ABSTRACT
Hydrostatic stability is associated with a condition where, for a small angular displacement of an object, that object will return to its equilibrium position without any external help.
This paper discusses the phenomenon whereby an object might not return to its original position due to the magnitude of the displacement involved. Such situations are possibly more a question of seaworthiness rather than stability. However, throughout the text the term stability has been used when discussing the phenomenon.
The first part of the paper briefly describes the basic philosophy currently used to verify stability, simplifications in the procedure and effects not being given explicit attention.
The next part gives a description of the work done recently by VERITEC.
The last part is a discussion of the possible consequences of the latest findings including some conclusions from a joint venture research project recently being finalized together with a short outline of proposed further work.
I. INTRODUCTION
For some years experts dealing with floating stability matters, either on the engineering, constructing or approving side have been aware of a certain inadequacy in the floating stability criteria. Effects which may be of significant importance for the platform stability are not properly taken care of. Although this is well known, few steps have been taken to upgrade the approach and criteria to incorporate these findings do not exist. In addition there is a trend to consider floating stability and seaworthiness not as just a separate property but rather as an important part of the total integrity of the buoyant platform. This is e.g. reflected in the 1984 revision of the Norwegian Petroleum Directorate (NPD) regulations for structural design of loadcarrying structures /15/. In our development of new concepts for floating and compliant structures VERITEC has found it necessary to further develop this field.
In order to establish the-state-of-the-art of floating stability and from there make further investigations into this field a research project was launched in January 1983. The Droject has continued through 1984 and will further continue through 1985. This paper is presented at this time to expand the philosophy
behind the work that
has already been completed.2. CURRENT DESIGN REQUIREMENTS 2.1 Background
The basic philosophy behind current stability 7equirements for mobile offshore units is to a great extent Dased on experience from ships in general. There appeared to be no need for major changes in the philosophy when stability requirements for semisubmersibles and jack-ups in transit were established. Only slight modifications were
TECHNISCHE UNIVERSITET Laboratorium voor
Scheepahydromechanica
ArchiefMakelweg 2,2628 CD Delft
Ca;016- 786873 -Fax 015-781838FLOATING STABILITY AND SEAWORTHINESS -TRENDS AND DEVELOPMENT
by
M. Morland., J. Wiborg it, S.A. Lotveit +, M. Sigurcisen Senior Engineer, VERITEC
Project Engineer, VERITEC SFnor Surveyor, VERITAS
introduced, as necessary based on the gained experience. After extensive studies on the behaviour of e.g. semisubmersibles, however, strong objections to this adoption were made. Significant effects were found to be disregarded to a greater or lesser extent when verifying the stability requirements.
2.2 Current Requirements
With the exeption of the 1984 issue of the NPD regulations there is in general a great similarity in the rules issued by the different governmental bodies and classification societies. The basic philosophy is the same, but the specific acceptance criteria may be somewhat different. To claim a platform designed according to one set of codes
to be
significantly more stable, compared to a similar platform designed according to another set of codes is, however, not possible. One reason for this is the fact that the different societies and governments try to coordinate their requirements in this respect. An example is the work carried out and conducted by IMO, a result of which is presented in the IMO Modu Code /16/.Harmonization of stability requirements is further on the agenda under the North-West European Certification Scheme (NWE).
The main elements of the requirements more or less recognised in all rules and regulations, are outlined below. Intact Stability
There are two main elements in the theoretical verification of intact stability properties.
To avoid any initial heeling of the platform, the rnetacenter must be situated above the center of gravity when the platform
is on an
"even keel". This is expressed as a, "positive initial metacentric height". The magnitude required, however, differs in the various codes.To establish safety against capsizing when exposed to e.g. environmental loading, the absorbed energy from the heeling moment must be less than the potential energy represented by the righting moment. The heeling moment is based on the static wind force. To take into account other possible heeling effects such as wind gusts, waves, current, moorings and associated dynamic effects the area under the righting moment curve shall exceed the area under the heeling moment curve multiplied by a factor greater than one (usually 1.3 or 1.4), see Fig. I. The unit is assumed free floating when calculating the wind heeling moment.
Damage Stability
Sufficient stability in the damaged condition is verified by assuming that a certain volume of the intact buoyancy bodies is flooded. This volume is normally limited to one or two compartments adjacent to sea.
To improve the compartmentation of the platform a collision with e.g. a ship will have to be assumed. The extent of the damage of the platform due to this collision is also
specified by a minimum extent of the damage, both lorizontal, vertical as well as inward.
In an assumed flooded condition the following general itability criteria are recognized in the major part of current -ules: When exposed to a certain static wind force, the waterline in the final equilibrium condition is to be below my opening through which progressive flooding may occur". rhe platform is assumed free floating.
2.4 Effects not being given Explicit Attention
The present requirements are all based on considerations of the "static properties" of the platform, and the only parameter considered creating heeling is the static wind for7.- The real dynamic behavior is not taken into account. In addition, the philosophy behind some of the present requirements seems to be too simple. In the following important parameters influencing the floating stability and not explicitely taken into account in current procedures is discussed.
Wave Action
Depending on e.g. natural periods,
the wave heel
amplitude may contribute significantly to the total heel angle. For many designs the 1st order wave response may contribute more to the total heel angle than static windforce.
In model tests with semisubmersibles a particular phenomenon termed "mean heel" or "steady tilt", /1/ and /12/, has been observed. This phenomenon is explained by a heeling moment caused by asymmetrical second order wave forces on the pontoons or footings. In irregular seas, the mean heel will appear as a low frequency oscillation. Under certain conditions the mean heel may be significant, in the order of 10 degrees, /11/.
Wind Action
As mentioned above only the heeling moment caused by static wind is currently considered. The response due to gusting winds may, however, be significant.
Several floating platforms operating today have eigenperiods (roll and pitch) in the range of 30 to 80 seconds. This range of periods is in the high energy range of the gust wind spectrum. For this reason moderate gusting may lead to significant angles of heel.
Relative Motion
As the present verification procedure is based on a static equilibrium approach in "still water" the relative motion between the platform and the sea surface is neglected.
In reality the possibility of flooding the platform through an opening may therefore be greater than that which a "static approach" with still water condition will indicate. This is particularly important for jack-ups in the floating mode and for similar structures where the freeboard is small. Current Action
For some concepts heeling angles of some importance may be experienced due to current action. If the underwater body is deep relative to the width, and the mooring lines are attached to the deepest part of it, the heeling moment due to current may be significant. However, for most of todays platforms this effect appears to be small.
Mooring Forces
Compared to the free floating condition, the attachment of mooring lines may lead
to a change
(inlocation) of the rotational axis depending on the fairlead position.
In addition, the mooring line forces applied on the platform may give a resulting moment.
Operational Load Handling
During operation of one of the various systems onboard the floating platform large loads may either controlled or
uncontrolled change position leading to important changes in stability properties.
(A -4-B) Kx(B+C) HEEL ANGLE
Righting Energy K x Heeling trorgy
rig. I. Explanatory Sketch, Area-ratio
.3 irnplificat ons Inherent in Stability Verification :alculation of wind heeling moment
rieeling moment cue to static wind is the only ,nvironmental effect to be considered explicitely.
Calculation of the wind heeling moment has been iscussed ever since the stability requirements were stablished. An exact analysis is
difficult due to
e.g. omplex geometry, interaction and shielding effects between he various modules and air flow under the deck. Lift forces re required to be included in some rules whereas others isregarci them. For units with ship shaped hulls the moment lay be assumed to vary as a cosine function to the angle of eel.Due to differences in, e.g. wind speed, wind gradient, rag coefficients, lift forces, definition of centre of lateral sistance and the model representation of the geometry, ere may be deviations of up to 30% in wind heeling moment
tween different rules for the same semisubmersible, /12/. looring_S ystem Influence
rhe pla,lorm is assumed freely floating in the culations. This simplification may have two major nsequences:
As the platform is assumed to be freely floating the rotational axis is supposed to go through the centre of lateral resistance. When exposed to steady wind a cored structure will rotate about an axis lying in the horizontal plane where the mooring lines are attached. The centre of lateral resistance will, in general, not lie
in this plane.
When moving the platform, the mooring lines will create a moment on the platform. In most cases the ,:loment
3. QUANTIFICATION OF EXPLICITLY DISREGARDED EFFECTS
3.1 General
In order to get a better understanding of the relative
importance of some of the effects not being given explicit
attention as outlined in the previous chapter, some examples of the influence of the real dynamic behaviour will be
presented.
The examples deals with response due to wave and wind effects. Concerning platform motions emphasis is put on the response in roll and pitch as it is, generally, these degrees of freedom that will dominate when the possibility of capsizing
or downflooding is considered. In two examples of relative
motion between platform and sea surface the combined
motion of heave and pitch/roll is considered.
Four different platform types have been considered in
the study. One platform type may represent a possible future
large oil production platform with oil storage capacity while
two platforms are of the semisubmersible type, one "footing"
and one "pontoon" type. All these three platforms are
column stabilized. The fourth platform is a jack-up in transit and can be said to represent a floating platform type with
relatively low natural periods in roll and pitch.
In the text the four platforms are denoted platform A, B, C and D as follows:
Platform A: Oil production platform with storage capacity
Platform B: Footing type semisubmersible
Platform C: Pontoon type semisubmersible
Platform D: Jack-up platform in transit
Typical data for the four platforms are given in table I.
Table I Typical data of platforms
The motion response analysis are all linear except for
platform A where nonlinear time simulations have been performed.
Response due to first order waves, gust wind and a
combination of both have been considered and compared with heeling angle due to steady wind.
The exercises have all been performed for intact
condition.
When evaluating the results there are certain relations
one should be aware of.
The area ratio requirements as explained previously are fulfilled for all platforms as shown in table II.
The natural periods in pitch and roll for platform A, B
and C are in the range of 40 to 60 seconds. This is far
from the energy rich par- of the wave spectrum and hence the dynamic amplification is rather small.
rable II Area ratios, intact condition
Platform , area ratio
A 10.20 I
1.50 3.40 1.46
Platform D, however has a natural period in roll of
15 seconds and hence a larger dynamic amplification
factor is anticipated.
When it comes to the gust wind spectrum most of the energy is gathered in the range 50 to 100 seconds. The wind should therefore be treated as a dynamic load.
The maximum expected roll/pitch amplitude is
calculated based on a spectral approach for a single
degree of freedom system /14/.
The resulting maximum expected heel due to a
combination of wave and gust wind is calculated as the
square root of the sum of squared maximum expected
heel of the gusting wind and waves. This is believed to
be a reasonable approximation /5/. 3.2 Results
From the results presented in table III one may observe the following:
None of the platforms will capsize or experience heeling angles which exceeds the down flooding angle provided the results are measured against "still water level". Heel angle due to gust wind is of equal magnitude as the heel due to the mean wind for the platforms A, B and C. Heel angle due to 1st order wave response is completely
dominating for platform A and D compared to
responses from gusting and steady wind. For platforms B and C response from 1st order waves is smaller than response from gusting and steady winds with mean wind speed 40 m/s.
With regard to table III it is interesting to observe the
equal magni, :de of heel angle due to wind, both gusting and mean, compared to 1st order waves for a mean wind speed of
40 m/s for platform B and C. The reason for this may, to a great extent, be explained by the relatively rough method for calculating the heeling moment due to wind. After having
determined this moment, however, the procedure for
calculating heeling angle due to both gusting and mean wind is relatively easy and straightforward.
From the fact that no platform had problems with reaching the area ratio requirement one may further
conclude that it is not necessarily the intact "extreme" load case that dominates the design when it comes to stability
properties. For platform A the need for fulfilling stability requirements is probably not any limiting design factor at all.
For A, B and C none of the platforms appear to reach
the downflooding angle and one may say today's
requirements regarding area ratio may be sufficient for the
considered platforms. On the other hand, requirements being
conservative may not necessarily be proper requirements.
They may simply be too conservative. This is especially the case for the intact condition where experience with
operating semisubmersibles shows that there
A B C D
Displacement (tons) 262000 18900 23600 12500
Natural period pitch (sec) 50.0 43.0 43.0
-Natural period roll (sec) 50.0 47.0 52.0 15.0
Metacentric height (m) 3.1 3.7 3.1 42.4
Table HI Max expected heel in degrees
Down Total Heel angle Heel angle Heel angle
fl. angle heel due to due to due to
Platform Condition angle mean wind gust wind 1st order waves
Intact 24.5 6.9 0.8 0.8 6.0 Intact 22.5 18.7 8.1 8.7 6.1 Intact 35.4 20.3 9.3 9.3 6.8 Intact 23.2 12.8 2.4 3.0 10.0 A B C D
Ire seldom problems.. One reason for this is the fact that the
stability master always will seek to neutrialize any mean,
Further the area requirements may be a relatively
uncertain measure for the stability properties. This is
further confirmed by model tests, where platform models
:alibrated for area ratios less than unity and exposed to
?..nvironmental conditions far above conditions reflecting a -ecurrence period of 100 years did not capsize, ref. /I/ and
The reasons for this are many, for column stabilized :ypes the most important being connected to the sharp .ncrease in stabilizing moment when the deck enters the
water.
If the natural periods in roll and pitch had been shorter
>ne should anticipate an even greater contribution from wave response relative to the response from wind and also totally.
Platform D is an example of a platform with a roll/pitch
latural period in the range of 10 to 20 seconds. This jack-up .n transit condition has a natural period in roll of 15 seconds. When exposed to steady wind with a speed of 46 m/s the heel
Ingle will be around 3.5 degrees. When exposed to waves with an average period of 15 seconds and height of around 10
meters the maximum expected heel angle due to first order
,/ave loading will be around 10 degrees. In other words, for
such a low natural period, in the same period interval as one iefinitely can expect large waves, the contribution to the
total heel from 1st order waves is several times the
:ontribution from steady wind. Ruelative motion
Platforms B and C have been exposed to regular waves with a period in the interval 8 to 20 seconds and wave heights, from 12 to 31 meters.
By combining the motion responses heave, roll and pitch
or the "worst" column, that is the column experiencing the
argest relative motion relative to the sea surface the -esulting maximum relative motion. (double amplitude) is:
latfoimi B: 12,4 hi,
3latform C: 18,8 m
These two figures corresponds to equivalent heeling
ingles (with still water) of 10.1 and 13.2 degrees,
-espectively. It is in this connection also worth mentioning :hat the relative motion has a relatively large magnitude for
large range of wave heights and periods.
Table P/ Severe sea states applied in the analysis of the intact platforms.
3 S_um mary
From the examples presented it is clear that there are
lotion responses not explicitly incorporated in today's
tability requirements that are of similar and even greater
lagnitude in relation to heel respons from steady wind. In order to reach a relatively constant safety level the ehaviour of each individual platform when influenced by the
arious important effects should be considered. A further iscussion. will be given in the summary of the paper.
RISK ASSESSMENT
iGeneral,
After one severe accident, public opinion ,creates a
pressure on those responsible for something to be done. In such situations it is often the most visible factor that gets all
the attention, and by eliminating that particular risk it is
believed that the causative pattern leading to the accident is broken. One also believe that overall safety has been increased.
However a closer examination of accidents normally
reveals that there usually are many technical causes of an
accident. Besides human factors are found to contribute to
the accident causation process.
For these reasons stability properties and seaworthiness
should not just be regarded as a separate property of. the floating platform but rather as a part of the total integrity.
The need for a modelled and systematic approach where
stability is evaluated as a main and total system
characteristic where all significant design factors can be optimized and subsequently evaluated and reviewed for approach becomes obvious. Such development is presently in progress within VERITEC where different ,departments
working with specific aspects of stability are trying to
coordinate and systemize their "tools" and skills in this respect.
Risk of Damaged Stability
For the floating platform as an object, the worst case
accidental event is capsizing and sinking (total loss). All kind
of major unwanted accidental events on the object such as hydrocarbon fires or explosions, collisions or other extreme condition affecting the primary structure or buoyancy will
under worst case conditions have a potential to end up as a ,capsizing/sinking scenario.,
All kind of design, construction and operational means will have the objective to prevent such situations. However, through subsequent incorporation of these means, which
today are quite sophisticated, the development has reached a
region where the compromising and optimizing between
different main conceptual aspects easy become complex and
difficult to compare.
The chance of some aspects being, overldoked or not
adequately incorporated can, possibly increase,
if
notsystemizing "tools" are incorporated as safety reference in
decision making. The term risk, which in an analytical sense
is the product of a defined unwanted consequence with the
calculated probability of such an occurence, may be used as
such superior systemizing reference. All significant means
which decrease the risk of unstability will thereby have a
comparision reference in the decision process.,
Analyses
Within the field of speciality disciplines as risk,
reliability and system analyses, several systemizing methods
for quantification purposes have been developed. The
usefulness of each method is highly dependant on how and where is applied.
Generally spoken, such methods are easier applicable on
high risk problems or higher functional criticality. To adapt these methods to effective results on the safety of floating platforms is on its way, and promising adaptions to critical
decision problems have been carried out by VERITEC.
To calculate the risk of unstability is an iterative
process, where different approaches and sources of statistics
are utilized and extrapolated to maximum confidence in the
results.
11
Platform II Significant Peak wave
wave height period
A 15.5 m 14.0 s B 1 16.5 m 14.5 s ,C ' 16.5 m 14.5 s Di , 001.0 m 15.0s 4. 4.1
A calculation formula for classified estimation of total instability risk can be performed according to the following formula. Rtor = p(C) C C3(TL) ° p (C3(TL)) n=1 1 + C2(PL) p (C2(PL)) 71=1 + C1 (CM/D) Tr: p (CI (CM/D)) where TL PL CM/D Cr, p(C)
k,I nd -n are the number of ways in which each consequence can occur.
ri the consequence is measured in money value and the )robability in frequence of occurrence per year, Rtot will epresent the average accidental expectancy value per year.
Approximately only the ci"minant probabilities have to >e taken into the calculation in order to establish the risk )icture.
The determination of such a total risk picture involve lifficulties due to the fact of uncertainties in data or lack -hereof. This will especially be a problem in the medium :onsequence class of critical damages and malfunctions. By ising alternative approaches to these determinations it is, iowever, possible to accumulate adequate confidence in the >robability factors.
DOMINATING RISK CURVE
4
Abb.,EplC(total (oss)] .\411
bbanEp[C
( partly loss)]
p,
'lg. 2. Classified Risk Calculation Curve
+.2 Reliability of Uprighting Functions Dependence and Redundance
To make a reliability model of the uprighting functions nvolve the inclusion of both structural reliability and the )perational active reliability, necessary for control of the )uoyancy parameters. Such a model will be additional to the )erformance and capability characteristics of the particular :boating platform. However, this model will follow and iccumulate during the design process. The final operational eliability model will mainly contain the resulting operational Ind maintenance considerations, while many of the reliability :onsiderations made in the design process already have been ntegrated and built into the construction.
Total loss Partly loss
Critical malfunction/damage Consequence level in
Probability of C.
Ep IC (critical malfunction /damage )1 p(C)
Stability have, among others, dependence to the structural reliability, and thus incorporated structural margins and redundancy will improve stability in the probabilistic sense of the word. The structure should withstand both design operational loads and hazardous events. The external and internal environment in a total sense has thus to be considered for optimum stability achievement in the design process.
Highly different types of such initiating events as extreme wind and waves, collisions, fire/explosions, falling objects, etc. have to be tolerated by the structure without jeopardizing the stability. Part of the structure or redundant members may be damaged, but without loss of the residual stability.
Stability will also have dependence to the active ballasting system, which shall accomplish intended adjustments and change of the rig's center of gravity and displacement. Unintended or reversed functioning of this system can be a hazard equal to such energetic events as mentioned previously. The stability has thus dependence to both active and passive reliability characteristics, and both have in a balanced way to fulfil the objective of stability. The achievement of structural reliability is mainly accomplished through design and inspection! maintenance systems, while active reliability characteristcs have additional link to the operational use of systems, as with the performance of the rig. Omissions and human errors during operation of the rig may thus also have jeopardizing effect on stability. Reliability measures and failure tolerances in the ballasting system may also have significant risk decreasing effect on unstability.
Accidental Examples
When looking into experienced marine accidents with semisubmersibles, usually a long chain of causative factors can be listed, each contributing to the escalating scenario development. The initiating event starting such a sequential chain of malfunctions and failures, is mostly some unexpected situational event causing common failure effects and loss of control, or of main safety functions. Such developments should not be probable within a proper developed system where also proper planning of operations and emergency procedures are implemented.
One example of a critical failure that may lead to capsizing/sinking of a rig if not proper counteractions are initiated, is blockage of hydraulic return line in the ballast valve control system. This failure would result in position problems with the remote controlled valves. Such a common control problem, which can be quite difficult to locate correctly, may easy lead to more stressed and overcritical situations. This is likely to continue a consequence escalation, which probably will lead to loss of floating platform.
The frequency of occurence of such an initial failure will be dependent on both the lay-out and maintenance of the respective hydraulic system, however, the probability of such malfunction in an average system is certainly comparable to structural damaging events which are encountered for in design calculations.
Reliability Improvements
The means for improving reliability, are additional to the general technology aspects, the effective incorporation of alternatives and mitigating means. Redundancy and damage tolerances are words central in this respect.
The most effective way to improve reliability is to improve those factors that contribute most significantly to unreliability or risk of malfunction. This can be done by increased safety factors, derating, increased redundance or improved maintainability factors in already redundant systems. Quality assurance factors throughout all phases of floating platform realization will mainly control the system reliability, and thereby reduce the chance of faults in design, construction, critical components, etc. The analytical reference to reliability improvement or control should have great potential in assurance of adequate stability characteristics of each constructed floating platform.
=
4.3 Systematization of Results
When working through different fields of speciality, which is the case when analyzing from a risk reference point of view, it is important to present the basis for results in an easy accessible form. The discussions and reviews can then be finalized around a proper basis. Different schematics are presented, which can be utilized under this objective.
Hazard Tree
A hazard tree fig. 3 is developed where capsizing is shown as the main top-event. Here a top-approach systematization of typical gross hazard types potentially leading to capsizing are presented.
Each of the initiating events can be analyzed further through fault trees development, however, as developing consequences can augment into other categories of accident-types, such high level fault-tree developments easy become cross-correlated. This problem can, however, be handled through calculation systematics.
Unplanned environmental forces Excessi anchor line forces Underwater blowout
Fig. 3. Hazard Tree
Capsizing/ :Foundering Loss of stab. due to knout caul non Or Deteriorations due to burning blowout on sea Unplanned environmental loading Unfavorable drifting not stopped Platform in-duced collision
Progr. col lapse
of anchor lines
4.4 Risk Picture
A risk picture is combining the consequence types with their expectancy frequency of occurrence. Selected and calculated unwanted top-events can be plotted into this picture, and trends and goals in design can be monitored. A schematic of such a risk picture is shown, fig. 4, and -,-ference is made to the calculation formula of total risk. T he further assessment of specific cause distributions should preferably be continued in a pure analytical sense.
Unplanned filling of buoyancy member Fixed obstacles in drifting direction Level of consequence TOTAL LOSS PARTLY LOSS OAMAGE / CRITICAL MALFUNCT.
Fig. 4. Estimated Contributions to Risk Curve
Criticality Matrix
However, also a semiquantitative assessment of failure causes can be performed. This can be more effective on greater detail level where the consequence of failures become more undercritical. Classification in degrees of criticality can define a scale of measuring for undercritical risks. For demonstration purposes the hazard tree shown has been transferred to a criticality matrix on the same level, fig. 5.
The further assessment of rig subsystems, where critical failure points could be revealed, will better show the potential strength of this summarizing method. This is usually based on a Failure, Mode, Effect and Criticality Analysis (FMECA). Only the classification of frequency and consequence within a proper scale of decades is necessary when using this method. Further quantifications can be reached through reliability analyses.
error pfC) Capsizing/ 03 Foundering condition C2 Unfav.combined accidental events Blowout conditions Buoyancy decrease
Ballast system Minor Anchor line
Cl critical malfunction collision rupture Ballast svstes unreliability Operational control seq. al aperat aul sad handling error/failure Ball a. system critical malfunctions Other extreme fault/failure condi tines Other extreme fault/failure conditions Collisions from ships/ yesse/s Falling loads/ Dropped obiects and/or OperatIonal
!Ballast system Collision Collision while
control seg. unreliability 1 le drifting navigating
WTOr
(Extreme tilt) (Extreme draught)
Stability Buoyancy
control failure loss/decrease
0.0001 VERY IMPR. 0 001 IMPROBABLE 0 01 LESS IMPR. 0,1 PROBABLE rate
:lass 0 1 2 3 per year
Fig.). Criticality Matrix
A
-4.5 CONCLUSIONS
Risk Analyses have already been utilized for about years on fixed installations on the Norwegian Continental shelf. Through accumulation of experience the benefit of this activity has been increasing and today the main hazards connected to these offshore installations are handled quite objectively and open.
Reliability Analyses have earlier been used in high technology, high risk fields, where reliability of systems are utmost important. To what extent these skills and methods are transferable to stability problems of floating platforms is presently under evaluation. The objective is to combine risk and reliability analyses into an analyzing system where adequate and balanced reliability criteria can be delineated and optimized into the specific design of a rig with specified stability.
5. SUMMAR
We feel there is a great need for revising the present stability rules. This revision should be considered on two levels as discussed in this paper.
Research carried out and experience gained throughout the world indicate that the present rules are too simple and does not reflect each design and its total behaviour.
During the last 4 years a comprehensive research program regarding safety of Mobile Offshore Units has been conducted. One of the projects was the Mobile Platform Stability (MOPS) project /3/. The project has been carried out as a collaboration between VERITEC/Veritas, Norwegian Hydrodynamic Laboratories, Norwegian Ship Research Institute and the Norwegian Institute of Technology. All research work is now finished and most of the results are now available.
Concerning the importance of the findings it is
concluded that it is not possible to make recommendations to specific changes in the present rules, the various investigations are not comprehensive enough for that. However, when it comes to focusing on the most important aspects regarding stability properties and associated matters the project has been very useful.
It is not the intention here to give details of all the conclusions but to focus on one aspect considered by VERITEC to be may be the most important and confirmed by the MOPS project.
During model tests in both intact and damage condition Lt appears to be impossible to capsize a platform regardless of initial established premises. The only "method" for -eaching this stage was to allow for progressive flooding. This can be achieved by water filling through open holes or by -nalfunction/misoperation of ballast system. Further efforts should therefore be laid down in the risk assessment field in order
to prevent any minor accident to develop into a
catastrophy.Important headlines in this respect should therefore be primary deck structure design, open/"closed" design hull design regarding compartmentation
ballast system design overall deck layout
design of water and weathertight closing appliances mooring system design
Another important area where great effort should be laid lown is on the development of efficient and accurate :omputer tools for estimation of platform motion response lue to environmental loading. For some platform types like
sernisubmersibles and ship shaped designs reliable tools lave been developed while for others more development is leeded.
However, if dynamic analysis are to be required for evaluation of stability propreties, relevant acceptance :riteria should be established. Presently VERITEC is working both internally and also as committee member of NPD's development of guidelines for design of floating platforms on the subject. We hope conclusions in this respect will be drawn in the near future.
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Numata, E., Michel, W.H. and McClure, A,C.:
Assessment of stability requirements for semi-submersible units. Trans. SNAME, 1976.
Moan, T.:
Note on Stability Regulations for Mobile Platforms NTH 1984.
Moan, T., Brevig, P., Soma, H., Dahle, L.:
Experiences and Results Gained During The MOPS and Ocean Ranger Projects. MOPS Report No. 23.
Numata, E., McClure, A., C.,:
Experimental Study of Stability Limits for Semisubmersible Drilling Platforms. OTC 2285, 1975. Morland, M., LOtveit, S.A.:
Floating Stability of Compliant Structures. VERITEC Report No. 84-3043.
Mot-land, M., Hansen, V.:
Relative Motion - Semisubmersibles. Internal Note VERITEC (VT/65-108-84).
Wiborg, J., Vebjoer, K.:
Risk Analysis of MOU's, Methodologies and Experiences, VERITEC Report No. 84-3587.
Wiborg. J.:
Risk Analysis of Mobile Platforms, Ballast System, VERITAS Report No. 83-0789 (in Norwegian).
Huang, X., Nmss, A., Dynamic Response of a Heavily Listed Semi-Submersible Platform, Second International Symposium on Ocean Engineei"i..g and Ship Handling,
Goteborg 1983. Hoff, J.:
Survey of stability rules for mobile platforms, NHL report 182047, March 82.
De Souza, P.M.F.M., Miller, N.S., The intact and damaged stability behaviour of two semi-submersible models under wind and wave loading, OTC/1978/Vol. 4 paper 3298 p2147.
Miller, N.S., Notes on wind heeling moments on semisubmersibles, NADE-HL-76-04, 1976.
Kuo, C., Martin, J.:
Calculation for the steady tilt of Semi-submersible in Regular waves. R.I.N.A. meeting April 13, 1978.
Davenport, A.G., The prediction of the Response of Structures to Gusty Wind_, Safety of Structures under Dynamic Loading, Vol. 1, Trondheim 1977.
The Norwegian Petroleum Directorate:
"Regulations for load-carrying structures for extraction of exploitation of petroleum", Stavanger, 1984.
Intergovernmental Maritime Consultative Organization Code for the Construction and equipment of Mobile offshore drilling units (MODU CODE) IMCO 1980. Det norske Veritas
Rules for the Construction and Classificaiton of Mobile Offshore Units, 1981.
Norwegian Maritime Directorate
Regulations for Mobile Drilling Platforms with Installations and Equipment used for drilling for petroleum in Norwegian internal waters, in Norwegian territorial waters, and in that of the continental shelf which is under Norwegian Sovereignty, issued 10th of September 1973. ten 1. 5. 7. ;II. '13. 15,
The Norwegian Maritime Directorate.
Sketch for modification of the stability requirements in ref. 4 dated 12.3.81.
Lloyd's Register of Shipping
Rules for the Construction and Classification of Mobile Offshore Units, 1972.
American Bureau of Shipping
Rules for Building and Classing mobile Offshore Drilling Units, 1980.
U.S. Coast Guard
Department of transportation
Requirements for mobile offshore drilling units, issued 4th of December 1978.
Department of Energy
The Offshore Installations (Construction and Survey) Regulations 1974
(5.1. No. 298).
Langfeldt, J.N., Egeland, 0., Gran, S., NV 407 B, Motions and Loads for Drilling Platforms, DnV report No. 74-63-5, Dec. 74. (With revisions).
Hulila Users Manual: NV 935 Short and long term response of wave induced motions and loads, DnV, Jan. 1977.
