Main features of Semi-Subs
Semi-Submersibles are used for producing oil from fields in harsh environments. Semi-submersible have no storage capacity and require export pipelines, thus semi-submersible are an attractive choise for fields near existing infrastructure (pipelines, onshore oil terminals etc). Oil is produced and stored on the FPSOs. Shuttle tankers offload the FPSO in frequent time intervals
1. Metocean conditions, Depth, Mooring
- Suitable for any harsh environments. Semi-submersible have heave periods in the order of magnitude of Tn=20 s and good response in severe environmental conditions.
- Conventional mooring systems
2. Production and Storage
Semi-submersibles have small deck space and small payload so production facilities are often restricted in size. There are always export pipelines for gas and oil with capacities larger than that of the production facilities so capacity is limited by the production facilities. There is only a very small storage capacity for well testing.
3. Existing infrastructure
FPSOs are very economical solution for fields in remote locations without oil and gas infrastructure (pipelines, oil terminals etc.). Oil is stored on the FPSO and is offloaded by shuttle tankers. For exploiting the gas from the reservoir gas pipelines to the short must be in place. Often this is not the case; thus, part of the gas produced simultaneously with the oil is used for to cover the energy needs onboard and the rest is reinjected to the reservoir.
4. Field size and type
Semi-submersible are used for large field with large field life since Semi are build specifically for the field. Semis are used for producing both oil and gas.
5. First Oil
The construction of Semis simultaneously with drilling has an huge benefit for first oil.
6. Wells, Drilling, Well Inervantion
Only wet trees
The wells are not drilled centrally so
Production is not helt by well intervention
1. Preliminary Design
The aim of the project is obtaining an understanding of the basics of the optimisation of heave characteristics of semi-submersibles, in relation to their main dimensions and configurations
The six column semi-submersible consists of following construction elements:
- Two floaters
- Six columns
- One buoyant self-supporting upper structure
- Braces
The optimisation of rig overall performance is based on factors such as:
- Motions
- Capacity (drilling load)
- Construction
- Stability
Design requirement of semi-submersible can be split up into owner’s and shipyard requirements. Owners’ requirements are mostly related to operational aspects such as:
- Environmental conditions
- Variable drilling load
- Motion behaviour
- Stability
Whereas shipyard requirement are mainly related to construction issues such as
- Optimum use of shipyard fabrication system,
- Maximum construction width due to dock facilities