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Seafastenings design

The scope

We were asked by a major UK offshore installation contractor to assist their structures team in the design of seafastenings for a piling spread.

The piling spread was to be installed across two offshore campaigns and seafastenings were required for both mobilisations. Due to the operational schedule of the vessel, the project needed to be completed in a short timeframe of six weeks.

Our scope of work involved the detailed seafastening fabrication, assembly and deck plan drawings for two deck spreads comprising of over fifty pieces of equipment. During transit to site, equipment has a tendency to slide around a deck due to vessel motion and environmental loads. This is a significant safety hazard for offshore workers and so all deck equipment must be adequately restrained by seafastenings. The deck equipment included:

  • A 127 te pilling fast frame
  • A hydro-hammer
  • Piles
  • Overboarding chutes
  • Winches
  • Containers

The solution

Our engineers used the DNV LASHCON tool to calculate the force of sliding equipment on the vessel deck and so the necessary restraining capacity of the seafastenings. As some of the equipment would be performing work from the vessel deck during the installation process, operational loads imparted on the seafastenings were also considered. This was prudent for such items as winches, overboarding chutes and the hydrohammer. To ensure safety in all designs, the structural calculations were completed in accordance with AISC 360-10.
Although designing seafastenings to restrain the loads is essential for the safe transportation and use of deck equipment, the design is constrained by both the deck strength and available space between neighbouring pieces of equipment. To avoid clashing of seafastenings or the welding of structures to weak points on the deck, we requested and reviewed all of the available deck drawings and deck plans.

To ensure that no undocumented modifications or features were present on the vessel deck, the drawing review was followed by a deck inspection. The busy schedule of the vessel made it challenging for the client to confirm a date for inspection but our proximity to the North Sea harbours allowed for rapid deployment of engineers when required.

When the client was eventually able to confirm the vessel’s arrival in harbour, one of our engineers travelled out of office hours to inspect the deck, taking measurements of key areas. The visit highlighted some challenging deck features which were absent from the deck drawings, resulting in modifications to the existing draft designs. The visit was invaluable to the project and avoided costly oversights which could have hindered or even halted the installation of some seafastenings.

Following the inspection and an intensive period of design, we provided the client with all of the deliverables for approval ahead of the mobilisation dates. During this time we liaised with the client to answer clarifications and responded to any questions from the fabricator. After client approval and fabrication, we continued to assist the client during the mobilisation phase, offering on-site support to oversee the safe landing and installation of seafastenings.

The benefits

From the initial kick-off meeting through to vessel mobilisation, we worked closely with our client to guarantee project delivery was on time, on budget and met expectations. This resulted in the successful completion of all design documentation and ultimately the safe transport and field operation of seafastenings for both offshore campaigns.

The project was performed and managed from our Aberdeen office, placing the expertise within walking distance of the client’s structural design team. This proximity allowed our engineers to hold face-to-face design meetings with our client on a fortnightly basis. Our engineers were also able to react quickly to project dynamics, which included the ability to visit the vessel and perform a deck survey at very short notice.

Seafastenings design

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