DRILLSTRING DESIGN

REF: DRILLING ENG’G. BY NEAL ADAMS

WELL PLANNING CONSIDERATION

Drill-Collar Selection• The drill collars are the first section of the drillstring to be

designed. The collars' length and size affect the type of drillpipe that must be used.

• Drill-collar selection is usually based on :1.) buckling considerations in the lower sections of the drillstring when weight is set on the bit or

2) using a sufficient amount of drill collars to avoid running the drillpipe in compression.

• The design approaches that satisfy these design criteria are the buoyancy factor method and the pressure-area method, respectively.

DRILL STRING DESIGN CRITERIA

• The controlling criteria for drillstring design are – Collapse and tension are used to select pipe

weights, grades, and couplings– slip crushing affects the tension design and

pipeselection.– dogleg severity is performed to study the fatigue

damage resulting from rotating in the dogleg angle change

Collapse Load

• The highest external pressure tending to collapse the string will occur at the bottom when the string is run empty into the hole. (This only occurs when running a Drillstem Test - DST tool).

• The highest anticipated external pressure on the pipe is given by• Pc = 0.052 x MW x TVD• where:

Pc = collapse pressure (psi)MW = mud weight (ppg)TVD = true vertical depth (ft) at which Pc acts

This assumes that there is no fluid inside the pipe to resist the external pressure (i.e. no back up).

Tension Design

• The tension load can be evaluated after the weights, grades, and section lengths have been established from the collapse designs. Buoyancy is included in the tension evaluation due to the manner in which biaxial stresses

• alter the collapse properties of the pipe. Since the string is designed with a maximum load concept, it is important that buoyancy be included in the design.

• The tension design line is established as the maximum load resulting from applying three different design considerations, including 1.overpull2.design factors,

3.slip crushing. Each consideration is applied separately to the load line

• a safety factor for slip crushing is generally added to the loadingline calculated above. This allows for the interaction of hoopstress (Sh) caused bythe slips and the tensile stress (St) caused by the weight of the string. This effect reduces the allowable tension load by a factor (Sh/St) which can be calculated as

• follows:

• where:• D = O.D. of pipe (in)• L = length of slips (in)• K = lateral load factor• Slips are normally either 12 in. or 16 in. long, and for normally lubricated slips K = 4.00.If used

pipe is being considered the actual D must be measured and Sh/St calculated from the above equation. Having obtained this factor it is applied to the tension loading line.

•

COLLAPSE DESIGN

BURST DESIGN

TENSION DESIGN

SLIP CRUSHING

Dogleg Severity Analysis

• The most common type of drillpipe failure is fatigue wear. It generally occurs in doglegs where the pipe goes through cyclic bending stresses. These stresses occur because the outer wall of the pipe in a dogleg is stretched and creates a greater tension load (see Fig. 15-20)