iso standards for jack up rigs

HSEHealth & Safety

Executive

Interpretation of full-scalemonitoring data for input to

ISO Standard on jack-up rigs

Prepared by MSL Engineering Ltdfor the Health and Safety Executive

OFFSHORE TECHNOLOGY REPORT

2001/036

HSEHealth & Safety

Executive

Interpretation of full-scalemonitoring data for input to

ISO Standard on jack-up rigs

MSL Engineering LtdMSL House

5-7 High StreetSunninghill

AscotBerkshire SL5 9NQ

United Kingdom

HSE BOOKS

ii

© Crown copyright 2002Applications for reproduction should be made in writing to:Copyright Unit, Her Majesty’s Stationery Office,St Clements House, 2-16 Colegate, Norwich NR3 1BQ

First published 2002

ISBN 0 7176 2329 7

All rights reserved. No part of this publication may bereproduced, stored in a retrieval system, or transmittedin any form or by any means (electronic, mechanical,photocopying, recording or otherwise) without the priorwritten permission of the copyright owner.

This report is made available by the Health and SafetyExecutive as part of a series of reports of work which hasbeen supported by funds provided by the Executive.Neither the Executive, nor the contractors concernedassume any liability for the reports nor do theynecessarily reflect the views or policy of the Executive.

iii

EXECUTIVE SUMMARY This report is concerned with the analysis and interpretation of data obtained with the instrumentation of the West Epsilon jack-up rig. The rig is operated by STATOIL in the Sleipner Vest field and has been modified by the addition of deep skirts on the spudcans. Measured data for the period 1998/1999 has been utilized and supplemented by further data from 1996 and 1997. In total, 15 sea states have been identified and processed in order to determine statistical and deterministic wave parameters. The study concerns the assessment of measured against predicted response and includes assessment of foundation stiffness. The following aspects are covered in this report: •= The measured data has been utilized to establish the degree of fixity for selected sea

states. •= Comparison between measured structural response with corresponding response

predicted by SNAME guidance. The structural model was validated against design analysis results and against previous data analysis study results. Good agreement was found between hydrodynamic loading and structural response. Following SNAME practice, the predicted response utilizing deterministic wave parameters correlates to measured response. Utilising statistical wave parameters generally over-predicts the structural response compared to measured response. The measured data indicates an increase in foundation stiffness as the sea state increases, indicating mobilization of the spudcan skirts. The results obtained from the study indicated the following primary findings: •= Foundation behaviour is elastic up to a minimum 10m Hs. The trend of increased

foundation stiffness as the sea state increases seems to extend to waves much larger than 10m Hs. Foundation fixity will need to reduce significantly to deviate from near fixity.

•= The SNAME recipe for regular wave analysis may over-predict structural response.

Printed and published by the Health and Safety ExecutiveC30 1/98

iv

v

CONTENTS EXECUTIVE SUMMARY iii CONTENTS v

1. INTRODUCTION ...................................................................................................... 1

2. STRUCTURAL MODEL........................................................................................... 3

3. COMPUTER MODEL VALIDATION...................................................................... 5

4. MEASURED DATA .................................................................................................. 8

5. DATA ANALYSIS................................................................................................... 11

6. RESULTS AND OBSERVATIONS........................................................................ 15

7. FINDINGS................................................................................................................ 16

REFERENCES ...................................................................................................................... 17


vi

1

1. INTRODUCTION The Sleipner West field in the Norwegian Sector of the North Sea has been developed by Statoil. As part of the field development Statoil have utilized the MSC CJ62 class jack-up, West Epsilon, Figure 1, for the drilling programme since early 1996. The water depth at the site is 107.3 m (LAT) and, because of unfavourable soil conditions, the jack-up spudcans have been increased in area and fitted with 5 m deep skirts. This introduces a degree of fixity that can be exploited in contrast to ‘normal’ conditions where inadequate information of the soil conditions, the possibility of erosion pumping and scour, and the non-linear behaviour of the foundations under large bending moments point to the use of pinned conditions as the basis for assessment. The consequences of the introduction of this fixity has been to reduce the stresses at the top of the legs under both extreme storm and boat impact conditions and increase the stresses at the bottom of the leg due to the fixity with a consequential impact on fatigue. With the aims to quantify the degree of fixity, the effect of dynamics, and the impact on fatigue, STATOIL initially extensively instrumented the West Epsilon, see Figure 2, as follows: •= Strain gauges on the braces and chords of the aft starboard leg in Bays 3 and 18. •= Current meters in Bays 4 and 13 of the aft starboard leg. Two meters are in Bay 13 and

they also measure pressure. One is in Bay 4 and measures current only. These meters record direction as well as speed.

•= Accelerometers at deck level and at the top of the spudcan of the aft starboard leg. •= Water surface meter. •= Anemometer and wind direction indicator. Following installation, problems were encountered with a few strain gauges and most of the current meters. Also, spray affected some of the recordings made by the water surface meter. However, high quality data has been generated and utilised in this study. In 1997, HSE commissioned MSL Engineering Limited to conduct a preliminary interpretation of data(1) from the winter of 1995-96. This demonstrated that: (i) The data can be used effectively and efficiently. (ii) The degree of correspondence between the predicted and measured responses can be

quantified and is reasonable. (iii) The prospects are good for dealing with non-linear soil response in the event of more

severe weather being experienced by the unit. (iv) A viable programme for further exploitation of the data recorded from 1996-1997

onwards can be undertaken.

2

The winter of 1995-1996 exhibited a maximum 9.3m Hs seastate and the foundations were interpreted as working within the ‘elastic’ range of soil response. The HSE-commissioned work concentrated on hydrodynamic and structural modelling aspects. A two-stage approach was adopted: (i) Stage 1 covered the creation and validation of a structural model beginning with a

SESAM model made available to MSL by STATOIL. (ii) Stage 2 in which a comparison was made between the measured responses and results of

a non-linear time-domain irregular sea simulation. This Stage also included an extensive Quality Assurance (QA) of the structural model as far as its dynamic/quasi-static response to regular/irregular waves is concerned.

This study has been undertaken in light of the above and following discussions with HSE, and relates to a study of the West Epsilon data over the winter period 1998-99. The objectives of the proposed study are to: (i) Use the instrumented data to establish the degree of fixity for selected sea states. (ii) To compare the measured structure loads with corresponding global loads predicted by

an accepted code recipe (SNAME(2) recommendations) applied to a structural model for the same environmental conditions.

(iii) Provide value input to ISO TC67/SC7/WG7 for the ISO Standard on Jack-up Rigs. In order to meet the above objectives, advantage has been taken of the lessons learnt from the earlier HSE/MSL(1) and earlier work carried out by SINTEF(3), the reports from which have kindly been made available to MSL by Statoil.

3

2. STRUCTURAL MODEL General The MSC CJ62 jack-up rig (Figure 1) is supported by three lattice legs which rest on spudcan foundations. The spudcans have been increased in area and fitted with 5 m deep skirts (not shown in Figure 2). The three legs are spaced 62 m apart and each consists of a triangular truss leg structure. The leg length to can tip is 165.076 m and the leg width is 16 m. The typical tubular diameter of the trusswork is 0.324 m with the thickness varying over the leg length (basic values are 28 and 30 mm). Typical span-breakers are 0.152 m in diameter with a thickness of 12.5 mm. The chords are of split tube type with radius of 0.3 m and thickness varying over the leg length (basic values are 55 and 65 mm). The thickness of the rack teeth is 210 mm. Computer Model As part of the previous study(1) a SESAM model was converted to CAP/SeaStar(4) format. In addition, two of the detailed legs were simplified to equivalent stiffness parameters. The simplified model was calibrated to a detailed model in order to validate the use of simplified legs. The original intention was to undertake this study using the CAP model, however in consideration of the following factors, a SACS(5) model was generated: •= Problems with the Unix based workstation on which CAP is mounted may have affected

timescales. •= SACS is a recognized analysis package and is mounted on a much faster PC based

workstation compared to the Unix machine. •= Dynamic analyses in SACS are readily executed and provide generation of combined

hydrodynamic forces and inertia forces. The SACS model also utilized detailed representation of all legs compared to the use of two simplified legs in the CAP model due to analysis run-times. Section 3 describes the calibration of the SACS model. Soil-Structure Interaction The structure-soil interaction is modelled by means of a linear-elastic rotational spring positioned 1m above the tip of the spudcan and with a rotational stiffness of 310,000 MNm/rad(1), representing a skirted spudcan foundation with a penetration of 5m(1). The associated horizontal and vertical stiffness are 8,600 MN/m and 11,000 MN/m respectively(1). These springs formed the basis of the validation analysis. Hull-Leg Connection The unit has a fixation system including lower guides, chocking system, jacking system and upper guides. In the previous SESAM and CAP models the leg-hull connection was modelled as a linear 6-degree of freedom link element(1). This link element was directly coupled to the leg at the point of contact (and elevation) of the rig chocking system (which is approximately at the same elevation as the vertical centre of gravity of the unit). Such modelling was assumed on the following basis:

4

•= Previous studies showed the stiffness of the jacking system had little influence on the overall results of the dynamic analysis.

•= Similar considerations led to the non-inclusion of the vertical separation of the upper and

lower guides. Guide clearance was also neglected. It is beyond the scope of the present work to refine the leg-hull connection model. However, potential influence of the hull-leg connection and soil spring stiffness on predicted response and natural periods is discussed in Section 5. Material Properties The majority of leg elements consist of steel with 690 MPa yield strength, with the exception of the span-breakers which consist of steel with 450 MPa yield strength. Young’s modulus is 210,000 MPa and Poisson’s ratio is 0.3. Mass and Weight The topsides mass is 16,900 tonnes(1). The dry mass of each leg, excluding spudcan, is 1,680 tonnes(1) and the dry mass of each spudcan is 450 tonnes(1). The total dry weight of each leg is therefore 2,130 tonnes. The model material density was assigned such that leg and hull mass and centre of gravity are representative. Added mass and buoyancy effects were generated in the FE model.

5

3. COMPUTER MODEL VALIDATION Structural Model The SACS model was validated against results obtained from the in-place assessment of the unit. The QA of the model is similar to that adopted in the previous West Epsilon study. The SACS model was generated principally from the SESAM model supplied by STATOIL. The CAP model was used to some extent to verify geometry, coordinates and mass. The SACS model was subsequently utilised to derive structural natural periods and in-place extreme storm loading. The validation analysis included the following: •= Self weight loads and buoyancy effects. •= Topsides mass. •= P-delta effects. •= In-place environmental data. As the primary objective was to compare natural periods and hydrodynamic loading, wind load was not included. Soil-structure interaction was represented by linear soil springs as defined in Section 2. Figure 3 presents the computer model. Environmental Data The in-place analysis utilised the following environmental data: •= Design operating water depth of 108 m. •= 100 year design wave height of 27 m and associated period of 14.5 s. •= 10 year current profile:

at mean water level 1.10 m/s - 30 m below MWL 0.75 m/s - 50 m below MWL 0.70 m/s - 105 m below MWL 0.70 m/s

•= Marine growth profile:

Above +2 m no marine growth Between +2 m and –40 m 100 mm thickness Below –40 m 40 mm thickness

Analysis Parameters A deterministic wave response analysis was undertaking utilising the following analysis parameters: •= Water level, wave height, wave period, current profile and marine growth profile as

defined in above. •= Stokes V regular wave theory. •= Wheeler current stretching.

6

•= Hydrodynamic coefficients based on SNAME and marine growth profile with an allowance of 10% for anodes etc, as follows:

Marine Growth Average Cd No growth 1.55 100 mm 1.36 40 mm 1.46

•= Relative velocity effects included. •= Kinematics factor and current blockage factor of 1.0. •= Damping ratio of 0.04: includes critical damping of 2 % for structure and 2 % for

foundation. Of particular note are the averaged Cd values used. Experience has shown there is little variation in predicted wave load if different Cd values are assigned to each chord depending on the relative wave attack direction to the chord. The averaged values have been assigned to all chords and therefore, do not alter with wave attack direction. However, this is only applicable for the assessment of global response rather than individual members, particularly for members in the wave zone. Validation Results The results obtained are based on the use of environmental data and analysis parameters as defined above. The model was not altered in any other way in order to achieve validation results close to previously reported values. The primary reason for this was to derive results based on SNAME recipe for comparison to previously reported CAP validation results. A confirmatory analysis was, however, performed utilising environmental data and analysis parameters as used in the previous validation analysis. The confirmatory analysis yielded results to within 5% of the previous validation analysis, although there is a difference in predicted natural period as presented below. Dynamic characteristics were derived for input to the analysis. The computed natural periods are presented below:

Mode CAP Validation Period (s)

SACS Validation Period (s)

Surge 5.14 5.26

Sway 5.11 5.24

Yaw 3.47 4.04

7

The analysis results are presented below:

Result CAP Validation BS (MN)

SACS Validation BS (MN)

Quasi-Static 15.43 15.00

Dynamic 17.60 17.86 The quasi-static results compare well with the in-place assessment(6) value of 15.8 MN. As stated previously, the analysis model was not modified in order to obtain a close correlation as the primary purpose of the study is to compare measured response to predicted.

8

4. MEASURED DATA A total of 15 seastates were selected. The seastates represent mild conditions with a significant wave height of 4.4m Hs to reasonable storm conditions with significant wave heights approaching 10m Hs. The majority of seastates were chosen based on containing the highest significant wave heights. The 15 seastates consist of the following: •= 1 seastate from February 1996, as used in the original West Epsilon study. •= 4 seastates from September 1997 to April 1998. •= 10 seastates from November 1998 to April 1999. These 10 seastates were obtained from

the retrieved hard-drive from the West Epsilon monitoring system. On review of the drive content it was established that data for 1999-2000 period was not present.

The data was provided and reduced by Fugro Structural Monitoring. Previous checks on the data have already established the current meters are faulty and that the spudcan accelerometer readings are in error by a factor of 10. Further checks on the data has established a number of the strain gauges are either consistently or erratically generating faulty data. Figure 2 presents the West Epsilon monitoring system. Seastates containing a range of wave heights were chosen to establish the variation in foundation fixity with hydrodynamic loading. Statistically and deterministically derived seastate parameters were utilised in the study. The derived parameters consist of the following: Statistical Parameters •= For each of the 15 seastates, extract significant wave heights (Hs) and wave zero

crossing period. •= Derive maximum wave height from Hs. •= Derive lower bound and upper bound wave periods. Deterministic Parameters •= Select 20 highest wave heights from the 15 seastates. •= Determine wave period for the 20 highest waves. The total number of 50 analyses consists of 30 utilising statistical parameters and 20 utilising deterministic parameters. Table 4.1 presents the parameters derived and utilised for each seastate. The following data was extracted and processed: •= Twenty minute statistics for:

Significant wave height Mean crossing period Wind speed Wind direction Natural frequency (surge, sway and yaw)

9

•= Time series data for:

Lower strain gauges Hull and spudcan linear/angular acceleration Hull and spudcan displacements (from accelerations) Wave height

•= Spectra for:

Hull and spudcan linear/angular accelerations. Where possible, the lower leg chord strain measurements were utilised to derive moment/rotation stiffness. Where the strain gauge measurements are faulty or erratic the spectrally derived natural period was utilised to establish foundation rotational stiffness. In general, however, the foundation rotational stiffness is comparable to that established from the spectral natural period. Time history plots for wave height and hull displacement are presented in Appendix A. Presented in Appendix B are the hull linear acceleration spectral plots and hull displacement plots.

10

Table 4.1

Statistical Seastate

Date Time

Record

Mean WaterElevation

(m)

Wave Attack

Direction(deg)

Hs (m)

Tz (s)

Hmax(m)

Tp (s) Lower Upper Tp/Tz

Lower Upper

DeterministicHmax (m)

Tp (s) Hmax/Hs Tp/Tz

1 9-2-96 810 -0.96 16 9.30 10.90 14.9 10.5 13.5 1.0 1.2 13.50 11.50 1.45 1.06 610 -0.96 16 11.80 10.40 1.27 0.95

2 10-9-97 675 -0.24 157 4.39 7.95 7.0 7.2 9.3 0.9 1.2 6.56 9.38 1.49 1.18 3 28-2-98 1150 -0.52 159 6.50 10.91 10.4 8.8 11.3 0.8 1.0 8.85 17.82 1.36 1.63 4 3-4-98 850 -0.24 40 7.46 9.23 11.9 9.4 12.1 1.0 1.3 9.78 10.30 1.31 1.12 5 31-12-97 550 3.07 142 8.13 9.80 13.0 9.8 12.6 1.0 1.3 13.04 9.38 1.60 0.96 6 9-11-98 715 -0.56 223 7.45 8.28 11.9 9.4 12.1 1.1 1.5 10.34 9.37 1.39 1.13 7 27-12-98 815 -0.56 248 8.05 9.52 12.9 9.8 12.5 1.0 1.3 12.64 10.31 1.57 1.08 75 -0.56 238 9.53 16.87 1.18 1.77

8 4-1-99 250 -0.56 305 6.65 9.09 10.6 8.9 11.4 1.0 1.3 8.61 12.19 1.29 1.34 9 17-1-99 1100 0.24 316 6.59 7.41 10.6 8.8 11.4 1.2 1.5 8.64 12.19 1.31 1.65

10 4-2-99 275 0.64 229 9.92 10.17 15.9 10.8 13.9 1.1 1.4 15.70 11.25 1.58 1.11 850 0.64 234 13.77 12.19 1.39 1.20

11 5-2-99 525 0.24 349 8.84 10.17 14.1 10.2 13.1 1.0 1.3 13.63 12.19 1.54 1.20 230 0.24 321 11.57 12.19 1.31 1.20

12 6-2-99 1055 -0.06 331 7.04 9.45 11.3 9.1 11.7 1.0 1.2 11.04 12.19 1.57 1.29 13 7-2-99 75 -0.06 341 7.01 8.51 11.2 9.1 11.7 1.1 1.4 14.84 8.44 2.12 0.99 14 17-2-99 80 0.14 342 9.09 10.91 14.5 10.4 13.3 1.0 1.2 10.80 11.25 1.19 1.03

740 0.14 338 9.39 17.81 1.03 1.63 15 21-4-99 330 -0.56 241 6.90 8.39 11.0 9.0 11.6 1.1 1.4 10.95 7.50 1.59 0.89

Mean 1.01 1.30 1.43 1.22 Std 0.09 0.12 0.23 0.26 Kurtosis 1.04 1.04 3.59 -0.09

11

5. DATA ANALYSIS Three specific areas have been assessed, namely: •= Effect of seastate on foundation fixity. •= Statistical ratio of measured response to predicted response utilising computed

maximum wave height with upper bound and lower bound wave periods.

Hmax = 1.6 × Hs Tlower = 3.44 × (Hs)0.5 Tupper = 4.42 × (Hs)0.5

•= Deterministic ratio of measured response to predicted response utilising measured

maximum wave heights and wave period. The statistical determination of wave parameters is derived using SNAME guidance for deterministic/regular wave force calculation. Foundation Fixity It is clear from the data obtained, that the foundation stiffness increases with an increase in wave height, as presented in Figure 4. The effect on structural period by varying foundation rotational stiffness was determined by undertaking eigen analyses for values ranging from pinned to fixed conditions. Figure 5 presents the analyses results. The plot does not quite reach the fixed condition, because the design vertical and horizontal foundation springs were maintained for all analyses. Statistical Comparisons Comparisons are made between measured and predicted response utilising the hull displacement response range. The statistical comparisons have utilised lower and upper bound derived wave periods. Table 5.1 presents the results for all 30 analyses. Table 5.2 presents the results separated between lower bound wave period and upper bound wave period. Deterministic Comparisons The deterministic comparisons utilise parameters derived from a wave-by-wave analysis based on measured wave heights and period. Table 5.3 presents the results for the 20 analyses.

12

Table 5.1

Seastate

Date Hs (m)

Measured Displacement

Response

Predicted Displacement

Response Meas/Pred

1 9-2-96 9.3 146 199 0.73 2 9-2-96 9.3 146 118 1.24 3 10-9-97 4.394 16 16 0.99 4 10-9-97 4.394 16 21 0.74 5 28-2-98 6.5 35 50 0.69 6 28-2-98 6.5 35 63 0.55 7 3-4-98 7.459 56 85 0.66 8 3-4-98 7.459 56 84 0.67 9 31-12-97 8.128 42 103 0.41

10 31-12-97 8.128 42 85 0.49 11 9-11-98 7.449 74 96 0.76 12 9-11-98 7.449 74 78 0.94 13 27-12-98 8.054 79 161 0.49 14 27-12-98 8.054 79 86 0.92 15 4-1-99 6.65 52 71 0.74 16 4-1-99 6.65 52 61 0.86 17 17-1-99 6.594 38 62 0.62 18 17-1-99 6.594 38 69 0.55 19 4-2-99 9.915 152 161 0.94 20 4-2-99 9.915 152 121 1.25 21 5-2-99 8.837 73 200 0.37 22 5-2-99 8.837 73 102 0.72 23 6-2-99 7.043 40 50 0.79 24 6-2-99 7.043 40 62 0.64 25 7-2-99 7.012 64 71 0.91 26 7-2-99 7.012 64 60 1.07 27 17-2-99 9.089 90 170 0.53 28 17-2-99 9.089 90 107 0.84 29 21-4-99 6.896 37 81 0.46 30 21-4-99 6.896 37 67 0.56

Average 0.74 Std 0.23 Kurtosis -0.06

13

Table 5.2

Lower Bound Period Upper Bound Period

Sea State

Seastate Date

Hs (m)

MeasuredDisplacement

Response


Response

Meas/Pred

Sea State

Seastate Date

Hs (m)


Response


Response

Meas/Pred

1 9-2-96 9.30 146 199 0.73 1 9-2-96 9.30 146 118 1.24 2 10-9-97 4.39 16 16 0.99 2 10-9-97 4.39 16 21 0.74 3 28-2-98 6.50 35 50 0.69 3 28-2-98 6.50 35 63 0.55 4 3-4-98 7.46 56 85 0.66 4 3-4-98 7.46 56 84 0.67 5 31-12-97 8.13 42 103 0.41 5 31-12-97 8.13 42 85 0.49 6 9-11-98 7.45 74 96 0.76 6 9-11-98 7.45 74 78 0.94 7 27-12-98 8.05 79 161 0.49 7 27-12-98 8.05 79 86 0.92 8 4-1-99 6.65 52 71 0.74 8 4-1-99 6.65 52 61 0.86 9 17-1-99 6.59 38 62 0.62 9 17-1-99 6.59 38 69 0.55

10 4-2-99 9.92 152 161 0.94 10 4-2-99 9.92 152 121 1.25 11 5-2-99 8.84 73 200 0.37 11 5-2-99 8.84 73 102 0.72 12 6-2-99 7.04 40 50 0.79 12 6-2-99 7.04 40 62 0.64 13 7-2-99 7.01 64 71 0.91 13 7-2-99 7.01 64 60 1.07 14 17-2-99 9.09 90 170 0.53 14 17-2-99 9.09 90 107 0.84 15 21-4-99 6.90 37 81 0.46 15 21-4-99 6.90 37 67 0.56

Average 0.67 Average 0.80 Std 0.19 Std 0.25 Kurtosis -0.96 Kurtosis -0.59

14

Table 5.3

SeastateDate

Hs (m)

Hmax (m)


Response (mm)


Response (mm)

Meas/Pred

1 9-2-96 9.30 13.50 146 101 1.44 2 9-2-96 9.30 11.80 110 108 1.02 3 10-9-97 4.39 6.56 16 20 0.78 4 28-2-98 6.50 8.85 35 49 0.70 5 3-4-98 7.46 9.78 56 68 0.82 6 31-12-97 8.13 13.04 42 93 0.45 7 9-11-98 7.45 10.34 74 67 1.10 8 27-12-98 8.05 12.64 79 178 0.45 9 27-12-98 8.05 9.53 60 56 1.06

10 4-1-99 6.65 8.61 52 37 1.42 11 17-1-99 6.59 8.64 38 45 0.84 12 4-2-99 9.92 15.70 152 133 1.14 13 4-2-99 9.92 13.77 127 91 1.40 14 5-2-99 8.84 13.63 73 96 0.76 15 5-2-99 8.84 11.57 73 75 0.97 16 6-2-99 7.04 11.04 40 59 0.67 17 7-2-99 7.01 14.84 64 119 0.54 18 17-2-99 9.09 10.80 90 60 1.51 19 17-2-99 9.09 9.39 99 53 1.87 20 21-4-99 6.90 10.95 37 69 0.54

Average 0.97 Std 0.39 Kurtosis -0.27

15

6. RESULTS AND OBSERVATIONS The scope of work for this study concerned the assessment of measured against predicted response. This included assessment of foundation stiffness utilising measured data and spectral analysis of hull accelerations. Predicted response has been generated by analysis utilising statistical and deterministically derived wave parameters. The primary results and observations are summarised as follows: •= Measured and predicted structural natural period increases with a decrease in wave

height. Although the maximum measured significant wave height was 10m, there is no indication the trend alters, ie. it is expected the trend will extend to much larger wave heights.

•= For a wave-by-wave analysis, the computer model predicts structural response

reasonably well. The mean measured over predicted response is close to unity. •= Utilising SNAME guidance recipe, the computer model over-predicts structural

response. For both upper bound and lower bound wave periods the measured over predicted response is generally below unity.

•= The design wave height (1.6 Hs) is generally higher than the measured maximum wave

height by an average 10%, as indicated in Table 4.1. •= The variation in measured wave period is consistent with upper bound and lower bound

design values. •= For the seastates analysed, the measured data indicates no long-term effects in

structural/soil interaction stiffness. Furthermore, the foundation stiffness alters wave-by-wave. This indicates the foundation stiffness is fluctuating elastically, as indicated by the trend in natural period against wave height.

•= The study has shown that a significant reduction in foundation stiffness is required in

order to reach pinned conditions. For the seastates assessed, the foundations are close to the fixed condition.

16

7. FINDINGS From analysis and review of the results obtained in this study, the following observations are made: •= The presence of skirts on the spudcans provide increased foundation stiffness as the

seastate increases, ie. mobilisation of the skirts is enhanced with increase in structural response.

•= The computer model predicted structural response reasonably well. •= Use of the SNAME practice generally yields appropriate conservative predictions in

structural response. A number of factors contribute to this, namely:

•= Design wave height higher than measured. •= Regular wave theory used. •= Long crested waves are assumed.

17

REFERENCES 1. MSL ENGINEERING LIMITED.

West Epsilon Jack-Up Rig-Data Analysis, Final Report. Document Reference C203R005, Rev. 1, December 1997.

2. SOCIETY OF NAVAL ARCHITECTS AND MARINE ENGINEERS

Site Specific Assessment of Mobile Jack-up Units SNAME, Jersey City, T&R Bulletin 5-5A, 1994.

3. SINTEF

Analysis of full-scale measurements from the West Epsilon jack-up platform - Vol. 2 Report STF22 F96733, Norway, October 1996.

4. PMB ENGINEERING

CAP/SEASTAR Manuals PMB, San Francisco, 1994.

5. ENGINEERING DYNAMICS, INC.

Structural Analysis Computer System (SACS). Offshore Version 5.0.10. 6. DET NORSKE VERITAS

NPD concept evaluation of the self-elevating unit ‘Galant’ for the ‘Sleipner Vest’ field location. DnV Rep. 91-0043, May 1992.

18

Figure 1

West Epsilon

19

Figure 2 West Epsilon Monitoring System

20

Figure 3

West Epsilon Computer Model

21

Spectrally Derived Period vs Hs

0.0

2.0

4.0

6.0

8.0

10.0

12.0

5.0 5.1 5.2 5.3 5.4 5.5

First Mode Natural Period (s)

Hs

(m)

Figure 4

22

4.00

5.00

6.00

7.00

8.00

9.00

1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07 1.0E+08

Foundation Rotation Stiffness (MNm/rad)

Stru

ctur

al 1

st M

ode

Perio

d (s

)

Fixed

Pinned

Design

Figure 5

23

APPENDIX A

TIME HISTORIES OF WAVE ELEVATION AND HULL DISPLACEMENT


24

25

February 9th 1996 21:20

-10

-8

-6

-4

-2

0

2

4

6

8

10

0 50 100 150 200 250 300 350 400Time (s)

Wav

e H

eigh

t (m

)


-10

-8

-6

-4

-2

0

2

4

6

8

10

400 450 500 550 600 650 700 750 800Time (s)

Wav

e H

eigh

t (m

)


-10

-8

-6

-4

-2

0

2

4

6

8

10

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Wav

e He

ight

(m)

26

September 10th 1997 00:20

-4

-3

-2

-1

0

1

2

3

4

0 50 100 150 200 250 300 350 400Time (s)

Wav

e He

ight

(m)


-4

-3

-2

-1

0

1

2

3

4

400 450 500 550 600 650 700 750 800Time (s)

Wav

e H

eigh

t (m

)


-4

-3

-2

-1

0

1

2

3

4

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Wav

e He

ight

(m)

27


-6-5-4-3-2-10123456

0 50 100 150 200 250 300 350 400Time (s)

Wav

e H

eigh

t (m

)


-6-5-4-3-2-10123456

400 450 500 550 600 650 700 750 800Time (s)

Wav

e He

ight

(m)


-6-5-4-3-2-10123456

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Wav

e H

eigh

t (m

)

28

April 3rd 1998 20:20

-7-6-5-4-3-2-101234567

0 50 100 150 200 250 300 350 400Time (s)

Wav

e H

eigh

t (m

)

April 3rd 1998 20:20

-7-6-5-4-3-2-101234567

400 450 500 550 600 650 700 750 800Time (s)

Wav

e H

eigh

t (m

)

April 3rd 1998 20:20

-7-6-5-4-3-2-101234567

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Wav

e H

eigh

t (m

)

29

December 31st 1997 07:00

-8

-6

-4

-2

0

2

4

6

8

0 50 100 150 200 250 300 350 400Time (s)

Wav

e H

eigh

t (m

)


-8

-6

-4

-2

0

2

4

6

8

400 450 500 550 600 650 700 750 800Time (s)

Wav

e He

ight

(m)


-8

-6

-4

-2

0

2

4

6

8

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Wav

e H

eigh

t (m

)

30

November 9th 1998 10:40

-10

-8

-6

-4

-2

0

2

4

6

8

10

0 50 100 150 200 250 300 350 400Time (s)

Wav

e He

ight

(m)


-10

-8

-6

-4

-2

0

2

4

6

8

10

400 450 500 550 600 650 700 750 800Time (s)

Wav

e He

ight

(m)


-10

-8

-6

-4

-2

0

2

4

6

8

10

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Wav

e He

ight

(m)

31

December 27th 1998 12:40

-8-7-6-5-4-3-2-1012345678

0 50 100 150 200 250 300 350 400Time (s)

Wav

e H

eigh

t (m

)


-8-7-6-5-4-3-2-1012345678

400 450 500 550 600 650 700 750 800Time (s)

Wav

e H

eigh

t (m

)


-8-7-6-5-4-3-2-1012345678

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Wav

e H

eigh

t (m

)

32

January 4th 1999 21:00

-8-7-6-5-4-3-2-1012345678

0 50 100 150 200 250 300 350 400Time (s)

Wav

e H

eigh

t (m

)


-8-7-6-5-4-3-2-1012345678

400 450 500 550 600 650 700 750 800Time (s)

Wav

e H

eigh

t (m

)


-8-7-6-5-4-3-2-10123456

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Wav

e He

ight

(m)

33

January 17th 1999 05:20

-8-7-6-5-4-3-2-1012345678

0 50 100 150 200 250 300 350 400Time (s)

Wav

e H

eigh

t (m

)

January 17th 1999 05:20

-8-7-6-5-4-3-2-1012345678

400 450 500 550 600 650 700 750 800Time (s)

Wav

e He

ight

(m)

January 17th 1999 05:20

-8-7-6-5-4-3-2-1012345678

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Wav

e H

eigh

t (m

)

34


-10

-8

-6

-4

-2

0

2

4

6

8

10

0 50 100 150 200 250 300 350 400Time (s)

Wav

e H

eigh

t (m

)


-10

-8

-6

-4

-2

0

2

4

6

8

10

400 450 500 550 600 650 700 750 800Time (s)

Wav

e He

ight

(m)


-10

-8

-6

-4

-2

0

2

4

6

8

10

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Wav

e H

eigh

t (m

)

35


-8-7-6-5-4-3-2-1012345678

0 50 100 150 200 250 300 350 400Time (s)

Wav

e H

eigh

t (m

)


-8-7-6-5-4-3-2-1012345678

400 450 500 550 600 650 700 750 800Time (s)

Wav

e H

eigh

t (m

)


-8-7-6-5-4-3-2-1012345678

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Wav

e H

eigh

t (m

)

36


-8-7-6-5-4-3-2-1012345678

0 50 100 150 200 250 300 350 400Time (s)

Wav

e H

eigh

t (m

)


-8-7-6-5-4-3-2-1012345678

400 450 500 550 600 650 700 750 800Time (s)

Wav

e H

eigh

t (m

)


-8-7-6-5-4-3-2-1012345678

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Wav

e H

eigh

t (m

)

37


-10

-8

-6

-4

-2

0

2

4

6

8

10

0 50 100 150 200 250 300 350 400Time (s)

Wav

e H

eigh

t (m

)


-10

-8

-6

-4

-2

0

2

4

6

8

10

400 450 500 550 600 650 700 750 800Time (s)

Wav

e H

eigh

t (m

)


-10

-8

-6

-4

-2

0

2

4

6

8

10

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Wav

e He

ight

(m)

38


-8-7-6-5-4-3-2-1012345678

0 50 100 150 200 250 300 350 400Time (s)

Wav

e H

eigh

t (m

)


-8-7-6-5-4-3-2-1012345678

400 450 500 550 600 650 700 750 800Time (s)

Wav

e H

eigh

t (m

)


-8-7-6-5-4-3-2-1012345678

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Wav

e He

ight

(m)

39

April 21st 1999 09:20

-8-7-6-5-4-3-2-1012345678

0 50 100 150 200 250 300 350 400Time (s)

Wav

e H

eigh

t (m

)

April 21st 1999 09:20

-8-7-6-5-4-3-2-1012345678

400 450 500 550 600 650 700 750 800Time (s)

Wav

e H

eigh

t (m

)

April 21st 1999 09:20

-8-7-6-5-4-3-2-1012345678

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Wav

e H

eigh

t (m

)

40


-100

-80

-60

-40

-20

0

20

40

60

80

100

0 50 100 150 200 250 300 350 400Time (s)

Disp

lace

men

t (m

m)


-100

-80

-60

-40

-20

0

20

40

60

80

100

400 450 500 550 600 650 700 750 800Time (s)

Disp

lace

men

t (m

m)


-100

-80

-60

-40

-20

0

20

40

60

80

100

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Dis

plac

emen

t (m

m)

41


-25

-20

-15

-10

-5

0

5

10

15

20

25

0 50 100 150 200 250 300 350 400Time (s)

Dis

plac

emen

t (m

m)


-25

-20

-15

-10

-5

0

5

10

15

20

25

400 450 500 550 600 650 700 750 800Time (s)

Disp

lace

men

t (m

m)


-25

-20

-15

-10

-5

0

5

10

15

20

25

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Disp

lace

men

t (m

m)

42


-40

-30

-20

-10

0

10

20

30

40

0 50 100 150 200 250 300 350 400Time (s)

Dis

plac

emen

t (m

m)


-40

-30

-20

-10

0

10

20

30

40

400 450 500 550 600 650 700 750 800Time (s)

Disp

lace

men

t (m

m)


-40

-30

-20

-10

0

10

20

30

40

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Dis

plac

emen

t (m

m)

43

April 3rd 1998 20:20

-60-50-40-30-20-100102030405060

0 50 100 150 200 250 300 350 400Time (s)

Dis

plac

emen

t (m

m)

April 3rd 1998 20:20

-60-50-40-30-20-100102030405060

400 450 500 550 600 650 700 750 800Time (s)

Dis

plac

emen

t (m

m)

April 3rd 1998 20:20

-50-40-30-20-100102030405060

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Dis

plac

emen

t (m

m)

44


-20

-15

-10

-5

0

5

10

15

20

0 50 100 150 200 250 300 350 400Time (s)

Dis

plac

emen

t (m

m)


-25

-20

-15

-10

-5

0

5

10

15

20

400 450 500 550 600 650 700 750 800Time (s)

Disp

lace

men

t (m

m)


-20

-15

-10

-5

0

5

10

15

20

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Dis

plac

emen

t (m

m)

45


-40

-30

-20

-10

0

10

20

30

40

0 50 100 150 200 250 300 350 400Time (s)

Hull

Disp

lace

men

t (m

m)


-40

-30

-20

-10

0

10

20

30

40

400 450 500 550 600 650 700 750 800Time (s)

Hul

l Dis

plac

emen

t (m

m)


-40

-30

-20

-10

0

10

20

30

40

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Hul

l Dis

plac

emen

t (m

m)

46


-50

-40

-30

-20

-10

0

10

20

30

40

50

0 50 100 150 200 250 300 350 400Time (s)

Hul

l Dis

plac

emen

t (m

m)


-50

-40

-30

-20

-10

0

10

20

30

40

50

400 450 500 550 600 650 700 750 800Time (s)

Hul

l Dis

plac

emen

t (m

m)


-50

-40

-30

-20

-10

0

10

20

30

40

50

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Hul

l Dis

plac

emen

t (m

m)

47


-50

-40

-30

-20

-10

0

10

20

30

40

50

0 50 100 150 200 250 300 350 400Time (s)

Hull

Disp

lace

men

t (m

m)


-50

-40

-30

-20

-10

0

10

20

30

40

50

400 450 500 550 600 650 700 750 800Time (s)

Hul

l Dis

plac

emen

t (m

m)


-50

-40

-30

-20

-10

0

10

20

30

40

50

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Hul

l Dis

plac

emen

t (m

m)

48

January 17th 1999 05:20

-50

-40

-30

-20

-10

0

10

20

30

40

50

0 50 100 150 200 250 300 350 400Time (s)

Hul

l Dis

plac

emen

t (m

m)

January 17th 1999 05:20

-50

-40

-30

-20

-10

0

10

20

30

40

50

400 450 500 550 600 650 700 750 800Time (s)

Hul

l Dis

plac

emen

t (m

m)

January 17th 1999 05:20

-50

-40

-30

-20

-10

0

10

20

30

40

50

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Hul

l Dis

plac

emen

t (m

m)

49


-100

-80

-60

-40

-20

0

20

40

60

80

100

0 50 100 150 200 250 300 350 400Time (s)

Hul

l Dis

plac

emen

t (m

m)


-100

-80

-60

-40

-20

0

20

40

60

80

100

400 450 500 550 600 650 700 750 800Time (s)

Hull

Disp

lace

men

t (m

m)


-100

-80

-60

-40

-20

0

20

40

60

80

100

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Hul

l Dis

plac

emen

t (m

m)

50


-50

-40

-30

-20

-10

0

10

20

30

40

50

0 50 100 150 200 250 300 350 400Time (s)

Hul

l Dis

plac

emen

t (m

m)


-50

-40

-30

-20

-10

0

10

20

30

40

50

400 450 500 550 600 650 700 750 800Time (s)

Hul

l Dis

plac

emen

t (m

m)


-50

-40

-30

-20

-10

0

10

20

30

40

50

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Hull

Disp

lace

men

t (m

m)

51


-50

-40

-30

-20

-10

0

10

20

30

40

50

0 50 100 150 200 250 300 350 400Time (s)

Hul

l Dis

plac

emen

t (m

m)


-50

-40

-30

-20

-10

0

10

20

30

40

50

400 450 500 550 600 650 700 750 800Time (s)

Hull

Disp

lace

men

t (m

m)


-50

-40

-30

-20

-10

0

10

20

30

40

50

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Hul

l Dis

plac

emen

t (m

m)

52


-100

-80

-60

-40

-20

0

20

40

60

80

100

0 50 100 150 200 250 300 350 400Time (s)

Hul

l Dis

plac

emen

t (m

m)


-100

-80

-60

-40

-20

0

20

40

60

80

100

400 450 500 550 600 650 700 750 800Time (s)

Hul

l Dis

plac

emen

t (m

m)


-100

-80

-60

-40

-20

0

20

40

60

80

100

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Hul

l Dis

plac

emen

t (m

m)

53


-80

-60

-40

-20

0

20

40

60

80

0 50 100 150 200 250 300 350 400Time (s)

Hul

l Dis

plac

emen

t (m

m)


-80

-60

-40

-20

0

20

40

60

80

400 450 500 550 600 650 700 750 800Time (s)

Hull

Disp

lace

men

t (m

m)


-80

-60

-40

-20

0

20

40

60

80

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Hul

l Dis

plac

emen

t (m

m)

54

April 21st 1999 09:20

-40

-30

-20

-10

0

10

20

30

40

0 50 100 150 200 250 300 350 400Time (s)

Hul

l Dis

plac

emen

t (m

m)

April 21st 1999 09:20

-40

-30

-20

-10

0

10

20

30

40

400 450 500 550 600 650 700 750 800Time (s)

Hul

l Dis

plac

emen

t (m

m)

April 21st 1999 09:20

-40

-30

-20

-10

0

10

20

30

40

800 850 900 950 1000 1050 1100 1150 1200Time (s)

Hul

l Dis

plac

emen

t (m

m)

55

APPENDIX B

HULL LINEAR ACCELERATION SPECTRA PLOTS HULL DISPLACEMENT X-Y PLOTS


56

57

September 10th 1997 00:20 H-LAY

0

100

200

300

400

500

600

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)

H-LA

X (m

m/s

^2)^

2/Hz

September 10th 1997 00:20 H-LAX

0

100

200

300

400

500

600

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)

H-LA

Y (m

m/s

^2)^

2/Hz

February 28th 1998 20:00 H-LAY

0

50

100

150

200

250

300

350

400

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)

H-L

AX

(mm

/s^2

)^2/

Hz

February 28th 1998 20:00 H-LAX

0

50

100

150

200

250

300

350

400

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)

H-LA

Y (m

m/s

^2)^

2/Hz

April 3rd 1998 20:20 H-LAY

0

500

1000

1500

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)

H-LA

X (m

m/s

^2)^

2/Hz

April 3rd 1998 20:20 H-LAX

0

500

1000

1500

2000

2500

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)

H-L

AY (m

m/s

^2)^

2/H

z

December 31st 1997 07:00 H-LAY

0

100

200

300

400

500

600

700

800

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)

H-L

AX

(mm

/s^2

)^2/

Hz

December 31st 1997 07:00 H-LAX

0

100

200

300

400

500

600

700

800

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)

H-LA

Y (m

m/s

^2)^

2/Hz

58

November 9th 1998 10:40 H-LAX

0

500

1000

1500

2000

2500

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)

H-L

AY

(mm

/s^2

)^2/

Hz

November 9th 1998 10:40 H-LAY

0

200

400

600

800

1000

1200

1400

1600

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)

December 27th 1998 12:40 H-LAX

0

2000

4000

6000

8000

10000

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)

H-L

AY

(mm

/s^2

)^2/

Hz

December 27th 1998 12:40 H-LAY

0

200

400

600

800

1000

1200

1400

1600

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)

January 4th 1999 21:00 H-LAX

0

1000

2000

3000

4000

5000

6000

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)

H-L

AY

(mm

/s^2

)^2/

Hz

January 4th 1999 21:00 H-LAY

0

100

200

300

400

500

600

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)

January 17th 1999 05:20 H-LAX

0

500

1000

1500

2000

2500

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)

H-L

AY

(mm

/s^2

)^2/

Hz

January 17th 1999 05:20 H-LAY

0

100

200

300

400

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)


0

1000

2000

3000

4000

5000

6000

7000

8000

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)

H-L

AY

(mm

/s^2

)^2/

Hz


0

500

1000

1500

2000

2500

3000

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)

59


0

500

1000

1500

2000

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)

H-L

AY

(mm

/s^2

)^2/

Hz


0

500

1000

1500

2000

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)

H-L

AX

(mm

/s^2

)^2/

Hz


0

500

1000

1500

2000

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)

H-L

AY

(mm

/s^2

)^2/

Hz


0

200

400

600

800

1000

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)

H-L

AX

(mm

/s^2

)^2/

Hz


0

500

1000

1500

2000

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)

H-L

AY

(mm

/s^2

)^2/

Hz


0

200

400

600

800

1000

1200

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)


0

500

1000

1500

2000

2500

3000

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)

H-L

AY

(mm

/s^2

)^2/

Hz


0

500

1000

1500

2000

2500

3000

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)

April 21st 1999 09:20 H-LAX

0

500

1000

1500

2000

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)

H-L

AY

(mm

/s^2

)^2/

Hz

April 21st 1999 09:20 H-LAY

0

500

1000

1500

2000

2500

3000

3500

0 0.1 0.2 0.3 0.4 0.5Frequency (Hz)

60


-100

-80

-60

-40

-20

0

20

40

60

80

100

-100

-90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100

X Displacement (mm)

Y D

ispl

acem

ent (

mm

)


-20

-15

-10

-5

0

5

10

15

20

-20 -15 -10 -5 0 5 10 15 20

X Displacement (mm)

Y D

ispl

acem

ent (

mm

)


-40

-30

-20

-10

0

10

20

30

40

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

X Displacement (mm)

Y D

ispl

acem

ent (

mm

)

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January 17th 1999 05:20

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April 21st 1999 09:20

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Printed and published by the Health and Safety ExecutiveC0.50 4/02

OTO 2001/036

£15.00 9 780717 623297

ISBN 0-7176-2329-7

iso standards for jack up rigs

Documents