The Solan oil field is located offshore in the hostile north Atlantic ~135 kilometres west of the Shetland Isles, UK, in ~135 metre water depth. The field commenced production in 2016. The relatively remote field was developed by four subsea wells tied back to a single slim jacket with a minimum facility topsides. The development includes several innovative features such as full automation, which enables full remote control from an onshore control room in Aberdeen, UK, but strikingly, a world development first involving the installation of an independent steel subsea oil storage tank (SOST) located ~300 metres from the platform. The concept selection, design, fabrication, installation and operation of this novel feature are the subject of this paper.
The concept selection of the SOST was driven by several factors including HSE, life cycle cost and operational considerations for this modest sized development in a harsh marine setting. An obvious competitor concept is a subsea well and FPSO alternate; the pros and cons of each approach are compared, using actual field performance now seen versus assumption at the time of concept select.
The multiple-compartment SOST is capable of holding almost 300,000 barrels of stabilised oil; it has an in-air weight of ~10,100 tonnes and internal dimensions of 45m x 45m x 25m. The oil is offloaded to a shuttle tanker through a single anchor loading (SAL™) system with the offloading hose stored on the seabed, which is picked up using a pennant line. Oil export to the tanker is driven by hydrostatic displacement by treated seawater from the topsides. This operation is conducted while still producing oil to the SOST. The tank design has novel features for installation, oil-water interface management and corrosion monitoring and subsea inspection.
The tank's detailed design and construction phase involved some significant changes as the structural and fatigue life issues were fully analysed and implemented. The installation required significant pre-planning with the use of a very large lifting sling arrangement and a smart air venting and water ballasting system, to then land its eight skirt piles to a tight tolerance on the seabed.
Following a commissioning program and trial tanker offload using treated seawater alone, over the past year the SOST has now successfully undergone numerous oil fill and tanker offload operations. There have been learnings regarding the offloading operation but to date the concept has worked in a manner very close to expectation with a full tank offload achieved in less than ~36 hours. The paper closes with a review of how the concept could be improved further considering the experiences now gained from both the project and operational phases.
