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:
·
Basis of Design 84506-60-00-2L-060
·
General Specification
84506-60-00-2L-000 - Definitions and Specification Directory
·
General Specification
84506-60-00-2L-010 - Quality Assurance / Quality Control
·
General Specification
84506-60-00-2L-020 - General Offshore Specification
·
General Specification
84506-60-00-2L-040 - Offshore Survey
·
DnV OS-F101 - Submarine Pipeline
Systems (2000)
·
DnV Guidline No.14
2. FREESPAN CORRECTION REQUIREMENTS
2.1 CONTRACTOR
should be aware that unsupported freespans are likely to exist along all
flowline routes, especially in the as-laid empty condition.
2.2 CONTRACTOR shall determine critical
freespan lengths by analysis of pipeline static and dynamic (in-line and
cross-flow vortex shedding) stresses and fatigue potential in the following
conditions:
·
Installation (empty)
·
Flooded
·
Hydrotest
·
Operation
Evaluation
of freespan length shall be in accordance with DnV Guideline No. 14.
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, and shall also take into account seabed soil
characteristics when determining the most appropriate freespan correction
methods. The CONTRACTOR shall be responsible for consequences arising from
undulations in the pipeline path which give rise to out of specification
spanning.
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,
for all the conditions specified in section 2.2 of this specification.
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, ROV surveys
along the pipeline, following installation and prior to flooding, to determine
the location, length, touch down conditions and separation off the seabed of
any freespans along the pipeline route.
2.7 Where freespans are predicted to occur
along the pipeline route, which exceed the allowable dimensions for the
installation case, CONTRACTOR in accordance with his responsibilities, 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 for Offshore Survey 84506-60-00-2L-030.
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 give location, 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 permit the COMPANY to promptly assess the need for
span rectification by the CONTRACTOR.
4.2 COMPANY
may make independent surveys to verify CONTRACTOR freespan survey results.
5. FREESPAN CORRECTION METHODS
5.1 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 specially designed
grout bags filled with grout from a surface vessel (generally where pipeline
separation off the seabed is less than 2.0 m).
·
Mechanical supports under the pipeline
(generally where pipeline separation off the seabed is greater than 1.0 m).
5.2 Freespan
correction methods shall be permanent and strong enough to support the pipeline
throughout the design life. CONTRACTOR
shall consider the effects of settlement and scour during design of the
freespan supports.
5.3 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 freespans based on experience and specific equipment or tools
available. Such proposals shall be
submitted to COMPANY in enough time to allow for technical evaluation.
7. RECORDS
Upon Work
completion, CONTRACTOR shall submit all records to COMPANY for permanent
retention, as specified in the Contract Documents. All records shall be clearly
identified and traceable.
All data records shall be submitted to the
COMANY in both hard copy and electronic format.