Introduction
The subsea drilling and production business is dependent upon a variety of vessels to support exploration drilling, development, production and workover of wells in shallow and deepwater.
New and innovative operational methods are continuously being envisioned and developed to support these efforts. In this section, you will be exposed to several of the most prevalent vessels.
There are four typical methods in which subsea well systems may be tied back in order to accommodate production. Note the Fixed Platform as the name suggests is fixed to the sea bed by fabricated columns. The MSV, TLP, and FPSO all float and do not have a sea bed support structure.
Fixed Platform: To 1,650 ft
Compliant Tower: 1,500 to 3,000 ft
Mini Tension Leg Platform: 600 to 3,500 ft
Tension Leg Platform: 1,500 to 4,500 ft
SPAR Platform: 2,000 to 7,500 ft
Floating Production Systems (FPSO and FPF): 1,500 to 7,500 ft
The structural and distribution technologies have adapted to the increasing challenges of producing in deeper and deeper waters.
Diving Vessel, Service Rig, and Semi-submersible Rig
For the “heavy” part of the installation, normally called a workover or completion, a semi submersible rig is typically used. For lighter jobs, often called intervention, it is normal to use diving vessels or smaller service rigs.
A semi submersible rig, as the name suggests, means that the columns and hull that support the rig can be filled with water to partially (semi) submerge the rig or emptied to float the rig on the surface.
Partially submerging the rig provides increased rig stability especially in heavy seas. Rig can be ballasted for transport by a vessel or can be towed to location.
A semi submersible deck and moon pool arrangement is ideal for handling the subsea equipment associated with subsea drilling and completion equipment.
This type rig allows use in deep water applications with dynamic positioning.
Drill Ships
Drill ships allow work to be completed in deep water without anchors using a dynamic positioning system. Dynamic Positioning (DP) is a system to automatically maintain a ship’s position and heading by using her own propellers and thrusters. This allows operations at sea where mooring or anchoring is not feasible due to deep water, congestion on the sea bottom (pipelines, templates) or other problems. Additionally, this vessel does not require towing between locations.
The drill ship, as the name implies, has a ship shaped hull with the derrick typically mounted over a “hole” in the center of the hull. Drill ships and semi-submersibles – also known as Mobile Offshore Drilling Units (MODUs), have this “hole” in the hull or deck to allow passage of the subsea equipment to the sea floor. This hole is
called the moon pool.
Moolpool Areas
The moon pool size is typically 6 meters square and as such all subsea equipment must be designed to pass through this size.
Supply Boats
The supply boat is the workhorse of the offshore industry and transports all supplies to the offshore platforms and rigs. They carry everything from food stuffs, chemicals, casing to subsea production equipment. The picture below shows a compact subsea manifold on the deck of a supply boat.
Other Support Vessels
Other vessel used include Dive Support vessels and Multi Service Vessels that provide services including diver operations for multiple operations, light weight intervention to subsea trees, flow line and flow line jumper installation and rock dumping to protect flow lines.
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