OTC 581 9

Computer Analysis of Heavy Lift Operations by H.J.J. van den Boom, J.N. Dekker, and R.P. Dallinga, Maritime Research Inst. Netherlands

Copyright 1988 Offshore Technology Conference

This paper was presented at the 20th Annual OTC in Houston, Texas, May 2-5, 1988. to copy is restricted to an abstract of not more than 300 words.

ABSTRACT -

construction^ and Offshore p la t foms are ca r r i ed out i n exposed a r e a s and at growing Not Only entire sides can be lifted but the l a r g e jackets can now be ca r r i ed out by ves se l s with dual cranes. ....

take advantage the large cOst-savings r e l a t e d to such opera t ions , the f e a s i b i l i t y , work-

and r i s k l e v e l have to be evaluated accu- rateb. this end computer simulations of the

behaviour of the t o t a l heavy l i f t system due t o wind, waves, cur rent and hois t inglde-bal las t ing a r e required.

A general purpose degrees Of simula- t i o n model covering the mechanism of motion has been developed. The computer model is ab l e t o provide the motions Of the crane load and barge and the r e l a t e d tensions i n ho i s t i ng wires, s l i n g s and mooring l i n e s a s well a s the barge-load impact forces. s p e c i a l a t t e n t i o n is paid t o the sin- u l a t i o n of p a r t l y submerged jacket-type s t ruc tu re s . he computer program has been va l ida ted with r e s u l t s of an extensive model t e s t program.

INTRODUCTION . v . . . .

Large S-emi-submersible crane ves se l s (SSCV's) with l i f t i n g c a p a c i t i e s of up t o 14,000 tonnes a r e used nowadays f o r the i n s t a l l a t i o n of platforms i n exposed a reas* A cons iderable i nc rease i n load weights and volumes i s expected because of t he proven savings that: r e s u l t from heavy of fshore l i f t s . ( see r e f . [l.] ).

By means of the dual crane option, i t is not only poss ib le t o l i f t complete tops ides , but a l s o t o per-

References and ' i l u s t r a t fons a t end of paper.

The material is subject to correction by the author. Permission . -

form i n s t a l l a t i o n of l i f t a b l e jackets (Fig. 1). The conventional way of jacket platform i n s t a l l a t i o n i s the t r anspor t a t i on barge launch procedure. I n ge- ne ra l t h i s launch causes an extreme loading on the s t r u c t u r e . Furthermore buoyancy requirements r e s u l t i n l a r g e diameter tubulars and e x t r a tanks. For upending b a l l a s t systems a r e needed. A l l launch r e l a t e d s t r u c t u r a l provisions increase the wave and cu r r en t loading. It is the re fo re evident t h a t t h e launch procedure has a s t rong impact on the design, engineering and cons t ruc t ion of jacket-type Flat- forms. A l i f t a b l e jacket can be designed f o r loads exerted to the s t r u c t u r e during i ts opera t ional l i f e t ime. Hence such jackets a r e outstanding from the poin t of view of design and cons t ruc t ion work, use of s t e e l and maintenance. It is gene ra l ly accepted t h a t l i f t a b l e jackets a r e 30 t o 40 percent more economic than conventionally launched jackets .

L i f t i n g opera t ions a t exposed loca t ions with r ap id ly increas ing weights, volumes and ope ra t iona l complexity cannot be performed on the b a s i s of ex- perience only. Evaluation of t he f e a s i b i l i t y and workabil i ty of such opera t ions r equ i r e s a de t a i l ed ana lys i s of the system's dynamic behaviour a s in- duced by waves, wind and current i n combination with the ho i s t i ng ac t ion and the (de )ba l l a s t i ng proce- dures.

For tops ide i n s t a l l a t i o n the following phases i n the dynamic behaviour of the crane system may be distinguished:

crane vessel in mooring system ;k side-by-side mooring of crane vessel and barge * pre-hoist condi t ion with pre-tensioned s l i n g s * hoisting/de-ballssting with load/barge impacts * pendulum condit ion with f r e e hanging load * posi t ioning and lowering on support s t r u c t u r e s

The most important items i n the dynamic behaviour during these s t ages a r e t he motions of t he crane t i p s and load, the ho i s t i ng wire and s l i n g tensions and the impact forces on the stabbing p i l e s . I n Fig. 2 t y p i c a l t i m e t r aces of the ho i s t i ng phase f o r a 8000 tonnes nodule are presented.

2 COMWTER ANALYSIS OF

For jacket i n s t a l l a t i o n by means of a dual crane l i f t the follawing typ ica l s tages can be recognized:

* l i f t - o f f -

* immersion of jacket * upending * posit ioning * mating

Besides the s l i n g loads, impact loads on the barge and docking p i l e loads, f o r l a r g e jackets the r e l a t i v e motions (clearances) between the jacket and the crane booms or the h u l l can be of importance.

Since the dynamic behaviour of the system is bas i ca l ly non-linear and non- s t a t iona r~ , a Proper l i f t i n g analys is can only be performed using exten- s ive time domain Simulation too l s* For t h i s Purpose MARIN, i n c lose co-operation with Shel l In ternat io- na le Petroleum Maatschappij (SIPM), has developed the LIFSIM simulation program, by which the be- haviour of t h ree bodies wi th eighteen degrees of freedom can be computed. The three bodies mentioned a r e a crane vesse l , a load and a t ranspor ta t ion barge o r platform subs t ruc ture . The e l a s t i c i t y of the crane systems, the hois t ing ac t ion and the (de )ba l l a s t ing of the crane vesse l a r e included i n the mathematical model. Any other l i n e a r or non- l i n e a r mechanical i n t e rac t ion force between the bodies ( fo r ins tance tugger l i n e s or dynamic con t ro l algori thms) can be formulated by means of a user i n t e r f a c e t h a t a l s o enables the user t o speci fy add i t iona l output records.

Recently the computer package has been extended f o r the ana lys i s of jacket i n s t a l l a t i o n . The mathematical model can be out l ined a s follows:

- the time-domain analys is with 18 degrees of f ree- dam is based on the equations of motion according to Cumminsa with convolution i n t e g r a l s which cope with the frequency dependency of the added mass and damping c o e f f i c i e n t s of barge and crane vesse l .

- the environmental exc i t a t ion forces may be arbi- t ra rys which means t h a t f i r s t order wave forces , slowly varying d r i f t forces , current and wind loading can be included. - s ince r e t a rda t ion functions and f i r s t and second order wave force records can be generated f r m the r e s u l t s of frequency domain analys is , on a bas i s of three-dimensional po ten t i a l theory, a r b i t r a r y h u l l shapes (sh ip , semi-submersible, catamaran) a r e allowed.

- add i t iona l a r b i t r a r y forces may be speci f ied a s funct ions of a l l s t a t e va r i ab le s [ including time) through a spec ia l user program in ter face .

- a change of mass and i n e r t i a as well a s r e su l t i ng forces and moments due t o (de)bal las t ing of the crane vesse l is taken i n t o account. - the hois t ing speed is opt ional to the user.

- non-linear and asymmetric mooring c h a r a c t e r i s t i c s can be d e a l t with. - the influence of t he f l e x i b i l i t y of the crane system on the motions is modelled. - impact loads due to s t rongly non-linear 'spring c h a r a c t e r i s t i c s ' of load/barge o r l oad l j acke t i n t e r f e rences can be handled. - f l u i d and windloading on jackets with a r b i t r a r y pos i t ion and or i en ta t ion a r e incorporated.

HEAVY LIFT OPERATIONS OTC! 5819

To inves t iga t e dynamic e f f e c t s during l i f t opera- t i ons and t o va l ida t e the LIFSIM-package, extensfve model experiments have been car r ied out i n the MARIFf t e s t f a c i l i t i e s . These t e s t s were sponsored by SIPM and HEEREMA [2 ] . COMPUTER MODEL

gguations of motion ----------------- The descr ip t ion of motions of the th ree body

heavy l i f t system due to environmental loads, mutual mechanical i n t e rac t ions and the l i f t i n g opera t ion is based on the impulse response theory to handle che f l u i d r eac t ive forces. This descr ip t ion has proved to provide accurate motions and mooring fo rces f o r s i n g l e body moored s t ruc tu res such as a j e t t y moored tanker ( ref . [ 3 ] ) .

.. t X [(%j+ % j ) ~ j + l % j ( t - ~ ) k ( ~ ) d ~ + c X ]=

j,l k j j

, t) with k = 1, .. , 6 . . . . . . . . ( 1)

where: = motion in the j-th mode

arbi t rar i ly time varyiog external f o r c e i n the k-th mode of motion

M = inertia matrix m = added i n e r t i a matrix R = matrix of r e t a rda t ion funct ions C = matrix of hydros ta t ic r e s to r ing

forces.

The left-hand s ide of eq. (1) contains the r i g i d body reac t ions and the l i n e a r hydromechanic r eac t ion forces . The right-hand s ide conta ins the environ- mental loads due to wind, waves and current and other forces which may be a r b i t r a r y functions of the motions of the s t ruc tu re . When i t i s assumed t h a t the in t e rac t ions i n f l u i d r eac t ive forces a r e negli- g i b l e and provided t h a t non-linear drag terms, mechanical coupling due t o the crane system and impacts and res tor ing forces due to mooring may be formulated as functions of the motions of the th ree bodies, eq. (1) fo r t h i s system yields:

. . . ( 5 ) + x ~ ~ E ~ Z ~ > X - ~ , ' )

. . . (2)

OTC 5819 -~ VAIT D g BOOM, DALLINGA, & DEKKER -- ~ -p---

3 l I l

i n which: Mi = i n e r t i a + added i n e r t i a matrix of body

No. i = matrix of r e t a rda t ion functions of body

-

NO. i p

Ci = matrix of hyd ros t a t i c r e s to r ing forces of body No. i

X = motion vec tor of body No. i = vector of ex t e rna l forces on body No. i.

-i

I n eq. (2) the indices 1, 2 and 3 stand fo r the crane ves se l , the t r anspor t a t i on barge and the load respect ive ly . The c o e f f i c i e n t s of the matr ices con- ta ined i n the left-hand s i d e of eq. (1) and (2) , which descr ibe the hydrodynamic reac t ion forces , a r e determined employing a three-dimensional, l i n e a r po t en t i a l theory. I n t h i s method, t he mean wetted pa r t of t he h u l l is approximated by a d i s t r i b u t i o n bf source s i n g u l a r i t i e s .

The formulation f o r the po ten t i a l of the sources complies with the equation of cont inui ty , t he l i n - ear ized free-surface condit ion, the bottom condit ion ( i f f i n i t e water depth i s considered) and a radia- t i o n condit ion. The unknown source s t rengths a r e ob- ta ined by imposing the water t ightness condit ion a t t h e mean pos i t i on of the cen t r e of each source panel.

The hydromechanic r eac t ion terms a r e pr imar i ly obtained from p o t e n t i a l theory a s frequency-de- pendent added mass and damping coe f f i c i en t s . Based on these c o e f f i c i e n t s the added i n e r t i a matrix mkj and the matr ix of r e t a r d a t i o n funct ions Rkj a r e obtained from:

2 m . .

R ( t ) = ;; b (w)coswtdw k j

. . . . . . ( 3) kj

i n which ak (U) , b .(U) = frequency-dependent added mass and da&ing c k ~ f f i c i e n t matrices, respect ive ly ; and w* = a constant frequency.

The advantage of descr ib ing the hydrodynamic r eac t ion fo rces due t o added mass and damping ef- f e c t s using r e t a r d a t i o n funct ions l i e s i n the f a c t t h a t a r b i t r a r y motions can be accommodated correc t - l y , i r r e s p e c t i v e of t he na ture of the motions. Equa- t i o n s of motion based on the d i r e c t app l i ca t ion of frequency-dependent added mass and damping coeff i - c i e n t s have t h e disadvantage of not being a b l e t o cope with, e.g., t r a n s i e n t motions t h a t can a r i s e during load-barge impacts.

The right-hand s i d e of eq. (1) conta ins , besides mooring or r e s t r a i n i n g forces , forces due t o wind, waves, and current . Wave forces may be s p l i t i n t o two cont r ibut ions : f i r s t order forces t h a t o s c i l l a t e with wave frequency; and second order mean and slow- l y varying wave d r i f t forces . F i r s t order wave fo rces a r e computed i n the frequency domain using three-dimensional d i f f r a c t i o n theory. The second

order wave d r i f t forces a r e computed i n t he form of frequency domain quadra t ic t r a n s f e r funct ions based on the pressure i n t eg ra t ion formulation..

I n order t o be ab l e t o compute the f i r s t and second order wave loads on a ves se l i n t he t i m e domain f o r a r b i t r a r y wave condi t ions , use is made of t he Vol ter ra s e r i e s formulation. According to t h i s formulat ion, t he t o t a l wave load, including f i r s t and second order cont r ibut ions , fol lows from:

i ~ ~ y h i c h = wave e l eva t ion time record and 4 (T) ,G ' f [T T ~ ) = f i r s t and second order impulse response funct ions r e l a t i n g the wave e l eva t ion t o the fo rce i n the k mode, respect ive ly .

The impulse response funct ions a r e found from:

i n which Hkl)(w) and H(Z)(wiw2) a r e the f i r s t order wave fo rce t r a n s f e r fkunction and the second order d r i f t force t r a n s f e r funct ion , respect ive ly .

The advantage of the Vol ter ra s e r i e s approach i s t h a t s imula t ion computations can be c a r r i e d out based on measured wave records a s well a s on synthe- t i c wave records.

From the o u t l i n e given above i t fol lows t h a t the simulat ion approach takes advantage of e f f i c i e n t l y computed frequency domain r e s u l t s . For t h i s reason l a r g e changes i n d r a f t , t r i m or hee l a r e not ac- counted for .

Wind and current fo rces a r e determined by t h e following empir ica l r e l a t i onsh ip :

i n which: p = mass dens i ty of the f l u i d V current/wind speed

Ak projected area f o r fo rce i n k mode C = drag coe f f i c i en t f o r the fo rce component dk i n k mode.

4 COMPUTER ANALYSIS OF HEAVY LIFT OPERATIONS OTC 5819

I I Current drag force coe f f i c i en t s a r e best obtained

from tests i n l a rge basins t h a t allow r e l a t i v e l y un res t r i c t ed flows i n the case of oblique current angles. The influence of water depth is another f a c t o r of importance with respect to current forces.

i n the hor izonta l modes of motion, however, w i l l be governed by the mooring system. Therefore the s t a t i c c h a r a c t e r i s t i c s of the mooring system a r e incorpora- ted while neglecting dynamic behaviour of i nd iv idua l components of the mooring system.

Crane szstem ----

The wind force coe f f i c i en t s may be obtained from wind tunnel t e s t s S empirical data bases o r separa te computer programs. The e f f e c t s of wind gus t s may be, incorporated by use of dynamic wind ~ e l o c i t i ' + s , derived from wind spect ra .

The crane system consis t ing of c r a n e ( ~ ) , hois t ing wires, hooks and s l i n g s , forms the mechanical con- nect ion between the crane vesse l and the load. In order t o der ive the instantaneous mutual forces between these two bodies, the t o t a l crane system is considered a s a set of l i n e a r spr ings thus neglec- t i ng dynamic behaviour of the crane system i t s e l f (Fig. 3 ) .

The mooring l i n e tensions a r e found from the instantaneous pos i t i on of the f a i r lead locat ions , and f r m the s t a t i c load elongation c h a r a c t e r i s t i c s of l i nes . The d i r ec t ion of the l i n e tension is taken equal t o the d i r ec t ion between the points of a t tach- ment of the l i n e .

Based on the r e s u l t s of f u l l s c a l e load-deflec- t i o n t e s t s , the crane i s schematized a s an i n f i n i - t e l y s t i f f beam ro ta t ing i n a v e r t i c a l plane around i ts heelpin due t o a l i n e a r support spring. Trans- verse f l e x i b i l i t y of the crane t i p is represented by a l i n e a r spring. The ho i s t ing wires and the indiv i - dual s l i ngs a r e a l s o represented by l i n e a r springs. From the instantaneous body pos i t ions the crane system forces a r e found. A continuous update of the hois t ing wire length accounts f o r the hois t ing ac t ion .

Impacts *- ---- Four 3-D s e t s of l i n e a r springs a r e chosen t o

model the impact forces between the suspended load 1 and the body 3. Since the geometry of the stabbing I points may be r a the r complex, gap width and depth

may be speci f ied fo r a l l impact points separa te ly .

For seve ra l SSCV's the use of dynamic b a l l a s t systems i s an important aspect of the l i f t i n g operation. Dynamic ba l l a s t ing not only secures the even keel condit ion but i t a l s o may cont r ibute t o the ac tua l l i f t offfon. Due to extreme l a rge b a l l a s t volumes the mass proper t ies of the crane vesse l may a l s o change considerably. For these reasons both changes i n b a l l a s t mass alid forces a r e accounted f o r i n the computer model.

The r e s to r ing forces due to the mooring system a r e provided by mooring l i n e s y ie ld ing tens ion forces.

Due to i ts l a r g e volume, the dynamic behaviour of the moored s t r u c t u r e a t wave frequencies w i l l hardly be af fec ted by the mooring. The low frequency motion

When the load suspended from the crane(s) is a par t ly submerged jacket-type s t ruc tu re , s eve ra l add i t iona l i t e m s have t o be accounted for . Large ro t a t ions r equ i r e l a r g e angle d i r e c t i o n a l trans- formations. The f l u i d forces have to be determined on the b a s i s of the instantaneous pos i r ion and o r i en ta t ion of the jacket. I n t h i s way the immersion is a l so incorporated.

The f l u i d forces a re pa r t ly of po ten t i a l o r i g i n and pa r t ly due to viscous e f f ec t s . Assuming s lender tubular members without hydromechanic in t e rac t ion , these forces may be approximated by the r e l a t i v e motion concept known as the Morison formulation:

The i n e r t i a and drag force shape coe f f i c i en t (CI,CI)) f o r cy l ind r i ca l elements may be formulated i n normal and tangent ia l components. For t h i s reason the f l u i d forces a r e computed i n a l o c a l system of co-ordina- t e s .

where : [R] = d i rec t iona l transformation matrix U = veloci ty vector i n loca l coordinates U = global ve loc i ty vector (G + % - ) - V = current ve loc i ty

= o r b i t a l ve loc i ty 7 w J member no. n - t o t a l number of submerged members. D = diameter of member 1 = length of member.

The instantaneous o r b i t a l v e l o c i t i e s and acce- l e r a t i o n ~ of the water p a r t i c l e s due t o wave ac t ion a r e computed by means of a Volterra s e r i e s formula- t i o n v iz . the l i n e a r f i l t e r i n g of the given wave t r a i n :

OTC 58 19 VAN DEN BQO_MM, DALLIWA, & DEKKER 5

I

where: Hv(@> = V(U>/C~(~> W = wave frequency 3 (t) = input wave train.

The transfer functions H (U) are readily found v

from linear wave theory taking into account the position of the jacket member and integrating the wave action over the length of this member.

The wind loads on the above water part of the jacket is also treated according to equation (9).

Additional forces ----------------- By means of a user's interface, additional forces

on each of the three bodies may be specified as arbitrary functions of all state variables and time. In this way automatic control procedures, tugger winches, fenders etc. may be accomodated.

Based on the mathematical description of motions and forces as outlined above, the computational procedure of LIFSIM as presented in Fig. 4, has been developed.

As already indicated by eq. (2), the computer model describes the motions of the three individual bodies including the mechanical coupling due to the crane system and impacts. Computation of fluid reactive forces by means of retardation functions may be considered as linear filtering of the structure's velocity history. First and second order wave forces are obtained from linear and quadratic filtering of the input wave record.

Knowing all forces on the individual bodies, the motions may be solved by numerical integration methods. The Runga Kutta algorithm has proved to provide efficient solutions for many situations. In case of heavy impacts the use of Gear's stiff inte- gration method is required.

VALIDATION -.

In 1987 an extensive model test program was carried out to study dynamic effects in heavy lift operations and to evaluate the simulation program LIFSIM*

The test series were carried out at scale 1:40 in the Seakeeping Basin of MARIN sponsered by Shell Internationale Petroleum Maatschappij and Heerema Engineering Services.

The models used during the test represented SSCV BERMOD, transportation barge H108, a rectangular module of 8000 tonnes and a lattice structure with a weight of 8000 tonnes at a displacement of 4000 tonnes. (Fig. 5)

The test program comprised single body motion tests, hydromechanic interaction tests, load-barge impact tests and hoisting/deballasting operations in irregular waves.

During the test series 62 signals were measured. Motions of SSCV, crane tips, module and barge were measured by optical tracking devices, gyroscopes and polarisation filter techniques. Tensions in support wires, hoisting wires, slings and mooring lines were monitored by means of strain gauges techniques. The impact forces between the module and transportation barge were measured by means of 3-component force transducers integrated in the stabbing piles (Fig. 5). These impact forces were recorded with a sample rate of 15 Hz (full scale).

Stationary tests ------ Obviously the dynamic effects during a typical

hoist period of 80 seconds strongly depend on the individual waves which pass the SSCV in this short time span. Because of the fact that the operation is carried out in irregular waves the level of impact forces and hoisting tensions are strongly related to the starting time of the operation. (Fig. 6).

Although the wave excitation is of random nature, due to the hoisting/ballasting procedure the be- haviour of the system is a non-periodic determinis- tic process. (Fig. 7).

Contrary to an ergodic stationary process, a single realisation of a deterministic process does not result in valuable statistical information.

In- order to derive statistical data for the hoisting operation two approaches may be followed. The operations may be repeated for many different starting times i.e. at different positions in the wave train. This so-called Monte Carlo approach will result in a distribution function for the required value e.g. the extreme impact force.

The alternative approach is based on the assumption that the hoistinglde-ballasting action itself will not contribute to the dynamic behaviour of the system. Hence this deterministic component of the process may be treated quasi-statically.

The operation is therefore studied by a number of stationary irregular wave tests. The load during such a. test is kept at a chosen pre-tension level or clearance from the supports. The same irregular wave test is repeated for other pre-tension levels and clearances. Each test can be considered as an ergodic stationary random process and can therefore be subjected to statistical analysis.

In the model test program both approaches were investigated.

6 COMPUTER ANALYSIS OF HEAVY LIFT OPERATIONS OrC 5819

I 1

( The model test program provided much information both from the points of view of the dynamic be- haviour itself and the correlation with LIFSIM simulations.

The time traces measured have been subjected to several post-processings and evaluations. Spectra and Response Amplitude Operators (RAO) were derived for the test situations without impacts.

In this paper only some selected results are pre- sented. Results for the SSCV only are shown in Fig. 8. These results clearly show the "resonant" re- sponse in swell waves with increasing non-linearity.

In the stationary irregular wave tests, the pre- hoist situation was modelled. This situation with 1000 tonnes pre-tension in the hoisting wires can be considered as a test case for linear theory as long as the modules keeps contact with all impact springs and no slack slings occur. Crane tip motions for this situation for both the barge and jacket are compared with the motions of the SSCV only in Fig.

Model test results for a realistic hoistinglde- ballasting situation are shown in Pig. 2. In this case the SSCV was heading irregular waves (signifi- cant height 2 m, mean period 8 seconds). From the pre-hoist condition (1240 tonf. pre-tension), the hoisting speed was 0.0234 m/s and the deballasting time 60 seconds. The presented hoisting wire tensions and support forces show characteristic wave frequency oscillation before lift-off. When the hoisting wire tensions equal the weight of the mod- ule, the wave induced motions of SSCV and barge re- sult in large impacts and tension variations. Though tension drops are most significant, peak values are of ten referred to by means of Dynamic Amplification Factors (D.A.F.). As stated before the dynamic ef- fects during a 60 seconds hoist strongly depend on the characteristics of the individual waves which pass the SSCV in this short time span. Because of the fact that the tests were performed in irregular waves repetition of the hoisting test for other hoist/deballast starting times resulted in very different levels of impact forces and hoisting ten- sions.

Stationary irregular wave tests with load at several "pre-tension" levels provided more reliable information on such dynamic forces from the viewpoint of statistics.

In Figs. 10 and 11 results of a stationary case with irregular seas are presented. The SSCV keeps the load in the still water lift-off position (pre- tension 78000 kN) just on a fixed jacket structure. The total system is subjected to irregular head waves with mean period of 8 seconds and a sig- nificant wave height of 2 metres.

The use of impulse response techniques to derive first and second order wave force records enables

the severest test of the quality of the simulated results, i.e. a direct deterministic comparison with experimentally obtained results. To this end the wave elevation measured during the model tests was used as input for the computational generation of wave forces.

Comparisons of motions of SSCV and module clearly show the accurate modelling of the hydrodynamics with some discrepancies in the low frequency com- ponents of surge. Hoisting wire and sling tensions also show a good quantitative agreement. With respect to the jacket impact forces {Fig. II) a satisfactory agreement was found.

In recent years the understanding of the be- haviour of floating structures and the ability to compute wave forces, motions and governing loading has increased considerably.

Extensive model test programs [2], [ 3 ] have clearly demonstrated the validity and applicability of the presented chain of hydromechanic theories, numerical techniques and computer implementations.

For heavy lift operations, including the installation of jackets and entire top sides, to be performed in the coming years, a thorough evaluation of the dynamic behaviour is of prime importance. The feasibility, workability, planning and risks involved can be investigated by means of computer simulations .

The presented LIFSM-package can be used for simulation of both deterministic and stationary situations. The pre-processing allows arbitrary hull shapes such as semi-submersibles, ships and barges while the water depth is accounted for. Geometry and elasticity of the cranes, hoisting wires, slings and stabbing piles may be fully specified. Liftlng, upending and mating of jackets can be simulated as well.

REFERENCES

1. Michelsen, F.C. and A. Coppens: "On the up- grading of SSCV Hermod to increase its lifting Capacity and the Dynamics of heavy lift opera- tions." OTC paper No. 5820, Houston 1988.

2. Boom H.J.J. van den, A. Coppens, R.P. Dallinga and J.G.L. Pijfers: "Motions and Forces During Heavy Lift Operations Offshore". Workshop on Floating Structures and Offshore Operations, Wageningen 1987.

3. Oortmerssen, G. van, J.A. Pinkster and B.J.J. van den Boom: "Computer Simulation of Moored Ship Behaviour". Journal of Waterway, Port, Coastal and Ocean Engineering Vol. 112, No. 2, March 1986.

Fig. l-Liftable jacket oonflguration. Flg. 2-Force records; hoisting 8,000 tonnes from barge in irregular waves.

Fig. 3-Schematization of lift mechanlca.

Tine step

CRANE SYSTEM t-czl= Time step

differential

IMPACT

Fig. 4-Flow diagram of LIFSIM.

Fig. 5-Model test setup and stabbing pile instrumentation.

XlmO m

:F' 0 0 l, - ,

. .

, , 4.' ' S , #d.. -. L.. I,. r.. 8w.o YmOI

S t a r t i n g t ime 1 S t a r t i n g time 2

Fig. 6-Hoisting from jacket In irregular waves (simulation).

I I

RANDOM DETERMINISTIC

STATIONARY PERIODIC

NON-ERGODIC

Fig. 7-Ciasslficatlon of a process.

I r regular wave tes ts

.5

Frequency o f wave encounter Frequency o f wave encounter in radls i n rad/s

Flg. 8-RA0 of heave and pitch of SSCV.

SSCV only ------ SSCV with mdule on jacket (1000 t pre-tension) ---.-- SSCV with module on barae (1000 t pre-tension) -

Frequency of wave encounter i n rad/s

Fig. 9-RA0 ot verttcal crane motions.

l

Model tes ts - - - - - - - - LIFSIM

MDdel t e s t s

k f l 1

l l l L I F S I M ,

Fig. 11-Forces durin'g stationary impact tests.

, ;.I m . )m,* , W O m.0 W.. m.. m cl.* m . m e xmm

Fig. 10-Yolfona durlng stationary impact teats.