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DNV GL © 2013

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02 November 2016 SAFER, SMARTER, GREENERDNV GL © 2013

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02 November 2016Mike White

OIL & GAS

Site-Specific Assessment of Jack-UpsPart 2: Background to Code Developments - SNAME & ISO

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Noble Denton marine services

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Overview

Some history.

Development of SNAME T&R Bulletin 5-5A.

The IADC Jack-Up Committee.

Development of ISO 19905-1.

Some Conclusions.

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The Early Days

Early designs, e.g., LeTourneau Scorpion (1955) …. limited calcs.

As early as 1957, considerable knowledge of the issues existed, but ...

the tools to address the important effects, dynamics, P-Delta & spudcan fixity remained limited.

The demands of rapid turn-round and the lack of tools made rigorous assessment difficult and results were inconsistent.

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The Early Days

The first Class Rules for mobile offshore units were published by ABS in 1968.

Marine Warranty Surveyors developed guidelines and made checks at one level or another.

The first IMO MODU Code was published in 1980.

By which time owners often obtained a 3rd party review of new designs, which were also Classed.

Few failures, mostly unrelated to site-assessment.

The simplified trade-off: Dynamics = Fixity + Conservative Structural Design

was usually OK !

Foundations & blowout greatest part of in-situ risk.

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The “Shell Study”

In late 1980’s Shell developed guidance for jack-ups, which was shared with industry. A step forward, but felt by some to be too conservative. Inconsistency between assessors was observed. In 1987 Shell commissioned a study:

‘Jack-Up Operations – Comparative Study of Assessment Procedures’with 51 companies completing a general questionnaire,

16 full day interviews and 14 of the interviewees doing 1 or 2 test-case analyses of fictitious jack-ups.

The study, presented late 1988, showed a large spread of results. Some of this was due to differing methods of reporting, and some due to differing

assumptions and methodologies. Some differences were resolvable, others not. A number of areas were identified

for further study. It was decided to launch a JIP, supported by industry and guided by a Work

Group, comprising 4 representatives from: Oil Companies, Designers, Drilling Contractors & Class.

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The Development of SNAME T&R Bulletin 5-5AThe Site-Assessment JIP

The aim: to develop a site-assessment procedure through consensus.

The Work Group met for at least 2 days about 4 times/year from 1989 to 1994.

A philosophical Guideline (published 1989) set out the overall approach; this was at a level that should need no revision. It formed the basis of the normative clauses in ISO 19905-1. The key principle was that the level of analysis should match the need. However, in all cases the foundation must be checked.

Guideline supported by more technical Recommended Practice, a Commentary and a “Go-By” example calculation that has been invaluable to many.

Developed by specialist sub-committees for: hydrodynamics, foundations, structural modelling, dynamics, and assessment criteria. Most undertook funded studies to improve knowledge in their area.

Acceptance criteria based on exemplary cases. When the Practice was published, it used a conservative environmental load factor of 1.25 (for use with 50-year individual metocean extremes), as the more logical load factor of 1.15 was likely to be seen as “overly adventurous”.

1.15 was eventually adopted in Rev. 2 in 2002.

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The Site-Assessment JIP (continued)

The Guideline, RP, Commentary & Go-By published as SNAME T&R Bulletin 5-5A. Represented a 5-year effort and US$3million (1993 values) plus the many self-

funded man-hours & expenses of the participants. Rev. 1 (1997) included the results of further work on foundation fixity. Rev. 2 (2002) reduced environmental load factor; more on fixity. Rev. 3 (2008) alternative wave spreading allowed, updated fixity & clarified LF. Final update/corrigenda to Rev. 3 (2015).

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Phase 0Guideline, RP, Commentary & Go-By

14 drafts! 491 pages!

1989 19

94

1997

Rev

1

2002

Rev

2

Guideline13 pages

2008

Rev

3

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The IADC Jack-up Committee (IJUC)Developments Post SNAME T&RB 5-5A Rev. 0

After publication of T&RB 5-5A the drilling contractors (IADC) formed a Jack-Up Committee (IJUC) to advance the site-assessment process. This funded much of the work undertaken in the following areas:– Hindcast analysis of jack-ups in hurricane Ivan – indicated the SNAME 1994 was

overly conservative.– Wave spreading / short crestedness – greater benefit than that given in SNAME.– Load factor assessment – the original 1.25 load factor (with 50-yr independent

extremes) conservative.– Pushover analyses – when the foundation is adequate jack-ups have a good RSR.– Leg chord test – the results are still unpublished, but indicated that the methods

developed for SNAME for hybrid chords with a single axis of symmetry are OK or conservative.

– Deep spudcan penetration soil (fixity) study – shallow foundation capacity and degradation model conservative for deep penetrations.

– Instrumentation and effects of foundation degradation on dynamics – foundation degradation less that expected - instrumentation & hurricane back-analysis studies still not finalized.

– Gulf of Mexico Sudden Hurricane Study – to determine metocean data for the “still just manned” case (used in the GoMex regional Annex to ISO 19905-1).

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The Development of the ISOWhere do the ISO TC67/SC7 ISO 19900-series standards sit?

19900-series is part of a much larger suite of standards for P&NGI

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Surface Equip. Materials

Mechanical Equip.

Piping & Valves

Pipelines

Subsea Equipment

Drilling Equipment

19900-series SC7 Standards

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The ISO TC67/SC7 ISO 19900 Series Standards

ISO 19900 General requirements for offshore structures ISO 19901-1 Specific requirements for offshore structures - Part 1: Metocean design and operating

considerations ISO 19901-2 Specific requirements for offshore structures - Part 2: Seismic design procedures and criteria ISO 19901-3 Specific requirements for offshore structures - Part 3: Topsides structure ISO 19901-4 Specific requirements for offshore structures - Part 4: Geotechnical and foundation design

considerations ISO 19901-5 Specific requirements for offshore structures - Part 5: Weight control during engineering and

construction ISO 19901-6 Specific requirements for offshore structures - Part 6: Marine operations ISO 19901-7 Specific requirements for offshore structures - Part 7: Stationkeeping systems for floating

offshore structures & mobile offshore units ISO 19901-8 Specific requirements for offshore structures - Part 8: Marine soils investigation (under preparation) ISO 19901-9 Specific requirements for offshore structures - Part 9: SIM Fixed Steel (under preparation) ISO 19902 Fixed steel offshore structures ISO 19903 Fixed concrete offshore structures ISO 19904-1 Floating offshore structures - Part 1: Mono-hulls, semi-submersibles and spars ISO 19904-2 Floating offshore structures - Part 2: Tension leg platforms (under consideration) ISO 19905-1 Site-specific assessment of mobile offshore units - Part 1: Jack-ups ISO 19905-2 Site-specific assessment of mobile offshore units - Part 2: Jack-ups commentary and detailed

sample calculation ISO 19905-3 Site-specific assessment of mobile offshore units - Part 3: Floating units (under preparation) ISO 19906 Arctic offshore structures

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Development of the ISOISO TC67/SC7/WG7 and ISO 19905-1 & -2

WG7 on site-specific assessment of MOU’s first met in 1996.

SC7’s remit was to use SNAME as the “base document” for ISO 19905-1.

The initial idea was to translate SNAME into “ISO-speak”.

Needed to harmonize with other 19900-series standards (and they with us), e.g.– Needed 50-year met data in 19901-1.– ISO 19902 fixed steel offshore structures standard includes advances in tubular

member checking, but this needed to be modified to the capacity-based approach that is used for jack-ups.

However it was recognized that the ISO should embrace the technical developments that were in progress. This proved challenging, especiallyfor the foundations panel.

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ISO TC67/SC7/WG7 and ISO 19905-1 & -2 (continued)

Technical Panels were formed:

P0 Preamble Mike Hoyle (DNV GL / GL ND / ND)

P1/2Metocean / loading (actions) Olav Mo (DNV)Graham Bagnell (Rowan)Dave Lewis (LEG)

P3 Structures (modelling, response, resistance) Bob Bowie (ABS)Pao-Lin Tan (ABS)

P4 Foundations Patrick Wong (ExxonMobil)

P5 Seismic (earthquake) Doug Stock (Digital Structures)

P10 Acceptance checks / p. factors / calibration Mal Sharples (ABS)John Stiff (ABS Consulting)

P11 Long-term applications (fatigue/inspection) Gregers Kudsk (Maersk Drilling)

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Benchmarking for usability, completeness and comparison with latest SNAME (Rev. 3 + final update/corrigenda) - see Stiff & Lewis, OTC 22949

Benchmarking of the new leg member structural checks. Phase 1: A-Z benchmarking of a draft. Phase 2: Full benchmarking of the Dec 2009 post-DIS / draft-FDIS

version.– ISO generally a little less conservative than SNAME Rev. 3 (including the effect

of the deep penetration foundation changes that did not make it into Rev. 3).– Some significant differences in foundation checks – partly due to differing

interpretations of SNAME. The DIS text was revised and the checks repeated, resulting in differences that were judged to be acceptable.

– The bench-markers also provided some 270 comments.

ISO 19905-2, provides back-up from SNAME commentary, and includes a detailed sample “Go-By” calculation by one of the benchmark cases.

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Developments in ISO 19905-1 - Loading – see Dowdy et al. OTC 23342

Effects of wave spreading (and randomness).

Application of intrinsic & apparent wave periods.

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Developments in ISO 19905-1 - Loading

Effects of spreading (and randomness) - see also Hoyle et al. OTC 20297

Directional spreading produces short-crested waves.

Wave energy propagates in directions around the principal direction.

Peak particle velocities reduce as the waves are more spread.

The loads are, therefore, reduced.

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Effects of spreading (and randomness)

ISO 19901-1: latitude dependent directional spreading factor (at a point).

Applied as Kinematics Reduction Factor (KRF).

SNAME uses a fixed wave-height reduction factor (Hdet = 0.86 Hmax).

Both are applied with regularwave theories.

Spatial effects are ignored in both.

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0.86

0.87

0.88

0.89

0.90

0.91

0.92

0.93

0.94

0.95

-90 -60 -30 0 30 60 90Latitude, (degrees)

ISO

Dire

ctio

nal S

prea

ding

Fac

tor,

Tropical Cyclones <40°

Low Latitude Monsoons <15°

=0.88

=0.867

Extratropical Storms 36°<| <72° =1.0193-0.00208|

=51° =0.913

=61° =0.892

Nor

th

S

ea

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Leg spacing Jack-Up > Jacket:

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A parametric study undertaken for a range of:

– Leg spacing (jack-up class), SY

– Feasible water depths, d

– ISO spreading factor,

– Wave heights, H

– Wave length, L0 (or L)

Formulation for KRF:

Limitations:

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22

maxmax22

2 303.1640.0658.080.045.1043.0426.0824.0

dH

dH

Ld

Ld

LS

LS yy

κ 43.008.0

LS y 76.014.0

Ld

58.007.0 max

dH

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Application of Intrinsic & Apparent wave periods

Use of the correct forcing period affects dynamic response.

Forcing period affected by current (Doppler effect).

Underlying wave kinematics function of no-current wave period (current does not change in physical wave length).

In SDOF/regular waves this can be easily addressed – but more challenging in random sea-states / irregular waves.

ISO-19901-1 includes some guidance that is wrong for random sea-states.

Guidance is included in 19905-1 to address this.

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Developments in ISO 19905-1 – Foundations – See Wong et al. OTC 23521

Embracing ongoing research proved challenging, particularly for the foundations panel. The foundation-related changes in 19905-1 include:

Site investigation requirements.

Bearing capacity formulations (accounting for backfill).

Initial spudcan foundation stiffness.

Yield surface models and stiffness reduction.

Spudcan foundation acceptance criteria.

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Developments in ISO 19905-1 – Foundations

Geotechnical site investigation requirements ISO 19905-1 provides non-prescriptive but specific guidance on:

– Bathymetry survey.– Sea floor survey.– Geotechnical site investigation.– Integration of geophysical and geotechnical

data. Annex D provides specific guidance on:

– Number of boreholes and PCPT test holes.– Position of boreholes and PCPT test holes.– Open location vs. platform location.– Geologic setting: simple, complex, very complex.

Notable clarification in guidance: site-specific geotechnical info is to be “obtained from or on behalf of the operator”.

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Bearing capacity formulations (accounting for backfill) Effects of soil buoyancy on spudcan below notional flat-plate bearing surface and

backfill included as explicit terms. At preload (VLo):QVo = VLo + WBF,o - BS

ISO includes the Hossain et al, 2006 method for determining backflow; SNAME uses a wall collapse method that is often wrong.

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Initial spudcan foundation stiffness SNAME Rev, 3 includes some

of the results of studies funded by IJUC that address the initial foundation stiffness. Cut off at G/Su = 400

removed in ISO.

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10

100

1000

1 10 100

Overconsolidation Ratio

G / s u

Case Histories

Recommendation for storm loading

Small Strain value for Fatigue

Rev. 2 Practice for storm loading

Case History, Adriatic III, including Tropical Storm JosephineRecommendation, extended to include T. S. Josephine CaseSmall Strain data, Gulf of Mexico clay

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Yield surface and stiffness reduction

ISO includes improved recommendations on yield surface and stiffness reduction, especially for deeper penetrations.

These changes also address, and benefit from, the change to using Gross capacity.

where:a = shaping parameter (from 0 at no penetration to 1 at deep penetration)

FV, FH, FM = applied loadsQV, QH, QM = maximum capacities

with QH and QM now taking much greater values (vs. prior SNAME Bulletin 5-5A) for deep penetrations in clay.

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0QF1

QFa4

QF1

QFa116

QF

QF

V

V

V

V2

V

V2

V

V2

M

M2

H

H

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Developments in ISO 19905-1 – FoundationsYield surface models

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0

0.2

0.4

0.6

0.8

1

0.5 0.6 0.7 0.8 0.9 1 1.1

Mom

ent C

apac

ity, R

elativ

eto

Max

imum

Vertical Force Capacity, Relative to Maximum

Force and Moment Capacity Interaction

Parabola (Bulletin 5.5)

Ellipse (Max El. EnergyTheory)

Finite Element Results

Fit to Finite Element Resultsusing Martin & Houlsby (2001)Equation 2

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V

QV

(QV-WBF,o+BS)/R,VH

+WBF,o-BS

WBF,o-BS

H

1

2

(FH,FV,)ORG = 0,5 QV / R,VH

(FH,FV)

QVH,f

4 3

QH/R,Hfc QH

(FH,FV)

QVH,f

5

U = | (FH,FV) - (FH,FV)ORG | / | QVH,f - (FH,FV)ORG |



Spudcan foundation acceptance criteria fully revised

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Developments in ISO 19905-1 – Modelling & Response – Tan et al. OTC 23040

Revised drag-inertia method – based on Perry et al, OMAE2011-50335.

– DAF from the Shell drag-inertia method enhanced when TN/TP > 0.6

– Not permitted when TN/TP ≥ 1.0

New restrictions on use of the SDOF method.

– DAF must always be ≥ 1.2

One-stage stochastic storm analysis.

– Process provided to scale inputs so that results are compatible with standard two-stage analysis.

Additional requirement for earthquake assessment in seismic areas.

Analysis precautions when operating close to resonance.

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Developments in ISO 19905-1 – Modelling & Response

Precautions for operating close to resonance:

SNAME only suggests inertial load in phase with wave/current load.

However, not always true as shown in a SDOF system.

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Developments in ISO 19905-1 – Modelling & Response

Precautions for operating close to resonance (continued):

To ensure critical load cases are addressed, ISO suggests:

– Regardless of the Tn/Tp ratio, run inertial load in phase with wave/current for all assessment load cases.

– When the Tn/Tp ratio > 0.9, consider additional assessment load cases covering the possibility that the inertial loading is 90 or 180 degrees out of phase with the wave loading.

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Developments in ISO 19905-1 – Member Checking – See Frieze et al OTC 23071

Harmonisation with other standards was part of the remit.

ISO 19902 fixed steel platform standard includes advances in tubular member checking, but needed to be modified to the capacity-based approach used for jack-ups (chords typically use a mix of yield strengths).

Improved SNAME treatment of:– Cross-sectional slenderness limits – were unfriendly, inconsistent, limited.– Effect of Stiffening to, e.g., side plates.– Excluding hydrostatic pressure from member checks.– Axial compression column curves (for HSS - often 700 N/mm2 / 100 ksi).– Lateral torsional buckling – new, more applicable (and less onerous)

formulation developed and adopted for ISO.– Circular/curved components of prismatic sections – present guidance ignores

benefit of central stiffening but no easy fix.

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Developments in ISO 19905-1 – Member Checking

Effect of Stiffening, e.g., side plates.

In SNAME, no guidance on how to deal with reinforcing plates in checking cross-sectional slenderness – what thickness to use for local buckling?

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a)

b)

c)

d)

341check efft t t

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SNAME excluded hydrostatic pressure from member checks. The ISO 19902 strength formulations are complicated by the inclusion of hydrostatic pressure.

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Study carried out to determine thresholds at which hydrostatic pressure can be safely ignored.

0.4

0.6

0.8

1.0

0 50 100 150

20 axial only20 bending only40 axial only40 bending only60 axial only60 bending only

Water depth (m)

D/t for Fy = 450 N/mm²

fa/Fy

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Guidance given in the form of limiting D/t versus hydrostatic head.

0

10

20

30

40

50

60

0 50 100 150 200

Data for 10% reduction in strength including safetyfactorsPower series approximation D/t = 211 d^(-0.335)

D/t ratio

Water depth d (m)

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Axial Compression Column CurvesIssues:

AISC reference document (used for chords / non-tubulars) reduced safety margin in resistance factor in 2005.

Wish to take advantage of more favourable Structural Stability Research Council (SSRC) P1 column curve for HSS (yield > 450 N/mm2 / 65 ksi).

Based on Industry practice and selection of SSRC curves P1 & P2, the revised resistance factor was adopted for both.

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Conclusions

Jack-up technology has developed considerably over the years, although many of the key features were known as far back as 1957.

Inconsistencies in the 1980’s resulted in the “Shell Study”, which led to the JIP that developed SNAME T&RB 5-5A, published in 1994.

Since then much research has taken place; SNAME has moved forward and ISO 19905-1 has been developed from it by the Industry’s technical experts.

The draft of ISO 19905-1 was subjected to considerable technical and editorial review and was successfully benchmarked.

There was a strong commitment to ensure the ISO is practical, technically rigorous and in conformance with ISO requirements.

Jack-up technology will continue to develop and 19905-1 will be updated accordingly.

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www.dnvgl.com

Thank you for your attention.

Any questions?

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Noble Denton Marine Assurance and Advisory

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