OTC 7493

On the Analysis of Mooring Systems Using Synthetic Ropes Albert Dercksen, Maritime Research Inst. Netherlands, and L.F.E. Hoppe, DSM

Copyrlght 1994. Offshore Technology Conference

This paper was presented at the 26th Annual OTC In Houston. Texas, U.S.A., 2-5 May 1994.

This paper was selected for presenlallon by the OTC Program Commlltee following review of information contained In an abstract subm.tled by the aulhor(s). Contents of the paper, as presented, have not been revlewed by Ihe Offshore Technology Conference and are subject to wrrecllon by Iha author($) The malerlal, as presented, does not necessar~ly re!!ect any ~asltlon of the Offshore Teohnolwv Conference or its officers;Permisslon to COPY is restricted to an abstract of not more than 300 words. lllustratlons mav not be coDled. The abstract ahbuld contaln conspicuous aoknowledgment of where and by whom the paper 1s brasentad.

ABSTRACT

Rapid developments in the field of synthetic fibres have lead to serious alternatives for the traditional catenary mooring systems in several applications. The use of synthetic fibres in mooring systems has received increasing attention, especially for deeper water. However, 8 design based on these fibres requires modification in the analysis methods.

Here the feasibility of mooring systems using a synthetic rope (with Dyneemcl9 compared to a wire rope is analyzed, for a water depth of 800 m. Differences between the systems are elucidated by showing results of a time domain analysis on a turret moored tanker. Different optimization criteria are considered.

Generally speaking optimum mooring systems based on synthetic rope are stiffer and impose lower vertical turret loads than a wire rope mooring at increasing water depth. In return for the smaller tanker excursions, the vertical forces at the seabed are considerable for a synthetic mooring.

References, tables and figures at end of paper

INTRODUCTION

Floating production facilities are being used at increasing depths. In deep water conventional mooring systems, based on wire rope and chain, have serious drawbacks and alternative systems (synthetic ropes) become more feasible, see e.g. ref. 111. However the dynamic behaviour of a synthetic mooring system is different from a mooring with wire rope, because of the absence of a catenary and the time-dependent properties of synthetic materials, Thus a conventional design may result in a sub-optimum for a synthetic material.

Dyneema is a High Performance Polyethylene (HPPE) fibre, with a tensile strength similar to steel on a diameter basis, see also 121. Ropes made of Dyneema are well suited for use in the marine environment under dynamic conditions (see ref. [31 & 141). On the basis of these successful applications this material has been selected for this study.

A turret moored tanker is a dynamical system subject to excitation due to wind, waves and current. For the design of a mooring system excitation forces and dynamic responses in terms of loading and tanker motion are required, The

O n the analysis of mooring systems using synthetic ropes OTC 7493

excitation depends on the environment, the reaction forces are a combination of hydrodynamic forces and restoring characteristics of the mooring system, see ref. I51 for a general discussion.

The excitation forces can be divided in three groups, namely: average load, wave frequency (typical period: 5 - 20 sec) and low frequency. The low frequency excitation is a combination of mean wind, current and wave drift forces, with a typical period of 50 - 500 sec. The natural frequency of the mooring system is generally in the low frequency range. The wave frequency loads are from the first order wave forces on the tanker and the resulting tanker motions are directly transferred to the mooring system. The average loads are from the steady wind, current and mean wave drift forces.

Here a 200 kDWT tanker has been moored in a water depth of 800 m, and the behaviour of the tanker plus mooring system is studied under survival conditions.

Different optimization criteria are used to evaluate a synthetic mooring system.

METHODOLOGY

In order to compare the response of different systems two criteria are used here:

similar restoring curve Systems with similar restoring curves will have similar low frequency properties and natural periods. The response of such systems will therefore only differ in the wave frequency range, i.e. the effect of the high frequency tanker motions on the mooring line tensions can be investigated more or less independent of the low frequency behaviour. This approach is especially suitable for assessing the influence of the layout and material properties of the individual mooring lines.

similar safety factor This method is generally used in a design in order to meet the requirements of classification societies. Systems with similar safety factors (and breaking strength) have similar mooring line loads. The mooring arrangements are designed on the basis of maximum allowable mooring line tensions. Generally the restoring curves between these systems will be different.

Similar restoring curves and similar safety factor are only two of many criteria. Others are lifetime, installation, fabrication, costs, etc. These are not considered here.

To allow a more direct comparison the conditions selected are for a representative situation and simplified mooring systems, namely wire plus anchor and synthetic rope plus pile. Also, the strength of the ropes has not been varied.

As the behaviour of synthetic materials is tirne- dependent the response to a load of a given line may vary from frequency to frequency. For Dyneema, the specific stiffness shows a small dependency on frequency, namely k 15% increase in stiffness per decade frequency increase. Thus for Dyneema laid ropes a specific stiffness of 46 Nltex at 0.1 7 Hz is used. Polyester (PES) ropes show an increase in stiffness with load-history. A detailed discussion on polyester is outside the scope of this paper, but the reader is referred to 11 l and 161. For the polyester parallel rope a specific stiffness of 16.5 Nltex is used at 0.1 7 Hz and it is assumed that the rope has gone through a l000 cycles, see 11 l. This results in a polyester parallel rope that is 45% stiffer and has a 51 % larger diameter than a Dyneema laid rope for equivalent breaking strength.

The mooring lines have been discretized in 40 elements and the simdations have been carried out using the program DYNFLOAT, see 171. For the wire rope and synthetic ropes a weight in water of 5585 Nlrn resp. 0 Nlrn is used. For the synthetic materials this can be done, because their specific weight in water is small or negative and the pre- tension is sufficiently high to prevent a catenary.

RESULTS

The simulations have been carried out using a 200 kDWT tanker in fully loaded condition, see table 1 for details. The dynamic behaviour of the system is evaluated for a three hour storm, with wind, waves and current colinear, see table 2 for the environmental conditions. These data are for a 100 year return period survival condition. The water depth is 800 m.

1. As a reference, a conventional mooring system based on wire rope is designed. The mooring configuration obtained is: 9 lines of 127 mm wire rope and a length per line of 5000 m,

OTC 7493 A. Dercksen and L.F.E. Hoppe 3

results show a maximum offset of 79 m and a maximum vertical turret force of 1656 tf. The safety factor is 1.68, see table 4 (wire).

2. Next a system with Dyneema ropes is designed, with a similar restoring curve. This results in 6 lines of 131 mm rope, length per line 7000 m, see table 3 (HPPE I). The maximum offset is similar to system 1 and the vertical turret force compared with wire is reduced by 90% (148 tf). The safety factor in the lines is increased by 1 2% (1.88). Thus the use of these ropes is not optimal with this criteria.

3. Another system with Dyneema ropes is designed with a similar safety factor as the wire rope mooring. The mooring configuration now becomes: 6 lines of 131 mm rope, length 4000 m. The maximum offset and vertical turret force compared with wire are reduced by 35% (51 m) and 83% (276 tf) respectively. By designing for a safety factor comparable to the wire rope system the line lengths have been reduced by 43%, see tables 3 and 4 (HPPE 11). Also the offset is reduced, but vertical loads have increased, especially at the anchor point.

4. A system is designed where the Dyneema lines are replaced by polyester (PES I) parallel rope with equivalent breaking strength. The layout is: 6 lines of 198 mm diameter with a length of 4000 m. The maximum offset and vertical turret force compared to wire are reduced by 55% (36 m) and 81 % (308 tf) respectively. Because of the difference in stiffness between HPPE and PES the loads on the ropes are higher for this system and the safety factor decreases from 1.66 to 1.59.

5. Thus another PES parallel rope system is designed where the restoring characteristics are comparable to system nr 3. The mooring configuration now becomes: 6 lines of 198 mm rope, length 6000 m. The maximum offset and vertical turret force compared with wire rope are reduced by 33% (53.1 m) and 88% (204 tf) respectively, see table 4 (PES 11). Also the vertical force at the anchor point is lower compared to the optimum Dyneema configuration. These differences between the two systems can be attributed to the higher stiffness of polyester. This system is not modified further, to obtain an identical safety factor, because of the uncertainty in the dynamic stiffness of this system,

The resulting safety factors for all systems are relatively low, but as the aim of this paper is to compare different moorings the systems have not been redesigned. The restoring curves for these five systems are given in figure 1. in the time traces of the horizontal displacement and horizontal and vertical turret force it is most clearly shown how the restoring characteristics affect the low frequency behaviour, see also figure 2.

In figure 3 the footprint of the wire rope system, system 2 (optimum Dyneema) and system 5 (optimum PES) are given. Comparing the wire rope with the optimum synthetic systems it can be seen that the dynamic effects due to wave frequency motions are more pronounced for the wire rope system, especially the turret forces, see figure 4.

CONCLUSIONS

From this study the most significant differences between wire rope and synthetic moorings are the low vertical turret forces and low displacements for the synthetic systems, This can be attributed to the absence of the catenary for these systems.

The Dyneema system with the same safety factors in the mooring line as steel resulted in a reduction of the number of lines, a shorter length per line and approximately the same breaking strength per line as compared to steel.

Direct comparison of a wire rope system and a Dyneema rope mooring systems with similar low frequency properties, will not give useful results as the Dyneema lines are too long.

Comparison of Dyneema and polyester moorings shows that the optimum Dyneema system has lower rope diameters and shorter lengths per tine. The optimum polyester system has lower vertical pile forces. However differences are not as clear, because input data from the literature are ambiguous.

4 On the analysis of mooring systems using synthetic ropes

REFERENCES

1 Del Vecchio, C.J.M, Light weight materials for deep water moorings, PhD. Thesis, Reading Univ. 1992.

2 Kirschbaum, R., Dingenen, J.L.J. van, "Advances in gel-spinning technology and Dyneema fibre applications", 3rd Rolduc Polymer Meeting, April 1988.

3 Haar, J. ter, "Emergency towing gear: How to avoid a 'BRAER' disaster", Rina Int. Conf. Escort Tugs, London, October 1993.

4 Street, A., Potter, D., "High Performance Steelite Ropes for Harbour towingw, 1 2th lnt. Tug and salvage Convention, Genoa 1992.

5 Wichers, J.E.W., A simulation model for a single point moored tanker, Thesis, Delft Univ. 1988.

6 Ratcliff, A.T. & Parsey, M.R., "Man-made fibre ropes for marine usew, April 1985,

OTC 7493

7 Dercksen, A., Huijsmans, R.H.M., Wichers, J.E.W., "An improved method for calculating the contribution of hydrodynamic chain damping on low frequency vessel motions", OTC paper No, 6967, Houston.

OTC 7493 A. Dercksen and L.F.E. Hoppe 5

Mass tfs2/m 24 553

Virtual mass tfs2/m 26 147

Table 2: Environmental conditions and tanker damping.

Wind force

Current force

Mean wave drift force

Significant wave height

Peak Period (PM)

Tanker damping (total)

Table 1 : 200 kDWT Tanker.

Table 3: Properties of different mooring systems.

tf

tf

tf

m

S

tfs/m

290

10

148

12

14

100

On the analysis of mooring systems using synthetic ropes OTC 7493

OTC 7493 A. Dercksen and L.F.E. Hoppe

Figure 1: Restoring characteristics for the different systems p00

A 0 20 U ) 00 &O 100 1 X )

displacement (m)

Figure 2: Ten minute time trace selected from a three hour storm

HPPE I I

wire HPPE l

e

g 250 - S CI

X U- 0 I t

300 time (S) 600

500' i

N

17 HPPE l1

* , , A , . ,%A..--' .*..--'w--~*~~u%u%#'. ,d~~**...-~.",t,,,,*w~~."~~.~".,%~-~-.,~--.~.*.~d.~~-~.,*d --***---------------------P.

L L o I i 300 time@) 600

261

On the analysis of mooring systems using synthetic ropes OTC 7493

OTC 7493 A. Dercksen and L.F.E. Hoppe

Figure 4: Results for different mooring systems in a three hour storm

Static Maximum displacement

Wire rope (system 1)

Synthetic rope (system 3)