Buoyancy Lifting of Offshore Platform Jackets

Abstract

In this thesis a model is set up to assess the economic viability of early stage buoyancy design concepts. The main research question is to find out if a versatile design approach to the buoyancy lifting concept, i.e. being able to take on multiple projects, can result in an economical viable solution for decommissioning offshore platform jackets in the North Sea. The economical viability is tested against the benchmark figure set by the current heavy lifting removal costs.
From a market assessment it becomes clear that 8,190MT is the most promising size for a buoyancy device with an unknown versatility range. The minimum versatility range that the buoyancy concept of 8,190MT should achieve, is 940MT. A removal cost of 3,500 €/MT is used as a benchmark, in accordance with the Oil & Gas Authorities' estimate and cost reduction milestone of 35%. Three buoyancy lifting concepts are set up using design requirements distilled from the market assessment. These are the single structure DeltaLifter concept, the double external buoyancy caissons (EBC) and a configuration of external buoyancy tanks (EBT). All three concepts are tested on their technical feasibility and the operational practicability.
The methodology used in this research to assess the economic viability is to calculate the removal cost per metric tonne for every concept and compare it to the benchmark. To find the removal cost per metric tonne the initial investment costs, the job related voyage costs and the time dependent operating costs are estimated in the model.
The investment costs consist of three parts; the building costs, capital costs and scrapping income. The building costs are constructed with a model of Carreyette and optimised for pontoons. Later ballast control system costs and clamping and skidding costs are added. The capital costs are omitted from this model as it is unlikely it will be financed with any form of debt. The scrapping income is modelled as the steel weight times the scrap rate per metric tonne. The DeltaLifter turns out to be the most expensive structure to build, at roughly 17 million euros. The other two concepts will costs between 10 and 12 million euros.
The voyage costs are build up by multiplying the day rates of the assets required to operate a buoyancy lifted removal by the number of days offshore and adding the lump sum costs for an operation. The voyage costs lay between nine and eleven million euros per job, for the DeltaLifter and the EBT concept respectively. The fixed voyage costs are what could be expected for a buoyancy lifted removal. The running costs are higher than expected, due to the large amount of assets modelled to run an operation.
The operating costs for buoyancy lifting concepts consist of two parts, i.e. the storage costs and the maintenance costs. The storage costs are modelled as quayside storage using quotes from different ports around Europe. The maintenance costs are modelled as a percentage of the investment costs. The yearly operating costs of the three concepts lie close together around 250,000 €.
Finally the removal cost per metric tonne is calculated for the three buoyancy lifting concepts. It can be concluded that a versatile design approach to the buoyancy lifting concept, i.e. being able to take on multiple projects, does result in an economical viable solution. All three concepts presented in this research break even under the benchmark for multiple jacket removals. From the three concepts presented in this report, the EBC concept turns out to be the most promising concept. It is the cheapest structure to build and operationally the best concept.