Offshore trestle bridges and Flare towers

A Trestle is a rigid frame typically composed of a horizontal top chord and bottom chord rigidly connected by diagonal and vertical supports to form a short span bridge to the designated area. Normally one end of spans is pivoted to a common deck and the other end rest on a sliding bearing. The deck is supported by piles and jacket arrangement. Provision is made for catwalks utility platforms on the bridge as per project requirement. The flue gases from the processing plant are connected to flare towers to burnout safely at pre- determined height via the trestle bridges.


3D model of Trestle bridge and Flare towers

offshore4_2Actual Trestle bridge with Flare towers under construction

 CES has involved in development trestle consists of 8 bridges over 80m span. It includes a 55m high LP flare tower and a 105m high HP flare tower. Pile depth of over 50m meters were supported by a jacket from mud level. The development includes supply of manufacturing drawings sub assembly and assembly drawing.  A separate drawing for nesting of plates issued category wise and issued pipe nesting and copping drawings. A detailed MTO (material take off) list was prepared for pipes, plates, round bars, fasteners, bought-out-item etc. Around 4000MT of steel were processed to fabricate flare trestle apart from supply item issued by client.

Deck structure
A typical duct structure consists of Tubular legs with braces and an interconnected plate girder to form a flat support to rest the one end of the bridge through a slide bearing. The other end of the bridge is pinned through a guide. The two end of the bridge is connected through walking platform.

offshore4_3A typical Deck and Jacket Structure

 Jacket Structure
Jacket structure consists of welded Tubular legs interconnected with horizontal and diagonal tubular braces. At the bottom of legs a dish shaped mud-mat is fixed to rest at the sand of sea bed.

Bridge structure includes bottm chord, top chord, veritical and digonal braces, walkways, pipe supports, lifting padeys, heatshields and with or without cantiliver platforms. The top and bottom chords are made  out of plate  rolled into cans and welded and joined through seam welding.  Can lengths are predetermined  and located in such a way to avoid the over lapping of welds with brace connections and/or padeys as well as conection platform etc, as per EEMUA (Engineering Equipment and Materials Users  Association) standards.

offshore4_4 A typical bridge and close-up showing details

A sturdy saddle mount is provided at bottom chord for resting the bridge over the deck.  A build in pre-chamber is part of the design to avoid sag due to its load. A bumper guide is in build on all bridges and jackets to keep proper alignment while erecting.

offshore4_5A typical bridge Close-up showing details of working platform

At bottom chord a walkway is provided for operation and maintenance personnel to carry out their day to day work. Also provision for heat shield is provided at key areas to protect instruments and fittings. At key areas, working platforms are fitted with cantilever structure on bottom and top chord. Provision for ladders safety gates, and handrails are provided all around on such structures.

Flare tower
Flare tower is normally a tripod shaped structure made up tubular cans connected with horizontal and diagonal braces to support flare pipe at the center. At the top a working platform with provision for davits to lower and mount the flare burners are provided. At one of the faces, at sea level, a boat landing facility is provided for easy access for man and material. At lower level a working platform is provided to connect the last end of the connecting bridge.

offshore4_6A typical Flare Tower and Close-up showing details

A set of Intermediate landing platform connects the lower platform with top working platform through ladders. Apart from insulation on flare pipe, depending upon radiations level of flare flame, the structure is divided in to various temperature zones to apply fire retardant paints to protect the structure.

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