​A new industry benchmark Case Study

The Need

Despite a booming offshore oil and gas industry with a growing number of complex and technically advanced solutions entering the market, experienced and ever-cautious asset owners and operators remained grounded in fiscal realities – and identified a market for a cost-effective but capable platform.

Vard Marine brought together industry leaders and innovators to collaborate on a design for a semi-submersible platform that would outperform competitors – with superior deadweight and deck cargo capacity, DP-3 capability and flexible mission fit.

What emerged was a compact but highly capable, configurable and cost-effective platform that set a new benchmark in the offshore industry.

The Challenges

Developing a smaller platform with equal or increased capability than traditionally sized semi-submersibles resulted in significant structural challenges. Low wave gap clearance, single strut design and high prying loads demanded novel solutions.

One of the greatest challenges was to design a hull structure that could withstand prying loads from the multi-hull configuration and maintain a fatigue life adequate for unrestricted operations worldwide. A comprehensive array of Finite Element Analyses – in combination with model test load data and other ship motion analyses – gave us the data we needed to develop the right structure.

Engineering a relatively simple design and hull form was essential to delivering a cost-effective solution that could be built in shipyards throughout the world.

Classification Society regulations not entirely relevant to the final solution required analysis and close communication in order to achieve rule modifications that would grant us the value of a fully approved and reviewable design.

Innovative Solutions

The team designed a relatively simple structure with large flat panels and large radius corners to limit high stress concentration areas while maintaining an acceptable steel weight.

The critical upper haunch area received the greatest focus. This location, which connects the inner face of the vertical strut to the underside of the main transverse wet deck/hull structure, generates the highest structural loads and carries the most potential for fatigue. The use of local insert plates in the shell and optimized framing helped alleviate stress concentrations and extend the fatigue life of this area of the structure.

We performed specific analysis to ascertain the wave impact loads on the underside of the wet deck, as this is a complex scenario with limited experimental or analytical data available on which to base a design. The design has now received full ABS approval with unrestricted service notation based on draft and wave height limitations.

Most of the new compact semi-submersible (CSS) designs include large capacity cranes that require the ship to be ballasted to its wet deck for maximum stability during heavy lifts. To accommodate this need, the vessels can be fitted with large ballast tanks and a comprehensive ballast system to quickly move great volumes of water, reducing the overall time to perform the lift. The CSS vessels were designed to be operable throughout the world and come with various ballast treatment options to suit the mission and client preferences.

Outstanding Results

CSS designs provide a configurable platform that can be adapted, and easily re-adapted, to changing customer needs while outperforming more expensive purpose-built ships in operability and time on station.

The CSS designs offers substantially reduced roll motions coupled with a slight increase in pitch and heave motions when compared to a mono-hull design with equivalent capabilities.

Operability studies for the CSS accommodations model, stationed in the Campos Basin off Brazil, indicate operability ratings that are comparable to a much large 200 m mono-hull. The accommodations unit can remain connected to a rig by gangway 98% of the time, suggesting that only the most extreme weather events will require the CSS to detach from the rig.

Due to the novel DP-3 arrangement and configuration of the CSS it can still remain connected 89% of the time even in the event of damage or loss of one thruster from fire, flood or equipment failure in adverse weather conditions. The CSS can therefore maintain service, maximise asset uptime and continue to meet its charter under nearly all foreseeable conditions, while remaining efficient and effective in both capital outlay and operational costs.

The wide deck spanning the twin hulls permits versatile use of deck space and a wide variety of options for unit configuration. This provides an open, stable platform for many applications, allowing owners to quickly reconfigure at little cost based on different charter requirements.