OTC 6948

Monitoring Offshore Lift Dynamics Rene Wouts and Ton Coppens, HeereMac, and H.J.J. Van den Boom, MARIN

Copyright 1992, Offshore Technoiwy Conferenor

This P v r was presented at the 24th Annual On: in Houston, Texas, May 4-7, 1m2.

This paper was selected for presentation by the OTC Program Commlttee tollowlng revlew of information contained In an &tract submined by the author(s), Contents of the paper, as presented, have not been revlswed by the Offshore Technology Conference and are subject to correction by the author(s). The material, as presented, doer not necessarily reflect any posltlon of the Offshore Technolcgy Conferenm or its offlcors. Permission to copy Is restflcted to an abstract of not more than 300 words. Illustrations may not be copled, The abstract should contain conspicuous acknowledgment of where ahd by whom the paper Is presented.

This paper illustrates the swcance of lift dynamic aspects observed during two major offshore heavy lift operations performed in 1991. Extensive offshore measurements provided further knowledge related to the dynamic behaviour of heavy lift systems offshore. The wntriiution of lift dynamics to the overall response in the medium frequency range was found to be of similar magnitude as the response in the wave frequency range. Initial correlation studies with wmputer models show that this aspect was underestimated by the analyses.

The large semi submersible crane vessels (SSCVs), currently active on the heavy lift market, have been specifically designed and equipped to perform very heavy lift operations at sea, wen in rather severe environmental conditions. The lift records of the SSCVs show clearly the trend of increasing lift weight versus time [1,2]. This fact is not surprising as the market takes full advantage of the available Wing capacity. Simultaneously with the lift weight, the dimensions of the lifted structures have also increased swcantly, resulting in minimal clearances between the load and the crane vessel.

For loads which are relatively light when compared with the displacement of the crane vessel, the wave induced motion behaviour of the SSCV is almost independent of the motion behaviour of the load suspended from the cranes. Furthermore, the relative motions of the load with respect to the crane vessel, thus also the clearance, may be controlled to some extent by means of control lines operated from tug winches. For relatively heavy loads,

howater, the wave induced motion behaviour of the SSCV can be strongly affected by the motion behaviour of the load, and viceversa [2,3]. The dynamic motion behaviour of both the crane vessel and the load, as well as of a cargo barge in case it is involved in the lifting operations, are in fact coupled. The crane vessel, the load and the cargo barge together form one integrated dynamic system,

Considering the phenomenon described above, it becomes evident that lift dynamics play an important role in the feasibility and workability of a lift operation involving a relatively heavy load. Workability restrictions related to motions of the crane vessel and the load are therefore dependent upon the effects of lift dynamics. Furthermore, the clearance between load and crane vessef or the tension variation in the hoisting wires or the slings, can become limiting factors of a lift operation also due to the phenomenon lift dynamics. Maintaining the minimal clearance between crane vessel and lifted load throughout the lift operation has become of vital importance with respect to the overall safety.

In 1985, The Netherlands, Shell Internationale Petroleum Maatschappij (SIPM), The Hague, and the Maritime Research Institute Netherlands (MARIN), Wageningen, commenced research work with respect to lift dynamics, which was focused on the development of a li€t simulation computer program (LIFSIM} able to calculate the dynamic motion behaviour of crane vessel, load and cargo barge during the lift operation [4,5,6]. Extensive model tests in MARIETs seakeeping basins were canied out, additionally sponsored by Heerema Engineering Sentice BV, Leiden, for verification and validation of the LIFSIM program. In subsequent joint industry projects referred to as "Lift Analysis Study f (US- I)" and "Lift Analysis Study TI (M-W, the results of different computer models were mutually compared and verified against the

References and illustrations at end of paper.

2 MONITORING OFFSWORE LIFT DYNAMICS OTC 6948

results of model tests. The model test series comprised dual crane installations of a heavy topside and a large North Sea liftable jacket.

Particular assumptions and simpmcations are, however, inherent to both computer models and model tests. The results of offshore measurements obtained during actual heavy lift operations provide data for ultimate validation and verification work. The offshore measurement program performed by MARIN and HeereMac v.0.f. (a joint venture of Heerema and McDerrnott) comprising two heavy lift operations is desmied in this paper. The offshore measurement program is referred to as MOL91 which stands for Monitoring Offshore Wts in 1991.

Two major characteristic dual crane lift operationswere selected for the MOL91 project:

- A liftable jacket for Gannet of about 8500 t (Figure l), which was installed for Shell Expro in June 1991.

- A Production and Utility Deck (PUD) for Piper B of about 10800 t (Figure 2), which was installed for Elf Enterprise Caledonia (EEC) in December 1991.

The Gannet field is located in the central part of the North Sea and the Piper field in the northern part.

su'J3Y CONSIDERATIONS

It is indispensable for al l parties involved and for the installation contractor in particular that heavy lift operations are performed with an acceptable level of overall safety. This implies extensive and thorough job preparatory work, especially for very heavy and/or complex lift operations. The job preparatory work should obviously comprise, but is certainly not limited to, aspects related to lift dynanucs.

The key-persons on board the crane vessel who are responsible for decision making whether or not to commence a lift operation, rely on experience, skitls, in-situ environmental conditions and the information achieved in the job preparatory phase of the project. Data related to lift dynamics should be practical and should indicate whether significant dynainic response characteristics become evident in progress of a lift operation when compared with the initial condition of free floating crane vessel and cargo barge. In other words, the data related to lift dynamics should ensure that the key- persons are not taken by surprise in progress of the lift operation due to resonant motions of, for instance, the suspended load.

It is for these reasons that emphasis has been placed on the reliability and completeness of the advisory data presented to the offshore key-persons. Offshore measurement programs have been initiated and performed to further increase the understanding of the phenomenon lift dynamics with all associated pertinent aspects. The present offshore monitoring program (MOL91) basically forms part of this development.

SCOPE OF OFFSHORe MEASUREMENT PROGRAM

A heavy lift operation is basically a three body system, comprising the crane vessel, the load to be lifted and the cargo barge. Both crane vessel and cargo barge respond to waves, current and wind. Although the cargo barge may be shielded by the crane vessel, or vice versa, hydrodynamic coupling between the two-bodies is normally small when compared to the mechanical coupling f7,8]. The heavy lift system is not only subjected to environmental loads but also to the lift operation itself. Hoisting actions, ballastkg procedures, mooring and Dynamic Positioning @P) operations introduce motions in the system. For practical reasons three major ranges of periods in the response are distinguished. Since the mechanically coupled system contains a large number of natural periods, each frequency range is of particular interest:

- High frequency response (2 to 12 S) which covers the wave spectrum.

- Medium frequency response (12 to 30 S) which covers the resonant response of SSCVs exposed to long period swells and second order wave effects.

- Low frequency response (30 to 200 S) which covers mainly the horizontal motions excited by wave drift forces and wind gusts, and responses due to operations on board the crane vessel,

The crane vessel, the load and the cargo barge were instntmented to monitor the motion behaviour of the three bodies in all six degrees of freedom. Further instrumentation was applied to monitor the tensions in the slings of the hoisting arrangement. The tenszons in the hoisting wires were recorded by means of the built-in load sensors of the cranes. For both lift operations, wave rider buoys were deployed to measure the encountered wave condition. Furthermore, video recordings were made to obtain a visual aid on the global dynamic motion behaviour of the lift system and to monitor the clearances between the load and the crane vessel as well as the clearances and contact behaviour between the load and the cargo barge during the lift-off phase, and between the load and the jacket structure during the set-down stage of the deck installation.

The liftable jacket was lifted from the cargo barge, lowered in the water, upended and set on the seabed. The followkg lifting stages, listed in chronological sequence and shown in Figure 3, were recognized and were of importance for further analysis:

- Free floating SSCV without pretension in rigging arrangement. - Pretension of rigging arrangement. - Lift-off jacket from cargo barge. - Jacket horizontally suspended from the cranes bendufum

condition). - Lowering of jacket into the water. - Upending of jacket to vertical orientation. - Positioning of jacket above its location. - Docking of jacket on seabed. - Jacket in final position with tension in rigging arrangement.

The deck structure was lifted from a cargo barge and set onto the

OTC 6948 WOUTS, COPPENS, Van den BOOM 3

platform jacket. The following lifting stages, shown in Figure 4, were of importance for further analysis:

- Free floating SSCV without pretension in rigging arrangement. - Pretension of rigging arrangement. - Lift-off module from cargo barge. - Module suspended from the cranes (pendulum condition). - Set-down of module on jacket. - Module on jacket with tension in rigsing arrangement.

DATA ACQUISZTON AND PROCE!SING

The monitoring program on board DB102 was aimed at the assessment of the dynamic behaviour of the heavy lift system due to waves and operational activities such as hoisting, ballasting and moving in the anchoring system. To measure the dynamic behaviour in each of the defined frequency ranges special purpose sensors, data acquisition and storage, as well as analysis procedures, have been adopted.

Measurement equipment Motions of the SSCV were measured by means of six linear servo accelerometers distniuted over the vessel. This set-up enabled an accurate measurement of accelerations in all modes of motion over the required frequency ranges. Similar sets were used to measure the motions of the cargo barge (Gannet) and deck (Piper). The liftable jacket and the transportation barge used at Piper were instrumented with a compact sensor box containing three linear accelerometers and three solid state angular rate sensors. The compact sensor box containing vulnerable sensors was adapted to offshore use and could be installed easily. All signals were transferred to the measurement centre in the port crane tub by means of cables. The analogue accelerations and angular velocities were fed into a compact measurement computer, low pass filtered to prevent aliasing, and then converted into digital format at a sample rate of 5 Hz. The digital signals were stored on hard disk.

As no in-line components in the slings themselves were allowed, the measurement of the tensions required instrumentation of the sling attachments on the loads. For this purpose both spreader bars of the liftable jacket were fitted with strain gauges on both ends. The deck structure was instrumented similarly at the vertical sling supports. This instrumentation was camed out at the construction yards. Tensions in the starboard and port side hoisting wires were measured by means of the existing load sensors in one of the ends of the hoisting wires. The signals were taped directly from the sensors, excluding corrective measures accounting for friction effects. To be able to identify high frequency components in the tension signals, these signals were taped on an analogue recorder. The tension measurements were calibrated during the data post processing stage by equalling the mean values and the weight of the load.

The wave elevation during the lifts were recorded continuously by a Datawell wave rider buoy located some 1000 m approximately windward of the SSCV. The wave signal and the motion signals

were sirnilarily treated. Another wave rider buoy supplied on-line wave spectra.

Each measurement series started with DB102 motion recordings in the free floating condition. These runs were made for assessment of the Response Amplitude Operators (RAOs), both for correlation and quality control purposes. The actual lift operations (from cargo barge alongside SSCV to set-down of the loads) lasted approximately 20 hours for the jacket and 6 hours for the deck installation. The measurements covered most of this time, with separate recordings of 1 to 2 hours.

Data pr- Linear and angular motions were derived from the measured accelerations and angular velocities by means of integration. Inevitably this led to "drift" in the derived signals which was removed by high pass filtering. This implied that low frequency components had to be derived separately. The gravity acceleration was present in the measured horizontal accelerations due to roll and pitch. A special procedure was developed to derive the high, medium and low frequency components of each mode of motion, The end results of this post processing consisted of the motions of each body at its centre of gravity or reference location.

Material strains measured in the sling attachments were processed to derive the sling tensions. For this purpose an approximative strain-force relation was adopted based on the slenderness assumption of the attachments. Spectral analysis of the analogue tensions was carried out prior to sampling the analogue data to guarantee sufficient sample rate, especially for high frequency components in the signals.

All defined motion and tension time traces were then plotted and subjected to statistical analysis. For comparisons with linear computational lift dynamics, stationary lift stages such aspretension and pendulum condition were selected to derive spectra and RAOs.

Display, Logging and Simulation @LS) system The crane vessel DB102 is equipped with a so-called DLS system The basic aim of this system is to assist the captain and ballast operators in (1) preparing lift and ballast procedures, (2) taking adequate ballast actions during actual lift and ballast operations and (3) controlling the vessel's stability throughout the lift and ballast operation. Logging of pertinent variables (4) is permanently possible in both on-line display and simulation mode of operation. The logging facility of the DLS system has been used for the MOL91 program to monitor the draft, heel and trim of the crane vessel, the hoisting wire tensions and the ballast and crane operations. This system has also been used to monitor other heavy lift operations, For example the installation of Saga's concrete foundation templates for Snorre field [9].

RESULTS

Encountered wave climate The waiting period, which had to be adopted prior to the

MONITORING OFFSHORE LIFT DYNAMICS

commencement of the heavy lift operations, was in the order of a couple of hours (summer) for the jacket installation at Gannet and about a month (winter) for the deck installation at Piper. In wintertime, the Atlantic is exposed more frequently to depressions generating fast travelling swell waves which &end as far as the northern part of the North Sea. The long waiting period was due to these swell waves causing relatively large heave and pitch motions of the crane vessel.

Throughout the actual lift operations, the encountered wave climate was moderate. The wave spectra show clearly two distinct contniutions of swell seas and wind driven seas (Figure 5). The signZcant wave height (Hs) related to the wind driven seas was about 1.0 m during the jacket lift operation and about 1.4 m during the deck installation. The associated peak period (Tp) was about 5.8 s and 5.9 s respectively. The swell characteristics, derived from the wave spectra, amount to Hs-values of about 0.5 m and 0 3 m and Tp-values of about 8.4 S and 12. s for the jacket and deck installation respectively.

Motion behaviour of SSCV The motion response of the SSCV consists of (1) wave induced motions, (2) motions at the natural periods of the vessel and (3) motions induced by crane and ballast actions. The spectra of the motions clearly show these contributions, see Figure 6 for the roll response in free floating, pretension, pendulum positioning and jacket on bottom condition. In the pretension stage, the roll motions of the SSCV were sign5cantly larger when compared with those observed during the free floating and pendulum condition.

Motion behaviour of cargo barge The roll motions of the free floating cargo barge were reduced wcantly when pretension was applied in the rigging arrangement. Increasing the pretension level did not further reduce the barge roll motions.

Motion behaviour of the loads The observed motions of the loads when fully suspended from the cranes were very small. The vertical motions of the jacket when submerged in water were smaller than the vertical motions of the crane vessel, although the jacket and the crane tips move vertically in unison. The additional viscous and potential damping of the submerged jacket in combination with the relatively low hydrostatic pitch stiffness, characteristic for SSCVs, are expected to implicate this behaviour.

H o w wire tensions The spectra of the hoisting wire tensions show contributions in the high frequency range only. The tension variation was in pretension condition about 25 % of the final hoisting wire tension, about 6 % in pendulum condition, and about 15 % when the jacket was placed on the seabed.

The Dynamic Amplification Factor (DAF) of the hoisting wire tensions remained below 10 %, typically 5 %, throughout the heavy lift operations. The DAF-value is defined with respect to the final hoisting wire tension when the load is fully suspended from the

cranes.

Naturalpaiods The natural periods of both crane vessel and cargo barge changed swcantly during progress of the lift operation, For the jacket installation, the natural roll period of the cranevessel changed from about 28 S in free floating condition to about 16 S in pendulum condition, and about 36 s during positioning of the jacket (Figwe 6). The natural roll period of the cargo barge changed from about 9.0 s in free floating condition to about 6.2 S in pretension condition.

Behaviour during lift-off During the lift-off stage of a lift operation, one is generally concerned about significant vertical impacts between the load and the moving cargo barge. It was observed that immediately after lift- off the cargo barge remained stabilised for some time. Because of this phenomenon, there was suffident time to lift the jacket and the deck away from the cargo barge, before the cargo barge started building up wave induced motions. Minor vertical impacts and some horizontal impacts were observed having only marginal effects on the hoisting wire tensions. Video recordings showed that the longitudinal flexiiility of the cargo barge may have a significant effect on the magnitude of the impact forces between the load and the cargo barge.

One of the most important aspects of the followed lift and ballast procedures is to avoid vertical misalignment of the crane tips, load and cargo barge prior to, during, and directly after lift-off. Any vertical misalignment results in horizontal motions (S-) of the load after lift-off. There were no significant horizontal motions of the loads obselved and measured just after lift-off. Good job preparation and an experienced and trained crew is, during this stage, of utmost importance!

The time traces of the hoisting wire tensions did not show a noticeable overshoot during lift-off (Figure 7). The Dynamic . Amplification Factor obtained from these observations is well below 10 % of the final hoisting wire tension.

Setdawn of deck struclure on platform jacket The set-down of the deck structure on the platform jacket was a very smooth operation. No impacts were observed between the deck and the jacket. The motion behaviour of the SSCV changed significantly when the lifted deck structure was @artly) set on platform jacket.

Available data For correlation purposes, results of calculations and model tests could be used which were generated in previous projects (LAS-I and US-n). The large North Sea jacket used for the calculations and model tests was similar to the Gannet jacket. Data comprised time traces, spectra, transfer functions and statistical quantities. Furthermore, lift dynamic calculations were carried out during the

OTC 6948 WOUTS, COPPENS, Van den BOOM

preparatory stages of the actual projects.

For selected cases, calculations were carried out after the offshore measurements using the measured wave spectrum as input.

correlation tecbniquea Initial correlation work presented in this paper is based on comparison of motion and tension spectra. Comparison of statistical quantities was not yet performed, as the recorded data comprised low, medium and high frequency components. Comparison of statistical quantities can be done realistically when these quantities are calculated for each of the defied frequency ranges. The ultimate comparison of time traces was not performed as, for instance, the wave direction was not measured.

Behaviour of SSCV in free floating condition The correlation of pitch motions of DBlM in free floating condition is presented in Figures 8 and 9. The model test results were obtained for a theoretical formulation of the (PM) wave spectrum characterised by a significant wave height of 1.5 m and a mean zero upcrossing period of 6.0 S.

SigniF~cant discrepancies were found when comparing the calculations based on the theoretical formulation of the wave spectra with the measured data (Figure 8). The differences were reduced to some extent when using the measured wave spectrum. In the high (wave) frequency range, a marked conformity was found between the calculations and measurements (Figure g), which was reported by others as well [10]. In the medium frequency range however, the calculations underestimated the actual response.

Behaviour during lift Operation The observed motion response of the SSCV (heave, roll and pitch) was very small throughout the lift operation, determined as well by the calculations. The variations in the hoisting wires were siwcantly overestimated by the calculations which were based on the theoretical formulation of the wave spectrum.

Naturalperiods The calculated and observed natural periods showed an excellent correlation. The calculated and observed natural periods of the SSCV in free floating condition are presented in Table 1. For all stationary lifting stages throughout the heavy lift operation, resonant responses appeared at periods close to previously computed natural periods.

CONCLUSIONS

The instrumentation applied for this offshore measurement program functioned well and the sensitivities of the used sensors were adequate to monitor the motions of relatively small magnitude.

It was possible to define stationary stages in the lift operation duringwhich the characteristics of the system remained more or less constant. Spectral analyses were performed on the recorded signals to derive statistical quantities and response spectra. The response

spectra were very helpful in understanding the dynamic behaviour of the heavy lift system. Furthermore, the response spectra could be used to compare and correlate these with spectra obtained by means of lift dynamic calculations and model tests.

The observed wave spectra comprised distinct contniutions of wind driven seas and swell seas. The motion behaviour of the SSCV and the suspended load was found to make a large contribution ro the medium frequency range, outside the wave frequency range, The significant response at the natural periods of the dynamic system is expected to originate from wave drift forces, long period swell waves (Tp > 12 S) and unforeseen aspects of lift dynamics. Tftis contniution is underestimated by the lift dynamic calculations and may be due to incomplete modelling of aspects with signip~cant effects.

Natural periods of free floating crane vessel and cargo barges, as well as of complex heavy lift systems, can be calculated suffessfully and show a good correlation when compared with the actual natural periods.

The hoisting wire tensions show high frequency contributions only. The effect of swell waves and wave drift forces on the hoisting wire tensions was found to be marginal. The maximum hoisting wire tension observed throughout the heavy lift operations remained well below a margin of 10 % on the static hoisting wire tension. No wershoot was observed during lift-off of the loads from the cargo barge.

Although of particular concern during the design and job preparatory stages of a heavy lift operation, the observed impacts between the loads and the cargo barges were hardly noticeable and certainly did not endanger the lift operation at any time. Based on offshore observations, it is expected that the global flexiiility of the cargo barge and the load may have a significant effect on the magnitude of the impact forces.

It is recommended to initiate research on how to account realistically for combinations of wind driven seas and swell seas in lift dynamic calculations to be performed in the preparatory phase of a lift operation.

Enhanced post processing techniques have to be adopted to be able to correlate realistically the statistical quantities of results of offshore measurements, model tests and lift dynamic calculations, as the recorded data comprised low, medium and high frequency components.

Research should be continued on the phenomenon lift dynamics by means of extensive correlation studies in which the results of computer models will be compared with the observed data. If required, the computer models have to be adjusted and input data defined differently. Efforts should be focused on the excitation and motion response in the medium frequency range.

6 MONITORING OFFSHORE LIFT DYNAMICS OTC 6948

It is recommended to adapt the computer modeh accounting for the flexibility of the cargo barge and the load for lift-off simulations. Measurements offshore, focused on this topic, may also be valuable in understanding this phenomenon.

The significance of offshore monitoring programs during heavy lift operations has been demonstrated and offshore monitoring should be performed on a permanent basis for major heavy lift operations.

The authors thank the oil companies Shell Expro and Elf Enterprise Caledonia (EEC) respectively for allowing this offshore monitoring program, carried out during the installation of their respective structures.

The assistance and cooperation of the offshore crew on board DB102 were of vital importance for the success of the offshore monitoring programs. The offshore crew as well as the project teams were obliged to perform additional work on top of their already high workloads. The authors appreciate their attitude and their contribution to MOL91.

This offshore monitoring program was made possible via a subsidy granted by the Stichting Maritiem Ondenoek (CMO; Coordination Maritime Research), Rotterdam.

REFERENCES

1. Michelsen F. and Coppens A., "On the Upgrading of SSCV Hermod to Increase its Lifting Capacity and the Dynamics of Heavy Lift Operations", paper OTC 5820, Proceedings 20th OTC (1988), Houston.

3. Clauss G., Riekert T. and Coppens A., "Limits for Operations of Large Crane Vessels", Jahrbuch der SchiMbautechnischen Gesellschaft (1990), Berlin.

4. Willemstein A.P., van den Boom H.J.J. and van Dijk A.W., "Simulation of Offshore Heavy Lift Operations", Proceedings CADMO Conference (1986), Washington.

S. van den Boom W.J.J., Dekker J.N. and Dallinga R.P., 'Computer Analysis of Heavy Lift Operations", paper OTC 5819, Proceedings 20th OTC (1988), Houston.

6. Baar J.J.M., "Developments in the Analysis of Offshore Heavy Lift Operations", Proceedings 1st ISOPE Conference (1991), Edinburgh.

7. Baar J.J.M., Pijfers J.G.L. and van Santen JA., "Hydromechanically Coupled Motions of a Crane Vessel and a Transport Barge", paper OTC 6949, Proceedings 24th OTC (1992), Houston.

8. Tong K.C. and Duncan P.E., 'Modelling the Dynamics of Offshore Jacket Lifts", Proceedings 1st ISOPE Conference (1991), Edinburgh.

9. Lange F.C., Hetland S. and Knudsen J.I., "Control and Dynamics During Lift Installation of the Snorre TLP Concrete Foundation Templates", paper OTC 6881, Proceedings 24th OTC (1992), Houston.

10.Standing R.G., Brendling WJ. and Jackson G.E., "Full-scale Measured and Predicted Low-frequency Motions of the Semi- submersible Support Vessel Uncle John", Proceeding 1st ISOPE Conference (1991), Edinburgh.

2. Clauss G.F. and Riekert T., "Operational Limitations of Offshore Crane Vessels", paper OTC 6217, Proceedings 22nd OTC (1990), Houston.

Table 1: Natural periods of free floating SSCV at 20 m draft.

heave

p i t c h 19.6

I. , - . -.

FREE FLOATING STAGE PRETENSION STAGE

LIFT-OFF STAGE SET-DOWN STAGE

Fig. 4: Installation stages of deck

Piper - H = l 4 m T = 3.7 S

--- Gannet - HI - l : ~ m' T' = 4.2 r Model tes ts - H: = 1.5 m: T: = 6.9 s

- - - W W - Free f l o a t i n g ( p r e - l i f t ) Pretension

--- Pendulum Posit ioning Jacket on bottom

ia l .5 WAVE FREQUENCY IN RADB

Fig. 5: Encountered wave spectra Fig. 6: Spectra of m 1 1 SSCV for various stages (Gannet)

I . . . . I . . . . , . . . . , . . . . , . . 0 m 100 It0 two

3ECONDS

Fig. 7: Hookloads during l i f t - o f f (Gannet)

8;OM H All- WXL33d8