Best Practice Cable · PDF fileSubsea Cables •Installation of cables in the marine...

Best Practice Cable Installation

March 2012

Outline

•Company Overview

•The only two things you need to remember

•General offshore cable considerations

•Cable design

•Burial techniques

•Protection systems

•Contracting strategy

Introduction to the Group

38,000

employees

Operations in

50 countries $6.1bn sales

John Wood Group PLC Servicing Energy Industries Worldwide

A Global Market Leader in:

• Engineering and Project Management for

Oil and Gas and Renewable Energy

Developments

• Field Operation and Maintenance

• Industrial Turbines

Wood Group Renewables

Engineering – Market Leading Positions

• 40 years of unrivalled offshore expertise

• We have worked on

– 75% of GOM deepwater facilities

– 30% of deepwater facilities worldwide

• We have engineered the WORLD’S DEEPEST

– TLP - Magnolia 4,744’

– SPAR – Perdido 7,800’

– Production platform – Independence Hub

Semi 7,920’

• LARGEST independent SUBSEA technology

group of its kind in the world

• Engineered more than 500,000km of cables &

pipelines

• Engineered, managed or appraised OVER

HALF of all subsea development worldwide

• We have DESIGNED and PROJECT MANAGED

subsea developments in water depths up to

3,000 METRES

• Experienced working in HIGH ENERGY areas –

large waves, currents, cyclones, etc

Structures & topsides Subsea solutions

• Involvement in access of 800 RENEWABLE

ENERGY projects to date

• Assessed OVER 60,000 MW of renewable

energy development internationally

Health, Safety & Environment

“Maintaining the highest standards in safety across

all of our operations worldwide at all times is of paramount importance”

As expressed in our Vision for HSE Excellence,

as an integral part of our business we will

Maintain a healthy workplace

Prevent accidents

Minimise adverse environmental impact

* Total Recordable Case Frequency per million man hours

7.1 5.8

4.6 4.2 3.9 3.33 3.26 2.94 2.49

2003 2004 2005 2006 2007 2008 2009 2010 2011Oct

Safety Cases*

5


Project Lifecycle – Influence on Expenditure

& Concepts

Advisory Services

INF

LU

EN

CE

EX

PE

ND

ITU

RE

S

Major Influence Decreasing Influence

Project Management

Measurement & Analysis

Construction Management

Concept, FEED and Detailed Design

Low Influence

ENGINEER PROCURE CONSTRUCT START UP &

COMMISSIONING PLAN SELECT DEFINE

The first thing


Project Lifecycle – Influence on Expenditure

& Concepts The second thing

1 day = 25 Weeks

•Engineering is cheap

•Engineering reduces the risk profile of the project


Subsea Cables

Cable Design Criteria

• Subsea cables typically comprise:

– Copper Core

– Fibre Optic Cables

– Amour Layer

– Fillers

– Outer Sheath

– Inner Sheath

Outer sheath

Armour Layer

Inner Sheath

Fillers

Copper Core

Fibre Optic Cables

Outer sheath

Armour Layer

Inner Sheath

Fillers

Copper Core

Fibre Optic Cables

General Cable Considerations


Subsea Cables

Cable Design Criteria

• For all subsea cables, the following aspects are considered during the

design process:

– Power requirements

– Resistance to anticipated in-service loads

• Free spanning

• Vortex Induced Vibration (VIV)

• On-bottom stability

• Hydrostatic pressure

• Exposure to trawl gear

– Resistance to anticipated transportation and installations loads

• Dynamic loads and curvature during installation

• Establish need for secondary protection


Reasons for Rock Dumping

Finertia

Fdrag

Flift

Wsub

Friction

Stability

wave

&

current

Vortices

Freespans


Cable Stability

0

500

1000

1500

2000

2500

0 5 10 15 20 25 30

KP

We

igh

t (N

/m)

Cable Weight

100 yr wave

10 yr wave

10 yr design

1 yr design

May

June

July

August

September

Required Weight

Actual

Cable

Weight

•Long term integrity of cable can only be achieved by protection.

•Even in 1-year summer conditions, temporary stability of cable is

marginal


Cable design

12

•Cannot be performed in isolation

•Need to consider on-bottom conditions as well as installation

constraints

•Vessel size

•Cost – additional armour or secondary protection?

•Joints

•Landfall


Subsea Cables • The cable route should be selected with consideration to:

– Safety of the public and personnel

– Protection of the environment

– Probability of damage to cable or other nearby facilities or

infrastructure

• Typical factors that are considered are:

– Environmental - archaeological sites and areas of natural

conservation interest

– Seabed characteristics –sand waves, rocks, susceptibility to

mudslides or seabed mobility, soil properties, subsistence

and seismic activity

– Facilities – offshore installations, subsea structure, subsea

pipelines and costal protection works

– Third party activities – shipping traffic, fishing activity,

dumping areas for waste and military exercise areas

• The route selection should also, where practical, consider future

installations

Route Selection


Subsea Cables

• Route selection is typically undertaken in the following stages:

– Desk Top Study

• Naval charts

• Similar projects in the region

• Seismic data

• Environmental constraints

• Block ownership

• Any other data available

– Route Surveying

• Seabed topography

• Seabed features and obstructions

• Sub-bottom profiling

• Magnetometry used to detect buried cables and other metallic obstructions

– Route Selection and Optimisation

• The preliminary routes are then refined again bearing in mind the Cost, Risk and Impact

Route Selection


Subsea Cables

• There are fundamentally four different aspects of interest during the cable

installation for an offshore wind farm:

– 1st end pull-in at wind turbine or substation

– Landfall/Shore Approach (export cables)

– Normal cable lay along cable route

– 2nd end pull-in at wind turbine or substation

• Cable lay is undertaken using specialist cable installation vessels with

ROV used to monitor operation subsea

• Installation procedures are typically developed using dynamic analysis to

assess cable integrity throughout proposed operation

Cable Installation


Subsea Cables

• Installation of cables in the marine environment is significantly more

complex than for onshore cables

– Limited visibility of operation, particularly at regions of interest

– Environmental challenges – depth, waves and current can all restrict

operations

• The cost of cable installation is likely to be more than the cost of cable

itself

– Vessel cost and availability can be highly volatile

Cable Installation


Cable Installation Vessel Choice


Landfall Construction – HDD

• Short HDD

– Onshore cable in Duct?

– No De-rating of cable needed

– Beach pit

– Smaller vessel

Intertidal

Zone Stand off

• Long HDD

– Offshore cable in Duct

– De-rating of cable needed

– No Beach pit – Need divers

– Larger vessel


Subsea Cables

• Normal lay of cables is generally well optimised and can be performed with

relative efficiency

• The cable installation is controlled by adjusting the vessel stand-off to alter

the cable tension

– Large stand-off distance - large bend-radius but high tension

– Small stand-off distance – small bend radius but low tension

– High residual tension can minimise potential for compression in cable (from

vessel motion) and provide better touchdown control (prevent loop formation)

– Tension restricted by cable design

• Where seabed condition permit, the cable may be buried simultaneously

using trencher system

Cable Installation – Normal Lay


Subsea Cables

• For cable trenching, cable is laid within a trench below

the natural seabed

• There are three distinct methods of trenching:

– Ploughing – virtually all soil including friable chalk and

loose rock although efficiency is dependent on

conditions

– Jetting – primarily for granular soil conditions although

also effective in clays

– Cutting – suitable for most conditions although boulder

or flint can significantly increase wear of teeth

Cable Burial - Trenching


Subsea Cables

• Trenches can be either:

– Pre-formed before the cable is installed

• Allowed to back fill naturally

• Backfilled with imported soil/material

– Formed simultaneously during the cable lay

– Post-formed after the cable is installed

Cable Protection - Trenching


Cable Burial or Protection?


Cable Protection

• Rock dump may be needed for

– On-bottom stability

– Freespan correction

– Cable protection

– Scour protection

– Protection/stabilisation

of structures

• Alternatives?

– Mattresses

– Uraduct

– Cast Iron Pipe

– Cable trenching


Subsea Cables

• The main considerations to made when specifying and quantifying rock

dump are as follows:

– Rock size

– Berm size

– Berm weight

• Rock is deployed using specialist rock dumping vessels that either tip rock

directly over the side of the vessel or via a fall pipe

Cable Protection – Rock Dumping


Subsea Cables

• Rock dumping vessels

– Fall Pipe

• Big (~20 000 Tonnes)

• Large working window

• Max rock size of 16” (400 mm)

– Side dump

• Small (~2000 Tonnes)

• Small working window

• Unlimited rock size

Cable Protection – Rock Dumping


Subsea Cables

• Concrete mattresses are widely used in the offshore

industry to protect unburied cables and umbilicals

• Mattresses can be provide an effective solution for:

– Providing on-bottom stability to the cable

– Protect the cable from dropped objects and trawl

gear

– Span correction

• Mattresses are generally regarded as resistant to

scour

– Further investigation required to assess suitability is

regions of high tidal current

Cable Protection – Concrete Mattresses


Subsea Cables

• Mattresses are typically installed from a DSV or

work vessel

– Crane capacity greater than 10-15 tonnes

• Deployment can either be ROV assisted or using

quick release mechanisms

• Concrete mattresses are relatively low cost and can

take between 30-60 minutes to install

• Typically preferred as a temporary solution for

stabilsiation prior to rock dumping

Cable Protection – Concrete Mattresses


Subsea Cables

• Bend stiffeners are used to support cables

connected to rigid structures to prevent over

bending and sometimes fatigue

• Bend stiffeners are conically shaped

mouldings (polyurethane) which increase the

bending stiffness towards the point of fixity

• They are well suited to dynamic applications

such as light cable exposed to the

environment

Cable Protection – Bend Stiffener

• They are not designed to provide protection against dropped object or trawl

gear interaction


Subsea Cables

• Bend restrictors are used to prevent over bending

of cables and prevent damage to conductive

cores, fibre optics and amour

• Usually comprise inter-locking joints which lock at

a particular bend radius

– Lock radius is governed by cable MBR

Cable Protection – Bend Restrictors

• Typically used on sections of cable that span and are vulnerable to over

bending as result of environmental loading

– Interface between rigid monopile and cable

• Bend restrictors are generally not designed to provide protection against

dropped object or dragging loads


Appropriate protection for the environment


• Which protection is most suitable?

• What are the residual risks from each type of protection?

• How will this affect installation time?

• What other vessels do you already have on site?

• What is the cost of maintaining the protection?

• Total project cost?


Contracting Strategy

34


Conclusion

35

•Project risks & costs can be lowered with increased

engineering

•There is no “one size fits all” solution

•Get the right people involved as early as possible in a

project.

Best Practice Cable · PDF fileSubsea Cables •Installation of cables in the marine...

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