Section
5 Buoy units
5.1 General
5.1.3 The hull structure of buoy units is to be divided into watertight
compartments and is to have adequate buoyancy and floating stability in all
conditions defined in Pt 4, Ch 4, 5.6 Structural design 5.6.2.
5.1.6 Any spaces filled with foam or permanent ballast is to be specially
considered with regard to the materials and their attachment to the structure.
5.1.7 Hull construction and arrangements of buoys used for the storage of oil
in bulk storage tanks are to comply with the requirements of the applicable Coastal
State Authority.
5.2 Environmental considerations
5.2.1 The Owner or designer is to specify the environmental criteria for which
the installation is to be approved. The extreme environmental conditions applicable
to the location are to be defined, together with all relevant operating
environmental limits. Full particulars are to be submitted with sufficient
supporting information to demonstrate the validity of the environmental parameters,
see
Pt 4, Ch 3, 4 Structural design loads.
5.2.2 A full list of operating and extreme environmental limiting conditions
is to be submitted. This is to include the following cases, as applicable, and any
other conditions relevant to the system under consideration:
- Extreme survival storm condition.
- Worst environmental conditions in which a ship/unit may remain
moored to an installation.
- Worst environmental conditions in which the main operating
functions may be carried out (e.g., transfer of product through riser).
- Worst environmental conditions in which a ship/unit may moor on
arrival at an off-loading installation.
- Worst environmental conditions in which a disconnectable
ship/unit may remain connected.
5.3 Water depth
5.3.1 The minimum and maximum still water levels at the operating location are
to be determined, taking full account of the tidal range, wind and pressure surge
effects. Data is to be submitted to show the variation in water depth in way of the
installation. This data is to be referenced to a consistent datum and is to include,
where relevant, the water depth in way of each anchor or pile, gravity base or
foundation, pipeline manifold, and in way of the radius swept by a ship/unit
attached to the mooring installation.
5.4 Design environmental conditions
5.4.1 The design is to be considered for the following environmental
conditions:
Table 4.5.1 Design loading
conditions
| Mode
|
Applicable loading
condition
|
| (a)
|
(b)
|
(c)
See Note 4
|
(d)
See Note 4
|
| Site installation,
see Note 3
|
X
|
X
|
|
|
| Operating, see Note
2
|
X
|
X
|
X
|
X
|
| Survival
|
X
|
X
|
X
|
X
|
| Transit (loadout),
see Note 3
|
X
|
X
|
|
|
| NOTES
|
| 1. For definition of loading conditions (a) to (d),
see
Pt 4, Ch 3, 4.3 Load combinations.
|
| 2. For operating conditions, the load cases are to
include those defined in Pt 4, Ch 4, 5.2 Environmental considerations 5.2.2, as applicable.
|
| 3. For loading conditions (a) and (b) for
installation and transit conditions, see
Pt 4, Ch 4, 5.6 Structural design 5.6.8.
|
| 4. For
loading conditions (c) and (d) as applicable to buoy units,
see
Pt 4, Ch 4, 5.6 Structural design 5.6.9.
|
5.4.2
Extreme storm survival condition. In general, the individual environmental
factors (wind, wave and current) are to have an average recurrence period of not
less than 100 years. The joint probability of occurrence of extreme values of
individual environmental factors is to be taken into account where sufficiently
accurate data exists.
5.4.3
Maximum connected conditions. The maximum environmental conditions during
which disconnectable ships/units will remain connected to the buoy.
5.4.4 Account is also to be taken in the design of the maximum conditions
during which particular operational activities or marine operations are intended to
be carried out, e.g., production through risers, transfer of product, connection to
or disconnection from single-point mooring. Appropriate limits are to be set and
defined in the Operations Manual.
5.5 Environmental loadings
5.5.1 The environmental loading on the installation and its motion responses
are to be determined and the dynamic effects are to be considered, see Pt 4, Ch 3, 4 Structural design loads. Account is to be taken of the following:
- Environmental loads and motions are to be established by model
testing and suitable calculation methods.
- Satisfactory correlation between the calculation method and
representative model test results is to be demonstrated.
- The possibility of resonant motion is to be fully investigated,
taking second order wave forces into account.
- In determining environmental loads, account is to be taken of
the effect of marine growth. Both an increase in the dimensions of submerged
members and the change in surface characteristics are to be considered.
- Shallow water effects are to be considered where
appropriate.
- consideration should be given to performing a full coupled
analysis of the buoy, mooring and transfer lines, or risers in the case of
deep water buoy units.
5.6 Structural design
5.6.1 The general requirements for structural design are given in Pt 4, Ch 3 Structural Design but the additional requirements of this
Chapter are to be complied with.
5.6.2 The structure is to be designed to withstand the static and dynamic loads
imposed on the unit in transit (loadout), site-specific installation, survival and
operating conditions. All relevant loads as defined in Pt 4, Ch 3 Structural Design are to be considered.
5.6.3 Account is to be taken of slam effects when calculating wave loads in the
splash zone.
5.6.4 Local forces from mooring lines and risers are to be included in the
analyses for normal operating conditions.
5.6.5 All bearings, guide rollers, etc., forming part of a turntable or other
swivel arrangement associated with risers, moorings or pipeline systems on the buoy
are to comply with the requirements given in Pt 3, Ch 13, 6 Mechanical items.
5.6.8 Although buoy units will not be classed during transit (loadout) and
during the installation procedure at the operating location, the transit condition
and the site-specific installation condition are to be investigated and submitted to
LR.
5.6.9 The general requirements for investigating accidental loads are defined
in Pt 4, Ch 3, 4.16 Accidental loads. In operating and survival conditions,
collision loads against the buoy structure will normally cause only local damage to
the structure and consequently loading conditions (c) and (d) in Table 4.5.1 Design loading
conditions need not be investigated
from the overall strength aspects.
5.7 Buoy structure
5.7.1 Buoys are to be designed to withstand the forces and moments resulting
from the overall loadings together with the forces and moments due to local
loadings, including internal and external pressures.
5.7.2 In general, internal spaces within the buoy are to be designed for the
pressure heads defined in Pt 4, Ch 3, 4.14 Hydrostatic pressures. The minimum head on shell boundaries is
generally not to be less than 6 metres, see also
Pt 4, Ch 4, 7.5 Structural design 7.5.5. Special consideration will be given to
accepting a reduced design head in benign environments where this can be clearly
demonstrated.
5.7.4 The general requirements for watertight and tank bulkheads are to comply
with Pt 4, Ch 6, 7 Bulkheads. The scantlings of the boundaries of internal
watertight compartments adjacent to the sea which are required for buoyancy and
stability to support the structure are to comply with the requirements for tank
bulkheads.
5.8 Topside structure
5.9 Lifeboat platforms
5.10 Fatigue
5.10.1 The structure of buoys and highly stressed structural elements of
mooring line attachments, chain stoppers and supporting structures are to be
assessed for fatigue damage due to cyclic loading.
5.10.2 The general requirements for fatigue design and the factors of safety on
fatigue life are to comply with Pt 4, Ch 5, 5 Fatigue design.
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