Maintaining Integrity of FPSO Mooring System · 2017-11-02 · of the mooring leg and, potentially, the whole mooring system depends on each component in the line. This combination

Maintaining Integrity of FPSO Mooring

System Dr Dmitry Sadovnikov, Piotr Sujkowski, Dr Yuriy Drobyshevski

INTECSEA (Perth), WorleyParsons Group

ABSTRACT

This paper demonstrates the importance of having ready a mooring integrity management

plan, including a mooring failure rapid response plan, to ensure that potential anomalies on a

moored FPSO can be addressed quickly with minimum impact on production. An overview of

a typical mooring integrity management plan that includes the sparing strategy, mooring

replacement procedures and plans for mobilisation of construction vessels is presented for

FPSO spread mooring and turret mooring systems. Two major components of the mooring

failure rapid response plan, the assessment of the mooring system residual capacity after

mooring leg failure and the development of the mooring line replacement procedures, are

illustrated using two examples. The mooring residual capacity study, undertaken for a generic

FPSO with a disconnectable turret mooring system operated in the North West Shelf of

Western Australia, has been used as an example. This paper shows the methodology and main

results from the advanced mooring analysis used to determine the residual capacity of the

mooring system and the associated limiting weather conditions, which may be used when

deciding whether FPSO can safely continue to operate after the loss of one mooring line until

permanent repairs are carried out. An example of generic mooring line replacement

procedures is shown for a spread-moored FPSO in 1000 metres water depth. Typical

methodology of replacing the mooring leg, including a suction pile and a long wire rope, is

presented. The paper concludes that preparedness of an FPSO operator, prior modelling and

analysis of the mooring and riser systems and availability of up-to-date analytical models are

essential for the rapid response activities to be efficiently undertaken after a mooring failure.

1 INTRODUCTION

Mooring system of floating production storage and offloading facility (FPSO) provides

station keeping for the asset during production, maintaining vessel excursions within

allowable limits for the operation of risers and hydrocarbons offloading under anticipated

metocean conditions.

There are two main types of FPSO mooring systems—permanently connected and

disconnectable—used for different types of metocean conditions. Permanent mooring systems

are typically designed for FPSOs staying on station in non-cyclonic environments for the

duration of its design life (for example, in West Africa, South-East Asia or North Sea).

Disconnectable mooring systems are typically used for cyclonic environmental conditions (for

example, in Western Australia‘s North West Shelf) and allow quick disconnection of the

FPSO from the mooring system to evade cyclones. Disconnectable mooring systems are also

used for FPSOs in Arctic areas to avoid ultimate ice loads or collision with icebergs.

The majority of permanently connected FPSOs in benign environments have spread mooring

systems, typically made up of four line clusters, each containing several mooring legs.

Permanently moored FPSOs, in harsh environments (for example, in the North Sea), require

weather-vaning capabilities and have mooring lines attached to a turret.

Disconnectable FPSO mooring systems typically have a turret with a disconnectable buoy

(DTM buoy) which can be released from the FPSO and then retrieved and locked in the turret

after reconnection. There are internal and external types of mooring turrets located inside the

FPSO hull and mounted on a cantilevered arm protruding forward of the hull respectively.

A typical mooring line make-up includes multiple interconnected components. The integrity

of the mooring leg and, potentially, the whole mooring system depends on each component in

the line. This combination of various components, their individual locations and exposure to

different loads, wear and fatigue make mooring lines a complex system in terms of integrity

management.

Despite the continuing mooring technology development, improvements in engineering

design standards and manufacturing practices, the offshore industry, each year, records

several incidents involving damage or failure of mooring systems. The evidence gathered

throughout the offshore industry suggests that mooring line failure is a relatively common

event during the life cycle of a floating facility and has a high potential of occurrence. North

Sea statistics, collected since the 1980s, indicates that, on average, an FPSO may experience a

mooring failure within 8.8 years of operation [Ref. 13].

The mooring system is an important safety critical component for the FPSO operation.

Therefore its failure—even partial—may have serious consequences, such as risk to the

personnel, damage of risers and possible spill of hydrocarbons caused by exceeding allowable

excursions of the vessel. Although design procedures for FPSO mooring systems require

application of factors of safety for tensions in mooring lines and analysis of a mandatory

damage case [Ref. 4], FPSO operation at the limits of the design environmental conditions

may need to be discontinued if any mooring line has failed or significant abnormalities of

moorings have been detected.

Mooring failure or defect may, therefore, lead to a shutdown and substantial production loss.

These reasons highlight the importance of maintaining the mooring system in good working

order, early detection of abnormalities and prior planning of activities in case of its failure.

This early planning is typically carried out through the development of a Mooring Integrity

Management Plan (MIMP) which is discussed in the following sections of this paper,

including two examples of the plan‘s components.

2 OVERVIEW OF MOORING INTEGRITY MANAGEMENT PLAN

An FPSO specific Mooring Integrity Management Plan (MIMP) typically comprises the

following main modules:

Mooring Inspection Plan (MIP)

Mooring Monitoring Plan (MMP)

Mooring Failure Rapid Response Plan (MFRRP)

Although each of these three modules can be considered as a stand-alone unit or part of the

MIMP, there is an inter-relationship between them as the inspection and monitoring activities

are usually the final triggers for invoking an MFRRP.

The MIMP, MIP, MMP and MFRRP structure is presented in a flow chart in Figure 1 below.

Figure 1: Mooring Integrity Management Plan (MIMP)

2.1 MOORING ANOMALIES

During its lifecycle the mooring system may experience a number of defects or anomalies,

which may lead to a mooring failure if they are not detected and rectified. Various mooring

components are prone to specific defects, some of which are well known [Ref. 13].

Overall, failure of mooring connectors and terminations is one of the most frequent problems.

These may occur due to exposure of connectors to loads not predicted in the design (for

example, side loading of shackles), incorrect specification or mismatch of materials (for

example, free-rotating or locked pins, sheared off or corroded away split pins), or omissions

during installation. Material properties not matching specifications or incorrect fabrication

methods (for example, welding techniques) are known to have caused mooring failures and

required replacement campaigns in the past [Ref. 2].

An example of a failed chain link

due to faulty weld is shown in

Figure 2. In accordance with [Ref.

14], [Ref. 15] Petrobras America

alleges faulty welds were the reason

for a mooring failure on their

facility and has sued a chain-maker

for at least $US180 million over

losses allegedly caused by a broken

chain that let parts of a floating oil

production system sink and drift

away in the Gulf of Mexico last

year.

Figure 2: Example of Failed Chain Link Due to Faulty

Welds [Ref. 14], [Ref. 15]

Generally, upper sections of mooring lines are prone to defects resulting from high tension

and contact loads (for example, out-of plane bending of chain links in a chain stopper) or

relative movement (interlink wear). Mid-sections of chain and wire may suffer from

mechanical damage caused during installation (damage of the spiral wire sheath). Regions

around the touch down area are subject to abrasion and corrosion, which in certain soil types

may be accelerated by trenching. Although corrosion allowances are usually included in the

mooring design, types and rates of corrosion are site-specific and they should be closely

monitored. Excessive trenching may result in redistribution of mooring tensions, increased

vertical load on the chain stopper and in some cases may require extensive remediation.

2.2 MOORING INSPECTIONS

Regular mooring inspections are critical to ensure the integrity of the mooring system and to

minimise the probability of mooring failure resulting from premature failure of substandard

components [Ref. 4]. Inspection of FPSO mooring components may be based on API-2I [Ref.

5], which provides guidance for moorings of MODU and also permanent mooring systems.

Similarly, the API offers a guidance on re-use of mooring components. As a minimum, the

following surveys are required:

As-build survey after the FPSO hook-up;

Periodic survey at least every five years (or more frequently); and

Special event survey after dropped objects, collision, extreme storms, etc.

It is recommended that the mooring inspection plan is set up to suit, specifically, the

particulars of the mooring system in terms of its design and operation. Application of risk

based inspection methodology, the principles of which are described in API RP 580 [Ref. 6]

and API RP 581 [Ref. 7], may be advantageous over the periodical inspection regime for

many systems.

Mooring inspections are carried out either by divers or ROVs as visual surveys and often

include measurements (e.g. thickness measurements). ROVs are usually used for inspections

of permanent deep water moorings in water depths beyond the limits for diver access and/or

in cases of potential hazards for divers during inspections of moorings in operation.

2.3 MOORING MONITORING

The in-service permanent monitoring of a mooring system is an efficient method for the early

detection of mooring failures. Despite that, however, many of currently operational FPSOs do

not have a reliable means of detecting a mooring line failure. Many mooring failures remain

undetected for prolonged periods of time until the next inspection, thus posing a risk of

cascade failure. Therefore, mooring monitoring should be considered by operators as an

important safety measure.

The detection of mooring line failure should not be a surprise occurring during an inspection.

Early detection of mooring failures can be successfully achieved by installing a permanent

monitoring system. These will continuously examine critical parameters (e.g. tension or line

angle) and provide a warning in case of recording zero or abnormally low tension in a failed

mooring line and excessive tensions in other lines. Some commercially available monitoring

equipment is briefly mentioned below.

Line Tension Monitoring

Mooring line tension monitoring is a direct, and probably the most reliable, method of

mooring failure detection. There are number of different devices that can be used to measure

the mooring line tension:

Instrumented chain stopper (load cells)

Strain gauges on chain links for monitoring deformations and computing tensions

Devices for measuring the natural frequency of a chain segment to calculate the load

on the chain.

The installation of load cells in chain stoppers is the most reliable method for permanently

monitoring line tension. This method does not require a cable for data transfer to the FPSO

nor data logger with batteries for line tension information storage.

Line Angle Monitoring

An inclinometer can be fitted on the mooring line near a fairlead or other location. The line

angle is measured and transmitted to the FPSO using a data cable or logger. This method is

easier to implement than the load cells in chain stoppers (especially when retrofitting), but it

may suffer lower accuracy due to calibration errors and tension data obtained from the

analytical model.

FPSO Position Monitoring

The position monitoring methods may indicate abnormality of the mooring system but they

cannot provide information on a particular line failure. An FPSO‘s position can be measured

via GPS. Sufficient accuracy (less than one metre) of the vessel position tracking can be

achieved using a GPS signal receiver on the FPSO along with an onshore reference GPS

receiver in a known location, which sends a correction signal to the FPSO via Inmarsat. FPSO

heading measurement and the analytical model are also necessary for computation of tensions

in moorings for a known position of the reference point on the vessel.

2.4 MOORING FAILURE RAPID RESPONSE PLAN (MFRRP)

MFRRP is intended to be a decision support tool and the means to reduce the potential HSE

and financial consequences of a mooring failure. MFRRP provides the framework and

relevant procedures for the rapid deployment of specialist services in the event of a mooring

emergency. During the course of MFRRP development, information is gathered to allow the

operator to select suitable service providers and set up the necessary contracts for rapid

response. The key objectives of the plan are:

Minimise or eliminate risks to personnel, environment and the asset resulting from the

mooring failure

Minimise or eliminate risk of production downtime

Reduce risk of subsequent failure of adjacent lines

Reduce risk of riser and other equipment damage

Provide readiness for deployment of suitable resources and equipment for intervention

Provide guidelines and procedures for failure evaluation, response and operation

whilst in a damaged condition

Provide procedures for mooring line replacement in the most efficient manner.

A flowchart showing MFRRP is shown in Figure 1. Its key components are described below.

2.4.1 Assessment of Residual Capacity of Mooring System after Failure

Determination of FPSO excursion limits and the residual capacity of the mooring system after

mooring line failure allows for development of procedures for continued operation and

avoiding or minimising production downtime. This involves development of the FPSO and

mooring system model and performing limiting offsets analysis for various scenarios of

mooring line failure. The analysis process takes into consideration limiting operational criteria

dictated by the FPSO interfaces such as risers, umbilicals, etc., and includes the minimum

bending radius, maximum tension, compression limits and clashing between risers and

moorings. The analysis also considers the environmental conditions, and, as the key outcome,

delivers a definition of FPSO safe operational limits with a damaged mooring line.

2.4.2 Preparation for Mooring Repair or Replacement Prior to its Failure

Initial planning and setting up of procedures and plans that form an MFRRP typically consist

of the following documentation:

HSE Plan

Sparing Plan

Mooring Line Defect Repair and Line Replacement Plan

Mobilisation Plan

Post-Replacement Activity Plan

MFRRP requires a dedicated HSE Plan covering the activities described in all procedures,

which will provide the minimum HSE requirements for the operator, contractors and third

parties being part of the rapid response team.

A Sparing Plan defines the strategy with regard to mooring system spare components. It

typically includes complete inventory and details of the components in the current mooring

system (including alternative spares and identification of long lead items), listing of the

nearest suppliers, procurement details (cost, delivery time and transportation details), spares

storage requirements (including inspection, maintenance, security) or spares sharing details

(for interchangeable use with own assets or shared with other operators).

A Mooring Line Defect Repair & Line Replacement Plan typically contains Failure Modes -

Response Matrix, Preliminary Removal and Installation Procedures, Equipment and Special

Aids and Marine Spread Requirements Plan.

Mobilisation plans provide a framework and relevant procedures for the rapid deployment of

an operator‘s personnel and specialist services required in the event of mooring emergency.

These will typically include inspection services, installation contractors, repair and

maintenance and engineering support.

A Post-Replacement Activity Plan defines the minimum actions that need to be undertaken

after completing the response to a mooring failure, i.e. updating documentation following the

replacement of a mooring component or mooring line, updating the list of spares, procurement

of new spares, and updating the MFRRP procedures (e.g. in case alternative components are

used, etc.).

2.4.3 Benefits of Prior Planning and Preparedness for Mooring Failure

Prior planning and preparation for an accidental mooring failure may pay substantial

dividends to the operator. This requires an initial investment in setting up the response

systems.

A Sparing Plan is not always included in the MFRRP. Despite offering the obvious benefits of

rapid availability and deployment of components, some operators choose not to include the

spares in their Asset Integrity Management plans.

The value of having spares and rapid response plans in place appears to be evident, especially

considering the high cost of daily production loss at the current oil price and the duration of

expected procurement time for a replacement mooring likely to take up to several months. An

example presented in Table 1 shows cost of spare mooring components against cost of

production loss calculated using methodology described in [Ref. 13]. The mooring leg

procurement cost equates to approximately one to several days of the lost production cost,

depending on the production rate, current oil price and design of the mooring system.

Table 1: Example of cost of production loss versus spared mooring leg cost

Daily production loss

Production rate per day 50,000 bbl.

Current oil price per barrel $US 95

Field life 20 years

Year of failure 7

Approximate lost value of production per day $US 3 million

Spared mooring leg cost

Approximate procurement cost of one mooring leg $US 1 million - $US 2.5 million

The decision to continue production with a partially failed mooring (e.g. one mooring leg

failed) depends largely on the residual capacity of the system. Therefore, assessment of

residual capacity of the mooring system, prior to an occurrence of failure and retention and

updating of the analytical model, are essential for decision support during emergency.

Retention of up-to-date analytical models of the hull, moorings and risers allows prompt

verification of the previously assessed residual capacity of the mooring system, taking into

account variations of design parameters accumulated during FPSO operation (e.g. change of

FPSO loading conditions; marine growth on the hull, moorings and risers; corrosion and wear

of mooring components; anchor drag; etc.). Table 2 demonstrates potential time savings

provided by early assessment of residual capacity (ARC) of moorings after failure and

retention of analytical models.

An assessment of residual capacity of the mooring system, prior to its failure in the example

case, may reduce the duration of the decision making on temporary production by more than

three months. Retention of up-to-date analytical models is essential to minimise the decision

making process in case of facility changes in operation from the ‗as-build‘ condition, which

may potentially result in about one month time saving. The up-to-date model retention

supplemented by re-assessment of the residual capacity of the mooring system for the ‗as-is‘

condition—after the facility condition has changed—may reduce the decision time by an

additional three weeks.

Table 2: Estimate of time for assessment of residual capacity of moorings (example only)

Case

Assessment of Residual Capacity

No p

rep

ara

tion

for

moori

ng f

ail

ure

‘As-build’;

Mod

els

not

up

date

d t

o

refl

ect

fa

cili

ty c

han

ges

‘As-build’;

Up

date

d m

od

els

refl

ect

faci

lity

ch

an

ges

‘As-is’

Retention of analytical models No No Yes Yes

Residual

capacity

analysed prior to

failure

For ‗as-build‘ facility

condition

No

Yes Yes Yes

For ‗as-is‘ condition

facility (i.e. reflecting

changes in operation)

No No Yes

Activity Duration [weeks]

Design information collection and review,

extraction of relevant input data 2 1 N/A N/A

Analysis of residual capacity of moorings

and excursions 4 2 N/A N/A

Riser analysis 4 2 N/A N/A

Verification analyses of moorings and risers

taking into account facility changes from

original design

N/A N/A 2 N/A

Company and operator review and

endorsement of temporary operation 2 1 N/A N/A

Approval of temporary operation by Class

and Safety Authority 2 2 2 1

Total duration of decision making on

temporary operation after detection of

mooring failure

14 8 4 1

3 EXAMPLE OF THE ASSESSMENT OF RESIDUAL CAPACITY OF FPSO

MOORING SYSTEM AFTER FAILURE

3.1 GENERIC DISCONNECTABLE FPSO MOORING SYSTEM

An example of the assessment of residual capacity of a mooring system, after the failure of a

single mooring line, is based on a generic disconnectable mooring system of an Aframax size

FPSO—typical in the North West Shelf (NWS) of Western Australia—in between 300m and

400m water depth. The generic mooring system consists of 3 mooring clusters with 3 mooring

lines in each cluster, an internal disconnectable turret buoy and drag anchors. Mooring lines

comprise a combination of spiral strand wire ropes (SSWR) and mooring chains. A plan view

of the mooring systems is shown in Figure 3, while the mooring line composition is presented

in Table 3.

Figure 3: Plan View of Generic Mooring System

with DTM Buoy

Figure 4: Directionality of Reduced Non-

Cyclonic Environment in the NWS

Table 3: Composition of mooring line of generic disconnectable mooring system

Segment No

(from anchor to fairlead) Segment type Segment length [m]

1 76 mm Studless Chain 700

2 140 mm Studless Chain 200

3 76 mm Studless Chain 50

4 Spiral strand steel wire rope 400

5 76 mm Studless Chain 20

Total paid-out line length 1370

The generic FPSO and shuttle tanker are assumed to be of the same size as shown in Table 4

below. The two vessels are connected with a nylon hawser during offloading of crude oil from

the FPSO in the tandem arrangement.

Table 4: Principal particulars of generic FPSO and shuttle tanker

Description Full load condition Ballast condition Units

Length between perpendiculars 230 m

Beam moulded 42 m

Depth moulded 23 m

Draft 15 9 m

Displacement 120,000

70,000

t

Assumed minimum breaking load (MBL) of the FPSO moorings and release load of the

hawser release hook on the shuttle tanker are shown in Table 5.

Table 5: MBL of mooring lines and hawser release load

Line MBL or Hook Release Load [t]

FPSO mooring 450 t (MBL)

Hawser between FPSO and shuttle tanker 250 t (hook release load)

3.2 MOORING FAILURE SCENARIO AND LIMITING ENVIRONMENT FOR

TEMPORARY OPERATION

The example study considers a failure scenario when the mooring line #1 has failed, and the

FPSO will continue operation at reduced environment (in comparison with the design

environment) until the failed mooring line is reinstated.

Maximum design environment for the connected FPSO in the NWS typically corresponds to

100 years return non-cyclonic conditions. The target design return period of non-cyclonic

environment for temporary operation of the FPSO was assumed to be 10 years. Selection of

the reduced return period depends on the maximum duration of the temporary operation and

may be made based on equal probability of occurrence of extreme environment for the

original design and reduced environmental conditions.

The target limiting environmental condition, for offloading from the FPSO to the shuttle

tanker after the mooring failure, was assumed to be unaffected by the mooring failure. A

summary of the environmental conditions for the study is shown in Table 6.

Table 6: Limiting environmental conditions after single line failure for connected FPSO in

production and offloading modes

Environmental parameter Production *)

Offloading

Significant wave height Hs [m] 4.0 – 6.5 4.0

Spectral wave period Tp [s] 8 – 15 11 –16

Wind speed Vw (1-minute average) [m/s] 10 – 25 15

Surface current speed Vc [m/s] 0.4 – 0.7 0.5 *)

Note: Value varies for different directions of environment

A combination of values of Hs, Vw and Vc and the directions of waves, wind and current for

the production cases were selected based on the review of typical metocean conditions in the

NWS. For the offloading cases, generic omnidirectional combinations of wind and current

directions relative to the waves were used based on the recommendations of Lloyds Register

[Ref. 11] and ABS [Ref. 12].

JONSWAP wave spectrum was used for all considered sea states, while constant velocities of

wind and current were assumed in the mooring analysis. Ranges of the wave spectral peak

period Tp covered probable contributions of swell and sea in the combined wave spectra.

About 100 base environmental cases were used for each of the connected FPSO mooring

analysis and offloading to the shuttle tanker, which covered the harshest combinations of

strength and directions of waves, wind and current.

3.3 ANALYSIS METHODOLOGY

3.3.1 Mooring Analysis Overview

Mooring analysis for the temporary operation of the FPSO after a single line failure can be

carried out in a similar way to the conventional analysis of moorings, but considering the

failed mooring system as ‗quasi-intact‘ and applying the standard mooring analysis

methodology to this ‗quasi-intact‘ system. The damage case for the ‗quasi-intact‘ mooring

system implies failure of any other mooring line in addition to the mooring line which has

already failed.

Mooring analysis for disconnectable mooring systems has to be carried for two cases:

FPSO connected to the mooring system (including offloading to shuttle tanker)

Disconnected DTM buoy

For the FPSO connected cases, an uncoupled mooring analysis in respect of the mooring

stiffness is usually sufficient due to significantly lower vertical stiffness of the catenary

mooring system in comparison with the hydrostatic stiffness of the FPSO hull. Partial

hydrodynamic coupling of the FPSO with the mooring and riser systems is achieved by

incorporating damping of the mooring lines, risers and umbilicals in the mooring analysis.

A fully coupled dynamic analysis of the FPSO, with the mooring and riser systems, may be

carried out using commercially available software, but it is more time consuming and not

mandatory. The fully coupled dynamic analysis allows application of lower factors of safety.

However, carrying out the faster quasi-dynamic mooring analysis allows consideration of a

longer time history (more wave seeds) and more environmental cases. Future progress in the

software and hardware development will likely make the fully coupled dynamic analysis a

preferred option even at early design stages.

The present example study utilises the partially coupled quasi-dynamic approach in the

mooring analysis for the connected FPSO. When the DTM buoy is disconnected from the

FPSO for tropical cyclone evasion, a fully coupled dynamic mooring analysis is required as

motions of the DTM buoy are highly dependent on dynamic loads from the mooring lines,

risers and umbilicals.

The mooring analysis in the present study has the following milestones:

Mooring analysis for connected FPSO at two loading conditions (ballasted and fully

loaded)

o Computation of response amplitude operators (RAO) of the FPSO motions and

quadratic transfer functions (QTF) of wave drift forces

o Calculation of wind and current forces and moments on the FPSO

o Calculation of additional damping due to mooring and riser systems

o Calculation of additional damping of the FPSO hull in yaw motions

o Time domain computation of the FPSO motions (at wave frequency and low

frequency) and tensions in mooring lines for the quasi-intact mooring system

o Same as the above but for damage cases

o Calculation of factors of safety for tensions in moorings and maximum

excursions of the FPSO

Mooring analysis for disconnected DTM buoy

o Time domain computation of the DTM buoy motions and tensions in mooring

lines for the quasi-intact mooring system

o Same as the above but for damage cases

3.3.2 Connected FPSO Models and Time-Domain Simulations

The mooring system was modelled in a time domain program, and models for production and

offloading cases are shown in Figure 5 . The program utilises the quasi-dynamic approach and

the time domain analysis for the low frequency and high (wave) frequency motions of moored

vessels.

The quasi-dynamic approach assumes that mooring lines behave as static catenaries, while the

FPSO and shuttle tanker (during offloading only) experience wave frequency motions in 6

degrees of freedom (prescribed by RAOs) and low frequency motions in the horizontal plane

governed by wave drift forces (accounted for by QTFs). Non-linear stiffness of the nylon

hawser was used in the computations.

The time domain analysis utilised a 3-hour simulation per each wave seed. Every

environmental case was analysed for 20 random wave seeds, while up to 100 wave seeds were

utilised for some sensitivity cases.

Figure 5: Model of Moored FPSO (Production) and Model of Moored FPSO and Shuttle Tanker

(Tandem Offloading)

3.3.3 Design Tensions, Factors of Safety and Allowable Offsets

Design tensions Td in mooring lines for each environmental case were calculated from

maximum tensions computed in the time-domain simulations for different wave seeds using

methodology described in [Ref. 9]. The minimum allowable factors of safety (FoS), defined

as

FoS = MBL / Td

for different types of analysis are presented in Table 7. FoS, specific for the quasi-dynamic

type of analysis, are recommended by Bureau Veritas in [Ref. 9]. More conservative FoS

applicable for the quasi-static type of analysis [Ref. 4] were used for the example case.

Assumed allowable excursions of the connected FPSO for operation of risers are presented in

Table 7 as percentage of the water depth (WD).

Table 7: Factors of safety for mooring tensions and allowable offsets

Mooring

Condition

Factor of Safety Allowable

Excursion Quasi-Static

Analysis [Ref. 4]

Quasi-Dynamic

Analysis [Ref. 9]

Dynamic Analysis

[Ref. 4]

Intact 2.00 1.75 1.67 12% of WD

Damaged 1.43 1.25 1.25 17% of WD

3.3.4 Damping of Low Frequency Motions

Damping due to Moorings and Risers

The quasi-dynamic mooring analysis method takes into account all components of the

damping except damping from moorings and risers. The low frequency oscillatory motions of

a moored vessel are caused by the second order wave forces, which are narrow-banded in

frequency and normally occur at the resonant frequency of the mooring system. The latter is

defined by the mass (including the added mass) of a vessel and the mooring stiffness. As

amplitudes of oscillations in the resonant mode depend strongly on the system damping,

accurate estimation of damping components is important for the mooring analysis. In some

cases, as much as 80% of the total surge damping in the system (vessel, moorings and risers)

can be attributed to that generated by the mooring and riser system moving through water or

dragging against the seabed.

Linear damping of low frequency motions was calculated using an in-house method which

takes into account properties of the actual mooring and riser system and anticipated offsets of

the vessel and applies the following major steps.

Step 1: Natural periods of the mooring system in surge and sway are determined from the

stiffness of the mooring system, mass and inertia of the vessel and the added masses.

Step 2: A time history is calculated of the force exerted on the vessel undergoing low

frequency motion oscillations with period and amplitude appropriate for the system. This is

done using software with full lines dynamics capability by setting up a model of the mooring

system and risers/umbilicals and imposing oscillatory motion with a given amplitude and

period. An example of the model is shown in Figure 7.

Step 3: The time history of the force from moorings, risers and umbilicals is processed to

extract the equivalent damping coefficient.

Calculated equivalent linear damping is presented in Figure 6 and compared with a generic

damping from [Ref. 10]. The calculated damping is specific to the mooring/riser system and

water depth and is also dependent on the vessel excursions. The mooring-specific damping is

significantly different from the generic damping. The latter does not depend on the FPSO

excursions and includes the hull translational damping, which is already accounted for in the

time domain analysis by the current drag on the hull. The use of the mooring-specific

damping is therefore preferred for the mooring analysis.

Figure 6: Linear Damping due to Moorings

and Risers

Figure 7: Model of Moorings, Risers and

Umbilical for Determination of Equivalent

Linear Damping

Calculation of damping, specific to some governing environmental cases, was made by

iterations, using an estimate of the vessel excursions from previous time domain simulation

and case-specific current speed and condition of the mooring system (intact or damaged

lines).

Hull Yaw Damping

An approximate value for yaw damping, associated with the viscous drag on the vessel hull,

can be automatically calculated by some software packages using input provided by the user

(e.g. the transverse current drag coefficient). The present study utilises liner yaw damping

calculated as:

NL BaB 3

8

where:

TLBN

40125.0 – nonlinear damping; L – hull length; T – hull draft; – circular

frequency of yaw motions; a – amplitude of yaw motions.

This formula for BN applies a typical distribution of the transverse drag along the hull and

effect of the Keulegan-Carpenter (KC) number due to periodic oscillatory yaw motions.

3.3.5 Analysis for Disconnected DTM Buoy

Checking the residual capacity of the mooring system and riser survivability is mandatory for

the DTM disconnected case – similar to the connected case. This analysis is executed for

reduced cyclonic conditions due to the temporary operation of the mooring system until its

repair. Previous experience shows that the DTM disconnected cases are usually less critical

than the FPSO connected cases, and they are not presented in this paper.

3.3.6 DTM Buoy Connection and Reconnection

Attention should be paid to the

disconnection and reconnection of the DTM

buoy to the FPSO during temporary

operation after a mooring line failure. This is

because the DTM buoy may clash with the

FPSO hull during reconnection due to the

buoy tilt caused by asymmetry of the

mooring forces and increased FPSO

excursions. An example from the time

domain simulations is shown in Figure 8.

Such behaviour of the DTM buoy could also

reduce maximum allowable FPSO

excursions and environmental conditions for

the disconnection and reconnection during

temporary operation after the single line

failure.

Figure 8: Example of DTM Buoy Clashing

with FPSO Bottom during Reconnection

3.4 RESULTS

Results of the mooring analysis showed that the mooring system, after the single line failure,

could withstand the reduced metocean conditions. The mooring system complies with

required factors of safety for tensions and maximum excursions in the connected case, as

shown in Table 8. FoS for anchor loads were also acceptable and the anchor uplift did not

occur for all environmental cases.

Table 8: Summary results for mooring tensions and offsets (connected case)

Mooring Case Min FoS Max offset [m]

Achieved Required Check Achieved Allowable Check

Quasi-Intact 2.55 2.00 Pass 36 42 Pass

Damage 1.62 1.43 Pass 55 60 Pass

Results for tandem offloading from the FPSO to a shuttle tanker are presented in Table 9.

Design tensions in the mooring lines and the mooring hawser are below allowable values, and

excursions are less than 30 m.

Table 9: Summary results for offloading from FPSO to shuttle tanker

Mooring Case

Min FoS in

moorings

Design tension in

hawser [t]

FPSO Excursion

[m]

Achieved Check Achieved Check Achieved Check

Quasi-Intact 2.9 Pass 220 Pass 27 Pass

The conclusion from this example study is that the mooring system meets the reduced design

criteria for a temporary operation. Because of the directional nature of metocean conditions in

the NWS with predominant directions of waves from the south-west, as shown in Figure 4,

this conclusion may be different if another mooring line is assumed to have failed. Therefore,

it is recommended to assess residual capacity of the mooring system further, considering

failure of any other mooring line.

As an alternative assessment method, the response based analysis of the mooring system,

using a site-specific metocean time history for several years, can be carried for the

identification of most critical cases and determination of maximum tensions and excursions

corresponding to a given return period.

4 EXAMPLE OF DEEP WATER MOORING REPLACEMENT PROCEDURE

4.1 GENERIC DEEP WATER SPREAD MOORING SYSTEM AND FAILURE &

REPLACEMENT SCENARIOS

A generic mooring system for an FPSO in 1000 m water depth comprises four clusters with

three mooring lines in each cluster. The mooring lines are connected to suction pile anchors

on the seabed and chain stoppers on the FPSO deck. Mooring lines consist of a short length of

ground chain, a long spiral strand wire rope (SSWR) with polyethylene sheath and a short

section of top chain where the paid-out length can be adjusted. The mooring line composition

is shown in Table 10.

Table 10: Mooring leg composition of generic deep water spread mooring system

Item

#

Mooring component

(from anchor) Description

1 Suction pile anchor Length: 22m; Diameter: 5m; Mass: 150 tonnes

2 Ground chain 130m of 110mm chain

3 Spiral strand wire rope

(SSWR)

1450m of sheathed spiral strand wire (102mm strand diameter);

closed spelter sockets on each end

4 Top chain 100m of 110mm chain

5 Shackles Shackles connecting: suction pile padeye to ground chain; ground

chain to SSWR; SSWR to top chain

Failure of any of the mooring

components in Table 10 may results in a

complete failure of a single mooring leg.

Defects of the mooring components

revealed by inspections may also require

immediate action for repair or

replacement.

Replacement of the entire mooring leg

may be considered as preferred option

after mooring failure for reducing the

mooring reinstatement time. A new

suction pile has to be installed at some

distance (20-30m) from the existing pile

closer to the FPSO to avoid any reduction

of the soil holding properties and

interference of the replaced ground chain

with the abandoned suction pile. The

mooring leg replacement procedure is

described below.

Figure 9: Minimum Diameter of Stern Roller

and winch Drum for Deployment of Sheathed

Spiral Strand

0

1

2

3

4

5

6

7

500 1000 1500 2000Ro

ller M

inim

um

Dia

me

ter [

m]

Water Depth [m]

102mm SSWR

102mm SSWR (with DAF=1.2)

102mm SSWR and 111mm studless chain

102mm SSWR and 111mm studless chain (DAF=1.2)

4.2 GENERIC MOORING REPLACEMENT METHODOLOGY

Requirements for installation of sheathed SSWR

Polyethylene sheath on SSWR is used in many deep water moorings as it significantly

reduces corrosion of the steel strand and allows its prolonged operation without deterioration

of SSWR mechanical properties for a period of up to 25 years. The sheath is prone to damage

during handling and installation, and imposes certain limitations for the installation methods

and equipment. Pressure on the sheath has to be limited during installation to prevent its

collapse or plastic deformation. This implies limits on allowable tension in SSWR and also

consequential limits for caterpillar-type tensioners, winches and stern rollers. A generic

formula below [Ref. 3] gives the minimum diameter of the winch drum or stern roller against

tension in sheathed SSWR.

where:

D = diameter of deployment stern roller or flat drum (mm)

W = line load (N)

b = allowable average bearing pressure on the sheath (21 N/mm2)

d = nominal sheath outside diameter (mm)

t = nominal sheath wall thickness (mm)

As illustrated in Figure 9, the limiting water depth for deployment of sheathed SSWR and

ground chain using a winch and stern roller with diameters typical for offshore support

vessels (OSV) and anchor handling tugs (AHT) is about 1000 m, while suspension of the

ground chain weight by a wire from the installation vessel may extend this limit to

approximately 1500 m.

Provision of a sheath repair and spiral strand re-termination kits on-board an OSV is

recommended for the prompt repair of SSWR in case of its damage during installation.

Marine Spread Selection

A preferred option, for the replacement of a single mooring leg after failure, is the use of

available support vessels in the area rather than the mobilisation of a large crane barge similar

to that probably used for the original installation of 12 mooring legs before the FPSO hook-

up. The ―use what is available‖ principle may bring challenges in the development of the

mooring replacement procedures, which could be overcome via detailed engineering work

involving dynamic analysis. As a minimum requirement an OSV with DP system, hull length

between 80 m and 100 m, SSWR winch, stern roller and deck crane (about 100 tonnes

capacity) may be sufficient for the installation of the new suction pile with mooring line in the

present example.

Suction Pile Overboarding

Overboarding the suction pile using a deck crane with sufficient capacity (about 300 tonnes in

the example case) may increase mobilisation time and cost for the installation vessel, so the

use of the stern roller on the selected smaller OSV is likely to be a less expensive and a faster

option. A risk of the suction pile top clashing with the OSV roller or hull during overboarding

using a lowering work wire at the pile centreline needs to be closely reviewed and may be

mitigated by attaching an additional work wire to the pile side. This technique was verified in

the installation analysis as shown in Figure 10, where the pile top is tilted away from the stern

roller.

Deployment of Suction Pile Anchor and Mooring Line

After disconnection of the additional work wire from the pile at the end of the overboarding

stage, deployment of the suction pile and mooring line to depth can be carried out using the

lowering work wire from the OSV winch as shown in Figure 11. The mooring line is

deployed simultaneously whilst lowering the pile, providing slack in the line for preventing

SSWR over-tensioning and pile tilt. Spelter sockets in SSWR should be lifted over the stern

roller to avoid their damage, using the deck crane and an ROV for disconnection of the crane

hook in the water.

The dynamic installation analysis indicated a risk of the suction pile heave and pitch

amplification in a water depth in excess of 700 m and wave periods under 8 seconds, caused

by the proximity of regular wave periods to the natural period of motions of the suspended

pile. The accurate modelling of this phenomenon requires correct estimate of the system

damping, which is addressed, for example, in [Ref. 1] and [Ref. 8]. Although such behaviour

of the pile was not evident in the irregular wave analysis, a check of the pile heave and pitch

is essential before the final stage of the pile deployment onto the seabed for its self-

penetration.

Figure 10: Overboarding Suction Pile

using Additional Work Wire on OSV

to Prevent Clashing with Stern Roller

Figure 11: Example of Deployment of Mooring Line

and Suction Pile from OSV

Embedment of Suction Pile Anchor

The OSV work winch, in a constant tension mode, lowers the suction pile and continues to

pay out the wire after the pile touches down to allow its penetration into the seabed under the

pile‘s own weight. A work class ROV docks on the pile top, closes ventilation hatches and

facilitates the pile full embedment using suction created by a pump. The ROV disconnects the

work wire from the pile, and the OSV moves away from the pile towards the FPSO and

simultaneously continues deployment of SSWR and top chain.

5 CONCLUSIONS

The development of a Mooring Integrity Management Plan (MIMP) facilitates early detection

of a mooring anomaly, may allow temporary operation of the FPSO after a mooring line

failure, and accelerate the mooring repair or replacement campaign, thus reducing or

eliminating production loss caused by the mooring anomaly. Development of an MIMP in

the beginning of the FPSO operation or even before its commissioning is recommended as

experience shows many mooring failures occur at early operation stages. The costs of lost

production in the early production stages are higher than in the later years, therefore the

importance of minimising any downtime is critical.

Retention of analytical models is important for the prompt verification and update of the

Mooring Failure Rapid Response Plan (MFRRP) in case of deviation of the FPSO, mooring

and riser systems and available marine spread from those initially assumed. Specifications for

spare mooring components, assessment of residual capacity of the mooring system after its

partial failure and mooring removal and installation procedures should be maintained up to

date to reflect actual operating conditions.

A possibility exists for operation of an FPSO in the North West Shelf of Australia after a

single line failure for up to several months at reduced design environmental conditions –

subject to the mooring and riser systems design, orientation of the failed line relative to the

predominant environment and necessary approvals. This possibility has to be established by

the assessment of residual capacity of the mooring, considering the mooring system after the

line failure as in ‗quasi-intact‘ condition and applying the advanced analysis methodology.

Replacement of a mooring leg in deep water, using available support vessels, imposes

additional challenges and potential risks which should be understood and addressed. The

replacement procedures require dynamic analysis and development of additional protection

measures for sensitive mooring components from damage during retrieval and installation.

6 REFERENCES

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the dynamic hook load during subsea deployment of suction can‖, OMAE2007-

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for-Service Assessment of Deepwater In-Service Low Toughness Mooring

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[Ref. 6] API, ―Risk-based Inspection‖, Recommended Practice 580, 1st Edition, 2002.

[Ref. 7] API, ―Risk-based Inspection: Base Resource Document‖, Recommended Practice

581, 1st Edition, 2000.

[Ref. 8] DNV, ―Modelling and Analysis of Marine Operations‖, Recommended Practice

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[Ref. 11] Lloyd‘s Register, ―Rules and Regulations for the Classification of a Floating

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[Ref. 12] ABS, ―Floating Production Installations‖, Guide for Building and Classing, July

2009.

[Ref. 13] Noble Denton Europe Limited, ―Floating Production System: JIP FPS Mooring

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[Ref. 14] Upstream News, ―Petrobras Sues over Chains in GOM Project‖,

http://www.upstreamonline.com/live/article1243028.ece.

[Ref. 15] U.S. Department of the Interior: Bureau of Ocean Energy Management,

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