Basics of Offshore Well drilling

Common drilling of wells are two basic types —exploratory, i.e. to find new oil or gas deposits  and development, i.e. to prepare the discovery for production. Water depths range from 50 to 400 feet for jack-up rigs to up to 10,000 feet for semisubmersibles and other types of vessels like drillships.

Before drilling an exploratory well, an operator will conduct geologic surveys of an area to determine the potential for oil or gas deposits. The operator then hires a drilling contractor to drill exploratory (also term “wildcat”) wells offshore. The oil company chooses the location and supervises the operation, which may take as little as 15 days or as long as 12 months to drill a single well depending on the complexity of the project.

Offshore rigs are designed for efficiency in living and working, with emphasis on keeping the rig steady in gulf or ocean waters. Offshore wells are drilled in much the same way as their onshore or typical landrigs  —with several allowances for the offshore environment. A string of tubes made from lengths of steel pipe permits drilling fluids to move between the rig—at the water’s surface—and the sea floor. This tube is called a “riser.” Such riser exists only on deepsea semi drilling, as for jackups, we normally call them drill string. The riser is fitted with ball-and-slip joints that permit the long string of riser pipe to move up and down and bend slightly with the wave-induced movement of the rig.

The well is drilled using a length of slender steel pipes and other tools that, connected, comprise a “drill string.” At the bottom of the string of pipes is a hole-boring device called a “drill bit.” Heavy sections of pipe, called “drill collars,” add weight and stability to the drill bit. Each ordinary pipe in the string is about 30 feet long and weighs few hundred pounds; drill collars can weigh 4,000 Pounds or more per 30-foot length.

As drilling proceeds, and the well gets deeper, the drilling crew adds new sections of drill pipe to the ever-lengthening drill string. Hydraulic devices keep constant tension on the drill string to prevent the motion of the rig and riser from being transmitted to the drill bit.

The drill string is lowered through the riser to the sea floor ( this is the case for semi drilling), passing through a system of safety valves called a “blowout preventer” (BOP, pronounced “B.O.P.”). This stack of multiple safety valves is designed to contain any natural pressures that the drillers might encounter beneath the Earth’s surface. Its purpose is to prevent a possible “blowout”—an uncontrolled eruption of oil, gas or wellbore fluids due to excessive natural pressure.

 
Basics to Drilling_choong
 

Offshore Drilling Mud solids control

The drilling fluid called mud only looks like mud. Actually, it is a complex mixture of water or oil, clays, and chemicals. It's composition and properties have been carefully studies and tested. The study is closely associated with chemistry, math, and physics. The term mud refers technically to a suspension of solids in water or oil, while drilling fluid is a broader term including air, gas, water, and mud.

Drilling fluid is the more appropriate term for including all types of fluid used, but term mud is preferred the field for in naming the most common type. The drilling mud basically perform the following functions:-

1. Removal of Cuttings

2. Control Formation Pressure

3  Prevent Caving

4. Caking off Per. Formations

5. Suspension of Cuttings

6. Release of Cuttings

7. Cooling & Lubrication

8. Formation Damage

9. Formation Evaluation

10. Corrosion

Common types of mud used are :

1. Polymer Muds - incorporating generally long-chain, high-molecular-weight polymers are utilized to either encapsulate drill solids to prevent dispersion and coat shales for inhibition increasing reducing loss inhibition, or for viscosity and fluid loss.

Various polymers are available for these purposes, including acrylamide,cellulose and natural gum-based products. Frequently, inhibiting salts, such as KCl or NaCl, are used to provide greater shale stability. These systems normally contain a minimum amount of bentonite. Most polymers have temperature limits below 300°F, but under certain conditions, may be used in wells with appreciably higher BHTs.

2. Oil-based muds. Oil-based systems are used for a variety of applications, where fluid stability and inhibition are necessary, such as high-temperature wells, deep holes, and where sticking and hole stabilization are problems. They consist of two types of systems:

a. Invert emulsion muds are water-in-oil emulsions, typically with calcium chloride brine as the emulsified phase and oil as the continuous phase. They may contain as much as 50% brine in the liquid phase. Relaxed, invert emulsion muds are a “relaxed” emulsion, and have lower electrical stabilities and higher fluid-loss values. Concentration of additives and brine content/salinity are rheological, filtration and varied to control emulsion stability.

b. Oil-based muds are formulated with only oil as the liquid phase and are often used as coring fluids. Although these systems pick up water from the formation, no additional water or brine is added. All oil systems require higher additional gelling agents for viscosity. Specialized oil-based mud additives include: emulsifiers and wetting agents (commonly fatty acids and amine derivatives) for high molecular weight viscosity; high-molecular-soaps; surfactants; amine treated organic materials; organo clays and lime for alkalinity.

3. Synthetic muds. Synthetic fluids are designed to mirror oil-based mud performance, without the environmental hazards. Primary types of synthetic fluids are esters, ethers poly alpha olefins and isomerized alpha olefins They are esters, ethers, olefins. environmentally friendly, can be discharged offshore, and are non-sheening and biodegradable.

Mud weight, or density, is the weight per unit volume of the mud. With simple water base mud a mud, density can be regarded as measure of the suspended solids.

Excessive solids can:
􀂾 cause wear on pumps bits drill strings; and 􀂾retard penetration rates;
􀂾cause a thick filter cake to be deposited on permeable formations;
􀂾cause fluids loss to the formation;
􀂾causes unnecessary work for the pump, having to push unwanted weight in the circulating fluids.


Solids Control

Offshore Marine Project Management

Offshore and Marine projects are generally large scale and their production is spread over several years. The planning of these projects is therefore diverse, and covers a wide range of activities. The cost implication is also very huge and risky and critical of management control over the overall design and construction phase. Cost, material, logistic and resources control are going to be very dynamic in such kind of projects.

Several levels of planning are usually identified.

Corporate Planning, which looks at the long term future of the whole company. A corporate plan may look five years ahead, and include major investment plans, product changes and other important issues. It is usual for the planning of a marine project to be done within the framework set by the Corporate Plan. In some cases, where small ships are built in a generally shorter timescale or in the case of repair and conversion companies, the Corporate Plan may have a shorter timescale. But there should be some idea of where the company is heading

For a company engaged in a number of offshore marine projects, the corporate plan may need to correspond to some recognisable programme management. Selection of the most appropriate projects and their co-ordination is important.

Strategic Planning, covering the duration of a project :

The strategic plan for a project has a timescale determined by the timescale of the project. For a typical ship this is about two years as the time between contract and delivery. Again the plan may have a longer or shorter duration depending on the project size.

Strategic planning is essentially a network plan for the construction of the ship (which may be likened to a civil engineering project in that it takes place usually on a fixed site to which the parts of the ship are moved.

Tactical Planning, covering the next few months in a department :

At this level the focus moves from a network plan to departments, which may be organised on batch or flow production lines. The tactical plan is a response to the demands set by the strategic plan. The tactical plan often includes work for several projects which are running in parallel.

Detailed Planning, covering the next few weeks for a work station :

The detailed plan is essentially a schedule for the individual work stations. This attempts to strike a balance between the internal efficiency of the work station activities and the need to produce parts and other items to a timetable which will allow the overall project timescale to be maintained.

The project plan is not limited to the production activities, but must also include technical elements, curement and other pre-production functions.

In order for a offshore or marine vessel to be produced, a number of key questions need to be answered.

What is to be produced?

When is it to be produced?

Where is it to be produced?

With what resources?

How will it be produced?

The production of a marine or offshore vessel depends on the generation of a large set of information. Historically, technical departments were concerned primarily with function, and other information was developed within the production departments.

Now most of the information is developed within technical and other departments,and must be included in the project planning and management process. The Korea and China yards are now applying Mega blocks concept to speeding up their construction processes and thus shortening the project lead time compared to other developing countries yards.

 
MAR8102 Marine Proj Mgt Post School

More on marine systems design...

Introduction to basic marine system concepts

• The various duties of an marine engineer relate to the operation of the ship or offshore rig in a safe, reliable, efficient and economic manner. The main power and propulsion machinery installed will influence the machinery layout and determine the equipment and auxiliaries installed. It includes the number of personnel on board in which the auxiliary services and living quarter onboard will provide the necessary comfort to the crew.  Hotel systems provide the hotel facilities. Examples are cabins, galley equipment, laundry equipment, drinking water systems and waste disposal systems.

• Support systems provide the support function, e.g: electric power supply systems, hydraulic power supply system, lubrication oil system and compressed air system.

• Operational systems provide the operational functions, e.g: cargo-handling and conditioning systems, combat system, fishing gear or oil drilling equipment or other specific vessel system.

• This will further determine the operational and maintenance requirements for the ship and thus the knowledge required and the duties to be performed by the marine engineer.

• A vessel or offshore rig is designed to perform a certain operational task: The mission of the vessel determines which functions are needed on board. Ship systems provide these functions. If necessary a system may be categorised into subsystems and ultimately into components. Example, the power, controls, heating and cooling, etc.

marine service system functions.doc 

Marine Systems

Ship Systems

This article briefly shows some typical ship's ancillary services. These info are well applicable not only at concept stage but also during basic design when actual components have not yet been chosen.

Auxiliary oil fired boiler

-Steam production ability:

Saturated steam, 7bar g, 170°C.
1 kW corresponds to about 1,6kg/h steam or
1 MW corresponds to about 0,42kg/s steam

-Fuel oil (FO) consumption:
1,0kW corresponds to 0,105kg of HFO/h
1,0kg/h steam corresponds to 0,066kg of HFO/h

Fuel oil systems

- Main engine (ME) and auxiliary engine (AE) fuel oil consumption
Normally engine suppliers give the specific fuel oil consumption (SFOC) based on ISO 3046/1 standard.

Emergency diesel generator :

SFOC: typical value 0,25kg/kWh
100 kW of power means about 28-litre fuel oil consumption per hour.
FO tank to be dimensioned at least for 36 h constant running according to SOLAS.

Heavy Fuel Oil (HFO) Tanks :

Storage tanks-
Minimum temperature in storage tanks depends on the pour point of the HFO. The temperature of HFO should always be kept higher than pour point to avoid filter blocking, and other similar problems.

Settling tanks-
To allow reasonable separation the tank should be sized for 24h consumption.
The settling temperature to be calculated to be about 70degC.
According to the latest SOLAS rules double settling tanks are needed.

Service tanks-
Service tanks should be sized for 10h - 12h consumption.
The temperature in service tanks to be calculated to be 75°C.

According to the latest SOLAS rules two separate service tanks are needed.

Lubrication oil systems:

- Lubrication oil consumption

LO consumption for medium speed engines in average is about 1,0g/kWh.
-System oil tanks

In case main engines are so-called dry sump engines, there should be a system oil tank on the double bottom.


Sewage systems:


It is highly recommendable to specify a biological sewage treatment plant for all types of ships because of the environmental reasons, further reading required on latest Marpol MEPC159(55) requirement. The plant is typically dimensioned to treat full black water load.

Galley waste water is normally not led to sewage treatment plant, because it slows down the biological process.

Grey waters have been discharged directly overboard or collected to grey water storage tanks. Some times grey waters have been chlorinated before discharging overboard but not really biologically or chemically treated onboard.

Other systems not described could be referred to below slides or will be continued in next blog article.

Ship Designs - General Info