Upload
dinhdung
View
215
Download
1
Embed Size (px)
Citation preview
HSE Health & Safety
Executive
Development of design guidance for neoprene-lined clamps for
offshore application
Phase II
Prepared by MSL Engineering Limitedfor the Health and Safety Executive 2002
RESEARCH REPORT 031
HSE Health & Safety
Executive
Development of design guidance for neoprene-lined clamps for
offshore application
Phase II
MSL Engineering Limited Platinum Blue House
1st Floor 18 The Avenue
Egham Surrey
TW20 9AB United Kingdom
The document is concerned with a test programme investigating the slip capacity of neoprene-lined clamps. In Phase I of the project, which is reported separately, a total of sixteen full-scale tests were conducted at Memorial University of Newfoundland, Canada. Based on the results of the Phase I tests, interim recommendations were made for the estimation of frictional coefficients. The results indicated some surprising effects, and further tests were recommended. The further tests have now been conducted under Phase II of the project and are reported herein.
This report and the work it describes were funded by the Health and Safety Executive (HSE), ExxonMobil, Shell UK Exploration and Production and MSL Engineering Limited. Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.
HSE BOOKS
© Crown copyright 2002
First published 2002
ISBN 0 7176 2577 X
All rights reserved. No part of this publication may bereproduced, stored in a retrieval system, or transmitted inany form or by any means (electronic, mechanical,photocopying, recording or otherwise) without the priorwritten permission of the copyright owner.
Applications for reproduction should be made in writing to:Licensing Division, Her Majesty's Stationery Office, St Clements House, 2-16 Colegate, Norwich NR3 1BQ or by e-mail to [email protected]
2
FOREWORD
This document has been prepared by MSL for three sponsoring organisations:
Health and Safety Executive
ExxonMobil
Shell U.K. Exploration and Production.
In addition, MSL themselves partly funded the work described herein.
The document is concerned with a test programme investigating the slip capacity of neoprene-lined clamps. In Phase I of the project, which is reported separately, a total of sixteen full-scale tests were conducted at Memorial University of Newfoundland, Canada. Based on the results of the Phase I tests, interim recommendations were made for the estimation of frictional coefficients. The results indicated some surprising effects, and further tests were recommended. The further tests have now been conducted under Phase II of the project and are reported herein.
A project steering committee including representatives of the sponsoring organisations oversaw the work and contributed to the development of this document. The following individuals served on the committee:
Mr P Bailey
Mr J Bucknell
Dr A Dier
Mr D Galbraith (Chairman)
Mr M Lalani
Mr B McCullough
The Project Manager at MSL was Mr J Bucknell who carried out the work with guidance and support from Dr A Dier and Dr K Chen. The tests were conducted at Memorial University, Newfoundland.
The recommendations presented in this document are based upon the knowledge available at the time of publication. However, no responsibility of any kind for injury, death, loss, damage or delay, however caused, resulting from the use of the recommendations can be accepted by MSL Engineering or others associated with its preparation.
The participants do not necessarily accept all the recommendations given in this document.
3
JIP – DEVELOPMENT OF DESIGN GUIDANCE FOR NEOPRENE – LINED CLAMPS FOR OFFSHORE
APPLICATION – PHASE II
FINAL REPORT
CONTENTS Page No
FOREWORD .............................................................................................................................3
1. INTRODUCTION..........................................................................................................6
1.1 General ...............................................................................................................6
1.2 Summary of Phase I Programme........................................................................7
2. OBJECTIVE AND SCOPE OF TESTS ......................................................................10
3. DESCRIPTION OF TEST CONFIGURATION AND PROCEDURES.....................12
3.1 Clamp Specimen ..............................................................................................12
3.2 Test Rig ............................................................................................................12
3.3 Test Instrumentation.........................................................................................15
3.4 Test Procedures ................................................................................................17 3.4.1 Pre-Testing Procedures ........................................................................17 3.4.2 Testing Procedure.................................................................................18 3.4.3 Post-Test Procedures ............................................................................19
3.5 Loading Schedule .............................................................................................19
4. RESULTS.....................................................................................................................23
4.1 Failure Criterion – Quasi-Static Load (T1A &T1B; T1, T4 & T4A) ...........................................................................23
4.2 Bolt Pre-Load (T1A & T1B;T1, T2 & T3) ......................................................23
4.3 Neoprene Hardness (T1A, T18 & T18A).........................................................25
4.4 Cyclic Loading (T4C & T1C) ..........................................................................26
4
5. DISCUSSION AND GUIDELINES FOR DESIGN/ASSESSMENT.........................30
5.1 Discussion ........................................................................................................30
5.2 Design/Assessment Guidelines ........................................................................33
6. CONCLUDING REMARKS .......................................................................................36
REFERENCES
FIGURES FOR SECTION 4
5
1. INTRODUCTION
1.1 General
This report is concerned with a second phase of a Joint Industry Project (JIP) investigating the experimental slip capacity of neoprene-lined clamps.
The use of such clamps in the offshore industry has been, and continues to be, widespread throughout the world. The following applications of such clamps may be given:
•= attachment of retrofitted appurtenances such as conductors and risers
•= temporary attachments for lifting purposes, especially pipelines
•= repair of damaged members (dented or corroded)
•= attachment of new structural members.
Neoprene-lined clamps contain a liner that lies between the clamp steelwork and the enclosed member. The liner provides tolerance against lack of fit of the clamp saddle around the tubular brace. In general, the linear is made of polychloroprene (neoprene) sheet that is bonded to the inner surface of the clamp saddle plates. The neoprene liner is usually plain for structural connections designed to transmit axial or rotational loads, although ribbed linings are sometimes used to accommodate potentially large lack of fit tolerances.
Stressed neoprene-lined clamps rely on applied stud bolt pre-loads to generate compressive forces normal to the interface between the clamp liner and the surface of the clamped brace. The strength is considered to be dependent on the magnitude of the normal force, the relative stiffness of the steel and liner and the effective coefficient of friction at the liner/brace interface.
Despite the widespread use of neoprene-lined clamps through the world, there were only limited data, and no data in the public domain, on the slip capacity of these clamps. As part of a Joint Industry Project conducted by MSL entitled “Demonstration Trials of Diverless Strengthening and Repair Techniques”, a static slip test on a neoprene-lined clamp exhibited a slip capacity significantly less than that expected from the guidance available at that time.
It was against the above background that MSL launched this current Joint Industry Project. The project was intended to generate test data so that more reliable design guidance could be formulated, both for the rational assessment of the reliability of neoprene-lined clamps currently in service and for the safe design of such clamps in future applications.
With the support of HSE and two major North Sea operators, Phase I of this current )JIP was concluded in May 1999 with the issue of a final report(1 to the sponsoring
organisations. Phase I covered a programme of 16 full-scale neoprene-lined clamp tests. The tests in Phase I encompassed both axial and torsional loading and were designed to investigate the influence of a variety of parameters, including the bolt
6
load, test repeatability, neoprene thickness, pipe surface condition, clamp length/diameter ratio and pipe radial stiffness. Interim guidance was prepared on the basis of the test results generated in Phase I. Further tests were recommended in the following three areas where new data would lead to a substantial and significant enhancement of the interim guidance created in Phase I:
•= Clamp tests with imposed interface pressures lower than those used in the Phase I programme.
•= Clamp tests with neoprene liners having hardness values different to that adopted in Phase I.
•= Clamp tests with loading rates different to those used in Phase I.
In light of the extensive use of neoprene-lined clamps, and the benefits that will result through generation of data in the above three areas, this Phase II of the subject JIP was instigated.
1.2 Summary of Phase I Programme
The Phase I testing programme involved a total of 16 tests. The programme was designed to investigate the influence of the following parameters on the slip strength of neoprene-lined clamps:
•= stud bolt preload (Tests T1, T2 and T3)
•= repeatability (Tests T1 and T4)
•= failure criterion (Tests T1 and T4A)
•= surface conditions of the clamped tubular member (Tests T1, T7 and T8)
•= brace stiffness (D/T) ratio (Tests T1, T9 and T10)
•= clamp length to diameter (L/D) ratio (Tests T1, T11 and T12)
•= torsional v. axial slip (Tests T1, T13, T14 and T15)
•= thickness of the neoprene liner (Tests T1 and T16)
•= clamp brace pinching (liner/brace interference) (Tests T16 and T17).
The Phase I test programme is summarised on the pullout table at the back of this document. The primary Phase I finding is that coefficient of friction for neoprenelined clamps is substantially below the range of values adopted in practice. Hence, some existing structural neoprene-lined clamps potentially have capacities that may be less than the design intent. In addition to the primary findings, the following results were also achieved during the Phase I tests:
•= The failure load for all axial tests was defined as the position at which the load-slip curve was seen to deviate substantially from the trendline defining its
7
initial slope. The failure torque was defined as the position with a relative rotation of 0.45° between the clamp and tubular for pure torsional load or when an axial displacement of 1.25 mm was reached under the combination of tensile and torsional loads.
•= The axial tests were repeatable and similar failure loads could be derived from tests with identical conditions according to the definition of failure.
•= The increase in the applied stud bolt load did not lead to a corresponding increase in the clamp axial capacity, at least for a preload level of 40% - 60% of the stud bold yield strength. For a given stud bolt pre-load level, significant drop in bolt load with the increase of the slip was shown, although such reduction was not so marked for the initial 4 to 5 mm slip.
•= The clamp axial slip capacity varied little with either the pipe surface condition or the pipe radial stiffness (i.e. pipe diameter to thickness ratio) according to the definition of failure criterion.
•= As the clamp length was reduced, with a corresponding reduction in total applied bolt load, the axial capacity of the clamp was also seen to reduce. The reduction in axial capacity was proportional to the reduction in total applied bolt load.
•= No increase in clamp slip capacity was obtained by increasing the neoprene thickness. Reducing the circumferential length of the pipe/neoprene interface could lead to an increase of the clamp slip capacity.
•= Application of the torsional load would reduce the clamp slip capacity.
With the above observations achieved in the Phase I test programme, interim guidance was formulated for clamp slip capacities under axial load alone, torsional moment alone and combined axial and torsional loadings respectively.
The developed guidance had to be considered as being of an interim nature until further data became available due to the unexpected slip behaviour of the clamp, particularly with regard to the relationship between applied bolt load and clamp capacity. There were insufficient test data to permit a proper clarification of the role of bolt load.
The interim guidance can be considered conservative, particular for small bolt preloads where no data exists in the Phase I test programme. For combined axial and torsional loadings, it could also lead to a rather conservative prediction of the torsional capacity at high values of co-existing axial load.
The combination of lower bolt pre-loads, applied loading rate effects typical of those due to wave action, and possibly liners of greater Shore hardness may give higher apparent coefficients of friction. It is for this purpose that Phase II of this current JIP was launched.
8
The remainder of this document presents the Phase II JIP test programme in detail, viz:
•= Section 2 – Objective and Scope of Tests
•= Section 3 – Description of Test Configuration and Procedures
•= Section 4 – Test Results
•= Section 5 – Design/Assessment Guidance
•= Section 6 – Concluding Remarks.
9
2. OBJECTIVE AND SCOPE OF TESTS
Objective
The objective of Phase II of the JIP is to conduct a programme of tests that, in combination with the Phase I results, will enable cost-effective, robust and safe guidance for the design of neoprene-lined clamps to be established.
Testing Programme
The testing programme involves a total of 6 tests, as summarised in Table 2.1. The programme has been designed to investigate the influence of the following parameters on the slip strength of neoprene-lined clamps:
•= Interface pressure - Tests T1A and T1B (along with Phase I Tests T1, T2 & T3)
•= Neoprene hardness - Tests T18 and T18A (along with Test T1A)
•= Cyclic loading effect (Test T4C)
•= Failure definition (full cyclic and half cyclic loading) (Test T1C)
Phase I test rig was utilised for the first two tests on T1A and T1B. In order to apply a sinusoidal-type loading in both tensile and compressive direction (tests on T4C and load steps 1 through 5 of tests on T1C), the Phase I test rig was slightly modified to remove slack in the bearings.
10
CH
1001
0R00
6 R
ev 1
Janu
ary
2001
Test
N
o.
Nat
ure
Of T
est
Load
ing
Cla
mp
Leng
th
(mm
)
Bra
ce
D/T
(m
m)
No.
O
f B
olts
Bol
t Si
ze
(nom
.)
Bol
t Lo
ad
(% f y
)
Bra
ce
Surf
ace
Con
ditio
n
Neo
pren
e Th
ickn
ess
(mm
)
Neo
pren
e H
ardn
ess
(IRH
D)
T1A
In
crem
enta
l 80
0 32
4/17
8
M36
20
%
Bla
ck o
xide
10
60
T1B
In
crem
enta
l 80
0 32
4/17
8
M36
10
%
Bla
ck o
xide
10
60
T4C
Fu
ll C
yclic
&
Hal
f Cyc
lic
800
324/
17
8 M
36
20%
B
lack
oxi
de
10
60
T18
Incr
emen
tal
800
324/
17
8 M
36
20%
B
lack
oxi
de
10
70
T18A
In
crem
enta
l 80
0 32
4/17
8
M36
20
%
Bla
ck o
xide
10
50
T1C
H
alf C
yclic
80
0 32
4/17
8
M36
20
%
Bla
ck o
xide
10
60
Not
es:
IRH
D In
tern
atio
nal R
ubbe
r Har
dnes
s Deg
ree
Tab
le 2
.1:
Sum
mar
y of
Tes
t P
rogr
amm
e fo
r P
hase
II
11
CH10010R006 Rev 1 January 2001
3. DESCRIPTION OF TEST CONFIGURATION AND PROCEDURES
A complete description of the test configurations and procedures is provided in the Annex. Here, a summary of the more salient points is given.
3.1 Clamp Specimen
The clamp test specimen, as used in the Phase I trials, is illustrated in Figure 3.1. The clamp was structurally typical of many clamps used for the retrofitting of risers to existing installations and for the handling of pipe spools.
bolt
800 mm
600
460
mm
saddle plates side plates
neoprene lining
stiffeners stud bolt centrelines (8 No. total) mm
Figure 3.1: Clamp Test Specimen
3.2 Test Rig
Phase II tests were restricted to pure axial loading of the clamp along the longitudinal axis of the pipe. The same test rig configuration used for application in axial tensile loading in the Phase I tests, as shown in Figure 3.2 and illustrated in Figure 3.3, was utilised in the Phase II tests. Modifications to the end connections have been made to permit load reversal. The modified end connection and load cell – tubular interface (flange joint) are illustrated in Figure 3.4.
12
CH
1001
0R00
6 R
ev 1
Janu
ary
2001
cell
tubu
lar s
ectio
n
clam
p te
st
spec
imen
hydr
aulic
ac
tuat
or
load
ce
ll
tubu
lar s
ectio
n lo
ad
hydr
aulic
ac
tuat
or
Fig
ure
3.2:
T
est
Rig
Con
figu
rati
on f
or A
xial
Loa
ding
Onl
y
13
Figure 3.3: Photograph of Axial Load Rig
14
Figure 3.4: Modified End Connection and Load Cell – Tubular Interface
3.3 Test Instrumentation
Strain Measurement
•= Strains were measured by means of linear 120-Ohm electrical resistance strain gauges.
•= A total of six strain gauges were mounted on the pipe to verify loading of the specimen. The locations and identification numbers for each of strain gauges are shown in Figure 3.5. The identification numbers of each of the stud bolts are also shown in Figure 3.5.
•= Each bolt was instrumented with two strain gauges in a half bridge configuration to monitor the total load on each bolt.
•= Dummy gauges were provided on blocks of steel to allow for temperature compensation of measured values.
Displacement Measurement
•= Displacements were measured by means of temperature compensating Linear Voltage Displacement Transducers (LVDTs).
•= A total of five LVDTs were positioned about the specimen to record the relative displacement of the clamp with respect to the pipe. Figure 3.6 presents the locations of the LVDTs.
•= LVDTs were aligned to ensure that displacement were measured perpendicular to specimen or reaction surfaces.
•= Sufficient travel lengths were specified for the LVDTs such that the entire loading regime could be recorded.
15
16
Figure 3.5: Bolt and Strain Gauge Locations
Figure 3.6: Locations of LVDTs
Data Acquisition
•= All transducers were connected to a data acquisition system that consists of a terminal block, a high-speed analogue/digital card mounted in an IBM compatible personal computer and a data acquisition software used to configure the system and set up data acquisition parameters.
•= Digital signals were conditioned to output as load, stroke (displacement) and strain.
3.4 Test Procedures
3.4.1 Pre-Testing Procedures
Calibration of Stud Bolts
Two strain gauges were mounted, diametrically opposite each other, at the mid length of each stud bolt. Each bolt was calibrated by installing it into a tensile testing machine and applying incremented tensile loading up to 80% of nominal tensile yield.
Preparation of Tubular Members
The existing uncoated specimen from the Phase I tests was utilised in the Phase II tests. The tubular was lightly manually wire brushed immediately prior to the clamp installation for each test.
Application of Neoprene Liner
The supply and application of the neoprene-liner (IRHD 50, IRHD 60, or IRHD 70) for the clamp was in accordance with MSL document entitled “Specification for Clamp Lining and Bonding” and consistent with procedures employed for the Phase I tests. Every effort was made to ensure the highest quality bond of the liner to the clamp.
Assembly of the Clamp Specimen
During steps 1 to 4, below, the neoprene liners of each of the clamp halves and the tubular member were liberally doused with water from a hose, ensuring that all contact surfaces remain fully soaked throughout the installation.
1. The lower clamp half was supported at a height of approximately six inches (152 mm) from the floor of the laboratory.
2. The tubular member was placed into the lower clamp half.
3. The upper clamp half was lowered onto the tubular member to align directly above the lower clamp half.
4. Each of the stud bolts was carefully inserted through the holes in the flange plates of the clamp halves, ensuring that the attached strain gauges were not damaged. Spherical washers and nuts were applied and hand tightened,
17
ensuring that the split line on each side of the clamp remained even along the length of the clamp and approximately equal either side of the clamp. A sufficient length of bolt protruded above the top plate of the upper clamp half to accommodate the hydraulic stud bolt tensioning system.
5. The leads from the strain gauges on each of the bolts and on the tubular were connected to the appropriate channels of the data acquisition system.
6. The operation of all instrumentation and data acquisition system was checked.
7. Initial datum (zeroed) readings of all instrumentation were taken.
8. The bolts were hand-torque, evenly, using a standard wrench.
Tensioning of Stud Bolts
The stud bolts were simultaneously tensioned using the Hydratight hydraulic tensioning system. The procedure for the tensioning involved a three-stage pressure application. A qualified Hydratight technician supervised the tension operation.
Strain gauge readings from each stud bolt were continuously monitored throughout the tensioning procedure to confirm:
(a) the desired average bolt load had been achieved to within 5%
(b) maximum variation of load between bolts did not exceed 10% of the target load.
Installation of the Test Rig
•= The specimen was installed into the appropriate test rig and the LVDT instrumentation was set up.
•= The operation of all instrumentation and data acquisition system was checked.
•= Initial datum (zeroed) readings of all instrumentation was again checked.
•= The specimen was bedded down by applying load cycles not greater than 5% of the estimated failure load.
3.4.2 Testing Procedure
After completion of the steps described in Section 3.4.1, the application of loading proceeded in accordance with the following procedure:
•= Throughout all loading and unloading operations, data from each instrumentation point, including all strain and displacement gauges, were continuously recorded, calibrated and logged by the computerised data acquisition system. Visible and/or audible events were manually recorded and photographed as appropriate.
18
•= With the exception of Tests T1C and T4C, each specimen was subjected to two loading and unloading cycles. Test T1C comprised of sinusoidal loading cycles (at 5 different loads) and half cycle loading cycles (again at 5 different loads). Test T4C contains 7 full cycle sinusoidal loading cycles. The actual loading schedules used are summarised in Section 3.5.
3.4.3 Post-Test Procedures
Following the completion of load application to the test specimen the following basic procedures were followed:
•= All instrumentation readings were recorded at the point of complete load removal.
•= All stud bolt nuts were completely slackened off using the Hydratight tensioning system and readings of all instrumentation were again taken.
•= The electrical connection leads from the stud bolts to data-logger were disconnected and all stud bolts were carefully removed, supporting clamp halves appropriately.
•= The two clamp halves were split and the interfaces surveyed, taking photographs and notes as appropriate.
3.5 Loading Schedule
Digital files were generated by the data acquisition system, then sampled and converted to the control signal by the Digital/Analogue channels of the data acquisition system. As mentioned above, for each specimen configuration except Tests T1C and T4C, the test contained two loading cycles. Test T4C was comprised of seven load steps and Test T1C contained ten. Table 3.1 presents an overview of the loading schedule.
Test Load Type Termination Condition
T1A Incremental Tensile 20 mm slip
T1B Incremental Tensile 20 mm slip
T4C Sinusoidal Tension-Compression Predetermined number of cycles
T18 Incremental Tensile 20 mm slip
T18A Incremental Tensile 20 mm slip
T1C Sinusoidal Tension-Compression & Tensile-half of sine wave
Predetermined number of cycles
Table 3.1: Test Overview
19
20
Details of loadings are given in the Annex in the form of load-time plots. Here, the loadings are summarised as follows:
Tests T1A, T1B, T18 & T18A
These four tests were comprised of two loading cycles each.
Loading cycle one was incremental loading of 5-minute durations commencing at 50 kN, proceeding to 75 kN, then 100 kN. After this, the load increments were reduced to intervals of 10 kN, continuing until a relative displacement of 4 mm of the tubular in the clamp was measured.
Loading cycle two involved a linear increase of the load, at a rate of 700 lbs (approximately 31.2 kN) per minute, from zero load to a point where 20 mm slip was measured between the clamp and the tubular, after which the load was gradually reduced back to zero.
Test T4C
A total of 7 loading steps were comprised in this test. All loading steps were similar, sinusoidal loading in tensile and compressive directions about a mean load of zero. The only variations between load steps were frequency and amplitude of the cyclic load and the total number of times each load was applied (cycle numbers). Figure 3.7 presents a typical loading cycle in Test T4C. Specifications of each of the 10 loading cycles are summarised in Table 3.2. The amplitudes, periods and the steepness of 1/16 are explained below.
-4 0 0 0 0
-3 0 0 0 0
-2 0 0 0 0
-1 0 0 0 0
0
1 0 0 0 0
2 0 0 0 0
3 0 0 0 0
4 0 0 0 0
1 67 133
199
265
331
397
463
529
595
661
727
793
859
925
991
1057
1123
1189
1255
1321
1387
1453
1519
1585
1651
T im e (s )
Lo
ad (
lbs)
-2 .5
-2
-1 .5
-1
-0 .5
0
0 .5
1
1 .5
2
Dis
pla
cem
ent
(mm
)
L O A D L B SA V E R A G E B O L T L O A D L B SL V D T 1 M M
Figure 3.7: Typical Load Step in Test T4C
Test T1C
Test T1C included a total of 10 load steps, the first five being sinusoidal and the latter being the tensile load, i.e. positive portion only, of similar cycles as shown in Figure
21
3.8. The variations in the frequency, amplitude and number of cycles are presented in Table 3.2.
-5 00 0
5 00 0
15 00 0
25 00 0
35 00 0
45 00 0
55 00 0
1 56 111
166
221
276
331
386
441
496
551
606
661
716
771
826
881
936
991
1046
1101
1156
1211
1266
1321
1376
T im e (s)
Lo
ad (
lbs)
-1
-0 .5
0
0 .5
1
1 .5
2
Dis
pla
cem
ent
(mm
)
L O A D L B SA V E R A G E BO LT LO A D LB SL V D T 1 M M
Figure 3.8: The 10th Loading Step of Test T1C
Test T4C
Loading Cycle
No. of Cycles
Amplitude(kN)
Period(seconds)
Comment
1 10 24 7.0 Steepness of 1/16 2 10 51 9.0 Steepness of 1/16 3 5 99 10.5 1-year return wave 4 5 150 13.2 Steepness of 1/16 5 5 188 12.0 100-year return wave 6 5 220 12.0 Steepness of 1/16 7 5 220 10.5 Steepness of 1/16
Test T1C (Loading Cycles 1–5: Tension–Compression; 6–10 Tension only)
Loading Cycle
No. of Cycles
Amplitude(kN)
Period(seconds)
Comment
1 & 6 10 24 7.0 Steepness of 1/16 2 & 7 10 51 9.0 Steepness of 1/16 3 & 8 5 99 10.5 1-year return wave 4 & 9 5 150 13.2 Steepness of 1/16 5 & 10 5 188 12.0 100-year return wave
Table 3.2: Specifications of Loading Steps in Tests T4C and T1C
The amplitudes, periods and steepness values in Table 3.2 were proposed, in consultation with the Project Steering Committee, to be similar to what existing clamps may experience. For this purpose, it was assumed that the test specimen had been used to attach a 26′′ retrofit riser to a typical UK Southern North Sea platform. The clamp was assumed to be located at an elevation close to the first horizontal frame below the waterline (-8.0 m). The water depth was taken as 35.4 m. It was assumed that the clamp had been designed with an interface pressure of 3.2 MPa (i.e. consistent with Test T1A). It was further assumed that, in the design of the clamp, a value of 0.2 was used for the coefficient of friction and a factor of safety of 1.7 was applied to the extreme event load. On this basis, the clamp notional design axial slip capacity is 188 kN.
The design wave height and period used for the 100-year event were 15.1 m and 12 seconds respectively. At 8 m water depth the lateral wave load on the riser was 25.9 kN/m. The clamp was therefore designed to resist the wave load on approximately 7.3 m of riser. The wave height and period used for the 1-year event were 10.8 m and 10.5 seconds respectively. At 8 m water depth, the load on the riser was 13.6 kN/m, therefore, the load on the clamp during the 1-year return period wave was 99 kN. For the determination of the wave periods associated with the intermediate load steps a wave steepness of 1/16 has been assumed.
22
4. RESULTS
This section presents the main results of the various tests conducted in Phase II. The results are grouped, according to the parameter under investigation, in the following subsections. Further details may be found in the Annex, especially of the condition of the liner following each test.
4.1 Failure Criterion – Quasi-Static Load (T1A &T1B; T1, T4 & T4A)
It has been observed from the tensile tests in Phase I that the slip behaviour of the clamp is extremely ductile. Typical load-slip curves obtained from Tests T1 and T4 in Phase I are reproduced in Figure 4.1. In order to provide greater accuracy in the definition of the failure criterion, Test T4A was also carried out in Phase I. Tests T1, T4 and T4A had a similar test programme except for the load application rate. Compared with Tests T1 and T4, where the load was gradually and continuously ramped at a slow rate, the load in Test T4A was applied in increments with a period of 5 minutes between each load increment. It was observed during Test T4A that sliding did not stop during the hold periods and equilibrium was never established, see Figure 4.2. Due to the creep effect observed in Test T4A, the failure load could not be defined by selecting a certain amount of limiting slip.
Based on the observations from Tests T1, T4 and T4A, a failure criterion was defined in Phase I for clamps under axial loading. The failure load was defined as the position at which the load-slip curve was seen to deviate substantially from the trendline defining its initial slope.
Tests T1A and T1B in Phase II have an identical test frame and specimen configurations to those of T1, T4 and T4A in Phase I. However, in Tests T1A and T1B, the total stud bold loads were lower. The load-slip curves of Tests T1A and T1B are presented in Figures 4.3 and 4.4 respectively. The figures show the mean slip of the two clamp halves for each test. The initial trendlines are also shown. Once again, a ductile form of slip can be seen in each of the tests.
In loading cycle one of Tests T1A and T1B, relatively low axial loads were applied with a low loading rate. These tests called for incremental loading until 4 mm of displacement was measured between the tubular and the clamp, whereupon the load was reduced to zero. It can be observed from Figures 4.3 and 4.4 that the unloading paths are approximately parallel to the initial slope.
On the basis of the above observations, the failure criterion defined in Phase I for axial quasi-static load is retained herein.
4.2 Bolt Pre-Load (T1A & T1B;T1, T2 & T3)
Tests T1A and T1B, together with Tests T1, T2 and T3 conducted in Phase I, represent an investigation of the influence of bolt pre-load on clamp axial slip capacity. For Test T1A the bolts were tensioned to a nominal pre-load of 20% of tensile yield. In Test T1B the nominal pre-load of each bolt was 10% of tensile yield. As reported in the Phase I final report, the pre-load of each bolt in Tests T1, T2 and T3 were 40%, 50% and 60% of tensile yield, respectively. It was observed in Phase I
23
tests that a pre-load of 60% of tensile yield caused excessive bulging of the liner during bolt preload application and damage of the neoprene liner during slip. Hence, 50% of tensile yield was taken as the design limit of the bolt pre-load level.
Bolt Pre-load
The bolts were simultaneously tensioned using Hydratight hydraulic tensioning tools. The loads in the bolts were continuously monitored during the tensioning operation by means of the attached strain gauges, each bolt having been previously calibrated to 80% of yield. The average applied pre-loads at the start of Tests T1A and T1B are presented in the table below, where the pre-loads at the start of Tests T1 and T2 in Phase I are also included for reference.
Test ID Applied Bolt Pre-Load at Start of Test (KN)
Average Total % of tensile yield
T1A 106 848 20% T1B 55 440 10% T1 212 1696 40% T2 274 2192 50%
Bolt Load Variation
The variation of the pre-load in each bolt for Test T1A is shown in Figure 4.5. Similar variations in bolt loads were observed for Test T1B. The variation of the average bolt load for each of Tests T1A and T1B is shown in Figure 4.6. The plots shown a significant drop in bolt load over the duration of the tests, however, for the first 4-mm of slip, the reduction is not so marked. The reduction of bolt load was also observed in Phase I tests.
In all quasi-static tensile tests the variation in the bolt loads followed a similar pattern. The bolts at the end of the clamp from which the pipe was pulled, numbers 1 and 8 (see diagram below), experienced an immediate fall-off in load. At the other end of the clamp bolts 4 and 5, initially, see a small increase in load, until about 3 mm of clamp displacement relative to the pipe. They then see a similar rate of load loss as the other bolts. The central bolts, numbers 2, 3, 6, and 7, see little or no loss in load until about 3 mm of relative displacement, at which time a similar rate of load reduction to the end bolts occurs. A similar bolt load variation pattern was also observed in Phase I tests.
24
Slip
The load-slip behaviour recorded in Tests T1A and T1B are shown in Figures 4.3 and 4.4 respectively. The load-slip curves for Tests T1 and T2 in Phase I are reproduced in Figure 4.1. The figures show the mean slip of the two clamp halves for each test. A ductile form of slip can be seen in each of the test, as already mentioned. The linear trend line through the initial slope of the curves has been plotted on Figures 4.3 and 4.4. The failure load for each test, based on the definition discussed in Section 4.1, is given in the table below, together with the corresponding apparent friction coefficient. Those obtained in Phase I for Tests T1 and T2 are also included for reference. The apparent friction coefficient is defined as the failure load divided by the total applied bolt pre-load per clamp half. As reported in Phase I final report, the design capacities for Tests T1 and T2 were estimated as 441 kN and 552 kN respectively (µ = 0.2, factor of safety Γ = 1.7). The corresponding estimated design capacities for Tests T1A and T1B are 220 kN and 110 kN respectively.
Test ID Failure Load Apparent Friction Coefficient
T1A (Fb = 0.2 Fy) 150 0.088 T1B (Fb = 0.1 Fy) 115 0.131 T1 (Fb = 0.4 Fy) 150 0.044 T2 (Fb = 0.5 Fy) 150 0.034
During the failure load determination for Tests T1A and T1B, the load cycle 2 curves in Figures 4.3 and 4.4, which more actually represent the practical load application rate, were utilised. The load-slip behaviour of each of the Tests T1A, T1B, T1 and T2 are shown, for comparison, in Figure 4.7. Given the feature that all these fours tests behaved very similar shown in Figure 4.7, the failure load of Test T1B presented in the above table may be considered to be conservative.
It can be observed that the increase in the applied stud bolt load does not necessarily lead to a corresponding increase in the clamp axial capacity. For a bolt load of 20% of tensile yield and above, the nature and magnitude of clamp failure remain similar. This, in turn, results in a reduction in the apparent friction coefficient for each of the Tests T1A, T1 and T2.
4.3 Neoprene Hardness (T1A, T18 & T18A)
Tests T18 and T18A were carried out to assess the effect of neoprene hardness on clamp displacement by comparison with Test T1A of Phase II. For Test T1A the neoprene liner had a hardness of 60 IRHD. The neoprene hardness in Tests T18 and T18A were 70 IRHD and 50 IRHD respectively.
25
Bolt Pre-Load
The bolts were simultaneously tensioned, using Hydratight hydraulic tensioning tools. The load in the bolts was continuously monitored during the tensioning operation by means of the attached strain gauges, each bolt having been previously calibrated to 80% of yield. The applied bolt pre-loads at the start of Tests T1A, T18 and T18A are given in the table below.
Test Neoprene Applied Bolt Pre-Load at Start of Test (KN) ID Hardness Average Total % of tensile yield
T1A 60 106 848 20% T18 70 100 800 20%
T18A 50 105 840 20%
Bolt Load Variation
Variation of the bolt pre-load for Tests T18 and T18A followed a similar pattern to that for Test T1A shown in Figure 4.5. The description of the bolt pre-load variation and observations therefrom are discussed in Section 4.2, above.
Slip
The load-slip behaviour recorded in Tests T1A, T18 and T18A are shown in Figure 4.8. The curves show the mean slip of the two clamp halves for each test. Once again, a ductile form of slip can be seen in each of the tests. The linear trend lines through the initial slope of each curve have been plotted on the figure and the slopes can be seen to correlate with neoprene hardness. The failure load for each test, based on the definition discussed in Section 4.1, is tabulated below, together with the corresponding apparent friction coefficient. The apparent friction coefficient is defined as the failure load divided by the total applied bolt pre-load per clamp half. The estimated design capacity for all three tests is 220 kN.
Test ID Neoprene Hardness
Failure Load
(kN)
Apparent Friction Coefficient
T1A 60 150 0.088 T18 70 160 0.100
T18A 50 130 0.077
The correlation between neoprene hardness and the apparent friction coefficient is presented in Figure 4.9. It can be seen that the apparent friction coefficient increases with neoprene hardness.
4.4 Cyclic Loading (T4C & T1C)
Tests T4C and T1C represent an application of cyclic loading on the neoprene-lined clamp to determine slip due to simulated wave action on a riser fitted to a platform in the UK Southern North Sea (see Section 3.5). Seven loading steps were conducted in
26
Test T4C with a neoprene hardness of 60 IRHD. The cyclic loading steps in Tests T4C followed a sinusoidal tension-compression pattern with different amplitudes and frequencies. For Test T1C ten loading steps were carried. A new neoprene liner with hardness of 60 IRHD was fitted for Test T1C. The first five loading steps for Test T1C were identical to those in Test T4C and represent the repeatability of the application of cyclic loading performed in Test T4C on the neoprene-lined clamp. The second five loading steps represent the application of half-cycle pulse loading on the neoprene-lined clamp with the same assumptions as the first five steps. Half-cycle wave loading shows the response of the clamp when subjected to a push-push action instead of the push-pull action of full-cycle loading. The comparison of the results of the two tests determined whether the clamp slip seen in the previous tests was a product of shear deformation of the neoprene lining or actual slippage due to the applied force. The load pattern is presented in Table 3.2 in Section 3.5.
Bolt Pre-Load
The bolts were simultaneously tensioned, using Hydratight hydraulic tensioning tools. The load in the bolts was continuously monitored during the tensioning operation by means of the attached strain gauges, each bolt having been previously calibrated to 80% of yield. The average pre-loads at the start of each step for Tests T4C and T1C are presented in the following table.
Test ID Neoprene Hardness
Applied Bolt Pre-Load at Start of Test (KN) Average Total % of tensile yield
T4C – Step 1 60 117 963 20% T4C – Step 2 60 117 963 20% T4C – Step 3 60 117 963 20% T4C – Step 4 60 117 963 20% T4C – Step 5 60 115 920 20% T4C – Step 6 60 113 904 20% T4C – Step 7 60 111 888 20% T1C – Step 1 70 114 912 20% T1C – Step 2 70 113 904 20% T1C – Step 3 70 113 904 20% T1C – Step 4 70 113 904 20% T1C – Step 5 70 112 896 20% T1C – Step 6 70 111 888 20% T1C – Step 7 70 112 896 20% T1C – Step 8 70 112 896 20% T1C – Step 9 70 112 896 20% T1C – Step 10 70 112 896 20%
Bolt Load Variation
The bolt load variation for step 5 in Test T4C is presented in Figure 4.10. Similar variation pattern was observed for other loading steps in Test T4C. It can be seen in Figure 4.10 that bolt numbers 1 and 8, which are the closest ones to the loading cell, experienced a fall-off in load during the tensile half-cycle and an increase in load during the compressive half-cycle. The rates of load increase and loss were similar. At the other end of the clamp bolts 4 and 5, an increase in load during the tensile half
27
cycle and a load loss during the compressive half-cycle were observed. The rates of load increase and loss were similar and approximately equalled to those of bolts 1 and 8. The central bolts, numbers 2, 3, 6 and 7 see little or no loss/increase in load during the entire tensile-compressive cycle. The above observations are similar to the findings of the quasi-static tests as already described in Section 4.2. The variation of the bolt pre-load for Test T1C followed a similar pattern to that for Test T4C.
Figure 4.11 presents the average bolt pre-load variations for each of the loading steps in Tests T4C. Comparisons of the average bolt pre-load variations among the loading steps in Test T4C, full-cycle steps in Test T1C and half-cycle steps in Test T1C are shown in Figure 4.12. As can be seen, there is little difference in response for full- or half-cycle loading.
Slip
As seen in the quasi-static axial loading tests, a ductile form of slip appeared in each of the load steps of Tests T4C and T1C. The load-slip response for step 5 of Test T4C is shown in Figure 4.13. It can be seen from Figure 4.13 that the load-slip response forms a closed hysteresis loop. Similar load-slip behaviour was observed for all load steps in Tests T4C.
Figure 4.14 presents a single cycle of each step in Test T4C. It can be observed that the slope and area of the load-slip hysteresis loop depend on the frequency and amplitude of the applied cyclic load. Generally, higher load amplitude results in a larger loop area.
The first five loading steps of Test T1C were carried out to assess the repeatability of the clamp slip behaviour under cyclic load. As addressed in the test procedures, the neoprene liner was to be replaced whenever visual damage became evident or, in any case, following three successive tests. Test T1C was conducted using a new neoprene liner with the same hardness as that in Test T4C. The comparative load-slip behaviour of Tests T4C and T1C is shown in Figure 4.15. Figure 4.15 reveals that the clamp slip response in Tests T4C and T1C are similar.
As mentioned above, Test T4C had the identical specimen configurations and similar bolt pre-load levels to Test T1A. The peak load-slip responses of each loading step in Test T4C are compared with that of Test T1A in Figure 4.16. It can be seen from Figure 4.16 that the clamp has similar slip behaviour under cyclic loading and quasistatic loading.
Full-cycle Tests T4C and T1C showed that the load-slip response formed a stable hysteresis loop. The amplitudes of Steps 1 to 4 in these tests did not exceed their static axial load capacity. It can be seen from Figure 4.15 that a residual displacement of clamp relative to pipe is within ±0.4 mm for Steps 1 to 4 when the applied load reduced to zero. It was observed during tests that even this residual displacement was recovered within a very short period of time.
In order to determine whether the observed residual displacements of the clamp are purely from the shear deformation of the neoprene lining or if they represent true slip movement (ie. sliding at interface) of the clamp, 5 half-cycle loading steps were conducted in Test T1C. The five pulse loading steps simulate the tensile half of the
28
five full-cycle steps respectively. Figure 4.17 presents one loading cycle of Step 5 in Test T1C.
The clamp load-slip response under the tensile half-cycle loading steps was again observed to form a stable hysteresis loop. A typical example is shown in Figure 4.18 for loading Step 10 of Test 1C.
Comparisons of the clamp load-slip response among the full-cycles tests in T4C and T1C and the half-cycle tests in T1C are shown in Figure 4.19. The neoprene liner under the half-cycle load is slightly stiffer than when experiencing a full-cycle load. In part, this may be because the loading rate in the half-cycle is slightly higher.
It can be seen from Figures 4.18 and 4.19 that the residual displacement of the clamp under the half-cycle loading is so small that it can be neglected. The residual displacement observed in the full-cycle tests can be taken as the pure shear of the neoprene.
Given the above observations, the following conclusions can be drawn:
1) A clamp under cyclic load has a similar deformation response to that when subjected to static load.
2) The clamp displacement, at least up to an amount of 2 mm, results purely from the neoprene undergoing shear deformation.
3) With the application of axial load less than the failure load, the shear deformation of the neoprene results in a small amount of residual displacement, which is recoverable within a short period of time.
29
5. DISCUSSION AND GUIDELINES FOR DESIGN/ASSESSMENT
This section is concerned with the development of guidance for the slip capacity of stressed neoprene-lined clamps based on the results of the Phase I tests and of the test programme described in Section 4.
5.1 Discussion
It is appropriate to begin with the tests that were not subject to cyclic loading. A most illuminating, albeit short, discussion of friction behaviour, as pertains to natural rubbers, can be found in Reference (2). It is not known how applicable it is to synthetic rubbers such as neoprene but it would seem to explain the results of most of the tensile tests in both Phase I and Phase II. Quoting from Reference (2) (underline inserted):
“The coefficient of friction is defined by µ = W F , where F is the/ tangential friction force and W the applied normal load. For rubber, the coefficient of dry friction is not constant, but falls with increasing normal load. At light loads the dependence is weak, but it becomes more pronounced at high loads. The friction force F is proportional to the real surface contact area, which for normally rough surfaces under light loads is much less than the geometric area of contact. At very high loads the relatively low modulus of rubber results in the real contact area approaching the geometric area, and F tends to a limiting, maximum value. For dry contacts, the constant of proportionality between F and the real contact area is of the same order as the shear modulus, but it is reduced by surface contamination.”
The above passage suggests that the bolts loads in the majority of tests, although typical of offshore practice, were sufficiently high so that the limiting value of slip load F was reached. The evidence of liner extrusion due to preloading the bolts tends to confirm that liner was highly stressed, and the real contact area was approaching the geometric area. Figure 4.7 illustrates the similarity of clamp slip loads over a wide range of bolt loads.
However, rather than taking the ultimate slip load (ie. the load occurring at a slip of 15 mm or more), a more conservative failure criterion has been used herein. The failure load has been taken as the load when significant departure from the initial linear elastic behaviour occurs. This corresponds to when relative displacement occurs under sustained loads (see Test T4A curve in Figure 4.2). With this failure criterion, most specimens again give a similar failure load, and hence approximately similar factors of safety against true slip. In only one test (Test T1B) were bolt loads so low that a lower failure load was inferred.
To assist in the development of design guidance, reference is made to Figure 5.1, which is a plot of the interface shear capacity against the interface pressure. The figure shows the results of Tests T1A, T1B, T1, T2 and T3 in which the preload was the parameter under investigation. Superimposed are lines corresponding to the apparent coefficients of friction inferred from the tests. Also shown is the line corresponding to µ=0.8 which is typical of values suggested by liner manufacturers.
30
The results from Tests T18 and T18A for different neoprene hardness are also presented.
Inte
rfac
e S
hea
r C
apac
ity,
ττ ττ=
Pc/
DL
(N
/mm
2 )0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
µ=0.8 µ
Τ18 =0.100 T1A, µ=0.088 T1, µ=0.044 T2, µ=0.034
Τ1Β µ=0.131 Τ18
µ=0Α.077
T3, µ=0.024
0 1 2 3 4 5 6 7 8 9 10 11
Interface Pressure, q=FB/DL (N/mm2)
Figure 5.1: Plot of Interface Shear against Interface Pressure
The design guidance given in Section 5.2 is formulated in terms of a limiting interface shear capacity (this is 0.29 N/mm2 for the results of Tests T1A, T1 and T2 all having neoprene hardness IRHD = 60). A limit of 8.5 N/mm2 is put on the interface pressure as the liner of specimen T3 in Phase I was extruded when the stud bolt preload was applied and it also suffered damage during the slip test. For interface pressures less than that corresponding to about that in Test T1A (q = 3 N/mm2 in fact), the limiting interface shear capacity is ramped down in a parabola form, effectively ending with a slope of µ= 0.19 at the origin. Although it is conservative compared with a slope of 0.8, uncertainties exist in the small interface pressure region, and there is no test data available for an interface pressure less than that of Test T1B (1.7 N/mm2). The parabola is given by:
q2 �τ = 0.29 ��
� q2 − … 5.1 9� 3
The limiting interface shear capacity was observed to be a function of the neoprene hardness, measured in International Rubber Hardness Degrees (IRHD), see Tests T18A, T1A and T18 (IRHD values of 50,60 and 70 respectively) in Figure 5.1. Assuming that these test results are indicative of their respective plateau regions, the limiting interface shear capacity can be plotted against IRHD as shown in Figure 5.2.
31
Inte
rfac
e S
hear
, τ (
N/m
m2 )
0.34
0.32
0.3
0.28
0.26
0.24
0.22
0.2
T18
T1A
T18A
40 50 60 70 80 IRHD
Figure 5.2: Effect of Neoprene Hardness on Interface Shear Capacity
The curve shown in the figure is a suitably simple yet accurate approximation to the data; its equation is given by:
τ= = 0.29 * 1.15 * sin(IRHD) ≈ sin (IRHD) … 5.2
3
in which the sine function is based on degrees angular measure.
Equations 5.1.and 5.2 are combined and the coefficients rounded off for the purposes of the design/assessment provisions in Section 5.2.
Two other sets of results, not shown on Figure 5.1, are worth mentioning. Firstly, the results of the tests conducted at Karlsruhe fall in the region of an interface pressure of 2.5 N/mm2 at a slightly higher shear capacity than Tests T1 and T2. Secondly, some ad hoc tests on flat plate specimens confirmed the µ=0.8 line for low interface pressures. Both sets of results indicate that the design guidance is conservative.
The design/assessment provisions include a factor of safety, Γ. For long term loads (eg. Gravity loads) applied to the clamp, Γ should not be taken as less than unity as such values could lead to creep of the liner material (recall Test T4A in Figure 4.2). Indeed, it should not be forgotten that the design/assessment provisions are essentially based on mean values of a relatively small data sample and therefore are not even characteristic or lower bound. However, as discussed below, environmental loads are short term in nature, and this allows a less onerous interpretation to be assigned to Γ.
32
Cyclic loading tests, especially those of half-cycle tests in T1C, reveal that the relative displacement of the clamp is almost entirely due to neoprene shear deformation. The response of the clamp took the form of closed hysteresis loops, with only negligible deformation following load removal, see Figure 4.19(e).
Rubber-like material is highly sensitive to creep, during which the material continues to deform under a given load. Figure 5.3, reproduced from Reference (2), shows that for a certain types of rubber, creep varies approximately linearly with the logarithm of time under load. It would appear that the durations of the cyclic tests were such that no substantial creep occurred, and that this is the essential difference between the cyclic and quasi-static tests. In the design/assessment provisions, it is therefore recommended that the factor of safety Γ may be taken as unity for designing clamps subject to environmental loads. For assessment purposes of existing clamps, a factor lower than unity may be justified for the storm event. This is because the storm event occurs infrequently and very minor slippage (certainly less than 0.1 mm) does not have any significant structural consequence. The data presented in Figure 4.19(e) would suggest that a clamp load of 200 kN should be perfectly acceptable which, when compared with the quasi-static limit of 150 kN, leads to an allowable Γ of 150/200 = 0.75.
Figure 5.3: Shear Creeping Curves for Different Rubber Materials
The provisions in Section 5.2 for torsional and combined axial/ torsional loads follow the Phase 1 findings and recommendations.
5.2 Design/Assessment Guidelines
Base on the above observations, design guidance can be formulated as follows. The clamp slip capacity under axial load alone has been updated with the test results in Phase II. The clamp slip capacities under either torsional moment or combined axial and torsional loading are reproduced from Phase I.
33
� ��
� � ��
(i) Slip capacity of one clamp half under axial load alone:
P = αDL when 3.0 N/mm2 ≤ q ≤ 8.5 N/mm2
c Γ
2 when q < 3.0 N/mm2α DL q2 q
3 9 Pc = −
Γ
In the above:
α is a limiting stress to be taken as = α sin (IRHD) (degree angular measure). 3
D and L are respectively the tubular diameter and length of the clamp, both to be expressed in units of millimetres to give Pc in unit of Newtons.
q is the radial pressure at the neoprene liner/tubular interface, to be calculated as:
9 = FB , where FB is the total stud bolt load. DL
Γ is a factor of safety which is selected according to the following:
i. For long term (gravity) loads, Γ should not be taken as less than unity.
ii. For designing new clamps for environmental loads, Γ = 1.0.
iii. For the assessment of existing clamps for environmental loads, Γ less than unity may be used with caution. On the basis of the project results, Γ = 0.75 may be acceptable.
The total axial capacity of a clamp is thus 2Pc, but note that in many situations the axial load is transferred to only one clamp half in the first instance.
The above formulation assumes that there is no interference, i.e. that the tubular outside diameter is not greater than the inside diameter of the neoprene liner. If there is interference, a lower capacity may result.
(ii) Slip capacity of clamp under torsional moment:
Mc = Pc D
Where Pc is defined in item (i) and D the tubular diameter.
Note that the value of Mc above is the total torsional capacity. Because the axial stiffness of stud bolts is much greater than the circumferential shear stiffness of neoprene, the applied torsion is effectively resisted by both halves of the clamps even where the torsional loading is applied to only one half in the first instance.
34
(iii) Combined axial and torsional loading:
The applied axial force P and torsional moment M should satisfy the following inequality:
P M+ ≤ 1.0Pc Mc
with Pc and Mc evaluated as above.
35
6. CONCLUDING REMARKS
A programme of slip tests have been carried out on neoprene-lined clamps as used in offshore applications. The programme of Phase II consisted of six axial slip tests under either quasi-static loadings or cyclic loadings that simulate wave action in the UK Southern North Sea. The following parameters were investigated in the Phase II test programme:
•= Bolt pre-load
•= Neoprene hardness
•= Cyclic loading effects
The Phase II tests, with lower bolt loads than those in Phase I, have allowed the conservatism of the Phase I design guidelines to be removed. This is important as many existing clamps have neoprene/steel interface pressures corresponding to lower bolt loads.
The tests with clamps having different neoprene hardness (IRHD value) have confirmed that hardness affects capacity.
The cyclic loading tests indicate that at the design capacity, the relative displacement of the clamp and member is recoverable. In other words the displacement is largely due to neoprene shear deformation as opposed to true slip. It is only when the loads are applied statically that time dependent phenomena such as creep are manifested.
Design guidance has been formulated based on the results of both Phase I and Phase II test programmes, see Section 5.2. The provisions encapsulate the above observations. It is recommended that the factor of safety be adjusted depending on whether quasi-static or dynamic loading is being considered. A further relaxation may be used if the clamp is existing, as opposed to a new design.
36
CH10010R006 Rev 1 January 2001
REFERENCES
1. MSL Engineering Limited. “Development of Design Guidance for Neoprene-Lined Clamps for Offshore Application”. JIP Phase I Final Report, Doc. Ref. CH10010R005, Rev 1, May 1999.
2. The Malaysian Rubber Produces’ Research Association, “Engineering Design with Natural Rubber”, NR Technical Bulletin, 5th Edition, ISSN-0956-3856, 1992.
Printed and published by the Health and Safety ExecutiveC30 1/98
CH10010R006 Rev 1 January 2001
FIGURES FOR SECTION 4
Printed and published by the Health and Safety ExecutiveC30 1/98
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
Fig
ure
4.1:
L
oad-
Slip
Res
pons
e fo
r T
ests
T1
and
T4
(Rep
rodu
ced
from
Pha
se 1
Fin
al R
epor
t)
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
Fig
ure
4.2:
L
oad-
Slip
Res
pons
e fo
r T
ests
T1
and
T4A
(R
epro
duce
d fr
om P
hase
1 F
inal
Rep
ort)
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
050100
150
200
250
300
350
400
450
05
1015
2025
Cla
mp
slip
(m
m)
Applied axial load (kN)
Load
ing
Cyc
le 2
(hi
gher
load
ing
rate
)
Load
ing
Cyc
le 1
(lo
wer
load
ing
rate
)
Line
ar (
Initi
al s
lope
)
Fig
ure
4.3:
L
oad-
Slip
Res
pons
e fo
r T
est
T1A
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
050100
150
200
250
300
350
400
05
1015
2025
30
Cla
mp
slip
(m
m)
Applied axial load (kN)
Load
ing
Cyc
le 2
(hi
gher
load
ing
rate
)
Load
ing
Cyc
le 1
(lo
wer
load
ing
rate
)
Line
ar (
Initi
al s
lope
)
Line
ar (
Initi
al s
lope
)
Fig
ure
4.4:
L
oad-
slip
Res
pons
e fo
r T
est
T1B
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
405060708090100
110
120
130
-50
510
1520
25
Cla
mp
slip
(m
m)
Bolt load (kN)
Fig
ure
4.5:
B
olt
Pre
-Loa
d V
aria
tion
dur
ing
Tes
t T
1A
3
24
6
7
1
8
5
1
58
4
CH
1001
0R00
6 R
ev 1
Jan
uary
200102040608010
0
120
-50
510
1520
2530
Cla
mp
slip
(m
m)
Bolt load (kN)
Fig
ure
4.6:
A
vera
ge B
olt
Loa
d V
aria
tion
for
Tes
ts T
1A a
nd T
1B
Tes
t T1A
(F b
=0.2
Fy)
Tes
t T1B
(F b
=0.1
Fy)
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
-1000
100
200
300
400
500
-50
510
1520
25
Axi
al D
ispl
acem
ent o
f Cla
mp
Rel
ativ
e to
Pip
e (m
m)
Applied Axial Test Load (kN)
Fig
ure
4.7:
L
oad-
Slip
Res
pons
e fo
r T
ests
T1,
T2,
T1A
and
T1B
Tes
t T2
(Fb=
0.5
F y)
Tes
t T1
(Fb=
0.4
F y)
Tes
t T1B
(F b
=0.1
Fy)
Tes
t T1A
(F b
=0.2
Fy)
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
0
100
200
300
400
500
600
05
1015
2025
3035
Axi
al D
ispl
acem
ent o
f Cla
mp
Rel
ativ
e to
Pip
e (m
m)
Applied axial load (kN)
Test
T18
(IRH
D 7
0)
Test
T18
A(IR
HD
50)
Test
T1A
(IRH
D60
)
Fig
ure
4.8:
Loa
d-Sl
ip R
espo
nse
for
Tes
ts T
1A, T
18 a
nd T
18A
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
0
0.02
0.04
0.06
0.08
0.1
0.12
Ap parent F rict i on C oeff icient
40
45
50
55
60
65
70
75
Neo
pre
ne
Har
dn
ess
(IR
HD
)
Fig
ure
4.9:
C
orre
lati
on b
etw
een
neop
rene
har
dnes
s an
d th
e ap
pare
nt f
rict
ion
coef
fici
ent
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
9095100
105
110
115
120
125
130
-3-2
-10
12
3
Cla
mp
slip
(m
m)
Bolt load (kN)
Fig
ure
4.10
: B
olt
Loa
d V
aria
tion
in T
est
T4C
(St
ep 5
)
1
58
4
18
4
5
3
7
2 6
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
110
111
112
113
114
115
116
117
118
-4-3
-2-1
01
23
4
Axi
al D
ispl
acem
ent o
f Cla
mp
Rel
ativ
e to
Pip
e (m
m)
Bolt load (kN)
Fig
ure
4.11
: A
vera
ge B
olt
Pre
-loa
d V
aria
tion
of
One
Cyc
le f
or E
ach
Step
in T
est
T4C
Step
1
Step
2
Step
3
Step
4
Step
5 Step
6
Step
7
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
118
Bo lt Load (k N)
110
111
112
113
114
115
116
117
Tes
t T4C
(St
ep 1
)
Tes
t T1C
(St
ep 6
)
Tes
t T1C
(St
ep 1
)
-0.3
-0
.2
-0.1
0
0.1
0.2
0.3
Axi
al D
ispl
acem
ent o
f Cla
mp
Rel
ativ
e to
Pip
e (m
m)
Fig
ure
4.12
(a)
: A
vera
ge B
olt
Pre
-loa
d V
aria
tion
of
24 k
N A
mpl
itud
e C
ycle
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
112
113
114
115
116
117
118
Tes
t T4C
(St
ep 2
)
Tes
t T1C
(St
ep 2
)
Tes
t T1C
(St
ep 7
)
Bolt Load (kN)
-0.6
-0
.4
-0.2
0
0.2
0.4
0.6
Axi
al D
ispl
acem
ent o
f Cla
mp
Rel
ativ
e to
Pip
e (m
m)
Fig
ure
4.12
(b)
: A
vera
ge B
olt
Pre
-loa
d V
aria
tion
of
51 k
N A
mpl
itud
e C
ycle
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
118
Bo lt Load (k N)
112
113
114
115
116
117
Tes
t T4C
(St
ep 3
)
Tes
t T1C
(St
ep 3
)
Tes
t T1C
(St
ep 8
)
-1.5
-1
-0
.5
0 0.
5 1
1.5
Axi
al D
ispl
acem
ent o
f Cla
mp
Rel
ativ
e to
Pip
e (m
m)
Fig
ure
4.12
(c)
: A
vera
ge B
olt
Pre
-loa
d V
aria
tion
of
99 k
N A
mpl
itud
e C
ycle
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
Tes
t T4C
Ste
p 4
Tes
t T1
4
Tes
t T1C
(St
ep 9
)
112
112.
5
113
113.
5
114
114.
5
115
115.
5
116
116.
5 Bolt Load (kN)
C S
tep
-2-1
.5-1
-0.5
0
0.5
1 1.
5 2
Axi
al D
ispl
acem
ent o
f Cla
mp
Rel
ativ
e to
Pip
e (m
m)
Fig
ure
4.12
(d)
: A
vera
ge B
olt
Pre
-loa
d V
aria
tion
of
150
kN A
mpl
itud
e C
ycle
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
112
113
114
115
Bolt Load (kN)
Tes
t T4C
(St
ep 5
)
Tes
t T1C
(St
ep 5
)
Tes
t T1C
(St
ep 1
0)
111.
5
112.
5
113.
5
114.
5
115.
5
-3-2
-10
1 2
3
Axi
al D
ispl
acem
ent o
f Cla
mp
Rel
ativ
e to
Pip
e (m
m)
Fig
ure
4.12
(e)
: A
vera
ge B
olt
Pre
-loa
d V
aria
tion
of
188
kN A
mpl
itud
e C
ycle
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
-250
-200
-150
-100-5
0050100
150
200
250
-3-2
-10
12
3
Cla
mp
slip
(m
m)
Applied axial load (kN)
Fig
ure
4.13
: L
oad-
Slip
Res
pons
e of
Ste
p 5
for
Tes
t T
4C (
188
kN A
mpl
itud
e C
ycle
)
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
0
100
200
300
-4
-3
-1
0 1
2 3
4
l
Step
1
Step
4
-300
-200
-100
-2
Appied ax ial load ( k N)
Step
2
Step
3
Step
5
Step
7
Step
6
Axi
al D
ispl
acem
ent o
f Cla
mp
Rel
ativ
e to
Pip
e (m
m)
Fig
ure
4.14
: L
oad-
Slip
Res
pons
e of
One
Cyc
le f
or V
ario
us T
est
T4C
Loa
d St
eps
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
-200
-1000
100
200
-2.5
-2-1
.5-1
-0.5
00.
51
1.5
22.
5
Axi
al D
ispl
acem
ent o
f Cla
mp
Rel
ativ
e to
Pip
e (m
m)
Applied Axial Load (kN)
T4C
T1C
Fig
ure
4.15
(a)
: L
oad-
Slip
Res
pons
e fo
r T
ests
T4C
and
T1C
(St
ep 1
)
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
-200
-1000
100
200
-2.5
-2-1
.5-1
-0.5
00.
51
1.5
22.
5
Axi
al D
ispl
acem
ent o
f Cla
mp
Rel
ativ
e to
Pip
e (m
m)
Applied Axial Load (kN)
T4C
T1C
Fig
ure
4.15
(b)
: L
oad-
Slip
Res
pons
e fo
r T
ests
T4C
and
T1C
(St
ep 2
)
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
-200
-1000
100
200
-2.5
-2-1
.5-1
-0.5
00.
51
1.5
22.
5
Axi
al D
ispl
acem
ent o
f Cla
mp
Rel
ativ
e to
Pip
e (m
m)
Applied Axial Load (kN)
T4C
T1C
Fig
ure
4.15
(c)
: L
oad-
Slip
Res
pons
e fo
r T
ests
T4C
and
T1C
(St
ep 3
)
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
-200
-1000
100
200
-2.5
-2-1
.5-1
-0.5
00.
51
1.5
22.
5
Axi
al D
ispl
acem
ent o
f Cla
mp
Rel
ativ
e to
Pip
e (m
m)
Applied Axial Load (kN)
T4C
T1C
Fig
ure
4.15
(d)
: L
oad-
Slip
Res
pons
e fo
r T
ests
T4C
and
T1C
(St
ep 4
)
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
-200
-1000
100
200
-2.5
-2-1
.5-1
-0.5
00.
51
1.5
22.
5
Axi
al D
ispl
acem
ent o
f Cla
mp
Rel
ativ
e to
Pip
e (m
m)
Applied Axial Load (kN)
T4C
T1C
Fig
ure
4.15
(e)
: L
oad-
Slip
Res
pons
e fo
r T
ests
T4C
and
T1C
(St
ep 5
)
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
050 0
1 2
3 4
5
i
il
i ll
l
li
-50
100
150
200
250
300
-0.5
0.
5 1.
5 2.
5 3.
5 4.
5
Applied Axal Tes t Loa d ( kN)
T4C
- T
ens
e P
eak
T4C
- C
ompr
ess
ve P
eak
T1A
- L
oad
Cyc
e 1
(ow
oadi
ng r
ate)
T1A
- L
oad
Cyc
e 2
(hgh
load
ing
rate
)
Axi
al D
ispl
acem
ent o
f cla
mp
Rel
ativ
e to
Pip
e (m
m)
Fig
ure
4.16
: L
oad-
Slip
Res
pons
e fo
r T
ests
T1A
and
T4C
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
250
Ap plied Axial Tes t Lo ad ( k N) 20
0
150
100 50 0
-50
-100
-150
-200
-250
0 1
2 3
4 5
6 7
8 9
10 11
12 13
14
Ste
p 10
of T
est 1
C
Ste
p 5
of T
est 1
C
Tim
e (S
econ
d)
Fig
ure
4.17
: A
pplie
d A
xial
Loa
d of
Ste
p 5
in T
est
T1C
(O
ne C
ycle
)
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
-1000
100
200
-0.5
00.
51
1.5
22.
5
Axi
al D
ispl
acem
ent o
f Cla
mp
Rel
ativ
e to
Pip
e (m
m)
Applied Axial Load (kN)
Fig
ure
4.18
: L
oad-
Slip
Res
pons
e of
Ste
p 10
in T
est
T1C
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
Applied Ax ial L oad (kN ) 50
-0.5
0.
5
-50
Axi
al D
ispl
acem
ent o
f Cla
mp
Rel
ativ
e to
Pip
e (m
m)
25 0 -0
.25
-25
0 0.
25
T4C
-Ste
p 1
T1C
-Ste
p 1
T1C
-Ste
p 6
Fig
ure
4.19
(a)
: L
oad-
Slip
Res
pons
e of
Ste
p 1
in T
ests
T4C
and
T1C
and
Ste
p 6
in T
est
T1C
(O
ne C
ycle
)
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
-100-50050100
0 1
-1
-0.5
0.
5
Applied Ax ial Load (kN)
T4C
-Ste
p 2
T1C
-Ste
p 2
T1C
-Ste
p 7
Axi
al D
ispl
acem
ent o
f Cla
mp
Rel
ativ
e to
Pip
e (m
m)
Fig
ure
4.19
(b)
: L
oad-
Slip
Res
pons
e of
Ste
p 2
in T
ests
T4C
and
T1C
and
Ste
p 7
in T
est
T1C
(O
ne C
ycle
)
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
Applied Ax ial L oad (kN ) 15
0
-1
1
-150
Axi
al D
ispl
acem
ent o
f Cla
mp
Rel
ativ
e to
Pip
e (m
m)
75 0
-75
-0.5
0
0.5
T4C
-Ste
p 3
T1C
-Ste
p 3
T1C
-Ste
p 8
Fig
ure
4.19
(c)
: L
oad-
Slip
Res
pons
e of
Ste
p 3
in T
ests
T4C
and
T1C
and
Ste
p 8
in T
est
T1C
(O
ne C
ycle
)
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
0 -2
-1
.5
-1
-0.5
0
1 2
-200
-100
100
200
0.5
1.5
Applied Ax ial Lo ad ( kN)
T4C
-Ste
p 4
T1C
-Ste
p 4
T1C
-Ste
p 9
Axi
al D
ispl
acem
ent o
f Cla
mp
Rel
ativ
e to
Pip
e (m
m)
Fig
ure
4.19
(d)
: L
oad-
Slip
Res
pons
e of
Ste
p 4
in T
ests
T4C
and
T1C
and
Ste
p 9
in T
est
T1C
(O
ne C
ycle
)
CH
1001
0R00
6 R
ev 1
Jan
uary
2001
-200
-1000
100
200
0 1
2-2
.5
-2
-1.5
-1
-0
.5
0.5
1.5
2.5
Applied Ax ial Load (kN)
T4C
-Ste
p 5
T1C
-Ste
p 5
T1C
-Ste
p 10
Axi
al D
ispl
acem
ent o
f Cla
mp
Rel
ativ
e to
Pip
e (m
m)
Fig
ure
4.19
(e)
: L
oad-
Slip
Res
pons
e of
Ste
p 5
in T
ests
T4C
and
T1C
and
Ste
p 10
in T
est
T1C
(O
ne C
ycle
)
Printed and published by the Health and Safety ExecutiveC30 1/98
Printed and published by the Health and Safety Executive C1.25 10/02
ISBN 0-7176-2577-X
RR 031
780717625772£25.00 9