Hazard identification methods for liquefied natural gas process safety

LNG Process Risk Safety: Modeling and Consequence Analysis

The Risk Assessment/Analysis

Checklist

A checklist is made up of guidelines that are placed in questions or bullets in order to assist a given methodological health and safety (EHS) risks analysis (Fthenakis and Tramell, 2003).It is used in the stimulation of group discussion and thinking. The checklist is appropriately developed by a team of experts who have a prior knowledge of hazard analysis. The checklist is then used in the process of risk assessment is includes a specified list of items that are specified using a group of codes, industry safety practices and regulations. The developed checklist is important in the LNG process by enabling the conducting of self -- appraisal as well as auditing of the LNG process and facilities. An analysis of the technique reveals that the checklist method has the disadvantage of heavily influencing the accuracy and the quality of LNG risk assessment (EASH,2010)

What-if Checklist

The what-if check-list is a rather broad-based technique of hazard assessment that combines elements of creative thinking of a team of specialists using a methodological focus of a checklist that is prepared (OSHA, 2010). A selected review team is then represented using a wide range of selected disciplines such as safety. The special team is then given the basic information on the specific hazards of materials, the process technology, equipment design, procedure, incident experiment, a review of previous hazards and instrumentation control. A thorough field is also conducted when the process in operation.

The selected review team then examines the process methodologically from the time of receipt of raw materials up to the time the final product is delivered to the intended destination. At every step, the group then generates in a collective approach a listing of all the what-if questions that concerns the hazards as well as the safety of the entire operation. After the review team has completed the listing process of forming spontaneously generated questions, it applies a systematic process of preparing a checklist in order to simulate the additional questions. Answers are then generated for each question. A consensus is then derived for each question and answer. Recommendations then follow.

The application of what if analysis in the context of LNG process lies in the process of hazard identification (Woodward and Pitblado, 2010). It is categorized as a quantitative technique of risk assessment (Bridges,2008). In our research data, it has been used in different situations such of LNG risk assessment. It can be used in the assessment of LNG fires and explosions scenarios as indicated by Biao et al. (2010).

The HAZOP (Hazard and Operability) Method

The HAZOP concept is on the basis on the principle that it is better to employ a team approach in the process of hazard analysis since it helps in the identification of more problems as compared to when just an individual's work separately then combines their results (Acutech,2009).

Extant literature has discussed this method as indicated in the works of Mannan (2005) and CCPS (2008).The technique is applicable to various LNG processes. More specifically, it can be used on onshore LNG production units, LNG reception, and ship transfer and at the regasification terminals (Woodward and Pitblado, 2010). At its simplest form, the method can effectively be used in challenging the detailed design comprising a wide range of various deviations from the otherwise normal operations as well as to confirm the level of design conformance with standard safety requirements. It can also be used in the testing of whether the safeguards that are in place are sufficient. The noted deviations could be mechanical ones such as corrosion, less temperature or more pressure (Woodward and Pitblado, 2010).They could also be human errors or a combination of events such as faults in the control system caused by a power failure. A sample of hazard identification for LNG is shown in the Table 1 below.

Table 1: HAZOP of an LNG facility

Source: Woodward and Pitblado (2010)

Failure Mode and Effects Analysis (FMEA)

This is a methodology employed in the analysis of the potential reliability problems at an early stage of a product development lifecycle when it is still easier to initiate appropriate actions intended at overcoming the issues involved and thereby improving the reliability of the product under design (Crow, 2002).The method is used in the identification of the potential failure modes and in the determination of the effects of the failure modes on the operation of a given product such as LNG so as to identify the necessary failure mitigation actions. A crucial element of this method is the anticipation of what may go wrong with a certain product. An extensive list of all the potential failure modes is formulated by the development team. The use of FMEA in the design process helps engineers in the designing product that are failure proof and reliable. They therefore become safe and pleasing to the customers. The FMEA process captures all the historical information to be used in the future improvements of a certain product (Crow, 2002).

The use of FMEA in the LNG process of identifying mechanical and electrical failure modes as pointed out by Woodward and Pitblado (2010).

Site Selection

The site for this analysis was selected from both sea and land-based LNP storage facilities. The process of risk management involves the analysis of all aspects of potential loss. The process of managing the risk. The process of LNG site selection must use community engagement in order to identify the perfect site that must have low social and environmental sensitivity as pointed out by a 2005 BHP Billiton report (BHP,2005).

LNG spill

An analysis of these hazards reveals that they have different severity indices in relation to the extent of damage they can cause to the facility, community and the business. Their rates of probability also vary. Their failure effect and hazard rates also vary. The failure effects of the various LNG hazards range from 5% to 90% which are considered critical and severe respectively in terms of severity class. This shows that a lot of care must be take to curtail this wide range of hazards resulting due to LNG incidents.

Risk Analysis

This is a systematic utilization of the available information used for the identification of hazards so as to estimate the level of risk to individuals, population, environment as well as property

Risk Assessment

The overall process that is involved in the risk analysis as well as risk evaluation and usually compares the risk analysis estimates

The Liquid Natural Gas Process Chain

It was until 1964 that the Liquid Natural gas followed a process of production, import, distribution and export that followed a due sequence as illustrated in the figure below.

The Processing of LNG form extraction to consumption (Source BV -2009)

The first step in the processing chain of a natural gas is extraction. Most countries with the large natural gas reserves export this product to other countries with no reserves. The total number of these countries is 15 but the total number of the LNG plants was 22 by the beginning of the year 2008. These countries include: Indonesia, Algeria, Egypt, Russia, Qatar, Yemen, Malaysia, UAE, Nigeria, Australia, Trinidad, Brunei and Norway. Although USA also produces the natural gas, it is mainly for domestic market as their reserve is not adequate to allow exportation. In most cases the gas supply may not be enough to meet intra-country needs hence the countries import the deficit from the countries with surplus.

Once a team of Geologists and geophysicists locate a field with potential to produce the gas, a special team is sent to drill the point the prospective field to establish the viability of the quantity of the gas and if verified, the next procedure is extraction as well as processing. It is important to note that before the commercial market of the gas was established, the gas which was associated with oil, was wasted in a flare but now its value has been established and being used as LNG. There is a procedure that the natural gas must pass through in order to be fit for sale to remove impurities that are usually associated with Natural gas, which is mainly methane. Such impurities include: ethane, propane, hydrogen sulfide (H2S), Carbon dioxide (CO2), Butane, Pentane, Helium and Nitrogen as well as water and oil. These impurities must be eliminated before liquefaction to become LNG.

The Liquefaction Plant

The second stage in the process is cleaning at the liquefaction plant where a series of processing steps ensure the removal of the impure and extraneous compounds from the raw material just before liquefaction.

Purification of the product

The main reason why this purification process is necessary is that before the LNG is loaded onto tankers, trucks or ships for transportation, the composition and combustion properties must be consistently provided. This is achieved through cooling and condensation of the gas. The consistency n the content of the LNG is critical so as to obtain pipeline-quality gas which usually contains between 86% - 99% methane. It is normally associated with long-chain hydrocarbons and other impurities that fail to be removed during the processing. The figure 2 below summarizes the stripping process of removal of the compounds from the natural gas as it leaves the ground before the start of liquefaction process.

The flow process for natural purification before liquefaction (source BV -- 2009)

Carbon dioxide and water are usually removed prior to liquefaction since they can cause the malfunctioning of the liquefaction equipment due to freezing properties. Hydrocarbons with longer carbon chain like ethane, propane butane and pentane are also removed and sold as fuel to petrochemical industries.

Liquefaction

After the removal of most impurities and long-term hydrocarbons, the gas that results is mainly methane that is ready to undergo the process of liquefaction. A refrigeration technology, which is able to cool of the gas to temperatures as low as -162oC (-259oF) is used. When it liquefies, the LNG becomes a non-corrosive liquid which is as colorless as water 50% less than the weight of water. That is to say, it is half less dense than water. The LNG is more portable than the natural gas to transport since one volume of LNG is equivalent to 600 volumes of natural gas at standard temperature and pressure. This is what makes economically lucrative to transport by truck or ship.

The authors Aldwinkle and Slater (1983) had a discussion of risk and reliability analysis of the appropriate methods to be used for certain type of offshore LNG terminals, liquefaction as well as the storage ships that are secured via a single point mooring attached to an underground pipeline (Woodward and Pitblado,2010).The use of other conventional risk assessment as well as reliability methods is presented with a caveat regarding the fact that it is very difficult to estimate the failure frequencies. The tree analysis can however be used in this case provided there is relevant data. This is used in conjunction with failure mode and effect analysis (FMEA) so as to relate the consequences of the various postulated failures.

Below are the major systems that have been identified for the fault tree analysis that leads to major events of LNG gas leakage and spillage;

Liquefaction process plant failures

Single - point mooring failures

Containment system failures

LNG transfer arm failures and LNG piping system failures

They are used in the analysis of the cost/benefit ratios for the mitigation measures that are proposed. For the single - point mooring failures scenario, the mitigation usually consists of the measures that are aimed at protecting the collision of shuttle ships involved in the loading the LNG for the subsequent delivery to various markets. The options to be evaluated in this case are;

Collision barrier

Sacrificial structure.

This involves the calculation of the structure's energy - absorbing capability. This technique can be used to effectively illustrate how risk analysis can be employed in order to design structures.

Other notable contributions have been advanced for the concept of FLEX LNG and the concept of Hoegh LNG as pointed out by Pastoor et al., (2009); Festen and Leo,(2009); Iversen and Hellekleiv,(2009 ).

Transport of LNG

There are three modes of transporting the LNG which are: sea, rail and like Japan use rail. In the sea, it is transported by using specialized LNG carriers. The first transportation of LNG was done in 1959 in the Lake Charles in Louisiana and was destined to Canvey Island in the United Kingdom. The name of the voyage was MV methane. The initial stages of using the sea to transport the LNG were accompanied by training on the safety systems and also the training of the ship crews that operated the vessels. This training process has undergone improvement over the periods and now is robust in its operation. From that period, over 45,000 voyages have been carried out without losses in the LNG cargo. These days, the cargo is transported by specialized double walled ships that are designed to carry cargo at the atmospheric pressure and at a temperature of about -162oC. The carriers blends a conventional ship technology with specialized designs for handling cryogenic-temperature cargoes that isolate the LNG from the bottom as well as the sides of the hull. This is meant to conform to the International Gas Codes regulations. Some safety layers are also added to cushion the ship in case of collision. These insulating layers have also a purpose to limit the amount of LNG that evaporates throughout the voyages. While, there are about 300 ships that transport LNG daily, the International Maritime Organization (IMO) has adopted about 40 conventions as well as the protocols together with codes of conduct in the construction of equipment carrying the Liquefied gases in the seas. Due to the demand for LNG exports, the construction of the vessels for transporting LNG cargo has increased. A 135,000m3 capacity carrier costs about between $225 -- 250 million and a larger carrier costs about $300 million.

In regions around the world where the liquefaction plant is closer to the regasification facilities, trucks come in handy and cost effective means of transportation. Transportation of LNG involves the use of double-skinned special trucks to transport the liquefied gas effectively and quickly. In many areas, of the world where such trucking is done, it has become a mature industry since it began in 1968. Using tanks to transport LNG can only be limited to 20 tons by the industry regulations. The following countries have adapted tank transportation: Belgium, China, turkey, Australia, Germany, Brazil, Portugal, UK, Korea, Japan, U.S. And Norway.

The use of trucks to transport LNG is important for small scale "peak shaving" operations. This provides flexibility to the gas pipeline network. In 2003, it was noted that there were about 240 LNG facilities the world over. Of the 240, 113 were located in the U.S. And included 96 that were connected to the United States pipeline grid. The satellite LNG storage capacity came to about eighty percent of the U.S. total (IELE, 2003).

The largest of the facilities has been noted to liquefy gas that is drawn from the extensive interstate pipeline grid. However, smaller facilities that lack liquefaction capabilities do receive the commodity by trucks as pointed out by Parfomak, (2004). The LNG trucks do have a very robust construction as compared to the usual trucks for transporting fuel. The place where LNG is majorly transported by trucks is China .This is because the pipeline networks are developed well Woodward and Pitblado (2010).The various satellite LNG tanks are normally surrounded by huge containment that act as limits to the spread of an unprecedented LNG spill as well as the size of the potential vapor cloud as pointed out by Gaul and Young (2003). The risks in this mode of LNG transport are of highway collisions, spills on loading, truck rollover as well as storage tank leaks. The scale is usually smaller but the event frequency is noted to be higher as compared for the ones for LNG import terminals as well as the regasification systems.

Receiving LNG and its regasification

This is the fourth stage in the LNG handling process. This chain involves the marine terminals where the LNG is stored before it is reconverted back to gas thus, regasification. There are currently about 60 LNG terminals used in the import handling of the LNG which operate worldwide. The largest importers of LNG are Korea, Japan, Taiwan, India in Asia while in the Americas, the largest importers are USA, Mexico, Brazil, Argentina, and Chile and in Europe, the list includes; Belgium, UK, Italy, Spain and Portugal.

There are also ships that are constructed to function as Floating Regasification Units (FRU). The regasification units are common in the U.S., Australia, Brazil and the UK. There is another facility that may be used to receive LNG known as the Peak-shaving facility. It is a system of plants operated by utilities. They store the LNG in the tanks until it is demanded when the demand is high. It is normally connected to the gas supply system and usually consists of the equipment whose function is to convert natural gas into LNG, as well as the storage tanks. And other equipment that converts back LNG into gas form. There exist different structures or designs of LNG terminals but the basic process is similar. The figure 3 below shows the major components of the LNG equipment for import and regasification. The import and Regasification equipment has major components as follows;

Unloading arms, Storage tank, Cryogenic pipes, Boil-off gas compressors and re-condensers Low pressure pumps, High pressure (HP) pumps, and Vaporizers.

Figure 3 an example LNG terminal for importation (Source: BV 2009)

A drop in the pressure and an increase in temperature usually cause the vapor to be produced above the LNG cargo. It is called boil-off. The natural gas is usually odorless but many countries have made a requirement that for safety purposes, a typical odorant known as THT or tetrahydrothiphene or mercaptan to warn the consumers in case there is a leakage that might cause fire.

Unloading LNG

The process of unloading NLG involves the use of arms which are designed to transfer the LNG safely from the ship to the terminal. The arms connect the ship system to the terminal via the piping. The diagram showing the arms that connect the ship and the terminal is shown below. The loading arms must be chilled to a temperature of nearly -162oC or -259oC before the loading operations begin. Therefore the arms must be made of materials that can withstand contraction and expansion over a wide range of temperature. The only risk is the possibility of a rupture of these arms due to ship movements hence these arms are usually equipped with emergency disconnect system to protect the ship and the terminal arms. This mechanism provides for abrupt disconnection of the LNG carrier from the terminal at the same time it limits the amount of Liquefied Natural Gas released. When the ship moves vigorously, that is likely to break the arms; disconnecting system is activated as a matter of emergency.

Storage of LNG

There are specially designed tanks that can accommodate the LNG at these Cryogenic or very low temperatures and also limit the extent of evaporation. The storage process begins by using the cryogenic pipelines to transfer the LNG to, offshore import facility terminals, onshore import terminals and peak-shaving facilities. During the storage process the tanks are maintained at constant temperature and pressure in order to allow a minimized amount of boil-off gas to escape from the tank. The gas may again be recaptured and then condensed after which it is re-injected into the tank. This helps maintain a positive pressure during the unloading of the ship. There exists a venting feature fitted into the storage tanks and this offers protection against overpressure that may be as a result of roll-over condition. Roll over is prevented by densitometers which are utilized to monitor the layers inside the tank when different compositions of LNG develop inside the tank which can cause the rapid release of LNG gases. The densitometers also allow the operator to mix the LNG thereby breaking the stratification that exists within the storage tanks. It is important for an import terminal to possess two or more storage tanks for the LNG. The types of tanks that can be used for storage are; single containers, membrane tanks, double container tanks, in-ground tanks, and full container tanks.

Occupational Health and Safety Management system (OHSM)

Occupational Health & Safety Management System (OHSM) is a special framework of processes as well as procedure in order to control various occupational and safety risks while improving the performance via a process of continuous improvement (SSA,2010). The key personnel of LNG production, transportation and storage should be trained on the health, safety and security concerns regarding LNG using specific guidelines provided by OSHA (OregonLNG, 2008).

Process Safety Management of Highly Hazardous Chemicals; Explosives and Blasting Agents (PSM)

OSHA has specific guidelines to be used in the LNG process. They come in the enforcement of PSM standards applicable at LNG facilities that are engaged in the transportation of LNG via pipelines that are subjected to pipeline safety laws and regulations (49 U.S.C. 60101 et seq.) as well as and Parts 192 and 193 of 49 CFR Chapter I that are to be enforced by the Office of Pipeline Safety (OPS) Department of Transportation (DOT).An analysis of the regulating and enforcing bodies' records reveal a high level of compliance to the PSM standards (Runyon,1998). OSHA has therefore determined that the existing OPS regulations do fully address fire and explosion hazards in the LNG distribution and transmission process.