Characteristic and requirements of electrical systems on modu class vessels

CHARACTERISTIC AND REQUIREMENTS OF ELECTRICAL SYSTEMS

ON MODU CLASS VESSELS

This article briefly introduces the characteristics of MODU vessels and of their electrical systems. Electrical systems of such vessels consist of: redundant power generation and distribution system, dynamic positioning, drilling/well intervention process system and gas detection system including electrical installations in hazardous areas. Offshore vessels classified as MODU are among the most complex and technically advanced. Additional MODU classification society requirements for electrical systems have been stated. MODU philosophy has been presented by examples of various systems.

Keywords: marine electrical systems, mobile offshore drilling units.

INTRODUCTION

Subsea exploration has, in the last years, taken a massive step towards the development of new technologies, in order to extract more oil and gas from offshore fields; therefore, many special purpose vessels and installations have been built and put into service. Marine safety regulations for ships did no longer match the complexity of installations designed for subsea processes. To address this issue, the International Maritime Organization (IMO), and subsequently the International Association of Classification Societies, have come up with new regulations that are adequate for these types of vessels [1–4].

The mobile offshore drilling unit (MODU) is a vessel capable of engaging in drilling operations for the exploration of resources beneath the sea bed, such as liquid or gaseous hydrocarbons, sulfur, or salt [1]. The MODU code refers to column-stabilised units (i.e. semi-submersible rigs), self-elevating units (i.e. jack-up rigs) or ship shaped units (i.e. drilling ships, well intervention ships).

MODU vessels may vary depending on their construction characteristics and type of operation in which they are engaged; however, they all have the common feature of being very complex industrial structures with expanded automation, redundancy and reliability, as well as extensive electrical installations in hazardous areas.

a) b)

Fig. 1. Jack-up rig profile: a) during sea-passage, b) in operation (self elevated)

a) b)

Fig. 2. Profile of semi-submersible rig (a) and drilling/well intervention vessel (b)

Jack-up rigs are designed for shallow waters (up to 200 m). They use their own drive and dynamic positioning systems to place the vessel in the right position prior to self-elevation. During drilling, the legs are jacked down onto the sea bed. Other types of MODU vessels are designed to operate in deeper waters. They use their own drive and dynamic positioning systems to approach the final position and conduct the drilling operation. Unlike jack-up rigs, they have to use stabilization (semi-submersible) or heave compensation (drilling vessel) systems.

Despite the special purpose of MODU vessels, they are also considered as seagoing ships and therefore have to fulfill the Rules for Classification and Construction of Seagoing Ships stated by the qualified classification society.

The mentioned regulations consist of specific demands and requirements (including those concerning electrical systems) that have been implemented to ensure safety of navigation and safety of ongoing offshore processes. Details can be found in the following regulations [11–13].

A ship’s electric power network is a three-phase, three-wire, alternating-current network insulated from the ship’s hull structure. In medium voltage systems, that are common on diesel-electric type vessels (due to power demand), an indirect grounding system (neutral point of generator grounded to a ship’s hull by impedance) may be used [16, 17].

In the marine network, the power received by single consumers is often comparable to the nominal plant power. The ship’s electric power network is therefore a flexible (‘soft’) one and is characterized by large changes of voltage and frequency, resulting from switching on and off heavy consumers, such as thrusters, pumps and compressors [15].

Environmental factors also cause changes of voltage and frequency in the shipboard electric power networks. Electric and electronic devices operating in a marine environment are exposed to influence of extreme external conditions such as high and low ambient temperature, salt mist, water wetting, high air humidity, vibrations, impacts and vessel oscillations (rolling and pitching) [15–17].

In addition to ordinary energy consumers, there are also critical loads linked to systems that are vital for the safety of navigation and of the ongoing offshore/subsea processes. Any unexpected issues with those systems pose a risk to human life and may cause marine pollution and high-cost damage.

2. MODU CLASS ELECTRICAL SYSTEMS

Mobile vessels designed for drilling operations that meet the requirements for the safety of seagoing also have to fulfill the MODU code regulations.

The MODU code has been issued to ensure a level of safety for MODU vessels and their personnel, equivalent to that required by the 1974 SOLAS Convention and the Protocol of 1988 [1].

2.1. Dynamic Positioning System

In order to carry out their tasks, MODU vessels have to be equipped with a Dynamic Positioning (DP) system. Semi-submersible rigs and drilling/well intervention ships use DP during subsea operations, whereas jack-up rigs/vessels use DP prior to the self-elevation process. General DP guidelines have been issued by the IMO [5].

The IMO defines a DP-vessel as a one which automatically maintains its position exclusively by means of thruster forces. The DP system is defined as a complete installation necessary for dynamic positioning a vessel, comprising the following sub-systems:

• power system (prime movers with necessary auxiliary systems and piping,

generators, switchboards, distributing system),

• thruster system (drive units with aux. systems, main propellers and rudders of

thrusters are under control of DP system, thruster control),

• DP control system (computer system, joysticks, sensors, position reference

system).

On the basis of the generic IMO document, each classification society has published more detailed documents for the classification of DP systems [6, 7]. Comparison of IMO classification and common notations has been presented in table 1.

The DP system components presented in Table 1 can be classified as active: including prime movers, generators, thrusters, switchboards and remote controlled valves, and passive: including cables, pipes and manual valves.

Table 1. Comparison of IMO DP classification and class societies’ notations

IMO

Class DNV-GL

Lloyd

Reg. ABS DP class description

DP class 1 DNV-AUT DP (AM) DPS-1 Automatic and manual position and heading control under specified maximum environmental conditions

DP class 2 DNV-AUTR DP (AA) DPS-2 Automatic and manual position and heading control under specified maximum environmental conditions, during and following any single fault excluding loss of a compartment (double redundancy of active element and unprotected passive elements)

DP class 3 DNV-AUTRO DP (AAA) DPS-3 Automatic and manual position and heading control under specified maximum environmental conditions, during and following any single fault including loss of a compartment due to fire or flood (double redundancy of active and passive elements with physical separation – flood and fire A60 class division)

of power electrical systems, have been more widely presented in an another article [14].

Due to the fact that MODU vessels are involved in complex types of subsea operations, they mostly belong to DP class 3. This means they must have at least double redundancy DP systems in separated compartments that are fire and flood proof (A60 class); however for better reliability and operation convenience, triple or even quadruple redundancy has been applied in many solutions.

Fig. 3 presents the DP philosophy of MODU DP3-class drilling/well intervention vessels. Vessel positioning is realized by using 7 thrusters and 2 main propellers. The electrical motors of the thrusters and main drives are supplied from 4 different main switchboards by variable speed drives (VSD).

T5 DP ZONE 1 DP ZONE 2 T7 E N G INE R OOM 1 E N G INE R OOM 2 D10 D11 T4 T1 T3 T2 T6 D8 D9 DP Z O N E 3 VSD VSD VSD VSD VSD VSD MSB A MSB B VSD VSD VSD MSB C MSB D VSD VSD VSD

Fig. 3. MODU class DP3 drilling/well intervention vessel DP example

The dotted line represents a simplified flood and fire separation, which is a DP3 requirement. The vessel has many compartments and bulkheads that can be assigned to 3 main zones: DP zone 1 is supplied from switchboards A and B (prime movers in an engine room 1), DP zone 2 is supplied from switchboards C and D (prime movers in an engine room 2) and DP zone 3 is separated from the 2 other zones but can be supplied from switchboard B or C.

Cross-connections between DP zones 1 and 2 are not allowed. This applies to power cables as well as to control and reference systems. The implementation of the three zones has to be analyzed during the design stage through Failure Modes Effect Analysis (FMEA) and tested during FMEA proving trials.

2.2. Electrical installations in hazardous areas

For the classification of hazardous areas, the MODU code uses definitions from the standard IEC 60079-10-1: 2015 Explosive atmospheres – Part 10-1: Classification of areas – Explosive gas atmospheres [2–4].

Table 2. Classification of hazardous areas

Zones Definition Examples from drill vessel

Zone 0 In which ignitable concentrations of flammable gases or vapors are continuously present or present for long periods

Methanol tank, special product tank, multipurpose tower (MPT) vent line

Zone 1 In which ignitable concentrations of flammable gases or vapors are likely to occur in normal operation

Drill floor and moon-pool area, flare boom vents, drilling derrick, areas around zone 0

Zone 2 In which ignitable concentrations of flammable gases or vapors are not likely to occur, or in which such a mixture, if it does occur, will only exist for a short time

Mud pump room, areas around zone 1, chemical stores, airlocks to non-hazardous areas

To reduce risks it is obligatory that electrical equipment meets protection methods stated in table 3. It is mandatory to fix or use portable electric equipment (i.e. electrical tools, multimeters, VHFs, etc.) in hazardous areas depending on their zone.

Table 3. Types of electrical apparatus that can be used in hazardous zones according to

the MODU code

Protection method Zone 0 Zone 1 Zone 2

intrinsic safety [ia] X X X

intrinsic safety [ib] X X

flameproof enclosures [d] X X

increased safety [e] X X

encapsulation [m] X X

protection [n] X

oil immersion [o] X X

pressurized enclosures [p] X X

powder filling [q] X X

Areas where flammable gas may be expected to be present must be monitored continuously. Typically, the drilling, mud processing and well fluid test areas must be always monitored. Each vessel has to have administration-approved Ex zones plan; however, due to the characteristics of MODU vessel operations, in case of major emergency situations, the explosion hazard may extend designed Ex zones.

2.3. Emergency shutdown of operation system (ESD)

In order to respond rapidly to the detection of explosive gases, toxic gases, fire or other major hazard emergencies and to prevent the escalation of abnormal conditions, the MODU code demands that vessels are equipped with an emergency shutdown system (ESD). Scenarios that may require the usage of the ESD system are:

• fire or explosion on the on-board installation;

• subsea or ‘topside’ release of gas following the failure of the installations’ well

control equipment;

• a ‘loss of position’ incident, where the movement of the installation is

significantly beyond its station, potentially causing a failure of the well control equipment, either subsea or topside;

• ‘impending vessel collision’ where an errant vessel is approaching the vessel on

a collision course and may require the installation to move off from its location;

• other installation emergency, where there is potential for escalation due to the connection to a subsea well and/or presence of well hydrocarbons on deck that can be mitigated by shutting in and disconnecting from the well.

The principal of the ESD system is that of shutting down the marine and process operations through the orderly and sequenced removal of power. The ESD is divided into levels and zones, so it is possible to shut down areas or processes that are a subject to risk. The highest level of protection is Abandon Vessel Shutdown (AVS) and it is used to protect personnel during abandonment of a vessel.

The ESD system may be activated both manually (by senior personnel) or automatically (by at least two sensors simultaneously). Automatic functionality will be executed by built-in logic control based on agreed Cause and Effect (C&E) charts for each ESD level. Automatic activation by confirmed fire or gas detection from the Fire and Gas detection (F&G) system will be in accordance to the C&E chart.

AVS push- buttons ESD push- buttons Gas

detectors detectorsFlame

Drill/well intervention process control station PA/GA MSB/ESB distribution boards circuit breakers trip signal

Generators Fire/ _gas

dampers Fire pumps Other fire fighting (i.e. deluge) ESD control system

Fire and Gas detection system Smoke/ heat detectors Manual call points Flame detectors PA/GA Fire dampers Fire detection central Equipment to be stopped i.e. thrusters, boilers, fuel pumps E m er ge nc y s top s

MODU systems Marine systems

Fig. 4. Comparison between MODU related and marine related safety system topology In Fig. 4, a comparison between MODU related and marine related safety system topology has been presented. The ESD control system receives as input the AVS and ESD pushbuttons signal, emergency signal from drilling/well process control station and confirmed (2 out of n sensors) gas and fire signals from the F&G detection system. Outputs come to the public announcement and general alarm (PA/GA) system, various fire fighting equipment, generators (including emergency generator) and circuit breakers. Circuit breakers are tripped in order to shut-down the affected zone or process. In addition, generators are de-excited and prime movers stopped in order to carry out AVS. Shut-downs are performed automatically according to the designed and software-implemented cause and effect chart.

For marine related safety systems, fire sensors and manual call points are in use as inputs. PA/GA systems and fire dampers are outputs. Emergency stops are individually associated to components.

to be taken before disconnection or shutdown of machinery and equipment associated with maintaining the operability of the dynamic positioning system. This rule is enacted to preserve the integrity of the well.

Shutdown systems that are provided to comply with the above should be designed in such a way that the risks of unintentional stoppages caused by a malfunction in a shutdown system and of inadvertent operation of a shutdown are minimized. This includes loop monitoring, sensor voting (for example, to take action we need high input on 2 out of 3 sensors) and ESD system fail safe design and logic.

For the purposes of ESD fail safe logic, all electrical equipment onboard is divided into two groups: essential equipment (i.e. DP related systems) and non-essential equipment. In case of loop fault or ESD system internal malfunctioning, non-essential equipment will be tripped, while essential equipment will not.

CONCLUSIONS

Over the last few decades, the exploration of resources beneath the sea bed has become more and more intensive. New technology allows reaching deeper layers of natural sources. Dynamic positioning systems have become milestones in deep water drilling. Thanks to DP, drilling units do not have to be attached to the sea bed as required in the past. The main disadvantage of the MODU solution is that it requires all machinery to be up and running to maintain position. Because of the effects of the position loss, there is a demand for redundancy and reliability and responsible authorities have to follow these needs and implement the appropriate regulations. The MODU code has been issued to secure levels of safety for vessels performing drilling and well intervention. These regulations are supplementary for regulations following 1974 SOLAS Convention.

It is critical for safety that rules and requirements follow the changes in gas industry and the latest technical trends and solutions.

REFERENCES

1. Code for the Construction and Equipment of Mobile Offshore Drilling Units – MODU Code, International Maritime Organization 2009.

2. 2009 MODU Code - Code for the Construction and Equipment of Mobile Offshore Drilling Units, Lloyd's Register of Shipping, 2009.

3. Rules for Building and Classing - Mobile Offshore Drilling Units, American Bureau of Shipping, 2016.

5. [a1] IMO Guidelines for vessels with Dynamic Positioning Systems, IMO MSC/Circ.645 June 1994.

6. Rules for Classification of Ships - Dynamic Positioning Systems, DNV-GL, 2013. 7. Guide for Dynamic Positioning Systems, American Bureau of Shipping, 2013.

8. Clarification of IMO MODU Code 2009, Classification Support Newsletter, DNV-GL, 2014. 9. Offshore Standard – Electrical Installations DNV-OS-D201, DNV-GL, 2013.

10. Offshore Standard – Safety Principles and Arrangements DNV-OS-A101, DNV-GL, 2013. 11. DNV GL rules for classification: Ships, DNV-GL, 2016.

12. Rules and Regulations for the Classification of Ships, Lloyd's Register of Shipping, 2016. 13. Rules for the Classification of Steel Ships, American Bureau of Shipping, 2016.

14. Bastian B. Sieci elektroenergetyczne na jednostkach pływających z systemem dynamicznego

pozycjonowania, Przegląd Elektrotechniczny 2/2010.

15. Mindykowski J., Assessment of Electric Power Quality in Ships Systems Fitted with Converter

Subsystems, Polish Academy of Sciences, Shipbuilding & Shipping, Gdańsk 2003.

16. Wyszkowski S., Elektrotechnika Okrętowa, Wydawnictwo Morskie, Gdańsk 1991.

17. Bastian B., Plachtyna O., Kutsyk A., Real Time Computer Tester for Automatic Voltage

Regulators Used in Marine Generators, Poznań University of Technology Academic Journals,

Poznań 2016.

18. Zhi Z.Z., The development and research method of dynamic positioning systems, The Ocean Engineering 20(1), 2002.

19. Morgan M., Dynamic Positioning of Offshore Vessels, Marine Division Honeywell Inc., 1978. 20. Kamal G., Saurabh S., Dynamic Positioning Power Plant System Reliability and Design, PCIC

Europe Conference, Rome 2011.

21. Lauvdal T., Power Management System With Fast Acting Load Reduction For DP Vessels, Dynamic Positioning Conference – 2000.

22. Savoy S., Power Management System Design, Functionality and Testing, Dynamic Positioning Conference – 2002.