Aker Solutions has unveiled a new, low-cost semi-submersible rig design for marginal field development
As marginal field development becomes a more serious concern for operators and engineers, new innovation in the rig market is beginning to reflect the changing concerns of the industry. With day rates and utilisation so low, many rigs, whether semi-submersible or jack-up, can be overdesigned and over-equipped for the jobs they need to do. This has already been seen in the intervention market, where newer and more capable vessels are replacing costlier workover rigs.
Marginal field development has, so far, been restricted to subsea tie-backs to nearby existing infrastructure. However, as these marginal fields are beginning to take priority – in the UK and Norwegian continental shelves especially – operators are after efficient solutions for field development, which can maximise production at minimal cost. A number of initiatives have already been introduced for fixed platforms such as subsea on Slim Legs (Statoil), Subsea on a Stick (Kværner), and STEP (Det norske and Aker Solutions).
However, a new approach has been suggested by Aker Solutions. Its latest Lean Semi™ design is intended to help bring fields of up to 300 million barrels of oil equivalent on stream. Suitable for water depths of 100-400m, it is lighter, cheaper and faster to produce than other semi-subs on the market.
Lean if you want to go faster
Aker embarked on the rig design tasked with achieving a 60,000 bpd production capacity with reduced investment and hire costs.
The topside itself weighs around 6,000 metric tonnes – making the Lean Semi up to 30% lighter than comparable rigs – and has a displacement of 22,500 tonnes. The platform design uses a combination of elements from two proven designs for the North Sea and the Gulf of Mexico and is the result, the company says, of “a painstaking review of every single platform item.”
It features an unmanned hull combined with a single flat top deck to ensure its structural integrity. The hull is designed without manned spaces and without connection between the pump rooms in the four quadrants. Ballasting and de-ballasting is performed by two caisson pumps in each column.
The deck is then integrated with the hull via a wet truss, which allows for wave impact with the lower part of the structure. This design also allows for more flexibility in the rig’s production schedule, as well as in deployment. The streamlined setup means that platform delivery time has also been reduced to around 29 months.
“We envisage these platforms will be ideal in developing marginal resources that are located next to bigger fields with excess processing capacity,” said Valborg Lundegaard, head of Aker Solutions’ engineering business. “The concept combines lean design philosophy with the highest levels of safety.”
The topside is also designed to use standardised equipment, meaning equipment skids and small modules can be placed easily and directly on the deck. In this case, skid-based topside equipment is provided by Fjords Processing and covers separation, electrostatic dehydration, water treatment and sand treatment.
Aker explained to InnovOil that its design philosophy in approaching the rig was to focus on “need-to-have, instead of nice-to-have.” This thinking will be vital as marginal development plans continue to contend with price pressures, but with a stripped-down approach, new ideas like the Lean Semi™ could ensure these plans are possible.