NO345160B1

NO345160B1 - A floating production unit

Info

Publication number: NO345160B1
Application number: NO20180709A
Authority: NO
Inventors: Kristian Mellingen; Robin Balas; Odd Olav Vatne; Henrik Georg Hannus
Original assignee: Aker Solutions As
Priority date: 2018-05-22
Filing date: 2018-05-22
Publication date: 2020-10-19
Also published as: NO20180709A1; WO2019224205A1

Description

Technical Field

The present invention relates to a floating production facility, in particular a socalled FPO (Floating Production and Offloading unit) or a floating production and storage facility, in particular a so-called FPSO (Floating Production, Storage and Offloading unit).

Background Art

Such facilities are in particular widely used in offshore production of oil and gas, especially for smaller fields and fields far from shore, where it is not feasible to install an export line to bring the oil or gas to shore or to another facility.

The floating production and storage facility has a hull which contains storage tanks for oil and/or gas. These tanks generally fill the majority of the volume of the hull. Ballast tanks are conveniently arranged in the outer regions of the hull. The facility also has at least one deck with production equipment to process the oil and/or gas, such as removing water, separating oil and gas and compressing gas. This deck is usually in the upper regions of the facility, separated from the storage tanks and living quarters. Due to the possibility of gas leaks from the production equipment, the production deck has to be well ventilated, which means that it is open to the environments. The production deck is usually above the expected level of a 100 year wave.

In order to protect against fire or an explosion in the production equipment spreading to the storage tanks, it has become well established practice to create a separate deck between the process deck and the top of the storage tanks. This deck is commonly known as tank deck or sometimes cargo deck.

The tank deck should ideally be empty, but it is common to use this deck for necessary cargo handling equipment, such as pumps, valves, pipes and similar, for pumping oil and/or gas to and from the storage tanks as well as ballast pumps and pipes for the ballast tanks.

There are some solutions where the cargo handling equipment, such as pumps and pipes, has been moved away from the tank deck. One of these solutions is shown in NO 320663, where the cargo handling equipment is arranged in a central space surrounded by cargo tanks. The purpose of this solution is to avoid having pipes inside the cargo tanks and have short pipe sections from the pumps and valves to each tank. In addition to the arrangement of the cargo equipment stealing a large proportion of the space otherwise used for cargo tanks, the publication is silent about the use and design of the tank deck.

To avoid that flammable gas collects on the tank deck, the deck is commonly ventilated via openings in the hull. However, the openings will also let green sea onto the tank deck when the wave height is above a certain limit. To avoid the green sea waves from splashing onto the deck, the openings are equipped with green sea protection in the form of a grille, grate or other type of deflector that is intended to take away most of the energy of the wave when it splashes against the opening.

However, it has proved that the grille, grate or deflector is not capable of preventing the wave from splashing into the tank deck with great force. The wave may then damage structures and equipment on the deck and be a hazard to people who inadvertently are present on the deck.

If the grille, grate or deflector is made sufficiently impermeable to waves, it will also at the same time prevent proper ventilation of the tank deck. When the ventilation is poor, flammable gases may collect on the deck and pose hazards, both due to risks of being ignited and as a suffocating or toxic substance to people on the deck. The tank deck is considered a category 1 hazard zone because of the potential risk of leakage of hydrocarbons, which requires an efficient ventilation of this area. This is quantified in the standards by a minimum number of air exchanges of the area.

WO 2017/186284 shows a floating production and storage vessel having a hull with a longitudinal well for a riser arrangement. The hull also contains storage tanks for oil and/or gas and ballast tanks, a main deck immediately above said storage tanks, and a processing deck above said tank deck. The publication does not describe the decks in any further detail.

WO 1993/17905 shows a tank extending all the way up to the ship's deck, which is the upper deck of the vessel. In the upper part there is an air gap that can be filled with inert gas. There is no physical partition between the load and the inert gas. Any gas from the load will therefore inevitably mix with the inert gas.

WO 2004/080791 shows a tank installation for the storage of liquids, especially oil in a floating storage or production installation, comprising a number of tanks which are connected to pipe systems for supply of liquid to or removal of liquid from the tanks. The installation comprises a central section around which the tanks are arranged.

The present inventors have found out that there is no optimum solution between the grille, grate or deflector being able to reduce the energy of the wave to an acceptable level and at the same time fulfill the requirements for ventilation of the deck.

JP S5531736U shows a vessel where a partition is made a slight distance below the main deck to create a space between the main deck and a storage section that can be filled with inert gas, fresh water or seawater. The space may be used for various piping. The purpose of this is to create a barrier between the storage area and processing equipment on the main deck. The reference is silent about where the cargo handling equipment, pumps and piping for loading and unloading the cargo, is arranged.

Summary of invention

The present invention offers a solution to the above dilemma by moving at least the majority of the cargo handling equipment etc. from the tank deck to a higher deck, such as the central area of the production deck. Room for this equipment can be made by reorganizing the layout of the process deck. According to the invention, the cargo handling equipment is placed on a separate pump deck elevated above the tank deck but depressed relative to the main deck. The tank deck is then made into a closed off area which is filled with inert atmosphere or is at a moderate overpressure. This is achieved by the tank deck being contained within a watertight and airtight barrier; that said hull forms an outer part of said barrier and extending along the height of said tank deck; that said tank deck is either filled with an inert atmosphere or kept at an elevated pressure; and that equipment that may pose an explosion hazard, such as cargo handling equipment, is arranged outside of said tank deck. This provides for controlled spillage handling. Any spillage will not reach the main deck, but can be confined to a limited area.

In a further embodiment, the tank deck is partitioned into multiple airtight compartments. This will ensure that even if there is a leakage of inert gas or reduction of overpressure in one compartment, it will not affect other compartments.

In a further embodiment, an outer portion of said tank deck, immediately above said ballast tanks, is equipped with auxiliary equipment. This provides for utilization of the less hazardous area above the ballast tanks for non-hazardous equipment.

The invention may be used both on circular production units, polygonal production units and boat shaped production units.

Preferably, the tank deck is equipped with gas sensors and/or pressure sensors.

The overpressure is suitably regulated to be between 0 to 0.5 barg relative to the cargo tank operating pressure.

The closed of tank deck area may contain non-hazardous equipment, such as auxiliary pumps and firewater gen-sets. As the area is closed off, this equipment will not be impacted by green sea.

The closed off tank deck area may be further divided by both vertical and horizontal partitions. Different areas of the closed off tank deck area may contain different types of equipment.

A moonpool may extend through the hull, closed tank deck area and main deck of the production unit. The moonpool is also isolated from the tank deck area by an airtight wall.

The invention will now be described in more detail, referring to an embodiment of the invention, shown in the enclosed figure.

When in the above and below, the term tank deck is used, it does not only mean the actual surface of the deck, but also covers the spatial room between the surface and the next deck above.

Brief description of drawings

Figure 1 shows a cross section through a production unit in an embodiment of the invention.

Detailed description of the invention

Figure 1 shows a circular floating production unit 10. It comprises a hull 11, a main deck 13 and production equipment 12 etc. arranged on the main deck 13.

The hull 11 contains cargo tanks 1 and ballast tanks 2. A moonpool 14 extends through the hull 11 centrally of the production unit. This moonpool may also be arranged elsewhere through the unit.

The line 3 denoted the top of tank level 3, where the cargo tanks 1 and the ballast tanks 2 are partitioned from the upper part of the unit by a tank deck 3.

The outer surface of the hull is water and airtight from the bottom of the hull to the main deck 13, including the part 4 that extends between the tank deck 3 and the main deck 13.

The tank deck 3 is divided into smaller compartments by horizontal partitions 5. As shown, it is convenient to position one of the partitions 5 in line with the inner limitation of the ballast tanks 2. This makes it convenient to place auxiliary pumps and other equipment to handle sea water in the area above the ballast tanks 2.

In the central area of the main deck 13, an area 6 for cargo pumps and other equipment associated with cargo handling, can be arranged. This area may be denoted cargo pump deck 6. The cargo pump deck 6 may either be at a lower level that the main deck 13 to allow for a continuous fire and explosion barrier above the cargo pump deck 6 or the cargo pump deck 6 may be arranged as a depression in the main deck 13 to allow for controlled spillage handling.

A caisson 15 extends from the cargo pump deck 6 through the tank deck 3 to the storage tanks 1. The caisson 15 is isolated from the tank deck 3 by an airtight shaft. The caisson 15 is used for hydrocarbon piping between the cargo pumps on the cargo pump deck 6 and the storage tanks 1 as well as access to the cargo tanks.

The tank deck 3 above the cargo tanks 1 and below the main deck 13, including the cargo pump deck 6, is either filled with an inert atmosphere, such as carbon-dioxide, argon, nitrogen, other non-flammable gases or a mixture of non-flammable gases, or it has a higher pressure than atmospheric pressure, so that any gases that may leak from the cargo tanks 1, or the pipes leading to and from the cargo tanks 1, will not enter the tank deck and create an explosion hazard.

To ensure that no hydrocarbon gas leaks enters the tank deck, gas leaks sensors may be arranged to detect any gas entering the tank deck. If the deck is kept under an elevated pressure, the deck may also be equipped with pressure sensors to detect any pressure loss. If the deck is filled with an inert gas, sensors may be present to detect any loss of inert gas.

Claims

1. A floating production unit having a hull (11) containing storage tanks (1) for oil and/or gas and ballast tanks (2), a tank deck (3) immediately above said storage tanks (1), and a main deck (13) above said tank deck (3), said tank deck (3) being contained within a watertight and airtight barrier; that said hull (11) forming an outer part of said barrier and extending along the height of said tank deck (3); equipment that pose an explosion hazard, including cargo handling equipment, being arranged outside of said tank deck (3), said tank deck (3) is either being filled with an inert atmosphere or being kept at an elevated pressure characterised in that said cargo handling equipment to pump cargo into and out of the storage tanks (1) is arranged on a depressed area (6) of said main deck (13).

2. A floating production unit according to claim 1, characterised in that said tank deck (3) is partitioned into multiple airtight compartments.

3. A floating production unit according to any of the preceding claims, characterised in that an outer portion of said tank deck (3), immediately above said ballast tanks (2), is equipped with auxiliary equipment.

4. A floating production unit according to any of the preceding claims, characterised in that said floating production unit (10) has a circular or polygonal cross-section.

5. A floating production unit according to any of the preceding claims 1-3, characterised in that said floating production unit is a longitudinal structure with a bow at each end.

6. A floating production unit according to any of the preceding claims, characterised in that said tank deck (3) is equipped with gas sensors and/or pressure sensors.