Local loads by pipes, hoses etc

ARCH1EF

r-Dr. P. KAPLAN OCEANICS, Inc.

Technical Industrial Park

Pl ainvieW NEW YORK 11803 U.S.A.

L

enclosed you will find my contribution to the ISSC-Report 76.

I had. to report about local loads by pipes, hoses,

etc. except by cables, that was the taskof another member. I preferred to pay attention to the whole system including stinger and pipe, more than to the vessel alone, both for a wider literature available and for the tendency to consider pipe or stinger failures as basically affecting the ship itself.

At present I am roughly available for our interim

meting being held on any day in April or the first half of

May.

Enel.

e

I

CENTRO PER GLI STUDI Dl TCN1CA NAVALE

Technische Hogeschool

Lab. y.

CETENA

Deift

Yours very sincerely,

c.c. TELEFONI: 590.521 539.349 53.481 GENOVA..1.0.th....March, 1975 VIA XX SETTEMBRE. 8/9 NOSTRO RIFERIMENTO

J DA CITARE NELLA RISPOSTA GSA/oj)

: Dr. 0. Faltinsen Dr. P. Holmes

Prof. V. Fer&inand.e Mr. J. Lundgren

Dr. 3. Fu.kuda' Dr. D. M.Bostovstev pMr. F. Van Gunsteren Dr. H. Soding

Unlike many other vessels, pipelayers undergo particular types of local loads, due to the appendage pipe + supporting structu

re.

The rapid evolution recently occurred in pipelaying teçhniqu.e has led on one side to fundamental changes in the laying

ves-sel, on the other hand to development of the pipe-supporting

structure.

Laying vessels are now falling into the third generation, star ting from conventional lay-barges through semisubmersibles,

aiming to lay large-diameter pipes at high speed in deep waters under the roughest environmental conditions and to undergo

ac-ceptable motions under any weather. Concurrently pipe-support Ing structures, designed to support the pipe in its way to the sea bed limiting the curvature, evolved from the conventional pre-tensioned stingers to articulated stingers; furthermore the hinge was shifted from the attachment stinger-barge to the last section of the stinger.

The 3rd generation of lay-barges will lay by a curvedstern ramp rigidly connected to the hull

El].

The previous considerations suggest to think of the system ves sel-stinger-pipe as a whole and of the local loads acting on

the pipes as directly affecting the vessel itself.

V1iile theoretical and full scale investigations will be revie-wed in the next sections of this report aimed to a better

un-derstanding of the static and dynamic behaviour of the system, mention must be made of the model testings carried out at the N.S.M.B. and at Netherlands National Aerospace Laboratory.

I

I - Pipe systems :

A noteworth simulation of the behaviour of the barge-stin-ger-pipe system on a hybrid computer has been carried out to verify the possibility of laying cables beyond 1500 ft using dynamic positioning : in the two-step procedure

de-scribed in [2} the physical system was taken into account by the analog computer, while the digital one stored the curves of the environmental conditions from tank model or

literature. The pipe was simulated into 2 supported and i

unsupported parts and haudled. as catenary or-alternatively as beam under tension, subjected to constant current and variable wind and waves. Three parameters (tension along the pipe, lateral load at the stinger lift-off point, lato rai position of the vessel)

were repeatedly calculated, and checked against their maxi mum permissible values : overcoming caused control system

to take places to evaluate the total correction in thrust and turning moment (to be later distributed among the thru sters with a special techniqye) on the basis of the absolu te and the differential error.

The. static and dynamic behaviour of the vessel-stinger-pi-pe system during laying ovessel-stinger-pi-perations has been exvessel-stinger-pi-perimentally investigated [4J.

A joint effort has been paid by Esso Production Research Company and Esso Australia Ltd to check the effect of the pipeline design parameters on the stresses arising along the pipe _3J. Regularly spaced sections of pipe were pro-perly instrumented either for a flat-bottom conventional barge and for a semisubmersible.

The static analysis showed the stinger-barge hinge and the nearly-middle point of the suspended portion as the most critical points, as far as the vertical stresses were

con-cerned : the figures showing the laterally- bent configar

tion of the pipe subjected to transverse current are quite

impressive, giving clear ideas of the mechanism of the mu-tuai interactions stinger-pipe.

Magnitude and spectrum of the dynamic stresses due to bar-ge-stinger relative motions were compared with the calcula ted ones : high values were found at the hinge, due to the

short radius of curvature of the pipe supports in conjunct ion with the dynamic oscillations between barge and

stin-ger, as well as at "lift-off" point for the semisubmersi-ble, showing the attitude of the column-stabilized stingers

to "ride the waves" and to let their last portions act as

.1

3

Several conclusions are drawn, like the possibility of im-proving the efficiency of the semisubmersible laying opera tions using short and curved stingers and higher tensions. Static and dynamic analysis of pipelines is also found in

[5]. The static configuration is reached through the

ite-rative procedure by Skop-O'Hara based on successive modifi cation of the reaction values till the equilibrium and the boundary conditions are met. The dynamic analysis of the pipeline is the classical one aimed to find the frequencies and modes of the line, uncoupling the governing eqaatiün by Foss method for system with non-proportional damping displacements, tensions etc. are obtained as forced respon

se.

Although such method is general enough to include the bar-ge and deal with barbar-ge-pipeline interactions, the paper is mainly oriented to the behavióur of the line itself.

I. Hicks and L. Clark presented in [6.) the mathematical mo

del of a subsurface buoy supporting a slender column of uniform cross-section, attached to the ocean floor, subjec ted to waves and to parallel or cross currents. The model fits very well to represent offshore loading and product-ion facilities (f.i. drilling risers supported by buoyant chambers) but it is clearly extendable to our vessel-pipe system from which mainly differs for the surface boundary conditions (zero moments at the buoy); the column was di-scretized in mass-spring system. Newton's second law yie-lds 3N equations for the N segments undergoing tensions at the end, buoyancy, weight and hydrodynamic forces along

every one. These last were computed by Morison formulae

as functions of the normal components of the relative velo cities and accelerations between the local water particles

and the pipe.

The 3N equations were solved for the three components of the segment accelerations in terms of time using finite-difference expressions,starting from the initial configur tion due to the current only. Static displacement as well as the superimposed pendulations were treated as large and 3-D deflections, using a zero-stiffness model. Bending

stresses were further computed at any point of the column by the simple beam theory applied to the segments of the

deflected shape.

The method lends itself to parametric nalysis of stresses in the column and forces interchanged with the surface su

4

A practical approach to the problem of laying pipes of gi-ven weight and stiffness keeping control of the bending mo ments along the line as weil as of the forces at the con-nection stinger-vessel is described in [8J.

Curves giving the actual vertical coordinates of the

su-spended portion of the pipeline (Y) against the local an-gle (e) can be built by segments, as functions of the ver-tical reaction at the bottom : they may be changed repeate dly till a satisfactory condition for the bending moment along the line is found.

The optimum lenght of the stinger is achievable at design

stage crossing a curve (Y, 9 ) typical of the stinger; a

similar procedure is used to determine the best radius of curvature for a stinger already built (given lenght).

II - Hose system

Flexible hoses are frequently used. in association with sin gle-point moorings for loading-unloading operations of lar ge tankers, to cope with the most unfavourable environmen-tal conditions.

While forces of no significance are conveyed to the ship, the hose itself as part of the system ship-hose-buoy is su bjected to stresses of òonsiderable magnitude mainly at the connection of the floating portion of the hose to the

buoy.

A great effort has been devoted to identify and measure the forces acting on the hose as well as to correlate thorn with the environmental conditions, by Dunlop Ltd. and

Shell B.P. Company of Nigeria Ltd. Detailed description of the three-units measuring system as well as practical problems like calibration etc. is given in [9J.

Recordings of pressure signals, axial stresses, torsional and vertical bending moments were taken for different orlen tations of the ship in respect of its hose, of the swell and the sea.

Peaks in axial forces and torsional moments were mainly due to swell, while local high frequency conditions were

found responsible for the peaks in bending moment.

A rough correlation appeared to be consistent between such stresses in the hose and the prevailing conditions, as a first set of analyzed tape signals showed : furthermore the results suggested that fatigue, rather than sudden

high stresses were responsible for hose failures.

It is felt that wider research involving different buoys and sites could confirm steadily the correlation between the forces on the hose and the prevailing environmental conditions, so that proper hoses be selected at design sta ge for given environmental parameters.

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III - Riser systems

'It. is felt to give additional considerations to the

drill-Ing environment, where long, slender risers support local loads and convey them to the connection point with the sur

face platform.

Concerns about the static behaviour of the riser were found (10] in the possibility of high deflections and stresses as well as buckling due to the slenderness. The static analysis was performed in 11 , where the general dif f

eren-tial ecjuation for a laterally - loaded beam under steady

current.

[El -'+

Ñ()Ç

+w(z)4

(i)

and under axial compression due to weight

(): N(o)+ ₍₂₎

o

were firstly reduced to a series of first order equations and then integrated by a 4st order Runge Kutta method. A further parametric analysis aimed to show the effects of top tension, top offset and current on riser bottom angle and stresses led to the following conclusions

addition of buoyant modules to the riser considerably re duces the stress levels along it;

- no great adjustment is required for the control of the top offset as the depth of the water increases, because the behaviour of the bottom angle and the minimum bend-ing stress, referred to the unit-deflection, change slow ly with the water depth;

the current induces greater stresses at lower water

le-veis.

The significance of the dynamic analysis of the risers at deep waters at design stage was evidenced by studies

con-cerning the Mohole project [12] , [131

Formula (i) was extended [ii] to handle the dynamic

beha-viour of the riser : use was made of a linearized Morison formula to evaluate the response either to sinusoidal wave or to vessel motion; the second one was found to be the bigger source for high dynamic responses, as already shown

in [121, [13j.

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Response to random wave forces was also investigated in [143 through a finite-difference model : the non-linearity

in the drag term was removed by the linearization

techni-çae : the riser was found very sensitive to the spectral

distribution and the total amount of wave energy; further-more further-more than one natural mode partecipate significantly to its response.

IV - Conclusions

Some conirnon difficulties in the mathematical models at the

present moment induce researchers to make various assumpt-ions; mainly they are as follows

- the unpredictable behaviour of the flow of the water par

tides around the oscillating pipe : this suggest to

as-sume the hydrodyìiamic forces acting on the pipe

follow-ing the Morison approach;

- the non-linearity arising in the velocity-dipendent term

of the Morison formula : it is usually removed by an equivalent linearization technique;

- the unknown boundary conditions for the ends of the pipe: usually limiting boundary conditions are replaced.

Future efforts are recommended to tackle such difficulties with more efficient tools.

Genoa, 7th March 1975

REFERENCES

[i) Larges-Bell : "The third generation Lay barge" - Offshore

Technology Conference OTC 1935 (1974)

[21 Harris-Morgan-Stuart : "Control simulation study of

dynami-cally positioned pipelay barge system for deep water operat tion" - Offshore Technology Conference OTC 1506 (1971)

131 Mc Phail-Finn-Rohmaller-Okaro.: "Offshore pipeline construc

tion stress measurement" - Offshore Technology Conference OTC 1573 (1972)

[43 Stewart-Fraser : "Experimental measurement of stresses whi-le Laying pipe offshore" - 6-PET-24, Petrowhi-leum Mechanical

Engineering Conference-New Orleans - (1966)

[) Dominguez : "Predicting behaviour of suspended pipelines in

the sea" - Offshore Technology Conference OTC 1574 (1972)

16) Hicks-Clark : "On the dynamic response of buoy-supported ca

bies and pipes to currents and waves" - Offshore Technology Conference OTC 1556 (1972)

[7) Lampietti "Pendulation of pipes and cables in water"

-ASME Paper No.63-WA-101

t8] Daley : "Optimization of tension level and stinger lenght for offshore pipeline installation" - Offshore Technology

Conference OTC 1875 (1973)

[9) Brady-Williams-Golby : "A study of the forces acting on ho

ses at a monobuoy due to environmental conditions" - 0f f-shore Technology Conference OTC 2136 (1974)

ioJ Fischer-Ludwig Milton : "Design of floating vessel drilling

risers" - Journal of petroleum technology (1966)

iii) Burke : "An analysis of marine risers for deepwater" - Of-fshore Technology Conference OTC 1771 (1973)

112] National Engineering and Science Co.

: "Dynamic stress

ana-lysis of the Mohole Riser System"- NSF Beport PB 175258(1965)

[13) National Engineering and Science Co. : "Structural and Dyna

mic analysis of the riser and drill string for project Moho le"- NSF Report PB 175364 (1966)

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