The Solan Oil Storage Tank (SOST) is a vital component of Premier Oil’s new Solan oil field, west of Shetland. The Solan field does not use a pipeline for oil export, instead, the SOST collects and stores the oil extracted from the field prior to export by shuttle tanker. Any delay in the tank’s installation would have had a major impact on the platform’s ability to transport oil to market. Aker Solutions was contracted to provide hook-up, commissioning and facility management services to Premier Oil for the whole Solan development.
DNV GL’s Noble Denton marine assurance and advisory team was selected by Aker Solutions to co-ordinate all stages of the SOST towage, mooring-up and monitoring operation from the Brei Wick area to Lerwick Harbour. The team also coordinated the controlled release of moorings prior to the handover of the tank for its final installation at the Solan field. The project team inspected and approved all tugs, workboats and crews associated with the marine operation. In addition, working directly for Premier Oil, the team assisted with the safe departure of the SOST from Dubai Drydocks prior to transportation to Shetland.
A bespoke mooring system
The operational phase included a challenging towing operation after the tank had been offloaded from Cosco’s Heavy Lift’s transport vessel, Xiang Yun Ko at Brei Wick. “This potentially difficult towage operation through the relatively narrow Loofa Baa channel was completed without incident under the direction of the Noble Denton marine assurance and advisory onsite master mariners,” said Martin Brown, DNV GL senior consultant naval architect.
The SOST was then carefully moored at Lerwick Harbour using a bespoke mooring system designed by the DNV GL project team to accommodate the worst anticipated weather conditions for the time of year. This bespoke mooring system included hand-operated winches to pre-tension the mooring lines to pre-calculated values that were measured using load cells on each mooring line. Aker Solutions on site commissioning team, Lerwick Port Authority, Premier Oil and Shetland Maritime have worked together as an integrated team throughout the operation.
Bespoke mooring system including hand operated winches to pre-tension the lines and load cells to continuously monitor mooring loads
“The Solan SOST tank is one of the largest assets to have been offloaded at Lerwick Harbour to date,” explained Brown. “Due to its large wind area, it was necessary to prepare in advance for its arrival. This included installing two additional mooring bollards on the quayside, so as to be able to resist safely the calculated maximum mooring loads.”
Although the offload and mooring up/release operation took place over a period of approximately four days, it was 10 months in the planning: “Quayside mooring of large floating objects is always an area of potential difficulty and also potential exposure to personnel concerned with establishing and tending the mooring system,” said Brown. “A key element for any successful marine operation is to plan ahead in good time.”
To ensure this, DNV GL undertook a site-specific mooring analysis early on in the project to identify the civil engineering work that would be required. “If this analysis and follow-on construction work had not been carried out, it is possible that the mooring system might not have been able to safely moor the SOST at the quay when required,” explained Brown. “In addition, checks were needed of under-keel clearance for all expected tidal conditions to minimise the danger of the danger of the deep- draught SOST grounding.”
Backed by decades of expertise
To facilitate the project, DNV GL’s experts worked to ensure risks were as low as reasonably practical (ALARP). This was supported by risk assessments carried out in conjunction with Premier Oil, Aker Solutions, Lerwick Port Authority and personnel from the tow tugs.
The project was completed on schedule and with no injury or incident of any description. “It was an example of how good engineering, sound marine practice and experienced marine personnel on site could all come together so that a relatively complex marine operation could proceed smoothly,” concluded Brown.
Developing a new recommended practice
DNV GL is currently developing a new recommended practice for quayside mooring. It will identify what factors need to be taken into account to ensure that quayside mooring is safe and fit for purpose; for example, the identification of all potential load paths and the investigation of required safety factors and analysis methodologies to accommodate possible maximum dynamic loading.
“The SOST project has identified the importance of addressing early on in a design how mooring line pre-tensions can be safely established,” said Brown, explaining the background to the engineering phase. “It also illustrated that with up-to-date waterproof instrumentation, such as load cells, it is possible to continuously monitor line tensions to confirm that analysis predictions and in-field performance are comparable. This helps to ensure that a balanced mooring system has been installed, with proper load sharing of individual lines, thus reducing the danger of excessive loads and possible mooring line failure.”