The bank asks when placement from a barge is more practical than placement from the bank. The answer is usually one of four conditions.
When it is the right call
The work is beyond reach from land. Out in the channel, at a berth face, offshore.
There is no standing position. A bank too steep, too soft, too narrow, or already failing.
Land access does not exist. An island, a site with no route in, a reach with no adjacent land in the right ownership.
Land access exists but is worse. Building a temporary causeway and working from it may cost more and disturb more than mobilising a barge. See temporary working platforms.
What has to fit on deck
The production moves onto the water, which means everything on the staging list has to fit somewhere:
- The lifting machine, secured, with the deck rated for it
- Filled units, or empty units plus aggregate plus filling operation. Filling on deck is possible and it takes a lot of space
- Working room around the machine’s swing
- Crew and welfare
The usual arrangement is filling on shore and transporting filled units, because a filling operation on a barge consumes deck area that lifting needs. That means a shuttle cycle, and the cycle time frequently governs productivity rather than the placement rate.
Stability
A crane on a floating platform is a different problem from a crane on land.
- The load list changes the vessel’s trim and heel as the unit swings and as deck stock is consumed
- Deck loading has to be within the vessel’s limits and distributed
- Free surface effects in tanks
- Anchors and spuds or dynamic positioning to hold station
Vessel stability calculations for lifting operations are specialist work belonging to the marine contractor and the vessel’s operator. This page is not a substitute for that.
Holding position
Placement accuracy depends on knowing where the vessel is, and holding it there.
- Spuds on a spud barge in shallow water, which is the most stable arrangement
- Anchors, with the anchor pattern planned around existing assets on the bed. Anchoring near a cable or pipeline is its own hazard assessment
- Dynamic positioning, offshore
- Positioning reference, because placement is measured rather than visual. See underwater placement
The bank covers how placement tolerances are established above a subsea cable or pipeline, and how safe separation is confirmed during placement. On a congested bed, the anchor pattern needs the same care as the placement.
Weather and tide
The constraint that decides the programme.
- Sea state or river conditions limit both lifting and vessel safety
- Wind limits crane operation
- Tide may govern access, draft over the work, and the workable hours per day
- Current affects both station-keeping and how a unit descends
- Visibility for the operation and for navigation
A marine spread waits on weather, and waiting is on the day rate. On a scope with an in-water work window as well, the intersection of window, weather, and tide can be a small number of workable days. See in-water work windows.
Navigation and other users
Working from the water means being in someone else’s navigation channel.
- Notice to mariners where required
- Marking and lighting the spread
- Coordination with the harbour authority or waterway manager
- Section 10 authorisation, since work in navigable waters is regulated on navigation grounds. See Section 10 of the Rivers and Harbors Act
- Keeping the channel usable, or agreeing the restriction
At an operational berth, the work happens between vessel calls, which makes the schedule someone else’s. See scour at marine piles and dolphins.
The cost shape
We publish no prices, but the structure is worth knowing.
A marine spread is a day-rate operation, and the day rate accrues whether you are placing, shuttling, or waiting on weather. That has two consequences:
Productivity per day matters more than usual, which favours arrangements that keep the crane placing rather than waiting on a shuttle.
Mobilisation is a large fixed cost, so a small scope carries it badly. This is the same economy-of-scale argument that governs fall-pipe rock installation, and it applies to any marine spread.
Where a construction vessel is already committed to the field for another reason, placing protection from it avoids a separate mobilisation entirely, and that is frequently the deciding factor.
Where to go next
- Underwater placement
- Equipment selection
- Site access and staging
- Marine and ports and offshore
- The rock bag FAQ for the marine and offshore section
- The installation hub for the rest