Section
3 Self-elevating units
3.1 General
3.1.1 This Section outlines the structural design requirements of
self-elevating units. Additional requirements for particular unit types related to
the design function of the unit are given in Pt 3 Functional Unit Types and Special Features.
3.1.4 The structural analysis and determination of primary scantlings are to
be on the basis of the distribution of loadings expected in all modes of operation
and the relevant combinations of global and local loadings.
3.1.5 A self-elevating unit may be designed for the site-specific environmental conditions
of one or more geographical locations. Where the unit is intended for operation at
more than one location, the most severe design environmental conditions are to be
considered in the design. The environmental limits for operating and survival
conditions should be defined in the Operations Manual.
3.1.6 For a self-elevating unit designed to leave the area of operation due to extreme
weather conditions such as cyclones, ice, etc., the limiting environmental criteria
for disconnection are to be defined in the design basis. The specific procedure for
site installation and the re-floating condition is to be stated in the Operations
Manual. For units operating in regions subject to sea ice and icebergs, the
requirements shall be supplemented with the provisions relating to ice actions and
specially agreed by LR.
3.1.7 The structural design of self-elevating units should consider possible changes in
topside weights, environmental conditions, leg penetration, water depth, soil fixity
and air gap during long-term operation in the elevated condition. A sufficient
weight allowance and variable centre of gravity may be considered for the design.
3.1.8 Site assessment for soil foundation capacity is not within the scope of
classification. It is the Owner’s responsibility for safe operation.
3.1.9 If the operation of a self-elevating unit is of limited durations, applicable
seasonal data may be used for the months under consideration.
3.1.10 Where appropriate, the influence of earthquake loading on self-elevating units is to
be fully accounted for in the design in relation to the particular site conditions.
Seismic design and seismic criteria should be considered in accordance with the
latest revision of ISO19901-2 Petroleum and natural gas industries – Specific
requirements for offshore structures – Part 2: Seismic design procedures and
criteria.
3.2 Air gap
3.2.1 An air gap is defined as the clearance between the underside of the hull
structure and the highest predicted design wave crest superimposed on the maximum
storm surge height over the highest astronomical tide in the elevated position. The
air gap and wave crest above the still water level is defined in Figure 4.3.1 Air gap. The hull elevations should also account for any
settlement due to extreme or abnormal storm events.
3.2.2 The maximum storm surge height and the highest astronomical tide should be based on a
design return period at the intended operating location of not less than 50 years
for mobile units and 100 years for fixed installations.
3.2.3 The minimum clearance is not to be less than 1,5 m. Calculations, model test results
or prototype reports are to be submitted for review. In cases where the unit is
designed without an adequate air gap, the scantlings of the hull and leg structures
are to be designed for wave impact forces and green seas. If any part of the hull
structure is waterborne during operation, the scantlings are to be specially
considered with site specific wave and current loads.
Figure 4.3.1 Air gap
3.3 Structural design
3.3.1 The structure is to be designed to withstand the static and dynamic loads
imposed upon it in all modes of operation including:.
- Transit condition;
- Site Installation condition (jacking condition and pre-load condition);
- Operating condition;
- Survival condition; and
- Re-floating condition.
All relevant distributions of gravity, functional and environmental loads are to be
considered, see
Pt 4, Ch 3, 4 Structural design loads.
3.3.2 For self-elevating units in transit condition, only wet transits, i.e.,
field and ocean transits, are to be considered in the structural design. Dry transit
on a heavy lift vessel, including planning and procedures, sea-fastenings, and
marine operations, is out of the scope of classification and normally covered by
marine warranty for the operation. After the dry tow a survey may be carried out at
the discretion of LR prior to installation of the unit. The survey includes a visual
inspection and NDT of the leg structures near the top of jack-house during the dry
tow and hull structures subjected to slamming impact during the dry tow on the
carrier vessel. The detailed scope is to be agreed with the attending Surveyor..
3.3.3 The pre-load condition is to be analysed for the structural design as
self-elevating units are pre-loaded in the elevated position to ensure that the soil
can withstand the maximum expected footing reaction without experiencing additional
leg penetration, sudden punch-through, or soil failure. The pre-load procedure and
the limiting environmental conditions are to be specified in the design basis and
the Operations Manual. The environmental conditions for the jacking condition are to
be used to assess the jacking capacity of the unit, see
Table 4.3.1 Design loading
conditions.
3.3.4 The operating and survival conditions are to be investigated separately
to provide for the mode of operation being changed from operating to survival mode
due to unfavourable weather forecast. The limits of operational loading conditions
may be dependent on the specific drilling or production operations.
3.3.5 For normal operating and storm survival conditions, variations in the
hull elevation, location of the cantilever and drill floor substructure, deck load,
etc. are to be considered in the design.
3.3.6 Global and local strength analyses for self-elevating units shall be performed for
the following objectives:
- To determine the correct response with respect to displacement, base shear and
overturning moment;
- To correctly simulate the linear and non-linear (P-Δ effect) characteristics of
legs, leg to hull connections, and leg to spudcan connections;
- To ensure adequate strength of the hull, the legs and the spudcan; and
- To ensure adequate capacity of the jacking and the fixation system, where
applicable.
3.3.7 Global strength analysis for the elevated condition is to consider the following
design loads.
- Gravity and functional loads;
- Environmental loads (wind, wave, current, snow and ice, etc.);
- Dynamic amplification due to wave loads (acting with current); and
- P-Δ effects due to hull lateral deflections including Euler’s
non-linear amplification factor and initial leg inclination.
3.3.8 Dynamic structural responses due to environmental loading are to be investigated
because the natural period of a self-elevating unit in the elevated condition is
typically within the range of wave period, so that the effect of dynamic
amplification is to be taken into account in both strength and fatigue analyses.
Natural period calculations for the elevated condition are to be submitted for
review. The three highest natural periods corresponding to surge (longitudinal),
sway (transverse) and yaw (torsional rotation) motions are to be included.
3.3.9 All modes of operation are to be investigated and the relevant design
load combinations defined in Pt 4, Ch 4, 3.3 Structural design are to be complied with. The design loading conditions applicable
to a self-elevating unit are given in Table 4.3.1 Design loading
conditions.
Table 4.3.1 Design loading
conditions
| Mode of operation
(see Note 4)
|
Applicable loading condition (see
Note 1)
|
| (a)
|
(b)
|
(c)
(See Note 2)
|
(d)
(See Note 2)
|
| Site installation (see
Note 3) and re-floating
|
X
|
X
|
|
|
| Operating
|
X
|
X
|
X
|
X
|
| Survival
|
X
|
X
|
X
|
X
|
| Transit
|
X
|
X
|
X
|
X
|
| NOTES
|
| 1.
For definition of loading conditions (a) to (d), see
Pt 4, Ch 3, 4.3 Load combinations.
|
| 2. For
loading conditions (c) and (d) as applicable to a self-elevating
unit, see
Pt 4, Ch 4, 3.3 Structural design 3.3.4. Loading condition (d) is to be
considered with a minimum 1-year return storm..
|
| 3. Site
installation includes jacking and preload conditions. Site
installation condition may be considered as load case (a) with the
environmental limits specified by the Operator or 1-year return
storm. Loadings associated with the maximum storm cases may be
considered as load case (b).
|
|
4. The following limiting environmental conditions for the global
strength analysis are to be specified in the structural design
basis:
- Wave height and a range of wave periods,
see
Pt 4, Ch 3, 4.9 Wave loads.
- Wind velocity (1 min. mean) and wind profile,
see
Pt 4, Ch 3, 4.6 Wind loads.
- Current velocity and current profile,
see
Pt 4, Ch 3, 4.7 Current loads.
- Water depth, air gap, and leg penetration depth (for
elevated condition only).
|
3.3.10 The permissible stresses and buckling capacity of all structural members
contributing to the global and local strength are to be in accordance with Pt 4, Ch 5 Primary Hull Strength and the minimum local scantlings of the
unit are to comply with the requirements of Pt 4, Ch 6 Local Strength.
3.3.11 For accidental assessments, the general requirements for investigating
accidental loads are defined in Pt 4, Ch 3, 4.16 Accidental loads.
- In transit conditions, collision loads against the hull
structure will normally only cause local damage to the hull structure and
consequently loading condition (c) in Table 4.3.1 Design loading
conditions need not be investigated from the
overall strength aspects.
- In the elevated position, accidental collision to the legs by a
supply vessel is to be considered in the design and the unit is to be
capable of absorbing the energy of impact in association with environmental
loads corresponding to the appropriate one-year storm condition. For legs of
the lattice type, damaged chords or bracings should normally be assumed
non-effective in the global strength analysis.
- In general, for loading condition (c) in Pt 4, Ch 4, 3.3 Structural design 3.3.3, the level of impact
energy absorbed by the local leg structure is not to be taken as less than 2
MJ. If the unit is only to operate in protected waters, as defined in Pt 1, Ch 2, 2.4 Class notations (hull/structure), the level of impact
energy absorbed by the local leg structure may be reduced, but should not be
less than 0,5 MJ. Collision loads by a supply vessel will, in general, only
cause local damage to one leg, but the possibility of progressive collapse
and overturning stability of the unit should be considered in the design
calculations.
3.3.12 In fatigue assessments,fatigue damage due to cyclic loading is to be
considered for transit and elevated conditions. Fatigue damage is considered
accumulative throughout the unit’s design life. The extent of the fatigue analysis
will be dependent on the mode and area of operations, see
Pt 4, Ch 5, 5 Fatigue design.
3.4 Hull structure
3.4.1 The hull is to be considered as a complete structure having sufficient
strength to resist all induced stresses while in the elevated position, including
the pre-load condition, and supported by its legs.
3.4.2 All fixed, variable and environmental loads are to be distributed and combined, by an
accepted method of rational analysis, from the various points of application to the
supporting legs. The scantlings of the hull are then to be determined consistent
with these load distributions.
3.4.3 Due account must be taken of loadings induced in the transit condition from external
sea heads, variable deck loads and inertia forces on the legs. The effects of
different ballast distribution are to be adequately considered for global and local
responses in the design of hull structure.
3.4.4 Hull integration structures in way of jack-houses (jackcases), fixation
systems, topsides, drill floor substructure, crane pedestals, moorings, and other
concentrated loads are to be analysed by direct calculations in compliance with
Pt 4, Ch 5 Primary Hull Strength and Pt 4, Ch 6 Local Strength, where applicable.
3.5 Deckhouses
3.5.2 Special consideration is to be given to the scantlings of deckhouses and
deck modules which will not be subjected to wave loading in any operating condition
such as units which are ‘dry-towed’ to the operating location.
3.5.3 The structural design, installation, and sea-fastenings of offshore
portable containers intended for storage or temporary accommodations are not within
the scope of classification. They are to comply with applicable codes and standards.
However, connections to the deck and the supporting hull structures are to be
analysed in compliance with Pt 4, Ch 5 Primary Hull Strength.
3.6 Structure in way of jacking or elevating
arrangements
3.6.1 Load carrying members in the jackhouses and frames which transmit loads
between the legs and the hull are to be designed for the maximum design loads in all
loading conditions and are to be so arranged that loads transmitted from the legs
are properly diffused into the hull structure.
3.6.3 Both elevated and transit conditions are to be analysed for the design of jack-houses
and supporting hull structures with respect to strength and fatigue. In the transit
condition, inertia loads due to wave motions in combination with wind induced
bending moments in the legs will be transferred to the jack-houses and the
supporting hull structures. Local detailed structural analysis is to be performed
and submitted for review.
3.7 Leg wells
3.7.1 The scantlings and arrangements of the boundaries of leg wells are to be
specially considered and the structure is to be suitably reinforced in way of leg
guides, taking into account the maximum forces imposed on the structure.
3.8 Leg design
3.8.1 Legs may be either shell type or lattice type. Independent footings may
be fitted to the legs or legs may be permanently attached to a bottom mat. Shell
type legs may be designed as either stiffened or unstiffened shells.
3.8.2 Where legs are fitted with independent footings, proper consideration is
to be given to the leg penetration of the sea bed and the end fixity of the leg in
the elevated condition as defined in Pt 4, Ch 4, 3.15 Foundation fixity.
3.8.4 For lattice type legs, the slenderness ratio of the main chord members
between joints is not to exceed 40, or two thirds of the slenderness ratio of the
leg column as a whole, whichever is the lesser, unless it can be shown that a
calculation taking into account beam-column effect, joint rigidity and joint
eccentricity justifies a higher figure.
3.8.6 Individual tubular and non-tubular members and joints in the lattice
type legs, including conical transitions, may be designed in accordance with
recognized Codes and Standards and the safety factors given in Pt 4, Ch 5, 2 Permissible stresses.
3.8.8 Fatigue analysis due to cyclic loading is to be performed for the
integrity of legs for transit and elevated conditions, see Pt 4, Ch 5, 5 Fatigue design. Fatigue critical locations in
the legs should include the following:
- Leg joints in way of the fixation system;
- Leg joints in way of the upper and lower guides;
- Leg joints in way of the splash zone; and
- Leg joints at lower part of the leg.
3.8.9 For self-elevating units permanently at a fixed location, leg structures in way of
the splash zone, leg structures and spudcans under the seabed are not considered
accessible for inspection and repair unless special arrangements are made in the
planned survey programme approved by LR.
3.9 Unit in the elevated position
3.9.1 When computing leg stresses with the unit in the elevated position, the
maximum overturning load and maximum shear load on the unit, using the most adverse
combination of applicable variable loadings together with the environmental design
loadings, are to be considered with the following criteria:
-
Wave forces: Values of drag coefficient,
 , and inertia coefficient,  , vary considerably with Reynolds number,  , and Keulegan-Carpenter number,  , and are to be carefully chosen to suit the individual
circumstances. In calculating the wave forces using acceptable wave
theories, the hydrodynamic coefficient values to be used are given below.
The effect of marine growth and other appurtenances (e.g., anodes) to the
legs should be considered in the wave force calculation.
- Tubular members:
| Surface
condition
|
Drag coefficient,
CD
|
Inertia coefficient,
CM
|
| Smooth (above MWL+2
m)
|
0,65
|
2,0
|
| Rough (below MWL+2
m)
|
1,0
|
1,8
|
|
Note 1.Cd
and Cm are considered at
post-critical Reynolds number and high
Keulegan-Carpenter number.
Note 2. Design assumptions
for marine growth is to be stated in the design
basis. Marine growth thickness should be at least
12,5 mm (25 mm in total) for all members below
MWL+2 m unless site specific data is available.
Marine growth on the teeth of racks may normally
be ignored.
Note 3. These hydrodynamic
coefficients are valid for tubulars less than 1,5
m in diameter.
Note 4. MWL refers to the
Mean Water Level.
|
- Cylindrical chord members with
protruding racks:
| Surface
condition
|
Drag coefficient,
CD
|
Inertia coefficient,
CM
|
| Smooth
|
0,65 for θ <
20o for 20o < θ
<90o
|
2,0 for all
directions
|
| Rough
|
1,0 for θ <
20o for 20o < θ
<90o
|
1,8 for all
directions
|
where
|
W |
= |
rack width |
|
D |
= |
distance between split tube
chords. For chords with rack teeth welded to the
outer surface the nominal diameter may be
used. |
|
CD1 |
= |
drag coefficient for wave flow
normal to the rack (θ = 90o) |
| = |
1,8 for W / D
< 1,2 |
| = |
1,4 + W / 3D for
1,2 < W / D
< 1,8 |
| = |
2,0 for 1,8 < W /
D |
|
- Triangular chord members:
| Surface
condition
|
Drag coefficient,
CD
|
Inertia coefficient,
CM
|
|
|
|
2,0 for all
directions
|
where
|
CDpr (q) |
= |
drag coefficient referenced to the
projected diameter. Linear interpolation is to be
applied for intermediate wave flow. |
| = |
1,7 for q = 0° |
| = |
1,95 for q = 90° |
| = |
1,4 for q = 105° |
| = |
1,65 for q = 180° -
q0 |
| = |
2,0 for q = 180° |
|
Dpr |
= |
projected diameter of the
chord |
| = |
Dcos(q) for q <
q0 |
| = |
Wsin(q) + 0,5D│cos(q)│ for
q0 £ q < 180 -q0 |
| = |
D│cos(q)│for 180 -
q0 £ q £ 180 |
|
q0 |
= |
angle where half the backplate is
hidden,  |
|
- Other shapes of non-tubular members:
 ,  values should be assessed based on the relevant
published data or appropriate tests. The tests should consider
possible roughness, Keulegan-Carpenter and Reynolds numbers
dependence.
Figure 4.3.2 Type of chord
-
Dynamics: Due account of dynamics is to be taken in computing leg
stresses when this effect is significant. The following governing aspects
are to be included:
- The mass and mass distribution of the unit. This
includes structural mass, mass of equipment and variable load on
board, added mass due to the surrounding water and marine growth, if
applicable, etc.
- The global unit structural stiffness. This includes
stiffness contributions from the leg to hull connections and the
footing interface, if applicable.
- The damping. This includes structural damping,
foundation soil damping and hydrodynamic damping.
- Fixity condition at bottom of leg.
-
P Δ effect:: Due accounts to be taken in computing leg stresses
include:
- Forces and moments due to initial leg inclination and
leg-hull clearances. Total horizontal offset should be taken as 0,5
per cent of the leg length below the lower guide, unless otherwise
specified.
- Forces and moments due to lateral frame deflections of
the legs under the wind, wave and current loads including the
dynamic effects.
- Euler’s non-linear amplification factor.
- Hull deflection: Bending moments at leg/hull connections due to hull
sagging under gravity loads.
3.10 Legs in field transit conditions
3.10.1 In field transit conditions within the same geographical area, legs are
to be designed for acceleration forces caused by a 6° single amplitude of roll or
pitch at the natural period of the unit, plus, 120 per cent of the gravity forces
caused by the legs’ angle of inclination, unless otherwise verified by appropriate
model tests or calculations.
3.10.2 The legs are to be investigated for any proposed leg arrangement with respect to
vertical position during field transit moves, and the approved positions are to be
specified in the Operations Manual. Such investigation is to include strength and
stability aspects. Field transit moves may only be undertaken when the predicted
weather is such that the anticipated motions of the unit will not exceed the design
condition.
3.10.3 The duration of a field transit move may be for a considerable period of
time and should be related to the accuracy of weather forecasting in the area
concerned. It is recommended that such a move should not normally exceed a twelve
hour voyage between protected locations or locations where the unit may be safely
elevated. However, during any portion of the move, the unit should not normally to
be more than a six hour voyage to a protected location or a location where the unit
may be safely elevated. Suitable instructions are to be included in the Operations
Manual. Where a special leg position is required for field moves, this position is
to be specified in the Operations Manual.
3.10.4 Where a special leg position is required for field moves, this position is to be
specified in the Operations Manual.
3.11 Legs in ocean transit conditions
3.11.1 In ocean transit conditions involving a move to a new geographical area,
legs are to be designed for acceleration and gravity loadings resulting from the
motions in the most severe anticipated environmental transit conditions, together
with corresponding wind moments.
3.11.2 Calculation or model test methods may be used to determine the motions.
Alternatively, legs may be designed for the acceleration and gravity forces caused
by a design criterion of 20° single amplitude of roll or pitch at a 10 second
period. For ocean transit conditions, it may be necessary to reinforce or support
the legs, or to remove sections of them.
3.11.3 The approved condition is to be included in the Operations Manual.
3.12 Legs during installation
conditions
3.12.1 When lowering the legs to the sea bed, the legs are to be designed to
withstand the dynamic loads which may be encountered by their unsupported length
just prior to touching the sea bed and also to withstand the shock of touching
bottom while the unit is afloat and subject to wave motions.
3.12.2 Instructions for lowering the legs are to be clearly indicated in the
Operations Manual. The maximum design motions, bottom conditions and sea state while
lowering the legs are to be clearly stated. The legs are not to be lowered in
conditions which may exceed the design criteria.
3.12.3 For units without bottom mats, all legs are to have the capability of
being preloaded to the maximum applicable combined gravity plus overturning load.
The approved preload procedure should be included in the Operations Manual.
3.12.4 Consideration is to be given to the loads caused by a sudden penetration
of one or more legs during preloading.
3.13 Overturning stability in-place
3.13.1 When the legs are resting on the sea bed, the unit is to have sufficient
positive downward gravity loadings on the support footings or mat to withstand the
overturning moment of the combined environmental and functional forces from any
direction, with a reserve against the loss of positive bearing of any footing or
segment of the area, for the elevated conditions.
3.13.2 The anticipated minimum variable load is to be considered for each loading direction
and, for the overturning calculation, in no case is the variable load to be taken
greater than 50 per cent.
3.13.3 The safety factor against overturning is to be at least 1,25 with respect
to the rotational axis through the centres of the independent footings at the sea
bed. For a unit with a mat type footing, the rotational axis is to be taken at the
maximum stressed edge of the mat.
where
|
Ms |
= |
stabilising moment due to the effects of weight and
variable load |
|
Mo |
= |
overturning moment due to the effects of wind, wave,
current, dynamic amplification, P-Δ, and crane operation |
3.13.4 For self-elevating units with independent footings, it is assumed that the vertical
point of axis of rotation at each footing is situated at a distance above the
spudcan tip equivalent to the lesser of:
- Half the maximum predicted penetration; and
- Half the height of the spudcan.
3.13.5 For independent footings, the safety factor against sliding at the sea
bed is to be related to the soil condition, but in no case is the safety factor to
be taken as less than 1,0.
3.14 Sea bed conditions
3.14.1 Classification will be based upon the designer’s assumptions regarding
the sea bed conditions. These assumptions are to be recorded in the Operations
Manual.
3.14.2 Full details of the sea bed at the operating location are to be
submitted to LR for review at the design stage. The effects of scouring on bottom
mat bearing surfaces and footings is to be considered, see
Pt 4, Ch 4, 3.16 Bottom mat.
3.15 Foundation fixity
3.15.1 For units with independent legs, foundation fixity should not normally be
considered for in-place strength analysis of the upper parts of the leg in way of
the lower guides unless justified by proper investigation of the footing and soil
conditions.
3.15.2 For in-place analysis, the leg to spudcan connections and the lower
parts of the leg with independent footings are to be designed for a leg moment no
less than 50 per cent of the maximum leg moment at the lower guides (assuming a
pinned footing), together with the associated horizontal and vertical loads.
3.16 Bottom mat
3.16.1 When the legs are attached to a bottom mat, the scantlings of the mat
are to be specially considered, but the permissible stress levels are to be in
accordance with Pt 4, Ch 5 Primary Hull Strength. Particular attention is to be given to the attachment,
framing and bracing of the mat in order that the loads from the legs are effectively
distributed into the mat structure.
3.16.2 Mats and their attachments to the bottom ends of the legs are to be of
robust construction to withstand the shock load on touching the sea bed while the
unit is afloat and subject to wave motions.
3.16.3 The effects of scouring on the bottom bearing surfaces should be
considered by the designer, with a stated design figure for loss of bearing area.
The effects of skirt plates, where provided, may be taken into account, see
also
Pt 4, Ch 4, 3.14 Sea bed conditions 3.14.1.
3.17 Independent footings and interfaces with
legs
3.17.1 Independent footings and the leg to spudcan connections and are to be
designed to withstand the most severe combination of overall and local loadings to
which they may be subjected, see also
Pt 4, Ch 4, 3.16 Bottom mat 3.16.3. In general, the primary structure of the footings, the
lower part of the legs, and the leg to footing connections are to be analysed by a
three dimensional finite element method.
3.17.2 The complexity of the mathematical model together with the associated
element types is to be sufficiently representative of all parts of the primary
structure to enable internal stress distributions to be established. A local model
may be developed for yield and buckling assessment and should be extended vertically
to at least two bays above the top of the footing.
3.17.3 The loading combinations considered are to represent all elevated
conditions so that the critical design cases are established, and are to include,
but not be limited to, the following:
- The maximum preload concentrated or distributed over the area
of initial contact.
- The maximum preload uniformly distributed over the entire
bottom area.
- The relevant preload distributed over contact areas
corresponding to intermediate levels of penetration, as required.
- The greatest leg load due to the specified
environmental maxima applied over the entire bottom area, with the pressure
varying linearly from zero at one end to twice the mean value at the other
end.
- The distribution in Pt 4, Ch 4, 3.17 Independent footings and interfaces with legs 3.17.3 applied in different
directions, depending on structural symmetry, to cover all possible wave
headings.
- Where it is intended to move the unit without the footings being
fully retracted, a special analysis of the leg to spudcan connections may be
required.
3.17.5 The minimum local scantlings of the bottom shell and stiffening and
other areas subjected to pressure loading are to be determined from the formulae for
tank bulkheads given in Pt 4, Ch 6, 7 Bulkheads. The loadhead  should be consistent with the maximum bearing pressure, determined
in accordance with Pt 4, Ch 4, 3.17 Independent footings and interfaces with legs 3.17.3, and the wastage allowance of the plating
should be not less than 3,5 mm, see also
Pt 4, Ch 4, 3.17 Independent footings and interfaces with legs 3.17.6.
3.17.6 Where it is intended to operate at a fixed location for the design life
of the unit, the footing/leg structure which is below the mud line or internal areas
of the footings which cannot be inspected are to have their structure designed with
adequate corrosion margins and protection. The corrosion allowance for wastage and
the means of protection are to be to the satisfaction of LR and are to be agreed at
the design stage.
3.17.8 Where the legs of the unit are made from steel with extra high tensile
strength, special consideration is to be given to the weld procedures for the leg to
footing connections. Adequate preheat should be used and the cooling rate should be
controlled. Any non-destructive examination of the welds should be carried out after
a minimum of 48 hours have elapsed after the completion of welding.
3.17.9 Fatigue analysis at the leg to footing connections is to be performed and submitted
for review.
3.18 Lifeboat platforms
3.18.2 If the lifeboat platform may be subject to wave impact forces in transit conditions,
the scantlings are to be specially considered and details are to be submitted for
review.
3.19 Topside structure
|