UNITEN ICCBT 08 Parametric Study of Environmental Load Impacts on a Jack-up

ICCBT 2008 - D - (43) 471-486

ICCBT2008 Parametric Study of Environmental Load Impacts on a Jack-up Structure

S. Saiedi, Universiti Teknologi PETRONAS, MALAYSIA S. Aghdamy, Universiti Teknologi PETRONAS, MALAYSIA

ABSTRACT The impacts of environmental factors on three major reactions of a jack-up offshore platform are analysed using computer simulations. The reactions are the total base shear, mud line overturning moment and platform maximum deflection. A simple in-house computer program facilitating the manual calculations of the total base shear is developed to ensure a sound application of a commercial software (SACS) employing finite elements analysis method. Three major structural reactions are chosen to represent the responses of the jack-up. The structure is a typical operating jack-up offshore platform in Central North Sea with a water depth of 97 m, a maximum wave height of 31 m with a period of 18 s, a current speed of 0.67 m/s, and 1-min mean wind speed of 40 m/s at 10 m above the sea level. The loads analysed are due to wind, current and wave. The first two environmental actions are computed by drag force equation while the third is expressed by Morrisons equation that accounts for both drag and inertia forces. Changes in all three reactions under a wide range of major design factors are monitored and careful conclusions are drawn. The factors are wave height (H), wave period (T), wind and current speeds, size (D) of the structural members (chords and braces) and the storm surge. The dominant role of wave forces, as compared to those of wind and current, is emphasised. It is shown that the shear force is more sensitive to H rather than to other wave parameters or to D. However, the dominant parameter in the overturning moment could be H or T depending on the range of the relative change of the respective factors. The overall behaviour of the platform deflection is similar to that of the overturning moment. The relative increase of D brings about a low relative change in the overturning moment but a high change in the base shear. The findings provide for guidance in the conceptual design of jack-up offshore platforms. Keywords: Jack-up offshore structure, parametric study, environmental loads, SACS *Correspondence: Associate Professor Dr. Saied Saiedi, Civil Eng. Dept., UTP, 31750 Tronoh, Perak, Malaysia. Tel: +60 5 368 7284, Fax: +60 5 36 56716. E-mail: [email protected] y ** Graduating Student (July 2008), Civil Eng. Dept., UTP, E-mail: [email protected]

Parametric Study of Environmental Load Impacts on a Jack-up Structure

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1. INTRODUCTION Great majority of the existing offshore platforms are of bottom-supported type as opposed to floating structures. The floating structures include tension leg platforms (TLP), semi-submersibles, Spars and ship shaped vessels. Bottom-supported structures can be divided into fixed (jacket, gravity base, jack-up) and compliant structures. While the latter are occasionally placed in waters as deep as 850 m, the former are usually constructed in shallower waters (

S. Saiedi & S. Aghdamy

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2. ENVIRONMENTAL LOADS AND BASIC ASSUMPTIONS A recent study [10] on a large database containing the overall causation data for 71,470 accidents recorded from 1991 to 2001, identified the following as the failure sources: human (mainly operational) errors 46%, engineering (mainly construction) failures 41%, extreme environmental (weather related) conditions 11%, and hazardous materials (fire, blast, etc) 2%. The total number of jack-up accidents in 1979-1988 was 226 with an average overall annual accident risk of 6% [11]. Figure 1 shows the distribution these accidents among which the direct contribution of storm is about 7%.

Figure 1. Distribution of 226 jack-up accidents in 1979-1988

BASFJOSEL requires the following structural-environmental date as input. Type of jack-up (hull type, leg type) Geometry of the structure : Members (length, orientation, position and diameter of all

members), topside (overall dimensions), air gap (height) Water depth and storm surge height Design wind (velocity at a referenced height-10 m- and choice of empirical parameters in

the formula for velocity profile Design current velocity at sea level Design wave height and period The program estimates marine growth thickness according to NORSOK [12]. It also computes empirical parameters for Morisons equation application such as drag coefficient, inertia coefficient and shape coefficient according to API Guidelines [13] and Coastal Engineering Manual [15]. Basic wave mechanics relations for small amplitude waves (Airy theory) are used to obtain wavelength, maximum horizontal and vertical components of the particle velocity and acceleration along the water depth [14, 15]. The wind velocity profile is automatically generated using power law to get the design velocities at specified levels. Wind drag force on hull and dray parts of legs of the structure is then computed incorporating velocity changes with depth.

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The current velocity profile is automatically computed with power law to obtain the current velocity at any required depth. The current drag force on the submerged part of lags is then calculated by the usual drag force formula incorporating velocity changes with depth. The wave forces on the structure are computed by the application of Morrisons equation to each exposed member. For members extending from the seabed to water surface, the total integrated formulas for forces and moments are employed. For a brace, the force for unit length of the structure at the member centre is computed. The total brace force is then obtained by multiplying this unit force by the member length. Knowing that the individual maximums for inertia and drag forces do not occur simultaneously, a conservative design approach has been adopted in which the individual maximum inertia and drag components during a wave cycle are added to get the total maximum force and moments (see pp. 253-4 of VI-5, [15]). The lift force on submerged structural members has insignificant effect on the total shear force and moment at the base of the structures. Therefore, its incorporation in BASFJOSEL is not reported here.

The software allows for user-defined values or automatic calculation based on built-in standards or guides. The equations for these three forces are as follows.

ACVF Swindairwind2

21 =

(1) ACVF Dcurrentwatercurrent

2

21 =

(2) Diwave FFF += (3)

iwaterMi HKDgCF4

2= (4)

DwaterDD KgDHCF2

21 =

(5) )2tanh(

21

LdKi

= (6)

nK D 41=

(7) )

]/4sinh[/41(

21

LdLd

CC

n g +==

(8) CD=1.2 Re < 105 CD decreases from 1.2 to 0.6-0.7 105 < Re < 4 105 (9) CD =0.6-0.7 Re > 4 105

CM =2.0 Re < 2.5105 CM =2.5 Re/5 105 2.5 105 < Re < 5 105 (10) CM=1.5 Re > 5 105

Cs= 1.5 beams Cs=1.5 sides of buildings Cs=0.5 cylindrical sections (11) Cs=1.0 total projected area of platform


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In Equations (1) to (11), Fwave is the total wave force consisting of drag FD and inertia Fi forces, is mass density, g is gravitational acceleration, D is member diameter, H is wave height, T is wave period, L is wave length, C is wave celerity, Cg is wave group velocity, n is C/Cg, KD and Ki are drag- and inertia- related coefficients in Morrisons equation (3), respectively, CM and CD are drag and inertia coefficients, respectively, d is water depth, Cs is wind shape factor, A is frontal area against wind and Re is Reynoldss number as defined by the ratio of (water velocity D)/ (kinematic viscosity of water). The total forces by wind, current and wave are translated to the base of the structure in terms of shear force providing. 3. THE CASE STUDY A typical jack-up structure in Central North Sea (called MSL here for ease of reference) is selected as the case study to monitor changes in the base shear force, overturning moment and platform deflection with varying environmental factors. The platform is adopted from a comprehensive investigation [16], performed by MSL Engineering Limited for the Health and Safety Executive (HSE) of United Kingdom to determine the effects of air gap and subsequent hull inundation when wave slams into the deck of a jack-up. The dimensions of major components and key environmental parameters, applied omni-directionally, are contained in Table 1. Figures 2 and 3 show the configuration of the case study structure. MSL is a three-legged jack-up structure with a triangular hull founded on them. The legs are triangular, consisting of three tubular chords braced with a K-bracing arrangement. At the bottom of each leg is a spudcan. The connection between the leg and hull consists of a set of pinions and rigid horizontal guides at the bottom of the hull and at the top of the yoke frame.

Table 1. Characteristics of the case study jack-up (MSL) MSL Model Given Dimensions

Hull 80m 72m 16m Overall leg length 146m Leg spacing 55m Vertical spacing between horizontal braces 6.96m Length of horizontal bracing 12.2m

MSL Model Assumed Dimensions Diameter of chords 0.85m Diameter of braces 0.6m Leg penetration 23m Spudcan diameter 14.3 m

Environmental Parameters of Central North Sea Water depth including storm surge 96.6m Maximum wave height 31.2 m Period for the 100-year wave 17.7 sec Associated current speed at the surface 0.67 m/sec Maximum wind speed at 10m above SWL 40.1 m/sec Soil type Sand


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a. 3d View

b. Plan View

Figure 2. Case study jack-up (MSL)

Figure 3. Environmental Loads on MSL

80 m

72 m 12.2 m

D brace (600mm)

Overall height (146 m)

D chord (850mm)

Hull Height (16m)

6.96 m


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4. RESULTS AND DISCUSSIONS The reactions of the structure to the environmental forces were analysed by the application of BASFJOSEL as well as SACS software. A full account of the analysis and an experimental setup to verify the theoretical findings are given in [17]. A summary of the shear force, overturning moment and platform deflection is contained in Table 2. Values of some primary factors are as follows: L = 433.2 m; C = 24.5 m/s; n = 0.67 Max. water particle velocity at SWL = 6.25 m/s Max. water particle acceleration at SWL= 1.97 m/s2 Ki = 0.44 ; KD = 0.17 Cs (Hull) = 1.5 ; Cs (dry part) = 0.5 Wind Frontal Area = 1603 m2

Table 2. Base shear, overturning moment and platform deflection for MSL

Base Shear from BASFJOSEL Reaction MN % of Ftotal % of Ftotal

Hull: 2.289 26.7 Vertical Members: 0.03 0.3 Horizontal Members: 0.039 0.5

Fwind

Diagonal Members: 0.075

Fwind on whole structure: 2.433

0.9

28.4

Vertical Members: 0.100 1.2 Horizontal Members: 0.049 0.6 Fcurrent Diagonal Members: 0.190

Fcurrent on whole structure: 0.339

2.2 4.0

Vertical Members: 0.641 7.5 Horizontal Members: 2.150 25.1 Fwave Diagonal Members: 3.013

Fwave on whole structure: 5.804

35.1 67.7

Ftotal 8.576 100 100 From SACS

Reaction Base Shear (FX)

Overturning Moment

(MY) Platform Maximum

Deflection (Dx)

MN % of Ftotal MN.m% of Mtotal

Wind 2.5 26.9 557 44 Wave + Current 6.7 73.1 699 56 Total 9.2 100 1256 100

340 mm

4.1 Breakdown of the total base shear force Figure 4 shows the breakdown of the total base shear force with wave loads accounting for 68%, wind loads forming 28% and current loads making only 4%. With respect to major structural components, diagonal braces, hull, horizontal braces, and vertical braces attract 38%, 27%, 26% and 9% respectively, of the total load by wave, wind and current. It should be pointed out that although diagonal members as a whole attract more environmental loads than the vertical members (chords), but the role of each diagonal member in taking environmental loads is less than that of vertical ones. This has been analytically substantiated elsewhere [17].


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Figure 4. General breakdown of the base shear 4.2 The ratio of drag force to inertia force Figure 5 gives the breakdown of the total wave force, Fwave on all submerged members (vertical, diagonal and horizontal braces) into the drag and inertia forces, FD and Fi, respectively. These proportions are related to the Keulegan-Carpenter KC number and are especially sensitive to member diameters and wave height.

Figure 5. Breakdown of wave force as in Morisons equation for braces

As waves bring greater contribution to the total load, it is interesting to look at the wave force in terms of the two components in Morrisons equation. The ratio of FD to Fi could be studied in terms of variation of D and H. It should be noted that for small Keulegan-Carpenter values (KC


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The ratio when D varies: While FD is directly proportional to D, Fi is proportional to D2. Therefore, increase of D has a greater impact on Fi than on FD. Noting that increase of D reduces the Keulegan-Carpenter number, this finding is also consistent with the established fact that drag becomes more dominant when KC increases. The ratio when D varies: The peak particle velocity is Umax= (H/2) (gT/2). Increase of the H leads to an increase of Umax, which in turn increases KC. Thus, the ratio of drag to inertia forces will also increase.

Table 3. Guide for wave load calculation; [13, 15] KC D/L0.2

KC>25 Drag dominated Morison equation with CM and CD

5


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(b) Base shear is the most sensitive to the wave height, H compared to T (and L). (c) The impact of the wave height to the total base shear is greater than that of the member diameters. A close look at equations (3) to (5) can physically support this observation. From equation (4), it is seen that Fi is proportional to D2 and H. Considering the exponents, Fi is then more sensitive to D than to H. With the same look at equation (5), it is seen that FD is more sensitive to H than to D. The discussion in (b) reveals that for most jack-up structures, FD is more significant than Fi. Therefore, the total base shear force is more sensitive to H (that is the core parameter in FD) than to D (that is the core parameter in Fi). (d) The variation of overturning moment (OTM) with the parameters does not show the same pattern in all ranges of relative change of the parameters. When increasing the parameters, the highest sensitivity of OTM is to H. However, in decreasing the parameters, the highest sensitivity of OTM is to T. 4.3.2. Leg Deflection: Figure 10 shows the variation of maximum deflection of the platform (at the top corner of the hull) with various environmental or size factors. The interpretation of Figure 10, follows the same as performed in (d) above for OTM. As the maximum deflection is directly proportional to the moment, this similarity of Figure 10 to Figure 9 is well justified. Figure 11 shows the maximum deflection of a jack-up leg at various elevations, from the mud line (with zero deflection) to the top (with 34 cm deflection). The deflection of the jack-up is due to moments from three force sources: (i) Wind force that is fairly concentrated at the hull (top side) with the largest arm. This

force is 28% of the total force (see Table 2). (ii) Current force that is fairly distributed with larger values near water surface and a

resultant force in the top half-water depth. This force is only 4% of the total force (see Table 2).

(iii) Wave force that is fairly distributed on a segment of legs near water surface with values diminishing rapidly with water depth. This force is 68% of the total force (see Table 2).

The largest moment is produced by wave, followed by the wind and current. For the sake of a conceptual discussion, it would be helpful if the whole jack-up structure is likened to a cantilever beam as shown in Appendix I. The Appendix contains the deflection characteristics of the beam (one end fixed, the other free) under various loads. Three cases (i) to (iii), resemble beam type 1 with x=beam length L, beam type 4, and beam type 1 with L/2


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An example of the approximations relates to SACS treatment of empirical values of drag and inertia coefficients, CD and CM, in Morrisons equation. While experimental observations show that these coefficients depend generally on Re (taking into account both water velocity and member size) as shown in equations (9) and (10), SACS determines these coefficients as functions of D only. Table 5 contains the tabular from of these functions. The resulting deference in the magnitude of CD and CM can partly explain the difference in the predicted base shear forces.

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Figure 6. Sensitivity of base shear to various parameters based on in-house software output

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Figure 7. Sensitivity of base shear to various parameters based on SACS software output


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-50 -40 -30 -20 -10 0 10 20 30 40 50

% Change in Parameter

% C

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e in

OTM

Wave height Member Diameter Wind Speed Wave Period

Figure 9. Sensitivity of overturning moment to various parameters based on SACS software output

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% C

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ion

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Figure 10. Sensitivity of platform deflection to various parameters based on SACS software output

Table 4. Comparison of results between SACS and In-house software

Force BASFJOSEL (col. 1) SACS (col. 2)

% Difference (col. 1-col. 2)/(col. 1)

Wind (MN) 2.43 2.46 1.14 Wave + Current (MN) 6.14 6.70 9.07 Total Base Shear (MN) 8.58 9.16 6.82


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Table 5. Assumption by SACS for CD and CM in Morrisons Equation (from SACS output) Drag Coefficient , CD(-) Inertia Coefficient , CM (-) Member size (D, cm) Normal flow Axial Flow Normal flow Axial Flow

30.48 0.6100 1.3900 0.6100 1.3900 60.96 0.6650 1.4000 0.6650 1.4000

121.92 0.7200 1.4500 0.7200 1.4500 182.88 0.7560 1.6000 0.7560 1.6000 243.84 0.7810 1.6700 0.7810 1.6700 304.80 0.7990 1.7100 0.7990 1.7100

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Figure 11. Deflection vs. elevation

5. CONCLUSIONS An in-house computer program and a commercial software (SACS) software have been applied to a typical jack-up structure to get insight into the sensitivity of these offshore platforms to environmental parameters and size of the structural members. A typical jack-up as designed for the Central North Sea was taken as the case study.

Mud-line


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The breakdown of the base shear force and overturning moment in terms of contributions from three environmental loads pertaining to wave, current and wind revealed the dominant role of the wave forces for real-life design conditions. The order of relative magnitude of these forces was also introduced. Examination of the values and ratios of wave action to the members, wind action to the hull and to the dry members in the air gap, and current action to various members showed that among the wave parameters (height H, period T), H causes greater impact on the base shear whilst the overturning moment and platform deflection show maximum sensitivity to H only when increase of the parameters is considered. With the decrease of the parameters, the overturning moment and platform deflection are the most sensitive to T rather than to H, D and wind speed. It also revealed that the significance of the wave height is even greater than the role of the member diameters in the total value of the base shear force. Referring to deflection of cantilever beams under various loads, a simple interpretation of the deflection of the jack-up leg is introduced. Numerous applications of in-house computer program have proved that in the absence of specialised expensive simulators such as SACS and ANSYS- ASAS OFFSHORE, the program is a handy tool in preliminary checks during the conceptual design of jack-up platform as well as for educational purposes requiring quick calculations of the total forces. REFERENCES [1]. Boon, B., Vrouwenvelder, A., Hengst, S., Boonstra, H., and Daghigh, M. (1997). System

reliability analysis of jack-up structures: possibilities and frustrations. Proc. of 6th Int. Conf. Jack-Up Platform Design, Construction and Operation, City University, London

[2]. Leijten, S.F. and Efthymiou, M. (1989). A philosophy for the integrity assessment of jack-up [3]. Sharples, B.P.M., Bennett Jr, W.T. and Trickey, J.C. (1989). Risk analysis of jack-up rigs. Proc.

of 2nd Int. Conf. on the Jack-Up Drilling Platform, City University, London, pp. 101-123. [4]. Young, A.G., Remmes, B.D. and Meyer, B.J. (1984). Foundation performance of offshore jack-

up drilling rigs. J. Geotech Engng Div., ASCE, Vol. 110, No 7, pp. 841-859, Paper No. 18996. [5]. Engineering Dynamics Incorporation (EDI), USA, Structural Analysis Computer System;

http://www.sacs-edi.com/ProductInfo.shtml [6]. SACS Release 5 User Manual. (2005). Engineering Dynamics, Inc. [7]. ANSYS Incorporation, USA, ANSYS ASAS-OFFSHORE version 14.04;

http://www.ansys.com/default.asp [8]. STAAD. offshore release 2005 manual, Research Engineers International, Bentley Solution

Centre [9]. Saiedi M.R., Saiedi S., Shafiqi N., (2007), Sensitivity of Bottom-Supported Offshore Platforms

to Environmental Forces; CUTSE07, Engineering Conference, Curtin University of Technology Sarawak, Malaysia, 26-27 November

[10]. Baker C.C., McCafferty D.B., 2005, Accident database review of human-element concerns: what do the results mean for classification?, ABS Technical Papers, also presented at Human Factors in Ship Design, Safety and Operation held in London, February 23-24

[11]. Le Triant. P and Prol. C. (1993). Stability and Operation of Jack-up, Technip, Paris [12]. Standards Norway. (2004) , N-003, NORSOK STANDARDS Actions and action effects, Draft 2

for Revision 2, February [13]. American Petroleum Institute (API). (1993), Recommended Practice for Planning, Designing

and Construction Fixed Offshore Structures - Working Stress Design, RP 2A - WSD, Twentieth Edition

[14]. Chakrabarti S.K. (2005), Handbook of Offshore Engineering, Vol. I, Offshore Structure Analysis, Inc., Plainfield, Illionois, USA


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[15]. US Army Corps of Engrs. (2006), Coastal Engineering Manual, EM 1110-2-1100, [16]. MSL Engineering Ltd. (2003), Research Repot 019, Sensitivity of Jack-up Reliability to wave-

In-Deck Inundation [17]. Aghdamy S. (2008), Impacts of Environmental Loads on a Jack-up offshore Structure, Final

Year Project (Thesis), Civil Engineering Department, Universiti Teknologi PETRONAS, Malaysia

[18]. Pilkey W.D. (1994), Formulas for Stress, Strain, and Structural Matrices, John Wiley and Sons, Inc., 1458pp.


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Appendix I. Deflection of Beams (Pilkey 1994)

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UNITEN ICCBT 08 Parametric Study of Environmental Load Impacts on a Jack-up