TECHNOLOGY Study shows turret-moored oil production vessel capabilities in typhoons

Takehiko Akiba, Haruhiko Komiya, Koichiro Matsumoto, Norihisa Kodan
NKK Corp.
Tokyo
Jun Mikami
Japan National Oil Corp.
Tokyo

A turret-moored, floating-type early production and testing system (EPTS) can weather the severe conditions for marginal field exploitation of East Asian offshore oil fields.

These vessels are an alternative to permanent facilities such as conventional jackets that often are uneconomical for marginal fields and deepwater, harsh environments.

The required production system must withstand the severe weather conditions of a typhoon. Weather downtime and the associated motion performance, weather-vaning capabilities, and mooring system requirements are main considerations in designing EPTSs for these environments.

An example of the EPTS concept is the Petrojarl 1 (Fig. 1), which operates in the North Sea and was built by NKK Corp. The results of 6 years' experience with the Petrojarl 1 have proven that such vessels can operate even in the harsh environment of the North Sea. The study, summarized in this article, verified that satisfactory operational efficiency can be achieved in environments where typhoons exits.

Specifications

Because East Asian offshore oil has high viscosity, the EPTS design will most likely include artificial lift facilities and have turrets positioned directly over the oil field. The EPTS will be outfitted with process equipment, storage, and other facilities.

An EPTS can enable early production, clarify the properties of the field by production testing, and lower producing costs because of its facilities and mobility.

The total system includes the hull structural configuration, subsea equipment, riser, workover provisions, and process equipment.

Table 1(12688 bytes) shows the typical environmental conditions. Case 1 considers the most severe conditions such as typhoons. The values indicate that the EPTS can maintain its position under typhoon conditions.

Case 2 includes the worst environmental conditions except for typhoons. In this case, the EPTS leaves the working area when a typhoon approaches. This mobility simplifies the mooring equipment and reduces construction costs.

Operations mode means the limit of weather conditions under which oil production can continue. Under the survival mode in Case 1, the environmental conditions exceed this limit and oil production is discontinued. The EPTS waits for the weather to improve with the mooring chains extended. But in Case 2, the EPTS leaves the working area when the survival mode conditions arise.

Principal specifications of the EPTS are:

Length 3 width 3 depth 3 draft: 193.6 3 30.0 3 15.5 3 10.5 m
Dead weight: 33,000 tons
Oil storage capacity: 33,000 cu m (220,000 bbl)
Oil production capacity: 20,000 bo/d
Complement: 60 persons
Main generator, diesel driven: five 2,500 kw
Emergency generator, diesel driven: one 700 kw

Mooring arrangement: Turret mooring with dynamic positioning (DPS)

To ensure weather vaning, the turret is positioned about 0.16 lengths ahead of midship (Fig. 2).(38460 bytes) Safety of life is ensured by having the accommodations on the forward, windward side, and the process equipment and ground flare on the aft. Oil is stored in tanks in a double hull structure.

Design steps

Fig. 3(82483 bytes) shows a general flowchart for determining the initial performance of a floating-type oil production system such as the EPTS.

First, a ship form is designed to provide stability in typhoon conditions. Then, the external force characteristics of wave, wind, tidal current, and ship motion characteristics in waves are created to scale. These characteristics are verified by tank tests and theoretical calculations.

The EPTS is moored at a single point by a turret that allows weather vaning. When subjected to wave, wind, or tidal current, the bow automatically turns to the wind or wave direction to minimize applied external force. Maximum estimated total force is applied.

The mooring system design is then altered to consider the motion of the EPTS in waves under these conditions. The ability of the EPTS to maintain production in any sea condition is calculated from the tested ship motions.

Study and development of the Petrojarl 1 confirmed that the ship motion characteristics of a ship-type floating oil production system such as the EPTS can be accurately estimated by strip-method calculations using the two-dimensional, sink-source distribution method.

Regarding motions in the horizontal plane (surge, sway, and yaw) of a slack-moored floating body such as the EPTS in irregular waves, low-frequency motion is produced in addition to high-frequency response corresponding to the incident wave period.

This low-frequency response is a kind of resonance phenomenon where a floating body/mooring system with a long natural period synchronizes with a higher-order, low-frequency wave drift force acting on the floating body in irregular waves.

Low-frequency motion is an important phenomenon to consider in designing a floating body and mooring system because the response can be equal to or greater than that from high-frequency motion. However, estimation of low-frequency response is not accurate enough. To investigate the weather vaning characteristics, combinations of inflow directions of the wave, wind, and current must be selected.

Sea conditions

Seasonal winds cause the sea to become rough, but in this case, the wave direction agrees approximately with the wind direction. On the other hand, the wind direction changes constantly during a typhoon, and the direction of waves generated by the wind cannot follow the changes in the wind direction, but sometimes lag in time.

Fig. 4(59685 bytes) shows the hind-casted history of wave, wind, and current inflow directions at Point A on the South China Sea during a typhoon. When a typhoon passes through this area, the direction of wind changes 180, but the wave direction changes several hours later. Therefore, waves could come in a direction opposite to the wind when a typhoon passes through this area.

The weather-vaning characteristics of the EPTS in the survival condition during a typhoon were studied for all possible combinations of wave, wind, and current inflow directions. The combination in which the total external force at equilibrium condition becomes maximum was determined and used in the mooring design.

Turret

The modeled EPTS is maintained directly over the subsea well by a turret that is anchored to the sea bottom. This minimizes the mooring facility, and improves the performance of the ship motion.

The turret is independent of the ship structure, as with the Petrojarl 1, and is supported both vertically and horizontally at the upper deck by turret bearings. The turret is anchored to the sea floor by an ordinary catenary mooring system using eight chain lines (Fig. 5)(26708 bytes).

The turret is the heart of the position-keeping system and also plays an important role in the passage of well fluid from the sea bottom. The EPTS has three flexible risers for production, test, and annulus, and two umbilical cables.

During operations, the turret is normally integrally locked to the ship hull; therefore, both the mooring chain lines and the flexible risers are twisted when the direction of the external force changes. A multiswivel was arranged on the turret to allow transfer of fluid, electrical power, and electrical signals between the ship and the turret while they are rotated and displaced from each other.

Well fluid is transferred to a pipeline end manifold (PLEM) set back from the subsea well and sent to the on-board, crude oil primary-treatment plant through the flexible riser and multiswivel.

Electrical power and control signals are supplied from the ship to the turret through a slip ring installed on top of the multiswivel and then sent to the electric submersible pump (ESP) and well equipment by means of the umbilical cable.

While the ship position is maintained directly over the well, a riser pipe is lowered through the moon pool at the center of the turret for maintenance of the ESP and the subsea well, allowing heavy workovers from the EPTS without outside assistance. Thus, workovers of adjacent wells can be carried out without interrupting oil production. This is the most important feature of the EPTS.

Mooring

Mooring design conditions are as follows:

For mooring chain tensions, safety factors of 2.0 in the survival mode and 3.0 in the operation mode are used with respect to the breaking strength.
Even if one mooring chain breaks, safety factors of 1.4 in the survival mode of 2.0 in the operation mode are assured.
During a typhoon, a minimum crew stays aboard to operate the dynamic positioning system (DPS), perform heading control, and adjust the mooring chain tensions.

Required capacity of the mooring system (Fig. 6)(37287 bytes) shows the worst balanced conditions of wave, wind, and tidal current for the survival and operation modes. The total external force acting on the ship in the survival mode is 4,280 kN (960,000 lbf). The equipment required to meet the design requirements outlined previously includes:

Stern nozzle propeller: two 2,800 kw
Fixed transverse thruster: four 1,600 kw
Turret mooring chain: Eight 79 mm diameter Grade 4.

Emergency evacuation

As shown in Case 2 (Table 1)(12688 bytes), the design environment conditions can be relaxed if the EPTS is evacuated when a typhoon approaches.

In the worst balanced conditions in Case 2 (Fig. 6b)(37287 bytes), the total external force in the survival mode is 1,580 kN (355,000 lbf), and only 54 mm diameter mooring chain is required. In addition, it was found that supplementary mooring force using a propeller or thruster is not necessary; therefore, the mooring facility can be significantly simplified.

Weather that exceeds Case 1 does not usually occur. Therefore, if the mooring facility is designed on the basis of this condition, evacuation from the working site would not normally be considered.

However, for Case 2, evacuation is necessary when a heavy typhoon approaches. In this case, the procedures for evacuation and reentry greatly affect the workability of the EPTS, and the establishment of means for rapid evacuation and reentry is important.

All of the required mooring chain cannot be stored on the ship because of limited space. Therefore, chain must be connected at the site to ensure the required length.

For evacuation, a "rig anchor release" that incorporates an hydraulic release mechanism was used at this connection to permit the mooring chain to be remotely released near the sea bottom by a signal from the ship.

For back-up, another emergency release system was provided that combines an emergency quick release function of a turret windlass and a weak link equipped on the way of the chain stored on board ship.

The flexible riser and the umbilical cable were connected to the piping and cable on the turret via a quick connector disconnector (QCDC) so that in an emergency they can be released immediately by an hydraulic mechanism.

Weather downtime

Weather downtime during a typhoon is based on the following procedure for evacuation and reentry.

1.During a typhoon, the EPTS moves to a position at least 130 nautical miles from the center of the typhoon. This distance allows for a storm zone radius based on past heavy typhoons having wind velocities of 25 m/sec or higher.

2.To evacuate the EPTS to a safety zone 130 nautical miles away from the typhoon, preparation for the evacuation starts when the typhoon is at a distance of about 400 nautical miles. This distance is based on the typhoons with the highest moving velocities over the past 40 years.

From these conditions, an alarm zone with a radius of 400 nautical miles is introduced.

When a typhoon enters this zone, preparations for the evacuation commence; for example, production stops and preparation for emergency release of mooring chain begins.

Whether the EPTS is actually evacuated is determined by whether the typhoon passes within 130 nautical miles from the working location. The time when the EPTS is evacuated is determined by considering the moving velocity of the typhoon and the evacuation speed of the BETS.

The probability of which path the typhoon takes through the alarm zone, and the probability of the typhoon's moving velocity is assumed to be normally distributed on the basis of data for heavy typhoons over the past 40 years. The downtime is determined for all possible typhoon paths and moving velocities by summing up the time for preparation, actual evacuation, reentry after evacuation, and restarting production.

Probable downtime during a typhoon is then determined (Fig. 7)(12514 bytes). It can be concluded that the average downtime from a single typhoon is 92 hr. Therefore, the annual workability is 95%, for five typhoons/year.

The Petrojarl 1 is an EPTS that has worked in harsh environments for over 6 years (Fig. 1).

The Authors

Takehiko Akiba is assistant to director of the ships and offshore structures division, NKK Corp., Tokyo. He has been with NKK since 1965 and has worked mostly in the ships and offshore structures design departments.

Haruhiko Komiya is a senior staff member in the ships and offshore structures planning department of NKK Corp., Tokyo. He joined NKK in 1969 and has worked mostly in the ships and offshore structures design departments.

Koichiro Matsumoto is manager of the ships and offshore structures basic design department of the NKK Corp., Tokyo. He has been with NKK since 1980 and has worked mostly in the ships and offshore structures design departments.

Norihisa Kodan is the head of the ships and marine structure laboratory, Tsu Research Laboratories, NKK Corp., Tokyo. He joined NKK in 1972 and has worked mostly in ship design and R&D departments of the Tsu Works.

Jun Mikami is a senior production engineer in the technical department of Japan National Oil Corp., Tokyo. He joined Japan Petroleum Exploration Corp. in 1987 and moved to JNOC in 1994.