Specification for Freespan Correction Ps3

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SPECIFICATION FOR

FREESPAN CORRECTION

WEST NATUNA PIPELINE PROJECT

INDONESIA TO SINGAPORE

PREPARED FOR

WEST NATUNA GROUP

INTEC PROJECT H-0879.01

MARCH 1998

0 30-Mar-98 ISSUED FOR CLIENT APPROVAL

LSW CDK RJP WNG

B1 27-Mar-98 ISSUED FOR CLIENT REVIEW

LSW CDK RJP WNG

REV. DATE DESCRIPTION BY CHKD APPD CLIENT

SPECIFICATION NO.

8791/09-PS3


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Specification No. 8791/09-PS3

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30-Mar-98

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TABLE OF CONTENTS

1. GENERAL................................................................................................................................1

2. FREESPAN CORRECTION REQUIREMENTS................................................................1

3. FREESPAN CORRECTION PROCEDURES .....................................................................2

4. FREESPAN SURVEYS...........................................................................................................2

5. FREESPAN CORRECTION METHODS ............................................................................2

6. ALTERNATE SOLUTIONS ..................................................................................................3

(This Specification consists of 5 pages)


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1. GENERAL

This specification defines minimum quality and technical requirements governing

inspection of offshore pipelines for freespans, and freespan correction methods. This

specification is to be used in conjunction with the following documents:

Design Basis Memorandum 8791/02-DBM001

General Specification 8791/09-GS1 - Definitions and Specification Directory

General Specification 8791/09-GS2 - Quality Assurance / Quality Control

General Specification 8791/09-GS3 - General Offshore Specification

General Specification 8791/09-GS4 Offshore Survey

General Specification 8791/09-GS5 Offshore Positioning Systems

2. FREESPAN CORRECTION REQUIREMENTS

2.1 CONTRACTOR should be aware that unsupported freespans are likely to exist along theWest Natuna pipelines, especially in the as-laid empty condition.

2.2 CONTRACTOR shall determine critical freespan lengths by analysis of pipeline staticand dynamic (in-line and cross-flow vortex shedding) stresses and fatigue potential in the

following conditions:

Installation (empty)

Flooded

Hydrotest

Operation

2.3 Freespans may occur on level or sloping seabeds, which can slope along the pipeline axis,

perpendicular to the pipeline axis, or a combination of both. CONTRACTOR shall

determine appropriate freespan correction methods for level and / or sloping seabeds, andshall also take into account seabed soil characteristics when determining the most

appropriate freespan correction methods.

2.4 CONTRACTOR shall be responsible for all pre-pipelay and post-pipelay rectification

work to ensure that allowable unsupported pipeline spans are not exceeded.

2.5 CONTRACTOR shall review pipeline route bathymetry and geophysical conditions,

prior to mobilization in order to optimize the pipeline route and reduce freespans.

2.6 CONTRACTOR shall perform diver, ROV and / or side-scan sonar surveys along the

pipeline, following installation and prior to flooding, to determine the location, lengthand separation off the seabed of any freespans along the pipeline route.


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2.7 Where freespans are predicted to occur along the pipeline route, which exceed the

allowable dimensions for the installation case, CONTRACTOR shall install supports

prior to pipelay.

3. FREESPAN CORRECTION PROCEDURES

CONTRACTOR shall prepare for COMPANY approval procedures for the correction of

freespans exceeding allowable limits. The procedures shall include the following:

Freespan detection and location (including video recording)

Freespan dimension determination (including seabed separation)

Determination of seabed slope and direction of slope relative to pipeline axis

Determination of seabed soil conditions

Appropriate span correction methods

Installation of span supports

Span reducing measures

4. FREESPAN SURVEYS

CONTRACTOR shall perform surveys in accordance with Project Specification forOffshore Survey (8791/09-GS4).

4.1 Following pipeline installation, and before filling with hydrotest water, CONTRACTOR

shall determine the extent of any pipeline freespans. Freespan surveys shall detect and

report all spans and list spans exceeding the critical dimensions. The list shall givelocation, length between supports and height of pipe above the seabed at 5 m intervals.These results shall be submitted to COMPANY immediately after pipelay, to allow

COMPANY to promptly assess any need for span correction.

4.2 COMPANY may make independent surveys to verify CONTRACTOR freespan survey

results.

5. FREESPAN CORRECTION METHODS

Unacceptable freespans shall be corrected by providing intermediate pipeline supports.Depending on the separation between pipeline and seabed, and water depth at the

location, one or more of the following methods may be applied:

Installation of sand-cement bags under the pipeline (generally where pipeline

separation off the seabed is less than 1.0 m).

Installation of specially designed grout bags filled with grout from a surface vessel(generally where pipeline separation off the seabed is less than 2.0 m).


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Placing rock under predicted pipe freespans prior to pipelay in accordance with

Project Specification for Pre-Pipelay Trenching and Backfilling (8791/09-ST1).

Mechanical supports under the pipeline (generally where pipeline separation off the

seabed is greater than 1.0 m).

Freespan correction methods shall be permanent and strong enough to support the

pipeline throughout the design life. CONTRACTOR shall consider the effects ofsettlement and scour during design of the freespan supports.

CONTRACTOR shall propose freespan correction methods for COMPANY approval, for

each individual freespan.

6. ALTERNATE SOLUTIONS

CONTRACTOR may propose alternate methods or designs to correct or reduce freespansbased on experience and specific equipment or tools available. Such proposals shall besubmitted to COMPANY in enough time to allow for technical evaluation.

Documents

Specification for Freespan Correction Ps3