Multi-module topsides and skidded packages

How does Multi-module topsides and skidded packages work, step by step?

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   Step 1: Decide the split and where the boundaries fall

   The split is an engineering decision made early, and it is driven by weight, by yard capability, by vessel availability and above all by where the fewest connections cross. Boundaries are placed along functional lines so that each module contains a complete set of related equipment and its supporting systems, and so that the interfaces between modules are as few and as simple as possible. A split that cuts through a complex system multiplies offshore work and is avoided even when it looks attractive on weight. The decision is documented with the interface schedule that will govern the rest of the project.

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   Step 2: Control the interfaces across separate fabrications

   Modules are commonly built in different yards to different programmes, and the only thing keeping them compatible is interface control. A single controlled set of dimensions, tolerances, connection details, cable and pipe schedules and system boundaries is issued to every yard, and changes to it are managed formally. Trial fitting of connection details, dimensional control surveys during fabrication and a common survey datum are all used to make sure the modules will actually go together. This work is unglamorous and it is what determines whether the offshore joining is straightforward or painful.

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   Step 3: Build and test each module complete

   Each module is fitted out and tested as far as it can be on its own, with its internal cabling installed and terminated, its systems commissioned to the module boundary and its records assembled. The aim is that each module arrives offshore proven up to its interfaces, so the only testing left is across the joints. Skidded packages are the extreme version of this: a complete assembly on its own frame, factory tested and shipped ready to be landed and connected. The more that is closed out at this stage the less exposure there is offshore.

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   Step 4: Weigh, load out and transport module by module

   Each module is weighed and its centre of gravity established before load-out, because each is a separate lift with its own plan. Load-out, sea fastening and transport are arranged per module, and modules may travel separately or together depending on the barge and the programme. Protection for the transit is fitted at every open interface so that connection faces arrive clean and undamaged. Sequencing matters here: the modules have to arrive in the order they will be installed, which constrains the fabrication programme as well as the shipping.

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   Step 5: Install the substructure and the first module

   The substructure is installed, piled, levelled and surveyed, and the survey is issued before the first module sails. The first module is landed and secured, and it becomes the datum for everything that follows, so its position and level are surveyed carefully once it is down. Any deviation is measured and its consequences for the next module are worked out before that module arrives. Getting the first module right is worth a great deal of care, because every subsequent alignment problem compounds from it.

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   Step 6: Land and align the remaining modules

   Each further module is lifted or skidded into place against the one already installed, guided by mating arrangements designed for the purpose, and aligned before it is secured. Skidded packages are landed and pushed into position on rails or skid beams, which is a controlled and repeatable way of moving a heavy package into a tight space. Alignment is checked by survey rather than by eye. Once a module is aligned and secured the structural connections across the joint are made permanent to the procedure the designer has set.

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   Step 7: Make and test the connections across the joints

   The connections that cross each boundary are made in turn: structural, cable, piping, cooling, control and safety systems. Each is made to a controlled procedure with its own records, and each is tested once made. This is the work that modularisation created and it is the part of the campaign most exposed to weather and to access constraints, so it is planned in detail and resourced properly. The interface schedule prepared at the start of the project is the checklist used to prove that nothing has been missed.

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   Step 8: Retest the systems end to end and hand over

   Once the joints are complete, systems that were only proved to a module boundary in the yard are tested end to end across the assembled platform. That retesting is a real scope of work and it is planned as such, using the module records as the baseline so that only what changed is repeated. The cables are pulled in and terminated and the platform is handed to the commissioning team. Energisation itself is a controlled step carried out with the network operator under a formal authorisation regime.