Method of Calculating Wellhead System Load Capacity

A method of calculating the load capacity of a wellhead system according to any preceding claim wherein the rated load capacity of the clamping system is given by:

• RL
• =
• (
• Q
• ×
• Φ
• ×
• A_s
• )
• /
• SF
• where,
• Q is the contact stress at the gripping interface,
• ϕ is the grip coefficient at the gripping interface,
• As is the surface area at the gripping interface, and
• SF is a safety factor with respect to the minimum load capable of causing slip between the inner tubular member and the gripping surface.

A method of calculating the load capacity of a wellhead system, the wellhead system comprising a clamping arrangement comprising a collar having an externally tapered surface, the arrangement also including an annular component with an internally tapered surface, the collar and the annular component being relatively axially moveable between a first position in which the tapered surface of the annular component exerts no radial force on the collar and a second position in which the tapered surface of the annular component exerts sufficient radial force to distort the collar inwardly in order to grip an inner tubular member within an outer tubular member to support a load of the inner tubular member, the inner tubular member having an outer surface including a ridged profile, and the method comprising:

• determining a first contact stress at a first interface between the annular component and the collar, by equating a radial deflection of an inner surface of the annular component with a radial deflection of an outer surface of the collar;
• determining a second contact stress at a second interface between the collar and the outer tubular member assuming negligible hoop stiffness of the collar;
• and
• using the first contact stress, the second contact stress and the axial movement of the annular component to determine a contact stress at a third interface between the outer tubular member and the inner tubular member, by equating a radial deflection of the outer surface of the inner tubular member with (i) a radial deflection of an inner surface of the outer tubular member plus (ii) a depth of penetration of a ridge of the outer surface of the inner tubular member into an inner surface of the outer tubular member plus (iii) a radial dimension of an initial gap between the outer surface of the inner tubular member and the inner surface of the outer tubular member.

A method of constructing a wellhead according to claim 25 further comprising selecting a material and dimensions for:

• the collar;
• the annular component;
• the inner tubular member;
• and
• the gripping surface.