Particular considerations for floating structures, storage and offloading systems and offloading systems

3.6.1 Introduction

Floating structures for production, storage and offtake have been used safely and reliably throughout the oil industry for many years. Early installations were primarily floating storage and offtake vessels, “FSO”, but today the modern floating production, storage and offtake vessel, “FPSO”, includes processing equipment and a higher level of sophistication.

Consequently, the FPSO becomes an offshore producing installation, storage facility, and loading terminal all rolled into one unit.

The early “ship-shaped” vessels, developed in the 1980s, took advantage of a severe downturn in the tanker market and were converted from relatively new tankers. More recently, the tendency has been to use new, purpose-built, ship-shaped hulls, particularly for FPSOs associated with long lived projects. Conversions of tankers, both old and new, continue to take place. There are many different types of design, weather-vaning with internal or external turrets or spread moored that maintain a fixed orientation.

The FPSO and the FSO present many of the same hazards to personnel and the environment, although the added complexity of production facilities on the FPSO increases associated risk.

The guidance in this section relies heavily on the published guidance of UKOOA, [3.28] and the draft guidance being prepared by OGP [3.29]. This Guidance will adopt the OGP nomenclature and when considering an issue applicable to both types of floating installation will use the term F(P)SO.

A number of features impact fire related hazards on floating installations; for example, the geometry of the layout, compartmentalisation, operations, fire scenarios, response characteristics of marine construction to fires and the vulnerability of marine systems associated with the motion, station keeping and stability. The effects of fire on these features are discussed further in the following sections.

3.6.2 Marine life cycle considerations

F(P)SOs usually consist of a marine structure supporting process and utilities decks of a conventional offshore construction. These differing methods of construction are governed by differing regulatory regimes. For the UKCS, the application of SOLAS and MODU codes without demonstration of validation by the additional risk assessments normally required by PFEER will be insufficient for the treatment of fire events.

Some specific attributes to be considered on F(P)SOs are;

• Fire-fighting in the enclosed compartments containing marine systems (e.g. engine rooms, DP control rooms, generators); fire fighting techniques will include inerting, with the ramifications this entails for personnel access and the requisite alarms

• As the process and utilities modules are normally located above the vessels deck (and the cargo storage), the process and utilities deck areas will be large, usually of one or two levels. Segregation to avoid escalation of a fire can be achieved by separation of modules occasionally further separated by fire barriers, (this may or may not help explosion overpressures but will impair the dispersion of released hydrocarbons).

• The fire risk analysis undertaken on F(P)SOs will consider the nature of the hydrocarbon fuel source as well. Due to the nature of the storage on F(P)SOs, the fields they are generally use for tend to be crude that can be stabilised fairly readily. F(P)SOs may be required to hold stored product in their cargo tanks for typically 3 to 7 days dependent upon their geographic location. The F(P)SO solution is therefore less favoured for more volatile reservoirs.

• The (potentially) long process and utilities decks and their orientation with respect to wind conditions will be affected by the weather-vaning of the F(P)SO. The top decks should be designed to follow a hazard gradient from the most hazardous area with respect to fires (and explosions) to the least hazardous. This will generally be from the turret outwards.

Due to the weather-vaning effects (either due to wind or current and their effects on the superstructure height and hull draft) the fires can escalate downwind and at the very least, toxic products of combustion will be distributed downwind. The layout should consider these additional hazards and the design should accommodate them to maintain levels of safety. (Some designs have used DP to adjust the F(P)SO orientation in the event of a release or a fire hazard although the DP then becomes a Safety Critical Element and subject to the development of Performance Standards and integrity assessments required in the UKCS.)

• On an F(P)SO, escape routes and piping runs may be very long and tortuous and personnel may need to pass the origin of the incident to reach the Temporary Refuge.

Consideration in the design of escape over long distances during incidents and incident escalation should be a key issue.

• Fire water mains will also be extensive and distant from the fire pumps in the process area. Correct fire-pump sizing and firewater-main hydraulic analyses will be required to ensure adequate pressure at deluge points, hoses and monitors.

• Buoyancy, stability and station-keeping must be maintained at all times, and the systems associated with these duties must be protected from fire hazards.

F(P)SOs also require specific consideration of major fire hazard and release scenarios unique to their design and operation.

• Oil storage tanks – May present hazards in the form of either large scale storage of stabilised crude or with empty storage tanks containing potentially explosive mixtures.

• Non-process hydrocarbon inventories – The F(P)SO is a power-hungry installation and requires substantial stores of diesel to maintain station, process and utilities power demands plus other life-support systems. The vessels are often located in difficult or

• Jet fires on main deck – The process decks on F(P)SOs are often lifted clear of the cargo storage tank roof for several reasons, (see bullet points below) a 5 m gap is not uncommon. The space provided also allows jet fires from the underside of the process to reach other process or utility modules without any impingement to reduce the effect of the flame. The gaps provide other risk reducing and operational benefits but steps can be taken to reduce the likelihood of jet fires by careful layout and orientation of the higher pressure equipment.

• A gap will allow “green water” to flow over the main deck without placing an excessive load on the process modules supports by creating restrictions and eddy current effects.

• A gap allows a clear and uninterrupted space for long piping runs (both process piping and storage tank vent and balancing lines)

• A gap allows personnel access across the vessel, both for normal operational and maintenance access as well as facilitating emergency response.

• Swivel connections, a source of releases – The turret contains a large number of swivel joints in order to function, these are often at the highest process pressure and pass the reservoir fluids prior to any cleaning or conditioning and are therefore subject to the F(P)SOs most onerous process duty.

• Offloading and pool fires on the sea – Offloading to shuttle tankers is a regular event and poses a significant risk both on the F(P)SO and the shuttle tanker. The risks comprise the breakage or leakage of the transfer hoses and the potentially flammable mixing of hydrocarbon and air in the storage holds of F(P)SO and shuttle tanker. During the offloading operation, the shuttle tanker and F(P)SO are in relative proximity and the risks on either vessel are compounded by increased potential for escalation to another vessel.

3.6.3 Application of fire and explosion hazard management to floating structures

3.6.3.1 Topsides considerations

The storage and transfer of hydrocarbons on F(P)SOs present particular hazards to personnel and the environment and some of these have been listed above. This sections describes further measures that can be applied to the management of fire hazards in the F(P)SO topsides.

There is a need to continuously vent hydrocarbon vapours during loading, it is important that the venting system be designed to accommodate the maximum volume of volatile organic compounds (VOCs) vented from storage. Allowance must be made the higher temperatures the vents will experience when venting during maximum production rates and as well as providing design allowances for possible process upsets. In some areas, local regulations or guidelines limit the amount of VOCs that may be released to the atmosphere. It is always good practice to adopt loading procedures that will minimise VOC emissions.

The atmosphere in the F(P)SO tanks is to be maintained in a ‘non explosive’ condition. The normal method is to supply low oxygen content combustion products to the tanks from boiler uptakes or from an independent oil or dual fuel generator.

F(P)SOs need special consideration due to the potential venting of hydrocarbons either near the process plant or near the flare stack. Calculations will have to be made at the design stage to ensure that carry over of hydrocarbons from the inert gas stack will not interfere with day to day operations.

It is recommended that the inert gas system comply in all respects with the requirements of SOLAS and the relevant IMO guidance notes. Prudent operators may also consider maintaining 100 % redundancy for this critical component.

After purging, the tank must be gas freed in order to remove the residual inert gas from the tank and replace it with a normal atmosphere containing 21 % Oxygen.

The issue of VOC return lines and their use during offloading represents a key safety issue.

The operation of VOC reclamation represents a highly hazardous situation where flammable mixtures of hydrocarbons are returned to the F(P)SO. An added complication is that the offloading and reclamation systems may often be combined as a dual hose system and for F(P)SOs with stern accommodation, the offloading and reclamation point may be located close to the accommodation and TEMPSC.

Due to the longer term storage (compared with most other offshore installations), water and other contaminants in the crude can accelerate corrosion of the F(P)SO storage structure and systems resulting in premature failure and, potentially, escape of hydrocarbons. Design allowances should not be based on ideal crude conditions but should consider a realistic appreciation of operational practices.

3.6.3.2 Vessel and marine considerations

The layout of surface and sub-sea facilities must be carefully considered early in the design to account for the following shipping related hazards (that may give rise to loss of integrity and fire):

• Passing ships and local community activities, such as fishing;

• Supply and maintenance vessels in relation to anchoring or dropped objects;

• Anchor mooring patterns of drilling rigs during locating and moving;

• Safe access by offtake tankers, avoiding interference with other moorings, flowlines and risers as well as other field operations.

The field layout must also consider the need for offtake tankers to approach the F(P)SO, moor, load their cargo, unmoor and proceed to open waters, always in safety. The parameters for achieving this, which will include manoeuvring areas and weather limits on operations for the tankers, may be derived by means of a risk assessment study as described in OCIMF Offshore Loading Safety Guidelines: With special reference to harsh weather zones [3.31].

Additional reference material for Offshore Loading may be found in UKOOA’s guidelines for tandem off-loading from FPSOs/ FSUs to shuttle tankers [3.32].

Thrusters may also be useful in fire or platform abandonment scenarios where the vessel can be rotated to clear fire or smoke from around production areas and living quarters and to provide a lee side for survival craft launch. When designed for a safety function, it should be noted that thrusters will be considered to be Safety Critical Elements.

Due to the vessels being in very close proximity, the risk of a fire or explosion on one vessel

It is important that the F(P)SO is equipped with emergency shutdown and release equipment that will allow the vessels to part in the event of an emergency on one vessel.

3.7 Particular considerations for mobile offshore units