large
diameter, supporting columns that are attached to submerged pontoons.
Sec.1 C
212 Corrosion allowance: Extra
wall thickness added during design to compensate for any anticipated reduction
in thickness during the operation.
Sec.1 C
213 Damaged condition: The
unit condition after accidental damage.
Sec.1 C
214 Deep draught floater (DDF): A
floating unit categorised with a relative large draught. The large
draught is mainly introduced to obtain reduced wave excitation in
heave and sufficiently high eigenperiod in heave such that resonant
responses in heave can be omitted or minimised.
Sec.1 C
215 Design brief: An agreed
document presenting owner's technical basis, requirements
and references for the unit design and fabrication.
Sec.1 C
216 Design temperature: The
design temperature for a unit is the reference temperature for assessing
areas where the unit can be transported, installed and operated.
The design temperature shall be lower or equal to the lowest mean daily temperature in
air for the relevant areas. For seasonal restricted operations the lowest mean daily temperature in
air for the season may be applied.
Sec.1 C
217 Driving voltage: The
difference between closed circuit anode potential and the protection
potential.
Sec.1 C
218 Dry transit: A transit
where the unit is transported on a heavy lift unit from one geographical
location to another.
Sec.1 C
219 Dynamic upending: A
process where seawater is filled or flooded into the bottom section
of a horizontally floating DDF hull and creating a trim condition
and subsequent water filling of hull or moonpool and dynamic upending
to bring the hull in vertical position.
Sec.1 C
220 Environmental loads: Loads
directly and indirectly due to environmental phenomena. Environmental loads
are not a necessary consequence of the structures existence, use
and treatments. All external loads which are responses to environmental
phenomena shall be regarded as environmental loads, e.g. support
reactions, mooring forces, and inertia forces.
Sec.1 C
221 Expected loads and response history: Expected
load and response history for a specified time period, taking into
account the number of load cycles and the resulting load levels
and response for each cycle.
Sec.1 C
222 Expected value: The
most probable value of a load during a specified time period.
Sec.1 C
223 Fail to safe: A failure
shall not lead to new failure, which may lead to total loss of the
structure.
Sec.1 C
224 Fatigue: Degradation
of the material caused by cyclic loading.
Sec.1 C
225 Fatigue critical: Structure
with calculated fatigue life less than three times the design fatigue
life.
Sec.1 C
226 Functional loads: Loads
which are a necessary consequence of the structure's existence,
use and treatment under ideal circumstances, i.e. no environmental
loads, for each design condition. All external loads which are responses
to functional loads shall be regarded as functional loads, e.g.
support reactions and still water buoyancy forces.
Sec.1 C
227 Gross scantlings: Scantlings,
including thickness as shown on structural drawings, i.e. the actual
(full) thickness provided at the newbuilding stage applies.
Sec.1 C
228 Guidance note: Information
in the standard added in order to increase the understanding of
the requirements.
Sec.1 C
229 Gust wind velocity: The
average wind velocity during a time interval of 3 s. The "N
years gust wind velocity" is the most probable highest
gust velocity in a period of N years.
Sec.1 C
230 Hard tank area: Usually
upper part of the hull providing sufficient buoyancy for a DDF unit.
Sec.1 C
231 High frequency (HF) responses: Defined
as rigid body motions at, or near heave, roll and pitch eigenperiods
due to non-linear wave effects.
Sec.1 C
232 Hindcasting: A method
using registered meteorological data to reproduce environmental
parameters. Mostly used for reproducing wave parameters.
Sec.1 C
233 Inspection: Activities
such as measuring, examination, testing, gauging one or more characteristics
of an object or service and comparing the results with specified
requirements to determine conformity.
Sec.1 C
234 Installation condition: A
temporary condition where the unit is under construction such as
mating or in preparation for operational phase such as upending
of DDFs, lowering the legs and elevating the self-elevating units
or tether pretension for TLPs.
Sec.1 C
235 Load effect: Effect
of a single design load or combination of loads on the equipment
or system, such as stress, strain, deformation, displacement, motion,
etc.
Sec.1 C
236 Lowest mean daily temperature: The
lowest value on the annual mean daily average temperature curve for
the area in question. For temporary phases or restricted operations,
the lowest mean daily temperature may be defined for specific seasons.
In the above definition:
Mean
daily average temperature: The statistical mean average
temperature for a specific calendar day.
Mean: Statistical mean based on
number of years of observations.
Average: Average
during one day and night.
Sec.1 C
237 Low frequency (LF) responses: Defined
as TLP rigid body non-linear motions at, or near surge, sway and
yaw eigenperiods.
Sec.1 C
238 Lowest waterline: Typical
light ballast waterline for ships, wet transit waterline or inspection
waterline for other types of units.
Sec.1 C
239 Material strength: The
nominal value of material strength to be used in the determination
of the design resistance. The material strength is normally based
upon a 5% fractile in the lower end of the distribution function
for material strength.
Sec.1 C
240 Mean: Statistical mean
over observation period.
Sec.1 C
241 Moulded baseline: A
horizontal line extending through the upper surface of hull bottom
shell.
Sec.1 C
242 Non-destructive testing (NDT): Structural
tests and inspection of welds with radiography, ultrasonic or magnetic
powder methods.
Sec.1 C
243 Offshore Standard: The
DNV Offshore Standards are documents which presents the principles
and technical requirements for design of offshore structures. The
standards are offered as DNV's interpretation of engineering
practice for general use by the offshore industry for achieving
safe structures.
Sec.1 C
244 One hour wind velocity: The
average wind velocity during a time interval of one hour.
Sec.1 C
245 Operating condition: A
conditions wherein a unit is on location for purposes of production,
drilling or other similar operations, and combined environmental
and operational loadings are within the appropriate design limits
established for such operations (including normal operations, survival,
accidental).
Sec.1 C
246 P-delta effect: Second
order effect due to vertical forces in combination with second order displacements.
For self-elevating units the P-delta effect describes non-linear
amplification due to second order bending of the legs for the unit
in the hull elevated mode. For DDF units the P-delta effect describes
global bending or shear effects due to relatively high roll or pitch
angles in harsh environment.
Sec.1 C
247 Potential: The voltage
between a submerged metal surface and a reference electrode.
Sec.1 C
248 Recommended Practice (RP): The
Recommended Practice publications cover proven technology and solutions
which have been found by DNV to represent good practice, and which
represent one alternative for satisfying the requirements given
in the DNV Offshore Standards or other codes and standards cited
by DNV.
Sec.1 C
249 Redundancy: The ability
of a component or system to maintain or restore its function when
a failure of a member or connection has occurred. Redundancy can
be achieved for instance by strengthening or introducing alternative
load paths.
Sec.1 C
250 Reference electrode: Electrode
with stable open-circuit potential used as reference for potential measurements.
Sec.1 C
251 Reliability: The ability
of a component or a system to perform its required function without
failure during a specified time interval.
Sec.1 C
252 Representative value: The
value assigned to each load for a design situation.
Sec.1 C
253 Resistance: The reference
value of structural strength to be used in the determination of
the design strength. The resistance is normally based upon a 5% fractile
in the lower end of the distribution function for resistance.
Sec.1 C
254 Retrieval condition: A
condition, normally applicable for self-elevating units only, and
for which the unit is lowering the hull and elevating the legs.
Sec.1 C
255 Ringing: The non-linear
high frequency resonant response induced by transient loads from
high, steep waves.
Sec.1 C
256 Riser frame: Framed
steel structures installed at different vertical elevations along
the hull or moonpool in order to separate the different risers.
Sec.1 C
257 Risk: The qualitative
or quantitative likelihood of an accidental or unplanned event occurring
considered in conjunction with the potential consequences of such
a failure. In quantitative terms, risk is the quantified probability
of a defined failure mode times its quantified consequence.
Sec.1 C
258 Self-elevating unit or jack-up: A
mobile unit having hull with sufficient buoyancy to transport the
unit to the desired location, and that is bottom founded in its
operating mode. The unit reaches its operating mode by lowering
the legs to the seabed and then jacking the hull to the required
elevation.
Sec.1 C
259 Service temperature: The
service temperature is a reference temperature on various structural
parts of the unit used as a criterion for the selection of steel
grades.
Sec.1 C
260 Shakedown: A linear
elastic structural behaviour is established after yielding of the
material has occurred.
Sec.1 C
261 Slamming: Impact load
on an approximately horizontal member from a rising water surface
as a wave passes. The direction of the impact load is mainly vertical.
Sec.1 C
262 Specified minimum yield strength
(SMYS): The minimum yield strength prescribed by the
specification or standard under which the material is purchased.
Sec.1 C
263 Specified value: Minimum
or maximum value during the period considered. This value may take
into account operational requirements, limitations and measures
taken such that the required safety level is obtained.
Sec.1 C
264 Splash zone: The external
surfaces of the unit that are periodically in and out of the water.
The determination of the splash zone includes evaluation of all
relevant effects including influence of waves, tidal variations,
settlements, subsidence and vertical motions.
Sec.1 C
265 Springing: The high
frequency non-linear resonant response induced by cyclic (steady
state) loads in low to moderate sea states.
Sec.1 C
266 Strake: Usually helical
devices (strake) welded to outer hull with the purpose of reducing
the vortex induced cross-flow motion of DDF hull due to current
(mainly). Also the term suppression device may be used to describe
the strake.
Sec.1 C
267 Submerged zone: The
part of the installation, which is below the splash zone, including
buried parts.
Sec.1 C
268 Survival condition: A
condition during which a unit may be subjected to the most severe
environmental loadings for which the unit is designed. Drilling
or similar operations may have been discontinued due to the severity
of the environmental loadings. The unit may be either afloat or
supported on the seabed, as applicable.
Sec.1 C
269 Sustained wind velocity: The
average wind velocity during a time interval (sampling time) of
1 minute. The most probable highest sustained wind velocity in a
period of N years will be referred to as the "N years sustained
wind". This is equivalent to a wind velocity with a recurrence
period of N years.
Sec.1 C
270 Target safety level: A
nominal acceptable probability of structural failure.
Sec.1 C
271 Temporary conditions: Design
conditions not covered by operating conditions, e.g. conditions
during fabrication, mating and installation phases, transit phases,
accidental.
Sec.1 C
272 Tensile strength: Minimum
stress level where strain hardening is at maximum or at rupture.
Sec.1 C
273 Tension leg platform (TLP): A
buoyant unit connected to a fixed foundation by pre-tensioned tendons. The
tendons are normally parallel, near vertical elements, acting in
tension, which usually restrain the motions of the TLP in heave,
roll and pitch. The platform is usually compliant in surge, sway
and yaw.
Sec.1 C
274 Transit conditions: The
unit conditions in wet transit from one geographical location to
another.
Sec.1 C
275 Truss spar: A spar
buoy with truss structure for the hull part below hard tank area.
Sec.1 C
276 Unit: A general term
for an offshore installation such as ship shaped, column stabilised,
self-elevating, tension leg or deep draught floater.
Sec.1 C
277 Usage factor: The ratio
between permissible stress and the characteristic strength of the
structural member.
Sec.1 C
278 Verification: Examination
to confirm that an activity, a product or a service is in accordance
with specified requirements.
Sec.1 C
279 Wave frequency (WF) responses: Linear
rigid body motions at the dominating wave periods.
Sec.1 C
280 Wet transit: A transit
where the unit is floating during the move from one geographical
location to another.
Sec.1 C
281 Ultimate strength: Corresponding
to the maximum load carrying resistance.
![[-]](/dwicons/i_colpse.gif)