HYDRODYNM{IC ASPECTS OF FIXED PLATFORM INSTALLATION
Prof.ir. Bart Boon Report OEMO no. 88/5 1 October 1988
CONTENTS
Introduction
Overview of Transportation and Installation 3
2.1 Lifted jackets, vertically transported 4
2.2 Lifted jackets, transported horizontally 6
2.3 Self floating jackets 7
2.4 Launched jackets 8
Load-out and Float-out 11
3.1 Lifting 11
3.2 Float-out 11
3.3 Skidding 12
3.4 Launching 13
Transportation 14
4.1 Liftable jackets, vertically and horizontally transported 14
4.2 Self-floating jackets 14
4.3 Launched jackets 15
4.4 Naval architectural aspects 16
4.4.1 Barges (launch and common) 16
4,4.2 Self-floating jackets 19
Launching of a Jacket from Barge 24
Upending 29
Jacket Lifting 33
Crane Vessel Downtime 35
Crane Vessel Position Mooring 36
Crane Vessel Stability 39
References 42
Appendix A Quasi-Static Mooring Analysis 45
Al Characteristics of single line 45
A2 Vessel moored to a single line in seaway 49
A3 Vessel moored to two lines 50
A4 Vessel in spread mooring
Hydrodynamic Aspects of Fixed Platform Installation
Prof.Ir. Bart Boon
Deift University of Technology
CHAPTER 1 INTRODUCTION
Steel fixed platforms constitute the vast majority of supporting structures for offshore production of oil and gas. In the installed situation they are normally considered as civil engineering works where the most important maritime aspect is the hydrodynamic loading of the structure by waves and current.
The steel support structures, jackets, are normally fabricated
onshore, then transported to and installed at their final location.
During those phases maritime aspects play an important role.
With "maritime" are meant the aspects governed by traditional
naval architectural disciplines such as stability, ship motion, anchoring, etc.
This paper describes which and how these naval architectural disciplines are used in the various stages of the installation of steel fixed platforms. Apart from the vessels and operations described hereafter, several more will play a role, such as ROV motherships (ROV = Remotely Operated Vessel), tugs, supply boats,
survey vessels, etc. Their operation, however, is either not very different from the handling of conventional vessels, or is a variation of the methods described in this paper.
Depending upon various considerations transportation of the jacket may take place either
- self-floating, or
-in vertical position on a normal transport barge (towed or self-propelled), or
-in horizontal position on a normal transport barge (towed or self-propelled), or
-in horizontal position on a launch barge, or - on board of a crane vessel, or
- on board of a (semi-) submersible barge or vessel. Related to the transport method are the load-out and installation procedures.
Hereafter will be given first an overview of the various ways of transporting and installing steel jackets with their
advantages and disadvantages and the limitations they
experience. The installation of modules is relatively straight forward and will be touched upon briefly.
Subsequently will be described:
-various load-out and float-out methods, ballasting and strength aspects
strength
- launching, stability, strength, upending -installation tasks of crane vessel
-mooring of crane vessels, choice of anchor pattern, anchoring calculations
-stability of crane vessels, ballast-systems
-motions of crane vessels, down-time and weather window, constraints for operation.
CHAPTER 2 OVERVIEW OF TRANSPORTATION AND INSTALLATION
From very early stages of offshore oil and gas production, it has been normal to preassemble the platforms as much as
possible, before installation at sea. The reason for this is that installation work done at sea requires a multiple of the
manhours for the same job executed on shore. Moreover, the
Fig. 1 Installation of a complete platform as one unit (HES tripod)
cost per manhour can be several times higher than manhours onshore, as a result of direct wages, higher insurance costs, overtime premiums, overhead costs, hotel facilities,
transportation, etc. Evidently, the financial benefits will be
maximised by decreasing the number of subassemblies for
offshore installation of a platform as much as possible.
Ideally, a complete platform could be installed as one complete unit. Although this has been performed for very small platforms
(fig.l). The common way however, is to install the steel support structure (jacket) as one unit on top of which
subsequently the steel support decks are placed, followed by subassemblies, the so-called modules, containing all process equipment and outfit (fig.2). This system has been applied for many platforms, ranging from very shallow to very deep water
combined decks and modules installed on top of the jackets) weighing up to 30.000 tons in total (topside weight up to 60,000 tons considered for concrete platforms).
NAB NAA
Fig. 2 Jacket, decks and modules [1]
The topsides must and can be subdivided in as many modules as is necessary in the light of the lifting limitations of the offshore cranes used. For the supporting jackets however, this is practically impossible because this would mean aligning and connecting sections underwater (Although this was done for Shell's Cognac platform installed in two parts).
The consequence of this is that jackets often are heavier than can be handled by the cranes available and that special
methods will be needed to install them.
For jacket installation the following systems are used:
2.1. Lifted jackets, vertically transported
The most straightforward and the cheapest way of installing a jacket is to transport it vertically on board of a barge or
the crane vessel itself, lift it off and install it (fig.3). Advantages:
-the jacket during 41 phases of installation is loaded in
practically the same direction as it will be loaded once in operation, i.e. in a vertical way (albeit in tension rather than compression). This means that for installation very few modifications are needed to the jacket as optimized for operational purposes. This reduces new building costs and time.
-the installation is performed in one phase: set-down of the jacket followed by pile driving.
Fig. 3 Jackets transported vertically (HES photo)
Disadvantages:
-During transportation the jacket is in an upright position. For deeper water jackets (relatively high) this may result in quite high loads resulting from large motions during
transportation and consequently the need for additional reinforcing steel.
-limitations are set to the weight of the jackets by the lifting capacity of the crane available for jacket handling.
Two cranes available will alleviate this, but the use of
those if on two barges will, be restricted to more benign conditions than when using one crane.
Fig. 4 Clearances for lifted jacket
-limitations are set top the dimensions of the jacket due to
Clearance to crane boom, vessel hull and mooring lines are important (see fig. 4). Theheight limitation may be more pronounced if a jacket is to be lifted from the deck of the crane vessel. In particular the deck of a semi-submersible crane vessel may be much higher than that of a transportation barge.
-the transportation barge may have to be of a relatively large size to prevent stability problems for the trans-portation phase.
2.2 Lifted jackets, transported horizontally
This method (see fig. 5) is comparable to the previous one, however the jacket is transported horizontally
(I) PREPARED FOR LIFT
(III) LOWERED AND RELEASED
cv) HOOK ASSISTED UPENDING
THE JACKET LIFT
(IV)
TUGS TURN JACKET FOR UPENDING
(VI) TOUCH DOWN
onboard the transportation barge. This results in some advantages and disadvantages compared to the vertically transported jacket.
Advantages:
-probably the jacket during transportation experiences less inertia loads, which may lead to a somewhat lighter platform (but see also the first disadvantage)
-stability of the transportation barge will be less of a
problem
-if the jacket is upended when partially in the water, less lifting height is needed for the crane(s).
Disadvantages:
-Because lifting loads will act on the jackets into various directions, the need for additional reiforcement may exist
(see also first advantage)
-the upending procedure which can be done either in air or partly in the water, may be difficult to perform with one crane, so either two vessels or two cranes on one vessel may be required for the job
-the procedure probably is more time consuming.
Note:Both methods with lifted jackets have been used in the offshore industry since its early days. For many years,
however, the systems played a minor role because most jackets had
weights far exceeding lifting capacities of the crane barges available. In recent years, however, the tremendous growth in lifting capacities gave an enormous stimulus to use these
methods. Liftable jackets are now considered for water depths up
to 175 in in North Sea conditions and jacket weights up to 12,000 tons [4]. Note that the weights of those jackets could only be attained by exploiting all possibilities for weight reduction offered by the lift installation method. Compared to launched jackets, lifted jackets may weigh 40 to 50% less.
2.3 Self floating jackets
In this method the jacket is transported to its final location floating in the water in a horizontal position. Buoyancy is provided by the jacket structure itself, by using temporary auxiliary buoyancy tanks or by a
combination. During the transport the jacket normally floats with a draft somewhat- less than the diameter of the lower
chords or of the buoyancy tanks (see fig. 6) or sometimes with only a small portion of the jacket above water. At location the jacket is upended and lowered to the sea bottom, using either a ballast system in the jacket or a crane barge or both.
Fig. 6 Jacket with buoyancy tanks under tow [5J
Advantages:
-no transportation barge needed (some jackets were of a size for which no launching barge existed or was available)
-less reinforcements needed to accept launching loads or crane loads
-no seafastenings needed
Disadvantages:
-jacket should be built preferably in a temporary or
permanent building dock. Other methods like launching from a shipway (difficult) or using submersible barges (or a ship lift) have, to the author's knowledge, not been used.
-minimum water depth needed may be more than when transporting on a barge
-towing speed will be low and more tugs may be needed, also
for manoeuvering
-jacket will be loaded by the waves in ways different from
those in the "as installed" situation
-ballast system required in the jacket may be rather extensive
-additional buoyancy tanks may be needed
-floating stability, both intact and damaged, may be difficult to attain.
2.4 Launched jackets
This still is by far the most common installation method. The jacket is transported on a launch barge. At location the
the launch beams, after the jacket's centre of gravity passes the aft end of the barge the jacket will start tilting being supported by the barge's tilting arms. When afloat in the water the jacket is upended and set on the seafloor using a
ballast system in the jacket or a crane barge or both (see fig.7). In one case a special "jacket" (the Lena guyed tower) has been launched sideways.
(I) TOW OUT CONWTION
(III) LAUNCH
CV) CONTROLLED FLOODED UPENDING
CIV) POST LAUNCH EQUILIeRIUM
Fig. 7 Launched jacket [15]
Advantages:
-nowadays launch barges exist for all jacket weights up to the largest
-procedure is well-proven -no severe cranage requirements
BALLASTED FOR LAUNCH
TOUCH DOWN
AUXILIARY
BUOYANCY TUBES AUXILIARY
Disadvantages:
-reinforcements needed for accepting the loads from launch ways and tilting beams
-during launching the jacket will be subjected to very high,
once only bending and shear loads leading to very high
additional steel requirements
-launching may require better sea conditions than upending.
Hence the weather window for launching is smaller than for the
CHAPTER 3 LOAD-OUT AND FLOAT-OUT
Jackets have to be transported from the building site to the transportation medium, be it a barge or the water. Various ways exist of doing this.
3.1 Lifting
The jacket sometimes may be lifted from the shore onto the transportation barge by means of a crane. Considering the weights involved this generally will mean a floating crane, either sheer-legs or an offshore barge. The jacket may be transferred into the water (although most liftable jackets will not be self floating), onto a transportation barge
(common or launch) or ship, or onto the crane vessel itself. The jacket may be lifted either vertically or horizontally.
Advantages:
- simple, fast procedure, little preparation necessary - few special reinforcements of the jacket needed
Disadvantages:
- jacket needs to be built within reach of the (floating) crane, or it has to be wheeled or skidded to such a
position
- floating cranes, especially offshore cranes, may need a minimum amount of water depth in front of the quay
- floating cranes of sufficient capacity may be difficult and expensive to mobilize.
Naval architectural aspects: Ballasting of floating crane Stability of floating crane Positioning of floating crane
None of these aspects will pose special problems
3.2 Float-out
This method is used only when the jacket is also
transported as a self-floating unit. The major advantages and
disadvantages have tobe seen as part of the total
transportation and installation system and have been
mentioned in section 2.
It is imaginable though, to use float-out in combination with barge transport by either lifting the jacket onto a barge (in this case floating would be the means to transport
the jacket to a suitable lifting location) or by using a submersible barge.
Naval architectural aspects: -ballasting
-stability, both intact and damaged
In case the jacket after float-out is transported in an afloat condition, no problems about this will exist in this phase.
3.3 Skidding
This is the most common way to transfer a jacket from shore onto a barge. It can be used both for liftable and for
launchable jackets. A skidded load-out may take as much as
two days [3].
Naval architectural aspects:
For a skidding operation some special naval architectural problems have to be taken care of:
-continuous ballasting of the barge will be necessary to keep draft and trim compatible with the skidding
arrangement. Not only the shifting load of the jacket must be accounted for, but also possible tidal changes of water
depth.
-Stability, intact and damaged, of the barge must be checked
for all stages of the skidding operation. In general no problems are expected as the requirements for the subsequent sea voyage will be far more exacting. Possibly free surface reductions for slack tanks may be more than for the sea
voyage, but probably this effect is small. When skidding onto the barge is in transverse rather than longitudinal direction (a method that has been used only once up till now, for the jacket of the Lena guyed tower) a heeling moment will be exerted by the weight of the jacket in intermediate phases, which has to be counteracted by water ballast.
-Longitudinal strength and deformations have to be checked for all phases of the load-out. Deformations may have to be limited in view of a proper skidding operation, or because of the loads they might impose onto the jacket.
- Mooring with sufficient stiffness for a good skidding operation can be a problem if large beam currents or winds are possible.
-Barge motions during load out seldom will pose a problem, but must be taken into account.
3.4 Launching
Launching of a jacket from a slipway into the water will in general be extremely difficult or impossible. If this ever
would be prefered, arrangements, operations and calculations
will have to be made comparable to the launch operation from
CHAPTER 4 TRANSPORTATION
In chapter 2 the various transportation methods have been
described as part of the total transportation and installation
system. The major advantages and disadvantages have been
mentioned. Some less basic choices still have to be made with
regard to the transportation means.
4.1 Liftable jackets, vertically and horizontally transported
Towed barges: Advantage:
-barge dayrate is low, which may be important if load-out is time consuming or if part of the fabrication work takes place on board the transportation barge
Disadvantage:
-speed is low compared to a self propelled barge (or ship)
Self propelled barges: See towed barges.
Crane barge: Advantages:
-less chances for waiting on weather during installation phase
-load out may be performed with own crane. Disadvantages:
-dayrate for crane barge is high
-towage of crane barge if not self-propelled is possibly
more expensive than that of a simple transportation barge
-clear height under crane hook above deck may be less than for a transportation barge, especially if the crane barge is of semi-submersible type.
4.2 Self-floating jackets
No variations exist except for the two drafts mentioned in section 2.3
4.3 Launched jackets
By definition these have to be transported onboard launch barges. These barges are comparable to normal towed
transportation barges, however equiped with specialities, such as launch beams, tilting arms and an extensive ballast system (fig. 8). Self-propelled launch barges do not exist.
r
Fig. 8 General arrangement launch barge [6]
I
. o900 0
0 0 0 0 0 c 0 o-r
L
---o ci COOLINGIWINCII WATER PUMP ROOM--
000
00 o 0 o 0 oo 0:0 0:0 0wArER(j
---I, C 0(0 00 0C, 00 COOeC
TILT BEAN PIN
c
PUMP -1TIc 8 TIc 7 TIc 6 TIc TIc 4 ROOM TIc 2 TIc
41 34 3 5 0
WIRE BRIDLES F AIR LE AD TANK VENT
TOWING BRACKETS
4.4 Naval architectural aspects
4.4.1 Barges (launch and common)
For transportation no basic differences exist between the
naval architectural aspects of normal barges and launch barges. For that reason aspects for both types will be dealt
with together. An example of a jacket on a transportation barge is given in fig. 9 (in this case a launch barge).
-r
'
______
- Stability
Stability requirements normally applied are: -range of stability 30-40 degrees
-area ratio for dynamic stability (as common in the offshore industry) more than 1.4 (see fig. 10).
Fig. 9 Jacket on launch barge [3]
UPRIGHTING ANN CUSVE
MA. I
C SNCL. .JACKST SUOYANCY CONYNIUTION)
NESL ANGLS ( OSONSCI)
WIND ARM CURVS
Ci-B'
Fig. 10 Stability requirements for barge transportThe windspeed to be used in calculating the dynamic stability depends on the season in which the transport
has to take place, the anticipated maximum duration of
the transport and the location of the transport. Detailed criteria normally are set by the warranty surveyors
issuing the towage certificate for insurance purposes.
- Motions
Motions must be calculated mainly in order to determine the inertia loads on jacket and seafastenings. Normally this will be done using strip theory or by performing model tests. The environmental criteria are as mentioned
for stability, however now also using wave heights from various directions. An example of maximum wave heights that may have to be used for various areas is given in fig. 11.
Fig. 11 Maximum wave height of common tow routes [3]
The very often large overhang of the jacket over the sides of the barge may cause slamming between the jacket and the wave. The jacket strength will have to be checked for this. Note that strip theory is based upon a linear motion response to wave height. In this last situation
this certainly will not be applicable. Yet the theory may still give a fair approximation of relative velocity between jacket and waves and hence the slamming loads. Inertia loads on jacket and seafastenings can be
determined from the calculated maximum expected motions and motion frequencies. Alternatively it is possible to base this upon a motion requirement set forward as a standard by the towage certification company, such as 20 degrees roll amplitude in 10 seconds (corresponding to ocean tow), or 10 degrees in 6 seconds.
- Resistance and propulsion
Resistance for the barges can be calculated directly from the information normally available for the barges. To this must be added a wind resistance acting on the
jacket. Because of the height of some jackets, this wind resistance may be considerable. Given a windspeed and direction applicable for the transport under
consideration the wind resistance can be calculated by summing the loads on each member using the wind load formula's as issued for instance by ABS (fig.l2).
te Sign. Wave ight (m)
Japan-!brth Sea 13.4
Japan-zithay 11.0
Japan-ering Sea 14.7
Japan-Calif ormia 13.2
Self of ?xico-Califormta 18.4
Values of C,
Static, or eon,brnatron, of shapes which ct, r,r,t readcls all rntcc the sjwe,fi,d catego-nec cci!! ic colt/oct to cpeeial ecincolenatoccc.
S/cope 2,
C Iivclr,ra! shapes all sizes (.5
Hclhcccrtaee npe 1.0
Dvii !ccccr,r Ill
ctrtcc?trIral chaps,
,craicc's angles cIcorrrrlc. rear,,, etc 1 5 trcder clerk areas socccccth curia....5! I
rcl,.kr. 1 Ins! gcrclers
0,c&dcrrcci, each dcc! I2.5
TABLE 3.2 Values of C1,
iii, height cc nc ft tithe serticul dislaicco' )rcc,lc lice design ccatcr sorl.ccr Ii thcecenter rh Ira cc) 4 cs clrtlncc'cl tic iSle
Fig. 12 Wind loads [7]
For a towed barge the speed attainable which of course depend on the tugs used, but six to eight knots is normal. Self-propelled barges will often have a higher speed.
Manoeuvering
As the jacket has a large windage area it may be
advisable for towed barges to use at least one extra tug to steer the assembly. Self-propelled barges normally do not need this during the transport voyage.
- Local strength
Local strength problems may arise from inertia loads due to ship motions acting on parts of the jacket (the
extremities in particular) and on the seafastenings. Damage may be excessive yield or rupture due to extreme motions or fatigue damage as a result of repeated
loading, particularly in case of long voyages.
Wind Pressure in the calculation of wine!lsr,ss11re. the
follow-ing equation is to be used and the vertical height is to be subdivided
t!cttcc )Fc'ightCc ic'rt (31cc 'cccl 1. ccercic,rcz O,rr 'cccl Exc'cc/rcccc
approximately in accordance with the values listed iii Table 3.2. - 153 3) .- 3(3 1,193
3.3 - 30.3 5)) - 10) 1.10
P = O.O623V C,, C, kg/ni2 P = O.(X)a38V C,, C ll,/2 3)1.5 - 433) 10)1 - 1311 I 20
P = pressure in kg/in2 (lMt2) 16.13
-(II.)) 13)) - 2)03 1.3)1
V0 wind velocity in n/s (kn) in.)) - 76))
2)6) - 3.d) 1 37 C,, height coefficient From Table 3.2
711.0 - 33.7 3.30 - 3)0) .43
C, = shape coefficient from Table 3.
4) 3 - 1)10.3 .5(X) - 13)) I 43
c Wind Force The wind force is tc) be calculated in asce,rdance 005 - 122 I) .330 - 4(5) 1 52
with the following equation for each vertical area and therest, Itant 22.1) - 137.0 4)6) - 430 1.56
force and vertical point cf application is to be detern, ned. 137.)) - 152.5 431) - 549) 64)
52.5 673 3)9) - 53)) 1)11
F = PA
3675 333.)) 350 - 646) I 67
F=force in kg (Ib) 1)23.)) 95)) 11)1) - 65)) I 70
P pressure in kg/icr (lh/ft2) 5)4.0 2)1.5 650 - 700 1.72
A = projected area in m2 (ft2) of all exposed surfiuces in either tin' 213.5 225.5 700 - 750 1.75
upright or heeled condition 2211.5 - 244.0 750 64)1)
I 77 244 33 - 3.56.0 14(0) - 55)1 1 79
- Global strength
Under influence of the varying buoyancy distributions in seaway the barge will deflect and twist. Compared to the barge the jacket often will be quite stiff.
In order to adequately determine the interaction between the two structures will have to be modeled as one
structure (fig. 13).
Fig. 13 Computer model of jacket-barge structure [3]
4.4.2 Self-floating jackets
- Buoyancy, draft, trim
In general, jackets if designed without special measures for self-floating, will possess a buoyancy- weight ratio well below 1.0. Both weight and buoyancy (assuming that
all tubular members are completely closed watertight) will be rather evenly distributed over the entire jacket structure, although always the centre of gravity (COG) and centre of buoyancy (COB) will have spatially
different locations.
A self-floating jacket thus normally will have to be provided with additional buoyancy. This can be done by either adding temporary buoyancy elements (fig.14)
Fig. 14 Jacket with temporary buoyancy elements [5] or increasing permanently the size of certain members, generally some of the main chords (fig.l5).
,.1
j
k
UIAN FIELD SOUTHERN PLATOR,
Fig. 15 Jacket with increased diameter main chords [8] In some special cases the jacket may have sufficient
buoyancy without such special measures (fig.16, where
chord diameter probvably had to be large to resist
bending moment from crushing ice acting on those
cantilevered members).
Fig. 16 Cook Inlet tower foundation under tow [9]
In view of float-out from the fabrication dock, possible draft restrictions on the route, towing resistance and stability during transport it is normal to transport self-floating jackets with a draft such that part of the lower choards is still above the water line (draft 5 to 10 meters). Of course the buoyancy provided must be
distributed relative to COG in such way that trim is near zero.
- Stability
Authorities and classification societies have no
explicit rules for the stability of self-floaters. The insurance and/or the warranty surveyors, however, will have requirements, explicit or implcit. To a large
extent, however, the designer will have to set his own requirements. The question may be raised how important stability is for a jacket under tow: no crew members are on board and a jacket upside down probably would not be lost. Draft, however, might increase unacceptably
(minimum water depth, towing resistance) and upending may become impossible (possible hitting of bottom).
Moreover, damage may be caused during the procedure of toppling over.
Blight [10] addressed the aspect of stability (together with many other aspects). Also Praught and Clifford
discuss stability requirements [8]. By the nature of
self-floating jackjet (two relatively small diameter
floats spaced widely apart) the transverse stability curves are characterized by a large initial CM-value, a
maximum righting moment at a small angle and a limited range of stability. The latter may be influenced by the amount of buoyancy in the upper chords, their height above the lower chords and possible additional buyancy
provided (see fig. 17).
(5
I!
\.\
\
"S Ij .01101 \:\
/,
WTh OFELING MOMENTS' -101IILATED . PDOEL .LTS -- CALQLATED W1Th BOTH TAM(S OF SPONSII FLOODED - CS IU ATED WITh TMIS NOD 6&T(PODS) FLOODED
I0 20 SO 00 50 RGLL ANGLE
Fig. 17 Intact and damaged stability curves [10]
Environmental conditions for the tow depend entirely upon the route and time of the year in which the transport is undertaken. Normally the transport will take a few days at most and good weather may be expected for upending at the end of the transport. It is likely therefore that benign weather conditions prevail during the tow. This
should be taken into account when determining criteria for stability. Reserve stability should be sufficient to resist the overturning effects of wind, wave induced motions, and towline effects. Praught and Williams concluded for tonnage of the Ninian Field Southern platform [8]:
- Range of stability 20 degrees
-Area ratio for 50 knots beam wind 1.4
Damaged stability should be considered with one or two compartments flooded. Praught and Williams [8] use one compartment with a 50 knots wind-heeling moment or two compartments without wind. Attention must be given to the hydrostatic head on watertight subdivisions as damage may
result in quite large trim and heel angles, bringing some subdivisions deep under the water line. This may lead to progressive damaging of separations.
For deep water jackets generally the transverse stability will be dictating, however, for platforms where the
length is not much more than the breadth, the
longitudinal stability may become governing. However, similar criteria will apply for stability.
- Resistance and manoeuvring
Towing resistance of a self-floating jacket is tremendous when compared to ship- or barge shaped structures or even to large displacement offshore structures like semi-submersibles or concrete gravity structures [10]. A
typical resistance curve is given in fig. 18. The main
reason for the high resistance is not so much the drag on the tubular members as it is the amount of wake generated in between the members [10].
6 400 2 0 o 2 d 4 - 200
I
Z u Ui 4 - '00 FT/SEC -0 20 3-0 40 20 6.0 7.0 4.0 30 00 SPED 0 20 00 40 00 60 0 20 .0Fig. 18 Still water resistance and power curves [10]
Towing resistance in waves may increase quite rapidly as the jacket starts pitching and slamming of waves against
transverse members may occur.
Towing resistance can best be determined from model tests, although scale factors may play an important role if members are fully submerged.
Also wind resistance may be an important factor. Wind resistance is normally calculated using a standard procedure as given by various classification societies. This means that relatively large tugs are needed for
towing a self-floating jacket. This may not be needed so much for attaining a high towing speed (the tow does
1 1 1
AHEAD R(SI$TAWCE - STILL rER DATUM WE1041-o-OISPLAC(M(4T. EXTRAPOLATED RESISTANCE NORSEP0ER (EUPS CONDNR 2760LTONS MODC. RESISTANCE S0N(R/OI
/
///
- - - EFEECOVE SPECIFIC 0 KNOTvit,
0. 0.2 03often not last more than a few days anyhow). More important may be the necessity to always have positive control over the towed jacket and to be able to
manoeuver it. For this reason also stern tugs may be provided.
Two head tugs of in total 23,500 IHP towed the Forties Field A platform at an average speed of 4.25 knots [10].
- Motions
Motions are important to be determined for stability
considerations and towing resistance as mentioned above. It is important to check the risk that equipment
installed on the jacket may be hit by green water. Slamming loads on jacket members also may have to be checked. Most important are the motions to determine the inertia forces on jacket members. Acceleration forces of 0.76 g at the extremeties of jackets have been mentioned
[8].
It is difficult to calculate motion responses for
jackets due to non-linearities, in particular submerging
members and the drag influence on fully submerged members. Nevertheless linearity in response has been
found. [8].
An example of a roll response curve is given in fig. 19.
7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
WAVE PERIOD (.}
Fig. 19 Roll response amplitude operator in regular waves [8]
- Strength
During the transportation the jacket will be loaded in a way quite different from the as-installed situation. The still-water loads must be checked, but generally will not pose a problem. The dynamic wave loads and in particular slamming may lead to some very high local loads and an inherent risk of fatigue.
CHAPTER 5 lAUNCHING OF A JACKET FROM A BARGE
Launching of a jacket from a barge is comparable with
launching a ship from a slipway in the sense that a floating structure is transferred from a dry position to the afloat condition by means of gravitational forces. There are, however, also big differences (see fig. 20):
Fig. 20 Stadia in launching a jacket [12]
- the "slipway" (barge plus launchway) is a movable structure. This leads to a varying angle between the launchway and the horizontal. Less important are the facts that the launchway has a variable height above the water line and the fact that the launch barge can move horizontally away from the jacket. - the water is deep immediately in front of the "slipway" - most important: no lift-off (float up) of the jacket
normally will have taken place when the centre of gravity of the jacket passes the aft-end of the launchway. This means that provisions have to be made to the launch barge to allow tilting of the jacket: the so-called tilting beams or rocker-arms (fig. 21). At the same time it means that maximum forces exerted on the end of the jacket structure are limited.
CRITICAL LAUNCH POSITIONS OF DEEP WATER JACIcET
Fig. 21 Double tilting arms [2]
The various phases of a jacket launch operation are:
The barge is ballasted to obtain a certain trim and sliding of the jacket is initiated by removing the last obstacles (including the sea fastenings), or by actively pushing to
overcome the sticking friction.
The jacket slides over the barge without hitting the water or tilting over the rocker arms. During this phase the trim of the barge increases gradually.
II The jacket hits the water and starts picking up buoyancy. The COG of the jacket (including its apparent shift due to the buoyancy) has not yet passed the rotation point of the tilting arms and thus the jacket does not yet tilt.
IliThe jacket tilts over the aft end of the launch barge and slides into the water in inclined condition till the aft end passes the tilting arms.
IV The jacket drops into the water picking up its full
buoyancy. Following a dynamic trajectory it comes to rest in a horizontal position, normally with a small freeboard
(reserve buoyancy in this condition normally is around 10-15 percent [11]).
The total launch may take about a minute and a half [3].
Launching calculations
The simplest launching calculation is one where a symmetric jacket is shifted relative to the launch barge in various stepos. For each step a static equilibrium is found between barge weight, jacket weight, barge buoyancy and jacket buoyancy. Results are draft and trim of barge, angle between jacket and
barge, draft of jacket, resulting force between jacket and
launchways, vertical force on tilting beams. A computer program for making these calculations is developed by Delft University of Technology [12]. Results of such a calculation are plotted in fig. 22. Such a quasi-static program may also calculate the final floating condition of the jacket and the bending moment in
the barge of each step of the procedure.
Fig. 22 Quasi-static launch calculation result [12]
An important draw-back of the quasi-static calculation is that, whereas it may sometimes give an idea about the maximum draft attained by the top of the jacket and of the jacket lower end in the final equilibrium conditions, it cannot handle the
transition period between the jacket leaving the barge and coming to rest in its final position. In this transient phase the jacket bottom may well overshoot its final draft, which may often lead to the maximum draft for the jacket during launching. A dynamic calculation introduces the accelerations of barge and jacket, both when still in contact as when sepe-rated. Normally the proghram will also include damping for barge and jacket. In this way maximum draft of the jacket can be calculated [fig. 24].
A further extension of the program would be to take non-symme-try into account [13].
Fig. 23 Dynamic launch calculation result [3]
In these calculations wave, wind and current actions are omitted as the launch will take place in good weather conditions and essentially free-floating. Barge and jacket
deflections mostly are omitted because they will be small compared to the rigid body motions, but for very large jackets this may no longer be permissible. Barge and jacket flexibility will then influence the distribution of the load transfer. Also
seaway can influence the load on the tilting arms and thus determine the maximum conditions in which launching is possible
[3]
During the process of launching itself two things should not happen:
- an uplift of the forward end of the jacket before tilting on
the rocker beams has started, as this would give too high loads at the aft end remaining on the launchways (too high both for the barge and for the jacket)
- a float off before the aft end of the jacket passed the
tilting beams, because in such situation the relative motion between barge and jacket might rapidly come to a stop and
impacting between jacket and barge might occur.
Furthermore it has to be checked that during the launching operation:
- stability of the launch barge remains sufficient, both transversely and longitudinally (although it might be acceptable to have negative stability in transverse direction during a short period of time)
- neither launch barge nor jacket is overloaded.
To make those checks extensive computer calculations will
be needed
- stability of the jacket during the transient phase till its final equilibrium position must be sufficient to prevent rolling over.
A general description of launch barges is given in [2] and [3]. Barge dimensions are closely related to the jacket weights which they might launch; typical examples are given in fig. 24. A deck arrangement of a launch barge is given in fig. 25, a tank
Fig. 24 Range of existing launch barges [3]
STERN
Fig. 25 Deck arrangement of barge H-85l [3]
-TILTREAM 'NO. OF SLMERS18LE PUMPS PER TANK
Fig. 26 Ballast tank lay-out [14]
Launching is normally performed free floating, heading in the direction of the current to prevent the risk of horizontal rotation of the jacket relative to the barge [3] [11]. Tugs are connected with slack lines to jacket and barge to be able to handle these immediately after launching.
In exceptional cases it oiay be decided to launch sideways. This will be done if the shape of the jacket permits and/or requires this as with the LENA guyed tower [14].
OP-8 OP-N 0P5 OP-4 OP-S
-.
OP-I
p-a
®
P-i k1 P-S P- IP-2 IP-I
0
®o@
IS-B IS-TA I-1 i$-S + HJIQHJIOI&4L!IIilhlE--'
flJ IflOJj[ S-I0
I iio
t4Ij
.*1:t
OS-Bo
OS-K®
0
OS-S0
OS-4 .0
OS-Iarrangement in fig. 26.
ManE LXBXD (m) Max. Jacket Wght. (t) Max. Laurch1action (t)
H-401 122x36.6x8.0 10 000 9000 1-600 152x36.6x10.2 16 000 10 000 H-hO 160x42.OxlO.7 20 000 17 000 BPR 376 177x48.8x11.O 30 000 17 000 H-109 183x47.3x11.6 30 000 17 000 M-44 190x50.Oxll.4 30 000 19 000 1-650 198x51.8x12.2 35 000 25 000 KSC 700 213x55.5x12.2 40 000 30 000 H-851 260x63.0x15.0 60 000 40 000
CHAPTER 6 UPENDING
Upending of a jacket is necessary to bring it from a horizontal floating condition into a vertical one. Both self-floating
jackets and launched jackets must be upended. As generally the draft of a self-floater is much smaller than that of a launched jacket, the procedure generally will be more complicated for the
latter. On the other hand all parts of a launched jacket will submerge for some time leading to special attention when installing control equipment on it. IJpending is done by sequential flooding with or without assistance of a crane.
Assistence of a crane alleviates the naval architectural
problems.
When a self-floater is upended the floatation members (either
increased diameter chords or auxiliary buoyancy tanks) will
probably submerge after a small pitch rotation. This will mean a tremendous loss of longitudinal stability leading to a rapid
further pitching of the jacket. This motion is slowed down or stopped when the upper chords get into the water (it is assumed that the upper chords also will have buoyancy or are fitted out
with auxiliary buoyancy tanks). This first rapid rotation may be
through 40 or 45 degrees, [8] [10] (see fig. 27, 28).
TOWING POSITION
FIRST HOLD POSITION
THIRD HOLD POSITION
FOURTH HOLD POSITION
LANDED ON BOTTOM
400 T 300 200 00 W&1ER SUAG( DATUM - 00 STAR
Fig. 28 Upending of a jacket with buoyancy tanks [5]
During this rapid rotation transverse stability may become
negative (fig. 29). Inertia of the platform will prevent rolling
over if the time rapid rotation takes, is short enough. On the other hand one should check what happens if asymetric floading accidentally would occur. Severe "heel" of the jacket may be
acceptable, "capsizing" not [8]. STAGE 2 CRANA - FLOOR
TNA30SE IAETmCC1*T (ii)
CENTER OF NRAVITY (COT
PITCH NIT CAL II3T ROLL IROTAGLITY 60 70 SO SO I00 PflCH ANGLE (OGGREEST STAGE 3 POST-FL000 .. SUBMERGED (4&3T STAGE 4 TRIM-VERTICAL STAGE S SET- OS .UCATON CENTER OF SP'ER 200 SUOTANCY ICR) ItflTFS
I. CG&CB BASED OS ADDED WEIGHT METHOD USING SEA WATER
2.M(TACENTRE HAND CALCULATED USD40 Tad MT OFAREA ABOUT AXIS IN WATERPL.TNs
Fig. 29 Metacentric diagram of transverse stability [10]
After reaching the intermediate equilibrium flooding is con-tinued in a controlled way leading to a gradual further pitch angle of the jacket (fig. 30). For this phase of the operation
transverse stability must be checked. When the jacket approaches the upright position a certain clearance to the seafloor must be
watched. It may be decided either to bring the jacket into a
completely vertical position and land all legs simultaneously or
to leave a small inclination and to land on one side of the jacket first.
HO
RO.3I
WI
ThtN..It Mt PlIDI MT C)
A D$SPP.ACEMVNT <RII.N HelM Se,c A SIedMg ANIORI
O COOHO HO ORTT s iHOt edee. .*l
. CENNIHO .00RCT CO * ti.pla..Z.t} - ORNOI. ThcM%rHO CF CONTROHO HO ONT1.MI1 AA HO OH_AF OF AONWNOLLORT O - PZORTM. fl1 RTS CF C TOIHO HOWl 3. IS AONW- - -S LA-SWl RONDO ANDN ND 101 HORIZONTAL U87
-T .ROlES A IS DIROCIfl ROOMflØ5 HO ROitaS I'fl 57 ZORO S
AIND HOTORNERt TO MiD. NT DROP C LOTPNDIPT - iRS IS A ISM SYND t
Fig. 30 Pitch stability during uprighting [10]
After upending at or near the final location, the jacket will be brought in position with help of tugs and/or lines to barges moored around the location. After locating and orienting the jacket it will then be lowered by ballasting to the seabed, after which the piling will immediately start to give positive stability to the jacket.
A similar procedure for a jacket after launching is already illustrated in fig. 7.
Instead of manually or remotely controlling the valves it is also possible to make a jacket self-upending after launching by opening up some members. The advantage is that upending is performed in a very short time [11]. Of course less control
exists and the procedure is completely irreversible (which to some extent is also true for the upending of self-floaters: the rapid, instable, rotation cannot be reversed) (see fig. 31).
SCO5TROCCEDAZP ZEHO StOFE OF ay 0
iii-iigii
r
xruyuu DTF 2OI030
Ff1 OJFA 6* K-1U51 161Ff 6/0 .1.fW!tflI 1*1* *16011
TO0 PINTICUI.RR5
TOWER WI/364T . 93'%71.40.6 (WI
COO OX) - 0.002 (XIJ
IT) . 0.1,43 IV*J)
(z) (zko)
,XTI LJ3C46/6OG COIJOITIOPJ
Dli' 206.03.0 OE6OOT5.. *EW4 (0UG44T 2.00.,
BNRGE TROM 4 0O. O6'I*.94C F6OCTl*j . 3003.
1 32.0 TRAJECTONY OP ONE PIECE OPTION IN PREIISYAILED PLES
Fig. 31 Launch trajectory for seif-upending jacket
Assistance of a crane greatly simplifies the ballasting
procedure. Also positioning of the jacket can be done with much
more accuracy and delicacy. This method is preferred when a
platform has to be stabbed over pre-drilled wells. Up till now
crane assist has been limited to 1000 to 1500 tons hookload, but there seems to be no reason why this should not go up with
todays large crane capacity [3].
lift 6 206.0 IWCflFNFW' 60 fly ',U
CHAPTER 7 JACKET LIFINC
The lifting of a jacket from a barge and setting it on the seabed sounds like an operation with little or no naval architectural impact other than mooring and stability of the crane barge (about which more in later sections of this paper). This would be true if the operation were to take place in
completely calm seas. Normally however, this will not be the case and dynamic effects will play a major role.
A typical example of how a jacket might be handled by a crane barge is already shown in fig. 5.
Various dynamic effects can be distinguished:
At lift-off of a jacket from a barge the relative motion
between hook and jacket due to motions of crane vessel and transportation barge may result in (repeated) shock loads in the hoisting tackle. The severity of this will depend upon
relative speed when the tackle becomes taut, the mass of the jacket and above all the flexibility of tackle, crane boom and crane foundation (fig. 32).
-Fig. 32 Shock loads in hoisting tackle [17]
With the load hanging in the crane the load may start
swinging, possibly in resonance with the vessel motions due to wave action [16J, or as the result of vessel acceleration when relocating the ship in its anchor pattern [18].
When a load is partly or completely submerged in the water
whilst hanging in the crane ship motions and the resulting crane hook motions result in inertia loads exerted by the accelerating mass on the crane. The mass to be accelerated
now not only consists of the mass of the load, but also of the added mass of water. At moments where the load is not accelerated the drag forces due to the relative velocity between the load and the water also may play a big role. The
relative importance of added mass forces and drag forces depends on the nature of the structure. Nevertheless these hydrodynamic effects may severely increase the loads acting on the crane. A 100% increase when compared to a load in air
The situation may be worse in case of a jacket with some
amount of buoyancy.This may greatly reduce the static load on
the crane. The dynamic loads however remain unaltered. As a consequence the dynamic load may be a multitude of the static load. An important aspect, when it is wished to use the crane capacity to its utmost.
Finally when setting a load onto the sea bed (in case of a jacket) or onto a jacket (in case of a module being set on a jacket) similar shock loads may occur as described above when
lifting a load from a barge.
All these dynamic load effects are of a highly non-linear
nature, not in the least because of the interaction that may
exist between the various excitations and responses. Computer programs are now being developed to simulate
hydrostatic, hydrodynamic, gravitational and inertia forces on crane barge, load and transportation barge taking into account
the relevant flexibility of crane boom, hoist tackles, etc. This probably is the only way to approach these problems. An example of such program is LIFSIM (see fig. 33).
Note that the comments given above apply not only to jacket lifting, but also to lifting of any other loads such as modules, templates, etc.
LIFSIM
l.a Stan In. EWIRICM. .. nt.qretion curren NELAIIOSS current arts, of 6 COupled different 1.1
RLSPON5 fortes
Fig. 33 Schematization for and flow diagram of LIFSIM [19]
PoSitIon Ifl! oriental On Positlo, and orientation CRANE VESSEL IMPULSE F
RESPOVISE oeqes Vetoc cEiCs Wind and
Ti step lnteqflt Ion EVIRICAL IWVnd Wed of 6 CedpInd
iii
fiat Ions p0SfltUO1 and correct dat. EATMP$
JCurrent Fe,...,
1st end 2nd ord.r
edceC1.0.100 IMPULSE ESPUS: OOfOrrS of MPtIOn OrIent.1041
TECHNIQUE
fld dt H
RELATI)I
Wind fprte
a... .l.o.to.. IMP(sLSE edsPoes4
TECI4UIQU( lt Led Qed0
ant forces MPORIIRS CRANE SYSTEM LedO TIen stop Intflrlt041 of 6 cOopl.d dcff.rtIWi eq...1 10,55 of ada IMPACT
CHAPTER 8 CRANE VESSEL DOWNTIME
Closely related to above description of lifting operations is down-time of the crane vessel.
Down-time may be caused by loads in the system becoming too high as a result of vessel motions. Also impact forces of loads
hitting a target structure (e.g. a module being set on a jacket
or deck). Either structural damage to equipment may be the result.
Quite often the limiting factor is safe working. In particular
handling of slings for heavy loads whilst the hook is swaying
may be difficult.
It may be required for the naval architect to determine from the environmental conditions expected, the vessel
characteristics and limit criteria for operability via calculations the down-time expected for the crane vessel in working on a certain job. Actually the results of such calculations may be used in selecting a crane vessel for a particular operation.
Computer methods for determination of down-time through simulation of the actual operation have been developed.
Down-time if caused by problems of setting down or picking up a load may be influenced by choosing the right moment of
performing such a short duration task. Normally in sea conditions periods (minutes) of relative severe motion are
alternated by calm periods (lulls). If such a lull period can
be predicted the risky job of setting down or picking up, a
load can be done at the right moment [201. This will shift one limit of operation.
CHAPTER 9 CRANE VESSEL POSITION MOORING
A crane vessel is used during a large part of the construction
of a fixed platform. During this period the ship will have to keep position near the platform.
The common way to do this is to moor the vessel in a spread mooring system with 8 to 16 anchor lines. Those lines mostly will be attached to anchors, but sometimes to permanent anchor
piles [21].
In selecting a mooring pattern several aspects have to be taken
into account.
Of utmost importance is of course a position of the crane
vessel relative to the platform (or platform site if the
jacket still is to be installed) that allows efficient
working, reachability for the crane of all relevant platform
areas with sufficient lifting capacity, heights and
clearances. Also of great importance is good orientation with respect to the prevailing weather conditions in order
to minimize down-time, good offloading locations for the transportation barges, possibility to bring a gangway to the fixed platform
The crane barge should not only have a working location, but
also a stand-off location for severe weather conditions where bigger motions can be accepted without hitting the
platform (fig. 33). Note that in this stand-off position the crane vessel may be orientated to some extent into adverse
weather conditions. For severe storms some crane vessels are
expected to leave the area completely and ride out the storm
at a more remote location.
PLE$ V. IN Pclrflow (IYO*N)
uI*
rn
VEIICI. IN P01*11CM (OP1*ATNN)Fig. 34 Typical anchor pattern for crane barge [21]
anchors may not be positioned within a certain distance of
pipelines. Also the anchor line if crossing a pipeline should have a minimum height above this, if necessary with help of an auxiliary floating buoy (this clear height is to be maintained, also when running anchor).
If another vessel is moored nearby care should be taken that crossing anchor lines have a certain clear distance to each
other. Finally anchor lines should not hinder the approach of
transportation barges and tugs and should have sufficient clearance to the jacket.
of course the anchor pattern must be such that maximum line tensions and vessel excursions are kept within limits, both in the operating and in the stand-off condition with all lines intact and with a broken line. The traditional quasi-static calculation procedures for this is outlined in
appendix A.
Appendix A also mentions the dynamic effects that occur to a vessel moored in an anchor pattern and which should be taken into account in evaluating the anchor pasttern. In particular transient motion after line failure and possible resonance to slow drift excitation may be important with a vessel moored
near a fixed platform.
The anchors must be run and laid by special anchor handling vessels. The anchor must be run with a certain line tension
depending upon an acceptable cable sag under the surface. MacDonald [22] gives a formula for the bollard pull needed
(fig. 35). The anchor may be marked by an anchor buoy and retrieved with a pendant wire (fig. 36) or with a chain chaser (fig. 37).
Fig. 35 Running anchor line using handling vessel [22]
CHAPTER1O CRANE VESSEL STA1ILITY
In judging the stability of a crane vessel distinction should be made for three conditions:
- survival condition
- operating condition, not lifting - operating condition lifting
The first two conditions are governed by the normal
requirements of authorities and classification societies for this type of vessel (We will forget at this moment that they often do not have requirements, in which case the naval
architect must follow his own judgement about the adequacy of certain rules.
The operation condition lifting poses some special problems.
De Vries, Kaldenbach and Suyderhoud treat the standard
stability of a crane vessel [24] see fig. 37.
Fig. 38 Symbols stability calculation [24]
When compared to normal ship stability some special aspects should be mentioned:
- In calculating the vertical centre of gravity the load
should be considered to act at the attackment of the hoist tackle to the crane boom.
- A consequence of heeling is that the outreach for the crane
changes
- Limit for stability is generally a maximum vessel heel of 5
or 6 degrees, being considered to be a limit for efficient
working
Limit for heel may also be the angle under which the crane boom may be loaded, e.g. 50 in plane of the boom or 20 out of plane. In particular when the crane is working in a diagonal direction this may become a governing criterium.
- In order to minimize vessel heel and cross loads to the
crane the barge will often be preballasted to the opposite direction before picking up a load.
- If preballasted attention must be given to correcting the
ballast condition when slewing with the crane. This may effectively reduce the usable slewing speed of the crane.
- If preballasted dynamic stability in case of hoisting wire
breakage may become problematic (see fig. 39).
COUNTER-HEELING ARM (COUNTER-BALLAST MOMENT
x cos B A
RIGHTING ARM CURVE (WITHOUT HOOI( LOAD)
RIGHTING ARM
EQUILIBRIUM HEELING ANGLE (OEO)2
WHEN THE HOOK LOAD LOST
HEELING ANGLE
HEELING ARM
-(BOOM HEEL
MOMENT-COUNTER BALLAST MOMENT) ç9! A
(BAG)1 EQUILIBRIUM HEELING ANGLE JUST BEFORE THE LOSS O HOOK LOAD
Fig. 39 Dynamic stability at line rupture [25]
A rapid ballast system as provided on some of the very large
lifting capacity cranes to day (Balder, Hermod, Micoperi 7000) has two functions:
- keep the vessel level during all phases of a lifting operation: picking up, slewing, setting down
- increase the effective lifting or lowering speed of the hook
during the critical phases of picking up or setting down [2]. Although a rapid ballast system can theoretically alleviate
many of the traditional crane barge stability constraints, and thus lead to a smaller vessel, it is considered prudent not to take the beneficial effect of such a system on stability into account. Neither is a negative effect as the result of
malfunctioning or maloperation taken into account.
The principle of Balder and Hermods rapid ballast system is
taking water into empty tanks under the waterline or emtying tanks above the waterline through gravity using computer controlled large capacity valves (fig. 40).
F.C. Michelsen, C.v.Zandwijk, J.L. Beguin P.C. Michelsen, C. v. Zandwijk, J.L. Beguin M.Lefranc, M.Brheim S.J. Hruska, A.M. Koehier, L.K. Shaw R.Szajnberg, W.Greiner, H.H.T.Chen, P.Rawstron 7] American Bureau of Shipping M.W.Praught, W.R.H. Clifford J.B. Daigle G.J. Blight References:
Care and maintenance of Aiwyn North, The Oilman, October 1987, p.27
On the installation of offshore platforms with emphasis placed on jacket launching, pile driving and lifting operations
PRADS '87 pp 462-491
Transport and installation of fixed deep water platforms. Deep Offshore Technology 1987
Design of a lift-installed
jacket for 175 m water depth; Veslefrikk.
BOSS '88 Addendum pp. 42-56 Engineering the procedure for installing the North Sea Forties Field platforms
Paper 2248, OTC 1975
Practical design approaches for the analysis of barge performance in offshore transportation and launching operations
Trans. SNAME vol.88, 1980,
pp . 195 -223
Rules for building Mobile Offshore Drilling Units, 1988 Naval Architectural
considerations in the design of the Ninian Field Southern Platform,
Paper 3045, OTC 1978 Cook Inlet, drilling and production platforms Offshore Exploration Conference 1968 Naval architecture
contribution to the transpor-tation and installation of the
North Sea Forties Field
platforms
[11] J.G. Mayfield, J.P. Haney Installation of platform Boxer
W.H. Luities Paper 5605, OTC 1987
J.A. Korteweg, A.Goeman
C. Aage, P.E. Christiansen, J .Moller B.W. Dearing, M.W. Lucas, C.K. Snell P.E. Christiansen, M.T. Clarke, M. Martin K. Sekita, H. Shiniada, M. Taniyama M.v.Holst K. Sekita, H. Kimura, M. Tatsuta H.J.J.v.d.Booni, J.N.Dekker, R.P.Dallinga D.Hoffman, V.K.Fitzgerald E.A.Lowe, V.C.Yin R.C.MacDonald Een quasi-statistische
computer simulatie betreffende het tewaterlaten van een
off-shore-jacket van een drijvende ponton (LAUNCH)
Report 729, Deift University
of Technology, Ship Hydro-mechanic Laboratory, Nov. 1986 A jacket launch computer
program compared with two full-scale launches
BOSS 1985 pp. 325-333
The design and development of
loadout procedures for the LENA guyed tower
Paper 5046, OTC 1985
The application of inflatable reinforced rubber bags as
auxiliary buoyancy for offshore installation operations
Paper 5603 OTC 1987
The operability of derrick
barges in construction of large offshore structures Paper 2635 OTC 1976
Lecture notes
Deift University of Technology
Dynamic lifting analysis of offshore structures
Paper 5287, OTC 1986
Computer analysis of heavy lift operations
Paper 5819, OTC 1988
Systems approach to offshore crane ship operations
Trans. SNAME, Vol. 86, 1978, pp. 375-412
Permanent mooring system for large construction and support
vessels in the Piper Field
Paper 3394, OTC 1979
Positional mooring offshore Lloyd's Register Technical Association, Paper 4, 1984-85
C.E.Zumwalt H.d.Vries, W.P.Kaldenbach, J.B.H.Suyderhoud S.Ghosh, F.S.Chou, E . W. Huang S.Klug
Deepwater mooring operations in the Gulf of Mexico
Paper 5249, OTC 1986 Design, construction and workability of a
semi-submersible derrick barge with 2,000 tons revolving crane Paper 3296, OTC 1978
A rational approach to the design of a pipelay/derrick semisubmersible barge
Trans. SNANE, Vol. 87, 1979, pp. 40-63
A control-system for an
ultra-high-speed ballasting
equipment
APPENDIX A QUASI-STATIC MOORING ANALYSIS A.l. Characteristics of single line
If a vessel is moored in still water to one anchor line and a
constant external horizontal force is exerted to the ship in plane with the anchor line, an equilibrium position will be
reached . If the anchor line is inelastic it wil attain a
catenary shape as indicated in fig.Al from [A.11. The shape will range from neutral through slack and taut to the geometric
maximum.
Fig. Al Various conditions of an anchor line IA.11 It is clear that the displacement of the vessel from the neutral position will depend upon water depth, line length (or
the scope, i.e. the ratio between line length and water depth), the external force and the weight of the line per unit length. For a given water depth, line length and specific line weight the relation between displacement from the neutral and the horizontal force may look like fig. A2. The curve will asymptoticaly
approach a displacement (excursion) equal to the geometric maximum.
If only the specific line weight changes, the load-excursion relation will also change, but the asymptote will remain the geometric maximum. If the line would be weightless in water no
load would be needed to reach the geometric maximum, after which the asymptote would be followed.
On the other hand if the specific weight increases the force needed for a certain excursion increases, but the asymptote would remain the same (fig. A3).
40
20
-20 40
Fig. A2 Force-displacement curve for one anchor line
100
SCOPE = 2.0
DEPTH = 1500
80
-DISPLACEMENT FROM NEUTRAL - DEPTH
Fig. A3 Effect of specific weight [A.1]
>c U C I-z C N C 60
-
SPECIFIC WT (W) 08.69Closed form mathematical solutions exist for determination of
the basic catenary relationships as in fig. A.4 from the API Recommended Practice for mooring systems [A.2].
rP
x ,2, (p,/W)2 Y (Ph/W)(cosh(WXIPh)_1)
, /W)inh(WX/P,)
Fig. A4 Basic catenary relationships [A.2]
In tabulary form those relationships are given in table A.l and fig. A.5 from [A.3].
Fig. A5 Catenary diagram [A.3]
In the catenary calculations it is assumed that the line
(anchor chain, steel wire or synthetic fibre wire) possesses no
bending stiffness. Hence the line tension is related to the top end horizontal force through the cosine of the angle between the top end of the line and the horizontal (see fig. A5). Line
tension rather than restoring force can be plotted against displacement. This would yield a curve similar to the one of
fig. Al. Often the two curves are plotted in one diagram. If the line instead of being inelastic has linear elasticity, the load-displacement curve changes, in particular the asymptote (see fig. A6 from {A.4]; note that this curve uses line tension
Table Al Non-dimensional catenary relationships [A.3} Of course all curves shown have an upper limit related to the breaking strength of the line.
Note that in practical mooring the anchor line in the vicinity of
the anchor itself should always remain on the seafloor as any uplift to the anchor will destroy its holding power. A
consequence of this is that hauling or paying out of anchor line shifts the neutral position relative to the anchor, but the
anchor characteristic relative to that position remains the same. L-x
r
T wii WITTH T 6'H s H '8' L-xri- wii
I
wiiT, T, WIT s H 4' .01 2223 2222 66.67 1.719 .5! 1.570 0.5702 .463 68.7! .02 556.3 555.3 33.34 3.436 .52 1.538 .5382 .441 69.52 .03 247.6 246.6 22.23 5.151 .53 1.508 .5082 .420 70.31 .04 139.6 138.6 16.6$ 6.862 .54 .480 .4800 .400 71.08 .05 89.61 88.6! 13.3S 8.567 .55 1.454 .4536 .38! 71.82 .06 62.44 61,44 11.13 10.27 .56 1.428 .4278 .362 7256 .07 46.05 45.05 9.545 1.96 .57 1.404 .4045 .345 73.26 .08 35.42 34.42 8.357 13.65 .58 1,382 .3818 .328 73.96 .09 28.13 27,13 7.434 15.32 .59 1,359 .3594 .3!! 74.67 .10 22.92 21.92 6.696 6.99 .60 1,340 .3398 .296 75,3! .11 19.07 18.07 6.094 8.64 .6! 1.32! .3205 .28! 75.96 .12 16.14 15.14 5,593 20.27 .62 1.303 .3026 .267 76.37 .13 13.85 2.85 5.167 21.91 .63 1.285 .2850 .253 77.19 .14 2,04 11.04 4.804 23.52 .64 1.26$ .2676 .239 77.82 .15 10.58 9.580 4.490 25.!! .65 1.253 .2528 .227 78.36 .16 9.383 8.383 4.215 26.69 .66 .238 .238! .215 78.9! .17 8.392 7.392 3.973 28.26 .67 1.224 .2236 .203 79,47 .18 7.56! 6.56! 3.758 29.80 .68 1,210 .2104 .192 79,99 .19 6.858 5.858 3.566 31.33 .69 1.197 .1974 .181 80.5! .20 6.256 5.256 3.393 32.84 .70 .185 .1845 .170 81.04 .2! 5.742 4,742 3.238 34,33 .71 1.173 .1728 .160 81.53 .22 5.296 4.296 3.097 35.79 .72 1.162 .1624 .151 81.97 .23 4.905 3,905 2.968 37.24 .73 1,15! .1509 .141 82,47 .24 4,56! 3.561 2.850 38.67 .74 1,141 .1407 .132 82.92 .25 4.259 3.259 2.742 40.07 .75 1.132 .1317 .124 83.32 .26 3.990 2.990 2,642 41.46 .76 1.123 .1227 .116 83.73 .27 3.754 2.754 2.55! 42.8! .77 l.I14 .1138 .108 84.14 .28 3.538 2.538 2,465 44.16 .78 1.105 .1050 .100 84,55 .29 3.346 2,346 2.386 45.48 .79 1.097 .0973 .093 84.9! .30 3.175 2.175 2.313 46.76 .80 1.090 .0897 .086 85.28 .31 3.018 2.01$ 2.244 48.04 .8! 1.082 .082! .079 85.65 .32 2.876 .876 2,180 49.29 .82 1.076 .0757 .073 85.97 .33 2,747 .747 2.120 50.5! .83 1.069 .0692 .067 86.29 .34 2.630 .630 2.064 51,70 .84 .063 .0628 .06! 86.61 .35 2.522 .522 2.011 52.88 .85 1.056 .0565 .055 86.93 .36 2.423 .423 1.961 54.04 .86 1.051 .0512 .050 87.2! .37 2.332 .332 1.914 55.17 .87 .046 .0460 .045 87.48 .38 2.247 .247 1.869 56.27 .88 1.041 .0408 .040 87,76 .39 2,171 .17! 1.828 57.36 .89 1.036 .0356 .035 88.03 .40 2.098 .098 1.788 58.44 .90 1.03! .0315 .03! 88.25 .4! 2.03! .03! 1.750 59.49 .9! 1.027 .0274 .027 88,47 .42 .97! 0.9706 1.715 60.49 .92 1.023 .0233 .025 88.69 .4344 .860.913 .9129.85% 1,681 61.50 .93 1.019 .019! .019 88.93 .649 62.47 .94 1.015 .0151 .015 89.15 .45.46 .809 .8090 1.61$ 63.44 .95 1.012 .012! .012 89.3! .762 .7625 1.589 64,36 .96 .009 .0090 .009 89.49 .47 .720 .7200 .562 65.25 .97 1.006 .0060 .006 8966 .48 .678 .678! 1.535 66 16 .98 .004 .0040 .004 89.77 .49 .640 .640! 1.510 67,03 .99 1.002 .0020 .002 89.89 .50 .604 .604! 1.486 67,88 .00 I .000 0.0000 .000 90.006O ooc oc oc a 0 DEPARTURE
Fig. A6 Tension characteristics of mooring line
A.2. Vessel moored to a single line in seaway
A vessel in a seaway experiences forces from wind, waves and current. The total force Fenv may be split into a constant mean force (wind, current and mean wave drift), a relatively high frequency oscillating force (the 1st order wave force) and
slowly varying forces (the slowly varying wave drift forces). The environmental force tends to displace the vessel, but this is counteracted by a horizontal restoring force exerted by the anchor line. This latter force Frest is related to the vessel excursion as indicated in section Al above.
From ship motion theory we know that the vessel motions in this situation are governed by the following differential equation: (m+a) + b
FenvFrest
Fconst+Fwaves+FsvdriftFrest (1)
where m is the ship's mass, a the added mass, b a damping constant and x the displacement from the neutral.
If a motion response x x1 cos w t is assumed, a solution is found with w equalling the wave circle frequency. (x1 is motion amplitende, w is motion circle frequency, t is time). It is easy to see that if w is high and the amplitude of the wave forces are high the solution of (1) will yield mainly ship motions as
response to the waves hardly influenced by the constant
environmental and slowly varying wave forces. If the stiffness of the mooring line (stiffness is restoring force-displacement
ratio) is small, the change in mooring force is also small and this will have little effect on the ship motions. In other words the wave forces are mainly balanced by inertia forces due to ship motions, On the other hand a constant environmental force,
