River Crossing Procedure

The purpose of this procedure is to ensure that construction of distribution gas lines is through open cut river crossings are carried out correctly and in accordance with recognized practices, codes and standards.

Trenching
Trenching Pre-construction checks.
Prior to trenching the Foreman completes the Trenching Preconstruction checklist whilst completing the following check:
(a) The foreman checks that the alignment is clearly surveyed and installs any trench markings by measuring from the trench centreline offset stakes previously on the riverbank installed by surveyor.
(b) Temporary access around the trench for equipment transportation and other requirements including safety signs are provided,.
(c) The foreman checks document for underground services and equipment requirements for trenching.

Trenching Operation
The Foreman ensures that the following requirements are satisfied:
(a) The trench is the required width and depth, and is configured in accordance with the minimum allowable roping radius of the pipe.
(b) Locations requiring extra width or depth are properly and safety trenched in accordance with accepted practice.
(c) The material excavated shall be stored clear of the crossing.

Tie-In Welds
Tie-in welds will occur at both sides of the river crossing and will be done by the mainline tie-in crew(s).

The alignment and gap set-up of the ends is considerably more difficult to achieve than for production welds, because the overlapped pipes require accurate cutting to form a one weld tie-in or an additional pipe length has to be cut where there is a gap between strings to form a two weld tie-in.

Localized enlargement of the trench (bell hole) will be provided to allow adequate welder access. Shoring will be installed where necessary to ensure safe working conditions in the bell hole.

An external applied line-up clamp is used to hold the ends for welding after cutting the sizes. Actual welding procedure is essentially the same as for production welds except that the external clamp is released when 50% or more of the root bead has been deposited and there is no pipe movement until the weld is complete.

Lowering-in
Preconstruction Checks
Prior to the lowering-in operation the Foreman completes the Lowering-in Preconstruction

Checklist whilst performing the following checks:
(a) Ensures he has up to date copies of Alignment Sheets, and all necessary permits covering R.O.W. access.
(b) Inspection of trench for the section being lowered to ensure that any loose rocks, sticks or other debris that may have fallen into the trench are removed and where required bedding material has been placed correctly to protect the pipe corrosion protection coating.
(c) Dewatering of the trench has been adequately carried out where necessary.
(d) That the holiday detector is calibrated and set at the correct voltage.
(e) That all field joint coating and repairs have been carried out in accordance with the relevant procedures.

Lowering-in Operation
On completion of the preconstruction checks the lowering-in operation commences as follows:
(a) The pipe is picked up at the starting point using excavator or Foco truck fitted with appropriate lifting belts.
(b) The holiday detector is attached to the pipe in a manner that provides full coverage of the pipe circumference and enables the detector to move along the pipe at the prescribed speed and the pipe string is electrically earthed.
(c) Immediately prior to lowering-in the pipe is holiday tested in accordance with the specified voltage and any defects detected are repaired in accordance with the relevant procedures and retested.
(d) The pipe is then lowered gently into the trench. Careful attention is paid to ensure the pipe is in the centre of the trench and rests on the bottom of the trench.
(e) External force shall not be applied to force the pipe to the bottom of the trench. Should the pipe not rest completely on the bottom of the trench, then the pipe shall be supported with sand bags before backfill

Backfill
Backfill Preconstruction Checks
The Construction Superintendent is responsible for ensuring that backfill is installed correctly. Notwithstanding this, the Foreman does not use any material he considers unsuitable.

Prior to commencing backfill in an area the Foreman performs the following checks and completes the Backfill Preconstruction Checklist:
(a) Ensures that he has the necessary Permits on the R.O.W. at all times.
(b) Ensures that backfill material is free of any material that could damage the coating of the pipe and in accordance with the relevant Authorities requirements for road base materials.
(c) Checks the location of any cathodic protection (C.P.) test point leads to make sure backfilling can take place without damaging the leads.
(d) Checks for sand bags supporting the pipe and ensures they are 'knifed' immediately before backfilling commences. 
(e) Checks that the As-Built Traceability Survey has been completed for the next section to be backfilled.

Backfill Operations
Backfill operations will generally be performed using suitable material in the steps listed below. Note that due to the variations in local conditions along the pipe alignment, the Foreman will use his discretion in applying the methods described here.
(a) Carefully place material into trench with excavator or front end loader progressively backfilling until sufficient cover over the pipe has been achieved to provide a working zone.
(b) Selected backfill material will be placed in increments in accordance the local Authority requirements.
(c) Place concrete weight (during the backfill process) in accordance with the drawings where required.
(d) Marker tape is installed during the backfill process at river crossings where required.
(e) Caution is exercised around cathodic protection cables to ensure that no damage is caused.

Use of Backfill Checklist
The Foreman prepares one Backfill Checklist for each day's operation. At the start of the day the Foreman fills out the planned work for that day. The Foreman then carries out the preconstruction checks, completes and signs the form.

The remaining items are signed off at the completion of the day's work. The checklist is then forwarded to the Construction Superintendent.

Inspection And Test Report
Inspection would be performed by QC Inspector to ensure that the whole activities of river crossing is in accordance with project specification. The inspection shall be cover but not limited to:
  • Check bottom of ditch, deep of pipeline to the riverbed and length of crossing pipe against reference drawing.
  • Check distance of pipeline to other structure, minimum clearance is 500 mm).
  • If additional backfill material beyond that available from trench excavation is required, borrow soil shall be furnished (check borrow soil from debris, foreign material).
The Construction Superintendent completes a Daily Progress Report for each day's operation at the end of the day's work. Checklists completed by the foreman for the various activities provide the details required to complete the Daily Progress Report. The report is to be forwarded to the Construction Engineer.

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