Pre Trenching services
Pre Trenching is often a key element of pipeline shore approaches and it is often cost effective to combine trenching in a single package with the other critical pipeline shore approach elements including tidal zone cofferdams, pull winches and holdbacks, pull wire buoyancy etc to take advantage of natural synergies.
OCS owns and operates a multipurpose utility barge, a bespoke customised 160’x40’x10’ (48768mm x 12192mm x 3048mm) vessel, constructed specifically for nearshore projects,which can be configured into the following operational modes:
- Pipeline pre-trenching or general near shore trenching work
- Pipelay Operations.
- Pipeline Post Trenching.
- Support vessel on which either Linear pull winch or Horizontal Directional Drilling equipment can be based
- Shallow Water Decommissioning projects.
- Miscellaneous Marine Construction support work such as dolphins & Jetty work, free span correction, etc.
Key features of the vessel include:
- 3 nos. spud wells with three 18m x 26”Ø spuds operating from 3 independent winches to keep Ms Pennie in position during pre-trenching using excavators;
- 8 nos 10T air operated winches below deck with centralised controlled – providing a total of 11 point mooring system (where necessary);
- 2 units – CAT 390 Excavator – with modified long arms and 2m3 bucket able to reach to 16m below the barge deck (critical for efficient pre-trenching operations)
- 1 unit 90T crane for spud handling and general lifting
- 1 unit – CAT 320 Excavator (convertible to amphibious mode) with standard and modified long arm with a reach of 8.5m below the track.
- Survey and positioning system
- Trench depth monitoring system
- Trailing Arm – This is a rigid arm designed for deployment from OCS Utility Barge UB-01 “Miss Pennie”. The arm provides a rigid frame to deploy a dredging head housing 2 submersible dredge pumps and working in water depths up to 25 metres. The trailing arm works with the robust 8-point mooring allowing the Miss Pennie/dredge head to track along the trench line as required. The system can be deployed diverlessly using scanners(provides a pre-trenching option in water depths beyond the reach of the CAT 390 excavators and also provides an efficient trench cleaning and maintenance solution to keep the trench open while waiting for installation of the pipeline).
In addition to any shore approach planning, crew accommodation, cofferdam requirements, pull winch and hold back requirements need to be considered during planning and preparation for pre trenching projects.
PRE TRENCHING EXPERIENCE
OCS has undertaken numerous major post-trenching projects for different customers;
- Shell Exploration Pte Ltd (SEPL) SBM Bukom Pipeline Rejuvenation Phase II Project (Ongoing). 75m pre-trenching to design profile from cofferdam to offshore side (Project is currently ongoing at the time of this document revision)
- ESSO Thailand Ltd – Construction, Installation and Precommissioning of 48” Subsea pipeline at Sriracha Refinery in Chonburi Thailand (Completed Oct 2021). 835m pre-trenching to design depth/profile from cofferdam to offshore up to water depth 25m (using trailer arm dredge)
- HCML (Husky CNOOC Madura Ltd) BD Project Pre Trench/Shore Approach Beach Pull/Post Trench (Completed Aug 2016) – 16” CWC pipeline, 52km long with 5.8km burial with 4.8km pre-trench and 1km post trench with 2m cover from T.O.P. Near shore location at Pasuruan City, Surabaya, Indonesia: Pre-Trenching of an open trench for a 4.8km beach pull. The trench depth was 2m T.O.P.
- Bukit Tua, Ketapang (Petronas, Completed Aug 2014) near shore at offshore Gresik in Surabaya, a 110km long of 12” CWC pipeline from shore LFP to offshore WHP, pre-trenching of first 4km pipeline for beach pull and post trenching of 27km of 12” CWC pipeline to 2m TOP: Pre-Trenching of an open trench for a 4km beach pull. The trench depth is 2m T.O.P.
- APD/Salamander (SericaKambuna Development project) 2009/2010, 14″ CWC pipeline x 13 km total burial distance (8km post trench), 2.0m TOP cover, near shore north of Medan, Sumatra, Indonesia. Pre-Trenching of an open trench for a 5km beach pull. The trench depth was 2m T.O.P.