FPSOs & Mooring Systems David Brown, BPP-TECH London€¦ · David Brown, BPP-TECH London. FPSOs &...

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FPSOs & Mooring Systems

David Brown, BPP-TECH London

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David Brown, BPP-TECH London

• Professional Naval Architect and Mechanical Engineer

• Engineering Director at BPP-TECH, specialises in FPSOs & moorings

• Chairman of ISSC Floating Production Systems Committee (from 2000 – 2006)

– Committee mandate:“Concern for the design of floating production systems. Specific emphasis shall be given to

FPSO hulls and the recent industry experience that influences the design methodology. Consideration shall be given to identification and quantification of uncertainties for use in reliability methods.”

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COMPANY OVERVIEW

History: Established in 1981

Sectors: Oil & Gas, Renewables, Insurance

BPP-TECH – Specialist Engineering Consultancy provides:

� Concept and feasibility studies for floating offshore systems (vessels,

moorings, risers, pipelines, umbilicals, power cables)

� Procurement support for subsea systems (SURF)

� Engineering for insured risk reduction & claims (failure analysis)

� Offshore monitoring and instrumentation

BPP-CABLES - Formed recently to satisfy needs of offshore power cable

market

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Introduction

• Introduction• Expanding market• Vulnerability• Hardware• Codes• Environment

• Life cycle - design, installation, operation & maintenance, retirement

• Failure modes• Losses• Risk reduction

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Introduction

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Introduction

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Introduction

Drivers:

• Global demand for oil growing

• Middle East/N Africa supply disruption

• Oil prices at $100/bbl

• Deep water drilling rigs will boost fleet by 38%

• Huge pre-salt discoveries offshore Brazil

Existing fleet:

• 250 units in service, including TLPs, Spars & Semis

• Doubled since 2001

(ref IMA rpt– 3/11, Upstream 5/11)

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Introduction

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Introduction

Source www.offshore-mag.com

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Introduction

Source www.offshore-mag.com

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Expanding market

Source: Upstream 5/11

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Expanding market


New units

55 - current bids or final design(next 18 mths)

139 – planning phase

194- in total

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Expanding market


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Expanding market


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Expanding market

Prelude FLNG• 488m by 74m hull

• 600,000 Tonne

• 100m high turret

Source: The Engineer 09

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Expanding market


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Vulnerability

Vulnerability:

• Class Rules evolving from seafaring background

• Tanker conversions to FPSOs

• New developments – Speedier, smaller operators, remote environments, deeper water, more subsea assets, increased asset values

• Existing developments - Equipment replacement considered as repairs , obsolete class codes continue to be used, many legacy systems in place for many years (eg SPMs & offloading tankers)

• Limited redundancy in mooring system, chain as strong as its weakest link

• Difficult to inspect & maintain, degradation & retirement issues

• Rapid incident escalation (eg cascading failure) in hostile environments

• Consequences can lead to major loss including damage to subsea architecture

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Hardware

Dahlia FPSO


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Hardware

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Hardware

Spread mooring

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Hardware

Typical single point mooring - external turret

Source: API RP 2SK

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Hardware

Catenary anchor leg mooring – hawsers & fixed yoke (Source API RP 2SK)

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Hardware

DNV-OS-E301, Oct 2010

Most permanent moorings use studless chain

• Studded chain: studs loosen, crack & fracture at weld• Studless chain: ~10% lighter, same Tbreak, lower fatigue life

Diameter upto 6”1 link weighs 0.5T, 0.9m longKenter shackle not allowed for long term mooring

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Hardware

Typical wire rope & permanent mooring socket• Galvanized jacketed spin resistant is best, 30-35 yr

life expectancy• Ability to inspect? (API RP 2SK)

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Codes

Various authoritative documents on mooring initial design, eg:

• API RP 2SK (Oct 2005), GoM 2008 addendum

• DNV E301 (Oct 2010)

• ISO 19901-7 (2005) - new standard being developed

There is little guidance on detailed design or mooring operational phase. Some inspection standard & retirement criteria given in:

• API RP 2I 3rd ed.(Apr 2008)

• API RP 2SM – synthetic fibre lines

• Intl Assoc of Class Socs (IACS) No 38 Oct 2010

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Codes

Oil & Gas UK: Mooring Integrity Guidance 08

• Develop Mooring Integrity Management

System

• Recognise that manuf. & deployment

contribute to reduced life

• Strategy on operational intent

wrtinspectability, op & survival limits AND

plan if breached

• Detailed risk review

• Monitoring to confirm actual behaviour &

deviation from ‘as designed’

• Inspection driven by the risk review

• System to track findings & anomalies

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Environment

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Environment

Average number of Storms of STS severity and higher per annum in the South China Sea (1945-2009)

100 yrHs (typhoon)

• S China Sea - 13.6m

• Timor Sea - 5.5m

• Mooring design limit = 8m

• STS wind speed > 48 kn

• Annually high probability of ‘unavoidable’ annual storms have wind speed > mooring design limit

• high probability of wind speed > Typhoon Koppu (65 kn)

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Life cycle - design

Directionality :• W Africa (non correlated)• Other: Wave 0 deg, wind 30 deg,

current 45 deg

Offshore Standard DNV-OS-E301, Oct 2010

• Position Mooring – Criteria, guidelines on design & construction

• Environmental conditions & its direction• Loads - wind, wave & low freq (drift),

current, VIV• Mooring analysisULS (extreme env)ALS (failure of 1 line, 1 thruster, etc)FLS (line cyclic loading, steel)• Thruster assisted moorings• Mooring equipment inc. anchors & piles• Testing

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Life cycle - design

Design criteria: Tension limits & safety factors (API RP 2SK)

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Life cycle – operation & maintenance

API RP 2I – In-service Inspection of Mooring Hardwar e, Apr 2008

• Covers steel permanent moorings & fibre ropes• Also MODU moorings in tropical cyclone areas

Inspection – 3 systems (O&G UK Mooring Integrity Gui delines 2008)

• Divers, successful history but safety risks (near surface & saturated), dedicated dive vessel required

• ROVs, large work class vehicles working from welded down platforms on FPSO or work vessel, smaller ROVs becoming more versatile (eg can be transported by helicopter)

• AUVs, specifically inspect detailed condition of mooring system, coming onto the market (Norwegian JIP)

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Inspection systems

- Vessel & seabed sonar- Hull mounted cameras- Pressure sensors- Tension monitoring by turret strain- Tension measurement in line- Strain measurement- ACSM (Alternating Current Stress)- Tension monitoring near touchdown- Seismic detection- Tension monitoring at stopper- Hydro-acoustic- Strain wire

O&G UK mooring integrity guidelines 2008

EgWelaptega Chain Measurement System

• ROV based• Wear, corrosion & pitting, plus link/link erosion• Establishes link length & bar stock diameter• Compares data with rejection criteria• Ageing & degradation can be established

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Visual Inspection:

• Link deformations

• Corrosion

• Interlink contact zones

• Welds

• Wear marks/cracks etc

Chain no.: Link no.:

Observations:

Images:

Factors noted on each link:

Stud condition:- missing stud: - loose stud: - Gap between stud and link:

Corrosion state of the stud:

Bend in link:

Abrasion/wear:

Corrosion state:

Are there any visible cracks:

Measurements (mm):

Length (L) = D2L =

Breadth (B) = D2W =

D1L = D3 =

D1W =

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Line failure

Hazard Failure Group

Corrosion General, Galvanic, Biological /SRB,Chemical, Hydrogen embrittlement

Contact Vessels, Seabed, Dropped object, During inspection

Motion of attachment point or external item, Seabed scouring

Manufacturing

Deployment

Hazard Failure Group

Input Data Seabed, Metocean, Vessel, Mooring comps

Design Code, Method, Spec

Strength SoF, Snatch load, Bend radius, Means of securing

Wear /erosion With connected item, Internal, with seabed

Fatigue Axial, Bending, Torsion


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Line failure

Failure Modes defined for each Failure Group, eg: Input Data

• Metocean (occurred & contributed to failure)

• Weather data not up to date• Data from too short observation period• Poor density or sea temp information• Insufficient data on LAT, tide & storm ranges• Squall risk not identified• No time series data (wind, wave, current combinations)


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Line failure

Failure Modes defined for each Failure Group, eg: Deployment

• Deployment (occurred & contributed to failure)

• Physical damage due to poor handling• Side loading on chain links & shackles, increased SCF• Cold bending or local heating (spot welding) causing reduced fatigue life• Wrong shackle sizes• Inappropriate deployment, lines dog-legged• Anchors non-aligned to pull direction• Worn pockets in chain gypsies, causing link bending


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Line failure

Common problems observed in used wire ropes

• Broken wires• Wear• Corrosion• Loss of lubrication• Change in rope diameter • Distortion of the rope

(kinking, bending, scrubbing, crushing, flattening, birdcaging)

• Thermal damage

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Source: HSE 2006 study

Main Factors Influencing Long-Term Mooring Integrity

Also inspection and maintenance, mooring jewellery


Line failure

Terminations:

• Hawse tubes - Highest tension with additional bending, twisting stresses &contact wear of the links

• Touchdown - Heavy contact with sea floor which may contain rock of comparable hardness to steel -> severe localized wear

• Touchdown - Accelerated corrosion (aerobic) as chain moves above & below mudline, parent metal exposure

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Line failure

Disconnectable assets

• Remaining on station during hurricane events

• Remaining connected during monsoons

• Inability to disconnect during short notice typhoons

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Line failure

Source: API

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Line failure

Mooring degradation

• disconnectable

vessels ‘riding-out’

storms causing accel

degradation

• Mooring systems

operating beyond

design life

• Moorings designed

to out-dated codes

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Line failure

Indicative stats (based on N Sea FPSOs)

• 50% of units cannot monitor line tensions in real time,

• 33% of units cannot measure offsets from the no-load equilibrium position,

• 78% of units do not have line failure alarms,

• 67% of units do not have mooring line spares available,

• 50% of units cannot adjust line lengths.

Single line failure costs

• 50,000 bpd N. Sea FPSO £2m

• 250,000 bpd W. African FPSO £10.5m

Source: HSE 2006 study

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Losses

Girassol Offloading Buoy Mooring

Spring 2002, buoy broke free of moorings

Chain failed in buoy's hawser, due to fatigue loading

Lessons learnt (Design) – new failure mode, chain out of plane bending under tension in hawser

(Ref HSE Report: Floating production system -JIP FPS mooring integrity)

44Case Study

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Losses

Recent GoM tropical storms

• 2004 Ivan, 2005 Katrina & Rita, 2007 Gustav & Ike

• Numerous drilling rigs broke moorings, 255 pipelines damaged in Katrina, 206 in Rita

• Lessons learnt – design to 10 yrenv, API GoM 2008 addendum, top end line failure, anchor drag (soils)

(Source - thomko.squarespace.com)

(Source – www.nasa.gov)

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Losses

Retrieved subsea arches (Upstream 9/11)

Gryphon

• Loss 2/11• Winds > 55 kn, 9m waves,

10 leg mooring• Cascading events.. low

tension line failure, DP miscalculates wind & wave forces, turns FPSO beam on, 21 deg roll, 3 more legs lost, blackout, 200m movement ripping prod & inj lines & umbilicals

• 2008 chain failure• 2009 DP problems (software)

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Losses

Cascade Chinook

• Loss 3/11• Faulty weld repair causes

fracture in single chain link near butt weld

• Post-heat treat made chain vulnerable to hydrogen induced stress cracking

• Chain was 6.25”• 440T air can supporting

hybrid riser released

• Upstream 5/11• Pictures: www.freepublic.com

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Risk reduction

Questions to the assured:

1. Are Oil & Gas UK Mooring Integrity Guidance (2008) procedures followed?

2. Are line tensions and offsets monitored & stored?

3. Are there functioning line failure alarms on board?

4. Are line spares available?

5. Can line lengths be adjusted?

6. What inspection/reclassification is carried out should line tensions exceed

design limits (eg drift off – DP issue or non-disconnect)?

7. What is the policy on monitoring and retirement of ageing assets?

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CLIENTS

For more info please contact:

David Brown

www.bpp-tech.com

Date post:	13-May-2018
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