Stress Distribution at the Fillet of an Internal Flange

Stress Distribution at the Fillet of an Internal Flange
Posted by rtyu1yu on 2021/08/24
Stress Distribution at the Fillet of an Internal Flange

    Stress Distribution at the Fillet of an Internal Flange

    This paper deals with the determination of the stress distribution at the fillet of a ANSI B16.5 flanges attached internally to a hollow cylinder. A load parallel to the axis of the cylinder

and of variable eccentricity acts on a bearing plate which rests on the flange. The strains are measured by means of

electrical resistance wire strain gages. The ratios of the mean cylinder diameter to the cylinder wall thickness and of the

mean cylinder diameter to the flange thickness are varied. The principal stresses at the fillet are given as functions of

these parameters. The experimental results are compared with the stresses calculated on the basis of an approximate

theoretical solution for both an axial and an eccentric load.


    Abstract Joining of steel pipes and pipe flanges use today the conventional method of fusion welding, where the flange is

girth-welded onto the pipe. However, fusion welding of flanges to pipes is associated with many disadvantages such as the

final quality of the weld, degradation of the mechanical properties of the base pipe near the heat affected zone, defects and

cracks appearing in the weld, misalignments, to mention a few. The current study proposes a novel pipe-flange connection to

replace the fusion welding process of steel pipes with a method based on cold working. The method is based on that the steel

pipe is inserted into the neck of the flange, in which two circumferential grooves are manufactured. An expansion tool having

two teeth is entered from the open side of the connection and is expanded hydraulically such that the teeth deform the pipe

and cold work it plastically into the grooves. This will provide a strong joint between the flange and pipe. In this study

the performance of the connection is maximized by optimizing the design of the flange and the expansion tool.


    The use of bolted flange connections in the offshore wind industry has steeply risen in the last few years. This trend is

because of failings observed in other modes of joints such as grouted joints, coupled with enormous economic losses

associated with such failures. As many aspects of bolted flange connections for the offshore wind industry are yet to be

understood in full, the current study undertakes a comprehensive review of the lessons learned about bolted connections from

a range of industries such as nuclear, aerospace, and onshore wind for application in offshore wind industry. Subsequently,

the collected information could be used to effectively address and investigate ways to improve bolted flange connections in

the offshore wind industry. As monopiles constitute an overwhelming majority of foundation types used in the current offshore

wind market, this work focusses on large ANSI

welding neck flanges
in the primary load path of a wind turbine foundation, such as those typically found at the base

of turbine towers, or at monopile to transition piece connections. Finally, a summary of issues associated with flanges as

well as bolted connections is provided, and insights are recommended on the direction to be followed to address these

concerns.


    As per recent reports, the offshore wind sector could bring in £17.5 bn investment to the U.K. economy over the next few

years after faster than expected cost-cutting slashed subsidies for the technology by half [1]. On top of that, the baseline

scenario for the United Kingdom’s installations by the end of 2030 is to reach the capacity levels of 40 GW, four times the

current state [2]. Additionally, the target of £100 per MWh set for the year 2020 regarding the levelised cost of energy

(LCOE) of offshore wind was achieved in U.K. projects four years earlier in 2016 [3]. The above figures reinforce the need

for new technological developments that will enable the utilisation of larger and more efficient offshore wind turbines

(OWTs). In this direction, one of the most important concerns is the support structure of the turbine’s tower, which

requires further study concerning not only the feasibility of future installations, but also current problems that need to be

better understood and addressed.


    OWT structures, which are quite large in thickness and diameter, operate in the hostile marine environment, where

variable amplitude loads are constantly applied on different parts of the structure [4,5]. In the offshore industry, grouted

connections were initially used to charge the transition piece (TP), with a certain overlap length, on the monopile (MP)

foundations. Therefore, there is a tube-in-tube connection, wherein the space between the two tubes is filled with grout

(Figure 1) [6]. Towards the end of last decade, numerous grouted connection joints between large diameter monopiles and

connecting tubular steel transition pieces at the base of overlying support towers were found to be failing. For the majority

of U.K. offshore MPs that experienced grout cracking and failures, the issue was recognised to be primarily owing to the

widespread absence of shear keys (or weld beads) on straight MP and TP surfaces. Bending moments as a result of complex wind

(which was the main difference in loading conditions compared with oil and gas platforms) and wave loading were important

design considerations that were not accounted for during design of grouted connections for OWTs. Furthermore, axial

connection capacity was found to be significantly lower than that assumed previously owing to the MP scale effect, lack of

manufacturing and installation tolerances, and abrasive wear due to the sliding of contact surfaces when subjected to large

moments. Typical failure modes included dis-bonding, cracking, wear, and compressive grout crushing failure.


    The number of bolts depends on the ANSI plate flanges

radius and thickness, type of tool used, size of the bolts, and predicted loads on the structure. These bolts serve the

purpose of exerting a clamping force to keep the joint together [20]. The behaviour and life of the bolted joint depend on

the magnitude and stability of that clamping force. The preload is created by the tightening process during the assembly of

bolt and nut in the joint to provide enough clamping force on the joint. Therefore, the bolts need to be preloaded at the

assembly stage in the flange connection. An intuitive analogy would be to think of the bolts and the joint members as elastic

parts. In that way, they can be modelled as spring elements, where the bolts are stretched in their elastic region when

tightened, in order to compress the joint. The joint has a much stiffer elastic constant compared with the bolts, depending

on material and dimensions.


    It is possible to consider the bolt as an energy storage device, which accumulates the necessary potential energy to

clamp the joint and is subjected to several environmental and operative conditions that may affect its behaviour [20]. The

objective is for the preload on the bolt to be maintained at a certain level, but, owing to a large number of influencing

factors, it is almost impossible to achieve or retain the desired state. It must be noted though, that the main concern is

not the value of preload on the bolt, but maintaining the sufficient level of clamping force that holds the joint together.

Moreover, if the clamping force is too low, the joint could loosen and be subjected to more severe consequences owing to

cyclic loads. On the other hand, if the bolt is over-tight, it could exceed its proof load and may break under external load.

In fact, during the tightening process, a torque is applied to turn the nut and the bolt stretches. This operation creates

preload in the bolted joint. This sequence of events, at any point, controls the preload. It is possible to control the

preload through torque or turn or stretch or through a combination of all of them. In all of the control strategies, the

torque is used to tighten the fastener even if other mechanisms are used to control the tightening. There are a lot of

uncertainties in the relationship between the control parameters like torque and the preload, which could be minimised by

measuring and controlling the build-up of bolt tension. This is the motivation for creating the family of tools called bolt

tensioners. Using the bolt tensioner is nowadays a common practice during the installation of offshore wind turbines.


    The employment of ANSI blind flanges

connections for OWTs has considerably increased in the past decade owing to the failures and subsequent economic losses

associated with grouted connections. In this study, the issues and opportunities associated with bolted flange connections

have been thoroughly reviewed and discussed for application in the offshore wind industry. The key conclusions drawn from

this study are as follows:


    The advantages of bolted flange connections include the provision of direct load path through the primary steel alone,

thereby avoiding slippage, reducing steel requirements compared with grouted connections, the absence of curing time, and

easiness to inspect and monitor the MP–TP connection.


    The challenges associated with bolted flange connections include material selection issues, short-term relaxation of

bolts, issues associated with load distribution in threads, and static failure of bolted flange.


    The main cause of short-term relaxation is the embedment that occurs mostly owing to surface irregularities as well as

time-dependent creep deformation.


    The consequence of temperature differential can either increase or decrease the clamping force depending on the thermal

expansion and contraction coefficient of the materials employed in bolted connections.


    The setups associated with bolted joint such as washers, lubricants, coatings, and gaskets play a pivotal role in

creating and maintaining integrity in bolted joints.


    The failure modes observed in bolted joints include self-loosening, fatigue failure, corrosion, and galling.


    An expected trend in the bolted flange connection is the increased usage of tensioning tools compared with torqueing

applications.


    Further studies in the offshore wind industry can enable the optimal use of ANSI threaded flanges connections in design, manufacturing, installation, operation,

maintenance, and decommissioning phases.


    Ring flange connections for tubular towers, like those for wind turbines or chimneys, are subjected to significant

fatigue loading. Next to the bolts, the weld connecting the flange to the tower shell also needs to be checked against

fatigue failure. The flange causes local bending moments in the shell, which increase the meridional stress, i. e. stress

concentrations occur. In this paper, the influence of geometrical imperfections on such stress concentrations is quantified

and the influence of flange geometry on resulting stress is investigated. Recommendations are given for flange dimensions and

the design procedure.


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