Risk Assessment of a Cross Country Pipeline Transporting Hydrocarbons

PAPER No. : M - 19

RISK ASSESSMENT OF A CROSS COUNTRY PIPELINE TRANSPORTING HYDROCARBONS

Dr. G. Madhu*

Division of Safety and Fire Engineering

School of Engineering

Cochin University of Science and Technology

Cochin 682 022, India

Tel : 91-484-2576167 , Fax : 91-484- 2577405

E-mail :

ABSTRACT

A major oil company in India proposes to lay two 600mm dia pipelines for transporting hydrocarbon products like naphtha, motor spirit, high speed diesel and superior kerosene from a South Indian port to their storage terminal about 15 kms away. There are five major river crossings, three railway crossings and one NH crossing along the proposed route. It is proposed to transfer about 3000 m3/hr of hydrocarbon product through each pipeline. A booster pumping station is provided at an intermediate location to overcome the pressure drop and to provide sufficient pressure at the storage terminal end.

National and international codes and practices are usually followed while laying hydrocarbon pipelines. The welded joints would be radiographically tested and cathodic protection would be given to the pipeline to minimize the effects of corrosion. The pipeline will be mostly laid underground except at the booster pumping station. It is proposed to incorporate advanced instrumentation and communication system based on supervisory control and data acquisition (SCADA).

Inspite of all the safety standards and practices, failure of pipeline resulting in release of hydrocarbons cannot be ruled out. The present paper discusses the result of a risk assessment study carried out for the pipeline system. As part of the study, the probable failure modes associated with different operational areas for the proposed facility were identified. The predominant causes of hydrocarbon release from the pipeline have been identified as failure due to external factors, corrosion, construction defects and human error.

Consequence analysis was carried out for the identified failure scenarios using empirical models. The impact distances for pool fires and explosion were estimated. The catastrophic failure of the pipeline at booster pumping station results in the maximum impact distances. An attempt has also been made in the study to assess the probability of failure of the pipeline. Based on the risk assessment study a few recommendations have been made for the safe operation of the piping system.

Key words : Risk assessment, Hydrocarbons, Pipelines, Failure modes, Consequence analysis, Probability of failure.

• Formerly with the Process Engineering Department of FACT Engineering and Design Organisation, Udyogamandal, Cochin, India

Introduction

Chemical process industries handle, store and process large quantities of hazardous chemicals and intermediates. These activities involve many different types of material, some of which can be potentially harmful if released into the environment , because of their toxic, flammable or explosive properties. The rapid growth in the use of hazardous chemicals in industry and trade has increased the risk to employees as well as the neighbouring community.

Under these circumstances, it is essential to apply modern approaches to safety based on good design, management and operational control (Wells, 1980). The major hazard units should try to achieve and maintain high standards of plant integrity with due regards to the probabilities of undesirable events. While assessing design and development proposals for plants which handle hazardous materials, it is essential to identify potential hazards. Risk assessment techniques have been recognized as an important tool for integrating and internalizing safety in plant operation and production sequencing (Hoffman, 1973). In India risk assessment is mandatory for all new projects in chemical process industries dealing with hazardous chemicals and severe operating conditions.

Risk assessment includes identification of hazard scenarios and consequence analysis. Scenario identification describes how an accident occurs, while consequence analysis describes the anticipated damage to environment, life and equipment. This paper presents the results of a risk assessment study carried out for a pipe line system proposed for the transportation of petroleum products.

Description of the proposed facilities

The proposed project involves laying of two 600 NB diameter pipelines for the transport of petroleum products from the tanker berth at a south Indian port to the marketing terminal of a major oil company which is located about 15 Km away from the port. One of these lines will be used for the transport for superior kerosene oil (SKO) / high speed diesel (HSD) and the other for naphtha / motor spirit (MS). About 3000 m3 / hr of each product available at the ship end at 10 kg/cm2g pressure will be transferred through the pipelines. The pipeline will be laid as per the guide lines of Oil Industry Safety Directorate (OISD 141). There are five river crossings, three railway crossings and one national highway crossing along the proposed route. The pipes will be designed for an operating pressure of 15 kg/cm2 as per ASME B31.4. The entire line will be hydrostatically tested at 1.5 times the operating pressure before commissioning.

(i)Facilities at the port

At present, there are two tanker berths at the port ; berths I and II. It is proposed to install two new 300 NB unloading arms in berth I which will be connected to the 600 NB headers ( existing ) through one of the 250 NB branches provided on the header . An interconnection will be provided between the arm connected to a 250 NB nozzle on a 600 NB header and a second 250 NB nozzle on the other 600 NB header. The interconnection will facilitate use of both the arms simultaneously for transferring either of the fluid. The interconnection will be made in such a way that the chances of mixing of the fluids are eliminated. It is proposed to use only one of the two 600 NB headers each from the berth upto the existing exchange pit. A tapping of 600 NB each is taken from these 600 NB lines at the existing exchange pit area and they join the new 600 NB lines from berth II at the new exchange pit, and is led to the marketing terminal via the booster pumping station located in between. Motor operated valves will be provided to isolate the other 600 NB lines during the operation of the new facility.

(ii)Booster Pumping Station

A booster pumping station is envisaged as part of the system to overcome the pressure drop in the long line and to provide sufficient pressure required at the terminal end. At the booster pumping station one pump with a standby is planned for each fluid.

(iii)Marketing Terminal

The petroleum product from the proposed 600 NB lines will join an existing 600 NB header at the terminal end from where the hydrocarbons can be directed to any of the respective storage tanks.

(iv)Pigging Facility

A pigging facility to pig the pipe line is also envisaged in the system. A launcher at the port end and a pig receiving station at the terminal end will form part of the facility.

(v)Instrumentation

All valves in the 600 NB lines will be motor operated. All the first block valves in the new exchange pit and the main block valves in the main 600 NB line running to the terminal are motor operated with provision for remote and local operation. These valves can be operated just before starting pumping from the ship.

The main block valves in the 600 NB lines after the new exchange pit will be interlocked with the leak detection system so that the lines can be isolated from the control room by closing these valves. The main line at the port end will be provided with pressure indicators, temperature indicator, turbine type flow meter. The flow meter will have indicator, integrator and low and high flow alarms. The flow meters are provided as part of the leak detection system. Thermal relief valves will also be provided at various locations.

Provision to start or stop the booster pumps locally or from the control room will be made. A panel indicator, a turbine flow meter with indicator, integrator and low and high flow alarms will be provided in the discharge line of the pumps.

The storage terminal will also be provided with all the necessary instrumentation. The motor operated valve in the main line is provided with two wire control system with local and remote operation. The smooth and safe operation of the systems will be ensured by incorporating a computerized Supervisory Control and Data Acquisition (SCADA) system.

Safety features of the proposed project

The safety features proposed to be incorporated in the pipeline project are outlined below :

1. The entire stretch of the pipelines is proposed to be buried underground except at the booster pumping station, which will be properly fenced, and the pumping station would be manned round the clock.

1. The lines are to be buried with a minimum cover of 1.2 m as against 1 m specified in the standards. At road crossings, the lines will be laid with a minimum cover of 1.5 m through hume pipe protection using horizontal boring/trenching technique.

1. At railway crossings, casing pipe protection as per the norms of Indian Railways will be provided. Minimum cover shall be 1.5 m. The casing pipe shall also be protected with anti corrosive coating. Pipeline insulators will be used to support the carrier pipe inside the casing pipe and electrically isolate the carrier pipe from the casing pipe.

1. River crossings shall be below the scour bed with a minimum cover of 4 m. Isolation valves with valve chamber shall be provided at upstream and downstream of major water crossings. Anti-buoyancy concrete weight coating will be provided on the pipelines in the water logged areas and river crossings to prevent lifting up of pipes due to buoyancy.

1. The buried lines will be protected with anticorrosive coal tar based coating and the entire section of the pipelines would be provided with cathodic protection.

1. All butt weld joints will be 100 % radiographically examined and fillet weld will be subjected to dye penetration test and ultrasonic inspection.

1. The entire lines will be tested hydrostatically at 1.25 times the design pressure. The sections for crossing road, rail and river shall be pre-tested before erection.

1. In all 16 numbers motor operated valves (MOV) shall be provided at critical locations along the pipeline some of which are connected to the interlock system. These valves can also be operated from remote location. This will ensure quick isolation of the pipeline during emergency.

1. The computerized SCADA to be incorporated in the system will ensure its safe operation. Any leakage in the pipeline will be immediately detected by the computer system and pumping of the fluid will be immediately cut off.

1. Communication between tanker berth, booster pumping station, and the marketing terminal is also achieved through SCADA. This will be in addition to telephones.

Identification of Failure Scenarios

A hazardous material either flammable or toxic is safe till it is fully contained and maintained at desired parameters during storage, operation and transportation. In the case of the proposed pipeline, the major causes of hydrocarbons from the pipe lines can be attributed to external factors like mechanical interference, material failure (corrosion) and other causes like construction defects, pipe material defects and human error.

The failure due to external factors generally caused by third party mechanical interference is a puncture or a gouge severely reducing the wall thickness of the pipeline or guillotine failure of the pipeline. The failure can be immediate or may occur sometime later by fatigue.

Pipeline failures by corrosion can be due to internal corrosion or external corrosion. External corrosion failures are due to moisture in the ground and salinity of the soil and can take two forms – small pin hole failures caused by pitting and more generalized corrosion leading to a reduction in pipe wall thickness over a plane area.

Pipe line can also fail for a variety of other causes like construction defects, pipe material defects and human error.

The following failure cases are identified as probable in the pipe line system under study by carrying out a preliminary hazard analysis and HAZOP study.

1. Unloading arm failure in HSD / SKO pipeline ( port area.)
2. Unloading arm failure in Naphtha / MS pipeline (port area)
3. Failure of 300 NB flange in each pipeline (port area)
4. Partial failure of booster pump discharge on each pipeline
5. Catastrophic failure of pipelines at booster pump discharge
6. Partial failure of 600 NB flange at the terminal on pipeline.

Consequence Analysis

Despite the universal acceptance of excellent codes of practice for design and operation of storage facility there have been instances of losses due to major accidents of varying degree of severity. The failure cases generally depend upon the availability of safety systems, instrumentation and their response time and the probability of human error. Thus, prior to identifying the failure scenarios for estimating the affected areas, the above mentioned safety systems have been studied in detail. Other parameters like material of construction and protection systems proposed to be provided at the facility have also been given adequate consideration.

In the present study, models for flash fire, pool fire and unconfined vapour cloud explosion (UVCE) and dispersion have been used for consequence analysis ( World Bank, 1985). Source models have been used to quantify the release scenarios by estimating the discharge rate and extent of flash and evaporation from a liquid pool.

UVCE and flash fires occur when a large amount of volatile flammable material is rapidly dispersed to the atmosphere, forms a vapour cloud which disperses and meets a source of ignition before the cloud is diluted to below lower flammability limit (LFL). The main concern for a UVCE is the shock wave that causes damage whereas for a flash fire the main concerns are the thermal radiation effects ( Gugan K, 1979). It is believed that the transition from flash fire to UVCE cloud be a function of the flammable mass, presence of confinement obstacles, burning velocity of the material and other factors.

Pool / jet fires generally tend to be localized in effect and are of concern mainly in establishing the potential for domino effects and employee safety zones. Issues relating to spacing of critical equipments can be addressed on the basis of specific consequence analysis for a range of possible pool / jet fires. The effects of a pool / jet fire depends upon factors such as flammability, combustibility, amount of material released, temperature, humidity and flame length ( Lees, 1996).

Dispersion modeling aims at estimating the distances likely to be affected due to release of certain quantity of flammable gas. Depending upon the properties of the material released and the release conditions, a dense gas dispersion or a buoyant gas release model is used for estimating the affected areas.

The following assumptions are made for estimating the impact distances for cloud dispersion, vapour cloud explosion and flash fires.

1. Simultaneous failure leading to more than one scenario is not considered.
2. Catastrophic failure of the pipelines is not generally considered in view of the high integrity of construction and safety measures that are proposed.
3. It is assumed that the ground surface is level and the roughness for a given surface is uniform.
4. It is assumed that the atmospheric conditions are constant for at least the time taken for the cloud to develop as a plume, to the lowest concentration of interest.
5. Concentration fluctuations within the cloud are ignored.
6. The flame speed through the cloud is constant.
7. Stoichiometric concentration of the cloud is uniform.

Damage Criteria

a)Thermal radiation

The flammable material released accidentally, from an orifice would form a vapour cloud. The cloud if encounters an ignition source would result in a jet fire. The cloud formed due to any failure, if finds an ignition source before reaching a concentration below lower flammable limit and the flammable mass in the cloud is less than 5 tonnes, a flash fire is likely to occur (Craven, 1976). The flame could also travel back to the source of leak. Any person caught in the flash fire is likely to suffer burns of varying degrees and at times could be fatal. Therefore, in consequence analysis, the estimated distance upto LFL value is usually taken to indicate the area which may be affected by the flash fire.

The damage effects of thermal radiation of varying intensity are shown in Table 1.

b)Explosion overpressure

Distances are estimated for unconfined vapour cloud explosion for overpressures of 14, 28 and 70 kg/cm2. These overpressures are the peak pressures formed in excess of normal atmospheric pressure by blast and shock waves.

Table 2 gives damage levels at various overpressures for both property damage and human injury.

Results of consequence analysis

The results of the consequence of the various failure scenarios are given in Tables 3 and 4.

Unloading arm failure at port area